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Basics – Absolute Encoders

Typical disk pattern showing radial scanning method used to read posit ion.

Absolu te encoder disk incorporated ina m ul t i - tu rn encoder.

Principle of OperationIn prin ciple, absolute encod ers aresimilar to incremen tal encoders, in thata rotating disk interrupts aphotodetector to produce an outputsignal.

However, absolute encoders aredifferent in two very importan t ways:

1. Every position of an absoluteencod er is uniqu e. Unlike anincremental encoder, where positionis determined by counting pu lsesfrom a zero mark or h ome base, theabsolute encod er reads a system of coded tra cks to establish p ositioninformation. No two positions arealike.

2. Absolute encoders do no t loseposition when power is removed.Since each position is unique, trueposition verification is available assoon as power is up. It is notnecessary to initialize the system byreturning to home base.

Absolute Encoder DiskAs oppo sed to th e simple, single track system of an incremen tal encoder, anabsolute enco der disk features severalconcen tric tracks, each consisting of apattern of transparent and opaqu esegments. These independent tracksprovide a unique combination of absolute values for each position. Thecoded format is a variation of Binarycode called Gray code. G ray code isused because on ly one "bit" chan gesbetween ad jacent words on the disk.This limits any transition erro rs of theword track sensors to plus or minusone coun t. In all other codes, such asBinary or Binary Coded D ecimal(BCD), several bits are requ ired tochange state between adjacent words.

Absolute Rotary Encoder

Photodetector

Stationary mask

LED Light source

RotatingEncoder Disk

Position

Tracks

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Programming with the Serial-to-Parallel Converter Module

The SPA 3 converter modu les areavailable for u se with multi-turnabsolute encod ers with serial dataoutput to convert information from theencod er to a parallel format. TheSPA 2 is used with absolute en codersthat have a synchronous serialinterface (SSI). In addition, severaloperating parameters can beprogrammed in b y the user orprogramming can be don e at thefactory prior to shipment. See pages70-71 for mo re information.

Advantages of AbsoluteEncoders

Non-volatile MemoryAn absolute encoder functions as a non-volatile position verification device. Trueposition is no t lost if the po wer fails orthe system moves while power is off. Acontinu ous reading of position is notrequired. This is particularly useful inapplications, such as satellite trackingantennas, where occasional positionverification is all that's necessary.

Programming FlexibilityBy eliminating the need for systemhoming, absolute encoders can becontrolled to provide positioning

programs based o n establishing referencefrom point-to-point, rather than from ahome position.

In ad dition, a microprocessor interfacemod ule enables the system designer toprogram in several operating parameters,including resolution. (See Serial-to-Parallel Converter Mod ule, pages 70-71.)

SafetyIn som e applications, a loss of positioncould result in damage to the machineryor injury to the operator. An absoluteencod er pro vides po sition verificationthe moment power is up.

Better Immunity to Electrical NoiseAbsolute encoders determine positionby continu ally reading a cod ed signal.Stray pulses, resulting from electricalno ise, will not accum ulate and accura teposition is available again o n th enext reading.

Long Distance TransmissionOu r Synchron ous Serial Interface (SSI)option is well suited to long cable runs.The encoder p rodu ces a parallel dataoutp ut which is converted to serial andis ideal for tran smitting information athigh speeds over long distances . . . upto 4,000 feet.

Single Turn and Multi-TurnAbsolute EncodersAs the n ame implies, single turnencod ers are best suited for shorttravel, motion con trol applicationswhere position verification isrequired within a single turn of theencod er shaft. A multi-turn en coderis recommen ded for applicationsinvolving lengthy or complexpositioning requirements.

A multi-turn encod er consists of aseries of disks conn ected to th e basichigh resolution disk thro ugh a systemof gearing. The ad ditional disksprovide for turns counting so thatposition d ata is available overmultiple revolutions. The Led exmulti-turn absolute encod er features4,096 turns with a resolution of 4,096positions/turn.

Single turn en coder

SPA 2 SPA 3

High precision m ulti-turn encod er