To attain accurate positioning, a servo system requires a feedback signal to close its feedback loop. Instruments that typically supply the feedback signal include optical encoders, resolvers, and quadrature magnetostrictive linear-displacement transducers (LDTs). Other instrument types for this purpose aren't discussed in this article. They include analog tachometer generators, induction generators, Hall-effect pickups, and potentiometric devices.
Optical encoders, which provide a digital square-wave feedback signal, include quadrature (incremental), absolute, and pseudorandom types. A typical optical encoder consists of the emitter side, the detector side, and a code wheel, which supplies a raw analog signal to the encoder's processing circuitry. A comparator stage then converts the analog to a digital output. Digital formats include open-collector outputs and, for single-ended outputs, 5- to 24-V logic. For noise immunity, the most robust outputs are the complementary, differential RS-422 types.
Quadrature optical encoders provide feedback signals in the form of A, B, and Z pulses. Therefore, the A and B signals (exhibiting a phase separation of 90° from the encoder's code wheel) are in quadrature, i.e., electrically spaced one-fourth of a period apart. When A goes positive prior to B, the encoder rotates clockwise, and vice versa for counterclockwise rotation. Thus, position, direction, and velocity data can be derived from these two signals. The Z signal indicates the motor's rotor position and whether or not the encoder shaft has rotated 360°. It also checks for miscounts of the A and B signals. For RS-422 connections, the encoder provides complementary signals for the A, B, and Z outputs.
Absolute optical encoders employ signal-processing components similar to those of the quadrature optical encoder, but their outputs produce one parallel binary word per increment of revolution. Typical outputs are 12 to 13 bits of BCD, gray, or natural binary code. The 13-bit outputs impose a lower frequency response (1200 rpm for 12 bits, versus 600 rpm for 13 bits) in exchange for the finer resolution per 360° rotation. This encoder type is normally used to monitor shaft position during power up and power down. Unlike quadrature encoders, the encoded output lets you read the shaft position without moving the encoder.
The new pseudorandom optical encoders give three output signals: A and B provide direction sense and spatial timing, and a third provides position data. Pseudorandom optical encoders require 1° to 2° of rotation to determine position.
Resolvers are feedback encoders that produce sine and cosine output waveforms, which can be processed to supply velocity and position data through the servo controller. A resolver's feedback signals represent absolute position when its shaft is rotating, but low-speed performance is poor. The main disadvantage of a resolver is the relatively expensive resolver-to-digital electronics necessary for processing its signals.
Finally, quadrature magnetostrictive LDTs are feedback encoder/transducers designed to measure linear motion, rather than the rotational motion measured by the encoders above. The analog position signal is developed from a current pulse sent down a magnetostrictive guide wire, interacting with a position magnet that moves with a linear-displacement rod protruding from the LDT. The reflected pulse is sensed by a pickup sensor. Then, the LDT processes and digitizes the pickup sensor signal to provide quadrature-output signals A, B, and Z, similar to those of a quadrature encoder.
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