10.3 Quadrature Encoding

Regardless of the sensor type, if it outputs a 50% duty cycle waveform, quadrature encoding can be used.

In quadrature encoding, two sensors of identical properties are used. In other words, both sensors must have the same 50% duty cycle output waveform. The two sensors are spaced out by a quarter period. In other words, if it takes $x$ rotation for one period (one half on, one half off), the two sensors are spaced by $(n+0.25)x$ rotation. $n$ can be any integer, as multiples of periods does not change the shape of the output.

Let us call one sensor A and the other one B. Once A and B are spaced out by a quarter period, we get four transitions instead of two per period. Furthermore, we can also tell the direction of rotation. If waveform A leads waveform B, it indicates rotation in one direction. If waveform B leads waveform A, it indicates rotation in the other direction.

This means that with two sensors, we can keep the same encoder disc (optical or magnetic), but we end up with:

• twice the resolution
• the ability to tell direction of rotation