Absolute Rotary Encoders

Absolute rotary encoders provide a unique output for every resolvable movement or shaft rotation. They do not lose their position after power-down, but instead provide absolute position upon power-up without requiring a home cycle or any shaft rotation. Traditional absolute rotary encoders have one track per encoder bit, and provide one fixed resolution. Consequently, a 1024-position encoder has 10 tracks with 1 bit per track. Multiple transmitter-receiver sets may be arranged to provide multiple counts per line. Lines or sections of varying angular width can be combined to determine absolute angular position rather than simply an incremental move referenced to zero at power-up. 

Absolute rotary encoders use four main technologies: optical, mechanical, fiber optic, and magnetic. 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.

Parameters to consider when specifying absolute rotary encoders include absolute encoder resolution, positional accuracy, and maximum update rate. Absolute encoder resolution is the number of bits used to encode the position. A higher number of bits indicates a finer resolution. For example, a 9-bit encoder will yield 2⁹ = 512 counts per revolution, while a 10-bit encoder will yield 2¹⁰ = 1024. Note that high resolution does not necessarily imply high accuracy. Resolution in bits is primarily a specification for optical encoders, but not all suppliers provide this information.  Positional accuracy is the maximum error of a reading, in arc seconds. Note that one degree is 3600 arc-seconds. The maximum update rate for absolute rotary encoders is the rate at which new position readings are generated and updated. 

Outputs, features, mechanical specifications, and environmental specifications are important considerations when selecting absolute rotary encoders. Outputs include:  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. Binary code represents the output as a series of ones and zeros. Binary coded decimal (BCD) and gray encoders are also available. Features common to absolute rotary encoders include: multi-turns, hollow shafts, programmability, and intrinsically safe (IS) construction. Modular kit encoders are also available. Common mechanical specifications include maximum mechanical shaft speed, diameter or width and rotor inertia. Environmental operating specifications include operating temperature, vibration rating, and shock rating. 

• 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...
  (read more)
• 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...
  (read more)
• 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...
  (read more)

Pages: 1 - 3 of 77

Part Numbers for Absolute Rotary Encoders