Input / Output Devices

Rotary Encoders - Rotary Encoder Information

While standard and hollow shaft rotary encoders are the most rugged, modular encoder makers say that users can easily make their rotary encoders more environmentally sound by simply mounting a bell or can over the rotary encoder to keep out contaminants.

Examining a modular rotary encoder used on a servomotor reveals a code wheel attached directly to the motor shaft. This makes the encoder sensitive to motor shaft run-out and axial movement. A large amount of play can break the code wheel or push it into the stationary parts of the rotary encoder, forcing the optics out of alignment. These problems beset even large, high-resolution rotary encoders that sometimes cost as much as the motor they attach to.

Newer rotary encoders are less likely to suffer from these problems. These rotary encoders, sold as kits that assembly piecewise onto a motor shaft, eliminate some of the components that cause run-out problems. The new rotary encoder designs have a lower part count as well, reducing costs and making closed-loop control practical for many applications.

Using rotary encoders with larger gaps between code wheels and housings may enable certain applications to use less-expensive motors with appreciable shaft play. If the rotary encoder also can withstand high temperatures (above 70°C), motors may also be sized smaller and run hotter than would otherwise be acceptable.

Most improvements in modular rotary encoders concern the rotary encoder optics and electronics. For example, until recently, rotary encoder light emitters (usually LEDs) and detectors were relatively large compared with the slots in the disk. As a result, a stationary mask is placed between the detector and code wheel to increase accuracy by sharpening the edges of light pulses falling on the detectors. But the mask presents problems. It increases component and assembly costs. It also increases the chance of interference with the spinning disk, and attenuates light reaching the detector.

To solve these problems, several manufacturers have designed rotary encoders that need no mask. Eliminating the mask allows more space between the disk and detectors, increasing the tolerance for phase error and edge jitter between channels. The key developments that make maskless rotary encoders feasible are miniaturized LEDs and detectors and the use of special lenses.

Eliminating the mask entails drawbacks as well. The biggest drawback is that detectors on maskless rotary encoders work only with specific code-wheel resolutions. Rotary encoders with masks, on the other hand, can be adjusted to fit wheels having different resolutions.

Code wheels also have improved. While many still consist of etched metal, Mylar, or emulsion on glass, these technologies are giving way to chrome on glass. Chrome on glass gives superior edge definition, greater immunity to condensation, and better durability. Superior edge definition also allows code wheels to be made smaller.

Rotary Encoders: Rotary Encoder Information

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