By Charles Faulk

As space becomes more of an issue on many plant floors, manufacturers are hard pressed to save space by going with smaller machines. The constant demand for smaller and faster machines means that the components that make up these machines must also get smaller.

This view of three generations of encoders show a dramatic reduction in size over the years.

For years, industrial machinery and equipment has been getting smaller. Many of the size reductions have been made possible by increasingly smaller electronic components. More powerful processors are able to pack more processing muscle into less space and the functions of several chips have been integrated together into one chip.

The encoders used for motion feedback, being largely electrical devices, have followed a similar path. Taking advantage of advances in electronic technology, encoders have been able to shrink in size by orders of magnitude. As a result, they’ve been able to fit into smaller machines. At the same time, they have found their way into many applications where they could not fit before, applications where nobody had ever thought of placing an encoder simply because they were too big.

The reasons encoders can get smaller are associated with technological improvements. There are two main factors at work: electronic components and optical components.

A cutaway view of an old encoder shows light sources and detectors.

A typical encoder packs a lot of parts inside. Optical encoders have a glass or plastic disk with a pattern of lines, a light source, photodetectors, and processing circuitry. In the past, encoders would have a light source for each track and a corresponding detector for the light source. So, for instance, a three-channel encoder would have three separate light sources and three detectors. Having so many parts took up a lot of space.

Improvements in light sources have meant that encoder manufacturers can use just one light source with a focusing lens and a photodetector array instead of discrete sources and detectors. This also means that the size of the actual disk can shrink significantly and pack more lines per track and boost resolution.

Another improvement is the use of LEDs instead of incandescent bulbs. LEDs generate a better quality of light and generate significantly less heat than incandescent bulbs.

Another space-saving advance has come in the form of application-specific ICs, known as ASICs. These chips combine the functions of many discrete components into one chip, saving board space, reducing complexity, and increasing reliability because fewer solder points are needed.

A side-by-side comparison of two generations of multi-turn encoders shows how older encoders used more PC boards to accommodate up to a dozen individual ICs.

Typical discrete components include comparators, op-amps, and digital gating circuits, all of which were individual chips that were soldered onto a circuit board. With the advent of ASICs, what once took six or seven chips for a three-channel encoder can now be contained in two chips: an ASIC and an output line driver.

Overall, this increases encoder reliability because there are fewer solder points and therefore fewer places for potential failure. In addition, the run of the signals on circuit boards is shorter. This is critical for low-level signals which are more susceptible to electrical noise in longer circuit-board runs. As a result, newer generation encoders can perform extremely well in noisy environments. The ASICs in effect increase signal-to-noise ratio by about 500:1.

Newer, smaller encoders also boast better accuracy. This is due to the increased quality of light, through better focusing of the light.

The other benefit is that heat generation is reduced. Fewer chips mean less generated heat. This means encoders can be used in applications where heat is a significant factor without having to worry about the heat generated
internal to the encoder. Newer encoders can also handle the higher temperature ranges required by smaller motors.
More processing power also means that encoders are smarter than ever before. Encoders can now perform diagnostic functions, are capable of storing data in internal memory, and can communicate with controllers or PCs.

In multi-turn absolute encoders, these same technology drivers of optics and electronics play an important role. Better light sources meant that fewer lights were needed inside of a multi-turn unit, while packing more circuit functions into a single ASIC meant fewer chips and PC boards. Newer units contain two PC boards that contain circuitry for both the single-turn section and the multi-turn section. Older multi-turn encoders needed as many as six or seven PC boards.

In addition, most gears used in multi-turn encoders are now plastic and not metal, which helps reduce weight. The gears also stay lubricated, which helps the gears mesh better, eliminating backlash and improving gear synchronization.

There are some adverse effects due to shrinking encoder sizes. One adverse effect is that as the encoders get smaller the bearings get smaller. Smaller bearings can’t handle the same loads that larger bearings can. In general, smaller bearings account for about a one-fourth reduction in the amount of load that newer, smaller encoders can handle.

About the Author
Charles Faulk is an applications engineering manager at Danaher Industrial Controls, 1675 Delany Rd., Gurnee, IL 60031. He has more than 30 years of experience in the electronics industry and received his technical education as a member of the U.S. Navy. The company can be reached at 800-873-8731.