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Shaft Speed & Vibration Phase Measnt. from Trigger?

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Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby musicman » Thu Sep 01, 2016 8:32 pm

I intend to purchase a 4ch scope (2405A) which would be used to measure vibration from rotating machinery. My question is regarding triggering the data collection. I need to make a decision regarding how I measure the speed of a rotating shaft and also obtain a phase relationship between the measured vibrations.

I would plan to use channels 2,3 & 4 to measure vibration using accelerometers. The decision I have to make is regarding what signal I can use in CH 1 to trigger the data collection. I can provide a once-per-revolution pulse signal using a displacement probe close to a shaft protrusion such as a screw. I assume this would work but the problem is the cost of this setup is high for a 'hobby'. An alternative is to use an inductive switch sensor which apparently produces a TTL signal.

My first question is very specific and is - "could the TTL signal be used to obtain shaft speed and to provide a phase reference between the vibration signals?"

My second question is more general and is - "how easy would it be to obtain a shaft speed measurement and relative phase measurement between the signals in channels 2, 3 & 4 if a suitable trigger signal could be supplied to channel 1?"
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby Martyn » Fri Sep 16, 2016 10:15 am

If you download the software and run it in demo mode you can see the basic features.

In the bottom right of the display are the phase rulers, green circle, which can be pulled onto the trace to indicate the start and end of a full cycle. In the bottom left are the time rulers, white square, which can be pulled on to the trace to see the phase relationship between any point on the screen and the phase rulers.

You can set set up a maths channel to calculate the frequency of a signal and convert this into revolutions per second/minute etc drawing the resultant trace on the screen.

If you require assistance in setting this up please email support@picotech.com
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby musicman » Tue Sep 27, 2016 8:32 pm

Hi Martyn,

Thanks very much for your response. I have since purchased the 2405A scope.

I am still a bit confused about the phase measurement. In demo mode I can trigger of channel A or channel B and this will freeze the signal and I can use the phase or time rulers.

However my requirement is to measure the relative phase of the signals between channels (eg between channels A & B). When I trigger off channel A the channel A signal is stable and does not move (as one would expect). However the signal in channel B is not stable and therefore it does not seem possible to measure the phase between these signals?

Could you please advise if it is possible to measure the phase between two signals on any of the four channels assuming, of course, the signals to be sine waves and of the same frequency?

Another issue I have is that I thought I would get a software development kit (SDK) with this scope but it appears tht this is not the case - could you please confirm?

Thanks again.

Irvin
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby Martyn » Wed Sep 28, 2016 8:40 am

You can do this reasonably well using Maths Channels and the following equation
Code: Select all
acos(integral(A*B)/(sqrt(integral(A*A))*sqrt(integral(B*B))))/Pi*180

although you have to take the value after the phase settles down as the start conditions are unknown and the first part of the trace will be inaccurate.

The SDK is available here
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby musicman » Wed Sep 28, 2016 5:58 pm

Thanks Martyn,

I take it there is no way, other than using your formula, to measure the relative phase from two channels? So I can't use the phase rulers in some fashion?

Thanks again
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby musicman » Wed Sep 28, 2016 6:09 pm

Martyn

When I go to your link for SDK I cannot find an SDK file for the 2405A scope?

Thanks

Irvin
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby musicman » Wed Sep 28, 2016 7:46 pm

Hi Martyn

Sorry for the separate messages.

Could you please cite a reference that explains the equation for phase calculation?

Thanks again.

Irvin
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Re: Shaft Speed & Vibration Phase Measnt. from Trigger?

Postby Martyn » Wed Oct 05, 2016 9:35 am

Please email support@picotech.com for updated SDK files to support your device.

I don't have a citation for the formula, but have worked it through from a mathematical viewpoint with some example waveforms to check that it works.
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