Why use an Optical Quadrature Encoder for a Motor Encoder ?

What is the purpose of an Optical Quadrature Encoder on the back of a motor?  Incremental Optical Encoders will often provide commutation “Hall signals” for timing of the BLDC windings, but I am going to forgo that discussion and turn instead to the Incremental Encoder channels, specifically A and B encoder signals.

When using encoder incremental channels, their signals are in “quadrature”, where one channel is shifted by 90 electrical degrees from the other.   Most people are familiar with mechanical degrees, and the idea that there are 360 of these in a circle.   Electrical degrees are much along those lines in that there are 360 electrical degrees in each pulse encoder cycle. For an incremental quadrature encoder a cycle is the duration of one high and one low pulse as can be seen in the encoder pic below:

One of an encoder pulse (either High or low) is nominally 180 electrical degrees in duration, as seen below:

The edge separation for an Optical Quadrature Encoder  (Motor Encoder) between any two adjacent edges is ideally 90 electrical degrees   Since there are four 90 degree periods in a standard 360 electrical degree cycle, this amount of separation is referred to as being in quadrature.

Why set up the Incremental optical encoder signals into quadrature?  The reason is to give us rotational directional information.  If we take either channel A or  channel B and count how many of the pulses have occurred, it tells us how far we have gone in distance.   If we were to compare that against a certain amount of time we can tell us how fast we are going (RPM for example), but it takes BOTH signals to tell us the direction in which the encoder shaft is rotating.

We are able to tell direction by finding which optical encoder channel transitions from low to high first.  We do this by finding the point at which both signals are low, then look for the next rising edge.

If the shaft on the rotary encoder is spinning in the opposite direction the relationship between these two optical encoder channels is reversed.

I have purposely ignored the third encoder channel which is the index pulse (also called Marker, I, or Z)  I have a previous post on that topic here.


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Quantum Devices, Inc. (QDI) Barneveld, WI, was established as a Wisconsin corporation in October 1989, as an outgrowth of activities in the general area of optoelectronics. The main goal of QDI is to provide customers with a complete source for all stages of product design and development, starting from concept to a marketable commodity. These products include application specific Silicon Photodiodes, Optical Encoders and Light Emitting Diodes (LED) for commercial, industrial and medical applications. QDI's photodiodes and LED's are used in instrumentation for photosynthesis and photobiological research, biomedical and medical instrumentation for measuring blood sugar levels (glucometer), oxyhemoglobin and pulse rate (pulse oxymeter). One of the most dramatic products developed at QDI was the application of LED lighting systems for use as the light source for Photodynamic Therapy (PDT). Sales of QDI products include both domestic and international markets. Quality and reliability are very important concepts in maintaining our company wide commitment to overall product performance. Quality simply means continuous process improvement. We are committed to continually increasing our product excellence through increased quality and reliability. Quantum Devices, Inc. is ISO 9001 certified and the ISO Standard will provide the guidance for the vital function of maintaining our commitment to constantly improve our product quality. Quantum Devices is now proud to introduce it's new line of rotary encoders. Incorporating QDI's patented sensor technology, our QPhase™ Family of Encoders feature high resolution, increased frequency response and superior reliability over temperature (0 to 120 degrees C). QDI incremental and absolute encoders set a new standard and are backed by a 2 year factory warranty. We invite you to request an evaluation unit, additional information or price quotations. Mission Statement Quantum Devices is dedicated to the principle that light provides the power for all life on earth. We believe the quality, delivery and control of light is essential to the wellness of man and his advancement into the future.

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