A rotary encoder is an electro-mechanical device that converts the angular position of a shaft to an analog or digital output signals. Rotary encoders are used to measure distance, speed, direction and position.
Rotary Encoders are either Incremental or Absolute. Incremental encoders transmit changes in rotational position of the shaft. It does not keep track of the absolute position and therefore upon loss of power, the last position information is lost.
Absolute Encoders retain the position information even loss of power occurs. When power is restored, the encoder transmits last position information without having to return to zero position.