Optical Encoder Waveform Triggering

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Seeing Encoder Quadrature with a two-channel scope

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I received an e-mail from a customer concerned about the  “out of control” optical encoder signals he was seeing on his Oscilloscope.

The photo below shows the type of signal he was seeing:

The encoder in question was a 10,000 Line Count optical encoder. I noticed that he was running relatively slowly, about 100 RPM.  At that speed a lot of BLDC motors will show some degree of motor cogging, which is irregularity in rotation due to the magnetic fields in the rotor.

The customer was also triggering on an incremental channel (A&B) and not the index (Z) channel .

I am sure he had omitted the index as he wanted to see if the A&B incremental encoder signals were in quadrature.

I knew that when triggering on an incremental channel, the oscilloscope triggers off of whichever ever edge happens to occur within the scopes timing window.  What the customer was seeing on the oscilloscope was overlapping screen shots of the incremental channels as the motor speed changed.

In other words, he was seeing the encoder report exactly what the motor was doing.

If the customer were to trigger on Optical Encoder channel Z (Index) with one scope channel, they could see a nice steady signal. If they wanted to check quadrature, they could then compare the phasing of A and then the phasing of B relative to where the index channel was located.


That’s a little bit of a hassle, it’s much nicer to see both A and B optical encoder signals on the scope at the same time.  The way to do this with a two-channel scope like the Tektronix TDS 210 we have, is to use the scope’s external trigger and trigger off of channel Z.

The video below compares the optical encoder signals being triggered off of channel A and then being triggered off of channel Z.


Jim is an Applications Engineer for the leading optical encoder manufacturer Quantum Devices Inc. He can be reached via E-mail at jmiller@quantumdev.com.


Inverted Flex Mount for QD145 Optical Encoder

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.Another way to mount an Optical Encoder

Instead of using an end bell,  a lot of our motor manufacturer customers recess the QD145 optical encoder into an extended motor housing and then seal the motor with an end plate.

This has the advantage of being a lower cost item to manufacture, as the  motor housing is often extruded  or cast.  Just making the housing longer and providing an end plate usually costs less than casting a separate end bell.

Conventional Motor End Bell

But the downside to a recessed motor housing is that mounting an optical encoder in a recess like this creates a problem when tightening the set screws to the shaft. It often has to be done through an MS connector hole, or blind, by reaching the Allen wrench around and under the encoder.

Cast recess in motor housing

Because of this, we occasionally get asked for a a version of our QD145 optical encoder that allows the assembler to  access the set screws above the body of the encoder.  The inverted flex mount turns the encoder upside down allowing easy access to the set screws that are normally underneath the encoder.

The QD145 inverted flex mounti9ng option is not currently listed on our web site, but can be ordered using the following part numbers under the mounting options:

Use an 06 Mounting option for the inverted 1.157″ Bolt Circle flex mount.

Use an 07 Mounting option for the inverted 1.812″ Bolt circle flex mount.

The wiring is changed internal to the encoder so that the QD145 maintains correct phasing for all channels.

Another option for a drop in recess mounted encoder is the JR12 Jam Nut style encoder which does not use set screws to secure the encoder to the shaft, but a compression nut instead.

Dimensions for the 07 version of the QD145 Inverted flex mount.