A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code.
There are two main types: absolute and incremental (relative). The output of absolute encoders indicates the current position of the shaft, making the mangle. The output of incremental encoders provides information about the motion of the shaft, which is typically further processed elsewhere into information such as speed, distance, and position.
Rotary encoders are used in many applications that require precise shaft unlimited rotation including industrial controls, robotics, special purpose photographic, computer input devices such as opt mechanical mice.