Absolute vs Incremental Encoders

In the world of feedback, AC motors will typically utilize one of an optical absolute encoder, an optical incremental encoder or a resolver. This article will focus on the primary differences between the absolute and incremental encoders.

Functionality of both optical encoders are similar. The encoder sits directly on the backside of the motor shaft. There is a disk with transparent lines or notches evenly spaced around the face of a circular disk. A light source shines throw from one end and a photo detector on the opposite side will transmit the signals to the encoder circuitry. Typically, there are two light sources that represent an A phase and a B phase, respectively, which create sinusoidal waves. The phases will be offset by a quarter phase (ninety degrees, electrically).  A third phase is common, known as the Z phase. This will provide a “home” reference position that outputs one pulse per motor revolution. The key difference between absolute and incremental encoders is how they reference to the Z pulse.

Incremental

The incremental encoder loses positioning anytime the power is removed. This requires the need to home out axes every time a machine is powered down. Therefore, the incremental encoder looks to provide a change in angle. It does not reference the current position to any particular reference point since the notches or lines are identical cutouts. No positioning data is saved and therefore, the incremental encoder counts up while rotating.

Absolute

The absolute encoder will save the positioning data even when the power to the motor and machine is removed. This is typically done utilizing a backup battery in the machine’s control, with a super capacitor on the encoder PCB, or both. The absolute encoder will have unique patterns on the disk to produce a specific code that indicates a unique position. Instead of having a home position to reference to, the unique pattern on the disk creates a binary code for each pattern that gives a specific position.

While the concept of optical encoder feedback is relatively simple, positioning control is a complex subject. The interpretation of that data is performed by circuitry on the encoder PCB and in the servo drive itself.  Encoders are sensitive equipment and can be difficult to troubleshoot in the field. They can lead to positioning issues, overloading if alignment is off, or even runaway issues due to problems with output signals. Taking care of the motors themselves and providing routine maintenance can help to limit problems with the feedback.

For more information on encoders or other servo applications, contact us today at PrecisionZone.net!