Incremental Rotary Encoders

Incremental rotary encoders or rotary pulse generators are multi-turn sensors that use optical, mechanical, or magnetic index-counting for angular measurement. They do not contain an absolute reference. Instead, incremental rotary encoders count relative to the turn-on position. The number of successive cycles (signals) corresponds to the resolvable mechanical increments of motion. The signal provides logic states "0" and "1" alternately for each successive cycle of resolution.  For example, optical encoders use a transmitter-receiver to count the opaque lines and thus the angular increment. Multiple transmitter-receiver sets may be arranged to provide multiple counts per line. One common technique is to offset two sets a half line-width apart. This results in four counts per line. This technique of enhancing resolution via out-of-phase signals is known as quadrature.

Important parameters to consider when specifying incremental rotary encoders include: resolution in counts per revolution, resolution in lines per revolution, positional accuracy, and maximum update rate. Resolution is specified in counts per revolution, including any interpolation, quadrature, electronic count multiplication, or lines per revolution; or the number of cycles per revolution of the raw square or sine wave output. Note that this is the same as the number of lines or tick marks on the encoder disk (or equivalent).  Positional accuracy for incremental rotary encoders is the maximum error of a reading, in arc seconds. One degree is 3600 arc-seconds.  The maximum update rate is the rate at which new position readings are generated and updated.

Incremental rotary encoders can have one of three signal types: quadrature, single channel, and pulse and direction. Quadrature signals are analog outputs that involve two channels that are 90° out of phase (quadrature). The sine and cosine signals may be read and combined. The theoretically infinite resolution is limited only by the resolution of external digitizing device. Single channel encoders are unidirectional or "tachometer" encoders. The single channel allows for one count per physical line. A direction channel may be combined with single channel counting or multiple channels counting with quadrature for a pulse and direction signal.  A reference or index channel has a once per revolution "mark" or "home" position.

Incremental rotary encoders use optical, mechanical, fiber optic, or magnetic technologies. With an optical encoder, incremental angular counting is achieved by a light emitter and receiver. Outputs can be sine-cosine waves or digitized square waves. Configurations include transmissive systems in which light passes though a transparent disk or is blocked by an opaque line, and reflective systems in which lines are segments of nonreflective space on a strip between reflective segments. With a mechanical rotary encoder, counting is achieved by disk engaging periodic spring-loaded detents or other mechanical switches. Fiber optic signal transmission is often combined with optical sensing. With a magnetic rotary encoder, position measurement results from the sequence of magnetic switches being activated, or by the resolution of interaction between magnetic fields. 

Features, mechanical specifications, operating environment, and output interfaces are additional considerations when selecting incremental rotary encoders. Multi-turn encoder, hollow shaft, programmable, and intrinsically-safe (IS) construction are common rotary pulse generator features. Modular kit encoders are also available. Mechanical specifications include maximum mechanical shaft speed, diameter or width, and rotor inertia. Environmental operating specifications include operating temperature, vibration rating, and shock rating. Output interfaces for incremental rotary encoders include: digital square wave, analog voltage, analog current, serial, parallel, serial synchronous interface (SSI), FOUNDATION Fieldbus, controller area network bus (CANbus), INTERBUS^®, DeviceNet, PROFIBUS^®, and SUCOnet. INTERBUS is a registered trademark of Phoenix Contact GmbH & Co. PROFIBUS is a registered trademark of PROFIBUS International.

• Pros and Cons of Absolute vs. Incremental Rotary Encoders
  An incremental encoder simply generates pulses proportional to the position. The principle of the incremental measuring system is the scanning of a line pattern on a glass or plastic disc (see Image...
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• Optical Incremental and Absolute Rotary Encoders
  The purpose of an encoder is to act as a position feedback device as part of an integrated control system. The encoder is an electro-optomechanical device that attaches to a rotating shaft providing...
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• Optical Rotary Encoders: The Basics
  On every factory, and on every shop floor, something is rotating and someone wants to monitor it. Enter the optical rotary encoder. Encoders transform mechanical rotary motion into a sequence of...
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Part Numbers for Incremental Rotary Encoders