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Participant

Join Date: Oct 2017
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Vibration Analysis of Rotating Machine

10/27/2017 5:34 AM

hello all,

I am working on vibration analysis project. I found problem while phase measurement. I am doing coding in MATLAB tool where i used FFT algorithm. so when machine starts, then there is rapid increase in rpm and then settled at its desired speed after some period. My problem is to measure amplitude and phase at that transient period accurately. Please help me and thank in advance for reply.

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Guru

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#1

Re: Vibration Analysis of Rotating Machine

10/27/2017 9:24 AM

Well, what you need is an encoder on the shaft of the motor that controls the vibration sampling. If you sample at a constant time rate and the motor is accelerating, FFT will not give you a good answer.

For example, if you have an encoder that triggers 1024 samples per revolution, you can then take those samples an perform an FFT to get your amplitude and phase spectra.

If you must sample at constant time rate, you still need the encoder. Also sample the encoder reading. Then using the MATLAB interpolation function get interpolated samples at encoder (motor shaft) positions so that you have the number of samples per revolution as the size of your FFT. Use these interpolated values for your FFT.

Motor vibration must be referenced to shaft position.

https://www.mathworks.com/help/matlab/ref/interp1.html

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Participant

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#4
In reply to #1

Re: Vibration Analysis of Rotating Machine

10/30/2017 1:10 AM

Hey thanks for your reply. My guide tells me to use a tracking filter . So how can i used it and how it simplifies the problem?

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#2

Re: Vibration Analysis of Rotating Machine

10/27/2017 9:56 AM

What many who are first delving into FFT analysis forget is that phase is a relative measurement not an absolute measurement. Nominally at t=0 all of the cosine functions are assigned a phase of zero.

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#3
In reply to #2

Re: Vibration Analysis of Rotating Machine

10/27/2017 11:17 AM

That's a good point about phase.

I have actually worked on this as a hobby project.

If you analyze the sound or vibration from a rotating machine, you will have two different types of signal, random noise and a periodic signal that generally will have harmonically related discrete components.

Assuming for now that your periodic signal has the fundamental present. Its peak (zero phase) will occur at some shaft position. The second harmonic, if present, will have two peaks 180 degrees apart oriented at some angle. The third harmonic will have three peaks 120 degrees apart oriented at another angle, and so on for the other harmonic frequencies present (360/n degrees apart).

If your FFT is synchronized to shaft position, you can use the phase spectrum to determine where all the peaks (zero phase) occur for the harmonic frequencies that are present. If the phase of the nth harmonic is Dn degrees, then related to shaft position, the peaks for that harmonic will be at

Dn / n degrees + k * 360 / n,

where k = 0 ... n - 1

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#5

Re: Vibration Analysis of Rotating Machine

10/30/2017 2:28 PM

So you need a three-dimensional presentation of the data:

(time axis) - (speed axis) - (vibration sensor amplitude data)

could still do the freq analysis with Matlab
FFT

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