Absolute vs Incremental Rotary Encoders

Positioning tasks require precise position values to monitor or control motion activity. In many applications position sensing is done using rotary encoders, also called shaft encoders or simply encoders. These sensors transform a mechanical angular position of a shaft or axle into an electronic signal that can be processed by a control system.

Absolute rotary encoders are capable of providing unique position values from the moment they are switched on. This is accomplished by scanning the position of a coded element. All positions in these systems correspond to a unique code. Even movements that occur while the system is without power are translated into accurate position values once the encoder is powered up again.

Incremental encoders generate an output signal each time the shaft rotates a certain amount. (The number of signals per turn defines the resolution of the device.) Each time the encoder is powered on it begins counting from zero, regardless of where the shaft is. Initial homing to a reference point is therefore inevitable in all positioning tasks, both upon start up of the control system and whenever power to the encoder has been interrupted.