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What is the general practice for selecting time interval for RMS calculation for vibration data (mm/s)

Should it be linked to the the frequency of the motor or the sampling rate?

Thanks!

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  • $\begingroup$ Sampling frequency should be at least 2* frequency you are interested in because of harmonics... $\endgroup$ – Solar Mike Jan 22 at 7:16
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  • Your sampling frequency $f_{samp}$ should be at least 2 - Times higher the highest expected relevant frequency $f_{max}$ because of the Nyquist - Shannon Theorem. However you minimize error by using a higher sampling frequency (Rule of thumb $f_{samp} = 5 f_{max}$)
  • If you want to get the RMS of smaller frequency bands (for example by performing a FFT), your measuring time $t_M$ should be long enough to get a high enough frequency resolution $\Delta f $ (difference between two interesting frequency bands, formula see next point).
  • Also if you are not aiming for a fft, you should measure long enough to measure at least one full cycle of the expected minimal frequency of interest $f_{min}$ ($f_{min} = \Delta f = 1/t_M$)
  • Longer measurements provide kind of a averaging.

Example:

  • You want to measure a fan with 9 blades rotating with 10 Hz.
  • In one measurement, you want to have at least 12 revolutions.
  • You definitly want to measure the 90 Hz blade pass frequency.
  • You also want to measure 5 harmonics -> 450 Hz

  • To make sure you get the 450 Hz you need to sample with at least 900 Hz, $f_{samp} = 2250 Hz$ would be better though.

  • To measure one cycle you have to measure at least 0.1 s. If you want to measure over 12 revolutions you need to measure at least $t_M = 1.2 s$

--> You end up with:

  • $f_{samp} = 2250 Hz$ -> $f_{max} = 1225 Hz$
  • $T_{m} = 1.2 s$ -> $f_{min} = \Delta f = 0.83 Hz$
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  • $\begingroup$ Does taking fsamp to be 5 instead of 2 introduce unnecessary noise into the signal? $\endgroup$ – zae Apr 11 at 1:49
  • $\begingroup$ You will get higher frequencies in your signal as well, when sampling with a factor of 5. They will be harmonics and/or noise. You could also add a Lowpass Filter with a cutoff around the factor 2. $\endgroup$ – gmc Apr 11 at 7:30

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