Rotary encoders make versatile motion feedback devices
`E ncoding' or converting angular position into electronic signals is the mission of rotary encoders. Ways to detect motion include mechanical (via brush contacts) or magnetic/inductive methods, but noncontact optical encoders comprise the most common feedback device used in industrial motion control.
Besides motor shaft position, rotary encoders also provide information for speed feedback, direction of rotation (see diagram), and electronic commutation in brushless servo systems. Position feedback is not limited to the motor. For example, an encoder can indicate valve position or sometimes be directly mounted to a rotary load.
All optical encoders work on the same basic principle. Light from an LED or other light source is passed through a stationary patterned mask onto a rotating code disk that contains code patterns (see below). The disk is the heart of the device. Photodetectors scan the disk and an electronic circuit processes the information into digital form as output to counters and controllers.
Incremental or absolute
Two basic types of optical encoders exist-incremental and absolute position. Incremental encoders are the simpler devices. Their output is a series of square wave pulses generated as the code disk, with evenly spaced opaque radial lines on its surface, rotates past the light source. Number of lines on the disk determines the encoder's resolution.
The simplest incremental encoder, called a tachometer, has one square wave output and is often used in unidirectional applications in need of basic position or speed information. The more useful quadrature encoder has two output square waves (Channels A and B), plus a reference pulse (Channel Z) generated as a 'home' marker once each revolution (not shown in diagram).
Incremental encoders provide only relative position; in case of power failure, the position count is lost. The Channel Z marker comes into play upon a restart to establish the home position, so that the new pulse count can begin.
Absolute position encoders are more complex and capable than their incremental cousins. They provide a unique output for every position. Their code disk consists of multiple concentric 'tracks' of opaque and clear segments. Each track is independent with its own photodetector to simultaneously read a unique position. The number of tracks corresponds to the binary 'bit'-resolution of the encoder. That is, a 12-bit absolute encoder has 12 tracks.
Also, the absolute encoder's nonvolatile memory retains the exact position without need to return to 'home' position if power fails. This is useful in remote applications where equipment runs infrequently with power turned off between uses.
Most rotary encoders are single-turn devices, but absolute multiturn units are available, which obtain feedback over several revolutions by adding extra code disks. The additional disk stages are geared to the main disk and have their own photodetectors.
Choice of incremental or absolute encoder is very application dependent, but price is a factor. Incremental encoders are less costly than 'absolutes,' but generally offer fewer functions and lower resolution. They're also more susceptible to electrical noise.
Some techniques can improve resolution and noise immunity. A controller counting the leading and trailing edges of Channel A allows the quadrature detection method to multiply basic encoder resolution by 2X. If this is done for both Channels A and B, 4X basic resolution is obtained. However, system bandwidth limits must be kept in mind.
Open-collector is one type of output circuit from an incremental encoder. The user must specify a pull-up resistor for this electronic interface to work properly with external controls. Another basic output circuit, the differential line driver, is used with long cable lengths and in electronically noisy environments. Complementary (differential) signals generated with this output take on an incorrect form in the presence of electrical noise. Information coming from these abnormal signals can then be rejected.
Absolute encoders work with various serial outputs that can be converted to parallel and fieldbus formats. Rotary encoders offer a rich variety of sizes, features, and capabilities. Models with speeds up to 30,000 rpm and operating temperatures well above 100 °C are available. For more information and product examples see July Online issue under this topic at www.controleng.com.