Quadrature

Digital quadrature signals

Quadrature digital signals consists two channels (wires) and it can carry incremental setpoint or feedback signal. Quadrature signal is commonly seen in optical encoders which is probably the most common feedback device type in modern servo motors. Quadrature signal can be also used as setpoint signal and it has similar properties with pulse and direction.

Operation

Each edge of signal (from A or B channel) will increment or decrement quadrature counter value by one.

Most optical encoder specify their resolution as pulses per revolution (PPR) which will equal 4X amount of position values per revolution. I.e. 1000 PPR encoder will yield resolution of 4000 counts per revolution.

Encoder PPR, CPR, LPR and resolution

Basic principle of quadrature encoder which produces quadrature signals when code wheel rotates and detection light passes through segments at different phase. The encoder in the picture has 4 lines per revolution (LPR) thus it's resolution after 4X decoding is 16 counts per revolution.

Most encoder suppliers specify quadrature encoder resolution as PPR (pulses per revolution) or LPR (lines per revolution). This specifies how many electrical pulses (0 to 1 transitions) there will be during one revolution. Some manufacturers (such as Avago and USdigital) uses name CPR (counts per revolution) to specify pulses per revolution which may seem counter-intuitive.

As each pulse will contain two edges (rising and falling) and there are two pulse channels (A and B) with 90 degree phase shift, the total amount of edges will be four times the number of PPR. Most quadrature receivers use so called 4X decoding to count all edges from encoder A and B channels yielding 4X resolution compared to raw PPR value.

Glossary

PPR, P/R, sometimes notated as C/T

Pulses per revolution

LPR, L/R

Lines per revolution (same as PPR)

CPR

Counts per revolution. Usually means encoder resolution after 4X decoding. However, some manufacturers (such as Avago and USdigital) specify PPR value as CPR.

An example encoder presented in different ways

Lines or pulses per revolution / LPR or PPR	2000
Counts per revolution / CPR	8000
Actual position control resolution	8000 steps per revolution
Mechanical servo accuracy including tracking error	Theoretically 1/8000. In typical case this would yield 1/500 to 1/2000 mechanical accuracy in dynamic conditions (not standing still).

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