Optical encoder Problem


Thread Starter

K Karthikeyan

We are facing a peculiar problem with optical encoders.

Application: Drive speed sensing of a material handling system (this speed signal is further processed along with instantaneous load cell value to arrive at feedrate.)

Sensor: BEI make dual channel incremental optical encoder. As per specification, two channels are expected to give identical pulse train but with a quadrature phase shift.

Problem: At any particular speed, when measured with a multimeter, the channel B frequency measures twice the value of channel A. However, if measured using an oscilloscope, the channels seem to have identical pulse train with quadrature phase shift (channel A freq=channel B freq).

Its perplexing why a multimeter & oscilloscope would read different frequency values at the same measurement point. Pl do help me out of this.


1) You might have a floating ground situation.
2) "any particular speed"; what does this mean? Most DMMs would AC couple into the frequency counter. If the base period is below the cut off then it would try to make something up.

Exactly how are you using a multimeter (voltage, current, resistance) to measure frequency? Are you getting the same results in both clockwise and counter clockwise directions? I would imagine the problem is in the measurement procedure and not the encoder.


Robert Scott

The output is a square wave, rich in harmonics. If the cable to the multimeter is not a good transmission line with a matched terminating impedance, some of these harmonics can be accentuated or phase shifted, which could make for more than one zero-crossing per cycle of the square wave. If the multimeter counts zero-crossings, it could double-count under such conditions. The difference between channels A and B might be due to small differences in drive impedance causing one channel to distort just slightly more than the other. Try shorter cables to the multimeter and maybe a terminating resistor and then you should get the correct frequency reading on the multimeter.

-Robert Scott
Real-Time Specialties
Embedded Systems Consulting
A DVM will lie to you because it does not have the frequency response of a scope. Most BEI incremental encoders (I've repaired) have "Open Collector Outputs" thus the controller's circuit design will have "Pull-Up" resistors on the inputs. If the scope shows a clean square wave, during operation, you should be OK. Many BEI's have trim pots within the encoder to clean up the square wave.

Hope this helps,

Curt Wuollet

That's simple, believe your oscilloscope. It uses carefully controlled transmission lines to deliver what's at the probe to the electronics. It has high frequency compensation to prevent the loss of the edges and maintain signal integrity. A multimeter with open leads has none of the above. In fact, if you use your scope to look at the signal at the multimeter jacks, you will probably be able to see exactly why you read twice the frequency. As I've mentioned before, if you are using typical open wiring for fast digital signals, you should be amazed when it works. It's remarkably difficult to get a square wave from point a to point b without a controlled impedance transmission line. Automation gear works due to slow sampling and "brute force" filtering. And often dumb luck plays a role. I've seen a lot of stuff work that really shouldn't judging from what the signals look like.



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