New encoders wrap around axes

MYRIAD features have been added to all rotary-encoder subtypes in recent years. These rotary encoders include those that output incremental or absolute signals to provide feedback on position and speed. Incremental signals are trains of high and low waves indicating movement but not specifi c position. In contrast, absolute encoders do indicate position of the rotary axis being tracked along with movement.

Mechanical encoders are actually more like rotary switches but with a digital output. They are common in consumer products for knob tracking and in medical and fitness equipment and avionics. Resolution is defined in terms of angle of throw — for a number of positions — such as 30 positions for a 360° turn for example.

Rotary encoders for true motion-control applications include capacitive encoders employing low current. These are an efficient option using a high-frequency transmitter that sends signals intermittently modulated by an etched disc before they get to a receiver. That receiver reads the capacitance variations and an ASIC translates them into angular values with resolution to forty ninety-six steps per revolution.

The most common rotary encoders today though are optical encoders. Early versions had fragile glass discs between a light source and sensing head. Increasingly common today are more rugged versions. The disc can be etched metal, marked tempered glass, or engineered plastic.

Integrated electronics have made these encoders more rugged as well. For example, Opto-ASIC technology from Encoder Products Co. is a single chip combining all sensor-board components including the photosensor on one circuit. That boosts resistance to particulates and electrical noise. Magnetic and magnetostrictive encoders have also seen increased use in recent years in part thanks to new designs based on such sensing. Common iterations of these sensors have an array of magnetic strips around a wheel. Two channels on the sensor tracks pairs of strips and output differential signal. Some designs use embedded microprocessors running signal handling software as well — to get accurate and dynamic response rivaling optical designs.

Robotics demand very high levels of repeatability — even down to 0.1 mm in some cases. POSITAL FRABA hollow-shaft encoders can deliver this performance on six-axis articulated (SCARA) industrial robots and cobots. They have center bores of 30 or 50 mm (to route cables and media hoses) and are 17.8 mm thick with an O.D. of 80 mm and weight of only 110 g.

Taking the measurement tasks from opto-electric components to solid-state electronics in some cases means these encoders can be smaller and more reliable. Plus users can change the performance characteristics of these encoders more easily through software updates rather than physical changes. So, it’s easier to tailor such encoders to projects with fewer if any design compromises. This plays into a broader industrial trend to Big Data and the internet of things … as well as the trend to more preventative maintenance.

Physical permutations for rotary encoders abound. These include various seal, bearing, and housing options, as well as modular setups.

Just consider hollow-shaft encoders with an open ring shape to install around shafts or cabling on servomotors, drives, and other rotating axes (such as ROTOR robotic joints) to report position. Most are single-turn devices. In contrast, POSITAL’s capacitive hollow- shaft kit encoders can track multiple turns using an integrated rotation counter that records every single revolution sans external power supply or batteries STATOR or even complex gears. Instead, the encoders use Wiegand energy harvesting already core to so many POSITAL FRABA encoders. The technology converts rotational motion into electrical impulses.

More specifically, mechanical motion is captured as magnetic energy in the components’ Wiegand wire and then released as pulses of electricity to power a rotation counter.

One catch is that traditional Wiegand harvesters rely on permanent magnets mounted on the centerline of the axis drive shaft … so the hollowshaft build necessitated a totally new turn-counting design. That’s why these new encoders have four permanent magnets on the rotor circumference for a stable magnetic field for the Wiegand sensor in the stator to detect and use. With every 360° rotation of the rotor magnets, a Wiegand wire in a copper coil generates a pulse of electric current that activates counting electronics — precisely recording every revolution. The multiturn counter has a 43-bit memory that provides a measuring range of almost nine trillion revolutions.

The MSAC200 flexCoder absolute encoder from SIKO allows for a customer-specific ring diameter and type; integration into small spaces; and resolution to 21 bits. Absolute position feedback is via an SSI or BiSS-C motor-feedback interface. Optional analog real-time sin/cos 1-Vss output serves industrial and medical applications needing motor feedback for automation or robotics tasks.

POSITAL has supported the motion industry’s increased use of magnetic encoder technology (instead of optical) but magnetic systems are hard to implement in hollow-shaft designs. That’s why the component manufacturer employs capacitive measurement technology in its new hollow-shaft encoder. Capacitive encoders have a flat rotor and flat stator with patterned conductive surfaces serving as capacitor plates. As the rotor turns, these surfaces experience capacitive-coupling variations. The variations modulate the amplitude and phase angle of highfrequency electrical signals through the system. ASIC processors collect the resulting output and decode them to accurately determine rotor angular position. Then the data goes onward to a central controller via open-source SSI or BiSS C interfaces. Capacitance is averaged around the component’s full rotor and stator circumference for feedback that’s relatively immune to dust, moisture, and minor alignment errors.

In another encoder design to optimize robotics and motor-driven axes on automated machinery, a sensor and rule adapt to the position and shape of the axis. Called flexCoder technology from SIKO, the most significant task is simply attaching the rule to the shaft to be tracked. Otherwise the system allows use of a standard magnet ring with a sensor array and stackable electronics adaptable to the design housing. Ring diameters are to 44 mm and repeatability is to 0.01° … high installation tolerances (and therefore simplified installation) are possible with the distance between the sensor and rule less than or equal to 0.6 mm and an axial tolerance of ±0.2 mm.