Quadrature Encoder Fundamentals

The most common type of incremental encoder uses two output channels (A and B) to sense position. Using two code tracks with sectors positioned 90 degrees out of phase, the two output channels of the quadrature encoder indicate both position and direction of rotation. If A leads B, for example, the disk is rotating in a clockwise direction. If B leads A, then the disk is rotating in a counter-clockwise direction.

By monitoring both the number of pulses and the relative phase of signals A and B, you can track both the position and direction of rotation.

Some quadrature encoders also include a third output channel, called a zero or index or reference signal, which supplies a single pulse per revolution. This single pulse is used for precise determination of a reference position.


Visit the how-to guide for in-depth guidance on making quadrature encoder measurements.

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Each increment (marked by detents on some shaft encoders, unmarked on others) is indicated by a single bit transition, whether you're talking about a 2-bit encoder like the one shown here, or a 3- or 4-bit. Gray code (http:// www.nist.gov/dads/HTML/graycode.html) is generally used to ensure that only one bit changes for each increment, so you never have to worry about tracking simultaneous or near-simultaneous transitions. The "not A" channel simply inverts the A channel. You only need to use one of A or not A. Losing a channel makes a 2-bit encoder useless. Missing a transition also makes it impossible to tell which way the shaft is turning. If your program reads both on: 11, followed by both off: 00, did the shaft turn clockwise or counter-clockwise? This may be an issue for software-based decoding.
- Jan 12, 2007

This was brief, but helpful. But, a link to an example problem would be perfect.
- Abdul Rahman, Northern Illinois University. shoaib_ar@hotmail.com - Nov 10, 2006

This explanation is too limited. For example, is one increment determined by two bit transitions, or 4? I have seen one system that used 3 bit transitions. Is there a standard?
- Oct 10, 2006

Quadrature Encoder
The information is great on the A and B signal but it doesn't explain the (A not) or (B not) that are with some encoders. Do you need to use all four signals (A, B, A not and B not)? What happens to the output signal if you lose one channel?
- Aug 16, 2005

Links needed
A link to applications would be useful. I.e. how do I set-up a FieldPoint counter and a Q-encoder to keep track of angular position.
- andres mencke, VI Research. amencke@panix.com - Dec 28, 2004

General/specific comment
Great description, however, it would be great to integrate this document with other articles that talk about integration. In general your "Developer Zone" would be more helpful if it had more detail, or linked to other pages. Its a bit difficult to filter through ~100 hits for a search on encoders. Great site though.
- Feras Habbal, UT austin. habbalf@mail.utexas.edu - Jul 18, 2002