Motion Systems Handbook 2020 by WTWH Media LLC

AUGUST 2020

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MOTION SYSTEMS HANDBOOK

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Just what exactly am I talking about? I’m talking about some common terminology used in the software, industrial controls and IT worlds, specifically two words that refer to control and networking functions. Two words so frequently used that it’s easy to not notice them or think twice about using them. The two words I’m referring to, of course, are master and slave. In the world of technology these words may have lost a lot of their original force. So much so that when people say them casually in conversation in which the context is entirely business or technical in nature, the other more common meaning which references the exploitative relationship between human beings may be enterly forgotten. But being someone who writes for a living and for whom words have a history and for whom words matter, I have never been able to entirely detach the original meaning of those words from their usage in the technical worlds. I can’t remember exactly when I first heard the pairing “master/slave,” most likely it was in college or one of the co-op jobs I worked as an engineering undergraduate. But I do recall thinking how odd and harsh and ugly of a usage it was — even way back then in the 1990s. Like almost everybody else, I didn’t dwell on it. Part of me quickly rationalized it, telling myself that of course it wasn’t about slavery or race but simply the terminology used in a technical context about controllers and control signals, and in networking about nodes and which node was in control and which one was passive and carried out orders or took some kind of action. The use of master/slave to refer to motors and drives and controls has been fairly standard in the industrial world for what may seem like forever. However, there has been growing recognition of this issue as well as debates on what to do about it, though most of that debate has been in the software and IT world, not so much in the world of industrial controls. In fact, just a few years ago, master/slave terminology was removed from the Python programming language after a developer raised the issue. Of course, retiring master/slave terminology isn’t going to magically solve the real problems of racism, nor will it undo centuries of racist thinking and policies. But it is a start in our own backyard, the one we are most familiar with, where we spend most of our days working the soil, so to speak. The bottom line is that these words reference the darkest chapters not only of U.S. history but the modern world’s global slave trade and the peculiar institution that denied groups of human beings their full human rights. So I have a proposal, a call to action — that we work to eliminate these words from our technical vocabulary. That we find a substitute that captures the essence of what is happening at a technical level, without calling to mind America’s original sin. I’m optimistic that we are all sufficiently smart, creative, and good-natured enough to come up with something better.

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DESIGN WORLD — MOTION

8 • 2020

Motion Control Solutions

Everything Between the Motor and the Actuator

WDS Bellows style distance couplings connect actuators without additional shafts

PMK Parallel mounting kits save space by mounting the motor parallel to the ballscrew actuator

DL-DC Right angle Dyna-Lite gearbox with hollow output design for easy mounting to linear actuators. Includes output adapter tailored to the actuator

EPL-H Inline gearbox with hollow output design for easy mounting to linear actuators

Broad Product Range - U.S. Manufacturing - Custom Solutions With U.S. manufacturing and one of the broadest product offerings in the industry, as well as the engineering expertise and manufacturing capabilities to develop customized solutions, GAM can help with your motion control applications. GAM Can.

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Precision Rack & Pinion Rack & Pinion paired with GAM Gearboxes for optimized system performance

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VOLUME 6 NUMBER 3

CONTENTS 2 8 16 18 20 25 32 38 41 50 54 60 67 70 75 81

Editorial Linear actuators Ballscrews Brakes • clutches • torque limiters Belts & pulleys Cables & connectivity Controllers Conveyors Couplings Encoders Gearing Servomotors & drives EC motors Positioning stages & tables Shocks • dampers • vibration mitigation Springs & retaining rings

25 54

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MOTION SYSTEMS HANDBOOK

Common linear-motion options in 2020 Typical linear-motion arrangements consist of rails or shafts, carriages and runner blocks, and some type of moving element. Engineers differentiate these systems by the type of surface interaction (sliding or rolling) and the type of contact points — as well as how the design’s rolling-element recirculation works if applicable. In fact, slides and rails are more advanced than ever, with advances in materials and lubrication setups (to help designs last longer in harsh applications), innovative rail geometries (to help designs withstand more misalignment and load than ever), and modular guide mounts (to boost load capacity and minimize deflection). One word of warning here: The term linear guide can refer to any one of several different component types depending on the context.

Unlike plain-bearing linear guide taxonomy (which is fairly consistent) the terminology for ball and roller linear guides is quite varied. That said, the term linear guide often indicates a standalone rod assembly, rolling-element slide, or other mechanism for guiding loads. In contrast, many (though certainly not all) manufacturers use the terms linear slide and linear rail to indicate linear-motion guide elements in builds complete with some mechanical drive. The term linear stage generally implies a design with linear-guide elements, a reinforced frame, and actuation components of some type — with or without motor. No matter the permutation, linear-motion rails, guides, and ways enable motion along an axis or rail either through sliding or rolling contact. Myriad moving elements can produce either sliding or rolling support. These include ball bearings, cam roller sliders, dovetail bearings, linear roller bearings, magnetic bearings, fluid bearings, X-Y

Shown here is a Cartesian gantry involved in the production of printed circuit boards (PCBs).

DESIGN WORLD — MOTION

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LINEAR ACTUATORS

tables, linear stages, and machine slides. One classic rail with sliding contact is a dovetail slide, and one classic rail with rolling contact is a ball rail with a recirculating system. Sliding-contact bearings are the more straightforward type of linear-motion component. These consist of a carriage or slide that rides over a surface known as a rail, way or guide. Sliding contact occurs when the moving part directly contacts the rail section. Older versions of these sliding-contact rails generated considerable friction during movement, so were only suitable for basic applications. However, newer versions have self-lubricating sleeves and other features to boost positioning accuracy and repeatability. In contrast, rolling-element linear-motion systems are either recirculating or nonrecirculating. Non-recirculating types use rolling elements such as bearing balls, rollers and cam followers for movement. Recirculating types use some type of moving platform that houses a bearing block. This bearing block contains raceways with rolling elements that let the platform move along the rail with little friction. Recirculating types include linear guides and ballbushing bearings. More speciﬁcally, rolling-element linear guides come in two basic versions — those with circular arc grooves and those with Gothic arc grooves. These groove choices are a result of industry evolution that’s enabled new geometries for better load handling. Circular arc grooves contact bearing balls at two points. The Gothic arch contacts the balls at four points for bidirectional load capacity. Another option for rolling-element linear motion is ball bushings that have a bushing nut lined with recirculating bearing balls. This nut rides along a round shaft to allow axial movement. History lesson: In 1946, ball bushings were introduced and established the basic mechanism of rolling-element linear-motion bearings. In today’s designs, the bushings may also have integral ﬂanges to support axial loads.

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MOTION SYSTEMS HANDBOOK

Common linear guide permutations Linear rails are linear assemblies that have tandem tracks (usually on one monolithic run) containing load-supporting balls or rollers. The backbone of many industrial applications, they provide low-friction guidance and high stiffness for loads that can range from just a few grams to thousands of kilograms. Their variety of sizes, accuracy classes, and preloads make linear rails suitable for virtually any performance requirement. The reasons for using linear rails are numerous, but their most obvious beneﬁts over other types of guides are load capacity, travel accuracy, and rigidity. For example, round shaft guides

can withstand only downward or liftoff loads, whereas linear rail guides can withstand downward loads and liftoff loads … as well as moment loads. Unlike crossed-roller guides for which travel is often limited to 1 meter or less, linear rails can provide very long travel lengths. When compared to plain bearing guides, linear rails also have higher stiffness and rigidity, and often have better load and life characteristics. Linear guides also provide a high level of travel accuracy, thanks to precise machining of one or both edges of the rail, which act as reference surfaces. With two, four, or six rows of rolling elements — either spherical balls or cylindrical rollers — stiffness is also high … and deﬂection of the bearing block is minimal. All of these attributes combine to provide a linear guide system that is perfectly suited for applications that require high precision, high rigidity, and long life. So here we outline common applications for these linear components in typical sizes. Single-rail applications: Because linear rails have load-supporting balls (or rollers) on each side of the rail assembly, they can withstand overhung loads, even when just a single rail is used. In contrast, round shaft linear guides must be used in pairs when overhung loads are present. Because of this feature, numerous applications use a single linear rail, to save space or to prevent issues with misalignment among other components in the system. Here are a few examples of applications that use a single linear rail.

Linear actuators are typically characterized by their drive mechanism — belt drive, ball or leadscrew drive, pneumatic drive, and so forth. But it’s not unusual for rod-style electric actuators to be classified by the input voltage — commonly 12 or 24 volts — of their integrated motors. 12-V linear actuator illustration courtesy Firgelli Automations

DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

Linear actuators: Linear rails are often the guide mechanism of choice for actuators that are driven with belts, screws, or pneumatic cylinders, because of their ability to withstand moment loads. They can also accommodate travel speeds up to 5 m/sec, which is important in belt or pneumatically driven systems. Overhead transport systems: When loads are centered below the rail and bearing block, as is often the case with overhead transport systems, linear rails are a good choice for guidance. Their high load capacity allows heavy loads to be transported, and the rigidity of the linear rail helps to stiffen the entire system. Gantry robots: The defining feature of a gantry is that it has two X (and sometimes two Y and two Z) axes. The individual axes typically incorporate a single linear rail and are driven by a screw or a belt and pulley system. With two axes working in parallel (X and X’, for example) very good moment capacities are realized, even though each axis has just one linear rail.

Dual-rail applications: When high moment loads are present, linear rails can be used in pairs, which allows the moment load to be resolved into forces on the bearing blocks. In this configuration, the drive mechanism can be mounted between the linear rails, making the overall system very compact. Applications include … Linear stages: Stages are typically very high precision systems, meaning that high travel accuracy and minimal deflection are paramount. Even if the load is centered on the stage with little or no moment loading, dual linear rails are often used to ensure that stiffness and bearing life are maximized. Machine tools: Like stages, machine tools require very high levels of travel accuracy and stiffness, to ensure that the tool produces high-quality parts. Using two rails in parallel — typically with two bearing blocks per rail — ensures that deflection is minimized. Machine tools also experience very high loads, so resolving the load over four bearing blocks helps to maximize bearing life.

Cartesian robots: Because Cartesian robots typically use just one linear system per axis, it’s important that each axis can withstand high moment loads. This is why most Cartesian robot axes are constructed from linear actuators that incorporate two linear guides in parallel. Robot transport units: Six-axis robots provide flexible motion for applications that require reach and rotation in many directions. But if the robot needs to move to another station or work area, dual-rail systems can act as a seventh axis, transporting the entire robot to a new location. A significant benefit of linear rails in these applications is the ability to join multiple rails for very long travel lengths — often exceeding 15 meters. Of course, linear rails aren’t the perfect solution for every application. For example, linear rails aren’t generally suitable for applications in the consumer space — such as door guides and drawer slides — often due to cost. And linear rails require very precise mounting surfaces, not only to reap

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LINEAR ACTUATORS

the benefits of their high travel accuracy, but also to avoid binding of the bearing block, which can lead to reduced life. They must also be fully supported, unlike linear shaft systems, which can be only endsupported. This means that not only is the up-front cost of a linear rail typically higher than that of a round shaft or plain bearing system, the cost of preparation and mounting is also higher. Linear rails can also be perceived as less smooth or notchy in their running properties than other bearing types. This is because of the contact that occurs between the load-carrying balls (or rollers) and the raceways. Preloading a linear rail system, which is often done to increase stiffness, can exacerbate the feeling of notchiness when the bearing block is moved along the rail. (This effect goes away as load is applied to the bearing, but the perception often remains.) For applications that don’t require the load capacity, stiffness, or travel accuracy of a linear rail, other linear guides such as round shaft systems, plain bearing guides, or even crossed roller slides may be suitable and less expensive. Linear guide rails and bearings offer high stiffness and good travel accuracy. And they can support not only downward, upward, and side loads, they can also withstand overhung, or moment loads. Of course, the larger the linear rail and bearing system, the more moment capacity it has, but the arrangement of the bearing raceways — face-to-face or back-to-back — also influences the amount of overhung load that it can support. While the face-to-face design (also known as the X arrangement) provides equal load capacities in all directions, it results in a shorter moment arm along which overhung loads are applied, which reduces moment load capacity. The back-to-back arrangement (also known as the O arrangement) provides a larger moment arm and gives higher moment load capacities. Many linear actuator designs include two rails in parallel with the drive mechanism — belt, screw, or linear motor — incorporated between the rails. While it’s not imperative that the drive be centered between the guide rails, doing so helps ensure even loading on all of the bearings, and reduces cogging, or uneven drive forces on each rail and bearing set. This arrangement also reduces the height of the actuator, making it relatively compact given the high load and moment capacity provided by the dual guide rails. Recirculating bearings (bushings) that ride on round shafts are unable to handle roll moments. This is why most linear bushing applications require that two shafts be used in parallel. DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

What constitutes a high-speed actuator Like many terms used in the linear motion industry — including heavy duty, miniature, and corrosion-resistant — there is no industry standard that specifies what constitutes a high-speed linear actuator. Nevertheless, there are some general guidelines that manufacturers follow when classifying and marketing their actuators as high speed. These guidelines are typically based on the drive mechanism, actuator type, and even primary use or industry. Understanding these distinctions can help you make an informed decision when your application calls for a high-speed linear actuator. Speed is primarily dependent on the linear drive mechanism: The limiting factor of a linear actuator’s speed capability is typically the drive mechanism. More specifically, ballscrews and leadscrews are limited in speed by their tendency to whip like a jump rope — which is a function of the screw diameter, length, and end bearing arrangement. Because leadscrew designs are based on sliding contact and generate high heat due to friction, they often have lower maximum speeds than ballscrews of a similar size. So of the screw technologies, actuators based on ballscrew drives are more likely to be deemed high speed than those based on leadscrew drives. Actuators based on belt drives or rack and pinion assemblies are typically able to reach higher speeds than ballscrews, provided they are properly tensioned (for belt drive versions) or preloaded (for rack and pinion versions). Actuators with steel reinforced belts can deliver speeds of 10 m/sec or higher while rack and pinion driven actuators can commonly reach speeds to 5 m/sec. Actuator type influences maximum speed: Another factor comes into play when discussing high speed linear actuators is the type of actuator. The high speed designation is most often applied to thrust-rod type actuators (also called electric cylinders) because their primary applications involve pushing-pulling and inserting operations, which typically require very short extension and retraction times. These actuators can be either ballscrew or leadscrew driven ... with speeds ranging from 0.1 m/sec to more than 1 m/sec. A few manufacturers even offer belt-driven rod-style actuators that can reach speeds to 2.5 m/sec. Slider or carriage-type actuators (also called rodless actuators) can achieve even higher speeds than rodtype actuators in many cases. But because their primary uses are for positioning and transport (typically with high loads as well) they are less often marketed as high speed. Rodless slider-type actuators have a wide range of drive options ... including leadscrew, ballscrew, rack and pinion, belt, and linear motor. Guide type limits top speed: Linear motors inherently provide the highest speed capabilities, with no mechanical parts to limit speed or create friction and heat. But when incorporated into a linear actuator, linear motor drives are limited by the speed of the guide mechanism. Similarly, steel-reinforced belt drives for linear actuation can deliver

DESIGN WORLD — MOTION

8 • 2020

Applied Motion Products TBSM11 closed-loop linear actuators give axes faster acceleration and shorter move times for more throughput than traditional stepper-motor linear actuators. With peak torques to 50% of holding torque, these linear actuators excel on designs that need axes to start and stop loads quickly.

speeds exceeding 12 m/sec. However, like linear motors, they are limited by the maximum speed of the guide. The most common guide systems used with linear motors and belt drives are recirculating profiled rail bearings for which maximum speeds typically reach up to 5 m/sec — limiting the overall speed of the actuator to 5 m/sec or less. Track-roller (wheel) linear guide mechanisms for still higher speeds: Faster stokes are possible when belt drives (and in some cases, linearmotor drives) are paired with wheel guides instead of recirculating profiled-rail bearings. High-speed linear actuators with a preloaded linearguide wheel assembly and a properly tensioned steel-reinforced belt drive win the race with travel speeds to 10 m/sec. Standard guide-wheel systems can reach speeds to 8 m/sec. Side note on miniature designs: For micro and nano-positioning applications, the actuators of choice are often based on voice coil or piezo technologies. Ultrasonic piezo actuators can reach speeds of 0.5 m/sec or greater, but they typically have maximum strokes of 100 mm or less. Voice-coil actuators operate at maximum speeds of 0.25 to 0.30 m/sec with strokes of 150 mm or less. While these specifications may not fit the general definition of highspeed linear actuators, considering the rapid acceleration that is required to reach these speeds in very short stroke lengths, piezo and voice coil designs can easily be classified as high acceleration actuators. motioncontroltips.com | designworldonline.com

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MOTION SYSTEMS HANDBOOK

Ballscrew update

Fixed-free arrangements provide the least rigidity.

Ballscrews include a screw and a nut housing recirculating balls to carry the axis load. Though recent years have seen more use of leadscrews in designs that once employed only

A fixed-floating arrangement has bearings at both ball screw ends, but a thrust bearing only at one.

ballscrews, four application requirements still necessitate a ballscrew.

A fixed-fixed arrangement provides maximum rigidity.

Vup = Permissible travel variation within useful travel V300p = Permissable travel variation within 300 mm I0 = Nominal travel V2πp = Permissible travel variation with 2π travel ep = Tolerance on useful travel Cup = Travel compensation LEAD I1 = Axial thread length Iu = Usable travel Ie = Excess travel

LEAD

POSITIONING BALLSCREW VALUES I1 Iu

300 +

GROUND VERSUS ROLLED BALLSCREWS

V300p C

V2πp Vup

Travel deviation

2π rad

Vup

ep ep

BALLSCREW NUT TUBE RECIRCULATION The standard and most economical option is tube recirculation.

BALLSCREW NUT DEFLECTOR RECIRCULATION

BRIDGE DEFLECTOR

Heavy loads: With recirculating steel balls to support the load, ballscrews have a higher load capacity than comparably sized leadscrews. Leadscrews with bronze nuts can drive heavier loads — but as the load increases, so does friction … and that lowers duty cycle. Note: Related to sizing is the L10 bearing life equation, which provides a statistically proven estimation of the screw’s life in meters or rotations traveled. The wear characteristics of a ballscrew make exact life hard to predict. Leadscrews with plastic nuts can be selected based on PV values, but these give pressures and velocities the screw can withstand — not an estimation of life. High accuracy requirements: Unlike leadscrews (which exhibit backlash between the nut and the screw) ballscrews can be and often are preloaded to remove backlash. This is typically done by using balls that have a diameter that’s slightly larger than the space between the raceways of the screw and nut. (Some leadscrews have nut designs that eliminate backlash, but these typically add friction and reduce efficiency.) Plus ballscrews are classified by standards dictating lead deviation, so choosing the proper ballscrew accuracy class is straightforward. High efficiency: Rolling balls in a ballscrew rely on point contacts to support a load. Point contact produces less friction than the line contact found in leadscrews, which boosts efficiency. More specifically, ballscrew efficiency is almost always 90% or better, while leadscrew efficiency is normally 50% or lower. High efficiency translates to lower required motor torque, so often a smaller motor (and components) may be used. Demanding duty cycle: The lower friction of ballscrews means they produce less heat than leadscrews … so can withstand higher duty cycles. In fact, duty cycle is only considered in ballscrew selection when determining the travel the screw will achieve in its calculated life. In contrast, duty cycle and heat must always be considered when selecting a leadscrew.

DESIGN WORLD — MOTION

Another comparison made in industry is that between ground and rolled ballscrews. What’s more important than this choice are the following three distinctions. DIN/ISO and JIS specifications define two ballscrew accuracy types: P is for precision and T is for transport, and lower numbers indicate better accuracy. Precision classes ranging from P0 to P5 and transport classes from T5 to T9 (T10 for JIS). JIS specifications denote accuracy classes with the prefix C for precision and Ct for transport. A common misconception is that accuracy class specifies manufacturing method, but the two are not intertwined. Rolled screws can be P5 and even P3 accuracy, and some ground screws only have T accuracy values. More important is whether lead error V300 accumulates over the screw length. P accuracy classes don’t allow lead error accumulation, while T accuracy classes do. Geometric tolerances are also specified by DIN/ISO and JIS standards: For ground ballscrews, both thread grinding and journal grinding are done using the same reference centers, making it easier to minimize radial runout and keep the screw threads and end journals concentric. When screws are manufactured by rolling, usually the end journals are machined and ground after the threads are rolled, so maintaining concentricity and runout is more difficult. Even so, if a ballscrew is manufactured to DIN/ISO or JIS standards, it will meet those lead accuracy and geometric specifications ... regardless of whether it was manufactured by rolling or grinding. Processes produce different surface finishes: Rough surface finishes cause increased friction and accelerated ball wear. Grinding and rolling both produce smooth surfaces, but rolled screws must also be polished to remove oxidation. So when considering surface finish, the comparison is actually between ground and polished rolled screws. The quality of the polishing step determines rolled-screw finish quality. Ground screws are usually necessary where P5 or better accuracy is required ... but both rolled and ground screws can deliver P5 and even P3 accuracy — as well as DIN/ISO 7 and 9 (JIS 10) accuracy. 8 • 2020

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Balancing design objectives with low-power braking As Plato once said, the measure of a man is what he does with power. Of course the ancient Greek philosopher wasn’t referencing motion control with that statement, but it holds true here as well: Electriﬁcation and batteries in our modern world demand top efﬁciency from all components, including clutches and brakes … because there’s no workaround for the fact that electrical power P = V×I … and electrical power dissipated Pd = I2R.

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In fact, power can be saved while using electromagnetic brakes. Cooler operation is better for efficiency and machine design longevity … and oftentimes, optimizing a brake’s performance is possible through customization. Consider how electromagnetic spring-applied brakes work: These brakes mechanically hold loads but electrically actuate to release. So without power applied, the brakes stop and hold a shaft in place to prevent rotation. In fact, some modern parking brakes in passenger vehicles use this design: The brake releases upon the application of power to allow wheel rotation. Where is more power a design objective? Rapid adoption of mobile robotics and batterypowered equipment has made power density more important than ever. Electromagnetic brakes for these applications must draw as little power as possible. This allows more power for essential equipment operation, longer battery life, lower cost of operation, and a healthier environment. Design engineers can limit power draw from electromagnetic brakes in three ways — by customizing to optimize the brake … and taking steps to keep the brake cool … and selecting a brake that’s lightweight … and using a power controller on the brake. How to customize a brake: Leading brake manufacturers offer motion designers supportive engineering and application expertise. Many have online application-submission forms to make the first reviews more convenient. Of course, customization doesn’t change the way electromagnetic brakes function … but design modifications to the brake’s airgaps, spring force, materials, coils, and flux paths can make for dramatic performance improvements. Pulse width modulation: Magnetic flux takes the path of least resistance. So within milliseconds of initial application of current to the brake coils, the brake disengages and inductance changes. The exact response time is a function of the brake size and design. In this disengaged brake state, the mechanical pressure plate is contacting the coil body. In addition, the magnetic flux no longer needs to travel across the airgap distance to close the electromagnetic circuit. Once the brake is disengaged, power can be reduced to hold the brake in a disengaged state. This is the most useful design approach in boosting efficiency … because after all, the maximum amount of electric current should go into performing work — and wasted power should be minimized. Here, pulse width modulation (PWM) is indispensable for drawing down power into the brake to keep it disengaged. In fact, brakes with PWM input can expect to consume 50% less power during holding … and some 24-V brakes can average down to about 7 V — which is closer to 70% energy savings. Over-excitation is another technique to make designs more efficient. It lets designers get more torque for a given size. For example, when disengaging a spring-applied 12-V-coil brake, an over-excitation routine might apply 24 V for a short duration while the brake’s coil magnetically attracts the pressure plate and compresses the springs. Required power drops once the pressure plate comes into contact with the coil body. Note: Over-excitation should only be used in applications where the brake has a coil specifically designed for this type of operation. After all, the goal is to have long-lasting products that meet life expectations.

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DESIGN WORLD — MOTION

8 • 2020

BRAKES • CLUTCHES • TORQUE LIMITERS

Keeping cool: Combining overexcitation and PWM reduces total average voltage on the brake, thus eliminating excess heat buildup (from wasted energy) in the coil. The benefit is compounded by the fact that as coil temperature rises, so does its resistance. Conversely, a constant-current situation with rising resistance will also see an increased power requirement. So to prevent wasted power, keeping the brake cool is the way to go. Competing design objective — eliminating weight: Unless the design at hand is a steamroller, there’s no point in engineering extra weight into a new build. While it would be easy to employ a brake with a large surface to dissipate heat, it just isn’t practical in most mobile applications. Instead, design engineers should focus on eliminating excess weight in mobile robotics and electrification projects. Lower weight necessitates less power from the drivetrain and can usually improve response time for better overall performance. Weight associated with a brake can be reduced by eliminating parts; by building a brake into the motor endbell; and by removing unneeded material from a brake flange. In addition, brakes should be designed for a particular application to meet torque, temperature, speed, inertia, duty cycle, and mounting. Customization leads to optimization in performance while minimizing weight.

BI-STABLE BRAKES ARE ANOTHER EFFICIENT OPTION There exists another braking solution called bi-stable brakes for applications needing power savings. Just consider how central duty cycle is to efﬁciency — because after all, fewer actuation events lead to greater energy conservation. Bi-stable brakes serve this function by only actuating when a pulse of power is sent to the brake — to change its state from engaged to disengaged or vice versa. This article was written by Brian Mather, industrial product manager at Ogura Industrial Corp. Read the rest by visiting motioncontroltips.com and searching Mather.

DESIGN WORLD — MOTION

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Ways to reduce synchronous belt noise Industrial belt drives

consist of 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 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 power-transmission devices.

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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. Finally, 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. So we know that synchronous belts are common in motion systems, providing smoother operation and better high-speed performance than chains and lacking the problems of slipping and stretching that can plague V-belts in precision applications. But one downfall of synchronous or toothed belts is the noise they produce. Although quieter than a chain drive, a synchronous belt can still generate noise that is unacceptable for some applications and environments. The noise from a synchronous belt is, for the most part, caused by the very feature that makes synchronous designs a better choice than chains or V-belts: meshing between the belt and the pulley. First, the simple impact of the belt engaging with the pulley creates noise often compared to a slapping sound, which is especially prominent at lower belt speeds. Second, as belt teeth engage with pulley grooves, air is trapped between the two components and then evacuated, making a sound that can be likened to air escaping from a balloon. This phenomenon is a significant contributor to belt noise at higher speeds. Another factor that contributes to synchronous belt noise is belt tension. Synchronous belts are typically operated under high tension and, therefore, easily resonate (like a plucked guitar string). Belt and pulley materials can also play a role in noise. For example, polyurethane belts typically exhibit more noise than neoprene (rubber) materials, and polycarbonate (thermoplastic polymer) pulleys tend to be noisier than metal pulleys. Noise generated by pulleys is also related to the dimensional accuracy of the pulley, which determines the smoothness of meshing between belt teeth and pulley grooves.

DESIGN WORLD — MOTION

8 • 2020

BELTS & PULLEYS

DW_LIE_halfpage 2020_X1a.pdf

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PRECISION, QUALITY AND VERSATILITY FOR THE

DESIGN ENGINEER Add together the effects of these various factors, and you can easily end up with a belt driven system that produces uncomfortable or even detrimental amounts of noise — especially when multiple belt systems are operating in close proximity. But there are ways to reduce the levels of noise produced by synchronous belts. From a sizing and design standpoint, the noise generated by a synchronous belt is directly related to the belt width and belt speed. Belts with larger widths tend to resonate more, and higher belt speeds generate more and higherfrequency noise. C Noise is also inversely related to the M diameter of the pulley. Therefore, a few easy Y ways to reduce noise (if the application allows) are to reduce the belt speed, use a smaller width CM belt, or use larger diameter pulley. From a mounting and operating standpoint, MY noise can be reduced by ensuring the pulleys CY are properly aligned, since angular misalignment CMY (parallelism of the pulley shafts) can lead to contact between the belt and the pulley flanges. K And if the belt isn’t properly tensioned, there can be unnecessary interference between the belt teeth and pulley grooves, which is another factor that contributes to unnecessary noise. Some manufacturers offer synchronous belts that are designed to be low-noise. From a manufacturing standpoint, noise can be addressed by applying a nylon covering to the toothed side of the belt, which reduces noise that occurs during meshing. In addition, cutting grooves into the pulley provides a low-pressure path for air to escape as the belt and pulley mesh. Another low-noise modification is to alter the geometry of the tooth profile to improve the “rolling action” as the belt teeth mesh with the pulley. One such design uses what is referred to as an “offset double helix pattern” for the belt teeth. In this design, the belt has DESIGN WORLD — MOTION

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BELTS & PULLEYS

two sets of teeth side-by-side but offset by 180° so the frequency of noise generated by one set of belt teeth (one side of the belt) is 180° out of phase with the frequency of noise generated by the other side, effectively canceling the noise.

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. As mentioned, belts are quieter than other power-transmission 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. Note that in a properly designed belt drive, there should be at least six teeth in mesh and at least 60° of belt wrap around the drive pulley. Also ... 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. Select the appropriate belt for the design torque. Select the appropriate belt material for the environment (temperature, chemical, cleaning agents, oils and weather). Beltand-pulley systems are suitable for myriad environments, but some applications need special consideration. Topping this list are environmental factors. Dusty environments do not generally present serious problems as long as 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 DESIGN WORLD — MOTION

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. Some rubber compounds may provide some marginal improvement in durability, but 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 in much the same way as excessive temperatures. 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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CABLES & CONNECTIVITY

Power over Ethernet (PoE) and M12 connectors in motion designs Though the internet of things is most visible to laymen in consumer devices, the IIoT is massive. The majority of applications are in business, factory, and healthcare settings … and more than a third of the nearly $6 trillion IoT market is in manufacturing. Connectivity is the basis of all this IIoT — providing the power and data transmission required by controls, devices, machines, and the cloud to execute IIoT functionalities. As we outlined in our six-part series on this topic (at motioncontroltips.com/category/networking-iot) the leading trends here currently include edge computing as well as cloud-connectivity services; components with wireless communications; Ethernet-based networks; open-source programming; software in the form of unifying development environments (UDEs) for large-scale interoperability; and standardized protocols, cables, and connector options. In this article we do a deep dive on just two IIoT-enabling connectivity standards so essential to today’s motion control systems — the humble M12 connector as well as power over Ethernet (PoE). Then we’ll briefly touch on a third related standard — that of single-pair Ethernet or SPE — as yet another standard to simplify automation.

M12 AND M8 CONNECTORS — SO COMMON ON MOTION COMPONENTS

Ethernet switches. In fact, the 802.3at (PoE+) standard provides 30 W for connected devices — sufficient for running a NEMA-14 step motor at full power. We’ll cover PoE in this article’s next section. But why the M12 connector on this step motor … and what does the “X-coded” designation mean? Recall that M8 and M12 and connectors are round cable terminations that securely connect automation devices to larger systems as well as common fieldbus networks. They dramatically trim installation time when compared to hardwired designs of yore and can even eliminate the need to hire electricians to install the automated machinery. It’s now difficult to imagine the wiring of automated systems before the advent of cordsets and connectors: After all, all sensors and actuators had to be hardwired via pre-attached wiring … often times unjacketed. These conductors required that an electrician fish them through conduit into controls. Upon some component failure, all component wiring had to be disconnected and pulled out — and then rewired by an electrician. The risk of faulty wiring necessitated expert testing before startup. Refer to Design World sister site www. connectortips.com for more on this topic. M8 and M12 connectors on jacketed cable avoid these complicated routines. The connectors trace their origins to the 1982 release of a

Consider the X-coded M12 connector on NEMA-14 step motors with closed-loop current control from Applied Motion Products. These motors employ power over Ethernet (PoE) connections. In the past getting energy sufficient to power a motor on PoE was challenging … but now that’s changing thanks to higher-power IEEE standards for industrial

Ethernet standards are ubiquitous — right down to their RJ45 socket connectors. While transformers in these RJ45 sockets are common, they’re rare in rugged M12 connectors. Now, HARTING M12 Magnetics includes M12 sockets with transformers to avoid the problem of needing electrical components next to the socket on the connected circuit board. That saves space and cost — and boosts reliability: Conductor tracks can be made lighter and more direct because they don’t have to be arranged around many different components. Plus these connectors are more reliable than combinations of sockets and transformers made by different manufacturers. M12 Magnetics interfaces are capable of 10Gbit and max PoE+ for reliable power and data transmission.

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8 • 2020

DESIGN WORLD — MOTION

MOTION SYSTEMS HANDBOOK

proprietary 7⁄8 in. three-pin connector with a free-spinning outer housing called a coupling nut. Just like most of today’s M8 and M12 connectors, this design’s outer coupling nut screwed down onto the male receptacles of devices and equipment for a maximally reliable connection of cable conductors — as well as mechanical shielding of dust and ﬂuids. M12 connectors were introduced a few years later and quickly became a dominant standard. The name M12 refers to the standard metric M12 thread (12 mm in diameter) of this connector’s coupling nut. The smaller M8 connectors — which came a few years later in 1989 — are also named for the metric thread of their coupling nuts … in this case a coarser M8 thread on a major diameter of 8 mm. Today, M8 connectors join industrial sensors actuators, switches, PLCs, and I/O to various automated systems. In contrast, M12 connectors are used in a wider array of factory automation, test and measurement, food and beverage, and robotics applications. They come with three

or four pins (as in initial versions to market — with the latter for advanced sensors and actuators) as well as ﬁve, six, eight, or 12 pins. They excel in harsh environments subject to washdown and corrosive substances … and in fact, it’s fairly common to see M12 connectors with IP65, IP67, IP68, and IP69K ratings. These connectors stay watertight even when submerged. That’s one reason they’re so common in food and beverage, rubber and plastic, and textile and printing applications. Most sensors and power applications require three and four-pin M12 arrangements. PROFINET and Ethernet use four and eight-pin counts … while most CANbus and DeviceNet connections use four and ﬁvepin counts. 12 pins are necessary for certain modes of signal transmission. M12 connector coding is a system of standardized mating and shielding geometry to ensure the many variations of the connector type are properly applied. For example, X-coded connectors (mentioned earlier in

This browser-configurable Opto 22 groov RIO I/O supports IIoT functionality with wired switches and sensors connections to Ethernet networks, software, and cloud platforms. The I/O eliminates the need for PLCs, PACs, or PCs to communicate sensor data … and can mount in hazardous locations without a power supply — because power over Ethernet (PoE) provides both power and data connectivity.

DESIGN WORLD — MOTION

8 • 2020

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CABLES & CONNECTIVITY Servo feedback cable from LAPP called ETHERLINE SERVO DQ FD P Cat.5e cable works with open SIEMENS DRIVE-CLiQ system interface for SIEMENS SINAMICS-S120 drive systems. 85% copper-braiding coverage ensure electromagnetic compatibility.

our step-motor PoE example) are a newer eight-pin option to meet CAT6A requirements for data transmission to 10 Gb/sec. Other common key (coding) variations are: • A coded M12 connectors for sensors, dc power and 1 Gbit Ethernet • B coded M12 connectors for PROFIBUS • C coded (dual keyway) M12 connectors for ac power to actuators and sensors • D coded four-pin M12 connectors for 100 Mbit Ethernet • S coded M12 connectors (to supersede C-coded connections) for ac power • T coded M12 connectors (which will eventually replace A-coded connections) for dc power

ANOTHER STANDARDIZED CONNECTIVITY OPTION — POWER OVER ETHERNET (POE) Power over Ethernet (PoE) is a standardized networking arrangement employing twisted pair- Cat5e, Cat6, and Cat6a Ethernet conductors to transmit data (with increasing levels of protection against signal noise) as well as electric power. Deﬁned by the IEEE, PoE versions include those for data transmissions of 10, 100, and 1,000 Mb/sec as well as electrical power transmissions of: • 15 W for IEEE 802.3af Type 1 devices • 30 W for IEEE 802.3at Type 2 devices • 60 W for IEEE 802.3bt Type 3 devices • 90 W for IEEE 802.3bt Type 4 devices

A, B, and D connectors are most prevalent, as they’re the original M12 connector variations. That said, X-coded connectors are increasingly common for high-speed industrial Ethernet and are replacing A and D-coded M12 connectors for Ethernet applications. Other newer M12 connectors include K-coded connectors for ac power and L-coded connectors for PROFINET dc power. Refer to Design World sister site www. connectortips.com for more on this topic. The design of M12 connectors (straight and right angle) are standardized under IEC 61076-2-101. Both PVC and PUR-jacketed cable pairs well with M12 connectors; myriad choices allow full customization of cordsets. Options include shielded and unshielded cable and connectors; armored cable; and stainlesssteel coupling nuts and other elements.

A newer Ethernet standard called single-pair Ethernet (SPE) aims to supersede the industry’s mishmash of fieldbuses to simplify and improve the connectivity of field devices and controls … and deliver on the potential of end-to-end industrial Ethernet protocols. Relatively high transmission rates and long cable lengths complement power over data line (PoDL) operation. Image via HELUKABEL courtesy of the SPE Industrial Partner Network

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8 • 2020

DESIGN WORLD — MOTION

MOTION SYSTEMS HANDBOOK SSI ENCODERS 2 1 8 3

8-PIN M12

4 7 6

GROUND (GND)

+Vdc

SSI CLK+

SSI CLK-

SSI DATA +

SSI DATA-

PRESET

DIR

SHIELD 5-PIN M12 +Vdc

GROUND (GND)

CAN

HIGH

CAN

LOW

CAN

GND SHIELD

HOUSING

CANopen ENCODERS

1 5

M8 and M12 connectors were first adopted by the automotive industry as a better alternative to hardwiring or relying on more delicate connector types. Today they’re used in all automated industries. Shown here are two M12 connector options for MA58H absolute encoders from Encoder Products Co.

Both the circa-2003 IEEE 802.3af and newer 802.3at (PoE+) standards along with the higher-power 802.3bt standards (including 60-W “ultra” UPoE) govern the specifics of the cabling and components for such connectivity functions. So PoE is mostly associated with sensors, cameras, and other feedback devices. However, 802.3at PoE+ (and what are called Type 2 devices accepting power from PoE+) is currently seeing the most rapid adoption in the motion-control space for its ability to power actuators. This aforementioned 802.3at standard is particularly relevant to moderately sized motion-control applications, as it defines systems with wattages sufficient to supply full power to electric motors (in this context, PoE powered devices or PDs) used in the actuation of motion axes. Eliminating the need for an ac power source is another PoE benefit at the remotest of equipment reaches … sans potentially dangerous electrical connections that would otherwise be necessary. In addition, PoE components impart IIoT functionality by allowing physical reconfiguration with connection and disconnection of components at the system switches (in stark contrast to the hassles of hardwiring) to complement the reconfigurability made possible with today’s UDEs and other software. There are some caveats: For each switch or hub in a PoE system, there is a power budget — the wattage of the switch divided by

DESIGN WORLD — MOTION

8 • 2020

the number of used ports. In addition, any installation with non-PoE components (and setup for separate power and data conductors) necessitates one of two subcomponents: • An injector: Often used in setups where PoE field devices are added to existing systems, these send power to PoE-compliant components accepting data through existing non-POE switches. In fact, PoE injectors are a cost-effective alternative to buying new switches to accommodate PoE components having modest power requirements. • A splitter: These split power and data to separate conductors to allow connectivity with components noncompliant with PoE. One final caveat: PoE without supplementation can only transmit signals over 100-m stretches off a given switch or hub … and compensatory power supplies as well as (data signal) extenders may be necessary on installations with many PoE devices.

ANOTHER NEW STANDARD: SINGLE-PAIR ETHERNET (SPE) So far we’ve detailed uses of standard four-pair (eight conductor) Ethernet for data connectivity and the relatively new adoption of PoE. Now another Ethernet standard called single-pair Ethernet (SPE) aims to overtake the mishmash of fieldbuses on the market to simplify and improve the connectivity of field devices and controls … and deliver on the potential of end-to-end industrial Ethernet protocols. SPE addresses the 100-m limitation of standard Ethernet mentioned above with the potential for runs to 1 km … which could prove quite useful in very large plants or automated settings. SPE cabling is also

MA58H absolute encoders from Encoder Products Co. are heavy-duty multi-turn hollow-bore absolute encoders. They work from -40° C to 85° C and are rated to IP65 on the shaft, with the balance of the unit rated to IP67. Notice the threaded connection at the top of the encoders for standard M12 connectors.

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HELUKABEL®’s extensive line of flexible control, VFD, servo & feedback cables, hook-up wire, industrial connectors and strain relief cable glands are ideally suited to meet the requirements of industrial machinery. Regardless of the application – continuous movements on multiple axes to working in a broad array of temperature environments – HELUKABEL® offers its customers a superior, cost-effective solution.

MOTION SYSTEMS HANDBOOK

UNMANAGED SWITCH

MANAGED SWITCHES

P1 P2

3 3

2 2

6 ETHERLINE®

1 ACCESS U08T

ETHERLINE® ACCESS U05T

PWR1 PWR2

FAULT

23.03.20 17 PWR1 MSTR/HEAD

PWR2

FAULT

CPLR/TAIL

4 MSTR/HEAD CPLR/TAIL

6 5

3 100M

4 1

10M

®ACCESS M08T

100M

ETHERLINE

2 1

10M

®ACCESSM08

ETHERLINE

MANAGED SWITCHES

UNMANAGED SWITCH

UNMANAGED SWITCH P1 P2

7 PWR1 PWR2

FAULT

MSTR/HEAD

PWR1

CPLR/TAIL

PWR2

FAULT

ETHERLINE

MSTR/HEAD

®ACCESS

U08T

6 5

CPLR/TAIL

ETHERLINE ACCESS ® U05T

3 100M

2 1

10M

®ACCESS M08T

ETHERLINE 100M

2 1

10M

®ACCESS M08T ETHERLINE

ETHERLINE ACCESS PROFINET switches from LAPP are configurable via a web interface. They support high transmission rates over long distances … and have RJ45 ports as well as small form-factor pluggable (SFP) ports for fiber-optic cables. Some convert the light signals of fiber optics into electrical signals for 100 Mbit/sec or 1 Gbit/sec and single-mode or multi-mode transmission. Otherwise an ETHERLINE ACCESS U04TP01T switch in this line (ruggedized for mounting near field devices) has four RJ45 ports with Power over Ethernet (PoE) and one port for fast Ethernet. Recall that unmanaged Ethernet switches are cost-effective points of field-device connectivity that serve as redundant power inputs and support plug-and-run functionalities of smart devices. In contrast, managed Ethernet switches actively monitor and troubleshoot the network and its traffic … and act as access points for other interfaces as well as plant personnel to investigate and adjust machine settings.

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8 • 2020

SPE addresses the 100-m limitation of standard Ethernet mentioned above with the potential for runs to 1 km … which could prove quite useful in very large plants or automated settings. SPE cabling is also about a quarter the diameter (and weight) of traditional Ethernet cable … which could prove quite useful where pieces of machinery have very tightly spaced banks of ﬁeld devices. Deﬁned by the IEEE 802.3cg 10 Mb/ sec standard, SPE is incapable of standard four-pair Ethernet data rates … though the 1 Gb/sec so useful in industrial settings is possible — and SPE maintains the built-in data security of all IEEE 802.3 standards. Note that PoE and SPE aren’t mutually exclusive: SPE can transmit power and data by leveraging existing PoE standards in a power over data line (PoDL) setup. Power distribution techniques are standardized for 10 Mb/sec twisted-pair conductors with wattages comparable to the IEEE 802.3af 15-W standard and beyond.

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Controllers for motion control and beyond The main task of any motion controller is to command the motion of some machine or system, including any motors and other mechanical components such as actuators. Whether control is needed for simple point-to-point positioning or for multi-axis interpolated moves, choosing the right motion controller can have lasting effects on the final designed system. For instance, a controller with insufficient computing power or capabilities that fails to meet a machine’s motion control needs can degrade the machine’s performance and abilities. On the other hand, overly powerful or robust controllers can add unnecessary system cost. More specifically, motion controllers calculate and generate the commands for the motor or actuator to move. These are known as motion trajectories and involve calculating the right speed and position. These commands can be sent to a drive which develops the necessary current to drive the motor. In a closed-loop system, there will also be some type of feedback from the process that helps to adjust the commands in order to meet system requirements. Controllers can use a number of algorithms needed to generate commands including control loops, interpolators and step generators. Choosing the right motion controller can be challenging because they range in complexity and capability — from simple indexing to full-fledged motion control systems. But knowing as much as possible about the immediate application and of the range of controllers on offer can vastly simplify the task.

CATEGORIZING CONTROLLERS Controllers can be categorized in a number of different ways depending on what is prioritized. For instance, they can be divided

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8 • 2020

The iQ-R Series from Mitsubishi Electric is a fully integrated controller designed to support a host of manufacturing applications. It integrates motion, safety, process, redundant, and other control disciplines on a single platform, interfaces with enterprise and visualization systems, and conforms with global standards such as IEC 61131-3 programming languages. by the physical format (plug-in board, stand-alone, embedded,) or by function (PLC, PAC, motion controller) or even by architecture (distributed or centralized.) If we focus first on size, we can start from the smallest motion controllers; embedded controllers. These are so-called because the control is literally embedded into the device or system being controlled. Most often these are circuit-board-sized controllers with a main processor on board, memory, and control circuitry. They also motioncontroltips.com | designworldonline.com

MOTION SYSTEMS HANDBOOK

include various bus interfaces as well as software for controlling actuators or receiving data from sensors. Embedded motion controllers can be connected to or built into drives or amplifiers or even onto the motor unit itself with drive circuitry to form what are often called integrated motors.

STAND-ALONE AND PC-BASED CONTROLLERS Standalone controllers are complete systems that include all electronics, power supplies and external connections that mount to one physical enclosure. These controllers fit into machines to command applications consisting of either a single motion axis or multiple motion axes. PC-based controllers include a basic PC motherboard or ruggedized industrial PC as well as PC-type hardware components and a high speed dedicated bus that transmits information to and from the processor. PC control requires the same inputs and outputs as a basic PC, as well as interfaces to factory floor devices. Here, typical I/O includes the electric motors and other actuators, as well as discrete sensors and other mechanical components. One key advantage of PCbased controllers is that they provide a readymade graphical user interface (GUI) for easier programming and tuning. PC-based controller software includes an operating system to manage internal processing and resources. Where once OS instability issues were a real concern, today with the proliferation of real-time operating systems, professional grades of Windows, and Linux application software, those concerns have largely been dealt with. Control programming languages include common general-purpose languages including C++ or Visual Basic but also more control oriented languages such as IEC-61131recognized Ladder Diagram, Instruction List, Function Block Diagram, Structured Text and Sequential Function Chart languages.

PLCs Programmable logic controllers (PLCs) are microprocessor-based electronic

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components that serve as the programmable smarts for simple or isolated applications. Now they’re also finding increased use on integrated machines and more complex automation setups. PLCs have evolved from their early days into controllers with a reputation for unbeatable reliability. Today, their logic and memory boards, backplane interfaces to I/O modules, and generally heavy-duty circuitry help make PLCs indispensable. The basic parts of any PLC system include the processor, I/O modules to handle inputs to the controller and outputs to the controlled devices, and some type of user interface which could be as simple as a keypad or a touchscreen interface or a programming link via a PC. The PLC’s processor is programmed via the user interface. The I/O modules are used to bring input signals into the PLC’s CPU and output control signals to controlled devices such as motors, valves, sensors and actuators. One core PLC strength is that they let engineers write application code to suit specific applications in which they’ll operate — on assembly lines and in machinery and elsewhere on factory floors, often times with other electrical, mechanical, and electronic equipment. Here, the original form of PLC programming known as ladder logic is still dominant. Another fact about PLCs is their durability. Despite industry predictions to the contrary, PLCs are still by far the most common choice for standalone or singleaxis motion. Such applications are seeing more inroads from motor drives sporting controller functions. These motor drives excel in machine designs that still need PLC functions with multiple interfaces — including Ethernet communications and digital I/O, for example. Because such motor drives can also incorporate motion controls delivering S-curve, camming, and freeform motion profiles, OEMs are more likely to pick them than programmable automation controllers (PACs) in otherwise-simple designs 8 • 2020

Small servo control modules, such as the TMCM-1617 single-axis servo controller with integrated field-oriented control implemented in hardware from TRINAMIC Motion Control, illustrate the idea of embedding controllers into machines and devices. They’re designed for servo drives with 18 A RMS and feature EtherCAT, CAN and RS485 interfaces all in a compact size of 36.8 mm long x 26.8 mm wide x 11.1 mm high. eliminating standalone PLCs. Part of the PLC’s endurance is related to cost. At least on the hardware side, the cost has consistently gone down over the decades, even as their capabilities have grown. For example, some PLCs are just a few hundred dollars. One caveat here is that PLC software sold through license agreements can increase overall design costs.

PACs A programmable automation controller (PAC) is an industrial controller that combines the functionality of a PLC with the processing capability of a PC. The term “Programmable Automation Controller” is generally accepted as having been coined by the ARC Advisory Group, which specified five characteristics that define a PAC: • Multi-domain functionality • A single, multi-discipline development platform • Flexible software tools that maximize process flow across machines or processes • An open, modular architecture • Compatibility with enterprise networks But with no industry-standard definition of a motioncontroltips.com | designworldonline.com

CONTROLLERS

Simplified Motion Series

PACs can operate in multiple domains simultaneously – such as motion control, process control, sequential control, logic, data management, and communication – using a single platform. Image courtesy of Opto 22

PAC, the distinction between PACs and PLCs is blurry. Higher-end PLCs now incorporate some of the characteristics described above and are encroaching on what was once considered PAC territory. In fact, many PLCs now include standard programming languages, the ability to expand functionality through add-on modules, and connectivity to various bus systems. However, PACs still differentiate themselves from PLCs by employing a more open architecture and modular design. They’re also more capable than PLCs at monitoring and controlling a large number of I/O, such as in a large processing plant or a complex automation system. They do this because data can be exchanged between devices and applications in different domains, such as motion and process control. And, a programmable automation controller can send and receive data to and from other PACs, creating a distributed control system of PACs. With large memory capacity, the ability to handle complex or high-speed analog I/O, and high-speed communication capabilities, PACs are wellDESIGN WORLD — MOTION

The simplicity of pneumatics combined with the advantages of electric automation The Simpliﬁed Motion Series is ideal for movements between mechanical end positions with a reduced number of parameters. • • • •

Operation without any software based on the plug and work principle Easy and quick commissioning without a computer, software or other accessories Connection via IO-Link for enhanced functionality including diagnostics Simple control with 2 digital inputs for basic functions or enhanced control with IO-Link for additional functions

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OMS Motion provides world-class motion control products and solutions for equipment and instruments requiring precise and dependable control of all types of motion and I/O. OMS offers 35+ years of proven industry trust and experience.

suited for vision applications, including vision-guided motion. The ability of PACs to gather, store, and track large amounts of data means they can handle predictive maintenance and operations monitoring. Data is often stored and accessed through an Ethernet network or a USB storage device. PACs also use the IEC 61131-3 programming languages (ladder diagram, function block diagram, sequential function chart, instruction list, or structured text), and some include standard PC programming languages such as C/C++, so familiarity is retained and the learning curve for programming is low, regardless of prior experience or expertise. PACs are best suited for applications that require complex controls — in automation, this often means multiaxis, coordinated motion or circular interpolation — while PLCs generally work well for simpler applications such as single-axis motion control.

PLC VS. DISTRIBUTED CONTROL

•

Multi-axis & Single-axis

•

Coordinated (or not)

•

Simple & full– Feature

•

Small Formfactor

•

Incremental and Absolute Encoders

•

Stepper and Servo motors

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(800) 707-8111

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While there are differences between a PLC and a distributed control system (DCS), there has also been a move towards greater convergence of the two technologies. In the early days of automation, PLCs dominated machine control – that is, applications calling for discrete control of machine processes. Meanwhile, a DCS was the predominant choice for process control in applications such as oil and gas and chemical plants. For the most part, the PLC/ DCS split still follows this historical pattern. PLCs still are used to control individual machines, while a DCS can control a larger number of machines or processes within a factory or plant. One hallmark of PLCs is their relatively simple operating system, designed to do a minimum of tasks like scanning inputs and updating outputs. This simple OS structure means they can execute programs quickly because there are not many other processes running in the background and demanding time

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PAGE TITLE CONTROLLERS WILL GO HERE

from the processor. PLC processing times are also quicker because they are naturally closer to the devices they control (motors, pumps, switches, etc.), so they are more responsive than a larger DCS would be because it is controlling much more than a single machine. PLCs are generally flexible and more readily customizable as well. On the other hand, a DCS can control many more machines and processes at the same time. In fact, they are often used to control entire systems within a factory. A DCS is likely to be more reliable as well, ensuring the continuous operation of a process or system. However, with more powerful PLCs and PACs available today, the line between a PLC and DCS continue to blur. For instance, PLCs are faster and can handle more complex control. They’ve also become more reliable, making them more like a DCS than ever before. As for the DCS side of the equation, they have become more nimble and adaptable like PLCs and are sometimes used in place of a PLC. So in terms of functionality, the two are growing more similar than dissimilar. The cost of a DCS is still more than a typical PLC, but even here the difference in cost is shrinking not expanding.

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MOTION SYSTEMS HANDBOOK

Heads or tails: A look at conveyor drive options Image courtesy mk North America

Conveyors are extremely versatile systems used for delivering bulk or discrete materials through processing operations or along assembly lines. One of the attributes that makes them so versatile is their ability to be conﬁgured in almost any layout imaginable, with curves, inclines or declines, and 90° or 180° turns to form complex travel paths. Another design feature that makes conveyors suited to a variety of layouts is that the drive (motor) can be placed at either end or, for some conveyors, at any location between the ends of the conveyor.

A drive located at the head or tail of the conveyor is generically referred to as an end drive. However, to designate which end the drive is located on, manufacturers typically refer to drives located at the head, or discharge end, as head drives and those located at the tail, or infeed end, as tail drives. But it’s important to note that some manufacturers use the term head drive in a more generic sense, simply to denote that the drive is located at either end of the conveyor, as opposed to somewhere along the length of the conveyor. A head drive — located at the discharge — is the preferred conﬁguration for most conveying applications, as it creates a pull force on the conveyor belt or media.

This is an ERT250 pallet-handling conveyor from Dorner Mfg. Corp. for cleanroom environments. Its use of rollers instead of belts avoids the release of dust. Energy-efficient gearboxes drive the rollers for positioning and zoning flexibility.

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CONVEYORS

A notable exception to this is a declined conveyor, where the weight of the transported load or the angle of the conveyor would create a pushing force that overrides the ability of the drive to control the conveyor’s movement. In these cases, a tail drive — placed on the infeed, or higher side, of the conveyor — would act as a brake to prevent the load from driving the conveyor.

Versatile. Flexible. Modular. Flat Top Chain Conveyors

CENTER DRIVES: LOCATED BETWEEN THE CONVEYOR ENDS A drive placed at the center, or anywhere along the conveyor’s length (but not at either end), is commonly referred to as a center drive, intermediate drive, or mid drive. Center drives are installed below the conveyor, directly under the transport media. Center drives are often used when there are space constraints at either end that prevent the use of end drives. A center drive may also be used when knife edges are required on both ends of the conveyor.

www.mkversaﬂex.com/dw The perfect solution for complex layouts: • Pallet or product handling • Inclines, curves and loops • Diverts and merges

better products. better solutions. DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

CENTER DRIVE

ERT 250 ®

Edge Roller Technology

NEW BELTLESS CONVEYOR

END DRIVE

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A conveyor with a center drive can also travel in both directions — sometimes referred to as reversing operation. Note that some of the terms used here — particularly infeed and discharge — are most commonly associated with conveyor systems that use belt or chain media to move products (either bulk products or individual items) from one location to another. The terms infeed and discharge are less applicable when talking about transfer conveyors or transfer systems — conveyors such as pallet conveyors that move discrete products through assembly, processing, or inspection stations. Typical nomenclature for these types of conveyors use the term end drive simply to designate a drive placed at either end of the conveyor.

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COUPLINGS

Applying couplings in motion designs 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 materialhandling 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. 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. 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 peak-torque 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.

Bellows couplings with tapered press-fit connections simplify connection of precision drive components by eliminating the need to access clamping screws inside mounting flanges and housings. Instead, the two coupling segments can be moun to their respective shafts at the correct axial location, and the “plug-in” bellows connection is completed once the driving and driven component frames mount together. The bellows provides a slight preload to the mating engagement of the two segments — eliminating backlash. Image courtesy R+W America

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MOTION SYSTEMS HANDBOOK FLEXIBLE COUPLINGS FOR MOTION

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 leadscrewdriven motorized axes or where there’s a need for attachment of a precision encoder, for example. Installation tip: With beam couplings, tighten one hub first and then (before tightening additional screws) rotate the coupling by hand to let it reach free length. Setting a beam coupling while it’s compressed or extended with shorten its life. Some particularly demanding designs may necessitate a flexible coupling type that maintains higher torsional stiffness. On the other hand, it’s also inadvisable 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. Specification tip: On any axis subject to jams, crashes, or emergency stops, safety couplings (though initially more expensive) are a better choice than standard servo couplings. The former can disengage to prevent damage and downtime. 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. That’s because overly stiff couplings of many designs pose an unnecessary risk of fatigue. Manage expectations: Not all servo couplings are meant to last as long as the machine. Where significant cyclical flexing to accommodate axis misalignment limits service life, couplings should be considered a wear item necessitating regularly scheduled replacement. Couplings for servo applications usually connect precision drives to sensitive loads, so they cannot induce any error. That’s why servo couplings should be zero backlash — to prevent issues with timing and predictability (not to mention failures due to hammering on reversing axes). Couplings for servo applications must also have high torsional stiffness while imparting slight forgiveness of misalignment (within specifications) of rotating shafts … even while holding transmitted rpm steady to motor output rpm. But these are just a couple servo coupling considerations. Consider a typical application for servo couplings — to connect a servomotor to a ballscrew. Here, couplings with low inertia let the axis deliver faster acceleration and deceleration without unnecessarily degrading overall system efficiency. In fact, couplings for servo designs must often compensate for subtle power-transmission issues to minimize errors down to 1 arc-min. or lower. That’s especially true where servo systems take the form of exacting positioning axes. Here’s a more complete list of parameters to consider for proper coupling operation in such designs:

SOME JAW COUPLINGS ARE FOR SIMPLER POWER-TRANSMISSION DESIGNS; LOOK FOR CURVED-JAW AND ZERO-BACKLASH DESIGNATIONS FOR MOTION.

STANDARD

τ MAX TO 7,000 LB-IN.

ALLOWABLE MISALIGNMENT

JAW (SPIDER)

ELASTOMERIC INSERT

BEAM

STANDARD

τ MAX TO 260 LB-IN. BEAM COUPLINGS ARE SINGLE-PIECE DESIGNS WITH NO ASSEMBLY POINTS TO LOOSEN. ATTACHMENTS ARE CUSTOMIZABLE.

BELLOWS STANDARD

τ MAX TO 900,000 LB-IN.

PHOTO COURTESY OF HUCO • ALTRA MOTION

TYPICAL TORSIONAL STIFFNESS

DISC ELEMENT STANDARD

Coupling type: Couplings shouldn’t be the last motion component speciﬁed because proper servo-machine function relies on having a suitable coupling in place. Torsionally rigid options (ideal for motion designs) include specialty bellows couplings, rubber-jaw couplings, and disc couplings. Curved-jaw couplings have good damping characteristics to optimize performance of axes with quick acceleration and deceleration. Elsewhere, both disc-type couplings and certain bellows couplings excel on high-speed axes. Other offerings abound to serve other design objectives. One caveat on coupling type though: Never use rigid couplings to replace

LESS

τ MAX TO 1,600,000 LB-IN.

RIGID RIGID COUPLINGS ONLY WORK ON EXCEPTIONALLY WELL-ALIGNED AXES.

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MOTION SYSTEMS HANDBOOK

ﬂexible servo couplings on axes where the latter seem to fail frequently. Here, this issue is most likely insufﬁcient alignment between the motor-output shaft and next component in the powertrain. There’s sometimes a misconception that rigid couplings are exceptionally strong, so can address issues in such situations. The fact is that rigid couplings only work when shafts are perfectly aligned, because these couplings transmit to connected motion components (potentially extreme) forces that arise from misalignment.

Sizing for torque and speed: After specifying the coupling type, the design engineer must select a coupling size. This is heavily dependent on axis speed (rpm) as well as the levels of torque the axis must transmit and the service factor the application requires. Axes that transmit steady torque are simpler to specify; in contrast, axes that transmit variable torque need additional consideration. Here, deﬁne the application’s average operating torque and the peak torque. Also consider the parameters listed in “Ability to handle reversals” below. One tip to avoid servo coupling oversizing: Quantify actual system requirements and base coupling selection on those values — and avoid deﬁning a whole axis by the connected gearmotor’s peak torque output.

Stiffness: Along with exacting control of position, force, or output velocity, it’s often essential to maintain high efficiency. Couplings that exhibit windup or backlash degrade this efficiency because they must overcome load inertia every move cycle. This can be a significant drawback in some setups ... which is why (especially on axes employing rigid variations) couplings should be prevented from inadvertently functioning as flywheels. Note that if a coupling’s torsional stiffness is insufficient, other system functions must compensate. One standard solution is to adjust PID controls and reduce servo gain, though that degrades system response and performance. In contrast, excessive torsional stiffness compromises the ability of an axis to withstand quickly reversing loads. That’s because servo couplings with excessive stiffness can be brittle and prone to failure on

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COUPLINGS ACCOMMODATE VARIOUS MISALIGNMENT TYPES Axial (sometimes called end float)

Parallel (lateral) sometimes called offset Angular

demanding axes that must make frequent and sudden directional reversals. Servo-application tip: Balance coupling characteristics for stiffness. Excessive torsional stiffness may induce premature failure. On the other hand, axes that must hold timing (as for positioning commands) beneﬁt from incorporation of torsionally stiff couplings.

Inertia: As mentioned, this is an important parameter for a few reasons. Applications with particularly aggressive motion proﬁles rely on low servo coupling inertia most of all.

Damping capabilities: Disc couplings, certain bellows couplings, and high-gain rubbertype couplings are all options for couplingbased damping in servo applications. In fact, the most demanding servo applications have in recent years spurred improved response frequencies ... but vibration (and hunting) arise with high gain settings on assemblies using torsionally stiff couplings. Visit couplingtips.com and search on “damping” for more on this issue and some solutions.

Shaft connections: Most servo couplings connect shafts with clamping or locking mechanisms (and not keyways). Though keyways are often offered as an option to prevent shaft slippage, the truth is that they can be a liability — adding concentrations of stresses in shaft connections, unnecessary cost, risk of imbalance, and other potential drawbacks.

8 • 2020

Torsional

Ability to handle reversals: Servo applications that must make quick directional changes require special consideration. Here, consider torque associated with system inertia starting and stopping. Service factors can often quantify the effect this value will have on assembly dynamics. Another aspect of reversing loads to consider is coupling-material fatigue. Keep in mind that some servo couplings that perform for years in regular applications with fail within weeks or sooner when forced to transmit power under reversing conditions.

Function to protect more expensive subcomponents: Though system failures are best avoided, couplings can be designed to protect the axis actuator or motor and gearbox by breaking if there is a machine crash or catastrophic overload. That’s especially useful in high-speed servo applications where drive-based current limits aren’t fast enough to address existing kinetic energy associated with the drivetrain and load upon a jam or sudden impact.

A realistic understanding of allowable misalignment: Flexible couplings for servo applications do accommodate misalignment. However, OEMs must be realistic about the level of permissible misalignment for a given axis — and specify assembly techniques and mounting that ensure levels that ever exceed the rating of the coupling. Otherwise, coupling or another component failure may occur.

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Disc couplings Disc couplings include two hubs of aluminum, stainless steeel, or other grades of steel that sandwich a pack of discs. Like a deck of cards, each thin element of this disc pack (sometimes called a disc spring) is the same size. This stack of discs is pressed together by bushing-fitted bolts that induce torque-transmitting friction between the discs. In some cases, the thin discs are made of stainless steel; in other instances, they’re made of engineered composite material. Because torque transmits through the faces of the discs, there’s no backlash or issues with stress concentrations that would arise in a coupling having only the bolt shanks to transmit torque. Friction operation also imparts torsional stiffness. Like jaw couplings, variations of disc couplings abound … with many satisfying applications characterized as heavy industry, power transmission, and process equipment. But other variations of disc couplings are applied in high-performance motion applications that require the precision transmission of torque and speed as well at the accommodation of shaft misalignment Typical maximum torques for all disc couplings are 100 to 678,000 Nm. Speeds from particularly specialized offerings are to 10,000 rpm.

DESIGN WORLD — MOTION

Single-disc couplings should perhaps be called single disc-pack couplings … because they include a single disc pack having many friction discs. In industry their less ambiguous name is single-flex couplings. In contrast, double-disc couplings (in industry referred to as double-flex couplings) have two disc packs — with an additional center hub between them. In some cases, this center spacer is of the same material as the two end hubs; otherwise the component supplier may offer center spacers made of something else — acetal for electrical isolation, for example. Other disc-coupling variations abound. Some include laminated discs; discs of circular or hexagonal or other shape • Attachments through the disc pack or alternating between the two hub flanges • Features to satisfy API, ATEX, and other standards. Although typical single-flex couplings incorporating steel discs accommodate no parallel misalignment, this limitation isn’t universal: Those with composite discs can accommodate a modest amount of parallel misalignment. That’s useful in compact machine footprints that don’t allow the extra length of a double-flex coupling. But where longer axes are acceptable and more dramatic parallel misalignment is unavoidable, double-flex couplings leverage the ability of two discs to flex in opposing directions for accommodating offsets.

8 • 2020

Zero-Max ServoClass couplings (available in single and doubledisc models) excel in servo and stepper-motor-driven applications — especially axes subject to shock loading from rapid acceleration and deceleration, start-stops, and torque reversals. In most cases, ServoClass couplings aren’t as radially stiff as bellows couplings, which reduces the reaction loads on connected components. In addition, the misalignment capacity and high torsional stiffness of ServoClass couplings avoid the issues associated with spider fatigue and torsional windup seen in some jaw couplings.

DISC COUPLINGS ≠ DIAPHRAGM COUPLINGS Diaphragm couplings — a completely different design than disc couplings — are nevertheless confused with and mistakenly substituted the latter. Note that diaphragm couplings transmit torque via one or more thin diaphragms. The diaphragm or diaphragm pack attaches to one shaft hub near its inner diameter and to the other near its outer diameter. Deflection on this attachment differential’s freespan accommodates both axial and angular misalignment.

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• Spring-loaded indexing plugers with or without lock-out. • Designed to lock devices in-place for adjustable positioning.

from

MOTION SYSTEMS HANDBOOK

Jaw couplings Jaw couplings are one coupling type that (depending on the subtype) is suitable for power transmission or servo designs. They come in an array of sizes to accommodate shaft diameters of 3 to 150 mm and beyond. • Standard industrial jaw couplings have spiders with straight spider arms. Such geometry introduces an oftennegligible amount of play to make assembly easier. These jaw couplings excel in many applications — though not on axes involving highly precise positioning of loads. Some manufacturers refer to these standard couplings as L-jaw couplings. Such couplings are common in process applications employing electric as well as hydraulic motors and turbines and even gasoline and diesel engines. • In contrast, zero-backlash jaw couplings include spider arms with a profile ... necessitating a press fit so there’s no clearance between the spider arms and jaws. Some manufacturers refer to these curved-arm couplings as C-jaw couplings. One representative use of these jaw couplings is on belt-drive linear actuators (tangential belt slides) for the positioning of loads. Such step and servomotor-driven actuators can join the belt pulley and motor … but where the use of servo controllers excite resonances (and other design changes are prohibited) a jaw couplings excel. Their ability to damp axes and move axis operating frequencies away from the assembly resonant frequency is indispensable.

In fact, some of the largest zero-backlash jaw couplings maintain zero backlash with a heavy press fit ... sometimes requiring assembly with precision press equipment. Despite more challenging assembly, large zero-backlash jaw couplings benefit assemblies by preventing any lost motion in the coupling. Many smaller zero-backlash jaw couplings go together by hand pretty easily. Have quick reversals? No worries. There are multiple spider materials to match jaw couplings to application needs. Consider how standard jaw couplings deftly handle shock loads. If the application at hand requires the coupled axis to quickly start and stop, a rigid coupling (or one without cushioning or damping characteristics — such as a zero-backlash jaw coupling) will transmit that shock and can actually accelerate the destruction of the assembly’s bearings on the ballscrew input or motor. In contrast, compliant jaw couplings handle shock loads quite well — even on axes with frequent stops and starts for high throughput. Here, requiring that the axis decelerate more gently or run softened move profiles just to accommodate a bellows coupling or a rigid coupling is unacceptable. Jaw couplings here impart excellent protection for the life of the

design’s motor or actuator — and lets the axis run aggressive move profiles without sustaining damage. Spider materials and spider hardnesses abound. For example, there’s 98 Shore A, 92 Shore A, and 85 Shore A — sometimes more casually expressed as just 85 durometer. Depending on the amount of cushioning or damping the axis needs, design engineers can fine tune the coupling behavior to suit by strategically choosing from these different spider materials. Jaw couplings are an inherently failsafe design: If the jaw coupling’s spider is catastrophically damaged, its arms tear off ... and the coupling will go metal-to-metal and still drive. This is useful on applications where it’s important to have a fail-safe design or positive drive. Even zero-backlash jaw couplings work in this manner. Case in point: The vertical (Z axis) of a semiconductor handling machine might be responsible for lifting and lowering an expensive boat of wafers for wafer processing. Here, suddenly dropping the valuable load could be an expensive error. So to prevent dropping that load in case of failure (and having the screw back drive) a jaw coupling here will act as the positive drive. One drawback of jaw couplings is that they have fairly low misalignment capabilities ... so require require precision alignment at installation. They do impart a bit of forgiveness, but nothing like beam or other couplings. So if an axis’ halves are misaligned beyond what the jaw coupling is capable of accommodating, that coupling’s will hubs will go in shear, and the spider will degrade ... which in turn results in excessive loading on the axis’ bearings.

Precision ball-detent torque limiters provide mechanical overload protection that’s indispensable on rotating axes at risk of damage from inertial shock loads resulting from binding and collisions … especially on more modern machinery with faster motor shaft speeds. By providing an overload safety breakaway, the limiters react to actual mechanical torque overload far faster than systems that merely monitor servo amperage draw. Shown here is an SLE torque limiter from R+W America with an elastomeric insert for misalignment compensation.

DESIGN WORLD — MOTION

8 • 2020

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MOTION SYSTEMS HANDBOOK

Fundamentals of encoders for motion control Encoders can be classified in several different ways – by the sensing method, whether the output is absolute or incremental, as well as by some common performance parameters. They’re also commonly divided into rotary (the more common) or linear types.

SENSING METHODS The most common sensing methods used by encoders include optical, magnetic, and capacitive sensing. 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. Optical encoder performance is influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity isn’t compromised. This means that mounting must be done carefully, and shocks and vibrations should be avoided. Optical encoders have historically been the only option for resolutions below 5 microns.

However, improvements in magnetic scale technology now allow them to achieve resolutions down to 1 micron. 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. Shock and vibration are also less likely to affect magnetic encoders. However, they are sensitive to metallic chips, such as steel or iron, as they may interfere with the magnetic field. Capacitive encoders offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. The basic principle behind capacitive encoders is that they detect changes in capacitance using a high-frequency reference signal. This is accomplished with three main parts—a stationary transmitter, a rotor, and a stationary receiver. The rotor is etched with a sinusoidal pattern, and as it rotates, this pattern modulates the high-frequency signal of the transmitter in a predictable way. The receiver disk reads the modulations, and on-board electronics translate them into increments of rotary motion. The main concern when using capacitive encoders is their susceptibility to noise and electrical interference, though there are methods to minimize these effects.

OUTPUT TYPES There are two types of encoder outputs; absolute or incremental. Absolute encoders have a unique code for each shaft position, so that every position is distinct. The encoder interprets a system of coded tracks to create position information where no two positions are identical. Absolute encoders also don’t lose position if there is power loss. Because each position is distinct, true position is verified as soon as power is restored, eliminating the need for a homing routine. Absolute encoders can be further classified as 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, multi-turn encoders are better for applications that involve complex or lengthy positioning requirements. Absolute encoders have a number of advantages, with safety being a key advantage. 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 wont build up so an accurate position is presented again on the next reading.

Encoders from Posital are available with IP69K level environmental protection. They’re designed to withstand repeated exposure to the hot, high-temperature water jets used in pressure washing systems, and are available with a wider range of mechanical options including more shaft diameters and more flange types, hub and square. Interface options includes analog, CANopen, J1939, SSI, Modbus and IO-Link, with HTL/TTL serial interfaces for incremental encoders.

DESIGN WORLD — MOTION

8 • 2020

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MOTION SYSTEMS HANDBOOK

NEW HOLLOW-SHAFT KIT ENCODERS

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Incremental encoders generally supply square-wave signals in two channels, A 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 is to use a battery backup. Such a system ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored.

COMMON PERFORMANCE METRICS

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Key to a good match between encoder and application is the resolution and accuracy. In other words, the ideal encoder has both the necessary resolution and is highly accurate. Resolution is the distance over which a single encoder count takes place – it’s the smallest distance the encoder can measure.

DESIGN WORLD — MOTION

Rotary encoders, such as these from HEIDENHAIN, come in a wide variety of configurations with different options such as with shafts, shaftless, or with a hollow through-shaft, and with or without an integral bearing. For rotary encoders, resolution is typically specified in terms of measuring units, or pulses, per revolution (PPR). Linear encoder resolution is most commonly specified as the distance over which the count takes place and is given in terms of microns (μm) or nanometers (nm). The resolution of an absolute encoder is specified in bits, since absolute encoders output binary “words” based on the encoder’s position. Accuracy is the difference between the true position (or speed) of the device being measured and the position (or speed) reported by the encoder. For rotary encoders, it is specified in arcseconds or arcminutes, and for linear encoders accuracy is typically given in microns. Note that higher resolution does not mean higher accuracy. Consider two encoders – one with 100 PPR resolution and one with 10,000 PPR resolution, but both with the same accuracy specification. The lower resolution (100 PPR) encoder can report a movement of 90 degrees just as accurately as the higher resolution (10,000 PPR) model. The higher resolution encoder just has the ability to break up that 90 degree movement into much smaller increments. An encoder’s resolution is based on the 8 • 2020

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ENCODERS These StepSERVO integrated motors from Applied Motion Products are equipped with HEIDENHAIN’s RENCO encoders. The RCML 15 rotary encoders are compact and reliable and provide easy-to-integrate motion control performance. The companies have collaborated to expedite critical components for the manufacture of ventilators, especially critical during the COVID-19 pandemic.

number of lines (for an incremental encoder) or the pattern (for an absolute encoder) on the encoder disk or scale. Physically, resolution is ﬁxed. Once an encoder is manufactured, there is no option to add more lines or patterns to the code disk. But the resolution of an incremental encoder can be increased through signal decoding. Incremental encoders output square wave signals, and by counting both the leading and trailing edges of one signal (signal A), the resolution of the encoder is doubled. When the leading and trailing edges of both signals (A and B) are counted – referred to as quadrature decoding – resolution is increased by a factor of four. While the number of lines or measuring

units determines resolution, accuracy is affected by the width and spacing of these lines or units. Inconsistent width and/or spacing will cause errors in the timing of the pulses. For absolute encoders, accuracy is inﬂuenced by the precision with which the pattern is placed on the code disk. Factors external to the encoder can also affect its accuracy. These include the rigidity of the assembly and mounting errors, such as a lack of concentricity between the encoder disc and the shaft to which it’s mounted. For linear encoders, thermal expansion of the scale and of the mounting surface can also degrade accuracy.

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MOTION SYSTEMS HANDBOOK

Pitch line velocity in gearbox sizing In gearbox sizing, it’s key to determine the required input speed in rpm and ensure that it doesn’t exceed the gearbox maximum-speed capability. But the linear velocity of the gear teeth known as pitch line velocity also plays a signiﬁcant role in gearbox performance. Pitch line velocity is measured at the pitch line of the gear, which is midway along the length of the gear teeth. For circular gears, the pitch line is more correctly referred to as the pitch circle — an imaginary circle that rolls without slipping when aligned with the pitch circle of the mating gear. Strictly speaking, pitch line is the correct term when referring to a linear gear rack, and pitch circle is the correct term when referring to a circular gear. However, the term pitch line is often used when discussing the equivalent linear velocity of a circular gear — as in pitch line velocity. Pitch line velocity is a function of the gear’s pitch diameter and its rotational speed:

Where PLV = pitch line velocity (m/sec); dp = pitch diameter (m); ω = rotational speed (rpm) Pitch line velocity is important for gear design and selection for several reasons. The American Gear Manufacturers Association standard 9005-D94 Industrial Gear Lubrication speciﬁes that a gear’s pitch line velocity is one of the primary criteria for selecting gear lubrication. Pitch line velocity also determines the contact time between gear teeth, which has a signiﬁcant impact on the required oil viscosity. High pitch line velocities are usually accompanied by light loads and short contact times, making low-viscosity oils suitable. However, low pitch line velocities are associated with high loads and long contact times, which make high-viscosity or even EP-rated oils necessary. In addition to lubrication considerations, pitch line velocity also affects the load capacity and service life of gear teeth. The

DESIGN WORLD — MOTION

8 • 2020

This AFXR series 90° helical bevel gearbox from Apex Dynamics features a short yet rigid housing and full compatibility with standard motor adapters.

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MOTION SYSTEMS HANDBOOK

MATING GEAR PITCH DEFINITIONS LINEAR PITCH PINION

RACK PITCH LINE RACK PITCH DISTANCE

PINION PITCH RADIUS

PINION PITCH CIRCLE

RACK

ability of gears to transmit the required torque for the desired operating life depends on the ability of the gear teeth to withstand bending stress. Tooth bending stress is determined according to the Lewis formula:

Thus the higher the pitch line velocity, the greater the bending stress on the gear teeth. Note: The AGMA has developed an equation for bending stress that replaces the Lewis Form Factor with a geometry factor J and includes factors for other conditions that affect gear service life such as overload, load distribution, and mounting.

Where σ = Tooth bending stress (MPa) Wt = Tangential force on tooth (N) and P = Diametrical pitch (mm-1) F = face width (mm); Y = Lewis form factor

TECHNOLOGY DEEP DIVE: HELICAL GEARS

But as gear teeth come into initial contact, they experience greater stresses, based on the velocity of the gear. In order to account for these stresses, a velocity factor Kv was developed. The velocity factor depends on both the pitch line velocity of the gear and the quality of the gear (Qv) and can be obtained from AGMA charts. This velocity factor Kv is used to modify the Lewis equation:

DESIGN WORLD — MOTION

Helical gears and spur gears are two of the most common gear types and can be used in many of the same applications. Spur gears are simple and inexpensive to manufacture, but helical gears offer some important advantages over spur gears. The teeth of a helical gear are set at an angle (relative to axis of the gear) and take the shape of a helix. This allows the teeth to mesh gradually, starting as point contact and developing into line contact as engagement progresses. One of the most noticeable beneﬁts of helical gears over spur gears is less noise, especially at medium- to high-speeds. Also, with helical gears, multiple teeth are always in mesh, which means less load on each 8 • 2020

The velocity factor depends on both the pitch line velocity of the gear and the quality of the gear (Qv) and can be obtained from AGMA charts as the one shown here. Of course, design engineers should require their gear suppliers to disclose in detail the methods used to ensure published quality values. individual tooth. This results in a smoother transition of forces from one tooth to the next, so that vibrations, shock loads, and wear are reduced. But the inclined angle of the teeth also causes sliding contact between the teeth, which produces axial forces and heat, decreasing efﬁciency. These axial forces play a signiﬁcant role in bearing selection for helical gears. Because the bearings have to withstand both radial and axial forces, helical gears require thrust or roller bearings, which are typically larger (and more expensive) than the simple bearings used with spur gears. The axial forces vary in proportion to the magnitude of the tangent of the helix angle. Although larger helix angles provide higher speed and smoother motioncontroltips.com

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MOTION SYSTEMS HANDBOOK SPUR GEAR VERSUS HELICAL GEAR

POWER SPECTRUM (dB)

The straight teeth of spur gears are easier to manufacture, so are cost effective.

Angled gears are longer for higher load capacity and quieter engagement.

FREQUENCY (Hz)

motion, the helix angle is typically limited to 45° due to the production of axial forces. The axial loads produced by helical gears can be countered by using double helical or herringbone gears. These arrangements have the appearance of two helical gears with opposite hands mounted back-to-back, although in reality they are machined from the same gear. Note that the difference between the two designs is that double helical gears have a groove in the middle, between the teeth, whereas herringbone gears do not. This arrangement cancels out the axial forces on each set of teeth, so larger helix angles can be used. It also eliminates the need for thrust bearings. Besides smoother motion, higher speed capability, and less noise, another advantage that helical gears provide over spur gears is the ability to be used with either parallel or non-parallel (crossed) shafts. Helical gears with parallel shafts require the same helix angle, but opposite hands (i.e. right-handed teeth vs. lefthanded teeth). When crossed-axis helical gears are used, they can be of either the same or opposite hands. If the gears have the same hands, the sum of the helix angles should equal the angle between the shafts. The most common example of this are crossed helical gears with perpendicular (90°) shafts. Both gears have the same hand, and 90° is the the sum of their helix angles. For

DESIGN WORLD — MOTION

FREQUENCY (Hz)

The teeth of a helical gear are set at an angle (relative to axis of the gear) and take the shape of a helix. This allows the teeth to mesh gradually ... starting as point contact and developing into line contact as engagement progresses.

configurations with opposite hands, the difference between helix angles should equal the angle between the shafts. Crossed helical gears provide flexibility in design, but the contact between teeth is closer to point contact than line contact. That means they have lower force capabilities than parallel-shaft designs — so are not the most suitable for heavy loads or axes needing very dramatic speed reductions. Helical gears are often the default choice in applications that are suitable for spur gears but have non-parallel shafts. They are also used in applications that require high speeds or high loading. Plus regardless of the load or speed, they generally provide smoother and quieter operation than spur gears.

8 • 2020

MOTION SYSTEMS HANDBOOK

Servomotor and drive fundamentals The 2G Motion System from Kollmorgen is based on the latest thinking about single-source, systems-based design. It harnesses the capabilities of Kollmorgen’s AKD2G servo drive and AKM2G servomotor. Motor and drive are precisely matched in every element, from drive switching frequency and commutation algorithms to motor magnetics. This avoids microincompatibilities that can occur when engineers select components from various manufacturers.

Perhaps the distinguishing feature of all servomotors is that they output motion under closed-loop control. They rely on feedback from encoders as well as control signals from a controller and drive to operate the motor. Most servomotors are rotary motors and produce precise torque and speed, most often used in positioning applications. Manufacturers classify motors for constant-speed tasks by horsepower or torque at base speed. In contrast, servomotors operate over varying

DESIGN WORLD — MOTION

8 • 2020

speed ranges and aren’t rated in this way. Instead they have speed-torque curves that express continuous torque capabilities (that won’t threaten to overheat the motor) and intermittent or peak torque for acceleration. Keep in mind that the term servomotor can mean different things depending on the context. Convention is that the term often (though not always) refers to what industry calls dc motors — both brushed and the costlier (but longer-lived) brushless servomotors. There are also instances of classifying induction-motor-based designs running off vector controls as servomotor setups where the design incorporates feedback (usually from an encoder) to track and control speed and sometimes even position. These induction motors motioncontroltips.com | designworldonline.com

SERVOMOTORS & DRIVES

typically adhere to NEMA or metric standards, whereas other servomotor offerings are less uniform. 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.

SELECTING A SERVOMOTOR Here’s a quick guide to selecting the right servomotor for an application. First, start off by using the application inertia information to deﬁne how much load the motor will have to move. Next, determine the application speed or velocity, and how far and fast the load needs to travel. Calculating torque is next. Then, plot them on the prospective motors’ torque-speed curves — as the servomotors’ continuous and peak torque limits over the axis’ full speed range. This bare-bones approach to picking and sizing a servomotor is a start, as the process may be a bit more complex. Still, plenty of manufacturers offer software programs to help simplify the selection process. What’s more, once a designer has 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. Servo systems are used when the application calls for precise control of position, velocity, or torque — or a combination of the three. Depending on the parameter being controlled, the servo system can be operated in torque mode, velocity mode, or position mode. Each mode requires control loops that allow the servo drive and controller to monitor the inﬂuencing parameters and provide the right commands to the motor to achieve the desired performance.

SERVO CONTROL – TORQUE MODE In torque mode (also referred to as current mode), the current loop controls the motor’s behavior. Since torque is directly proportional to current, the servo controller obtains the actual motor current from the servo drive and uses this to determine actual motor torque. It then compares the actual torque value with the desired torque and adjusts the current delivered to the motor to achieve the desired torque. The current control loop is typically tuned with a PI (proportionalintegral) controller, and current loop parameters are often set by the manufacturer. Applications that require torque mode control range from winding, where constant tension must be held on a web of material while it’s wound, to injection molding, where constant clamping force must be applied to the mold. The amount of torque a motor produces depends on the amount of current it receives. And torque determines the motor’s acceleration, which affects velocity and position. Thus, servo systems always include a current control loop. 8 • 2020

DESIGN WORLD — MOTION

Newer hybrid motor designs, like the TSM14POE StepSERVO integrated motor from Applied Motion Products, offer closed-loop servo control using high torque step motors. It uses a power over Ethernet (PoE) single-cable to satisfy motion-control applications requiring minimal wiring and a small footprint. Combining power and Ethernet communications in a single connector simplifies wiring to the integrated motor so it can mount almost anywhere.

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MOTION SYSTEMS HANDBOOK

SERVO CONTROL – VELOCITY MODE When an application requires that the motor maintains a set speed, even under varying loads, velocity mode is used. In velocity mode, the motor speed is controlled by the amount of voltage sent to the motor. But to change the motor’s velocity (to accelerate or decelerate) requires an increase or decrease in motor torque, so a current control loop is also required in velocity mode. When more than one control loop is used, the loops are cascaded, with current control being the innermost loop and the velocity control loop added “around” the current loop. When a position control loop is used, it’s added around the velocity loop, forming the outermost loop. Tuning is done from the inner to the outer loop, so the current loop is tuned ﬁrst, then the velocity control loop, then the position control loop. Many advanced servo controllers can switch between control modes “on the ﬂy” — transitioning from velocity mode to torque mode, for example, while the system is in operation without creating instabilities or interruptions. The velocity control loop obtains speed information from an encoder or resolver to determine the error between actual and commanded velocity and uses this error to determine what current (torque) is required for the motor to correct the speed error.

RELIABLE MOTION SOLUTIONS.

Why is the bandwidth of a servo control loop important? A servo drive can include any combination of three types of control loops—a

position loop, a velocity loop, and a current loop. While each loop’s purpose is to control a different aspect of the motor’s performance, they are all characterized by a common parameter: bandwidth. The bandwidth, or response time, of the system is a measure of how fast it responds to the changing input command. In other words, the bandwidth of the control loop determines how quickly the servo

system responds to changes in the parameter being controlled—position, velocity,

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or torque. In servo drives, the bandwidth of a control loop is defined as the frequency at which the closed-loop amplitude response reaches -3 dB. At this point, the output gain (ratio of output to input) equals approximately 70.7% of its maximum, and the output power (power delivered to the load) equals 50% of the input power. While higher bandwidth generally provides stiffer motor performance, decreases error, and improves transient response time, there are also drawbacks to high bandwidth in servo systems. Specifically, the higher the bandwidth, the higher the frequency at which the motor responds to disturbances, which typically requires higher accelerations and forces. Power dissipation has a squared relationship to force, so any increase in bandwidth significantly increases power dissipation (i.e. heat), and therefore, the temperature rise of the motor. And because temperature is a limiting factor in motor operation, the motor characteristics may actually limit the allowable bandwidth of the servo drive. Note: Other components in the system — including the resolution of the feedback device, the update rate of the drive, the motor-load inertia ratio, and the rigidity of the motor-load coupling — also affect the maximum achievable bandwidth of the drive.

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8 • 2020

SERVOMOTORS & DRIVES

Feedback gains determine how hard the servo tries to correct or reduce the error between the commanded position and the actual position. Proportional gain (P) is dependent on present error, integral gain (I) is dependent on past error, and derivative gain (D) is dependent on predicted future error. Image courtesy of Thorlabs, Inc.

The velocity control loop is typically a PI controller, and servo systems operating in velocity mode sometimes include parameters that smooth acceleration and deceleration to minimize the effects of jerk. Examples of applications that use velocity mode are conveyor tracking, dispensing, and machining processes such as grinding or polishing, where motor load varies but velocity needs to be maintained throughout the process.

WHEN THE APPLICATION CALLS FOR ALL THREE CONTROL LOOPS Servo systems can also be operated in position mode, allowing the motor to move the load to a precise location, either relative to a starting location or based on an absolute position. To achieve position mode in servo control, all three control loops are typically required: torque, velocity, and position. This is because the motor’s speed must be monitored to determine its position, and torque must be monitored to determine how much current the motor needs in order to reach the commanded position, without undershoot or overshoot. The position control loop uses a PI or a PID (proportional-integral-derivative) controller.

WHAT ARE SERVO FEEDBACK GAINS, OVERSHOOT LIMITS, AND POSITION ERROR LIMITS?

NEWe!nt Kit

Servo tuning can be accomplished by several methods, but the most common way is to use a PID algorithm. The PID algorithm uses three feedback gains—proportional gain, integral gain, and derivative gain—to compare the commanded position (or velocity) with the actual value and issue commands to correct errors between the two.

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FEEDBACK GAINS Proportional gain (Kp) determines the amount of restoring force (generated by the command voltage) that is applied to overcome the position error. The term “proportional gain” is used because 8 • 2020

DESIGN WORLD — MOTION

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SERVOMOTORS & DRIVES

Parameter increased

Rise time

Overshoot

Settling time

Steady-state error

Stability

Decrease

Increase

Small change

Decrease

Degrade

Decrease

Increase

Decrease significantly Degrade

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No effect

its value is directly proportional to the positioning error. For example, if the proportional gain is 1.2 volts per encoder count, and the motor is 10 encoder counts from the commanded position, the command voltage will be 12.0 volts. Proportional gain is the most important component of the PID algorithm, but a Kp value that is too high can cause the system to oscillate, to become under-damped, or to become unstable. Often, as the servo controller works to decelerate the motor by reducing the command output, system friction overcomes the command voltage and causes the motor to fall short of the target. Integral gain (Ki) overcomes this by producing a command that “pushes” the system to zero positioning error at the end of the move. The term “integral gain” is used because its command increases over time at the end of the move. If sufficient positioning accuracy is achieved with the proportional gain, then integral gain may not be necessary. However, it is useful when steady-state (static) positioning is difficult to hold due to system disturbances, or when constant velocity motion is required. Derivative gain (Kd) determines the restoring force that is proportional to the rate of change (derivative) of the positioning error. It works in conjunction with proportional gain to dampen the system response and reduce overshoot and oscillations. Servo instability can occur if the derivative gain value is calculated too frequently, as it will begin to work against, rather than with, the proportional gain. To avoid this, the derivative sampling period can be increased.

OVERSHOOT LIMITS Derivative gain is used to set the overshoot limit, or the acceptable amount by which the servo can exceed the target position. While perfectly accurate positioning is desirable in theory, in real world applications, an overshoot limit that is too small will likely cause the system to be over-damped. Conversely, a very high overshoot limit gives good system response, but can lead to oscillations.

POSITION ERROR LIMITS The principle behind servo control is the comparison of the system’s actual value versus the commanded value, and the difference between the commanded and actual position is the position error (also referred to as the following error). The purpose of feedback gains is to reduce the position error. The position error limit, however, is used to indicate a problem with the servo or with the tuning algorithm. The position error limit should be set to a value that would not be seen during normal operation, and the controller response should be programmed to stop the axis when this limit is reached or exceeded.

8 • 2020

DESIGN WORLD — MOTION

Improve (for small Kd)

Each parameter of a PID loop affects the servo response differently, but in the end, they work together to minimize position error and overshoot. Image courtesy of Thorlabs, Inc.

EC MOTORS

Update on EC motors In the motion control world, a relatively new motor distinction that is becoming more widespread in motion-control circles is the term electronically commutated or EC motor. So how does this new EC motor fit into the existing hierarchy? The short answer is that EC motors are not a new technology. They are simply permanent magnet brushless dc (BLDC) motors that are being distinguished by their method of commutation (electronic) rather than by their physical characteristic of lacking brushes. These motors — whether called brushless dc or electronically commutated — have a permanent magnet rotor with a wound stator. Electronics determine the sequence for commutation or energizing of the stator windings based on the rotor position, which is most often provided by either three Hall sensors or a rotary encoder ... although there are methods of determining rotor position without additional feedback devices. The term electronically commutated motor is already used extensively in the HVAC and refrigeration markets. That’s especially true for equipment such as fans, blowers, and compressors — all of which have traditionally been powered by ac induction motors … the inefficiency of which has not gone unnoticed by the United States Department of Energy. The DoE’s most recent Small Motor Efficiency Rule and Integral Horsepower Motor Rule — with various DoE rules specific to HVAC and refrigeration equipment (think blowers and compressors) as well as to pumps and fans — have compelled equipment OEMs to transition from ac induction motors to EC motors for many types of applications and equipment.

SLOTLESS VERSUS SLOTTED BRUSHLESS DC MOTOR SLOTLESS MOTORS DELIVER SMOOTH MOTION.

Reduced eddy currents and no detent torque for smooth rotation Compact encapsulated windings

Rotor diameter and rare earth magnets that are larger than those of traditional brushless designs (for power density) TRADITIONAL BRUSHLESS MOTORS INCLUDE HIGH-TORQUE VARIATIONS.

DIFFERENCE BETWEEN SLOTTED AND SLOTLESS MOTORS The original BLDC motors were designed with slotted stators, and the majority of BLDC motors are still made this way. But this design produces cogging torque, which makes it difficult to achieve smooth motion, especially at slow speeds. To eliminate this effect, a new design was developed to eliminate the slots in the stator (which are the root cause of cogging torque) and slotless motors were born. In a slotted motor design, the stator is made of slotted steel laminations that are stacked together, and copper windings are inserted into these slots. The design of the stator is sometimes referred to as having teeth. The slotted motor design is simple and inexpensive to motioncontroltips.com | designworldonline.com

8 • 2020

DESIGN WORLD — MOTION

PAGE TITLE WILL EC GO MOTORS HERE

motioncontroltips.com | designworldonline.com

TORQUE (FORCE) RIPPLE VERSUS ELECTRICAL ANGLE FOR SLOTTED MOTORS PHASE-TO-PHASE BACK EMF

RESULTING TORQUE FROM ALL THREE PHASES OPERATING TOGETHER

0.5

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60° 90° 120° 150° 180° 210° 240° 270° 300° 330° 360°

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-1

COMPROMISED RESULT OF COGGING TORQUE

TORQUE (FORCE) RIPPLE VERSUS ELECTRICAL ANGLE FOR SLOTLESS MOTORS 1

PHASE-TO-PHASE BACK EMF

manufacture, but it has a major drawback—cogging torque. Cogging torque (also referred to as detent torque) is a result of the permanent magnets in the rotor attempting to line up with the slots, or teeth, of the stator. The primary effect of cogging torque is that it causes motor rotation to be jerky (not smooth), especially at low motor speeds. The term iron core is commonly used when discussing slotted linear motors. This originates from the fact that the assembly of stator windings is encased in a steel — so the slotted stator is sometimes referred to as an iron core. Slotless linear motors on the other hand are often called air core motors. In a slotless motor, there are no iron teeth to support the windings. Instead, the stator lamination is constructed of steel rings that are stacked together, and the windings are encapsulated in an epoxy resin, which gives the winding structure shape and rigidity. This “self-supporting” winding is placed in the air gap between the stator lamination and the rotor. The primary benefit of a slotless motor design is that the lack of teeth in the lamination eliminates cogging torque and results in a motor with very smooth running characteristics. Torque production is predictable and highly controllable, because in the absence of these uncontrolled disturbances (such as cogging torque) motor torque production is directly related to the current supplied to the winding. There are other benefits to the slotless design. First, the elimination of cogging also significantly reduces audible noise. Because there’s no iron core, inductance is very low and current can get into the stator windings very quickly, making slotless motors good for applications that require high acceleration and dynamic response. But slotted motors still hold some advantages. For example, the air gap in a slotted motor is smaller than the air gap in a slotless design — which must accommodate the self-supported winding assembly. This means that the flux density is higher in a slotted motor, and torque production is more effective and efficient. One way that manufacturers of slotless motors overcome the effects of the bigger air gap is to use larger and stronger permanent magnets in the rotor — though they’re costlier. In addition, the cost to manufacture the self-supporting winding structure is typically higher than that of a conventional slotted design. For these reasons, the traditional slotted motor design is still the first choice for applications in which smooth running and the elimination of cogging are less critical than other design objectives.

RESULTING TORQUE FROM ALL THREE PHASES OPERATING TOGETHER

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THEORETICAL SINUSOIDAL TORQUE VERSUS ANGLE ELECTRICAL ANGLE (DEGREES)

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DESIGN WORLD — MOTION

MOTION SYSTEMS HANDBOOK

Positioning stages and tables One of the most common types of integrated motion systems

TABLES

is positioning stages or tables. These systems consist of a number of

So-called X-Y tables are similar to X-Y 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 X-Y Cartesian systems and X-Y tables lies in how the load is positioned. Instead of being cantilevered as in a typical Cartesian system, the load on an X-Y table is almost always centered on the Y axis, with no significant moment created on the Y axis by the load. X-Y 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. Designs referred to as

common motion components including motors (either rotary or linear) and linear actuators, as well as controllers, encoders and other sensors.

POSITIONING STAGES Positioning stages provide one of several different types of motion. They can be linear, rotary or even lift types (Z-axis positioning stages). Among these, they can be configured in many different ways including movement in one direction (or axis) only, in multiple directions (X-Y positioning), or for extremely small and precise movements, as in nanopositioning applications where moves are in the micro- or nanometer range. Depending on a number of factors including cost and desired accuracy, the drive mechanisms for positioning stages and tables can vary significantly. For instance, stages can be direct-drive types driven by linear servomotors or by a combination of motors, gears and couplings. They can be linear or rotary actuator driven, either using electric actuators or other types. Some other common methods include belt and pulley systems, ball screws or lead screws. Precision and accuracy requirements can also dictate design decisions such as what components to use in assembling a positioning stage. For stages requiring reliability and high accuracy, air bearings are often used to minimize friction. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. They’re typically referred to as aerostatic bearings, because a source of pressure rather than relative motion supplies the film of air. For instance, so-called planar stages are typically constructed of air bearing guides and linear motor drives. Unlike ordinary bearings, the surfaces of an air bearing do not make mechanical contact, so these systems don’t need lubrication. Because the surfaces do not wear, the systems don’t generate particulates, which makes them suitable for clean-room applications. When supplied with clean, filtered air, the bearings can operate without failure for many years.

DESIGN WORLD — MOTION

8 • 2020

Many manufacturers offer software selection tools to help engineers size and pick the right stage or table for a given application. For example, simulation software from PI (Physik Instrumente) helps determine if a hexapod (or Stewart platform) is suitable for a specific positioning task, in terms of workspace, load, center of mass, and operating orientation.

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POSITIONING STAGES & TABLES

How to define roll, pitch, and yaw for linear systems Linear guides and systems —

including Cartesian robots, gantry systems, and X-Y tables — are typically subjected to both linear forces due to downward, upward, and side loads and rotational forces due to overhung loads. Rotational forces — also referred to as moment forces — are typically defined as roll, pitch, and yaw, based on the axis around which the system tries to rotate. A moment is caused by a force applied at a distance. A moment force does not cause rotation, although it is often confused with torque, which is a force that does cause a body to rotate about an axis. To define roll, pitch, and yaw in linear systems, we first need to establish the three primary axes: X, Y, and Z. The two axes of the horizontal plane are typically defined as X and Y, with the X axis

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being in the direction of motion. The Y axis is orthogonal (perpendicular) to the direction of motion and is also in the horizontal plane. The Z axis is orthogonal to both the X and Y axes, but it is located in the vertical plane. (To find the positive direction of the Z axis, use the right-hand rule: point the index finger in the direction of positive X, then curl it in the direction of positive Y, and the thumb will indicate positive Z.) In multi-axis systems, the direction of travel of the bottom axis is typically defined as the X axis. If the next axis above it is also horizontal, that axis is defined as Y, and the vertical axis (even if it is the second axis, directly on top of X), is defined as the Z axis. Roll, pitch, and yaw are rotational forces, or moments, about the X, Y, and Z axes. Just like pure linear forces, these moment forces need to be considered when calculating bearing

8 • 2020

life or determining the suitability of a linear system to withstand static loads. Roll: A roll moment is a force that attempts to cause a system to rotate about its X axis, from side-to-side. A good example of roll is an airplane banking. ...Continued on page 72

DESIGN WORLD — MOTION

MOTION SYSTEMS HANDBOOK

Pitch: A pitch moment attempts to cause a system to rotate about its Y axis, from front to back. To envision pitch, think of the nose of an airplane pointing downward or upward. Yaw: Yaw occurs when a force attempts to cause a system to rotate about its Z axis. To visualize yaw, imagine a model airplane suspended on a string. If the wind blows just right, the airplane’s wings and nose will remain level (no rolling or pitching), but it will rotate around the string from which it’s suspended. This is yaw. Both pitch and yaw moments put excess loads on the balls located at the ends of a linear bearing, a condition sometimes referred to as edge loading. Recirculating bearings with a “back-toback,” or “O,” raceway arrangement have higher roll moment capacities than bearings with a “front-to-front,” or “X,” arrangement, due to the larger moment arm formed by the contact lines between the balls and the raceways.

“open-frame” or “open aperture” 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 back-lit inspection applications and insertion processes. Because X-Y tables are primarily used for high-precision applications, the guideway of choice is crossed roller slides, which provide extremely smooth and flat travel. Drive mechanisms are typically ball screws or linear motors, although fine pitch lead screws are also common.

Pitch and yaw moments can cause edge loading on the bearing. Image courtesy of NSK

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SHOCKS • DAMPERS • VIBRATION MITIGATION

Back to basics: Machine vibration and components to address it Machine vibration is a normal, and typically unavoidable, result of moving and rotating parts, often caused by standard manufacturing and assembly tolerances that lead to clearances between mating parts or imbalances in rotating parts. And while routine machine wear can cause vibration to increase over time, when a machine experiences a significant or sudden increase or change in vibration, this can be an indicator that something is wrong and the machine or its components are being subjected to increased forces, loss of stiffness, and premature wear. Although it’s not difficult to detect when vibration is approaching (or has reached) detrimental levels, pinpointing its causes can be challenging, since numerous components and operating factors contribute to a machine’s vibration. But an analysis of the vibration’s frequency and amplitude can help engineers and technicians root out the causes of worrisome vibration and determine its severity.

Machine vibration frequency: Frequency indicates the number of times an object oscillates, or vibrates, per unit of time and is often expressed in either cycles per second (referred to as Hertz, Hz) or cycles per minute (CPM). Frequency is probably the most fundamental parameter in machine vibration analysis. It is especially useful for narrowing down the potential causes of vibration, since every part vibrates at a distinct frequency or frequency range. Machine vibration amplitude: Amplitude defines the magnitude of the machine’s oscillation and is used to judge the severity of the vibration. Oscillations with large amplitude indicate that the vibratory movements are large, fast, or forceful, resulting in more stress on the machine, components, and structure. Amplitude can be measured and specified for three aspects of oscillation: displacement, velocity, and acceleration. Displacement amplitude: Displacement amplitude measures the distance the vibrating part travels in one direction from a reference position during oscillations. (Note that the peak-to-peak displacement value, which measures total travel in both directions, is sometimes used.) This vibration measurement is important because vibrations with a high

TIM

COMPLEX VIBRATION

FREQ

VIBRATION SPECTRUM motioncontroltips.com | designworldonline.com

8 • 2020

UEN

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MOTION SYSTEMS HANDBOOK

displacement amplitude can cause machine components to exceed their yield point and experience catastrophic failure. Displacement measurements are typically used when vibration frequencies are low. Velocity amplitude: Velocity amplitude measures the speed of the oscillation. This measurement is typically considered the industry standard for evaluating the condition of a machine based on its vibrations, because it takes into account both vibration frequency and displacement. (Recall that velocity is the rate of change of displacement.) In fact, ISO standards refer to velocity amplitude when specifying the severity of machine vibration. Velocity amplitude can be expressed in terms of peak value or, more often, in terms of the root mean square (RMS) value, which is an indicator of the vibration energy. Acceleration amplitude: Acceleration amplitude is directly related to the force imparted by the vibration and is especially useful for assessing the likelihood of fracture for equipment that rotates at high speed. The high forces associated with acceleration can also cause lubrication breakdown, which can lead to excessive wear, heat, and premature failure. Acceleration is typically measured in g or multiples of earth’s gravitational acceleration.

HOW FAST FOURIER TRANSFORMS ARE USED IN VIBRATION ANALYSIS Fast Fourier transforms are mathematical calculations that transform, or convert, a time domain waveform (amplitude versus time) into a series of discrete sine waves in the frequency domain.

SIMPLE 60-HZ SINE WAVE 1

AMPLITUDE (g)

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PERIOD = 1 f

PEAK TO PEAK

-0.5

-1

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0.016

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Depending on the vibration parameter being analyzed, the amplitude, peak-to-peak or RMS value may be used.

DESIGN WORLD — MOTION

8 • 2020

Machine vibration is typically analyzed with measurements of the vibration frequency, displacement, velocity, and acceleration. The latter three — displacement, velocity, and acceleration — are time domain measurements, meaning their amplitudes are plotted versus time. But these vibration signals contain useful information, such as noise and harmonic content, that are difficult or impossible to detect when their amplitudes are plotted in the time domain. However, when displacement, velocity, and acceleration amplitudes are expressed in the frequency domain —that is, amplitude versus frequency — abnormalities, in the form of high amplitudes at certain frequencies, become visible … … and because many vibration-related issues occur at specific frequencies, the cause and location of the vibration can be narrowed down or identified based on variations in amplitude at certain frequencies. Note: A time domain plot is referred to as waveform, and a frequency domain plot is referred to as a spectrum. Every waveform can be expressed as the sum of simple sine waves with varying amplitudes, phases, and frequencies. A Fourier transform is a mathematical process that converts a time domain waveform into these individual sine wave components in the frequency domain — a process often referred to as spectrum analysis or Fourier analysis. To understand fast Fourier transforms, it’s helpful to first understand the underlying process, known as discrete Fourier transform (DFT). A discrete Fourier transform tests the time domain waveform for discrete, or individual, frequencies based on the length of the signal (N). The number of frequencies, or samples, required is equal the signal length squared (N2). Even for small signals, this can take significant time and computing power. To make the Fourier transform faster and more efficient, a method known as the fast Fourier transform is used. Fast Fourier transforms (FFT) significantly reduce the number of complex calculations that must be undertaken by assuming that N (the length of the signal) is a multiple of 2. The underlying mathematics of this assumption eliminates redundant calculations and those that have no value (multiplying by 1 for example) which provides significant computational efficiencies and reduces the number of required samples to N·log2(N) — an amount significantly less than N2. This allows fast Fourier transforms to provide close approximations of the more timeconsuming discrete Fourier transforms, but with significantly faster computing time. The sampling rate must be greater than the highest frequency component of the signal to ensure the sampled data accurately represents the input signal, according to the Nyquist sampling theorem. The instrument for analyzing signals via fast Fourier transforms is the digital signal analyzer (also referred to as a spectrum analyzer). This device captures the vibration signal, samples it, digitizes it, and performs the FFT analysis. The resulting FFT spectrum helps pinpoint the location, cause, and severity of the vibration, based on the amplitude of the displacement, velocity, and frequency spectra.

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MOTION SYSTEMS HANDBOOK

COMPONENT TYPES USED FOR VIBRATION AND SHOCK MITIGATION Most shock absorbers common in industry achieve 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 generally made of high-strength steel to handle the pressures from the internal hydraulic forces. Elastomeric seals prevent the fluid from leaking out of the cylinder, and special plating and coatings keep the units protected from harsh operating environments. Recent and ongoing developments in sealing technologies and in the internal designs of shock absorbers and dampers 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 microhydraulic designs. In contrast, most 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; however, a vast selection of compounds and compound blends can be used to achieve different characteristics specific to the application. Air springs are comprised of metallic end fittings coupled by a composite elastomericbased bladder that contains the compressed air used to provide isolation. These single-acting designs are comprised of a pressurized bladder and two end plates. As air is directed into the air bladders, they are expanded 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

SHOCKS • DAMPERS • VIBRATION MITIGATION

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 will define the natural frequency (sometimes referred to as resonant frequency) of the isolation system and are important in achieving the desired performance. Gas springs, also called gas dampers, tension springs and 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 machinetool 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 8 • 2020

cylinders; spindle-only designs; and cable, return, adjustable auto-return, nonrotating, stage, and multi-mode cylinders. Elastomer and other synthetic and rubber pads can also damp vibration and isolate shock loads. They are available in a number of shapes, including tubes, bushings, blocks, pads and washers. These components are commonly used in heavyduty applications to create strong cushioning plates or foundations in heavy machinery such as cranes, presses, and also for vibration reduction in lab and testing equipment, aerospace, and for pipelines and bridges. The rubber-like materials with which they are designed allow these padding materials to meet speciﬁc requirements, such as natural frequency, load, and area. And because they are soft, they are forgiving in most environments. 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 (69 N/mm2) depending on plate form and size. Another custom product called Sorbothane (from a company with the same name) is a thermoset that attenuates shock with near-faultless memory. That means its 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 damping, acoustic damping and isolation. Sorbothane works by turning mechanical energy into heat as the material is deformed. Molecular friction generates heat energy that translates perpendicularly away from the axis of incidence.

APPLICATION EXAMPLE: HOW AN ARBOR CAN REDUCE MACHINE TOOL VIBRATION On a typical machine tool, the tool that performs the machining operation (cutting, milling, or boring, for example) is attached to a rotating spindle that drives the tool. DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

But on machines that require long machining lengths, such as milling and boring, the tool is mounted to an arbor. The arbor is driven by the spindle and provides the necessary length for the tool to reach the workpiece in these operations. Horizontal milling machines are often referred to as “arbor milling machines” because their design requires the use of an arbor to achieve the proper tool position. In machining processes, vibration can cause poor surface ﬁnish and machining accuracy as well as noise and reduced tool life. But machining operations require high forces and high rotational speeds, which naturally induce vibration. Short of changing the part requirements or modifying the machining setup — unrealistic solutions in most cases — the most common way to reduce vibration in machine tool operations is to slow the machining rate. But a slower machining rate means fewer parts produced in a given amount of time (parts per minute, day, or hour) and lower productivity. However, one factor that affects the amount of vibration at the tool is relatively easy to modify. This factor is the design of the arbor — particularly its rigidity. The arbor’s rigidity is especially important when the

PROVEN SHOCK, VIBRATION & NOISE REDUCING SOLUTIONS

machining length is long and the arbor has a high length-to-diameter (L/d) ratio — also referred to as overhang. To address the problem of vibration in machine tools, some manufacturers have even developed vibration-damping arbors that significantly reduce vibration, allow longer machining lengths (L/d ratio of 8 or greater), and cut down on machining times. While each manufacturer uses a proprietary damping technology, most vibration-damping arbors are based on passive tuned mass damping systems — consisting of a mass, a set of springs, and a damper. Multiple springs (or, similarly, several materials with frequencydependent stiffness) are used to address various frequencies that occur during machining. The frequencies of the tuned mass damping system are designed to match the structural frequencies to be eliminated. So when any of the specified frequencies is excited, the damper resonates out of phase with the structure, absorbing or dissipating the kinetic energy caused by vibrations. “Connected” versions of vibration-damping arbors have also recently become available. The connected arbor designs have embedded sensors and use Bluetooth or other wireless technology to transmit tool performance data — such as temperature or cutting status — to a dashboard. This insight is especially helpful for internal machining processes that can’t easily be monitored or inspected, such as boring or internal turning. Manufacturers indicate that machine tool vibration amplitudes are up to 1,000 times lower when using a vibrationdamping arbor versus a standard AL I R E T arbor. This allows machining rates to MA be increased — leading to significant productivity improvements without sacrificing machining quality or tool life.

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SPRINGS & RETAINING RINGS

How to avoid wave-spring fatigue in dynamic applications Wave springs are gaining acceptance in motion applications, thanks to their ability to provide similar force and deﬂection characteristics to traditional coil springs, but with signiﬁcantly reduced working height. Although wave springs can be used in static applications — such as preloading a bearing or mechanical seal — many applications require dynamic movement — for example, to assist with the operation of a valve. When springs are subjected to dynamic loading,

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the constant cycling between different work heights and forces can cause the spring to experience fatigue — weakening and

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MOTION SYSTEMS HANDBOOK

SPRING WORK

When friction is taken into account, load necessary for equivalent deflection is increased and follows the upper line.

RESTORED ENERGY

FORCE

DAMPING

to both material thickness (t) and number of waves per turn (N) so increasing either of these will provide a signiﬁcant decrease in working stress. However, wave spring geometry also affects spring deﬂection and spring rate, so these must be taken into account.

SPRING TRAVEL Theoretical (purely elastic) spring behavior assumes that the taper faces interact without friction.

The fatigue life of a wave spring depends on the working (operating) stresses on the spring and on the spring material’s tensile strength. In dynamic applications there are multiple operating stresses (one at each working height), but in most cases, it’s sufﬁcient to use the operating stresses at minimum and maximum loads. These operating stresses are used to determine the wave spring’s fatigue stress ratio.

S1 = Calculated working stress at lower working height (must not exceed σ) S2 = Calculated working stress at lower working height

The fatigue stress ratio is then used to determine the spring’s estimated cycle life. The most straightforward way to increase a wave spring’s fatigue stress ratio (and in turn its estimated cycle life) is to use a material with a higher tensile strength σ. Wave springs are offered in a broad range of steel, stainless steel, superalloy, and bronze materials, so designers often have the choice of several materials that will meet the Where X = fatigue stress ratio application’s tensile strength requirements σ = Material tensile strength and withstand any special environmental conditions. For example, 17-7 stainless FATIGUE STRESS RATIO ESTIMATED LIFE IN CYCLES steel (a type commonly used for wave springs) provides both corrosion resistance and better fatigue resistance than carbon steel. And Elgiloy, a cobaltchromium-nickel-molybdenum superalloy provides excellent fatigue resistance and can withstand saltwater and acidic environments. Spring geometry also plays a role in fatigue by determining the working stress the spring sees. Working stress has an inverse-squared relationship

DESIGN WORLD — MOTION

8 • 2020

Where S = Working stress P = Load Dm = Mean diameter b = Material radial width t = Material thickness N = Number of waves per turn Another way to avoid fatigue and improve cycle life in dynamic applications is to use an interlaced wave spring. In this design, two springs with similar thickness, amplitude, and frequency are interlaced so that the turns of each spring align. This increases the thickness of each wave turn, which provides higher load capacity and better resistance to fatigue. It’s important to note that in dynamic applications, wave spring manufacturers generally advise that working stress should not exceed 80 percent of the material’s tensile strength. This is to reduce the possibility of the spring relaxing … or taking a set. Setting is a condition where the spring is stressed beyond its elastic limit and does not return to its original length. Although setting is usually undesirable, in some cases, a process known as presetting is used to increase the spring’s apparent elastic limit. Presetting is achieved by manufacturing the spring to a longer free height and higher load than is required. The spring is then compressed to a solid — that is, to the point where the various waves touch. This removes permanent deformation that would occur when the elastic limit is exceeded and creates beneﬁcial stresses that cause an apparent increase in the spring’s elastic limit, improving its load capacity at a given height.

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AD INDEX

M OT I O N SYS T E M S H A N D B O O K ACE Controls..........................................79 AllMotion..................................................2 Apex Dynamics, USA.............................57 AutomationDirect..............................Insert Bansbach Easylift....................................77 Beckhoff Automation.............................33 Bodine Electric Company.........................1 Bosch Rexroth US.....................................9 BRECOflex CO., L.L.C............................21 Bunting Magnetics.................................61 Carlyle Johnson......................................18 CGI Inc....................................................55 Clippard .................................................11 CMT .......................................................20 Del-tron..................................................73 Diequa.................................................. IBC Digi-Key..................................................24 Dorner....................................................40 Emerson ASCO......................................13 Emerson Machine Automation Solutions........................BC

Encoder Products Company..................53 Fabco-Air, Inc.........................................15 FAULHABER MICROMO............ Cover, 68 Festo.......................................................35 Fluid Line Products.................................37 GAM.........................................................3 HELUKABEL USA...................................29 HIWIN.....................................................17 Intech......................................................58 ITT Enidine.............................................78 Lee Spring Company..............................83 Mach III...................................................19 Master Bond...........................................65 maxon.....................................................66 mk North America, Inc...........................39 Moog Animatics.....................................62 NBK America LLC..............................43,45 New England Wire Technologies & New England Tubing Technologies....31 Nippon Pulse Americas, Inc...................63 OMS Motion, Inc....................................36

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DESIGN WORLD — MOTION

8 • 2020

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Motion Systems Handbook 2020

Articles inside

How to avoid wave-spring fatigue

Back to basics: Machine vibration and components to address it

Positioning stages and tables

Update on EC motors

Servomotor and drive fundamentals

Pitch line velocity in gearbox sizing

Fundamentals of encoders for motion control

Jaw couplings

Disc couplings

Applying couplings in motion designs

Heads or tails: A look at conveyor drive options

Controllers for motion control and beyond

Power over Ethernet (PoE) and M12 connectors in motion designs

Ways to reduce synchronous belt noise

Balancing design objectives with low-power braking

Ballscrew update

What constitutes a high-speed actuator

Common linear guide permutations

Common linear-motion options in 2020

Terminology: A modest proposal