A device that generates electrical signals by means of a rotating disk that passes between a light source and photo detectors. Incremental encoders have two output signals, or channels, commonly referred to as channel A and B. The A and B outputs are nominally 90º out of phase with each other and are interpreted by a motion controller to determine position/velocity information. The lead/lag relationship between the A and B channels provides directional information. It is important to understand that each mechanical position is not uniquely defined. When the incremental encoder is powered on, the position of an incremental encoder is not known, since the output signals are not unique to any singular position. Incremental encoders often provide a third output that pulses once per revolution of the disk. This is typically called the Index, or Z-channel, and is commonly used for homing/reference moves.
Absolute encoders have a unique value (voltage, binary count, etc.) for each mechanical position. When an absolute encoder is powered on, the position is known. Absolute encoders most commonly provide digital data in a parallel or serial format to the motion controller which is used to determine position/velocity information. Since they provide absolute position information when powered on they eliminate the need for a homing/reference move in a motion system.