Aug. 1, 2004
The sensing mechanism in an incremental optical rotary encoder consists primarily of a light source, code wheel, and optical detector. Here we explain how their quadrature signals are used.

The sensing mechanism in an incremental optical rotary encoder consists primarily of a light source, code wheel, and optical detector. As the code wheel turns, a ring of alternating opaque and transparent regions shutters the light between the source and detector, creating a series of pulses.

Measurement precision reflects the mechanical precision of the pattern on the code disk, but is not limited to it. The reason is that, in a quadrature encoder, each opaque region or “line” produces not one, but four distinct reference points. Two points correspond to the leading and trailing edges of the line itself; two additional points correspond to the leading and training edges from the perspective of a second detector. This not only provides higher resolution, four times that of the code disk, but also indicates direction based on which detector responds first.

Q: What is the resolution of a 1,000-line encoder?
A: The base resolution is 0.360°. Resolution obtainable by quadrature signaling is four times better, or 0.090°. Electronic interpolation – mathematically predicting position between data points – can achieve even higher resolutions.

Q: What other signals do quadrature encoders produce?A: Many encoders produce a once-per-revolution index pulse that serves as a reference to a known (home) position. Some encoders also produce differential signals, the digital inverse of each of the three standard signals (quadrature A, quadrature B, and index). Differential transmission minimizes the risk of noise when sending signals over long distances.

Q: How is direction encoded in quadrature signals?
A: In the typical quadrature relationship, counterclockwise (CCW) rotation causes the signal designated as “A” to lead the one designated “B.” B leading A, on the other hand, indicates clockwise (CW) rotation.

Larry Berardinis

For more than two decades, Lawrence (Larry) Berardinis served on Machine Design and Motion System Design magazines as an editor and later as an associate publisher and new-business development manager. He's a member of Eta Kappa Nu, and holds an M.S. in Solid State Electronics. Today, he is the Senior Manager of Content Programs at ASM International, formerly known as the American Society for Metals.

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