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MOTION SYSTEMS HANDBOOK
Schadenfreude for Theranos
— and satisfaction in how engineering doesn’t lie
HOW DOES IT WORK? That was the question few people asked about the mythical supertechnology supposedly at the core of Theranos — a Silicon-Valley unicorn overvalued by some $9 billion during a 2013-2014 hype height. Not that it would’ve mattered anyway, as the fictional heart of secretive Theranos changed continuously over its 12 short years. As John Carreyrou of the Wall Street Journal details in Bad Blood — a book to delight technical types — official engineering focus took multiple abrupt turnabouts … going from concept to commercialized product with scant R&D between. The company’s last offering, which one expert in laboratory medicine called “theater … not science” includes technologies quite common to medical diagnosis and motion design. The staid components for the latter are quite familiar to the Design World and linearmotiontips.com audiences: Linear guides and dc motordriven actuators form a small three-axis Cartesian setup for the movement of samples to various stations inside the testing cube. Check out this Handbook’s section on stepper motors for a similar pipetting example. But it was a long and winding road to the final Theranos design.
Subsystems in the final Theranos design are common to motion design and medical diagnosis. A cartridge (7) allows initial loading of samples into the machine, and a camera (10) captures sample data for indexing processed results. A traditional Cartesian robot (1) moves samples around to subsystems in the machine. The sonicator (8) is a straightforward sample agitator. Other thermocycler (3), centrifuge (4), cytometer (5), thermal system (6), fluorescence-based isothermal detector (7), and spectrophotometer (11) offerings exist in industry; luminometer and fluorometer (9) biotechnologies have been around for decades. As Carreyrou details in Bad Blood, one company leader was so unfamiliar with the technologies inside the miniLab that its engineers were able to trick him into calling its sample-moving end effector an endofactor as a prank.
2005 — Theranos begins with the concept of skin-wearable blood diagnostics capable of drawing tiny amounts of blood through microneedles and responding with real-time drug therapy. Then it scales back its ambition to build a machine to execute batteries of tests on tiny volumes of blood using micro and nanotechnology. Ultimately, this takes the form of the erratic Theranos 1.0 — a machine
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that’s supposed to read data on bloodcarrying cartridges using microfluidics and biochemistry. 2007 — Focus shifts away from the microfluidics to the humbler aim of traditional lab diagnostics — but still using drop-sized blood samples. The company modifies and reprograms Fisnar adhesivedispensing machines to make the Edison.
8 • 2018
The unreliable machine only executes tests based on immunoassays and no other tests based on general chemistry, cytometry, and DNA amplification. But its look is sleek — created by former Apple designers. 2011 — Prompted by partnerships with Safeway and Walgreens, the company begins work on its miniLab (4s), an
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Ultimately it is a few engineers, scientists, and lab workers who triumph over deception, threats, intimidation, and triangulation to bring the fraud to light. Of course, questions about the engineering as well as financials should have been asked sooner. Francine McKenna of marketwatch.com marvels at how investors duped out of some $700 million by the Theranos fraud failed to ask for audited financial information — a pretty standard request for verifying the monetary health and promise of a company. And British professor and experimental physicist Richard Jones adds at softmachines.org that innovation in the physical and biological realms is fundamentally more costly and difficult than that in the purely digital world — a fact for which realistic expectations on Thernos’ commercialization timeline should have accounted. Jones also laments that the original Theranos patent (on the use of silicon microneedles from drawing blood and realtime treatment of detected conditions) was “astonishingly broad” — so roadblocks scientists who may have otherwise advanced related technologies for inexpensive yet reliable diagnostic testing. But alas on this front at least, there is hope. Visit our sister sites massdevice.com and drugdeliverynews.com and search on point-of-care, wearable, and microfluidics
for an abundance of news on promising research, devices, and technologies. Many of the announcements detail successful trials that quantify results or demonstrate the functionality of technologies like the ones Theranos initially claimed to have in development. J-Pac Medical announces lab-on-chip reagent blister development kits for diagnostic applications. A company called Genalyte is developing a diagnostics device to accurately perform more than a dozen tests on just a few drops of blood. Nemaura shares data from home-use study of wearable glucose monitor. Abbott’s i-STAT handheld blood analyzer allows point-of-care testing with an ever-increasing array of test cartridges and connectivity options. Raumedic shows solutions for wearable drug-injection devices at Pharmapack 2018 … and there’s lots more good news from the field. In fact, one standout technology from DNA Medicine Institute has just become a subject of NASA’s Civilian Commercial Readiness Pilot Program (CCRPP) — granted to vetted designs demonstrating potential maturation aligning with NASA interests and eventual marketability. The program aims to give International Space Station astronauts point-of-patient care through rHEALTH smallvolume blood analyzers and SKYE Sensor vitals monitors. Only more peer reviews, research and development, testing, and regulatory clearances will prove rHEALTH or the other designs out. But it’s a delicious reality that the stubborn old fields of science and engineering ultimately demand verifiable results.
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MOTION SYSTEMS HANDBOOK
Are the best engineers
born or made?
YOU MIGHT THINK the way to make better engineers is to attack problems from a different angle when they had to improve engineering education. Although there may be diverse but overlapping backgrounds. something to that idea, it is becoming apparent that factors “When someone thinks they understand something, outside the formal learning process may outweigh anything the mind edits reality so efficiently that errors can be done on a campus. hard to perceive. But when someone observes a scenario For example, it increasingly looks as though the they are unfamiliar with, a part of the brain operates engineering mindset is a function of preschool experiences. inefficiently, giving us time to see outliers and consider Consider this fun fact: For every 15 minutes of play, their significance,” summarizes Harvard University assistant kids tend to use a third of that time learning about professor Sarah Lewis in The Rise: Creativity, the Gift of mathematical, spatial, and architectural principles. Thus the Failure, and the Search for Mastery. new wave of thinking is that the best engineers are those And though it’s clear that good engineers are who were allowed to play unsupervised much of the time. persistent in the face of difficulties, even the best Studies by M.I.T. professor Laura Schulz reinforce this engineers are likely to go off course if they’re stubborn idea. She ran tests on four-year-olds wherein one group about the wrong things. Here’s how Angela Duckworth, was formally instructed in how a squeaky toy worked, author of Grit: The Power of Passion and Perseverance, while the other was shown the feature “accidentally.” puts it: “You have to figure out when to give effort and When both groups of kids were left alone to play with when to withdraw it. High-functioning people are able to the toy, the group that “discovered” the squeaky feature do both and somehow have an eye for asking whether played with the toy longer. That group of kids also they are in the game too long. The higher level the goal, uncovered hidden features at which the instructor the more you should be tenacious. The more concrete or hadn’t hinted. particular it is, the more you should be willing to give it up.” The takeaway of this exercise was that the fourNone of this stuff, of course, is taught in school. It year-olds taught how the toy worked never uncovered probably helps explain why some of the best engineers everything it could do. Their curiosity had been deadened. have never seen the inside of an engineering classroom. And kids who aren’t terribly curious don’t grow up to be great engineers. At least that is the thinking among an increasing number of people responsible for hiring engineers. At NASA’s Jet Propulsion Lab, for example, candidates for engineering jobs now get asked about their hobbies and how much they played in their childhood, regardless of their academic record. This line of questioning has its origins in a realization that younger JPL engineers were not as comfortable using their hands as their older colleagues, or as good as solving mechanical problems and puzzles. But even if you hire a bunch of creative technical personnel, they won’t be much good if they all have the same background. That’s the conclusion of researcher Kevin Dunbar who followed four molecular biology labs LEE TESCHLER EXECUTIVE EDITOR and recorded the human dynamics he noticed. His key finding concerns solving problems: When all members of the lab had roughly the same knowledge, they tended to lapse into group think. Researchers were better able
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1
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MOTION SYSTEMS H A N D B O O K VOLUME 4 NUMBER 3
EDITORIAL:
CO V E R P H O TO CO U R T E S Y O F i S TO C K
Actuators Pneumatic . . ........................................................... 15 Electrical .............................................................. 20 Ball screws ....................................................................26 Bearings Plain ...................................................................... 30 Rolling Element . . .................................................. 33 Belts & Pulleys . . ............................................................ 35 Brakes & Clutches . . ......................................................39
8
Cables, Power & Data Connections. . ......................... 42 Chain ..............................................................................46 Compression Springs ................................................. 50 Controllers .. ................................................................... 52 Conveyors .....................................................................56 Couplings .. .....................................................................62 Drives AC ..........................................................................68 DC........................................................................... 72 Encoders .......................................................................74 Gas Springs & Kinematic Holds ................................ 80 Gearing .......................................................................... 82 Gearmotors ...................................................................92 HMI . . ................................................................................96 Lead screws . . ............................................................. 100 Linear Guides, Slides & Ways . . ................................ 102 Motors DC ........................................................................104 Integrated . . ......................................................... 108 Linear .................................................................. 109 Servo .................................................................... 112 Stepper.................................................................114 Networking & Connectivity ....................................... 116 Positioning Stages, Gantries & XY Tables ............ 122 Robotics ...................................................................... 128 Seals . . ........................................................................... 130 Sensors ........................................................................ 134 Shocks & Vibration Damping ................................... 136 Torque Limiters ..........................................................140 Wave Springs ............................................................. 142
DESIGN WORLD — MOTION
Contents – Motion Systems Handbook 08-18.indd 8
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PAGE 2
Schadenfreude for Theranose — and satisfaction in how engineering doesn’t lie PAGE 6
Are the best engineers born or made?
30
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P N E U M AT I C A C T U ATO R S
Pneumatic
actuators
Pneumatic actuators are mechanical devices that transform a source of static power into useful output motion such as blocking, clamping or ejecting. They can also apply forces to motion axes and workpieces.
PNEUMATIC CYLINDERS and actuators are sometimes referred to as “bang-bang” devices, making quick moves from one end of their stroke to the other, with limited regulation of the force or move profile. On the other hand, electromechanical actuators with servo controls offer high levels of refinement in positioning, force/ torque, and accuracy. Generally speaking, pneumatics offers a cost-effective solution for rather crude, point-to-point moves, while electromechanical actuators provide high precision, at a higher cost. However, there’s a spot between these two solutions where a relatively high level of control is needed, but without the complexity and cost of electromechanical servo driven systems. Bridging this gap are pneumatics that operate in a closed-loop system—in other words, servo pneumatics. A servo system is one that uses a feedback device and a controller to monitor and correct the system’s error (in position, speed, or torque/force). Hence, integrating a pneumatic cylinder or actuator with a feedback system and a controller that can issue commands based on that feedback, gives us a servo pneumatic device. Another key component of a servo pneumatic system is a proportional valve, which precisely regulates air delivery to ensure that the commanded position and/or force is achieved. Traditional pneumatics enable rapid, highforce, point-to-point motion. Servo pneumatics provide the same speed and force capabilities, with the advantage of higher accuracy positioning, not only at the ends of the stroke, but also at intermediate points along the travel. In addition to obtaining feedback on position, servo pneumatics also monitor and regulate air pressure, which enables precise control of the force that’s produced.
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One drawback (real or perceived) to traditional pneumatics is air consumption. Air preparation and delivery costs money, and pneumatics can use a significant amount of air even when they’re not working. Servo pneumatics, on the other hand, control air flow based on the required position and force. This leads to less air consumption than standard pneumatics, by as much as 30%. It’s important to note that servo pneumatics require higher quality air than standard pneumatics. In addition to industry-standard filtration, a 5-micron filter is typically recommended for servo pneumatic systems. Why servo pneumatics instead of electromechanical actuators? Of course, electromechanical actuators can provide accurate position and force control, but servo pneumatics have a much higher power density—that is, force capability for a given size. A servo pneumatic cylinder or actuator typically provides many times the force capability of an electromechanical actuator of a similar body size, which is a significant advantage in pressing, inserting, and tightening applications. Servo pneumatics also operate with 24 Vdc power supplies, which allows them to be used in low-power applications. Lower power also reduces heat generation and thermal buildup, so they perform well in continuousduty applications and high-temperature environments. While electromechanical servo systems have been in use for decades, the adoption of servo pneumatics in industrial applications hinged on advancements in controls and software. Air is compressible, and this variable is much more difficult to define and model than the “compliance,” or backlash, in an electromechanical system. Before servo pneumatics could be commercialized, pneumatic control algorithms capable of taking into account this non-linearity had to be created. But the past ten years or so have seen the development and integration of high-response valves and digital signal processors (DSPs) that can perform high-speed computations, making servo pneumatic systems a reality, capable of providing accurate, highly responsive positioning and force control.
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P N E U M AT I C A C T U ATO R S
What are the benefits of pneumatic actuators? Pneumatics is the technology of compressed air. Pressurized gas—generally air that is dry or lubricated— is used to actuate an end effector and do work. End effectors can range from the common cylinder to more application-specific devices such as grippers or air springs. Vacuum systems, also in the pneumatic realm, use vacuum generators and cups to handle delicate operations, such as lifting and moving large sheets of glass or delicate objects such as eggs. Pneumatics is common in industries that include medical, packaging, material handling, entertainment and even robotics. By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock, a feature useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 to 5,000 psi that some hydraulic systems see. As such, pneumatics is generally used when much smaller loads are involved.
Many industrial applications require linear motion. One of the simplest and most cost-effective ways to accomplish this is with a pneumatic actuator, often referred to as an air cylinder. The most common pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be subdivided into two types based on the operating principle — single acting and double acting.
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Compact pneumatic cylinders may be up to 50% shorter than typical cylinders but maintain capacity to exert the same force as larger counterparts. Selection of compact cylinders is based on general, dimensional, performance, and material parameters.
A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hoses and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life. How do these fluid-power actuators work? Many industrial applications require linear motion during their operating sequence. One of the simplest and most cost-effective ways to accomplish this is with a pneumatic actuator, often referred to as an air cylinder. An actuator is a device that translates a source of static power into useful output motion. It can also be used to apply a force. Actuators are typically mechanical devices that take energy and convert it into some kind of motion. That motion can be in any form, such as blocking, clamping or ejecting. Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas. There are many styles of pneumatic actuators, including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders. The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders use one air port to let compressed air enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed. Double-acting
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P N E U M AT I C A C T U ATO R S
cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high pressure air. In a typical application, the actuator body connects to a support frame and the end of the rod is connected to a machine element that is to be moved. An on/off control valve is used to direct compressed air into the extended port while opening the retract port to atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston. If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element. Reversing the valving and the compressed air flow will cause the assembly to retract back to the “home” position. Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on/off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings. Pressure and flow requirements of the actuators in a system must be considered when selecting these upstream system components. Undersized upstream components can cause pneumatic actuators to perform poorly, or even make them incapable of moving their loads.
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Pressure and flow requirements of the actuators in a pneumatic system must be taken into account when selecting upstream components. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all.
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Shown here is a U-shaped linear actuator from THK America. Some in industry refer to these rugged builds as linear stages.
The dif ference between
linear actuators and linear stages ALTHOUGH THERE ARE NO INDUSTRY STANDARDS that define linear actuators and linear stages, generally accepted terminology indicates that a linear actuator is constructed with an aluminum extrusion or base ... while a linear stage is typically built on a flat machined-steel or granite base. This distinction implies that linear actuators can typically provide longer strokes and use a variety of drive mechanisms (belt, screw, rack and pinion) while stages generally have higher rigidity and use high-precision linear guides and drive mechanisms (typically a ball screw or linear motor) for excellent travel and positioning accuracies. But one actuator design — the U-shaped linear actuator — defies these specifications, using an extruded steel base to provide rigidity and travel accuracy specifications that rival some linear stages. Note: This actuator design has various names, depending on the manufacturer. But for this discussion, we’ll refer to it as a U-shaped linear actuator in reference to its defining feature — a U-shaped extruded-steel profile. The use of a steel (rather than aluminum) profile makes the U-shaped design extremely rigid and allows manufacturers to offer a linear actuator with the high
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travel and positioning accuracies typically found in more precise (and expensive) linear stages. The steel base can also be machined to provide a reference edge— for precise alignment with other machine components — or with other actuators in a multi-axis system. And with very high rigidity, the U-shaped linear actuator is much better suited than other designs for applications where the actuator is supported only on one end, such as two and three-axis Cartesian systems. In the U-shaped actuator design, the linear guide system is integrated — so there’s no guide rail. Instead, the raceways that would normally be found on the guide rail are ground into the inside of the base. The carriage or table is analogous to a linear bearing block turned inside out, with the balls riding on the outside. This leaves the center portion of the carriage available to accommodate the ball screw nut. Such construction makes the entire actuator extremely compact, with a width-to-height ratio of approximately 2:1. For example, a U-shaped actuator with a width of 60 mm is only 33 mm high. The most common cross-sections (width x height) are 40 x 20 mm, 50 x 26 mm, 60 x 33 mm, and 86 x 46 mm … although other sizes are offered as well.
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MOTION SYSTEMS HANDBOOK
Tolomatic IMA and SWA linear servo actuators come in an array of versions, including stainless-steel variations.
Despite their compact dimensions, U-shaped linear actuators have good load and moment capacities. This is because the raceways are spaced relatively far apart. So the carriage geometry serves as a bearing block much larger than what the actuator could accommodate in its standard form. Some manufacturers offer U-shaped linear actuators made from extruded aluminum profiles, with steel inserts for the linear guide raceways. Aluminum versions lack the rigidity of steel designs, but they offer a very compact profile and are often dimensionally interchangeable with steel versions, for a lowercost option. While steel versions of U-shaped linear actuators use ball screws drives almost exclusively, aluminum designs are more likely to be offered with both ball screw and lead-screw drive options.
Originally developed for high-precision applications such as semiconductor wafer handling and medical diagnostic dispensing — for which space constraints don’t allow a typical linear stage — U-shaped linear actuators are now used in a wide variety of industries and applications. These include plasma welding, automated assembly, and optical inspection. One of the driving factors behind the widespread adoption of U-shaped actuators is that they are the only linear actuator design with dimensional interchangeability between manufacturers. But note ... due to differing guideway and ball screw designs, technical specifications (such as load capacity, speed, and rigidity) can vary between manufacturers and product lines, even for products with the same crosssectional size and mounting dimensions.
VARIATIONS IN ACTUATOR LEADSCREW GEOMETRIES Length calculations
NON-CAPTIVE SHAFT PRODUCTS
ML•14 AND ML•17 SCREW DIMENSIONS 3.0 TO 18.0 (77.5 TO 455.0)
MOUNTING SURFACE PLATE THICKNESS + DESIRED STROKE LENGTH
0.25 (6.35)
+ (A/B/C)
0.25 (6.35)
SCREW LENGTH
ML•14 — ADD 1.4" OR 35.6 MM ML•17 — ADD 1.4" OR 35.6 MM ML•23 — ADD 1.8" OR 45.7 MM
EXTERNAL SHAFT PRODUCTS
Screw end options
STRAIGHTNESS 0.002 PER INCH
ML•23 SCREW DIMENSIONS 3.0 TO 24.0 (77.5 TO 610.0)
THREADED END
MOUNTING SURFACE PLATE THICKNESS – SCREW LENGTH
SMOOTH END
0.375 (9.52)
– NUT LENGTH AVAILABLE STROKE LENGTH
0.50 (12.7)
NONE
STRAIGHTNESS 0.002 PER INCH (0.05 PER MM)
Precision-rolled leadscrews are quiet — especially when paired with a self-lubricating nut. Leadscrews made of premium non-magnetic stainless steel are also corrosion resistant. With options from Schneider Electric Motion USA shown here, engineers can also specify the linear actuator’s screws with a Teflon coating.
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MOTION SYSTEMS HANDBOOK
Common linear-actuator drive mechanisms
2018 has seen the introduction of new linear actuators. This cylinder from Rise Robotics includes an electric-motor-driven windlass with steel belts. Recall that a windlass includes a solid bar or cylinder that rotates; cord wound around the drum attaches to load and fixpoint for power transmission. Rise Cylinders come in sizes to 650 mm in diameter; deliver thrust loads to 450 kN; and speeds to 1.0 m/sec.
BELT AND SCREW DRIVES are the most common types of electromechanical actuators. Most manufacturers of linear actuators offer both belt and screw-driven options. Belt-driven actuators pair with a variety of guide mechanisms, with plain bearings, cam-roller guides, and recirculating bearings (riding a profiled rail or round shaft) the most common. Because their strengths are high speed and long stroke, belt-driven systems are often housed in an aluminum extrusion or in open configurations without protective housing. Within the screw-driven category, there are ball-screwdriven and lead-screw-driven actuators. Ball-screw actuators have higher repeatability and thrust forces than lead-screw actuators, but both provide inherent gearing through the screw’s lead (pitch). The most common guide system for screw-driven actuators is profiled rail, although plain bearings sometimes guide lead-screw-driven actuators. Because screw-driven actuators need rigidly mounted end bearings, aluminum extrusions often enclose them. However, applications needing high travel accuracy benefit from ball-screw types with a machined steel housing. One design that doesn’t exist is a ball screw actuator with cam rollers as the guide mechanism. This is because the forte of cam-roller guides is high speed, whereas ball-screw actuators are primarily for high repeatability and high thrust force — with limited speed capabilities. Pneumatically driven actuators aren’t electromechanical devices like the other actuator types, but their prevalence in automated equipment makes them important. For more information on the two types — including slider and rodtype offerings — see the section in this handbook covering pneumatic actuators. For extremely long lengths and robustness against contamination, rack-and-pinion-
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driven actuators are often the most suitable choice. However, finding a comparably suitable guide can sometimes be difficult. Joined profiled rails sometimes work for extremely long lengths. However, where contamination is a significant concern, metal wheels are usually preferred. A unique feature of rack-andpinion-based actuators is their ability to drive multiple carriages independently. A common application for rack-and-pinion actuators is in overhead gantries in automotive production. Linear-motor actuators are also capable of long travel lengths with multiple carriages, but they’re primarily used for high-precision strokes and very dynamic motion. To complement the strengths of linear motors, these actuators use high-precision profiled rails, crossed roller guides, or even air bearings as their guidance. Linear-motor-based actuators can mount in an extruded housing or on a machined aluminum plate, but to meet the highest travel accuracy specifications, they can also mount on a machined steel plate or granite base.
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E L E C T R I C A C T U ATO R S
OFTEN THE EASIEST WAY to begin selection of a linear actuator is to use manufacturer sizing software. Results often include choices that can be narrowed by considering non-quantitative criteria such as ease of maintenance, integration with existing components or systems, and space constraints. Most sizing and selection software runs through several traditional steps. The first step is to define the motion profile to establish the demands for velocity and time as well as force (or torque) and the needed travel distance. This is also when engineers typically determine the maximum stroke needed — and maximum and minimum speed requirements. Next calculate the load. This can include the externally applied load, system inertial load, friction load, and the gravitational load. Load calculations also depend on the orientation of the actuator itself, whether it’s horizontal or vertical. Next define duty cycle. This the ratio of operating time to resting time expressed as a percentage. The cycling rate may be in seconds, minutes, hours or even days; knowing the operating hours per day may also be necessary. Knowing the duty cycle helps engineers estimate system
life requirements and can eliminate problems such as overheating, faster wear and premature component failure due to incorrectly sized actuators. Know the positional accuracy and precision demanded by the application. The actuator’s precision should meet or exceed the application’s requirements for accuracy, backlash, and straightness and flatness of linear motion. This directly impacts system cost. If the application doesn’t demand high accuracy or precision, then there’s no need to buy a more expensive actuator when a less expensive one will satisfy the demands of the application. Aside from the technical specifications mentioned above, select the proper configuration for the actuator in the final design. This includes mounting considerations and the need for any other external components, such as holding brakes and communication and power cables. Finally — consider the operating environment. What are the temperature requirements? Are there any contaminants such as water, oil, or abrasive chemicals? Contaminants can damage seals and shorten actuator life. Select actuators with sufficient IP ratings to guard against this.
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Ball screws A BALL SCREW is composed of a screw with helical grooves, a nut, and balls made of steel, ceramic, or hard plastic material that roll between the nut, the screw and the grooves when either the screw or nut rotates. Balls are routed into a ball return system of the nut and travel in a continuous path to the ball nut’s opposite end. Ball screw performance is gauged by a number of factors including lead accuracy, axial play and preload, and life/load relationship. Lead accuracy refers to the degree to which the shaft’s rotational movements are translated into linear movement. With lead accuracy and axial play determined by the manufacturing method of the ball screw shaft and the assembly of the nut, high lead accuracy and zero axial play is usually associate d with relatively higher-cost precision ground ball screws, while lower lead accuracy and some axial play is associated with lower cost rolled ball screws. Axial play in a ball screw is the degree to which a ball nut can be moved in the screw axis direction without any rotation of either nut or screw. Preload is typically applied to eliminate axial play, which also has the effect of removing backlash and increasing stiffness. Precision and repeatability are also important factors, which can be impacted by ball recirculation inside the ball nut. For this reason, ball nuts are available with a range of preload options to reduce or remove the axial play as they rotate around the screw. Minimal axial play allows better accuracy because no motion is lost from the clearance in the balls as they reengage. Ball screws have one significant advantage over other screw actuation methods; their high efficiency that can be well over 90%. Acme lead screws, by contrast, average about 50% efficiency or less. Ball screws also offer smooth movement over the full travel range. While ball screws typically cost more than other screw types, this can be offset by decreased power requirements for similar net performance.
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As a bearing system, ball screws do need some type of lubrication to avoid metal-to-metal contact of the balls in the raceway. While the lubrication choice can be either oil or grease, it’s advisable to avoid solid additives (such as graphite) as they will clog the recirculation system. An NLGI no. 2 type grease is recommended but it should also depend on the application, whether food-grade or another special type of lubrication is required. For applications requiring high thrust force and rigidity along with good positioning accuracy, a preloaded ball screw assembly is often the best choice. Manufacturers offer ball screw preload amounts ranging from just 1 or 2% for applications where backlash would be detrimental, up to 10% or greater for applications that require very high stiffness. Ball screw preload: What you need to know Though preload is beneficial for some applications, it produces additional loads that must be addressed when sizing the screw assembly, motor, and other related components. There are several ways to preload a ball screw, but the three most common methods use oversized balls in the ball nut, an adjustable-diameter ball nut, or a double nut system. Creating preload by using oversized balls allows the manufacturer to set the preload to a precise amount, although this technique is generally Miniature used for preload amounts of 5% or less. ball screw such However, this is the most common as the SP from SKF method for preloading feature smoother ball recirculation for quieter and faster operation in medical, laboratory, and automation applications.
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MOTION SYSTEMS HANDBOOK
because it can be used with a variety of nut styles and is relatively low-cost. Adjustable-diameter ball nuts allow the user to increase or reduce the preload manually, via an adjustment screw that changes the inner diameter of the nut. This method is the preferred option when the required preload may change over the life of the screw assembly — for example, to compensate for increased wear or clearance in other machine components. Although it doesn’t facilitate setting the preload to a specific level, the benefit of the adjustable nut is that if a ball nut needs to be replaced or interchanged (on the same screw) the preload of the new ball nut can easily be set to the required level. Double ball nut designs produce preload in one of two ways — through tension between two single nuts or through an offset in the lead of a single nut. The first method uses a spring or a spacer to create tension between two nuts that are mechanically coupled. In the second method, during machining of the ball nut raceways an offset is created in the lead ... approximately halfway down the length of the nut body. This lead offset creates preload within the ball nut. Because there are no spacers or springs, the lead offset method is more compact than a double nut design, but reduces load capacity. With greater complexity and higher cost, double nut designs are typically used when high preload (7% or greater) is required.
Axial play or backlash is from clearance between the balls and raceways. Using oversized balls in the ball nut can reduce or eliminate backlash. Image courtesy NOOK Industries
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MOTION SYSTEMS HANDBOOK
plain bearings Review of
PLAIN BEARINGS are cylindrical sleeves that bear light to moderate radial loads. The bearings are made of many graphite, bronze, and plastics that include PTFE, nylon, and polyacetal. Material improvements have made plastic plain bearings increasingly common, even in demanding motion applications. In short, plain bearings slide radially or axially over shafts to allow rotary motion or linear motion (or sometimes both) of these loads. Plain bearings of all types are compact and lightweight with high strength-to-weight ratio. Plain bearings have none of the moving parts that rolling-element bearings have, so minimize fail points; they’re also cost effective for even fairly rugged applications. Common variations are metallic sleeve bearings (which often ride loads on a hydrodynamic or full film of lubrication) and selflubricating plastic bearings in an array of geometries for bushing, thrust bearing, and integral-slide applications. Plain-bearing ratings are based in part on test results and its material modulus of elasticity, flexural strength, shore-D hardness, maximum surface pressure and running speed, rotating, and maximum load capacity — with the latter related to the plain bearing’s material compressive limit. (Here, recall that the compressive limit is the point at which 0.2% permanent deformation occurs.)
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This igus drylin W carriage for curved rails includes plain bearings with rounded ODs. Called spherical bearings, they pivot in their carriage mounts mountstotoautomatically automaticallyadjustadjust to straight to straight and curved and curved rail radii rail—radii down — to down a tobending a bending radius radius of of250 250mm. mm.
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MOTION SYSTEMS HANDBOOK
FRICTION
EFFECTS ON PL AIN BEARINGS OF SHAFT SURFACE ROUGHNESS
WEAR
In addition, a pressure-speed (PV) value expresses plain-bearing load capacity — usually in in psi times the shaft rpm. However, note that PV values are only one to help determine a plain bearing’s overall load capacity — especially where a PV expressions might mislead engineers into thinking that a plain bearing can bear excessively high loads if the speed is very low. In other words, use of PV values requires concurrent consideration of real-world speed and load limits.
Considerations for plain-bearing mating shafts The shafts on which plain bearings ride have 96 0 32 0 32 64 64 significant impact on plain-bearing performance and life. One common option is cold-rolled ROUGHNESS (RMS) carbon steel. This shaft material makes for a suitable mating surface for plain bearings made of polymers. Ceramic shaft surfaces induce more Shaft surfaces for mating with plain bearings wear, though are sometimes chosen for their shouldn’t be too smooth or rough. ability to withstand harsh environmental conditions. Though aluminum shafts are lightweight and easy to machine, they also induce accelerated plainbearing wear. Aluminum shaft made of anodized slightly improves the assembly performance. In fact, shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough. Overly smooth surfaces will cause stickslip adhesion variations — in turn causing higher friction resistance to bearing movement. More of a disparity between dynamic and static friction will make for faster bearing wear and jerkier motion. In contrast, overly rough shaft surfaces quickly abrade plain bearings. In fact, the rates of wear induced by shaft-bearing interfaces can vary a hundredfold. Some manufacturers recommend shaftsurface finishes to 64 root mean square (rms) for precision applications needing low friction; a smoother shaft with roughness of 20 rms or so is more suitable where long plain-bearing life is a design objective. Recall that the rms expression of surface roughness is derived from measurements of a surface’s microscopic peaks and valleys. Ra is an alternative measure some in industry use to quantify roughness — in this case, as an Simplicity average roughness bearing image of a surface’s peaks courtesy PBC Linear and valleys. In other words, the two measures express the same quality … only in different formats. Note that large and outlying peaks or flaws on a shaft surface will affect the RMS value more than its equivalent Ra value.
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BEARINGS • ROLLING ELEMENT
Rolling-element
rotary bearings:
Summary of types and variations
ROTARY BEARINGS are common components in motion systems. They’re used in gearboxes, motors, pulleys, fans, pumps — virtually any time a shaft is rotating, rotary bearings are there to reduce friction and support radial or axial loads. The classification of rotary bearings begins with the type of bearing that supports the load— ball or roller. Within the ball and roller types, the next distinguishing feature is the primary load direction that the bearing can withstand—radial or axial (aka thrust). Then, ball bearings are further classified by the configuration of the outer ring, while roller bearings are further categorized by the shape of the rollers.
While there are dozens of variations of rotary bearings — including designs made for specific environments, mounting configurations, and applications — the most common categories are covered here. Rotary bearings are categorized by whether they use balls or rollers … and then by the type of load they’re designed to support — radial or axial (thrust). Further classification for ball bearings is based on the configuration of the outer ring, whereas roller bearings are further classified by the shape of the rollers.
Ball bearings
Radial ball bearings
Roller bearings
Thrust ball bearings Radial roller bearings
Deep-groove ball bearings Four-point-contact ball bearings Angular-contact ball bearings
Single-direction thrust ball bearings
Cylindrical roller bearings
Double-direction thrust ball bearings
Angular-contact ball bearings
Thrust roller bearings
Needle thrust roller bearings
Needle roller bearings Tapered roller bearings
Cylindrical thrust roller bearings
Spherical roller bearings
Tapered thrust roller bearings
Spherical thrust roller bearings
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Ball bearings for radial loads Within the ball-bearing family, deep-groove ball bearings are the simplest type, with raceways that nearly match or conform to the diameter of the balls. This type is suitable for radial loads and for axial loads in either direction, although their load capacity is relatively small compared to other bearing types. For higher load capacity in a relatively small footprint, deep groove ball bearings are commonly available in a double-row design, with two rows of balls instead of one. Angular contact ball bearings are designed so that the line that connects the points of contact of the inner ring, ball, and outer ring runs at an angle radially to the bearing. The contact angle increases load capacity, but it means the bearing can withstand an axial load only in one direction, so these bearings are typically used in pairs or in a double-row design (essentially two angular contact bearings mounted backto-back) in order to withstand axial loads in both directions. Four-point contact ball bearings are single angular contact bearings that are designed to withstand high axial loads in both directions. They can also withstand combined axial and radial loads, as long as the axial component is predominant. Ball bearings for thrust loads These are made of two relatively thin bearing rings (sometimes referred to as bearing plates or washers), with raceways designed to take axial loads, in either one direction or both directions. Radial loads are typically not permissible for thrust bearings, and speed capabilities are limited (as low as 20 to 30 percent of their radial bearing counterparts in some cases). Roller bearings for radial loads With rollers supporting the load, these have higher load capacities and higher rigidity than ball bearings of similar sizes. The type of load they can support depends primarily on the shape of the roller. Cylindrical roller bearings can withstand high radial loads, while doublerow cylindrical versions having extremely high radial load capacity and high rigidity in the radial direction. Although standard cylindrical roller bearings are not capable of taking axial loads, some designs include internal ribs or collars that allow them to
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handle relatively small axial loads in one or both directions. Needle roller bearings use long, thin rollers, with a length that is anywhere between three and ten times the diameter. They have good radial load capacity, but the primary benefit of the needle design is that it has a thin cross-section, for applications where high radial capacity is required but space is limited. Unlike cylindrical and needle roller bearings, tapered roller bearings (which use conical rollers to support the load) can withstand axial loads in one direction. Tapered roller bearings can be thought of as the roller version of the angular contact ball bearing ... and like their ball-bearing counterparts, tapered roller bearings are often used in pairs or in a double-row design to counteract axial forces that are produced in the bearing when a radial load is applied. The tapered design also improves rolling properties and reduces friction within the bearing. Spherical roller bearings use barrel-shaped rollers and have two inner raceways inclined at an angle to the bearing axis, and one sphericalshaped outer raceway. This gives them a self-alignment capability, very high radial load capacities, and the ability to withstand axial loads in both directions. Roller bearings for thrust loads Tapered thrust roller bearings have pure rolling motion, so they produce less heat and less wear than other options. Thrust versions of roller bearings generally follow the strengths of their radial counterparts but with thrust (axial) loadcarrying capabilities. Cylindrical thrust roller bearings can accommodate high axial loads and can withstand impact loads, although radial loads are not permissible. Similarly, needle thrust roller bearings can withstand high axial loads, but in a much smaller form factor due to the use of needle bearings. Tapered thrust roller bearings can be singledirection or double-direction type, indicating whether they can accommodate axial loads in one direction or in both directions. Like their radial counterparts, tapered thrust roller bearings have pure rolling motion, which means less heat generation and less wear than other bearing options. Spherical thrust roller bearings can take very high axial loads and small to moderate radial loads, with the ability to withstand some misalignment.
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BELTS & PULLEYS
belt and pulley technologies Review of
This Festo ELGA-RF is accurate enough to function as the main axis on a machine that produces printed circuit boards. Two synchronized belt drives extend the length of travel.
INDUSTRIAL BELT DRIVES include rubber belts that wrap around drive pulleys, in turn driven by electric motors. In a typical setup, the belt also wraps around one or more idler pulleys that keep the belt taut and on track. The main reasons that engineers pick belt drives over other options is that modern varieties require little if no maintenance; they’re less expensive than chain drives; and they’re quiet and efficient, even up to 95% or more. In addition, the tensile members of many of today’s belts — cords embedded into the belt rubber that carry the majority of the belt load — are stronger than ever. Made of polyester, aramid, fiberglass or carbon fiber, these tensile cords make today’s belt drives thoroughly modern powertransmission devices. Manufacturers generally describe belts and pulleys with five main geometries. Pitch diameter is the drive pulley’s diameter. Center distance is the distance between the two pulleys’ centers. Minimum wrap angle is a measure of how much the belt wraps around the smallest pulley. Belt length is how long the belt would be if cut and laid flat. In the case of toothed belts (also called synchronous belts) the pitch is the number of teeth per some length — so a 3-mm pitch means that the belt has one tooth every 3 mm, for example.
Pyramid Connectable (PC) belt from Pyramid Inc. can replace line shaft-to-roller or roller-to-roller belts on live-roller conveyors. Also called twisted, quick-connect, or quick-fix belts, these work as conveyor replacement belts and in myriad other applications. While intended to be a temporary fix, PC belt can approach the performance of original-equipment belts.
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MOTION SYSTEMS HANDBOOK Manufacturers of Power Transmission and Motion Control Components Basics on how to integrate synchronous belts Some general guidelines are applicable to all timing belts, including miniature and double-sided belts. First of all, engineers should always design these belt drives with a sufficient safety factor — in other words, with ample reserve horsepower capacity. Tip: Take note of overload service factors. Belt ratings are generally only 1/15 of the belt’s ultimate strength. These ratings are set so the belt will deliver at least 3,000 hours of useful life if the end user properly installs and maintains it. The pulley diameter should never be smaller than the width of the belt.
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Environmental considerations for belt drives Belt-and-pulley systems are suitable for myriad environments, but some necessitate special consideration. Dusty environments don’t generally present problems if the particles are fine and dry. In contrast, particulate matter can act as an abrasive and accelerates belt and pulley wear. Debris should be prevented from falling into belt drives. Debris caught in the drive is generally either forced through the belt or makes the system stall. In either case, serious damage occurs to the belt and related drive hardware. Light and occasional contact with water—during occasional washdowns, for example—has little serious effect. However, prolonged contact with constant spray or submersion can significantly reduce tensile strength in fiberglass belts and make aramid belts break down and stretch out. In the same way, occasional contact with oils doesn’t damage synchronous belts. But prolonged contact with oil or lubricants, either directly or airborne, significantly reduces belt service life. Lubricants cause the rubber compound to swell, break down internal adhesion systems and reduce felt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it’s best to prevent oil from contacting synchronous belts. The presence of ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials imuch like excessive temperatures do. Although the bumper materials used in belts are compounded to resist the effects of ozone, eventually chemical breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation depends on the ozone concentration and generation of exposure. Rubber belts aren’t suitable for cleanrooms, as they risk shedding particles. Instead, use urethane timing belts here … keeping in mind that while urethane belts make significantly less debris, most can carry only light loads. Also, none have static conductive construction to dissipate electrical charges.
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As mentioned, belts are quieter than other powertransmission drive options … but they’re not silent. Noise frequency increases proportionally with belt speed, and noise amplitude increases with belt tension. Most belt noise arises from the way in which belt teeth entering the pulleys at high speed repeatedly compresses the trapped pockets of air. Other noise arises from belt rubbing against the flange; in some cases, this happens when the shafts aren’t parallel. Pulleys are metal or plastic, and the most suitable depends on required precision, price, inertia, color, magnetic properties and the engineer’s preference based on experience. Plastic pulleys with metal inserts or metal hubs are a good compromise. Tip: Make at least one pulley in the belt drive adjustable to allow for belt installation and tensioning. Also note that in a properly designed belt drive, there should be a minimum of six teeth in mesh and at least 60° of belt wrap around the drive pulley. Other tips: Pretension belts with the proper recommended tension. This extends life and prevents belt ratcheting or tooth jumping. Align shafts and pulleys to prevent belt-tracking forces and belt edge wear. Don’t crimp belts beyond the smallest recommended pulley radius for that belt section.
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Select the appropriate belt for the design torque and pick a belt material that’s compatible with the environment — accounting for the effects of temperature, chemicals, cleaning agents, oils and weather. Pulley balancing for belt drive systems: Is it always necessary? As a pulley rotates, centrifugal forces act on the pulley. So if its mass is not evenly distributed around the axis of rotation — that is, if it is unbalanced —– these centrifugal forces will also be unbalanced and cause the pulley to vibrate. (Uneven mass distribution can be due to imperfections in machining or inconsistencies in the material structure.) Pulley vibrations can transfer to the support bearings and other components of the machine, causing premature or even catastrophic failure. This is why pulleys used in belt drive systems almost always undergo some form of balancing. Static balancing: Virtually all pulleys undergo static balancing — also called one-plane balancing — after manufacture. This method ensures that the pulley’s weight is equally distributed around its center of rotation. As its name implies, static balancing can be done while the object is at rest and is relatively easy to demonstrate through a simple experiment. Rotate the pulley by hand and let it come to rest on its own. Mark the point at the very bottom center of the pulley. Rotate it again and let it come to rest. If it stops with the same point at the bottom center, then its weight is not balanced — and the pulley is heavier at that bottom point. Correcting this is typically done by one of two methods ... by removing mass from the “heavy” point (commonly by drilling a small hole in the pulley) or by adding mass to a point 180˚ from the heavy point. Static balancing is typically sufficient for pulleys that travel at 6,500 ft/min. (33 m/sec) or less. For speeds above this, or when the pulley diameter is less than 7 to 10 times the face width, dynamic balancing is recommended. Dynamic balancing – also called two-plane balancing — goes one step beyond static balancing and ensures that the pulley’s center of mass is on the same axis as its center of rotation. It is possible for a pulley to be statically balanced but dynamically unbalanced (although the reverse is not true) ... so dynamic balance must be measured while the pulley is turning. Because it involves forces in two planes, dynamic balancing requires that masses be added in two planes to counter the imbalances and prevent pulley vibration. The measure of unbalance is given in units of g-mm (oz-in.) based on the mass of the pulley and the eccentricity ... the distance between the center of mass and the center of rotation. The Mechanical Power Transmission Association provides guidelines for both static (one-plane) and dynamic (twoplane) balancing in their MPTA-B2c-2011: Standard Practice for Sheave/Pulley Balancing.
pyramidbelts.com • 641.792.2405 • sales@pyramidbelts.com 38 201702_Pyramid4x475_lineshaftR2.indd 1 Belts & Pulleys – Motion Control HB 08.18 V3 FINAL.indd 38
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2/9/2017 10:43:58 AM 8/23/18 8:54 AM
BRAKES & CLUTCHES
Where
clutchesare&indispensable brakes
AT THE CORE of most motion axes are electric motors. Stopping loads on their axes can be done with the electric motor itself — called internal braking in certain contexts — or with an external clutch or brake. For the former, one simple approach is to simply cut motor voltage input and allow the axis to coast to stop. That’s acceptable where stops are infrequent — from a few times a minute (for designs running off small motors) to a few times an hour (for larger motor installations). Another option is to use controls to generate stopping torque in the motor via regenerative braking t convert kinetic into electrical energy; dynamic braking — the injection of dc current into the stator; or electric reversal as plugging. But where such approaches are too slow — including all modern motion designs for high throughput — external brakes and clutches are required to get sufficiently quick stops or disengagement. This applies to conveyors, airport-baggage handlers, escalators, and elevators … as well as other axes that make frequent stops and starts — even as few as 10 cycles a minute in some cases. Where stops and starts happen at much higher cycle rates, motor inertia may degrade the quickness with which starts and stops are possible. So here, clutch-brakes are often more suitable — as they disengage the driven load from the motor to allow the former to run even while the brake
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Brakes & Clutches – Motion Control HB 08.18 V3 FINAL.indd 39
Miki Pulley’s BXR-LE electric springapplied brakes are suitable for small and precise servomotor designs. A lightweight build optimizes servodrive performance. A voltage controller means brake power consumption is stepped down to 7 Vdc after a split second of 24 Vdc for actuation. When compared to most other electric brakes, the BXR-LE brake consumes just a third the power (and generates just a third the heat) — though it’s half as thick. Speed is to 6,000 rpm; static friction torque is 0.044 ft.lb. to 2.36 ft.lb. and ambient operating temperature is 14° to 104°F. BXR-LE brake applications include those on end effectors, ballscrew actuators, XYZ positioning tables, and 3D printers.
engages and stops the load. Of course, though we focus on responsiveness here, failsafe design features are another main driver of brake and clutch inclusion. Mechanical, electric, fluidic, and self-actuated clutches and brakes are suitable for different applications. For example, spring-set brakes benefit motion designs that slow loads with the motor before the brake engages … and they’re suitable as holding mechanisms. Control of electric brakes is easy, and they can keep pace to a thousand cycles per minute. Most air-actuated brakes and clutches are cool-running and hold with minimal input. Friction brakes with drum, disc, and cone geometries deliver e-brake functionality with failsafe holding. Brake or clutch size and type depend on whether the axis at hand will make emergency stops or softer stops that sacrifice the clutch or brake to protect systems and loads from shock. Or sometimes it’s more essential that the brake deliver soft stops to
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CUP OVERFLOWING?
MOTION SYSTEMS HANDBOOK
Searching for components eats up valuable time. For more than 45
Brake modules such as the mayr ROBA brake-checker monitor the condition of electromagnetic safety brakes — and can facilitate smooth deceleration of machine axes. Shown here is a ROBA brake-checker module that works without sensors. Electronics track current and voltage and recognize movement of the armature disc to assess brake condition as well as temperature, wear, and tension path or tensile-force reserve. This tracks whether the brake’s magnet has sufficient force to attract the armature disc. On reaching the tensile force reserve, a warning signal sounds to prompt service. There is a design for ac voltage; soon another version will integrate brake input power (to eliminate the need for a separate rectifier) and combine switching, condition monitoring, and brake control.
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prevent shifting loads and misalignment. After that, other criteria — cycle rates, thermal capacity, machine envelope, and MRO schedules — dictate final selection. Some tips: Size clutches and brakes to the machine axis’ motor torque. Where a brake must stop vertical loads, account for how motors can briefly draw current to output in excess of their rated torque. Consult performance curves in manufacturer PDFs for dynamic torque ratings at set speeds to match the brake or clutch to peak motor-output torque. Case in point: Consider an inclined conveyor with regularly spaced on-off cycles. Here, a power-off spring-set brake may suffice to prevent load crashes during power failures. But more complex conveyor installations to position discrete product of varied size — without jerking — may need multiple deceleration rates may need more sophisticated spring-set brake plus drive on the motor for stopping … or even a permanent-magnet brake for quick yet soft starts and stops. Comparing clutch and torque-limiter functions Torque limiters are not clutches, as they’re not designed to continuously slip. That’s an important distinction when design engineers are specifying slowing and disengaging technologies for mechanical designs. Some tips for optimal design selection: First consider whether a torque limiter will only protect against catastrophic failure or semi-regular overloads. That will indicate whether an economical friction-style torque limiter is sufficient or if the design necessitates a ball-detent design. The latter is usually more costly but capable of slipping and triggering a limit switch for shutoff and resetting to resume operation. Ball-detent torque limiters here can slip multiple times in their application … unlike friction torque limiters requiring reset. When applying a torque limiter, confirm whether a zero-backlash design is needed. Note that torque limiters can generate heat at their frictional connections, so upon their activation, system shutdown is key. After it’s activated, the end user should inspect the torque limiter for wear and heat damage. He or she should also check its setting torque … as in some designs, that value will be diminished if the torque limiter has been run too long. As long as the torque limiter is still within design range, it’s safe from exhibiting more and more slipping. With some motion designs, a limit or proximity switch paired with controls can detect when the torque limiter slips — and shuts the system down to let the end user address the issue the torque limiter was protect (or protect against). Insight into torque-limiter functions provided by KTR engineering services manager Chris Scholz. For more information on torque limiters, refer to the separate section on that technology.
BRAKES • CLUTCHES TORQUE LIMITERS 40
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MOTION SYSTEMS HANDBOOK
Making connections with
industrial cabling and cable management
Cable carriers can house a large volume of cables and wires while supporting their weight without sagging or putting stress on the cabling. Here, Dynatect’s Gortrac Articulated Roller Support (ARS) long travel system eliminates traditional guide troughs, reducing system weight and costs. It is intended for applications up to 300 ft and eliminates the expense, potential damage, and depot storage area of a traditional trough.
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ELECTRICAL CABLING is often overlooked in motion control system design. If you use something other than industrial cabling—which is designed to flex continuously— premature failures can result. Continuous-flex cables are designed to cope with the tight bending radii and physical stress associated with motion control applications. A regular cable typically manages 50,000 cycles, but a flexible cable can complete between one and three million cycles. Flexible cables can be divided into two types: those with conductors stranded in layers inside the cable, and those that have bundled or braided conductors. Cables with stranded layers are easier to produce, and usually less expensive. The cable cores are stranded firmly and left relatively long in several layers around the center and are enclosed in an extruded tubeshaped jacket. However, this design can lead to core rupture. Bundled designs feature braided conductors that surround a tension-proof center. A pressure filled outer jacket ensures the cores cannot twist, so these cables are often stiffer than standard cables, but last longer in constant flex applications. Flat cables can incorporate power, signal, and video conductors as well
DESIGN WORLD — MOTION
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as air or liquid tubing and fiber optics in a single compact cable suitable for continuous flexing. Flat cables with silicone jacketing resist abrasion and other harsh environmental impacts and will even self-heal minor nicks. To select the right cables, determine the application type, such as whether it will be stationary or moving, and what type of movement it is—flexing, torsion, or both? If there is bending or motion involved, the bend radius must be specified. The bend radius depends on the gauge of the wire and the kind of conductors used in the cable. With continuous flexing, conductors containing multiple strands of fine-gauge wire generally last the longest. Making the connection Most cable assemblies use some type of interconnect device to transmit the signals from the cable to the motor or drive. Commonly used connectors include M12 and M8 connectors, which are circular connectors with a 12- or 8-mm locking thread used for sensors, actuators, Fieldbus and industrial Ethernet, primarily in factory automation applications. Often with ratings of IP65, IP68 and IP69K, they are ideal for washdown and corrosive environments. Additionally, high-speed, miniature connectors are growing in use in industrial automation, where signal density, electrical performance, and mechanical dependability in smaller package sizes are critical for efficient, compact and accurate machines. Modular connectors, micro connectors, heavy-duty connectors with locking systems, and more are used to provide constant, reliable connectivity for high-vibration environments. Where rotating motion is present, slip rings should be used. These electromechanical connectors transmit electrical signals, power and data from a stationary structure to a rotating machine.
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MOTION SYSTEMS HANDBOOK
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Eliminate cable entanglement Where there is a myriad of cables and wiring, cable management becomes an issue. A simple, lowcost form of cable management uses twist-tie type bundlers that tie together groups of wires and cables. However, bundling them together may pose weight problems that cause sagging and put undue strain on the cables. For stationary applications, cable trays safely and cleanly route cables. They are made of an U-shaped open channel into which cables can safely lay. They can be open or closed for protection against contaminants. When selecting a cable tray, consider what type of cable is to be routed, including its diameter and weight, the span between supports, the distance of the cable run, and environmental conditions. Finally, cable carriers can house a large volume of cables and wires and support the weight of
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them all without sagging or putting stress on the cabling. They can be made of plastic, steel, or a metal alloy in open or closed designs. Knowing length of travel, number of cables or hoses, size and weight of the cables, required speed and acceleration and environmental factors such as exposure to any debris, excessive heat or chemicals is critical to selection. A key specification factor is knowing the carrier’s bend radius, which is measured from the center of the Motion curve loop to the control systems center of the require the use of continuous-flex cables, pivot pin on which are designed with the side link. tight bending radii.Pictured A larger bend here are HELUKABEL’s radius means TOPSERV servo and feedback less stress cables that offer higher on the cable heat resistance for use in and a longer motor environments. service life.
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MOTION SYSTEMS HANDBOOK
Chain drive basics ROLLERCHAIN SELECTION CHART Chain strands 3
2
900 1,000 700 1,000 800 500 800 600 400 600 400 300 400 300 200 300 200 200 100 100 80 60 40 30
100 80 60
30
30
20
20 10 8 6 4 3
10 8 6
1 0.8
500 400 300 200 100 80 60 40 30 20
10 8 6 4 3
4 3
2
2
1 0.8 0.6
2
2 1 0.8
40
40
20 10 8 6 5 4 3
80 60
1
1 0.8 0.6
0.6
0.4
0.6
0.4
0.3
0.4
0.3
0.2
19 T 19 22 T T 2 25 21T 5T 22 19 T T T 21 25 25 T 25 T 23 23 T T 2 17 T 21 25 T 1 20 40 T T T 25 1 9T 0 17 21 7T T 1 T 21 T 16 80 T 25 0 14 23 17 T 0 T T 12 2 15 25 1T 0 T 10 T 17 19 0 T T 19 80 T 23 60 T 15 50 T 40 35
ROLLER-CHAIN DRIVE CAPACITY (HORSEPOWER)
4
0.4 0.3 0.2
10
30 20
50 80 200 500 1,000 3,000 7,000 40 60 100 300 700 2,000 5,000 10,000
SPEED OF ROLLER CHAIN’S SMALL SPROCKET (RPM)
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CHAIN DRIVES are used to actuate machinery axes and convey products. Now, advances in precision and technology let designers use chains in more applications than ever. For instance, remote installations can benefit from long-life chain that requires no lubrication. Though chain-based setups vary, the most common industrial designs use roller chain. This type of chain consists of five basic components: pin, bushing, roller, pin link plate and roller link plate. Manufacturers make and assemble each of these subcomponents to precise tolerances and heat treat them to optimize performance. More specifically, modern roller chains exhibit high wear resistance, fatigue strength and tensile strength. Roller-chain applications generally fall into two categories: drives and conveyors. In cases where chains operate as powertransmission components only (without doing double-duty as a conveyor) ASME/ANSI roller chain wraps around a driver sprocket connected directly to the motor or reducer. The driven sprocket is often connected to a machine’s conveyor head-shaft. This portion of the drive lets the designer build a system that’s either faster or slower by simply changing the ratio of teeth between the drive and driven sprocket. The ratio of the teeth determines the reduction in rpm so to reduce speed, the driven sprocket must be larger than the driver sprocket. For example, if the driver sprocket has 15 teeth and the driven sprocket has 30 teeth, the ratio is 2:1, so the rpm is halved at the driven sprocket. The simplest way to select a roller chain is using horsepower charts. First, obtain the motor horsepower and the rpm of the small driver sprocket. From this, determine the chain size and number of teeth for the driver sprocket. Where roller chain must drive applications that need long life without contamination, pick chain with self-lubricating subcomponents. Where roller chain must drive applications that need high precision, pick chain with precision roller bearings at each link connection. Power-transmission chain challenges Chain drives are often picked for their ability to withstand harsh environments. Some require clean operation without the contamination risk of lubrication. Others expose chain-driven machinery to weather,
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CHAIN
DESIGN WORLD — MOTION
Chain – Motion Control HB 08.18 V2 FINAL.indd 47
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water or chemicals. So, chain manufacturers offer several products to meet these challenges. For example, one critical area where roller chains need lubrication is the pin-bushing contact zone. Self-lubricating chains stay cleaner because the exterior of the chain is free of excess lube. These chains also attract less dust and particulates than regular chains. Such roller chains are useful where oil contamination is a concern, including paper-product or wood-processing industries. Nickel-plated chains offer another alternative for chain coatings, providing some protection for mildly corrosive environments. Stainless-steel chains offer superior corrosion resistance; however, designers must be aware that regular stainless steels cannot be hardened in the same manner as carbon steel. Therefore, the load carrying capacity of stainless steel is lower than carbon steel. Proper chain maintenance requires periodic inspection. All chains must be checked for damage, wear and chemical corrosion on a regular basis. Wear elongation is another issue. Eventually roller chains wear so much that they necessitate replacement—typically at 1.5 to 2% (12.180 in./ft to 12.240 in./ft) elongation. Chains may work until they reach 3% elongation but are at increased risk for suboptimal performance. Aside from chain use for power transmission, there are also conveyor chains that do double duty to accept a power input and move product horizontally, vertically or even around curved radii. The most common conveyor chains are ASME-style (ANSI-style) attachment chains. These have pins or plates with tabs onto which parts or product-holding shoes can bolt. Common versions are single-pitch attachment, doublepitch attachment, hollow-pin, curved-attachment and plastic-sleeve chain. The attachments let engineers put special fixtures or blocks onto the chain to serve specific conveyor functions. One subtype — accumulating chain conveyors — stop discrete products even while the chain is still moving without a lot of friction. Accumulating conveyors are suitable for applications such as assembly lines where products move through several work stations.
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MOTION SYSTEMS HANDBOOK
Chain actuation —
rigid type
RIGID-CHAIN actuation works by pairing a drive (usually an electric motor) with a length of chain sporting shoulders on each link. The motor output shaft — fitted with a specialty sprocket or pinion — applies tangential force to the chain. Then the chain comes out and straightens, and its links’ shoulders lock to form a rigid series. When the motor runs in the opposite direction, the chain shoulders disengage and allow for coiling. Inside the actuator body, reaction plates and guides counter thrust resistance and keep the chain on track. Links travel around the pinion to exit the actuator body along the stroke path. Here, the motor’s torque comes to act as forward thrust via the link
Common
shoulder to the rest of the links’ shoulders. The last link in the chain before the load has geometry that puts the thrust higher than the articulation axis. This makes a moment that effectively locks the link shoulders. In reverse, pulling force acts along the links’ cross axes. Rigid-chain actuators have the mechanical benefits of conventional chain but can act in horizontal push setups or vertically as jacks. Plus they’re compact. In contrast, traditional chain drives can only pull, so need two drives for bidirectional motion. Traditional screw jacks for vertical power transmission need space for retraction that’s as long as the working stroke itself. Before specifying a rigid-chain actuator, determine the application’s total load, including the transported load, rigid-chain arrangements acceleration forces, external environmental forces, and that due to friction — with a coefficient between 0.05 and 0.5 for Chain link shoulders typical rigid-chain actuator setups. Next, determine what type of actuator body and chain-storage magazine the application can accommodate. Determine whether the Unguided chain with shoulders up coils downwards ... chain will need to change direction on its way from the magazine to actuator body. Actuators usually feed chain around 90° or 180° turns. Note that rigid-chain actuators can work alone or in tandem. Twin-chain ... but guided chain is most stable. setups deliver high positioning accuracy and stability where loads are large or Actuator body bulky. Here, a pushing bar acts as a yoke to keep loads steady, with optional hooks for pulling as well. Optimized geometry Pinion
Input drive shaft Choose a rigid-chain actuator to satisfy the design geometry.
Unguided chain with shoulders up coils downwards, which is useful but not always stable enough for long strokes. That with shoulders down (here bottom) is slightly more stable. Use guided chain wherever space permits.
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CHAIN • RIGID TYPE
Common rigid chain has has the force vector act on the load’s two rows of link plates center for balance. If twin-chain setups and shoulders; duplex are impossible, consider adding chain has three; other framework to guide awkward loads. options abound. Image Guides on the chain also help courtesy iwis Drive maintain stability — even over very long Systems strokes — because they address side and buckling forces. Such guides come in different shapes with different crampons and subcomponents to engage the chain. Where use of chain guides is impossible, most designs run the chain with link shoulders down for moderate stability. Some last design notes: Standard chain is carbon steel to withstand heat to 200° C, but stainless, high-temperature, and coated chain for long life are other options. The required length of chain is total design stroke plus a few links to engage the actuator pinions. As with any powertransmission setup, consult the manufacturer for tips and guidance on determining necessary drive power and other details.
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MOTION SYSTEMS HANDBOOK
Photo courtesy Lee Spring
Review of
compression-spring variations ENGINEERS USE COMPRESSION SPRINGS in designs that need linear compressive forces and mechanical energy storage— designs such as pneumatic cylinders and push-button controls, for example. The most conventional compression spring is a round metallic wire coiled into a helical form. The most common compression spring, the straight metal coil spring, bends at the same diameter for its entire length, so has a cylindrical shape. Cone-shaped metal springs are distinct in that diameter changes gradually from a large end to a small end; in other words, they bend at a tighter radius at one end. Coneshaped springs generally go into applications that need low solid height (the total height when compressed) and higher resistance to surging. Whether cylindrical or cone shaped, helical compression springs often go over a rod or fit inside a hole that controls the spring’s movement. Other configuration types include hourglass (concave), barrel (convex), and magazine (in which the wire coils into a rectangular helix). Most compression springs have squared and ground ends. Ground ends provide flat planes and stability under load travel. Squareness is a characteristic that influences how the axis force produced by the spring can be transferred to adjacent parts. Although open ends may be suitable in some applications, closed ends afford a greater degree of squareness. Squared and ground end compression springs are useful for applications that
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specify high-duty springs; unusually close tolerances on load or rate; minimized solid height; accurate seating and uniform bearing pressures; and minimized buckling. The key physical dimensions and operating characteristics of these springs include their outside diameter (OD), inside diameter, wire diameter, free length, solid height, and spring rate or stiffness. Free length is the overall length of a spring in the unloaded position. Solid height is the length of a compression spring under sufficient load to bring all coils into contact with adjacent coils. Spring rate is the change in load per unit deflection in pounds per inch (lb/in.) or Newtons per millimeter (N/mm). The dimensions, along with the load and deflection requirements, determine the mechanical stresses in the spring. When the design loads a compression spring, the coiled wire is stressed in torsion and the stress is greatest at the wire surface. As the spring is deflected, the load varies, causing a range of operating stress. Stress and stress range affect the life of the spring. The higher the stress range, the lower the maximum stress must be to obtain comparable life. Relatively high stresses may be used when the stress range is low or if the spring is subjected to static loads only. The stress at solid height must be low enough to avoid permanent damage because springs are often compressed solid during installation.
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MOTION SYSTEMS HANDBOOK
MotionHowcontroller FAQ: to address overshoot? THREE CHARACTERISTICS indicate when a servo system is properly tuned:
Response time Settling time Overshoot
Response time is the time it takes the system to reach a specified percentage of the target value, while settling time is the time it takes for the target value to be settled, within a specified percentage. Overshoot is how much the system exceeds the target value. The goal of servo tuning is to minimize response time, settling time, and overshoot. Regardless of response and settling time, a system that cannot reach the specified value for position, velocity, or
torque — that is, a system that overshoots or undershoots — is not particularly useful. The system’s damping ratio indicates whether a system is likely to overshoot or undershoot. A system that is overdamped will undershoot its target value. In other words, an overdamped system has long rise and settling times and falls short of the target value. Conversely, an underdamped system will overshoot its target value. Underdamped systems often produce oscillations, with a short rise time and a long settling time. When tuning PID servo loops, increasing the integral gain (Ki) improves system response, but because the integral gain increases over time at the end of the move, it can cause overshoot and
Underdamped
160
Command
Output value (%)
140 120
oscillations. But if integral gain is too low, the system will be slow to respond to changes. Derivative gain (Kd) works in conjunction with integral gain to reduce overshoot and provide damping to the system, while keeping response time and error to acceptable levels. But derivative gain actually slows system response, so a Kd value that is too high will degrade response time and cause the system to oscillate. Another method for addressing overshoot (or undershoot) is to use a cascaded position-velocity loop with feedforward gains. In contrast to PID gains, which are reactive, based on error that has already occurred, feed-forward gains are proactive, predicting the commands
Overshoot
100
Critically damped
80 60
Delay
Overdamped
Steady-state error within tolerance band
40 20 0
Time
Rise time Settling time
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Overdamped systems fail to reach the target value, while underdamped systems overshoot.
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MOTION SYSTEMS HANDBOOK
This is a HPSC control module from maxon motor. Sensorless speed and torque control allow field-oriented control (FOC) of brushless motors. Such control is from full stop to full speed under load ... a signal-injection algorithm determines the rotor position for smooth starts.
Jon Schmidt, Chief Engineer Neff Press, Inc. St. Louis MO
Complex motion? Can do. Why does Neff Press®, an industry leader in high-speed hydraulic production presses, integrate Delta RMC controllers into its precision can-making line?
“Delta provides some highly advanced tools for tuning axes very quickly and accurately. That has been our best experience versus other motion controllers we’ve used.” “…the smoother motion and synchronization between axes enabled by the Delta controller has allowed us to increase our output by 25%.” Look to Delta RMC motion controllers and graphical RMCTools software to make complex motion design so much easier than any alternative. Give yourself a break and call 1-360-254-8688 or visit deltamotion.com Find the Neff can-making case study or one about your own industry or application. Watch a training video to see how easily Delta can put complexity in the can for you.
needed to achieve zero error and injecting them into the control loop. There are two types of feedforward gains: velocity feed-forward and acceleration feed-forward. Velocity feed-forward minimizes following error and improves response time during the constant velocity phase of the move. It does this by multiplying the derivative of the position (its velocity) by a velocity feed-forward gain. While velocity feed-forward minimizes error and improves response, it has the drawback of inducing overshoot. Acceleration feed-forward minimizes tracking error during the acceleration and deceleration phases of the move by adding an acceleration feed-forward gain to
Delta RMC Motion Controller Family
1-2 Axis
Up to 8 Axis
Up to 32 Axis
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MOTION CONTROLLERS
the second derivative of the position command (its acceleration). Acceleration feed-forward eliminates the overshoot caused by velocity feed-forward ... and because it operates outside of the control loop (as does velocity feed-forward) acceleration feed-forward doesn’t affect servo stability. By combining velocity feed-forward and acceleration feed-forward with standard PID servo control, both fast response and minimal error can be achieved without inducing overshoot.
A WindEDIT Lite app for iOS and Android devices from IDEC Corp. gives two-way access to MicroSmart FC6A PLCs — and is easier to use than browser-based access. Users can monitor any PLC parameter and change set points and other values … including data register, input, output, timer and counter values. Connectivity is through Bluetooth or Wi-Fi. Logged data, user programs, firmware, and recipes can be sent and retrieved from databases and common Cloud-storage platforms.
Arcus Performax motion-controller image courtesy Nippon Pulse of America
DESIGN WORLD — MOTION
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MOTION SYSTEMS HANDBOOK
conveyors
Technical review of
1.
2.
CONVEYORS to move discrete products serve as material-handling arteries to improve efficiency and throughput … and have become an integral component in nearly all automated facilities and applications. They come in widths of less than 2 in. (for moving extremely small parts) to several feet wide. All discrete product handling requires conveyors that can advance product with precision. However, the arrangement of how the conveyor interacts with nearby equipment and robots partially dictates the design. Conveyor Class 1 includes materialhandling uses in which the conveyor serves as an artery to transport product in a steady stream (with little interaction along the way). Class 2 includes conveyors that take product from a set location to a machine or other location. Class 3 includes conveyors that
1. Plastic modular conveyors (as shown here) and other conveyor offerings from mk North America satisfy FDA requirements. 2. Dorner’s Hartland, Wis. location became ISO 9001:2015 certified earlier this year. Conveyor-fabrication image courtesy Dor ner 3. Activated Roller Belt (ARB) conveyor technology from Intralox is based on modular plastic belting with in-belt actuation to move discrete products laterally across the conveyor. It is suitable for quick on-the-fly tasks related to product merging, sorting, and switching.
3.
4. This is another conveyor from mk North America with a food-grade fabric belt.
4. Conveyors – Motion Control HB 08.18 V4 FINAL.indd 56
5.
5. Motion Index Drives Stainless Steel Precision Link Indexing conveyors exceed requirements for washdown common in food-processing facilities. Heavy chemicals in such settings would corrode standard steel components.
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Drive And Controls For Any Application Whether youâ&#x20AC;&#x2122;re designing a simple conveyor or planning a complex system, you can rely on Interroll for your conveying needs. Our drives and controls are crafted with precision and engineered to deliver outstanding performance and reliability. The Interroll EC310 24V brushless RollerDrive can be paired with different Control Cards, depending on your application: EC310 & DriveControl Card EC310 & ZoneControl Card EC310 & MultiControl Card
Simple motor control. Simple ZPA conveyor line, some external I/O. State-of-the-art conveying and controls, monitor signals, track packages, adjust speeds on the fly, interface to a PLC, create custom package movements. UL Listed.
Visit interroll.us to learn more about our solutions.
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MOTION SYSTEMS HANDBOOK
Interroll DC0080 systems excel in hygienic food processing and dynamic storage.
NEED A QUIETER BELT ?
take materials into or out of machines or stations. Class 4 includes conveyors that run right through machinery without break. The first two classes generally prioritize ruggedness or throughput. The last two classes need positioning and (in many cases) custom workpiece pucks to steady product while machines perform work on the product pieces. Many conveyors for light to medium-duty discrete transport use belt that’s wrapped around two or more pulleys. A motor powers the pulleys that in turn engage the conveyor belt. In many cases, toothed belts engage synchronous drive pulleys while serving as the conveyor surface as well. These provide excellent belt-movement control for accurate part or fixture positioning. However, variations abound to meet specific applications. In many cases, servo drives start and stop belt conveyors to provide precise part location. They also let engineers control acceleration and deceleration, so are useful in assembly operations. Encoders on conveyor drive shafts track pulley revolutions for accurate control of the belt for feeding or indexing. Some belts are low friction to let product slide for accumulation. In contrast, high-friction belts have more grip to better hold products. Other options are magnetic conveyors (for applications that need parts to adhere to the belt during processing or elevation changes); pivot conveyors (to swing out of the way when workers need to walk through the line); and vacuum conveyors (which work with a perforated belt that draws air through grooves in the conveyor bedplate to hold light or flimsy parts during fast transport). Singledrive multi-belt conveyors serve two or more lanes of product for the sake of efficiency. Here, two or more conveyors run off a single gearmotor on a common drive shaft or coupled shafts. In some arrangements, the belts even mount to a single conveyor frame.
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ONE DEVELOPMENT in the area of logistics is tighter integration of warehousing and assembly — a trend that’s especially evident in the automotiv e industry but on the rise elsewhere too. That’s according to Maurizio Catino, industry sales manager at Interroll, a material-handling technology supplier. “Traditional facilities have a warehouse (from which parts originate) and an area for production. External logistics — what trucks bring into and out of a facility — are associated with the warehouse,” said Catino. But new production approaches are spurring different strategies.” Best-case setups in the past used automatic guided vehicles (AGVs) traveling the plant floor and feeding assembly lines at
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CONVEYORS
TRANSFORMING CONVEYOR AUTOMATION regular intervals. Personnel working each station were normally tasked with a set of fairly specific operations — picking pieces and fixing these pieces to the main assembly at the right position and so on. But such material-handling approaches are falling by the wayside. Why? “It’s because production plants are increasingly automated — relying on fewer people for a more reliable process. This in turn is being driven by product customization — as end users today expect increasingly customized product,” added Catino. Just consider shopping on Amazon: No longer does one need to visit a store and face a limited selection of options. Today’s consumers want and get an array of choices in colors, options, and cost. That means a significant increase of parts in production ... and old ways of handling increases in part numbers are insufficient. It’s impossible to handle the proliferation of today’s product variations and part numbers with legacy production logistics. Read on for more of what Catino had to say on the changes in logistics and approaches to material handling.
UNTIL RECENTLY it was sufficient to use AGVs for running parts from the warehouse to the production plant. But consider a situation where a facility needs a consistent supply of 200 or 300 different parts. This demand would require so much AGV traffic that it would leave no floorspace for personnel to walk ... and there would be a lot of safety issues along with general logistic problems.
DESIGN WORLD — MOTION
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MOTION SYSTEMS HANDBOOK
better products. better solutions.
Regardless of what makes your material handling application unique mk can provide a standard or custom solution — that meets your needs and keeps your business moving. h Aluminum framed conveyors h Featuring plastic modular belt, roller, chain and belt conveyors h Pallet-handling conveyor systems h Stainless steel conveyors h Extruded aluminum t-slot framing
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Conveyors – Motion Control HB 08.18 V4 FINAL.indd 60
That’s why Interroll and other materialhandling system suppliers are now working to support 21st-century logistics. We think that sooner or later, all warehouse and production facilities will be connected in better ways. How so? Well, just consider product cost. Normally baked into this price are the sum of the costs for all the parts in the product as well as the time for the labor it took to assemble the product. In fact, the latter is usually a large portion of final cost. Getting around this problem are new production concepts already in operation — and in fact, Tesla’s Gigafactory is a great example of this. The first objective is to minimize time wasted by personnel in production. Of course, the best way to do this is to totally automate all processes using robots. But robots aren’t suitable for all assembly tasks ... so at the end of the day, the real objective is to eliminate the manual execution of rote tasks while still keeping personnel to complete valueadded activities. Reconsider a traditional production approach. Assume we have an employee assembling a portion of a car engine on an assembly line. Normally he fits an array of parts to the main assembly while an AGV intermittently supplies racks of parts bins with fresh supply. Other pallets and boxes sit around his work cell — and so our employee must constantly step away from his assembly position to these boxes. Sometimes, the boxes are in slightly different positions — which means he might not automatically knows where to get a given engine piece. Then our employee must look and walk around for parts ... and that’s a waste of time ... not value-added activity. In fact, any time this employee spends walking around and retrieving parts isn’t time that customers want to cover in their product’s final purchase price. So new logistics concepts are based on the idea that our employee shouldn’t have to walk around for pieces ... but rather that parts should come to him. The main (and best-known) concept of this approach is the so-called World Class Manufacturing (WCM) concept invented by Toyota. This concept defines a space called the Golden Area — the area around the operator in which time is perfectly used.
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CONVEYORS
Image courtesy Interroll In an optimized assembly setup, all pieces that this employee must assemble into the engine must enter this Golden Area. That lets him efficiently pick and attach pieces one at a time to the engine before him. Complicating this task is that such operations involve lots of parts, product codes, and delivery dates. Our Motor Conveyor Platform (MCP) is one suitable solution to this challenge. Modular rollers allow Lego-like piece-by-piece builds that let plant operators put conveyors together for production. It’s very easy, only requiring the plant operator’s target layout plan. In fact, in a few installations, the conveyors interface with automated warehouse cells that load boxes of parts onto the conveyors. Then these boxes are conveyed to production and arrive in front of the operator. The employee receives the right piece at the right time, in the right position, so his work is extremely efficient — I’d say 100% efficient. He can stay put and doesn’t need to hunt for parts, so concentration is on the actual assembly task. Such conveyor installations also help to save floor space because there aren’t pallets stored on the assembly floor. Plus employee time is strictly focused on the valuable portion of his former suite of mixed-value tasks.
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Design World: Have any larger facilities implemented this kind of integrated conveyor concept? Of course. Several major players in automotive manufacture have successfully implemented quite complicated installations based on this new concept. Design World: Do these installations use overhead storage? No, storage isn’t overhand — these are automated warehouses including Interroll subsystems and equipment from other automation suppliers. The concept is based on box storage in a dedicated building where parts also happen to be sorted ... because the volumes of these parts are enormous and growing all the time ... so facilities actually need significant space to sort parts. Design World: Are there iterations that incorporate parts bins or pallets? Does our hypothetical assembly employee get groupings of parts fed to his work cell? Let’s say that there’s a flow — so-called one-piece flow. Here, two conveyor lines (the assembly line and the parts-feeding line) run in parallel. Engines on the (totally independent) assembly line run in front of our employee, while the logistics line is behind
8 • 2018
him. Everything works with software. Each engine-carrying conveyor pallet sports an RFID to let the conveyor system continually track every engine in realtime. Because everything is automatic, just as a given engine comes into our employee’s work cell, he gets the box of parts needing assembly onto that engine. We see warehouse automation and production logistics becoming increasingly reliant on conveyor systems — in no small part because of the exponential rise of ecommerce and how parts distribution is increasingly complex. Consider another application — that of automotive tire warehousing and distribution. In the past, third-party suppliers sold tires to dealerships and garages that assembled the tires to wheels as needed. But middleman involvement added cost. Today we see an uptick in online car-tire sales through ecommerce platforms. Users go online and buy preassembled and balanced tires without visiting a garage. The only catch is that the warehouses for such modes of distribution require advanced logistics. We sell roller conveyors into this growing industry to optimize facility automation. Design World: It’s striking how pickto-order approaches in ecommerce are coming to displace batch approaches — and how much the former resembles new modes of assembly we’re discussing. Are they very different? They share many similarities. There’s a parts-picking philosophy guiding them both ... so even the software that drives these applications is similar. Essentially, a product-picking employee must pick pieces to populate a box for an order and then ship that order onward ... which very much resembles the optimized tasks of our assembly employee.
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MOTION SYSTEMS HANDBOOK
Flexible couplings for motion axes COUPLINGS CONNECT rotating shafts in equipment powered by electric motors and other drives. All transmit torque and angular velocity. Flexible variations compensate for misalignment. Many of the latter even address vibration and improve system dynamics. Design considerations include machine or installation construction and backlash, torsional stiffness, damping, inertia, torque ratings, maximum rpm, size, misalignments, ease of installation, robustness, and cost. For power transmission (as in motors for pumps and large material-handling setups) common choices are gear, disc, elastomeric tire, grid, jaw, and Oldham couplings because of their ruggedness and ability to transmit very large torques. Motion-control applications (as for axes employed in precise positioning of loads, for example) typically employ couplings capable of oft-more modest but far more precise torque transmission. These include curved-jaw, beam (slit), bellows, disc, and other zero-backlash couplings. As we’ll explore, any misalignment that couplings
accommodate should be what’s otherwise unavoidable even after proper machine-axis squaring and installation adjustments. That’s because misalignment — manifest as parallel, axial, and angular misalignment— degrades efficiency, induces bearing wear, and excites machine natural frequencies. To review, the maximum amount of angular misalignment for which a coupling can compensate is expressed in degrees; parallel misalignment between the shafts a coupling connects is expressed in inches or millimeters. Axial misalignment is also a length value; it’s the maximum permissible spread between coupled shafts — and in fact, a misalignment permutation often most affected by thermal effects. Flexible couplings for motion control are often less forgiving of misalignment than those for more straightforward power transmission, and resolve it with specialty design features.
Bellows couplings have a low moment of inertia and are suitable for dynamic applications — as those on precision motion axes. Shown here is an example from R+W America. It has bores on the hubs to ensure balanced installation; frictionbased clamping and high-grade stainless-steel bellows maintain zero backlash. Some versions of these couplings work on axes to 10,000 rpm; some finely-balanced variations can run even faster.
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COUPLINGS
A related phenomenon and a coupling consideration specific to motion-control installations is backlash. In applications for strict power transmission, backlash is far less of a concern than that of efficient torque transmission — and actually a characteristic that (in normal moderate quantities) helps make some couplings in these settings more efficient and forgiving of misalignment. In contrast, couplings on the outputs of steppers and servomotors are designed to prevent the lost motion that can degrade output-product quality or overall machine throughput. Note there’s a difference between backlash (which is true mechanical clearance) and the torsional deflection or windup that all loaded rotary components exhibit. Most couplings for motion applications are inherently backlash free or preloaded to eliminate backlash — but they all have different torsional stiffnesses, which is sometimes a tradeoff for lateral flexibility. Pitfalls to avoid during selection of couplings for motion Design engineers often run into trouble when they neglect to account for environmental effects on couplings — particularly flexible couplings installed in gritty or caustic areas, vacuum environments, or places that are extremely hot or cold. Beyond that and the common design considerations already listed, designers must account for dynamic forces to which a coupling will be subject. Steer clear of using published an axis’ gearset or motor peaktorque values for setting its coupling’s nominal torque rating. That’s because this approach usually makes for an assembly with an oversized coupling and an unnecessary inertial increase. Designers should also avoid the application of a coupling type simply because it’s a familiar technology. For example, beam couplings are extremely well known in industry, and they excel on axes transmitting moderate to light torque — as on leadscrew-driven motorized axes or where there’s a need for attachment of a precision encoder, for example. However, some particularly demanding designs may necessitate a flexible coupling type that maintains higher torsional stiffness. On the other hand, it’s also unadvisable to simply pick a coupling based on high torsional stiffness. Many flexible couplings have an inherent stiffness that exceeds application requirements for servo tuning and motion accuracy. Even in motion designs requiring high stiffness for the shortest possible response time (as in equipment for electronics manufacturing, for example) couplings with good damping characteristics often offer more effective optimization than more torsional stiffness.
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DESIGN WORLD — MOTION
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MOTION SYSTEMS HANDBOOK Shown here are some zero-backlash coupling options.
FLEXIBLE COUPLINGS FOR MOTION SOME JAW COUPLINGS ARE FOR SIMPLER POWER-TRANSMISSION DESIGNS; LOOK FOR CURVED-JAW AND ZERO-BACKLASH DESIGNATIONS FOR MOTION.
STANDARD
JAW (SPIDER)
τ MAX
TO 7,000 LB-IN.
BEAM STANDARD TO 260 LB-IN.
τ MAX
BEAM COUPLINGS AS THESE FROM HELICAL PRODUCTS CO. ARE SINGLE-PIECE DESIGNS WITH NO ASSEMBLY POINTS TO LOOSEN. NOTE THE ATTACHMENTS AND OTHER CUSTOM VARIATIONS.
RUBBER INSERT
THIS COUPLING DELIVERS TORSIONAL STIFFNESS PLUS DAMPS VIBRATION FOR EASIER SERVO TUNING. THE CENTER MOLDED PORTION IS HNBR. IMAGE COURTESY NBK AMERICA
BELLOWS τ MAX
LESS
ALLOWABLE MISALIGNMENT
MORE
ELASTOMERIC INSERT
STANDARD TO 900,000 LB-IN.
APPLICATION EXAMPLE COURTESY GAM ENTERPRISES
τ MAX
STANDARD TO 1,600,000 LB-IN.
The importance of alignment No coupling — no matter how engineered — can correct for shafts that are excessively misaligned. The nature of flexible couplings occasionally misleads design engineers and assembly personnel (or more often, end users) into believing that they’re a fix-all for compromised or less exacting machine builds. But flexible couplings put into designs with excessive misalignment exhibit material stresses and fatigue and premature failure. Though coupling failures do occasionally originate from couplings themselves, it’s far more common that coupling issues arise as a symptom of other design problems. If a motion design does exhibit coupling problems, avoid the temptation to simply upsize or upgrade that coupling. Such upgrades are often unnecessarily expensive and short-lived solutions that actually put system bearings as well as gearing and connected motors at risk of collateral damage. Instead, make a holistic analysis of the design and consult the coupling manufacturer for assistance. Note that when motion systems exhibit coupling issues long after a proper installation and run of service, it’s sometimes a result of some other change in the drive assembly. Even small changes to the motor, drive, or programming can be to blame — especially if a new motion sequence demands higher transmission of motor torque or the elimination of a previously held electronic limitation. What are reaction forces? All flexible couplings compensating for misalignment cause reaction forces, and their effect is significant if misalignment is excessive. These often-overlooked reaction forces transmit to connected shafts and support bearings, and can cause damage to motion axes — especially on precision designs with delicate bearings and slender shafts. Though couplings get their compliance from elastomeric deflection, sliding contact, and flexing coupling members, here we focus on the types most common for motion designs employing stepper or servomotors. Ultimately, reaction-force magnitudes depend on the level of misalignment and the coupling type in use. Bellows couplings, so-called membrane couplings such as disc couplings, and beam couplings have thin sections of various designs capable of radial flexing. Resistance to misalignment — a spring-rate reaction defined as a force per unit of deflection — increases proportionally with shaft deflection. Because these couplings bend to accommodate misalignment, reaction force depends on the thickness of the flexible element. But bellows and beam couplings have multiple coils or convolutions, so work as flexible shafts that sweep through complementary bows as the mode of misalignment compensation. Torque transmission is through members in shear, so the convolutions can be thin and keep radial forces low while maximizing torsional stiffness. In contrast, membrane-coupling variations transmit torque via bending members, so need thick members to get high torsional stiffness. Such couplings’ bending (through complementary directions) also compensates for shaft misalignment. The catch is that these torsionally stiff
This is a Zero-Max composite-disc (CD) coupling for high-speed and high-torque servomotor and motioncontrol applications. CD couplings have high dynamic load capacity, and excel in precise positioning — even on axes that make high-speed reversals. Though comparable couplings of other designs may have high torsional stiffness, some are too brittle to withstand such reversing.
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That’s because overly stiff couplings of many designs pose an unnecessary risk of fatigue.
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RIGID COUPLINGS ONLY WORK ON EXCEPTIONALLY WELL-ALIGNED AXES.
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MOTION SYSTEMS HANDBOOK
Courtesy Miki Pulley
This EKM elastomer coupling from GAM is for motion-control applications needing to transmit torques from 2 to 1,000 Nm.
couplings can induce significant radial-reaction forces if excessive misalignment is present. Because radial-force magnitude depends on bend severity, minimizing bending angles reduces the detrimental forces on support bearings (though can reduce misalignment capacity, too). Some membrane couplings address more misalignment with a central member between the flexible members; the added distance imparts an ability to turn while making shallower bends (and lower radial forces) for a radial shaft offset. In fact, aforementioned beam and bellows couplings with divided flex-element arrays also sometimes leverage more distance between flexure points to get shallower bends for a given radial shaft offset. Short models can connect via an intermediate shaft. In contrast, elastomeric couplings have myriad torsional-damping properties and transmit torque in shear, bending, and compression. Coupling class is key with this design: Some versions exhibit zero backlash while others rated for use in power-transmission applications can exhibit minute inter-hub
rotation. Most variations transmit torque (and address misalignment) through compressible elastomeric-insert spiders trapped between jawed halves ... and induce reaction forces when connecting shafts with excessive radial shaft deflection. Jaw designs can accommodate more misalignment (and minimize detrimental forces on the shafts’ support bearings) with softer elastomer spiders, though that sometimes reduces torsional stiffness. Remember that if excessive misalignment is a concern, consult with coupling manufacturers on the design. Their engineers may suggest design improvements; offer coupling types to resolve the misalignment without inducing unacceptable reaction forces; and supply charts of reaction forces that a given coupling is projected to induce under a given set of conditions.
FAQ: How does coupling misalignment cause vibration? Misalignment introduces rpm limitations, vibration, and noise. Consider beam couplings: Once failed and unwound, these couplings won’t go back to their original form. So under misalignment, the machine will be at high risk of exhibiting resonance issues ... and in fact, bent beams running at high rpm even emit noise. A system that is not tuned properly (with a stepper or servomotor system untuned, for example) can cause any beam coupling in the assembly to vibrate. Other common causes of misalignment-induced vibration include the use of parts from different manufacturers. The design engineer may have a motor or a motor bracket — or maybe an actuator from another company. When mixing and matching components, there can be inaccuracies in the system assembly, mounts, and more ... all can introduce misalignment. Many motor mounts on actuators have pilot holes into which a motor slides. A common misconception is that as long as the assembly team puts the motor in and bolts it down, the system will be in alignment. In fact, that’s not always the case. If those are clearance holes on that motor (or motor bracket) during assembly, the motor could drop down in the holes ... bolting it down in that position will cause misalignment. Motor movement during operation can be another issue.
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Occasionally, a system running over time will put itself out of alignment. System components wear. Things wear down. So for example, input bearings on the motor and actuator wear out ... and that can cause any attached ballscrew (locked into the endmount bearing) to actually drop a little bit. It doesn’t take much to throw these systems off. To review, assemblies can exhibit angular misalignment (in which shafts are off at an angle); parallel misalignment (the most common form of misalignment); and complex misalignment — a combination of angular and parallel. Another type — axial misalignment — is an interesting subtype and one to guard against. Axial misalignment involves coupled shafts moving closer together and then apart during operation. All coupling types have a listed amount of misalignment in axial that they can handle. What causes maximum axial misalignment? Typically it’s high-speed motors running fast enough to cause heat and thermal expansion of that shaft — and dimensional changes which the coupling must accommodate. Tip: For any high-speed motor running between 6,000 and 10,000 rpm (anything over 3,000 actually) watch out for issues with axial misalignment due to thermal expansion of the motor shaft. Insight into misalignment and vibration provided by Bobby Watkins of Ruland Mfg.
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MOTION SYSTEMS HANDBOOK
Fundamentals of
ac drives
AN AC DRIVE controls the speed of an ac motor. The motor’s speed is determined by the number of poles and the frequency so a common way to control frequency is via pulse width modulation (PWM). A PWM drive outputs a train of pulses to a motor and by modulating the pulse width, making it either narrower or wider, delivers an ac current waveform to the motor. A variable frequency drive (VFD) is one of the most common types of ac drives. It operates by switching output devices — which can be transistors, IGBTs (insulated gate bipolar transistors), or thyristors — on and off. VFDs can be either constant voltage or constant current. Constant voltage types are the most common type of VFD. It uses PWM to control both the frequency and the voltage applied to the motor. One of the benefits of VFDs is the actual energy savings. Controlling the amount of current drawn by the motor can decrease energy costs because the motor will not run at full load all of the time. This is more important as motor efficiency continues to be a top design priority. For instance, single-phase induction machines (specifically, permanent split-capacitor motors) and universal motors are managed with simple voltage-control techniques. Contrast this with high-end, high-performance machines where three-phase motors are more common and which are controlled by VFDs. On motor start-up, VFDs can provide a crucial advantage. Without a VFD, an induction motor on startup has to handle a high initial in-rush current. As the motor speeds up and approaches a constant speed, the current levels off from the peak in-rush values. With a VFD, the motor’s input starts off with low voltage and a low frequency, avoiding the problem of high in-rush currents.
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Other benefits of VFDs include reducing mechanical wear on the motor components. Eliminating the in-rush currents upon start-up gets rid of the excessive torque on components, increasing the life of the motor and reducing maintenance costs and the need for repair. In addition to reducing wear, mechanical stresses on the entire system are greatly reduced. In many cases, mechanical controls such as throttles, valves, dampers and louvers can be completely removed, thereby reducing mechanical wear and maintenance costs. Further, with reduced mechanical wear, due to precise motor speed control, the Now Unitronics system output quality may be sells variable frequency improved and production drives (VFDs) to optimize times reduced. machine performance, save energy, and lower machine lifecycle costs across a range of applications. Unitronics’ VFDs seamlessly integrate with the manufacturer’s PLC + HMI All-in-One controllers: UniStream, Vision, and Samba. The line includes both single and three phase VFDs for 0.4 to 110 kW.
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MOTION SYSTEMS HANDBOOK
FAQ: What are medium-voltage ac drives and where are they used? RECALL THAT THE BASIC EQUATION for electric power is P = VI — power = voltage multiplied by current. This means that for a given power level, voltage and current are inversely proportional. So the higher the supply voltage, the lower the current draw. For ac motors used in high-power applications, operating at a low voltage causes the motor to draw very high current ... resulting in higher energy usage, lower efficiency, and higher cost. So current draw can be reduced by increasing the voltage supplied by the drive. The full equation for threephase power is: P = V · I · PF · 1.732 Where P = Power (W), V = Voltage (V), and I = Current (A) ... and PF = Power factor from the manufacturer. In fact, the power factor is the ratio of working power to apparent power — essentially, how effectively the electricity is used. The constant 1.732 is the square root of 3, and is used for threephase power to account for the fact that all three phases don’t produce the same amount of power at the same time. Ac drives are generally classified as low voltage, medium voltage, and high voltage, although classifications for each type vary by manufacturer and even by standard. The ANSI C84.1 standard defines low voltage as 240 to 600 Vac while medium voltage is typically 2,300 or 4,160 Vac ... or 3,300 or 6,600 Vac in Europe and most of the rest of the world ... but can be as high as 69,000 Vac. High voltage (according to ANSI C84.1) is 115,000 to 230,000 Vac. Based on the three-phase power equation above, here’s an example of the difference in current
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Watercooled medium-voltage variable-speed drive system image courtesy ABB
draw between a low-voltage and medium-voltage drive. If a 1,000 hp (746 kW) motor operates at 480 V, current draw is: 746,000 W = 480 V · I · 0.87 · 1.732 for I = 1,031 A The same 1,000 hp (746 kW) motor operating at 4,160 V will have a current draw of: 746,000 W = 4160 V · I · 0.87 for I = 206 A Medium-voltage drives can be either current-source inverter or voltage-source inverter types, although VSI types are more popular due to high reliability and low harmonic distortion. Ac drives in this configuration are referred to as medium voltage multidV/dt refers to change in voltage over time. level voltage-source PWM drives. In PWM applications, it describes the rapid voltage Recall that inverters convert dc power back to ac power at the rise at each pulse of the PWM waveform. required frequency and voltage. The term “multi-level” refers to the output of the inverter, and PWM is pulse-width-modulation — which switches the dc voltages to create the ac power. In the multi-level design, the inverter uses multiple dc voltages (rather than just two voltages, as with a typical two-level inverter) to synthesize the ac waveform ... making it nearly sinusoidal in nature. This allows the PWM switching frequency to be reduced — and reduces the dV/dt too, which reduces harmonics. Most medium-voltage ac drives include a multi-phase transformer on the front end, which works in conjunction with the multi-level inverter to reduce harmonics. For applications with highly dynamic loads, an active front end may also be used. The active front end reduces harmonics on the line (input) side by monitoring the harmonic level and actively filtering the waveform to provide dampening. Medium-voltage ac drives generally benefit applications that require motors from 500 to 20,000 hp, where the difference in current draw between low voltage and medium voltage becomes substantial. Motors in this range often operate in large compressors, fans for power stations, petrochemical plants, and mines.
8 • 2018
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MOTION SYSTEMS HANDBOOK
Basics of
dc drives
A DC DRIVE controls the speed of a dc motor. The best way to control the speed is by varying either the armature voltage or the field current. This is because the speed of dc motors is directly proportional to the armature voltage and inversely proportional to motor flux, which is a function of field current. The two types of dc motors — brushed and brushless (BLDC) — have different drive techniques. For instance, one can drive a brushed dc motor by applying a voltage or pulse width modulated (PWM) voltage. The motor increases in speed (while reducing torque) until the torque and speed match the load. A common dc-motor drive technique is to use a thyristor-based control circuit. These circuits consist of a thyristor bridge that rectifies ac into dc for the motor armature. Varying the voltage to the armature controls the motor’s speed. Thyristors or silicon-controlled rectifiers (SCRs) provide controllable power output by phase angle control ... so called for the firing angle that synchronizes with the ac source’s phase rotation. BLDC motors are a bit more complicated to drive. A typical brushless motor will have three sets of windings connected in a star or Y or Delta configuration. The motor drive energizes each of the windings to turn the motor so the motor position must be known to control the windings properly. The controller can be a sensorless type if relies on detecting the windings’ back EMF (electromotive force) to track position and provide the sequence information for the controller. A sensorless AutomationDirect IronHorse GSD4 series dc drives drive allows the use of a motor without Hall effect sensors, making the provide reliable control of permanent-magnet, motor less expensive and requiring fewer connections. shunt-wound, and universal motors. The drives Most BLDC motor drives use speed as a control signal — in the run off voltage of 24/36 Vac to prompt motors to form of a PWM input signal rather than an analog signal, for example. output up to 1/6 hp. Standard features include an The most common types of PWM drives are sinusoidal and trapezoidal. inhibit circuit for start-stop operation and 1% speed regulation over a 50:1 speed range. Dual-voltage Sinusoidal PWM drives increase and decrease the current to each models are available. winding to follow a sinusoidal curve to smooth drive power and produce a smoother motor torque. Simple on-off control of the windings tends to produce an uneven torque through the rotation of the motor ... and tends to generate more audible noise due to the uneven torque. Similar to sinusoidal drives, trapezoidal PWM drives increase the current to each winding in a straight line (based on motor position) and then decrease it in a straight line while increasing current to the next winding. To use a trapezoidal or sinusoidal PWM drive, The Gold Twitter motor controllers must track the motor-rotor NANO servo drive position with more accuracy than what a simple from Elmo Motion Control Hall-effect switch position provides. Such provides more than 5,500 W tracking is possible by monitoring the motor with output voltage to 96% of dc velocity and predicting the position with bus voltage. Weighing 18 to 22 grams, the tiny drive is designed time. While not perfectly accurate, its results to be mounted on a PCB with are considerably better than that of a simple networking via EtherCAT or on-off drive. Plus trapezoidal drives are quieter CANOpen. and smoother than simple on-off drives though not as smooth or quiet as a sinusoidal drive.
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MOTION SYSTEMS HANDBOOK
Encoder basics ENCODERS provide position information and are classified by the method of sensing (i.e. optical, magnetic, or capacitive sensing) or whether the position output is absolute or incremental. Another common distinction is whether they are rotary or linear. They can also be classified by how much force they can handle on the shaft, so there are light duty, medium duty, and heavy-duty encoders. Optical or magnetic The most common sensing technologies are optical and magnetic. While optical encoders have historically been the only option for resolutions below 5 microns, improvements in magnetic scale technology now allow them to achieve resolutions down to 1 micron.
Optical encoders use a light source and a photo-detector to determine position, but their use of light makes them sensitive to dirt and debris, which can disrupt the signal. The performance of optical encoders is highly influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity is not compromised. This means that mounting must be done carefully, and shocks and vibrations should be avoided. Magnetic encoders use a magnetic reader head and a magnetic scale to determine position. Unlike optical encoders, magnetic encoders are mostly unaffected by dirt, debris, or liquid contamination. Shocks and vibrations are also less likely to affect magnetic encoders. They are, however, sensitive to
magnetic chips, such as steel or iron, as they may interfere with the magnetic field. While linear encoders are often an add-on component to a system, in many cases their benefits outweigh the additional labor and cost. For example, in ball screw driven applications, a lower accuracy screw can be chosen if a linear encoder is used, since the encoder feedback allows the controller to compensate for positioning errors introduced by the screw. Absolute or incremental A fundamental distinction is whether an encoder is absolute or incremental. Absolute encoders have a unique code for each shaft position, so that every position is distinct. The absolute encoder interprets a system of coded tracks to create
These heavy-duty encoders from Leine & Linde come in incremental and absolute versions and are designed for harsh environments. They feature Ethernet I/P connectivity.
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MOTION SYSTEMS HANDBOOK
POSITAL KIT ENCODERS DELIVER THE ABSOLUTE ADVANTAGE!
Magnetic encoder image courtesy Renishaw
Accurate Position Feedback for Servo or Stepper Motors n
n
n
n
Same form-factor as US Digital or Broadcom incremental encoders position information where no two positions are identical. Absolute encoders also do not lose position if there is power loss. Because each position is distinct, true position is verified as soon as power is restored and so does not require a homing routine. Absolute encoders can be either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion applications where position verification is needed within a single turn of the encoder shaft. On the other hand, multiturn encoders are better for applications that involve complex or lengthy positioning requirements. Absolute encoders have a number of advantages. Safety is one. In some applications where a loss of position could lead to operator injury or machine damage, an absolute encoder automatically provides position verification when power is restored. Absolute encoders also have good immunity to electrical noise because they determine position by reading a coded signal. Stray pulses from electrical noise will not build up and accurate position is presented again on the next reading. Incremental encoders generally supply square-wave signals in two channels, A
Easy upgrade to absolute position feedback for superior position control Rugged magnetic measurement technology for long, maintenance free service lives. Multi-turn measurement range with Wiegand energy harvesting technology - no backup batteries required!
www.posital.com 76
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and B, which are offset (or out-of-phase) by 90 degrees. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position like an absolute encoder. To provide useful position information, encoder position has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function. A traditional limitation of an incremental encoder comes with a loss of power. For instance, if a machine with an encoder is turned off, the encoder will not know its position when switched on again. The encoder has to perform a homing routine in order to know its exact position. Then, a counter or buffer will be zeroed and the system will determine where it is relative to fixed positional points. One way around this issue of loss of power is to use a battery backup system. Such a solution ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored. Even if the load’s actual position at startup or re-start is not critical, performing
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ENCODERS
a homing sequence may be undesirable from a time and productivity standpoint — especially for applications with long strokes and slow speeds, as on machine tools. Rotary or linear Linear encoders monitor linear movement and provide position feedback in the form of electrical signals. In servo driven systems, linear encoders supply the precise position of the load, typically in addition to the speed and direction feedback provided by the motor’s rotary encoder. For stepper driven systems, which typically operate in open-loop mode with no position feedback, adding a linear encoder increases the accuracy and reliability of the positioning system without the cost and complexity of a servo motor.
Rotary encoders measure resolution in pulses per revolution. Similar to linear encoders, a typical rotary encoder contains an internal coded disk and a sensing head. Think of a linear encoder as a type of tape measure while a rotary encoder is more like a measuring wheel. For instance, a rotary encoder with a 100 point per revolution resolution would have 100 marks on its coded disk. Encoder performance Accurately measuring position is the most critical performance parameter for an encoder. The right encoder also should have the necessary resolution required by the application. However, resolution and accuracy are not the same. For instance, an encoder may have a high resolution but low accuracy, or vice versa. The ideal encoder has both the necessary resolution and is highly accurate.
Generally speaking, magnetic encoders are more resistant to shock and vibration than optical encoders. They’re also better able to withstand environmental contaminants such as dust, grease and moisture. As for electrical interference, optical encoders are fairly immune to it because position measurement is not electrical-based but optical. Also, magnetic technologies may be subject to strong magnetic fields that could impact readings. Encoder resolution can be programmed through a USB interface via computer. Other parameters can be changed on the fly through simple software commands. Common encoder communication protocols include SSI (Synchronous Serial Interface), BISS (bidirectional serial/synchronous), ProfiBus, DeviceNet and Ethernet I/P. These protocols are open — meaning they
For encoders in packaging, EPC is your motion control expert. Tru-Trac™ Linear Measurement Solutions
When you need a linear measurement solution, the Tru-Trac™ line of encoders are your all-in-one solution. An encoder and measuring wheel in one unit, Tru-Trac™ encoders come with a spring-loaded torsion arm that offers adjustable torsion load, allowing Tru-Trac™ encoders to be mounted in almost any orientation – even upside-down. Widely used in the packaging industry, the compact Model TR1 is available with several different types of measuring wheels. For product specs and installation videos, or to configure 3D models and download files, visit encoder.com.
Encoder Solutions for Any Packaging Application When you call EPC, you talk to engineers and encoder experts who can help you find and spec the right encoder for your application. From compact to heavy-duty stainless steel, thru-bore or shaft, incremental or absolute, we’ve got the encoder you need for motion feedback. Stainless Steel Encoders – For those tough, dirty applications needing a durable encoder that can tolerate washdown and extreme environments. Absolute Encoders – Mechanically and electronically versatile, EPC’s selection of absolute encoders is your solution when you need position information after power-off scenarios.
Programmable Encoders – Not sure of the exact configuration you need? No problem. EPC’s highly-configurable Model 25SP Size 25 shaft encoder and new thru-bore Model 58TP allow you to program the electrical specifications on-site.
EPC’s standard lead time is just 4 - 6 days And we offer same-day shipping on many models. Call us today. We’ll help you find your motion feedback solution.
1-800-366-5412 • www.encoder.com
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MOTION SYSTEMS HANDBOOK Encoder Products Co. Model TR1 all-in-one Tru-Trac encoders excel in linear-measurement applications. The compact encoder is widely used in the packaging industry. It can even be installed upside down.
are non-proprietary and not tied to a specific manufacturer’s products. Closed or proprietary protocols include Hiperface (High Performance Interface) and EnDat (Encoder Data). FAQ: What is TTL output for incremental encoders? Rotary encoders are used to track the angular position of a shaft, providing either incremental or absolute position information. Incremental rotary encoders work by generating pulses as the shaft rotates. By counting the number of pulses, the position of the shaft relative to its
starting position can be determined. Incremental encoders output two square-wave signals in quadrature, meaning they are phase-shifted by 90˚ relative to each other. Most incremental encoders also include an index signal, which is typically denoted Z. The Z signal is a single pulse that occurs once during each shaft rotation and can be used to indicate a zero position. With two output signals A and B, the receiver or controller can determine whether the shaft is rotating in a clockwise or counterclockwise direction, based on which signal is leading, or rising. Quadrature output refers to the waveform of the signal, but there are also different types of output circuits, with the most common types being open collector, HTL, and TTL. TTL output circuits provide not only the A, B, and Z signals, but also
their complementary, or differential, signals, denoted /A, /B, and /Z. When long cable lengths are used, stray electromagnetic fields or currents induce unwanted voltages, which cause noise. TTL output reduces or eliminates this noise through its use of differential signals. In a differential pair of signals (wires), the unwanted voltages are the same in each wire. Because the TTL output signals are complementary (opposites), the controller can sum the voltage of each signal to remove the noise and retain the original voltage. Square wave signals have two states — high or low. TTL output generally conforms to the RS 422 standard, which specifies that the high state will be 5 VDC regardless of the supply voltage, and the low state will be 0 Vdc.
VIONiC™ digital encoder series Designed for the designer
The VIONiC digital incremental encoder series has been specifically designed with the machine builder in mind. Its enhanced ease of use, superior metrology capability and multiple configuration options all ensure optimal machine performance. VIONiC really has been designed for the designer.
For more information visit www.renishaw.com/vionic
Renishaw Inc. West Dundee, IL
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ENCODERS
FAQ: What are encoder pull-up resistors and how do they work? Recall that incremental encoders can provide several types of digital output — including open collector, push-pull, and line driver. Open collector output is considered sinking, while the line driver type is considered sourcing; push-pull output is both sinking and sourcing. Line driver output is called sourcing because it supplies current to the load. Line driver outputs are differential, meaning that they produce pairs of signals. The complementary signals are exact inverses of their main signals, so when the A signal is in the high state, the A- signal will be in the low state. This arrangement spreads any noise across both signals, which are evaluated at the receiver as differential, rather than absolute, levels. This gives the line driver output a high level of noise immunity. A push-pull output driver can either sink or source current to the load. The high level output is equal to the power supply, effectively pushing or sourcing current to the load from the power supply. Low-level output equals the power supply common voltage (0 V) level — effectively pulling or sinking the load to ground. The simplest type of output driver is the open collector or NPN (sinking) output. An open collector driver sinks current from PNP (sourcing) inputs — either the PLC or motion controller. In the on state, it supplies a path to ground, and in the off state, it floats ... so is neither high nor low. So there must be a way to manipulate its voltage to indicate a logic high signal. This is where a pull-up resistor comes in. The basic function of a pull-up resistor is to ensure that when there is no input, a circuit takes on a default value. In other words, it sources current from the power supply to pull the voltage up to a specified value (typically the operating voltage) so that it can be read as a logic high signal. The output causes the voltage to drop to its low or ground state (0 V). This switching of the output pulses on and off creates a square wave. In contrast to pull-up resistors, pulldown resistors limit the current that can flow between the supply and ground and are sometimes used in encoder line driver and push-pull outputs.
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Encoders – Motion Control HB 08.18 V4 FINAL.indd 79
Signal diagram for a TTL output encoder 360° 90°
90°
90°
90°
A A
B B
Z Z
This is a signal diagram for a TTL output encoder showing complementary (differential) signals. TTL output is often called differential line driver output.
TTL differential wiring to cancel noise Encoder with single-ended wiring
Receiving device
Noise spike occurs
Single-ended wiring lets noise remain.
Encoder with differential wiring
Clean signal
Noise pickup on signal
Noise is cancelled out.
TTL output uses differential wiring (A with /A and B with /B) to cancel noise.
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MOTION SYSTEMS HANDBOOK
Gas springs GAS SPRINGS, also called gas dampers, tension springs, or gas-pressure springs depending on the setup and context, are compressed-air or oil cylinders that install in motion designs to damp forces and return kinematic linkages (and more complicated assemblies) to default positions. Gas springs work through a piston on the end of a rod that protrudes from a steel cylinder body; usually compressed gas (often nitrogen) within the cylinder exerts force on this piston to reassume and maintain set positions. Nitrogen is common here because it’s inert and nonflammable. In such designs, oil or grease between the piston and other contacting parts minimize friction. In fact, the small amount of oil in these gas springs serves another function — to further damp and gently decelerate gas springs during full extension or compression. Some setups even include a fine hole in the piston for damping that’s still slower than with other designs; such slow-damper springs are common on safety gates and doors. In contrast, extended-reach gas springs usually leverage telescoping mechanisms pairing multiple cylinders on one rod; then the smaller cylinder extends from within the larger cylinder. Consider one particularly long-stroke application: Passive heave compensators — systems on ships or offshore oil-rig systems that reduce the effect of waves on engineered structures — use gas springs with strokes to many meters long. Still other gas-spring applications include those for medical beds and hoists; industrial equipment such as machine-tool presses; off-highway and automotive equipment for hatches, hoods, and covers; office equipment and furniture; and general strut and support applications. Fast-acting gas springs find use in weaponry and aerospace design. Specific variations include
gas springs with standard or fixed-height cylinders; spindle-only designs; and cable, return, adjustable auto-return, nonrotating, stage, and multi-mode cylinders. No matter the iteration, gas-spring extension force — a value that usually ranges from 1 to 5,000 N — depends on piston-rod cross-section multiplied by fill pressure. Manufacturers commonly express extension force with two values — for rod extension and rod retraction — at normal ambient temperature and with the piston rod pointing downward. (Note that typical ranges are only those most common; some gas-spring applications in heavy industries use gas springs delivering several hundred-thousand Newtons cases.) Other gas-spring definitions include two pullin forces — at rod extension and rod retraction — and overall friction force. These values depend on the gas spring’s gas and damping-oil volumes. Various nozzle orifices and oil quantity allow control of push-out and push-in speed. If design parameters are unknown, look for manufacturers capable of prototyping — especially for designs requiring an exact force that’s hard to pre-estimate — as in lifting a frame in a set time, for example. Here, some manufacturers sell prefilled cylinders sporting bleed valves. Then installers can bleed gas from the cylinders after system setup to get the correct forceacceleration actuation profile. The only caveat here is that if too much gas is bled, the assembly will need a new spring. That’s why OEM-level quantities of gas springs justify pre-engineered cylinders with preset pressurization. Or gas springs can offer full in-design adjustability via bleed valves and movable-endstop pressurization mechanisms, Bowden cables, knobs, and more. Some emergency-use gas springs also employ gas-generator cartridges that resemble those in airbags.
F₃
FORCE, N
FORCE, N
GAS-SPRING FORCES AND STROKES GAS-SPRING FORCES AND STROKES
F₃
G A S -S
F₁G A S -
F₁
XTENS P R IN G E
F₄
IO N
N O M P R E S S IO S IO XGTAESN E P R IN G C G -S IN R SP GAS-
CO M P R S P R IN G
N
F₂
E S S IO N
F₄ F₂
STROKE, MM
F1 (and F₂) = Ex tension forces with ex tended (compressed) piston rod F₃ (and F₄) = Pull-in force with ex tended (compressed) piston rod
STROKE, MM F1 (and F₂) = Ex tension forces with ex tended (compressed) piston rod F₃ (and F₄) = Pull-in force with ex tended (compressed) piston rod
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These are force-output parameters to define the capabilities of gas springs. Note that if the design is setup as a tension spring, then what’s classified here as compression (retraction) is the extension position, and vice versa. The value difference between extension and compression forces for a give rod position is equal to the gas spring’s friction force.
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MOTION SYSTEMS HANDBOOK
SUMMARY OF GEARING:
Strain-wave gearing STRAIN-WAVE GEARING for speed reduction uses the metal elasticity (deflection) of a gear to reduce speed. (Strain-wave gearing sets are also known as Harmonic Drive® sets, a registered trademark term of Harmonic Drive Systems Inc.) Benefits of using strain-wave gearing include zero backlash, high torque, compact size, and positional accuracy. A strain-wave gearset consists of three components — wave generator, flexspline and circular spline. The wave generator is an assembly of a bearing and steel disk called a wave generator plug. The outer surface of the wave generator plug has an elliptical shape machined to a precise specification. A specialty ball bearing goes around this plug to conform to the same elliptical shape of the wave generator plug. Designers typically use the wave generator as the input (attached to a servomotor). The flexspline (usually acting as output) is a thin-walled steel cup. Its geometry makes the cup walls radially compliant but torsionally stiff (because the cup has a large diameter). Manufacturers machine the gear teeth into the outer surface near the open end of the cup (near the brim). The cup has a rigid boss at one end for mounting. The wave generator goes inside the flexspline so the bearing is at the same axial location as the flexspline teeth. The flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This conforms the teeth on the outer surface of the flexspline to the elliptical shape. That way, the flexspline effectively has an elliptical gear-pitch diameter on its outer surface.
Strain-wave gear illustration courtesy Harmonic Drive
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The circular spline is a rigid circular steel ring with teeth on the ID. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the flexspline. The tooth pattern of the flexspline engages the tooth profile of the circular spline along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse contacts a circle at two points. However, gear teeth have a finite height, so two regions (instead of two points) engage. The pressure angle of the gear teeth transforms the output torque’s tangential force into radial force acting on the wavegenerator bearing. The teeth of the flexspline and circular spline engage near the ellipse’s major axis and disengage at the ellipse’s minor axis. The flexspline has two less teeth than the circular spline, so every time the wave generator turns a revolution, the flexspline and circular spline shift by two teeth. Tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that lets up to 30% of the teeth (60 for a 100:1 gear ratio) engage at all times. This contrasts with maybe six teeth for a planetary gear, and one or two teeth for a spur gear. In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Backlash is the difference between the tooth space and tooth width, and this difference is zero in strain-wave gearing. As part of the design, the manufacturer preloads the gear teeth of the flexspline against those of the circular spline at the ellipse’s major axis. The preload is such that stresses are well below the material’s endurance limit. As the gear teeth wear, this elastic radial deformation acts like a stiff spring to compensate for space between teeth that would otherwise increase in backlash. This lets the performance remain constant over the life of the gear. Strain-wave gearing offers high torque-to-weight and torqueto-volume ratios. Lightweight construction and single-stage gear ratios (to 160:1) let engineers use the gears in applications requiring minimum weight or volume ... especially useful for designs with small motors. Another tooth profile for strain-wave gearing is the S tooth design. This design lets more gear teeth engage for a doubling of torsional stiffness and peak torque rating, as well as longer life. The S tooth form doesn’t use the involute tooth curve of a tooth. Instead, it uses a series of pure convex and concave circular arcs that match the loci of engagement points dictated by theoretical and CAD analysis. The increased root filet radius makes the S tooth much stronger than an involute curve gear tooth. It resists higher bending (tension) loads while maintaining a safe stress margin.
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247 Lynnfield Street, Peabody, MA 01960 • 978.532.1800 • www.HarmonicDrive.net Harmonic Drive is a registered trademark of Harmonic Drive LLC. Valkyrie image courtesy of NASA.
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MOTION SYSTEMS HANDBOOK
SUMMARY OF GEARING:
General gear design
Spinea TwinSpin (TS) cycloidal reducers from DieQua Corp. have a unique speed-reduction mechanism and radial-axial bearings to output precision motion. The reducers are more compact than many other cycloidal reducers. THE MAIN FUNCTION of a gear is to mesh with other gears to transmit altered torque and rotation. In fact, gearing can change the speed, torque and direction of motion from a drive source. When two gears with an unequal number of teeth engage, the mechanical advantage makes their rotational speeds and torques different. In the simplest setups, gears are flat with spur teeth (with edges parallel to the shaft) and the input gear’s shaft is parallel to that of the output. Spur gears mostly roll through meshing, so can be 98% or more efficient per reduction stage. However, there is some sliding between tooth surfaces, and initial tooth-to-tooth contact occurs along the whole tooth width at once, causing small shock loads that induce noise and wear. Sometimes lubrication helps mitigate these issues. In slightly more complex setups, parallel-axis gearsets have helical gears that engage at an angle between 90° and 180° for more tooth contact and higher torque capacity. Helical reducers are suitable for higher-horsepower applications where long-term operational efficiency is more important than initial cost. Helical gear teeth engage gradually over the tooth faces for quieter and smoother operation than spur gearsets. They also tend to have
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higher load capacities. One caveat: Angled tooth contact generates thrust that the machine frame must resolve. No matter the subtype, most parallelaxis gearsets have gear teeth with tailored involute profiles—customized versions of the rolled trace off a circle with an imaginary string. Here, mating gears have tangent pitch circles for smooth rolling engagement that minimizes slipping. A related value, the pitch point, is where one gear initially contacts its mate’s pitch point. Involute gearsets also have an action path that passes through the pitch point tangent to a base circle. Besides parallel-axis gearsets, there are non-parallel and right-angle gearsets. These have input and output shafts that protrude in different directions to give engineers more mounting and design options. The gear teeth of such gearsets are either bevel (straight, spiral or zerol), worm, hypoid, skew or crossed-axis helical gears. The most common are bevel gearsets with teeth cut on an angular or conical shape. Hypoid gears are much like spiral-bevel gearsets, but the input and output shaft axes don’t intersect, so it’s easier to integrate supports. In contrast, zerol gearsets have curved teeth that align with the shaft to minimize thrust loads.
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MOTION SYSTEMS HANDBOOK
SUMMARY OF GEARING:
Gearboxes, specialty gearheads & servogear sets SERVO DESIGNS are precision-motion setups with feedback and (in most cases) fairly stringent accuracy demands. So for these designs, engineers should pick servogear reducers with good torsional stiffness, reliable output torque and minimal backlash. OEMs tasked with integrating servo systems should look for quiet reducers that easily mount to the motor and require little or (if possible) no maintenance. In fact, a lot of advanced machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have specific labels. Here is a look at some of the most widespread. Gearmotor: This complete motion component is a gear reducer integrated with an ac or dc electric motor. Usually the motor includes the gears on its output (typically in the form of an assembled gearbox) to reduce speed and boost available output torque. Engineers use gearmotors in machines that must move heavy objects. Speed specifications for gearmotors are normal speed and stall-speed torque. Gearbox: This is a contained gear train … a mechanical unit or component consisting of a series of integrated gears. Planetary gears are common in integrated gearboxes. Planetary gears: Particularly common in servo systems, these gearsets consist of one or more outer planet gears that revolve about a central, or sun, gear. Typically, the planet gears mount on a movable arm or carrier that rotates relative to the sun gear. The sets often use an outer ring gear, or annulus, that meshes with the planet gears. The gear ratio of a planetary set requires calculation, because there are several ways they can convert an input rotation to an output rotation. Typically, one of these three gear wheels stays stationary; another is an input that provides power to the system, and the last acts as an output that receives power from the driving motor. The ratio of input rotation to output rotation depends on the number of teeth in each gear and on which component is held stationary. Planetary gearsets offer several advantages over other gearsets. These include high power density, the ability to get large reductions from a small volume, multiple kinematic combinations, pure torsional reactions and coaxial shafting. Another advantage to planetary gearbox arrangements is power-transmission efficiency. Losses are typically less than 3% per stage, so rather than waste energy on mechanical losses inside the gearbox, these gearboxes transmit a
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high proportion of the energy for productive motion output. Planetary gearbox arrangements distribute load efficiently, too. Multiple planets share transmitted load between them, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness. Disadvantages include high bearing loads, inaccessibility and design complexity. In servo systems, besides boosting output torque, gearboxes impart another benefit—reducing settling time. Settling time is a problem when motor inertia is low compared to load inertia … an issue that’s the source of constant debate (and regular improvement) in the industry. Gearboxes reduce the reflected inertia at the controls by a factor equal to the gear reduction squared.
EPR Series right-angle bevel planetary gearboxes from GAM Enterprises deliver the quality and cost advantages of the manufacturer’s EPL inline gearboxes but in a right-angle configuration. The gearboxes come with precision-ground spiral bevel gears on the input stage and EPL or PE inline gearbox elements on the planetary stage. EPR planetary gearboxes are suitable for an array of motion-control applications (including pairing with both servo and stepper motors) and come in 3:1 to 1,000:1 ratios. PER Series rightangle bevel planetary gearboxes are a valuepriced alternative for where there are minimal radial or axial loads.
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The DieQua Advantage We offer the widest range of servo gearheads and gearboxes, along with the experience and expertise to help you select the best solution for your requirements. Right angle or inline, economy or precision, we have the gearhead to meet your needs!
Planetary Gearheads
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• • • • • •
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MOTION SYSTEMS HANDBOOK COMMON GEAR OPTIONS Spur gearsets are simple ...
SUMMARY OF GEARING:
Pitch circle
Consultation, custom gear design & gear analysis
Reaction force ... but helical gearsets are more efficient. Cross-axis sets are another option.
CUSTOM GEARBOXES are increasingly common, mainly because they’re easier than ever to manufacture to specification. That’s not to say that the design work isn’t challenging. However, modern manufacturing lets some suppliers make gearboxes and components to meet specific application requirements. New supplier approaches to giving engineering Planetary gearsets support as well as new machine tools, are compact and automation and design software now run to 10,000 rpm. let OEMs and end users get reasonably Here, a lightweight priced gearing even in modest volumes. Schaeffler differential for a hybrid vehicle has an When enlisting help from a axial spline to boost efficiency. consultant or manufacturer, an engineer is more likely to get gearing that mounts Zerol bevel gearsets are a special veriation of properly and performs to specification straight right-angle after answering as many of these bevel sets. questions as possible: What’s the input speed and horsepower? What’s the gearbox target output speed or output torque? This partially defines the required gear ratio. What are the characteristics of use? How many hours per day will the Worm gearsets are rugged gearbox run? Will it need to withstand and don’t let designs backdrive ... which can eliminate the need for brakes. shock and vibration? How overhung is the load? Is there Note there’s some overlap between bevel internal overhung load? Remember that and worm applications. Case in point: The bevel gears usually can’t accommodate MS-Graessner DynaGear below is a multiple supports, as their shafts single-stage bevel gear with a 30:1 ratio. intersect … so one or more gears often overhang. This load can deflect the shaft which misaligns the gears, in turn degrading tooth contact and life. The One potential fix ratio of a here is straddle helical or bevel bearings on gearset is simply the each side of number of teeth in the the gear. larger gear divided by the Does number of teeth in the smaller the machine gear. Other gear types such as planetary gears have need a shaft more complex ratio or hollowrelationships.
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bore input ... or a shaft or hollow-bore output? How will the gearing be oriented? For instance, if specifying a right-angle worm gearbox, does the machine need the worm over or under the wheel? Will the shafts protrude from the machine horizontally or vertically? Does the environment necessitate corrosion-resistant paints or stainless-steel housing and shafts? Service factor: The starting point for most gearbox manufacturers is to define a service factor. This adjusts for such concerns as type of input, hours of use per day, and any shock or vibration associated with the application. An application with an irregular shock (a grinding application, for example) needs a higher service factor than one that’s uniformly loaded. Likewise, a gearbox that runs intermittently needs a lower factor than one used 24 hours a day. Class of service: Once the engineer determines the service factor, the next step is to define a class of service. A gearbox paired to a plain ac motor driving an evenly loaded, constantspeed conveyor 20 hours per day may have a service class 2, for example. This information comes from charts from gearbox manufacturers that list classes of service. To use these charts, the design engineer must know input horsepower, application type and target ratio. For instance, suppose that an application needs a 2-hp motor with a 15:1 ratio. Find the point where 2 hp and 15:1 ratio intersect. In this case, that indicates a size 726 gearbox. According to one manufacturer’s product-number system, size 726 defines a gearbox that has a 2.62 center distance. Such charts also work in reverse, to let engineers confirm the torque or speed of a given gearbox size. Overhung load: After the designer picks a size, the gearbox manufacturer’s catalog or website lists values for the maximum overhung load that is permissible for that sized unit. Tip: If the load in an application exceeds the allowed value, increase the gearbox size to withstand the overhung load. Mounting: At this point, the designer or manufacturer has defined the gearbox size and capability. So, the next step is to pick the mounting. Common mounting configurations abound, and gearbox manufacturers offer myriad options for each unit size. A flanged input with hollow bore for a C-frame motor combined with an output shaft projecting to the left may be the most common mounting, but there are many other choices. Options such as mounting feet for either above or below the body of the gearbox, hollow outputs, and input and output configuration are all possible. All gearbox manufacturers
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1
1/10/17
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PLQE
Call us: Charlotte, NC 980-299-9800 Neugart 1-17.indd 89
www.neugart.com
Neugart_REV (2)_1-17.pdf
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MOTION SYSTEMS HANDBOOK
Spiroid® gear
Worm gear
Spiroid® or Helicon® gear Helicon® gear Hypoid gear
Spiral bevel gear
1.
2. 3.
list their mounting options as well as dimensional information in catalogs and websites. Lubricant, seals and motor integration: Most manufacturers can ship gearboxes filled with lubrication. However, they default to shipping units empty to let users fill them on site. For applications where there is a vertical shaft down, some manufacturers recommend a second set of seals. Because many gearboxes eventually mount to a C-frame motor, many manufacturers also offer to integrate motors onto gearboxes and ship assemblies as single units. Work with consultants and even use custom gear designs if the application needs a unique motorgearbox combination. Some combinations are more efficient. Getting a pre-engineered geamotor ensures that the motor-gearbox combination will perform to specification. Also remember that today’s custom and standard gearing aren’t mutually exclusive. Where fully custom gearboxes aren’t feasible (if quantities aren’t high enough, for example) consider working with manufacturers that sell gearboxes built to order from modular subcomponents. Otherwise, look for manufacturers that leverage the latest CAD and CAM software and machine tools to streamline postprocessing work and reduce the cost of one-offs. One final tip: Once the gearmotor has been chosen and installed in the application, perform several test runs in sample environments that replicate typical operating scenarios. If the design exhibits unusually high heat, noise or stress, repeat the gear-selection process or contact the manufacturer.
1. Spiroid and Helicon brand gearing operate on non-intersecting and non-parallel axes. Compared to traditional right-angle bevel and worm gearing, the gearcenterline offset of Spiroid and Helicon branded gearing allows for more tooth-surface contact and higher contact ratios. This boosts torque capacity and smooths motion transmission. Spiroid gearsets deliver ratios from 3:1 to 300:1 and beyond. 2. Lubricated for life with Beacon 325 premium grease, Zero-Max Crown drives feature heat-treated AGMA Class 10 spiral-bevel gears. This combination of bearing design and lubrication formulation ensures long-term, maintenance free operation for high performance, industrial applications. The drives feature long-life, precision-hardened and ground ball bearings that handle speeds to 2,000 rpm in most environments. 3.The WITTENSTEIN XP+ planetary gearbox is specially designed output for transmitting extremely high torque and can withstand high tilting torques. The output pinion on the version shown here engares a rack.
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4. Input and output on coaxial gearboxes are on the same axis. Image courtesy Neugart USA
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MOTION SYSTEMS HANDBOOK
Gearmotor fundamentals GEARMOTORS are electromechanical devices that integrate a gear reducer and either an ac or dc motor into one physical unit. Gearmotors simplify design implementation because they save engineers from integrating motors with gears, which in turn reduces engineering costs. If the application requirements are known, engineers can order the right gearmotor from a supplier directly. A properly sized gearmotor with the right combination of motor and gearing can prolong operating life and boost overall design efficiency. Gearmotors also eliminate the need for couplings and potential alignment problems that come with those components. Such problems are common when a design includes the connection of a separate motor and gear reducer — which in turn increases the potential for misalignment and bearing failure, and ultimately reduces useful life. Motors and gears can be mixed and matched as needed to fit application requirements. However, housing design, assembly gearings, gear lubrication, and the specific mode of integrating the pinion gear and motoroutput shaft all affect gearmotor performance. Motor and gear-reducer combinations are many and varied. For instance, right-angle wormgear, planetary and parallel shaft gears can combine with permanentmagnet dc, ac induction, or brushless dc motors to form a gearmotor unit. There are five basic types of gears that can be paired with a motor to form a complete gearmotor. Those are bevel, helical, hypoid, spur and worm types. Though it’s possible to combine many different motors and gearsets, not just any one will work for every application, because certain combinations are more efficient and costeffective than others. Knowing the application and getting an accurate estimation of its required torque and operating speeds is the foundation for successfully integrating a gearmotor into a system. Gearmotors can be classified by the physical arrangement of the final complete unit. So for instance, there are so-called in-line gearmotors where the gear shaft is parallel with the motor shaft, also called a parallel shaft. These can either be offset from the output shaft or completely in line with the output shaft. The other configuration is the right-angle gearmotor, where the output shaft is at a 90-degree angle to the motor shaft.
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A critical tool for evaluating a gearmotor is the performance curve. These plots relate torque and speed and sometimes efficiency. Use speed and torque measurements to identify manufacturer’s performance curves that match the application needs. Gearmotor curves unify information (such as speed, torque and efficiency) to summarize the performance of the motor-gearset combination. If an OEM or end user buys a complete gearmotor assembly from a manufacturer, the latter supplies its performance curve. However, once the speed and torque requirements are identified, that may not be the end of the story. That is, proper selection of a gearmotor is not all science. The fact is that this is merely a beginning, a starting point. This is because often times the manufacturer’s data may not be derived from empirical testing, so there may be some variation between calculated requirements and the actual application. This is why it’s important to test a sample load under the actual This operating conditions gearmotor of the application. can withstand extreme environmental conditions. Image courtesy ABM Drives
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MOTION SYSTEMS HANDBOOK
These gearmotors from the Allied Motion Mil-Aero series satisfy military and aerospace applications. The gearmotor series includes versions incorporating planetary gearing and fractional-horsepower brush (PMDC) and brushless motors to output torque to 62 Nm (8,800 oz-in).
Efficiency requirements for continuous and intermittent-duty gearmotors Recent changes in motor-efficiency regulations now dictate that continuous-duty integral gearmotors be Premium Efficient. Motors rated for intermittent operation (for non-continuous applications) aren’t required to satisfy the standard. Premium Efficient motors have more rotating mass (or inertia) and this increased inertia takes energy to start and stop. That means Premium Efficient motors can actually consume more electrical energy than low-inertia (lower efficiency) designs when run in dynamic or highcycling applications. Rated motor efficiency doesn’t tell the whole story about energy consumption. Ultimately, consumed energy is that for the whole system ... and high motor efficiency doesn’t guarantee lower energy use.
Continuous and intermittent-duty definitions The classification of a motor as either continuous or intermittent duty is based on duty cycle ... how long the motor is on and off. So picking between continuous or intermittent duty is fairly straightforward: If the motor is switched on and runs continuously then the motor should be rated for continuous duty. If the motor is switched on for a short time or has a cyclic operation, then the motor can be rated intermittent operation and the motor has no efficiency requirement. The rule of thumb is that if the motor is running more than one hour continuously with no switching on or off during that time, that’s considered continuous ... and a Premium Efficient continuous-duty motor is required. If the axis typically runs for less than an hour continuously, then the application can be considered intermittent. Note: Picking the wrong motor here could cause motor overheating and failure as well as damage nearby components. Common examples of intermittent applications include packaging equipment, metering pumps, palletizers and winders and coilers. Typical continuous applications include bulk-material-handling conveyors and continuous-flow pumps and continuous mixers.
Bodine Electric Co. now sells 22B4-60P planetary gearmotors. These integral gearmotors combine Bodine’s high-performance type 22B brushless dc motor with the new 60P (60-mm) planetary gearhead. They excel in applications that need higher torque than conventional helical or spur gearheads of a similar size can provide ... and where a verylow-backlash gearhead isn’t required. Typical applications include conveyors, pumps, packaging, industrial automation, and solar and battery-powered equipment.
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Don’t get torqued off.
Get TorqLOC®! Can’t remove your corroded gear unit from your Assembly
machine shaft? Well, stop fretting about it. Man up and get TorqLOC® from SEW-EURODRIVE. With its stainless steel construction, keyless hollow shaft, and taper bushings, TorqLOC® disassembly is fast and easy. The same bolts used for assembly are used for disassembly.
Disassembly
Not only does TorqLOC® reduce your maintenance headaches, it also eliminates belts, chains, and sprockets to reduce energy costs.
seweurodrive.com / 864-439-7537
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MOTION SYSTEMS HANDBOOK
Update on
HMI software
and hardware innovation
HUMAN-MACHINE INTERFACES (HMIS) are the original IIoT component that evolved from man-machine interfaces (MMIs) and batch, command line, and graphical user interfaces (GUIs). Consumer-grade GUIs for computers, gaming consoles and mobile devices have spurred HMI iterations that allow for input beyond text — with touchscreen and image-based interfaces we now take for granted. The simplest HMI iterations replace pushbuttons and light towers to allow for centralized operator control and indications of machine functions. Other data-handling HMIs serve to communicate system feedback on production — often through
plots and charts of subsystem status, throughput, recipes, data logs, and critical alarms. More complicated still are HMIs with supervisory control and data acquisition (SCADA) functionality as well as manufacturing execution systems (MES) functionality. MES modules convert raw data from machines, controllers, operators, and enterprise resource planning (ERP) software into actionable information for individual machine tasks and whole production runs. HMIs capable of processing and communicating such information are called controller HMIs or overseer HMIs. These HMIs (integrated with machine controls for motion) often serve as command centers for whole production lines. In fact, many manufacturers today are focusing on increasing the SCADA functionality of their HMIs. This has meant some migration away from strictly proprietary architectures — especially for HMI and SCADA systems that see worldwide application. The software that runs on HMIs is for either machine-level HMI or supervisory-level HMI, with applications suitable for both types. Such software can be costly upfront but is inexpensive long-term thanks to the way it reduces redundancies. For example, even lower-tech applications (in which most machine interaction is via switches and pushbuttons) entry-level HMI offerings are making inroads — as they often reduce interface-part count and simplify controls. More sophisticated applications benefit in a different way: Pharmaceutical and medical machinery use the latest HMI features to differentiate from competitive offerings. But no matter the performance grade, selecting HMI software Most machine starts with an analysis of monitoring and control product specifications and is near the machine ... but features. What kind of GUI Weintek’s cMT3090 HMI provides remote access. It works as an HMI will the machine operator with data and graphic processing via a need? Will operators dual-core CPU. Plus it works as a server need to view diagrams, to allow remote access. That also lets the digital photos and detailed cMT3090 act as a gateway between the system schematics? Other plant floor and outside world. Built-in considerations include EasyAccess 2.0 provides a secure system architecture, VPN connection and encrypted data performance requirements, transmission between any remote integration, cost of PC (or handheld device) and the cMT3090. procurement, and operations. HMI software editors let
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MOTION SYSTEMS HANDBOOK
This Maple Systems Graphic HMI is a performance operator panel with a high-speed processor.
designers add touch-screen functions and configure control functions for industrial automation. Usually, programming is through Windows-based software or screen-editor software. That lets designers quickly edit schematics and set the right communication protocols in a familiar programming environment. Standard HMI software satisfies the needs of simpler machinery that’s not task-intensive. Highperformance HMIs run software that lets users customize the interface to meet specific operator requirements. Sometimes, HMI software lets users program advanced control functions as well — for editing servomotor parameters and issuing global commands to other control axes on a machine, for example. (For motioncontrol applications, the visual GUI can range from simple 4 line x 20 column text displays to color monitors with touchscreen controls.) Such motion functions go well beyond basic HMI tasks observing processes or making very simple changes to some individual variables or parameters or setpoints. One iteration of HMI software that’s increasingly common is programs that let users remotely monitor and control HMIs from smartphones, tablets, or offsite PCs. Traditional setups only let users get to the HMI on the factory floor, but cloud-based HMI software gives operators remote access lets them check machines from anywhere. Sometimes called web-based visualization, this is particularly helpful where machines run in hard-to-reach places. Related innovations in HMI software even let remote users make on-the-fly changes to machine functions.
Beckhoff TwinCAT HMIs omit proprietary systems for engineering and communication and employ IT standards — including Microsoft Visual Studio for engineering; design and Websockets with HTML5; and secure communication via HTTPS. Its user interface executes on any HTML5capable browser. TwinCAT HMI programming isn’t required, as the graphic editor handles configuration.
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MOTION SYSTEMS HANDBOOK
Lead screw basics ONE OF SEVERAL METHODS to achieve linear motion, a lead screw is a threaded rod or bar that translates rotational motion into linear motion. Lead screws generate sliding rather than rolling friction between a nut and the screw. Sliding generates higher friction, which results in lower overall efficiency. Lead screws typically have higher ratings than comparable ball screws, thanks to more contact between the nut and screw load surfaces. Specific manufacturing processes can determine the performance and cost of the finished lead screw. Lead screws can be manufactured in one of three ways; by machining, rolling, or grinding. Ground lead screws are the most expensive and are generally considered to also be the highest performing. The type of thread also impacts efficiency. Acme threads are the simplest to produce and the most inexpensive, but also among the least efficient. Other types include buttress threads and square threads, which generally have the least amount of friction and higher efficiencies. There are a few advantages lead screws have over other kinds of linear actuators, including a relatively high load carrying capacity. They’re also compact and simple to design into a system with a minimal number of parts. The motion is also generally smooth and quiet and requires little maintenance. Lead screws also work well in wash-down environments because the materials used and the lubricant-free operation allows total immersion in water or other fluids. On the downside, lead screws do not have high efficiencies. Due to lower efficiency ratings they’re not used in applications requiring continuous power transmission. There’s also a high degree of friction on the threads meaning that the threads can wear quickly. Because a lead screw nut and screw mate with rubbing surfaces they have relatively higher friction and stiction compared to mechanical parts that mate with rolling surfaces and bearings. A lead screw’s performance can be evaluated according to a few parameters. These include thrust, speed, accuracy and repeatability. The two most
Shown here are integrated linear actuators with precision leadscrews and programmable motion controllers, drive electronics, and optional encoders to form MDrive Plus actuators. Image courtesy Schneider Electric Motion USA
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heidenhain LEAD SCREWS
Lead Screw Technology Rolls Over Ball Screw
important factors in determining the performance of a lead screw are the screw pitch and lead. The pitch is the linear distance between the threads while the lead is the linear distance the nut travels. Speed is another critical parameter. Lead screws have a critical velocity, which is the rotational velocity limit of the screw. Reaching this limit induces vibrations in the lead screw. Accuracy and repeatability are also important. The accuracy of a lead screw is a measure of how close to a desired end point the assembly can move a load to within a given tolerance. The accuracy of the lead screw will mostly determine the system’s accuracy. On the other hand, repeatability is a measure of how well a lead screw assembly can repeatedly move a load to the same position. Sizing a lead screw involves determining the diameter and lead that provide the required load capacity, speed, and back driving torque for the application. Beyond the basics of diameter and lead, the final lead screw selection often comes down to three factors: backlash, material, and customization.
Lead screws offer the latest technology for motion control, and can be used where ball screws are used, with many distinct advantages besides better cost and lead times:
Backlash Caused by clearance, or play, between the screw and the nut, backlash affects the repeatability of the screw assembly, preventing it from returning to the same position over repeated movements. Although backlash was once taken as an unavoidable trait that prevented lead screws from being used in precision applications, manufacturers now offer anti-backlash versions of most lead screw nuts, greatly improving their repeatability. A common method to eliminate backlash in a lead screw nut is to incorporate a spring or other compliant part (such as a washer) between two nut halves. Other methods use a stiff spacer that can expand or move relative to the nut halves, continually compensating for clearance that develops as the nut wears. For applications that require high repeatability, look for an anti-backlash nut, keeping in mind how wear will affect its performance over time. Also note that drag torque may be higher due to the preload that’s placed on the nut, and that additional space may be required to accommodate the anti-backlash design.
• Loads up to 500lbs with standard product and larger with Kerkite® engineered polymers • Lead accuracies to 0.0001in/in • A full family of anti-backlash nuts for zero-backlash operation over the life of the product • Efficiencies over 80% without grease (lead dependent)
Material Lead screws are able to operate in very challenging environments, including some conditions that would be impossible for ball screws to withstand. One reason for this is that lead screw nuts don’t have recirculating elements, which are highly susceptible to debris and contamination. But another reason is that lead screw nuts can be made from a wide variety of materials, including bronze and various plastics, to suit the environmental conditions — whether they include caustic liquids, abrasive particles, or temperature extremes. Lead screw shafts can be made from carbon steel, stainless steel, or aluminum, and they’re commonly supplied with a surface coating for additional protection against contamination. In their standard offerings, many lead screw manufacturers supply PTFE-based coatings, which provide durability in harsh environments and supply lubrication to the nut, eliminating the need for grease or oil.
• Leads from 0.012in to 3.6in (linear travel per revolution)
Learn more | Order online
haydonkerkpittman.com
Customization In addition to the standard customization options for screw end machining — to accommodate fixed or floating end bearings, or to add keyways for couplings, for example — most lead screw manufacturers offer truly customized designs. These can include special leads, thread forms, or nut bodies. Going one step farther, some manufacturers offer integration of the lead screw with a linear guide to provide a custom slide or actuator. This level of customization is especially helpful when an OEM or end user is replacing legacy parts that are no longer offered, or changing an existing design that has fit, form, or function constraints.
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USA: Europe: Asia: Email:
+ 1 603 213 6290 + 33 2 40 92 87 51 + 86 21 5763 1258 info.haydonkerk@ametek.com
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MOTION SYSTEMS HANDBOOK
OLLER DE ASSEMBLY
When to consider
crossed roller slides
1.
2.
3.
1. Crossedroller bearings don’t exhibit the oscillations and vibrations associated with recirculating bearing balls entering and exiting load zones. This makes for extremely smooth and quiet motion.
2. Cages wrap around crossedroller guide rollers to maintain their spacing — and prevent skidding, premature wear, and failure. Various anticreep mechanisms prevent cage creep. Shown here is a Schneeberger crossedroller guide that uses a rack and pinion feature to maintain cage position. 3. This SKF Anti-Creeping System (ACS) is one of several variations for precisionguide types.
4.
4. Linear slides with crossed-roller arrangements maintain high rigidity. Image courtesy Del-Tron Precision Inc.
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WHEN AN APPLICATION requires a linear guide with high load capacity, high stiffness, and good travel accuracy, the range of suitable products is typically narrowed down to two designs: recirculating linear guides and crossed roller slides. Where recirculating linear guides use balls to support the load, crossed roller slides use rollers. The rollers ride in V-shaped grooves between two parallel guides and are arranged in a crisscross pattern — and each roller is at a 90˚ relative to the one next to it ... hence the term “crossed” roller slide. The rollers are held by a metal or resin cage similar to caged-ball or ball-chain designs now found in recirculating linear guides. While recirculating guides and crossed roller slides both provide high load capacities and rigidity, crossed roller slides typically outperform recirculating linear guides in both areas. This is due to the line contact that rollers have with the raceways, as opposed to the point contact that balls experience. The increased contact area of the rollers also gives crossed roller designs higher stiffness ... and so less deflection under load than recirculating linear guides. But crossed roller slides are less forgiving of imprecise mounting surfaces. This is one reason why crossed roller designs are suitable for use in linear stages —because stages are characterized by precise mounting surfaces and extremely flat and accurate travel. Note that recirculating linear guides are also available with rollers in lieu of balls, for higher load capacities and stiffness. However, recirculating roller guides are typically offered only in larger sizes, suitable for heavy machining and pressing equipment. Another advantage of crossed roller slides is that they have extremely smooth motion. This is because crossed roller designs are non-recirculating, meaning they don’t suffer from the pulsations that recirculating bearings experience as the balls (or rollers) enter and exit the load zone. The lack of impacts due to recirculating elements greatly reduces noise generation, making crossed roller slides noticeably quieter than recirculating bearings. However, the non-recirculating design of crossed roller slides means their travel is limited, typically to 1 meter. And the crossed roller slide assembly must be twice as long as the maximum stroke, because the two guides move in opposite directions from each other. Unlike recirculating guides, the static and dynamic coefficients of friction for crossed roller slides can be very similar, which minimizes the effects of stiction. This makes crossed roller slides work for applications that require minute motions, such as cameras and microscopes. Crossed roller slides are sometimes considered alongside air bearings. Although air bearings have extremely smooth motion and high load capacity, they’re unable to provide the same level of stiffness as mechanical bearings. Air bearings are also fundamentally more expensive and complicated to operate, requiring multiple components — such as compressors, filters, tubing, and fittings — to handle air preparation and delivery to the bearing surface.
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ISO 9001:2015/AS9100D CERTIFIED
I’ll carry that for you! with precise straight line accuracy
CU S SL TO AV ID M AI ES LA BL E
Del-Tron’s Ball & Crossed Roller Linear Slides
carry loads up to 779 lbs! These light weight slides offer engineers and designers the flexibility to choose the right linear slide technology for their application. Manufactured in the USA, our ball and crossed roller slides offer several levels of precision ranging from a straight line accuracy of 0.0005” per inch of travel in our standard precision slides to 0.0000040” per inch of travel in our high precision slides. Available with inch or metric holes and hole locations these factory preloaded, low friction slides are ready for installation right out of the box!
Other
Precision Linear Motion Products
Precision Linear Motion Required. Del-Tron Preferred. • Accuracy ranging from 0.0005”/” to 0.0000040”/” of travel.
Non-Magnetic Ball Slides
• Low friction straight-line design reduces the coefficient of friction to 0.003. • Factory preload adjustment prevents side-play and backlash. • Lightweight aluminum carriage and base with high load capacity. • Built-in holes simplify installation and component mounting.
DL Linear Actuators Lead Screw/Ball Screw Driven
• Available with ball slide or crossed roller slide technology, varying precision grades and corrosion resistant materials.
Micrometer Positioning Stages
Order Today! 800.245.5013 Solid Models Available for all Del-Tron Model Numbers
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Micrometer Driven Ball & Crossed Roller Stages
Designed & built in the U.S.A.
www. b u yd e lt ro n .co m
8/22/18 12:57 PM
MOTION SYSTEMS HANDBOOK
Brushless dc motor image courtesy Lin Engineering
dc motors Basics of
THE BASIC DEFINITION of a direct current (dc) motor is one that takes dc input. But variations abound. Some dc motors operate using magnetic fields produced in the motor via electromagnetic windings; others include permanent magnets. Plus there must always be a mechanism that commutates or switches the current flow — via some electromechanical or electronic means. Common dc motor types include brush, brushless, and permanent magnet (PM) motors. Brushed dc motors are sometimes referred to as wound-field motors, because a wound and lacquered coil of copper wire produces the electromagnetic field. There are also permanent magnet, shunt, series, and compound-wound brushed dc motors. Brushed dc motors are also referred to as permanent magnet dc (PMDC) motors, because the stator incorporates permanent magnets on its inner diameter. The rotor (also referred to as the armature) consists of a slotted iron core with windings that are attached to a commutator. PMDC motors are mechanically commutated, with brushes that carry current to the windings. This is in contrast to brushless dc (BLDC) motors — a type commonly used in servo applications — which are commutated by electronic means rather than by a commutator and brushes.
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maxon control electronics for strong DC brushed and brushless motors.
maxon motor control
Power under control.
When good control properties and fast startups are needed, maxon motorâ&#x20AC;&#x2122;s servo controllers are the ideal choice. The 4-Q PWM servo controllers have fast digital current and speed controllers with a large range. They offer highly efficient control of permanent magnet-activated DC motors.
Maxon Motor 2-18_Robot 228.6x276.225.indd 13 Report.indd 105
maxon precision motors 101 Waldron Road, Fall River, MA 02720, USA Phone 508-677-0520 info@maxonmotorusa.com, www.maxonmotorusa.com
8/22/18 13:40:12 1:01 PM 04.08.2016
Stepper Motors
Piezo Motors
Brushless DC Motors
Linear Actuators
10mm ø
FAULHABER coreless (brush) DC motors feature compact dimensions, low power consumption, high efficiency, and low noise, making them ideal for challenging medical equipment applications requiring high-precision positioning and durability. FAULHABER motors can be configured with gearheads, high-resolution encoders, and/or leadscrews for precise linear motion. LEARN MORE
As the exclusive provider of FAULHABER products for North America, MICROMO creates value through advanced design and engineering services. Precision Actuation Systems Design & Integration of Components Drive & Control Integration Performance & Cost Optimization
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Clearwater, FL USA www.micromo.com 800-807-9166 ISO 9001, ISO 13485 certified
8/22/18 1:02 PM
D C M O TO R S
Stall torque
For motion control innovation, Solution City never sleeps.
TORQUE-SPEED CURVE FOR EXAMPLE DC MOTOR
Torque
Speed is propor tional to voltage (and torque to current).
2
3 1 Speed
No-load speed
The torque-speed curves for dc motors demonstrates a linear relationship — something engineers often leverage for simple speed control. Driven under openloop speed-control, the motor exhibits a torque-speed curve drop proportional to command rpm. Under closed-loop speed control, feedback tracks rpm to prompt drive-current adjustments. Consider an example from Pittman of AMETEK: An axis moves through three operating conditions — a no-load condition (1) to one that’s loaded but too slow (2) — until input voltage increased (3) to reach target rpm.
While the mechanical commutation of brushed dc motors is often seen as a drawback — the brushes wear out and so must be maintained and replaced — this design is typically less expensive than that of a BLDC motor. And in terms of performance, brushed servo motors have very smooth speed output (especially at low speeds), a wide speed range, and good speed control, with the added benefit of using regenerated energy to brake the motor. (Alternatively, the regenerated energy can be sent back to the power supply). They can also produce very high torque for startup or acceleration. Compared to other dc motors, brush types are generally smaller and more energy efficient, since there is no field coil. Reading torque-speed curves Dc motors are relatively simple — as when the load on the motor is constant, speed is proportional to supply voltage. Plus when supply voltage is constant, speed is inversely proportional to the load on the motor. This second relationship — between speed and load (or torque) — is typically shown on the motor’s torque-speed curve. The inverse relationship between speed and torque means that an increase in the load (torque) on the motor normally causes a decrease in speed. The relationship also means that the torque-speed curve is a descending line, with a negative slope. So the torque-speed curve begins at the crossing of the Y axis, where torque is maximum and speed is zero. This is the stall torque — the maximum torque when the motor is running at nominal voltage. The curve slopes downward until it intersects the X axis — that is, zero torque and maximum speed. This point is known as the no-load speed ... the speed when running at nominal voltage and zero load. Because the torque-speed curve is a straight line, it’s simple to find the torque that the motor can produce at a given speed ... or to find the motor’s speed for a given load (torque) on the shaft. For a more detailed explanation, visit motioncontroltips.com and search on torque equation.
Whatever keeps you up at night, we’ve got a solution—the largest selection of motors, pumps and air-moving devices available. Plus, one-of-a-kind solutions ready to be custom-engineered for your precision industrial, commercial, combustion or transportation application. If you can dream it, you’ll find it at Solution City. ametekdfs.com
100 East Erie Street • Kent, OH 44240 © 2017 by AMETEK Inc. All rights reserved.
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MOTION SYSTEMS HANDBOOK
Integrated motor basics AN INTEGRATED MOTOR is one in which a motor is paired with other motion components. A typical integrated motor can contain a number of common components including the motor itself, a controller or drive with communication ports, and feedback devices such as encoders. With such a range of possible configurations, what constitutes an integrated motor can vary. Among the chief benefits of integrated motors are greater reliability ... mainly because there are fewer parts to connect together. And fewer external connections mean less cabling and wiring, which reduces costs, as does the fact that the components that would be purchased separately such as the motion controller and the drive are integrated into one physical unit. There are a few drawbacks however, including reduced configuration options due to lack of customizability as well as the issue of potential vendor lock-in. Integrated motor-drives can reduce development times via quick and easy programming. Communication options range from simple serial communication links to more advanced network topologies suited to complex motion control tasks such as CANopen or Ethernet protocols. Designing machines with integrated motors has a number of benefits including helping to reduce machine size, cost and complexity. In some cases, integrated motors can also eliminate external controllers such as PLCs. Such integrated systems can significantly reduce the amount of space required for a machine by consolidating components, eliminating cabling, and possibly the need for entire enclosures. Integrated motor use has become more common with the rise of decentralized motion-control architecture. An alternative to centralized motion control, decentralized architectures distribute motion control to a number of individual axes (in this case, to individual integrated motors), eliminating the need for a central controller. This means that individual motors can execute the control closer to the actual axis of motion, taking the computational burden off of a central controller and distributing it to individual integrated motors.
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These integrated motor-drives from Quicksilver Controls are a prime example of the benefits of integrated motors, with their compact size and quick and easy network connections.
motioncontroltips.com | designworldonline.com
8/22/18 2:34 PM
aventics L I N E A R M O TO R S
linear-motor technologies Update on
The ECO-LM series direct-drive linear motor stages include precision noncontact linear encoders for minimum incremental motion to 10 nm with micrometer-level repeatability. Optional HALAR factory calibration improves positioning accuracy to ±1.5 µm.
LINEAR MOTORS CONSIST OF a stationary platform that manufacturers call a platen or secondary (with electromagnetic windings) and a moving forcer or primary that sometimes includes permanent magnets. Linear motors are fast and precise for positioning but can move slowly and steadily for material processing. Speeds range from a few inches to thousands of inches per second. Unlimited strokes are possible and (with an encoder) accuracy is to ±1 μm per 100 mm. Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive. So, they avoid the gradual wear; relatively lengthy settling times; backlash and mechanical windup; whip and vibration; and speed limits of 15 ft/sec of traditional electrotechnical arrangements for linear motion. In short, linear motors use electromagnetic flux to produce motion — where flux density is the magnetic flux through the airgap area. In linear motors, the latter is proportional to magnetic and electrical loading as the vector quantity of flux lines between platen and forcer.
motioncontroltips.com | designworldonline.com
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Typical air-gap flux densities range from fractions to a few Tesla. Linear stepper motors have a toothed forcer consisting of laminated steel cores wound with coils. The platen also has teeth cut into a steel bar. Linear stepper-motor platens mount end-toend for unlimited travel. Thrust originates from reluctance force. Speeds are suitable for relatively quick-acting pick-and-place and inspection machines. Most hybrid linear motors have low-cost ferromagnetic platens. (Those with solid steel platens move to 3 m/sec; those with laminated platens move faster.) Much like linear stepper motors, they vary magnetic saturation from the platen to shape opposition to magnetic flow … so thrust originates from reluctance force. Feedback plus a PID loop with positioning control helps the motor output servo-grade performance.
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Linear motors include a coil and stationary platform. Linear-motor subtypes include brushless ironcore and ironless designs as the ones from Chieftek Precision USA shown here.
Key to hybrid linear-motor performance is a yoke on the platen that makes paths through which flux travels and closes flux loops between platen teeth and forcer. The moving forcer includes laminated stacks with three-phase coils wound into them. Some have permanent magnets on the platen for higher force output. Those that don’t rely solely on phase current in phase coils and the forcer’s magnets. These need coil current to electromagnetically balance magnet loading for optimal motor performance. Hybrid-motor drawbacks are limited output and cogging from reluctance coupling between the forcer and platen. Two solutions here are phase-teeth offset or driving to get partial saturation of platen teeth and sections of forcer teeth. Here, the drive only magnetically saturates working teeth sections. Some hybridcore motors also use external cooling to get more output during continuous operation. Linear ac induction motors run to 2,000 in./sec work for people movers, rollercoasters, and large aerospace applications. General-purpose types can move a few inches to 150 ft/sec or faster to drive gates, people movers, and other material-handling conveyors. Linear ac synchronous motors include iron-core or ironless-core motors. The former have an epoxy forcer plate holding copper coils.
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This in turn slides inside a U-shaped magnetic platen to output up to 3,000 N and speeds to 230 in./sec or better. Sometimes called brushless cog-free linear motors, these are lighter motors with potentially unlimited travel and quick acceleration. However, their main benefit (particularly in semiconductor applications) is smooth output. Their speed is helpful in flying-shear applications and long-stroke pick-and place machines. Other applications exist for waterjet and laser cutting and robotics tasks. Iron-core motors have slotted steel lamination stacks (insulated to reduce Eddy currents) for output of 7,000 N or more. The forcer coil assemblies include steel laminations and windings in a single or three-phase configuration. This allows for control directly from a line or through an inverter or vector drive. Some such linear ac motors use water-cooling to boost force output — enough to let the motors drive large baggage handling and amusement-ride axes. Iron-core motors are suitable for certain machine-tool applications as well. One last option is cylindrical or linear shaft motors with steel rods and moving coils — or rods filled with stacked magnets. With the same footprint as a traditional linear actuator, these work in myriad machines that need quick and accurate strokes.
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MEET DIGITAX HD
Introducing our next generation servo family. Perfect for dynamic applications where high peak torque is required. It packs a real punch from a tiny package; just over 1.5” wide. Minimum size, maximum performance. Helping you reduce your costs and maximize your space. A specialist drive, from the drive specialists.
DRIVE SPECIALISTS SINCE 1973
Eddie Hall
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Control Techniques | 7078 Shady Oak Road | Eden Prairie | MN | 55344 | USA Tel : +1 800 893-2321 | Email: info.cta@mail.nidec.com
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MOTION SYSTEMS HANDBOOK
Servomotor fundamentals A SERVOMOTOR SYSTEM is characterized by the use of closed-loop control. It takes any error in position, speed, or torque and corrects it through the use of feedback. A controller compares the system’s actual performance with its commanded performance and takes corrective action to eliminate error. Servomotors work for machine axes that need to make complex moves or position loads with high precision. Servomotors can also run at zero rpm while holding torque to keep a load at a set position, if that’s the goal. The term servomotor means different things depending on the context. Note these terms are complicated by the fact that functionally dc brushless motors run on ac power ... as any dc input is shaped into square-wave current that’s fed to the motor’s phases in a predefined sequence for electronic commutation. What the industry calls an ac brushless motor is the same permanent-magnet synchronous design ... just optimized to accept sinusoidal ac current into its phases instead. Brushed dc servomotors deliver linear and predictable performance that makes them easy to apply. Brushless motors usually run applications needing more torque; the only catch here is that their drives are more complex because commutation is done electronically. What’s more, industry categorizes motors in part by their number of electrical phases. Brushed dc servomotors and voice-coil motors are in fact single-phase motors, whereas brushless servomotors most commonly have three phases. Sometimes, induction-motor-based designs running off vector controls are classified as as servomotor setups where the design incorporates feedback (usually from an encoder) to track and control speed and sometimes even position. These induction motors typically adhere to NEMA or metric standards. Other servomotor offerings are less uniform.
Shown here are two operating zones of servomotors — continuous and intermittent or peak duty. Image via Moog Animatics
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This frameless brushless dc servomotor from maxon motor is an electronically commutated motor with a rotor and stator — but no bearings or motor shaft — for integration into robots and other applications.
Manufacturers classify motors for constant-speed tasks by horsepower or torque at base speed. In contrast, servomotors have speed-torque curves that express continuous torque capabilities and intermittent or peak torque for acceleration. To select a servomotor for an application, define the load using application inertia. Next, determine application speed or velocity, and how far and fast the load needs to travel. Then calculate torque and plot along with speed on the motor’s torque-speed curve. Sizing the right servomotor may be complex, but most manufacturers have sizing software to help simplify the task. Plus, once designers have the parameters for an axis and its motor, they can setup the drive to protect the rest of the system’s components by preventing excessive torques and other problematic conditions. Many non-direct-drive servomotors have top speeds up to thousands of rpm. To better leverage their full capabilities, designers will often combine such motors with gearing to trade an increase in output torque with lower output speed. Much of the time, this gearing takes the form of planetary or harmonic gearheads, precision arrangements with high accuracy and efficiency. In many instances, gearing even lets machine builders use smaller motors on some axes. This equates to cost savings that may even offset the price of the additional gearing.
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8/22/18 1:04 PM
MOTION SYSTEMS HANDBOOK
Stepper motors: the basics Integrated stepper motor drives such as the Sure Step series from AutomationDirect feature micro-stepping position and speed control.
A TYPICAL STEPPER MOTOR uses open-loop control as opposed to closed loop like many other motor types, including servomotors. One way that stepper motors are classified is by the number of steps they can be commanded to move. For instance, a 1.8˚ step motor is capable of 200 steps/revolution (1.8 x 200 = 360˚ or one full revolution) in full-step mode. If operated in half-step mode, each step becomes 0.9˚ and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements. Two common types of stepper motors are the permanent magnet (PM) motor and the variable reluctance (VR) type.
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Traditional VR stepper motors have a large number of electromagnets arranged around a central gear-shaped piece of iron. When any individual electromagnet is energized, the geared iron tooth closest to that electromagnet will align with it. This makes them slightly offset from the next electromagnet so when it is turned on and the other switched off, the gear moves slightly to realign. This continues with the energizing and de-energizing of individual electromagnets, thus creating the individual steps of motion. Combining the best features of both PM and VR type stepper motors are hybrids. The rotor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide a path
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to help guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding, and dynamic torque characteristics of the motor when compared with both the variable reluctance and permanent magnet motor. Hybrid stepper motors are usually more expensive than PM stepper motors but can provide better performance with respect to step resolution, torque and speed. Control techniques such as half-stepping and microstepping let designers get even finer movements, which make for more exact output than that from VR stepper motors, which usually can’t be microstepped. Hybrid stepper motors also have higher torqueto-size ratios and higher output speeds than other stepper-motor types, and are also quieter than VR stepper motors.
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S T E P P E R M O TO R S
Thomson Industries stepper-motor linear actuators include models with built-in anti-rotational guidance. The manufacturer’s motorized leadscrew actuator (MLA) saves designers of high-precision and shorter-stroke applications the cost, time, and maintenance worries related to designing and building externally guided systems themselves. Combining lead screws and stepper motors is a simple and common setup for precise linear motion.
The low-speed torque of a stepper motor varies directly with current. How quickly the torque falls off at higher speeds depends on a number of factors such as the winding inductance and drive circuitry including the drive voltage. Steppers are generally sized according to torque curves, which are specified by the manufacturer. Sufficient documentation of a stepper motor includes a torque-curve that shows both pull-in and pullout torque, two critical stepper motor parameters . Cogging torque basics Cogging torque is a product of the magnetic interaction between the poles of the rotor’s permanent magnets and the steel laminations of the stator’s teeth. In other words, when the poles of the rotor line up with the teeth of the stator, a force is required to break the attraction, and this force is referred to as cogging torque. Cogging torque is positiondependent, according to the location of the stator teeth relative to the permanent magnets, as the magnets constantly search for a position of minimum reluctance. A motor’s cogging torque profile depends on the number of permanent magnets in the rotor and the number of teeth in the stator and can be minimized through mechanical means by optimizing the number of magnetic poles and teeth, or by skewing or shaping the permanent magnets to make their transition between stator teeth more gradual.
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Cogging in stepper motors Stepper motors also exhibit cogging torque, also called detent torque. Like cogging torque, detent torque is a result of magnetic equilibrium in a nonenergized motor. This magnetic equilibrium must be overcome before the motor will rotate, which means that the detent torque reduces the amount of running torque the motor can produce. The amount of detent torque a stepper motor experiences is proportional to the motor’s speed, so the effect of detent torque on running torque is more substantial at higher motor speeds. PM and hybrid stepper motors (which use a permanent magnet rotor) exhibit detent torque. VR steppers (which use a non-magnetized soft iron rotor) do not. Of PM and hybrid types, hybrid steppers have higher detent torque due to their toothed rotors, which allows them to better manage the magnetic flux between the stator and the rotor. Detent torque is typically 5 to 20% of the motor’s holding torque, which is the amount of torque the motor can produce when the windings are energized but the rotor is stationary. But detent torque is not always a liability: When the motor is decelerating, it counters the motor’s momentum and helps it to stop more quickly.
Stepper motor construction
Shown here is a typical stepper motor and its parts ... including the motor shaft, rotor with teeth, and the stator — composed of pairs of poles which represent a single phase. Some stepper motors (such as PMX series motors from Kollmorgen) have several advantages over servo systems ... including design flexibility in a high torque-to-dollar package. Image courtesy Kollmorgen
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More details on the trend towards
Ethernet-based
networking protocols
IN A RECENT SURVEY of industry experts, many mentioned Ethernet-based networking protocols overtaking traditional fieldbuses. Recall that industrial communication protocols (including those based on Industrial Ethernet) are messaging systems for automated installations. Some fieldbuses are more reliable than nondeterministic Ethernet offerings, but Ethernet speeds now lead. “Some old bus systems have data rates in the range of 1 to 20 Mbit and no real-time reaction,” said Horst Messerer, HELUKABEL data, network, and bus technology sales manager. “Certain types of manufacturing can tolerate these rates … so bus protocol won’t go away entirely — but they’re seeing reduced market share.” In fact, industrial Ethernet-based networks now connect some 40% of industrial communication networks. “That’s why more bus systems are being replaced with Ethernet-based equipment,” said Messerer. “The advantage of Ethernet is its faster data rate — with a minimum of 100 Mbit — which lets industrial-automation manufacturers view manufacturing and production process data in realtime. Having this kind of data quickly allows for improved reaction time to optimize processes for higher output per hour.” Another benefit is that even though older variations such as 802.3c is slower than 802.3ab (at 1 Gbit/sec, which is called GbE) all can use Cat5e cable. That makes Industrial Ethernet implementation easier. Both Cat5 and newer Cat6 cables for Ethernet work with Gigabit Ethernet, though Cat6 Ethernet cable is displacing the former. “We’ve seen continued robust interest in both Cat5 and Cat6 flat cables,” said Rich Buchicchio of Cicoil Corp. “Our versions of these cables typically go in applications that benefit from high cable flexibility and durability to survive extreme environments better than common versions
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Shown here is a maxon motor EPOS04 Compact 50/8 EtherCAT smart drive-controller for programmable positioning. of these Ethernet cables.” The cables meet (and typically exceed) platform requirements. Besides speed, other reasons for Ethernet’s rise include its ease of use, and ubiquitous design; multiprotocol network acceptance; familiarity thanks to commercial off-the-shelf (CotS) Ethernet; and consolidation of network-interface hardware. Plus unlike traditional device and fieldbus networks (that can have multiple physical layers) Ethernet networks run on one. Controls and related motion hardware often dictate which fieldbus communications an installation will use. “We continue development of integrated motors that can be powered over Ethernet (PoE),” said Eric Rice of Applied Motion Products. “Combining power and Ethernet over a single cable is an excellent example of the evolution of integration in motors, drives, and industrial Ethernet communications.”
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MOTION SYSTEMS HANDBOOK
PoE-enabled integrated motor offerings from the manufacturer include NEMA 14 and NEMA 17 frame motors. Traditional industrial components feature Ethernet connectivity for an increasing array of functions. “Today, there are scalable I/O options for PLC control … and new hardware to communicate with remote I/O includes multi-Ethernet products to cover major Ethernet such as SERCOS, EtherCAT, PROFINET, and EtherNet/IP,” Joaquin Ocampo, product manager of motors and drives at Bosch Rexroth. In fact, five real-time protocols — CC-Link, EtherCAT, EtherNet/IP, PROFINET, and SERCOS III capitalize on Ethernet advantages and deterministic networking.
“One of three smart Han modules available is the Mini Ethernet Switch Module,” said Mark Kojak, V.P. of product management and business development in the Americas at Harting Inc., manufacturer of interconnect components. This module incorporates a four-port Ethernet switch within the connector. The module can replace or reduce the port count in systems need1. The CC-Link protocol (supported by the CC-Link ing a multi-port Ethernet switch Partner Association) includes field-device, controller, … and excels in systems that and information networking and gets speeds to 10 and integrated motors for use on use a daisy-chaining Mbps at 100 m. Versions include CC-Link, CC-Link Safety, PROFINET networks,” he added. architecture. CC-Link/LT, and CC-Link Industrial Ethernet (IE) Control and Field. One CC-Link standard is a Seamless Message Protocol 5. Serial Real-time Communications to let Ethernet-ready devices work on CC-Link IE networks. A new System (SERCOS) is an open protocol for serial version of a Control and Communication System Profile (CSP+) simplifies communication of closed-loop real-time data — over integration of engineering tools and device profiles (in part by outlining the Ethernet cable for SERCOS III. Specifically for motion data necessary for setup and operation of CC-Link-compatible devices). controllers, drives, actuators, sensors, I/O, the IECAutomation-component manufacturers primarily offer CC-Link functionality standard protocol closes servo loops in the drive to for servomotors, drives, robots, digital and analog I/O modules, and reduce motion-controller burden (and synchronize controllers including PCs and PLCs. more motion axes than otherwise possible). Such closing of servo loops also avoids delays between 2. The Ethernet for Control Automation Technology (EtherCAT) motion controls and drives. protocol uses Ethernet and twisted-pair or coaxial cables with BNC adapters over distances to 1,000 m and optical cable beyond that. It 6. SERCOS III gets deterministic bidirectional real-time gets Ethernet transfer rates and can control up to 65,535 nodes. Data motion and control. It also enables higher-level I/O processing through dedicated software and hardware allows deterministic and transmission of conventional protocols over one communications. EtherCAT accommodates any topology and handles Ethernet network in parallel with SERCOS real-time 1,000 I/O points in 30 μsec — capable of communicating with 100 servo communication — with cycle times to 31.25 μsec. axes in 100 μsec. Axes get values and control data and report position and status. A simple version of an IEEE 1588 distributed clock technique 7. PROFINET is from the same organization from the synchronizes axes with less than 1 μsec of jitter. same organization that gave the motion industry PROFIBUS (PROFIBUS & PROFINET Intl. or PI) 3. Ethernet/IP is an industrial application-layer protocol for communication PROFINET use of Ethernet means larger messages, between industrial controls and I/O. It uses standard IEEE 802.3 TCP/IP and better bandwidth, and theoretically unlimited address communications; Ethernet hardware pairs with TCP and UDP ports. Certain space … as well as avoidance of network collisions. permutations enable synchronized and deterministic multi-axis motion Deterministic communications are possible through control. real-time and isochronous real-time (IRT) operation. The latter gets update times down to 250 μsec, while 4. In fact, this and other industrial-Ethernet standards drive many productPROFINET IRT gets them down to 31.25 μsec for development decisions that Applied Motion Products makes. “This year synchronizing networked components. we enhanced our support for EtherNet/IP networks by introducing a set of improved Add-On Instructions for Rockwell’s RSLogix 5000 software,” Industry-specific communication buses endure said Rice. “We also introduced a new family of cost-effective motor and even grow, as connectivity imparts design drives for EtherCAT networks … and have begun development on drives sophistication even to turnkey operation.
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GIGABIT INDUSTRIAL ETHERNET
REST ASSURED SECURE THE PROCESS The industry requires a high level of protection in its processes because of the importance that its information represents. That is why CC-Link IE has the necessary safety certifications implementing corrective measures, control methods and safety curtains to generate confidence and efficiency. CC-Link IE is the future in communication technologies today.
info@cclinkAmerica.org / am.cc-link.org
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“Products with integrated industrial communications protocols simplify operation,” said Laura May of Thomson Industries. “For example, integration of J1939 CAN bus communications — most commonly used in mobile off-highway markets — means a system can programmed to let each of its actuators run with real-time awareness of speed and position of other actuators — enabling self-adjustment and deploying multiple actuators to handle heavy loads,” May added. She added the manufacturer has work in progress to better support protocols commonly used in plant-floor automation. Of course, an ever-growing (and costeffective) array of gateways simplify network connections where J1939 CAN (and other buses) must interface with Ethernet. It’s common for fieldbus components to connect to Ethernet blocks for connectivity via Cat5E cable and Ethernet RJ45 connectors. Ultimately, Ethernet can even facilitate IIoT functionality to connect enterprise-level systems with machine, I/O, and device-level operations. Ethernet-driven cloud-based IIoT along with (IEEE-802-compliant) wireless variations unify data streams for: I/O and motion communications and controls Safety functionality to protect equipment and personnel Diagnostics, machine-condition monitoring, and energy management Final product tracking and documentation of plant-operation statistics
Slip rings use CC-Link IE for gigabit data transfer JOHN WOZNIAK, P.E. MANAGER OF CC-LINK PARTNER ASSOCIATION (CLPA) AMERICAS
Manufacturer of mobile electrification and data transfer systems for industrial machinery Conductix-Wampfler now uses CC-Link IE open gigabit Ethernet (GbE) for high-performance connections in its slip rings. Traditionally used to supply continuous power and data communication to equipment on rotating assemblies, the slip rings are for new and existing machinery needing Industry 4.0 data connectivity. Conductix-Wampfler specializes in power, signal, and datatransmission systems for container handling, tunneling, mining, and bulk material handling. Their slip rings find use on rotating arms and gantries, cranes, wind turbines, manipulators, and cable reels. As a CLPA member, Conductix-Wampfler has technical specifications to develop compliant product … even for custom connectivity solutions that work with Ethernet up to 1Gbps. “Our slip ring technology excels in everything from industrial machinery to theme park rides … and we see need to transfer an increasing amount of data in a secure and easily accessible manner,” said product manager at Conductix-Wampfler Matteo Mosconi. So the company joined CLPA to focus on CC-Link IE because it’s the first and only open protocol that works at 1 Gbps with real-time characteristics — and the organization that supports it offered the technical support Mosconi wanted. “Slip rings certified for CC-Link IE can transmit power … as well as data at very high transfer rates.” CCFounded in 2000, CLPA is an Link Partner Association (CLPA) international open-network organization member Conductixwith more than 3,000 members (and Wampfler used CC-Link IE open GbE technical the leading open industrial-automation specifications to network technology in Asia) dedicated to develop compliant technical development of CC-Link open slip rings. automation networks. Main activities include the development of CC-Link IE and CC-Link technical specifications; conducting conformance tests and development support. CC-Link IE is an open GbE solution for Industry 4.0 automation with bandwidth for performance applications.
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We move. You win.
EasyHandling. Easy to Build. Easy to Use. Rexroth’s EasyHandling system is a complete platform for the easy design, construction, and commissioning of Cartesian motion robots. Combining open, user-friendly programming with Rexroth’s proven, world-class linear motion products, EasyHandling is ideal for single- or multi-axis applications ranging from pick-and-place to dispensing and tool handling. With a wide range of load and speed capabilities, the system is scalable from small laboratories to large warehousing or aircraft assemblies and anything in between. Get your next Cartesian robot up and running faster. Make your move to EasyHandling. Learn more at www.boschrexroth-us.com/EasyHandling.
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MOTION SYSTEMS HANDBOOK
positioning stages, Update on
gantries, and XY tables
POSITIONING STAGES as well as linear and rotary tables are integrated systems consisting of motors and mechanical power-transmission devices or linear or arc-shaped motors and actuators — complete with encoders, sensors and controllers. Consider the most common iteration: Traditional linear stages combine axes in X-Y-Z actuator combinations. Where serial kinematic designs are too bulky, integrated setups (in Cartesian or hexapod and Stewart platforms) are increasingly common. These output more accurate motion with no mechanical error accumulation. Better mechanical components and feedback and control options are enabling stages capable of motion that’s more accurate than ever. So, positioning stages today can execute
tasks with tighter synchronization than in the past — useful for complicated axis commands. One new option that leverages the speed of today’s motion controllers is positioning stages that fit a finer piezo-based axis to a courser and more traditional motion axis based on a rotaryto-linear mechanical devices and electric motor. Such tandem axes are useful for fast execution of tasks that need superfine movements once workpieces (such as medical devices or semiconductor chips) are in place. Note: When a gantry or XY stage is designated as a “planar” type, this indicates that the system has been designed to provide extremely high travel accuracy. Planar stages are typically constructed of air bearing guides and linear motor drives.
Aire bearings are useful on stages that must deliver high reliability and accuracy. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. Sometimes called aerostatic bearings, the source of pressure the air film. Unlike mechanical bearings, the surfaces of an air bearing make no physical contact, so don’t need lubrication. Surfaces don’t wear, so the systems don’t generate particulates. That makes them suitable for cleanroom applications. In fact, with a clean and filtered air supply, these linear bearings operate for years without fail. Different options for stage, table, and gantry installations There are myriad ways to build linear systems for motion in the X, Y, and Z
Physik Instrumente (PI) now sells an H-811. F2 high-dynamics hexapod. Designed for photonics alignment applications, the hexapod provides six degrees of freedom, actuator resolution of 5 nm, and repeatability to 60 nm … at speeds to 20 mm/sec. Bearing loads to 5 kg, the hexapod features a removable magnetic kinematic plate to accelerate workflow with fiber-optics components. Travel is to 34 mm and 42° of rotary motion.
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The Standard in High-Performance Linear Motion Just Got Better
Aerotech’s new and improved PRO-Series stages • Performance specifications improved by as much as 98% • New linear-motor stage sizes – PRO115LM and PRO190LM • Linear motor stages now with an absolute encoder option • Ball-screw stages now available with a linear encoder option • Direct mounting to English or metric optical tables Proven linear motor and ball-screw positioning stages now with new sizes, features, and improved performance. Call or email an Aerotech Applications Engineer today to discuss your requirements, or go to www.aerotech.com to request a quote.
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MOTION SYSTEMS HANDBOOK
directions — also known as Cartesian coordinates. The terms that industry uses to refer to these systems depend on how the axes are assembled, where the load is positioned, and (to some extent) the application for which the systems are designed. In many industrial applications, Cartesian and gantry-style robots are prevalent ... but in precision applications, XY tables are often the better choice because they have a compact and rigid structure — and very high travel and positioning accuracies as well. Cartesian systems consist of two or three axes for X-Y, X-Z, or X-Y-Z setups. They often incorporate an end effector with a rotational component for orienting the load or workpiece … and always provide linear motion in at least two of the three Cartesian coordinates. Where Cartesian systems are used, the load is usually cantilevered from the outermost axis — Y or Z. For example, in an X-Y gantry the load is mounted to the Y axis — either to the end of the axis or at a distance from the axis to creating a moment arm on the Y axis. This can limit load capacity, particularly when the outermost axis has a very long stroke (and creating a large moment on the lower supporting axes). Cartesian systems are suitable for a wide range of applications with maximum strokes on each axis typically one meter or less. The most common of these applications include pick-and-place, dispensing, and assembly. To address the issue of outer axes causing a moment load on the inner axes, gantry systems use two X axes or (in some
cases) two Y and two Z axes. Gantries almost always have three axes … X, Y and Z. The load on a gantry system is located within the gantry’s footprint and the gantry is mounted over the working area. However, for parts that cannot be handled from above, gantries can be configured to work from below. Gantry systems are suitable in applications with long strokes (greater than one meter) and can transport very heavy payloads that are not suitable for a cantilevered design. One of the most common uses for gantry systems is overhead transport, such as moving large automotive components from one station to another in an assembly operation. XY tables are similar to XY Cartesian systems in that they have two axes (X and Y, as their name implies) mounted on top of each other — and typically have strokes of one meter or less. But the key difference between XY Cartesian systems and XY tables lies in how the load is positioned. Instead of being cantilevered, as in a Cartesian system, the load on an XY table is almost always centered on the Y axis, with no significant moment created on the Y axis by the load. This is where the principle of “how the system is used” helps distinguish between the various types of multi-axis systems. XY tables generally work only within their own footprint, meaning the load does not extend beyond the Y axis. This makes them best suited for applications where a load needs to be positioned in the horizontal plane (X-Y). A typical example is a semiconductor wafer being positioned for inspection or a part being positioned for a machining operation to take place.
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So-called open-frame or open-aperture designs have a clear opening through the center of the table. This allows them to be used in applications where light or objects need to pass through, such as backlit inspection applications and insertion processes. Because XY tables are primarily used for very high precision applications, the guideway of choice is that based on crossed-roller slides, as these provide extremely smooth and flat travel. Drive mechanisms are typically ball screw or linear motor, although very fine pitch lead screws are also common. FAQ: What’s the difference between serial kinematics and parallel kinematics in the context of multi-axis motion designs? Today’s positioning tables and stages include hardware and software that’s more customized than ever to satisfy specific output requirements. That’s made
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MOTION SYSTEMS HANDBOOK Aerotech ECO-LM direct-drive linear motor stages deliver high precision. They include noncontact linear encoders for incremental motion to 10 nm with µm-level repeatability. Optional HALAR factory calibration improves positioning accuracy to ±1.5 µm. The ironless forcer coil provides high force with zero cogging for smooth velocity and position control — for delivering high contour accuracy and velocity profiling. The linear motor has excellent dynamic responsiveness. ECO-LM stages are available in 17 different models with travels of 100 mm to 800 mm and speeds up to 2 m/sec.
for motion designs that move accurately through even complicated multi-axis commands. Precision feedback is key to such functionality — often taking the form of optical or (electronics-augmented) magnetic encoders for nanometer-scale resolution and repeatability … even over many inches of travel. In fact, miniature stage design is spurring the most innovation from feedback and control algorithms to move even very large loads with sub-sub-micron precision. Use of pre-engineered stages and Cartesian robots continues to rise with rapid prototyping, automated research applications, and tighter timeto-market pressures. That’s especially true for photonics, medical-device, and semiconductor R&D and manufacturing.
In the past, building multi-axis motion for automating or otherwise improving tasks meant design engineers had to source and combine linear stages into X-Y-Z combinations … in-house. Any more degrees of freedom necessitated the after addition of goniometers, rotary stages, and other end effectors. Called serial kinematics, such machine builds sometimes result in bulky setups with accumulated error due to tolerance stackup. In some cases, bearings also limit such assemblies to one rotational center. These are non-issues when the design satisfies its motion requirements ... but miniature motion designs in particular aren't so forgiving of such factors. Contrast these builds with hexapod or Stewart platforms — forms of parallel kinematic actuators for motion. At least
for miniature multi-axis motion assemblies, these outperform serial kinematics. That’s in part because hexapod output motion isn’t limited by bearing (linear and rotary) ratings. Instead, the motion controls execute algorithms to an applicationdefined pivot point (center of rotation) unencumbered by error accumulation. Lower component count, lower inertia, and higher stiffness are other benefits. As in linearmotiontips.com articles on the topic, traditional motion systems can often work in conjunction with positioning stages having parallel kinematics — and traditional electromagnetic motors with piezoelectric motors … even on common assemblies or installations.
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PIglide HS, 3-motor planar XY air bearing stage with yaw control Physik Instrumente www.pi-usa.us 508-832-3456 (East) 949-679-9191 (West)
PI designs and manufactures precision motion systems at locations in the USA, Europe, and Asia. With over 40 years of experience developing standard and custom products based on piezoceramic and electromagnetic drives and more than 1,200 employees in 13 countries, PI can quickly provide a solution for your positioning and automation projects in industry and research.
PRECISION | SPEED | STABILITY - MOTION CONTROL & POSITIONING SOLUTIONS
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One Integrated Platform for All of Your Automation Needs One platform for electric automation means seamless connectivity. From electromechanical actuators when joined with servo motors and servo drives to complete positioning systems, motion control solutions as well as entire handling systems and decentralized control solutions - always with the right software and interface. www.festo.us/ea
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MOTION SYSTEMS HANDBOOK
robotics
Coordination of
with motion automation
FA Integrated engineering software iQ Works Enterprise SCADA software level
LAN (Ethernet)
IT system
Twisted-pair cable Control level
MELSEC iQ-R
MELSEC iQ-R Series safety CPU Seamless data coordination
Robot
GOT HMI
Field level
Safety remote I/O module
Sensor level
Safety switch
Light curtain Servoamplifiers and motion axes
Integrating robotics with other machinery allows top overall equipment effectiveness (OEE) by going beyond simple handoffs. The trend is towards complementary control, motion, robotics, and data management. Image courtesy Mitsubishi Electric
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AUTOMATED INSTALLATIONS feature more robots than ever. But unless it’s a custom or Cartesian setup, robot controls are traditionally segregated from those for automated motion axes. So connectivity between robot and system controller or PLC is via a network — usually based on Ethernet. Then a control system coordinates the machine functions of nearby conveyors or other workcell systems with those of the robot controller. Synchronization can be limited, and integration requires use of separate motion and robot programming — with the latter (if not code generated by teach modes and pendants) in proprietary software. In contrast, some newer functionalities are helping engineers more tightly coordinate robot operations with motion systems — especially for common applications. These also simplify integration. Epson Robots T3 SCARAs integrate controllers in their bases that (in some versions) run conveyor-tracking software for simple setup of assembly and other automation. DENSO Robotics, Mitsubishi Electric, and Universal Robots also offer conveyor-tracking functions that leverage either sensor networks or machine vision. Such synchronization is tight enough to allow for execution of pickand-place, sorting, and other motion-torobot automation tasks. Integration of motion and robots (including SCARAs and parallel robots such as delta robots and hexapods) is also possible through unified or unifying software and hardware. That includes robot programming for converting to and from coordinate reference frames more commonly used by motion design engineers. Case in point: Galil motion controllers with an onboard Coordinate Transformation function execute code to control robots without the latencies
8 • 2018
of other options. Design engineers work with an application engineer to define reverse and forward transformations; then equations are written into the controller’s firmware for quick execution. Beckhoff TwinCAT Kinematic Transformation software also allows programming and reading robot moves in Cartesian coordinates. Some controllers for automated installations can command coordinated motion axes while concurrently controlling a robot set to run off a remote host. Lenze Americas modules for controller-based automation incorporate delta robots essentially as motion axes. These unifying controllers and a single programming environment maximize cross-functionality between motion axes and robot. In a similar way, an Adept Robot Control Library allows control of SCARA, parallel (hexapod) and articulated robots by Omron NJ/NX controls or NY-series Industrial PCs with common instructions and programming. Consider still another example: Mitsubishi Electric Pak/iQ offerings for the packaging industry include integrated robotics and servo and PLC programming with one piece of software. Onboard analytics help operators avoid unplanned shutdowns; machine learning and continuous-tuning algorithms optimize servo performance. A typical setup might include a robot controlled via a MELSEC iQ Platform ... with a Q series PLC and integrated robot controller on the same rack. Then communication between robot controller and PLC CPU is across that rack for top speed and throughput — and simplified robot-cell setup. Networking connects other components such as inverter drives and dedicated safety controllers. Stay tuned to therobotreport.com for more on this topic and other open solutions.
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Raise Your Design iQ
Engineering for packaging applications just got easier. With the Pak/iQ design library, built on years of extensive experience in the industry, Mitsubishi Electric brings common applications into a condensed pre-engineered library for consistent machine designs with reduced development time. No need to re-engineer for each machine. Save hours of design time and improve repeatability in machine design, freeing you to innovate more. Innovate with Mitsubishi Pak/iQâ&#x201E;˘ . www.mitsubishipackaging.com
AD-VH-00105
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MOTION SYSTEMS HANDBOOK
Casette seals are rotary-type seals that protect against contamination. These designs are easy to install, as they don’t require shaft grinding or hardening. Photo courtesy of Trelleborg Sealing Solutions
sealing technologies Update on
IN ANY MOTION CONTROL SYSTEM, the risk from dirt and other ingress materials is great. Contaminants like these can damage bearings and other rotating equipment, destroy whole machinery systems and cause unwanted downtime and costs. To prevent these catastrophic events, all motion systems require some type of sealing system. In addition to stopping the ingress of contamintants, seals also prevent leakage of necessary lubricants, such as oil, grease or hydraulic fluid. Molded seals and v-shaped seals are two of the most common seals found in power transmission applications. V-shaped seals, such as wipers, are used most commonly in fluid power systems to prevent contaminants from entering a system while allowing lubricating oils to return to a system on inward stroke of the hydraulic piston. Molded seals, which are more common in power transmission applications, can be further divided into O-rings, radial lip seals and shaft seals. O-rings are one of the most common types of seals because of their simple and inexpensive construction. They are designed to create a seal between the interfaces of two or more components. They generally consist of
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an elastomer ring with a circular cross section and are usually placed in a groove. They are used frequently in hydraulic components, particularly on cylinder pistons and rotating pump shafts. Mechanical face seals, or heavy-duty seals, are used in extreme applications, such as bearings, gearboxes, turbines and machinery used in extremely tough and dirty environments, such as mining and agriculture. They feature two metal seal rings identical in nature that mount separately on a lapped face seal. A flexible, elastomer element centers the metal rings, allowing one half to rotate while the other remains still. While many seals are designed primarily to prevent debris from entering a machine, radial lip seals are designed to keep lubricants within a machine that has rotating or oscillating parts. These seals are available as one of two types—spring loaded and non-spring loaded. Each is suited to a particular type of lubricant, grease or oil. Non-spring loaded seals are suited for applications that use a highly viscous lubricant and operate at slower shaft speeds. Spring-loaded seals are best paired with lubricants with low viscosity and higher speeds. The spring helps the seal
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SEALS
lip maintain its contact with the shaft even as the seal material itself breaks down. In addition to keeping contaminants out and fluids in, rotary and shaft seals have the extra benefit of providing low friction and resistance to wear, thus extending component life. To reduce efficiency and power loss, bearing users can turn to non-contact seal designs. These seals eliminate efficiency and frictional power loss, and also reduce maintenance and contamination problems associated with contact or rubber seals. Several different styles of non-contact seals exist, including labyrinth and centrifugal seals. Designed to eliminate the physical contact between a machine’s stationary and rotating elements, they don’t suffer parasitic drag or wear.
More Cycles, More Uptime. Because it’s engineered to provide superior protection against leakage in a broad range of temperatures, pressures, and media, the Bal Seal® spring-energized seal can dramatically improve the service life of your motor. What’s more, its balance of low friction and sealing effectiveness can help you achieve new levels of power efficiency and reliability.
Centrifugal seals like this design from Centritec Seal from the Carlyle Johnson Machine Co. can be used in a rotating machine’s vertical shaft. The centrifugal pressure of this design helps prevent weepage.
Learn how a Bal Seal® can elevate your new or existing motor design. Call or click today.
800.366.1006
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How do canted coil springs reduce EMI/RFI? Canted coil springs shield sensitive electronics from the harmful effects of EMI/RFI. Featuring independent coils that serve as multiple contact points for optimal conductivity and/or grounding, these springs ensure consistent, reliable connection with mating surfaces, even under shock and vibration. These springs feature precisely engineered coil angles. Canted coil springs are ideal for use in highfrequency, small-package applications. Because they can be used to shield and mechanically fasten with precisely controllable insertion and removal forces, they reduce system weight, size and complexity. In addition to shielding, they hold, latch and lock. They feature customizable mechanical forces, and their electrical properties can be “tuned” to meet specific impedance requirements. Offering unique deflection and force behavior upon compression, the springs are available in a variety of configurations and coil shapes, and with various options: • As closed rings or straight lengths • In a variety of sizes • In a variety of materials, including copper alloy and stainless steel • With gold, silver, nickel or tin platings for optimal conductivity, galvanic compatibility and corrosion resistance Supplying near constant force over a large compression range, these springs provide consistent shielding despite surface irregularities and tolerance variations.
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Additional features of canted coil springs include: • They act as a latching component, providing an audible click that confirms secure connection with no threading and easy removal • Conductive/shielding properties provide protection against cross-talk, and interference from other devices in use Key benefits of canted coil springs include: • 3-in-1 functionality: Shielding, conducting and connecting while reducing weight, size and complexity of designs • Resistance to compression set for longer service life (as compared with elastomers and fingerstock) • Individual coils exert near constant force • Compensation for misalignment and surface irregularities • Ability to enable tool-less modular designs • Easy maintenance, part-switching
Canted coil springs offer lower transfer impedance and more effective EMI shielding in critical applications where crosstalk and interfence can be damaging to the machine and system. Image courtesy of Bal Seal Engineering
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SEAL and
DELIVER
Don’t Get Caught In a Labyrinth of Bad Seals Our Patented Centrifugal Pressure Seals: ✓ ✓ ✓ ✓ ✓ ✓
Keep lubricants in & contaminants out Reduce downtime Support horizontal & vertical applications Prevent friction & overheating with a non-contact design Create a dynamic pressurized barrier Support extremely low-viscosity fluids
A Carlyle Johnson Company
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MOTION SYSTEMS HANDBOOK
Sensors
in motion control
SENSORS USED IN MOTION CONTROL applications can be a number of different types such as rotary or linear encoders or resolvers for position feedback, sensors such as tachometers for speed sensing, and even proximity switches to initiate or halt some machine action. In motion control, the measured variable is typically position or speed. In brushless dc motors, for instance, Hall-effect sensors are used in place of a mechanical commutator and brushes. Halleffect sensors are solid-state magnetic field sensors. Here’s how it works: When a conductor with current flowing through it is placed in a magnetic field, the magnetic field induces a transverse (or sideways) force on the charge carriers, which pushes them to the sides of the conductor—negative to one side and positive to the other side. This buildup of charge on the sides of the conductor induces a voltage. This effect is referred to as the Hall-effect after its discoverer Edwin Hall. Hall-effect sensors are the most common method of determining rotor position in brushless dc motors, due to their low cost and ease of use with the permanent magnets of the rotor. And because the commutation happens in 60˚ increments, high-resolution sensing and output are unnecessary. Picking the right sensor Some key considerations for selecting a sensor include the desired variable to be measured, the required accuracy or resolution, the type of output, as well as any size or space restrictions, environmental factors, and product lifetime and cost. Ask about the motion-control application: Is the motion rotary or linear? Rotary encoders or resolvers measure rotary position
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SGAM and DGAM incline sensors from Joral take input by a gyroscope, accelerometer, and magnetometer to provide a 3-axis output for X, Y, and Z ... as well as feedback for pitch, yaw, and roll. while tachometers track speed. For linear motion, linear encoders use a variety of sensing technologies — including optical, capacitive, inductive, and magnetic. Also consider the needed precision for the application ... including linearity, resolution, and repeatability. Generally speaking, higher accuracy necessitates more expensive sensors. Conversely, knowing the needed precision (and specific accuracy) can help in lowering the design costs by avoiding buying more resolution than the application demands. What is the required measuring range? For linear measurements, is the range on the order of nanometers, a
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SENSORS
few millimeters, or several feet? For rotary applications, if measured in degrees, is the angular distance more or less than 360Ë&#x161; ... and does the setup require an encoder that is a single-turn or multi-turn device? Next to ask: What type of output is needed? Is it voltage or current? Digital or analog? Many transducers are programmable through a simple data connection such as a PC-to-USB link. Other interface options include encoder-specific communication links such as SSI (synchronous serial interface), BiSS (bidirectional serial/synchronous), or PROFINET. There may be other considerations such as any physical size or weight restrictions or special installation or mounting requirements. Environmental conditions are another factor. The sensor should be able to withstand the environmental conditions of the application. Other common issues to consider are EMI/RFI noise, shock and vibration disturbances, extreme heat or cold, and environmental contaminants such as dirt, dust, moisture, and corrosive chemicals.
Sensors with high precision such as LIF 400 linear encoders from HEIDENHAIN excel in semiconductor, metrology, micromachining, and medical applications. The encoder here includes a custom ASIC built into the scanning unit to minimize interpolation error.
Transforming vision. Reducing complexity. Reinventing technology.
Light Section Sensors with SmartRunner Technology
Transformation of complex image data into a simple digital signal for fast and easy integration Simplified profile comparison and area monitoring with a unique combination of 2D vision and light section technologies Easy installation and configuration via Data Matrix control codes or teach-in
www.pepperl-fuchs.com/smartrunner
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MOTION SYSTEMS HANDBOOK
shocktechnologies & vibration Update on
WHEN HIGH-SPEED, heavy-duty industrial automation systems must decelerate and stop, damage to the load and the machine itself can result. The cause of this damage is the release of kinetic energy, which induces long-term vibration and fatigue and sudden shock in a system. Smooth deceleration of a system should be done with shock and vibration attenuation components. Based on the type of inputs present in the application, vibration and shock attenuation components can include shock absorbers, linear dampers, wire rope or spring isolators, elastomeric isolators, air springs, or structural damping treatments. These devices help manufacturers reduce equipment downtime and costly cycle time limitations. These products work in a broad range of applications — from the rate control mechanisms that slow the motion of overhead luggage bins or seat recline on commercial aircraft, to the isolators that keep GPS from losing signal or becoming damaged on farm and construction equipment as they harvest crops or pave roadways. Shock and vibration damping Most shock absorbers deliver their damping characteristics through the use of hydraulic fluids. The fluid is pushed by a piston and rod through small orifice holes to create damping, and this action compresses some type of gas. This in turn creates a spring force to return the rod back to its starting position when the load is removed. Shock absorbers and dampers are Shock generally made of high-strength steel to absorbers safely handle the pressures from the internal decelerate and stop a hydraulic forces. Elastomeric seals load. Pictured here is ACE prevent the fluid from leaking out of the Controls’ Magnum series, cylinder, and special plating and coatings which the company says keep the units protected from harsh offers the lowest braking operating environments. force and shortest Recent and ongoing developments braking time. in sealing technologies and in the internal designs of shock absorbers and dampers
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Slow down to speed up
...with ACE industrial shock absorbers. · Decelerate moving loads · Protect against wear and tear on machines · Increase throughput & reduce downtime 800-521-3320 | www.acecontrols.com 23435 Industrial Park Drive, Farmington Hills, MI 43885
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MOTION SYSTEMS HANDBOOK
MIniaturization is a growing trend in industrial shock absorbers, as they must work within shrinking machine footprints. Here is Koganeiâ&#x20AC;&#x2122;s adjustable type KSHP series which has a 3 million cycle operating life.
have allowed for longer service life and more compact designs. Miniaturization is a growing trend in these devices, as systems require tighter tolerances and smaller machine footprints. In machine automation and robotics, motion stabilization requires the use of hydraulic dampers, particularly micro-hydraulic designs. Vibration isolation Vibration-isolation products rely generally on mechanical designs to achieve their isolation characteristics. A spring function provides support for the mounted equipment, while decoupling it from the vibration source. Friction and elastomeric material properties give the isolators their damping characteristics. Isolators can be made from a variety of materials. Wire rope and spring isolators can be made from carbon steel, stainless steel or aluminum. Elastomeric isolators generally have metallic components that function as mounting brackets, separated by an elastomeric material that provides the stiffness and damping desired. Common elastomeric compounds include natural rubber, neoprene, and silicone but many compounds and blends deliver various characteristics specific to satisfy different applications.
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Elastomer and other synthetic rubbers can also help damp vibration and shock loads. Here is a selection of custom designs from Sorbothane, which provide vibration and acoustic damping and isolation.
S H O C K S & V I B R AT I O N DA M P I N G
Air springs include metallic end fittings coupled by a composite elastomeric-based bladder that contains the compressed air to provide isolation. These single-acting designs have a pressurized bladder and two end plates. As air is directed into the air bladders, they expand linearly. All of these reusable designs are selfcontained, offering a number of advantages over any other technology that may require outside componentry. For example, hydraulic systems may require plumbing while electrical systems may require wiring and power. Energy or power dissipation is key when selecting a damper or shock-absorbing device. The size and characteristics of the device are based on these inputs, so it is generally the first consideration to make. Dynamic spring rate and damping are the two biggest considerations when selecting an isolator. These characteristics define system natural frequency (resonant frequency) and the most suitable isolation system.
Elastomer, rubber pads for vibration and shock reducing Elastomer and other synthetic and rubber pads can also damp vibration and isolate shock loads. They come in tubes, bushings, blocks, pads and washers. These components work in heavy-duty applications to deliver strong cushioning for cranes, presses, and pipelines and bridges; they also excel in vibration reduction for lab and testing equipment and designs for aerospace. Rubber-like materials let these padding elements satisfy specific requirements related to natural frequency, load, and area. Because they are soft, they are also forgiving. Predicting the natural frequency of an application lets material manufacturers target known disturbance frequencies to dissipate energy. The lower the ratio of natural system frequency to disturbance frequency, the more it’s possible to isolate problem vibrations. These cushioning plates can protect machinery subsystems against impacts and isolate vibration and structure-borne noise. For example, PAD plates from ACE Controls withstand compressive loads to 10,000 psi or 69 N/mm2 depending on plate form and size. Another product called Sorbothane (from a company with the same name) is a thermoset that attenuates shock with near-faultless memory. Deformation is elastic and not plastic, so pads of the material reliably return to their original shape. Custom pieces of the material work for vibration and acoustic damping and isolation. Sorbothane turns mechanical energy into heat as the material is deformed. Molecular friction generates heat energy that translates perpendicularly away from the axis of incidence.
8 • 2018
DESIGN WORLD — MOTION
IMPACT CAN DESTROY AN OBJECT OR A SINGLE ELEMENT OF THAT OBJECT
INNOVATING SHOCK & VIBRATION SOLUTIONS Visit sorbothane.com for Design Guide and Technical Data
800.838.3906
sorbothane.com
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MADE IN THE U.S.A.
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MOTION SYSTEMS HANDBOOK
torque limiter? What is a multi-position
TORQUE LIMITERS are a kind of ball-detent clutch using a series of hardened balls (spring loaded into a detent plate) to clutch the load. Then the balls disengage (release from their detents) once the drive line torque reaches a value sufficient to actuate the spring set. Torque limiters are suitable for applications needing high accuracy and repeatability with rapid response to torque overload. These mechanical devices protect machinery and equipment from torque overload, resulting from malfunctions, jams, and crashes. They come in a variety of designs — including shear pins, slip clutches, magnetic clutches, and balldetent clutches, and more. In recent years, low inertia and zero-backlash versions have become increasingly common for servo-driven systems. That’s according to Andy Lechner, Product Manager R+W Coupling Technology. Multi-position torque limiters are a type of high-performance ball-detent clutch. They’re particularly compact, backlash free, and capable of releasing driveline inertia much more rapidly than electronic controls would otherwise be able to react. While not all machines need them, multi-position torque limiters excel in preventing machine damage from interference or malfunctions. The balls and detents in the clutch are arranged in a pattern that allows reengagement after overload with detents at multiple intervals — such as 30, 60, 90 or 120° — over a single rotation. Though singleposition torque limiters are more common, multi-position variations excel in certain key applications. For example,
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they work on very slow-speed axes where there’s no need to wait for the shaft to rotate one full turn before reengagement. This is applicable when the slow speed results from a significant mechanical speed reduction ... often accompanied by mechanical advantage sufficient to cause excessive torque. They’re also useful when a machine must start with the torque limiter disengaged. Here, the drive line stops after an overload with the balls oriented away from the detents. If there is a rapid acceleration at startup, it becomes possible for the balls to jump over their detents and fail to re-engage in time. Use of a multi-position torque limiter ensures that the balls are never far from their detents, reducing the amount of time before an opportunity to re-engage occurs. Manual indexing is another possibility. In this situation a dial plate, or round indexing table, can be made to disengage from a back stop, and freely rotate until the next set of detents is reached. “The two most common commercially available multi-position torque limiters include simple mechanical designs (in which the disengagement torque is set manually prior to use) and pneumatically controlled designs,” says Lechner. With the latter, spring pressure on the ball detent can be controlled remotely during operation. However, there are other uses on machine axes wherever there’s a need to disengage and reengage at a set angular interval less than 360° full rotation. When selecting a torque limiter for a new or existing application, allow a sufficient safety factor for upward adjustment. Because their intended purpose is to protect against uncontrolled torque spikes, torque-limiter application is often on axes for which the real required peak torque for operation is unknown. According to Lechner, undersizing a torque limiter can exacerbate machine-downtime issues by preventing the machine from running at all — especially in designs with no margin left to increase the disengagement torque as needed. As always, consult with manufacturer and their applications engineering to ensure a suitable torquelimiter solution.
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WHEN SAFETY IS A REQUIREMENT. 0.1 - 2,800 NM.
RW-AMERICA.COM
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MOTION SYSTEMS HANDBOOK Image courtesy Smalley Steel Ring Co.
wave springs
Common challenges that
address
Strain energy (MJ)
} }
Deformation (mm)
SPRINGS ARE FLEXIBLE MECHANICAL COMPONENTS to the spring material provides store and release energy or apply and release forces on machine little damping and the spring axes. Wave springs combine flat (non-coiled) bow springs (as operates on an axis that must their waves) with traditional compression-spring coil geometry. make fast reciprocating strokes. For the design’s compactness (as a high force-to-work height Well-chosen wave-spring ratio) and other benefits, some motion systems have migrated geometry and material can help from traditional helical or coil springs to flat-wire wave springs. avoid issues of spring surge and Design engineers typically work with manufacturers to resonance excitation. customize wave springs to specific operating conditions by Mean, cycling, and localized stresses on the spring each load material, thickness, number of turns, and other geometric cycle (and the load cycle itself) dictate when spring fatigue failure features. That helps address or avoid the following springcould occur. For any spring design, more turns make for a longer installation challenges. MTBF … while longer springs generally have shorter MTBF. But no Column buckling in springs occurs with long free lengths and matter the variation, wave springs fatigue more slowly and have spring ends that can’t evenly distribute load around the spring longer MTBF than traditional springs. circumference. Buckling mainly depends on geometry and not Springs under normal operation exhibit no permanent spring material properties. Traditional springs tend to buckle spring-rate or dimensional changes — called relaxation or set. when deflection (for a set free length) is excessive. Such buckling But deflection under full load with stresses exceeding the spring is preventable by keeping the design to below critical deflection material’s yield strength will induce permanent deformation and length values. The former is the ratio of deflection to spring that compromises the spring ability to deliver full design force free length; critical length is the ratio of that length to the spring or energy. This is often a concern in designs that must operate diameter. One rule of thumb for avoiding buckling in traditional in extremely hot ambient conditions. Wave-spring deflection is springs is to keep free length to less than quadruple the spring about 25% lower than that exhibited by traditional springs. Wave diameter — and load to less than the product of spring rate, springs are also more resistant to relaxation for comparable length, and buckling factor. diameters, free lengths, and turns. One caveat: In designs Wave-spring buckling factors vary greatly, but the value is necessitating fewer turns and shorter free lengths, sometimes nearly always higher than those of other springs. That means traditional compression springs outperform wave springs by they readily hold their centered cylindrical shape and often selfexhibiting less deformation. locate into assembly bores — even operating reliably in machined Any spring under a twisting moment load exhibits shear assembly features held to relatively loose tolerances. stress and stores strain energy. Wave springs exhibit less Spring surge is strain energy than a potential concern traditional spring in assemblies with a variations (and lower DEFORMATION AND STRAIN ENERGY FOR WAVE SPRINGS compression spring equivalent stress having one free values) so better 70 end. Depending on withstand such 35 Traditional the motion input, loading. spring 60 such springs can 30 exhibit resonance Wave 50 25 that’s large enough spring to cause temporary 40 20 loss of contact Wave springs outperform other with the assembly 30 15 springs under housing — and the application damage surrounding 20 10 of normal (nonmachine elements. twisting) forces. That’s an issue of 10 5 Data courtesy P. highest concern if Ravinder Reddy and V. Mukesh Reddy 0 400 800 1,200 1,600 2,000 2,400 2,800 Load (N)
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Ad Index
M O T I O N
S Y S T E M S
ABB Motors & Mechanical ..................................................................................... 9 ACE Controls Inc. ..................................................................................................137 Aerotech .................................................................................................................123 All Motion .. ................................................................................................................. 4 Altra Industrial Motion Corp. . . ............................................................................... 7 AMETEK - Haydon Kerk Pittman . . ..................................................................... 101 AMETEK DFS ........................................................................................................ 107 Apex Dynamics, USA . . ........................................................................................... 91 Applied Motion Products . . ...................................................................................... 1 AutomationDirect ................................................................................................... 71 AVENTICS Corporation ......................................................................................... 17 Bal Seal Engineering .............................................................................................131 Bansbach Easylift ................................................................................................... 81 Beckhoff Automation ...........................................................................................117 Bison Gear and Engineering ............................................................................. IBC Bodine Electric Company .................................................................................... 93 Bosch Rexroth ........................................................................................................121 Carlyle Johnson Machine Company . . ................................................................. 41 CC-Link Partner Association .............................................................................. 119 Centritec Seals ......................................................................................................133 CGI Motion . . ............................................................................................................. 85 Chieftek Precision .. ............................................................................................... 110 Cicoil ......................................................................................................................... 43 Clippard . . ................................................................................................................. BC CMT ........................................................................................................................... 36 ContiTech ................................................................................................................ 58 Del-tron Precision, Inc. ....................................................................................... 103 Delta Computer Systems .................................................................................... 54 Deublin ...................................................................................................................... 13 DIEQUA Corporation ............................................................................................ 87 Dorner ...................................................................................................................... 59 Dynatect Manufacturing, Inc. .. ............................................................................ 28 Elmo Motion ............................................................................................................ 73 Encoder Products Company . . ............................................................................. 77 Festo. . .......................................................................................................................127 Fluid Lines Products .. ............................................................................................. 19 GAM ............................................................................................................................ 3 Harmonic Drive . . ..................................................................................................... 83 Heidenhain Corporation . . ..................................................................................... 75 HELUKABEL USA ................................................................................................... 45 HIWIN . . ...................................................................................................................... 29 IDEC Corporation . . ................................................................................................. 53
H A N D B O O K
igus . ............................................................................................................................ 31 Intech . ....................................................................................................................... 84 Interroll ..................................................................................................................... 57 ITT Enidine . .............................................................................................................138 Lee Spring Company ............................................................................................. 51 Lenze Americas ...................................................................................................... 69 Lin Engineering ........................................................................................................ 21 Mach III Clutch Inc. .................................................................................................40 Maple Systems ....................................................................................................... 97 maxon precision motor ...................................................................................... 105 Micromo .....................................................................................................cover, 106 Mitsubishi Electric Automation .........................................................................129 mk North America .................................................................................................60 NBK America LLC .................................................................................................. 63 Neugart .................................................................................................................... 89 Nidec ......................................................................................................................... 111 NSK Precision ......................................................................................................... 27 OMS Motion ............................................................................................................ 55 PBC Linear . .................................................................................................... 124, 125 Pepperl+Fuchs ......................................................................................................135 PHD Inc. ..................................................................................................................... 16 Physik Instrumente . ..............................................................................................126 POSITAL-FRABA Inc. . ............................................................................................ 76 Pyramid Incorporated .......................................................................................... 38 R+W America . ................................................................................................. 65, 141 Renishaw .................................................................................................................. 78 Rotor Clip Company, Inc. .....................................................................................143 Ruland Manufacturing . .......................................................................................... 67 Schneider Electric Motion USA .......................................................................... 23 SDP/SI-Stock Drive Product ............................................................................... 37 Serapid Inc. ............................................................................................................. 49 SEW Eurodrive ....................................................................................................... 95 Smalley Steel Ring . .................................................................................................10 Sorbothane ............................................................................................................139 THK TOS ..................................................................................................................IFC Tolomatic, Inc.......................................................................................................... 25 Traco Power ............................................................................................................ 12 US Tsubaki .............................................................................................................. 47 Weintek USA . .......................................................................................................... 99 Wittenstein . .............................................................................................................. 14 Yaskawa Electric America ...................................................................................113 Zero-Max, Inc. . .......................................................................................................... 5
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