Optical Encoder Incremental Signal Measurement

While most of us are familiar with how to measure something in mechanical degrees, incremental optical encoder signals are measured in electrical degrees.

360 electrical degrees is the period in which a signal completes one high to low transition.  This can be in any sort of waveform, but for encoders electrical degrees are usually used when referring to digital signals

Why Measure in Electrical Degrees?

We measure Incremental Optical Encoder signals in electrical degrees because this gives us a number that is not depended on other information.  If you were to talk about the high time of your encoder signal being 10 milliseconds, It doesn’t say anything about the quality of the signal unless you also talk about total cycle time.

An electrical degree specification gives us information about the signal with just one number.

Incremental Optical Encoder Symmetry

The ideal output for an incremental channel is a waveform with a 50/50 duty cycle.  This specification is termed “symmetry” and called out in electrical degrees.  Ideal symmetry is one where the high and low times of a channel are each 180 electrical degrees.

Some oscilloscopes will have a way to measure electrical degrees directly by setting a  full cycle to 360 degrees. In this post I am going to assume that your scope doesn’t have that feature and calculate electrical Degree symmetry measurements using the high, low and cycle durations as measured in time.

In the photo above the duration of a full cycle is 50.00 uS.

In this next photo we adjust the cursors to check the High time of the A channel we see that it is 24.80 uS

To find what this time would be in electrical degrees, we calculate as:

(high time/ cycle time) *360

Or

(24.80 uS/50.00 uS)* 360  = 178.56 Electrical Degrees

To find the low time you could measure it and again repeat the calculation (substituting in Low time for high time, but as long as the high time measurement was done carefully it is just as easy to subtract the high time from 360 Electrical Degrees to get the low time in Electrical Degrees.

360 – High pulse in electrical degrees = Low pulse in electrical Degrees

Or

360 – 178.56 = 181.44 Electrical Degrees

Note that to get the most accurate measurement you will want to widen out the cycle as far as possible.

Channel B is measured in the same way.

Incremental Optical Encoder Minimum Edge Separation.

Minimum A to B edge separation is an important measurement as the more time you have between the edges, the easier it is for a drive or controller to “see” the data coming in. This is particularly true with older equipment and when running at high speed.

Ideal A to B edge separation is 90 electrical degrees.  To find edge separation in an incremental optical encoder look for the smallest separation between adjacent edges of all six edges within an overlapping set of  A & B cycles.

To change from time to electrical degrees, we use a calculation similar to the one used for symmetry.

(Smallest Measurement/cycle time) *360 = Minimum Edge Separation in Electrical Degrees.

or

(12.00 uS/ 50.00 uS) * 360 = 86.4 Electrical Degrees Minimum A to B edge Separation


Video of an incremental optical encoder being tested for Symmetry and Minimum Edge Separation.

If you have any questions, I can be reached at jmiller@quantumdev.com.

For Optical Encoders with excellent  symmetry and edge separation go to Quantum Devices Encoder page

Quantum Devices Inc. is a leading manufacturer of optical rotary encoders their main website is at www.quantumdev.com They can be contacted via e-mail at info@quantumdev.com.

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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.