Glossary · Sensors and measurement
Absolute encoder
Also known as: Absolute rotary encoder, Absolute position encoder
German: Absolutwertgeber
In industrial automation, an absolute encoder is a position encoder that outputs a unique code for every position within its measuring range. The position is available immediately after power-up, without a reference run.
- Sensors
In one sentence
An absolute encoder reports a unique position value for every shaft or axis position, so the position is known right after power-up.
Example
A single-turn absolute encoder on a rotary indexing table tells the PLC the table angle after a power failure, so the machine can resume without homing.
How it applies
- Engineering: Single-turn absolute encoders code one revolution; multi-turn encoders also count revolutions, using gears or a buffered counter. The resolution is stated in bits per revolution and number of turns. The position is usually transmitted over a serial interface such as SSI or a fieldbus rather than as pulses.
- Commissioning: The mechanical zero of the axis must be aligned with the encoder value once, usually by setting a preset or offset in the drive or controller.
- Maintenance: Replacing an absolute encoder changes the position reference. The procedure must include re-adjusting the offset before the axis runs in automatic mode.
- Documentation: State in the maintenance manual which axes use absolute encoders, how the offset is set after replacement, and whether a reference run is still required after certain events.
Absolute encoder vs. incremental encoder
An absolute encoder knows its position at any time. An Incremental encoder only outputs pulses per movement, so the controller has to count them and needs a reference run after power-up to know the absolute position. Both are kinds of Rotary encoder when they measure rotation.