Rotary encoder

What is rotary encoder ?

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 analog or digital output signals. 

There are two main types of rotary encoder: absolute and incremental. The output of an absolute encoder indicates the current shaft position, making it an angle transducer. The output of an incremental encoder provides information about the motion of the shaft, which typically is processed elsewhere into information such as position, speed and distance.

Rotary encoders are used in a wide range of applications that require monitoring or control, or both, of mechanical systems, including industrial controls, robotics, photographic lenses,  computer input devices such as optomechanical mice and trackballs, controlled stress rheometers, and rotating radar platforms.

Basic types
Absolute
An absolute encoder maintains position information when power is removed from the encoder. The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy.

An absolute encoder has multiple code rings with various binary weightings which provide a data word representing the absolute position of the encoder within one revolution. This type of encoder is often referred to as a parallel absolute encoder. 

A multi-turn absolute rotary encoder includes additional code wheels and toothed wheels. A high-resolution wheel measures the fractional rotation, and lower-resolution geared code wheels record the number of whole revolutions of the shaft. 
Incremental

Incremental encoder
An incremental encoder will immediately report changes in position, which is an essential capability in some applications. However, it does not report or keep track of absolute position. As a result, the mechanical system monitored by an incremental encoder may have to be homed (moved to a fixed reference point) to initialize absolute position measurements.