Glossary term
Quadrature Encoder
A position encoder producing two phase-shifted signals to infer direction and displacement.
Definition
deviceA position encoder producing two phase-shifted signals to infer direction and displacement.
A quadrature encoder provides two digital position signals, commonly called channels A and B, shifted by approximately 90 electrical degrees. The relative phase indicates direction, while edge counts provide incremental position and speed information for motion-control and measurement systems.
A quadrature encoder converts motion into two square-wave signals whose phase relationship carries direction. If channel A leads channel B, the controller interprets one direction; if B leads A, it interprets the opposite direction. Counting transitions gives displacement, while measuring transition rate gives speed.
Many encoders also include an index channel, often called Z, which produces one pulse per revolution or per reference position. The index helps establish a repeatable home reference after startup, since an incremental encoder does not know absolute position until referenced.
Counting and resolution
Depending on electronics, the system may count only one edge per cycle, both edges of one channel, or all four edges of channels A and B. This is often called 1x, 2x, or 4x decoding. The final position resolution therefore depends on encoder line count, decoding mode, gear ratio, and mechanical coupling.
Electrical implementation matters. Encoder outputs may be open collector, push-pull, differential line driver, or sine/cosine analog. Long cable runs, electromagnetic interference, poor shielding, weak pull-ups, and ground offsets can create false counts. High-speed axes require counters and firmware fast enough to avoid missed edges.
Engineering use
Quadrature encoders are used in motors, robots, machine tools, conveyors, printers, laboratory instruments, and hydraulic or pneumatic actuators. In closed-loop control, encoder latency, quantization, backlash, coupling compliance, and filtering affect stability and tracking accuracy.
Common mistakes
A common mistake is swapping A and B channels and then compensating by changing signs elsewhere without documenting it. Another is treating encoder resolution as system accuracy while ignoring mounting eccentricity, backlash, missed counts, interpolation error, and homing repeatability. A good design review states counts per revolution, decoding mode, maximum edge rate, electrical interface, index handling, noise immunity, and failure response for lost counts.