Angular encoders measure the rotational position of an item as it is revolving.
An angular encoder is mounted directly on the rotating shaft so that there are no additional position measurement errors associated with gear back-lash. An angle encoder may consist of a simple potentiometer or a more sophisticated non-contact Hall-Effect technology. Angular encoders are available with analog and digital outputs that are typically found in industrial equipment.
For additional info about ASM angular encoders, please see the following link:
Angle and Inclination Sensors