Making octave plot in SVT to use g/SQRT(Hz) units

[Anatoly2]

I have been trying to measure some random vibrations, and it appears the main analysis approach required mesurement of Amplitude spectral density, typically expressed in units of um/sqrt(Hz), plotted on a 1/3rd octave plot vs frequency.

Is there a way to use the Sound and Vibration Toolbox to do this? The default octave analysis plots power spectrum vs frequency. I can only use the IEC 1260 analysis module (since ANSI does not go below 20 Hz) and it does not appear to have the spectral density mode...

Am I missing something, is there a way to do this?

[chericks1]

Hi Anatoly2,

Will the example SVXMPL_Vibration Analysis(DAQmx).vi work for you? Do you need an octave plot?

Please let me know!

Chad AE
Applications Engineer

[Anatoly2]

Chad:

I was able to get the SVXMPL_Vibration Analysis to work after upgrading the drivers to v7.4. The errors got a lot less frequent, and disappeared completely after about a day.

Now I just can't figure out how to setup a vibration analysis VI to look at random vibrations. According to the vibration literature, I need to plot the amplitude spectral density in terms of EU/Sqrt(Hz) and I can't figure out how to make the SVT do this.

Help, please, we would really like to analyze our site...

[chericks1]

Hi Anatoly2,

Sounds like you're looking for a power spectral density function. Luckily, there is one in the Sound and Vibration Toolkit called SVT Power Spectral Density.vi. There are a couple of examples that demonstrate how to use this VI. Check out any of the Frequency Analysis examples such as the Baseband or Zoom FFT. They all do a PSD measurement.

SVXMPL_Baseband FFT (DAQmx).vi
SVXMPL_Subset FFT (DAQmx).vi
SVXMPL_Zoom FFT (DAQmx).vi

They're all at the following location:

C:\Program Files\National Instruments\LabVIEW 7.1\examples\Sound and Vibration\Frequency Analysis

Let me know if this does not solve your issue!

Chad AE
Applications Engineer

[Anatoly2]

PSD ends up in units of (EU)^2/Hz

I want the AMPLITUDE spectral density, not the POWER spectral density, ASD has units of EU/Sqrt(Hz)

I understand I can take the square root of the PSD function, but the real pickle is that I need octave analysis performed on the ASD. All the octave VI's take EU's, square them internally, and then do the octave analysis on power (EU)^2, that is not what i need.

any ideas?

Message Edited by Anatoly2 on 04-25-2005 02:14 PM

[Anatoly2]

okay, i found the option in the PSD plot to turn it into EU/sqrt(Hz) Units, very nice.

now I just need to figure out how to plug the EU/sqrt(Hz) into octave analysis without squaring it... the octave VI's appear to take in EU an then they want to square the signal and dislpay EU^2...

[chericks1]

Hi Anatoly2,

I've forwarded your request to the Product Support Engineer for Dynamic Signal Acquisition and Analysis for assistance with this issue. I must be mistaken, as I thought the Octave VIs input EU and display EU. Thanks for your patience!

Chad AE
Applications Engineer

[Anatoly2]

Thanks, Chad, I appreciate it

[Anatoly2]

one additional comment: I would like to be able to do the PSD / ASD analysis before the octave analysis, so... to summarize here's what i need (most vibration analysis tools are set up this way:)

raw signal capture
scale voltage to EU
perform ASD analysis (does FFT internally, converts to freq space, units of EU/sqrt(Hz))
perform octave analysis (don't change space or units, just calculate averages by band)
dislpay

The octave VI's as is are black boxes that
intake scaled EU
perform Power FFT analysis (turning into freq space, units of EU^2)
perform octave analysis (average by band)

I need the last two steps separated so i can swap out the power fft for the ASD, any way to do this?

[chericks1]

Hi Anatoly2,

The attached VI will convert an octave spectrum into EU^2/Hz or EU/sqrt(Hz). To use it, just do whatever flavor of octave analysis you want in front of it, then convert it with this VI. There is no need to do a PSD unless you want to see it separately.

Please respond if you have any questions!

Chad AE
Applications Engineer