Specifications

Encoders provide motion control systems information on position, count, speed, and

direction. As the encoder shaft rotates, output signals are produced, proportional to the

distance (angle) of rotation. The signal may be in the form of a square wave (for an

incremental encoder) or an absolute measure of position (for an absolute encoder).

Due to the performance and reliability advantages of the semi-conductor technology they

incorporate, optical encoders are the preferred solution in many common computer, industrial,

and automotive applications. Optical encoders also benefit from ease of customization, are

suitable to numerous environments, and suffer no effects from high levels of stray magnetic

fields.

The basic construction of an incremental encoder is

shown to the right. A beam of light emitted from an LED

passes through a transparent disk patterned with opaque

lines, and is picked up by a photodiode array. The

photodiode array (also called a photosensor) responds

by producing a sinusoidal waveform which is transformed

into a square wave, or pulse train.

Incremental encoders are available in two basic output types, single channel and quadrature.

A single channel encoder, often called a tachometer, is normally used in systems that rotate

in one direction only, and require simple position and velocity information. Quadrature

encoders have dual channels (A and B), phased 90 electrical degrees apart. These two

output signals determine the direction or rotation by detecting the leading or lagging signal

in their phase relationship. Quadrature encoders provide very high speed bi-directional

information for very complex motion control applications.

Incremental encoders can provide a once-per-revolution pulse (often called index, marker, or

reference) that occurs at the same mechanical point of encoder shaft revolution. This pulse

is on a separate output channel (Z) from the signal channel or quadrature outputs. The index

pulse is often used to position motion control applications to a known mechanical reference.

Resolution is a term used to describe the Cycles Per Revolution (CPR) for incremental

encoders, or the total number of unique positions per revolution for an absolute encoder.

Each incremental encoder has a defined number of cycles that are generated for each full

360 degree revolution. These cycles are monitored by a counter or motion controller and

converted to counts for position or velocity control. Absolute encoders generate a unique

code word for every resolvable shaft angle (often called bits or counts per revolution).

PHOT O SENSOR

SQUARING

CIRCUIT

DISK

LED

ENCODER BASICS