Encoders are used to provide feedback on position, angle, speed and, when coupled with a draw wire or linear conversion mechanism, length. They fall into two main categories: incremental and absolute.
Incremental encoders generate a train of pulses; position and length are obtained by pulse counting, while speed is derived from pulse frequency. Absolute encoders assign a unique digital code to every shaft position, so the true position is retained even after power loss. Absolute types include single turn (within one revolution) and multi turn (tracking angle plus revolution count).