Quadrature Encoder Overview

A quadrature encoder uses two output channels, Phase A and Phase B, to track the position of a rotary shaft. Generally, this shaft is coupled to a motor drive that controls the movement of an object of interest. By monitoring the encoder Phase A and Phase B signals, you can obtain a precise measurement of the object's position.

To generate Phase A and Phase B signals, the quadrature encoder uses two code tracks with sectors positioned 90 degrees out of phase. The phase difference indicates the position and direction of rotation. If Phase A leads Phase B, the shaft is rotating in a clockwise direction. If Phase B leads Phase A, the shaft is rotating in a counter-clockwise direction. The following figure illustrates the Phase A signal leading the Phase B signal.

Compatible NI image acquisition devices include hardware that can be used to track both the position and direction of rotation of the Phase A and Phase B signals. For example, this information can be used in conjunction with a line scan camera to acquire lines synchronous to the movement of a conveyor belt. This gives you the ability to specify your line rate in terms of positional units (such as inches or centimeters) rather than time.