Quadrature allows a length control system to count four counts for one pulse from a bi-directional (two-channel) encoder.
A two-channel encoder is capable of reporting direction as well as movement. Typically, there is an A channel and a B channel. As the shaft of the encoder turns, pulses from both channels are being sent to the length control system. The two channels are always offset by 90 degrees, so one channel will always lead the other channel depending on direction.
When the A channel leads the B channel, that is typically the forward direction:
From left to right the first rising edge after the trigger (the vertical, light blue dashed line) is the A+ channel. Half way through the A+ pulse, a pulse from the B+ channel begins. A+ leads B+, therefore forward direction is indicated.
When the B channel leads the A channel, reverse direction is indicated:
In both images there are two pulses transmitted for each count from the encoder. Each pulse has a rising and falling edge.
As illustrated above, each optically resolved encoder pulse results in two rising edges and two falling edges – four transitions – seen by the length control system. In this way, four counts are accumulated from one pulse of the encoder. Typically, you may simply multiply the model number of the encoder by 4 to calculate it’s post-quadrature pulses per revolution.