(a.k.a. absolute encoders, incremental encoders, linear encoders, rotary encoders, shaft encoders)
The basics: encoders are used to determine the speed and direction of a motor shaft or other mechanical motion. This information is used for precise control in a variety of applications. Rotary encoders, also called shaft encoders, measure rotational motion while linear encoders measure motion along a path. Absolute encoders give position values by scanning the position of the coded element. Incremental encoders generate an output signal each time the shaft rotates a certain amount, the resolution is defined by the number of signals per turn.