Not getting change in natural frequency due to change in structural stiffness

[mech]

hi all,
I am using SCXI-1531, SCXI-1000 for frequency measurement along with LabView 7.1 . I want to see the change in natural frequency of pipe due to development of a crack. But in my expt I am not getting proper change. Change obtained is very less than I got by the FFT on the same pipe. So please anyone can advise on the same?

Thanks.

[rpursley8]

Maybe your FFT resolution isn't small enough. Acquire for a longer period of time to increase the resolution of your FFT. How much change are you expecting in the natural frequency?

[mech]

I am using SCXI-1520 and LabView for frequency measurement. I have used FFT instrument earlier for same purpose. But now I am not getting change in natural frequency which I get from earlier. In 4mm thickness pipe for a crack of 3 mm depth, second or third natural frequency should change at least by 5-6 Hz. But I am getting only by 1 or max 2.

[rpursley8]

Can you post your vi and data?

[E Murphy]

Hi mech: I agree with Randall about the resolution issue. Resolution is equal to 2 X WindowFactor X Fmax / NumberofLines.
WindowFactor equals 1 for Rectangular (no window), 1.5 for Hanning and 3.5 for Flat Top.
If you are displaying multiples of 400 lines then, for example, for a 800 line spectrum with Fmax = 1 kHz and a Hanning window, your resolution will be 3.75 Hz. This value is not to be confused with "cursor jump" or delta frequency indication. Assuming you use 1 pixel per line, one cursor jump will have a delta frequency value of 1000 Hz / 800 lines = 1.25 Hz / line
Resolution will be increased if you lower Fmax or increase number of lines. We are assuming you are using a suitable Anti Alias filter.
Data points needed for a 800 line spectrum is 2048 points (Fmax X 2.56)
Regards and Happy Holidays, Eduardo Murphy

[mech]

Dear Sir,
I have constructed this VI as per instructions given by NI person. I am forwarding it to you for your help. In this no of points taken are 10000 and sampling frequency is 3000. Window used is Hanning.

Thanking you,
Nandkumar.