Problem with RPM measurment via laser sensor

[FFXX]

Hi, experts!

Can you help me to solve the problem? I want to measure RPM of shaft with a laser sensor (see figure below). 

At first I tried to use PXI-6602 module. But using of standard DAQmx Frequency measurement task had bad results (numbers were jumping randomly). So I decided to use one of free analog inputs of PXI-4461 to watch signal shape.

In theory diagram of the signal is supposed to be something like this:

But I received some unwanted pulses:

I’ve tried to eliminate them via filters, but I don’t receive acceptable results (I’ve used Chebyshev filter).

How can I eliminate them?

Any ideas would be appreciated.

[johnsold]

My guess is that your Laser Tape Strip is not applied perfectly smoothly. You either have a wrinkle or bump, or possibly a scratch or dirty spot on the tape.  I have worked on quite a few devices with reflective tape targets for optical sensing and find that they are quite sensitive to small imperfections.  I do not recall using a laser in such a system, but if the spot size is small, that could aggravate the situation. If the focus is adjustable, try defocusing enough that the spot diameter is about the width of the tape (in the direction of rotation) or slightly larger.

If the gaps are smaller than the pulse width at the highest shaft speed, you could use a minimum width discrimination to get better data. If the shaft speed cannot vary much between one pulse and the next, match the pulse width approximately with the width of the previous pulse or perhaps an average of several previous pulses.

The best choice may depend on the range of speeds, the pulse widths, the nature of the dropouts, and other factors.

Lynn

[FFXX]

Thanks for your prompt answer, Lynn, and of course I'll try to follow your recommendations.

But let me clarify the terms. I've been working with laser tapes for a long time and I think I've mastered the technique of their installation. So I’ve no doubts about the applying of laser tape strips. But I’ll check it one more time anyway.

I think the problem is placed in sphere of reflections because the amount of dropouts significantly increases when I cover the shaft with protective case.

Also I’ve to say that the shaft can vary his speed in range from 0 to 6000 rpm.

And the sensor cannot change the focus.

I hope this information will be useful.

[johnsold]

From your reply I would tend to agree that you have a reflections issue.

With the constraints you have I would look into a timing discrimination process.  Look at the longest "recent" pulse.  Any pulse which is significantly shorter than that or which consists of multiple pulses with an aggregate length approximately equal to that longest pulse has a dropout. How you define "recent" depends on how fast the speed can change from one revolution to the next.  Without seeing some real data it is difficult to propose a detailed procedure for dealing with this.

Lynn

[Henrik_Volkers]

You can try the tone detection vi to find the fundamental frequency of the signal 🙂 (Would work best if the duty cycle is around 50%)

[FFXX · Accepted solution]

I've solved the problem!

All this time I've been using Baumer optical sensor and I noticed that the quality of the signal depends on excitation. I decided that applies to laser sensor too. So I've tried to raise the level of excitation of it and the dropouts went away.

Thanks for everybody!

P.S.Henrik Volkers, it is very inaccurate method, i've tried.

[henk1000]

Hello,

I have that you used a Baumer sensor. May I ask you if it was with LABVIEW and modbus? 

I am having issues to read the values from a Baumer laser sensor via Modbus with Labview.

Thanks.