How to time Optical Encoder commutation channels to a BLDC motor

Optical Encoder Brushless DC Motor timing using Back EMF

The optical encoders we work with have a set of three Commutation tracks (U,V,W) which correspond to the three phases of the motor (R,S,T).

Alignment of the Optical encoder commutation signals to a BLDC motor could be thought of as being comparable to timing the distributor of an auto engine. Just as the distributor tells the spark plugs when to fire, the optical encoder tells the amplifier/drive when to fire the windings in the BLDC motor.

And just like a car engine, if the timing alignment is off,  the motor will not run correctly, will run inefficiently, or will not run not at all.

If two phases are accidentally reversed, the motor may even run backwards.
Keep in mind that you will need to have the information that describes which motor winding corresponds to which encoder commutation signal before trying to align an optical encoder to a BLDC motor.

Basic Steps:

1) One phase of the motor is energized locking the motor into position.

2) The encoder is rotated to a given position, which is usually the start of one of the commutation signals (I.E. leading edge of U). Often times this corresponds with the encoders index pulse.

3) The encoder is assembled to the motor and the shaft is locked in place. (via encoder set screws) The encoder flex mount is not yet secured.

4) The motor winding is de-energized.

5) The Optical Encoder is powered.

6) The motor/encoder is back driven by another motor and the two waveforms are displayed on an oscilloscope. One waveform is back EMF from the motor phase, and the other is the encoder commutation channel.

Back driven motor set up

Below the Motor back EMF and Encoder Commutation (Hall) signals are shown. They have been separated for clarity, when timing a motor you will want them to overlap.

7) While the motor is rotating, the assembly is fine tuned by rotating the encoder body to align the encoder signal to the Motor waveform.

Video of BLDC motor Back EMF to Optical Encoder Hall Phasing

Proper timing typically calls for aligning the zero volt level of the back EMF Sin wave with the edges of the commutation signals.  That level is shown below by the red line.

8 ) Once alignment is achieved, the encoder flex mount is secured, locking in the phase relationship between the motor and encoder.


About Quantum Devices Inc.
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.

2 Responses to How to time Optical Encoder commutation channels to a BLDC motor

  1. Pingback: Interfacing a 5Volt Incremental Encoder to 12 Volts « Quantum Devices INC

  2. Pingback: Understanding Incremental Encoder signals « Quantum Devices INC

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