Sound and Vibration Toolkit

[WantToGolf]

We have LabView 7.1 and the latest Sound and Vibration Toolkit. I am interested in finding the natural frequency of metalic parts AND entire mechanical assemblies. To obtain the natural frequency, we have purchased an accelerometer and impulse hammer to perform bump testing. From the reading I've done, I believe I should use the FRF functions from the S&V toolkit. Can someone confirm this or direct me to the proper method?

thanks for your help

[chericks1]

Hi WantToGolf,

Sounds like you're on your way!

Cable the output of the impulse hammer to analog input 0 and the output of the accelerometer connected to your device under test (DUT) to analog input 1. The frequency response functions (FRFs) included with the Sound and Vibration Toolkit are the methods you'll use to find the natural frequency of the DUT.

I would recommend using Baseband Frequency Response (DAQmx - No excitation).vi, located at:

C:\Program Files\National Instruments\LabVIEW 7.1\examples\Sound and Vibration\Frequency Analysis\Dual Channel Analysis\SVXMPL_Baseband FRF (DAQmx - no excitation).vi

This VI continuously acquires data from two channels in the same device. No source is provided from the DAQ board, as you will be providing one from the impulse hammer. Triggering from either channel is supported.

The data from each channel is scaled to engineering units, windowed, and the frequency response is computed. The results of the frequency response measurement can be displayed on a Bode plot or a Nyquist plot. The coherence is also graphed to evaluate the validity of the measured frequency response.

You could then use the frequency response to determine the natural frequency of the DUT. Let me know if you have further questions or if this does not resolve your issue!

Chad AE
Applications Engineer

[WantToGolf]

Thanks, I'll give it a try.

[WantToGolf]

Chad,

Why do they call it "No excitation" when the vi contains the "POPUP" vi that can enable excitation?

[chericks1]

Hi WantToGolf,

You are correct in stating the VI can enable the DAQ board (or DSA) to supply an IEPE excitation current for the accelerometer, however the "no excitation" in the name of SVXMPL_Baseband FRF (DAQmx - no excitation).vi stems from the fact the stimulus signal is not coming from the DAQ/DSA board. Rather, you are providing the stimulus from the impulse hammer.

In comparison, a VI such as SVXMPL_Baseband FRF (DAQmx AI and AO).vi will send a stimulus signal from Analog Output 0. This signal would be cabled to both Analog Input 0 and the DUT. The response, detected by the accelerometer, would be cabled to Analog Input 1 just like in your application.

Best regards,

Chad AE
Applications Engineer

[WantToGolf]

Great explanation Chad. Many thanks.

[WantToGolf]

OK, I setup a flat piece of steel about (0.1" x 3" x 48") in a vice. It vibrates on it's own for some time after an initial bump. I'm not getting the type of Bode plot that I expected and I'm sure it's something wrong in my setup. I have the hammer on channel 0, the accelerometer on channel 1 and have set the trigger on and to the stimulus channel, rising and at levels from 0.1 to 1.0. With 1024 samples I've tried high scan rates 51,000 sps to 1024 sps and I seem to get the same results. I believe the natural frequency of the flat steel as clamped is around 17 Hz because with another program, an FFT, I see a spike at that frequency if I push the steel to one side then let go. You can see the steel move. I believe that I must first try to get the coherence for at least the range of frequencies of interest near 1.0 but so far I haven't had much luck. The coherence is not flat at all and around 17 Hz is more of a wave than a constant line.

Any suggestions?

[chericks1]

Hi WantToGolf,

Nothing comes to mind right away. From my experience and research, it seems everything is configured correctly. I've forwarded your request to the Product Support Engineer for Dynamic Signal Acquisition and Analysis for more assistance. Thanks for your patience!

Chad AE
Applications Engineer

[chericks1]

Hi WantToGolf,

Is it possible for you to send screenshots and/or your code? We would like to verify the configuration is correct. Accompany these with an outline of your procedure, and I will be able to assist you further.

Thanks!

Chad AE
Applications Engineer

[WantToGolf]

I tried to attach some screen shots in a rtf format. I received this message:

"You may include a limited set of HTML in your messages."

What should I do?