Why do I get so many phase jumps in the FRF of an impact hammer test?
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Hello to the community!
I am performing an experimental modal analysis and the following problem occured: I use NI Labview to get my measurement signals from an impact hammer and our laser vibrometer; so for those well familiar with modal analysis - its a mobility Frequency Response Function (FRF = Velocity/Force).
Afer loading the data into Matlab I am doing software triggering and use just the spike of the hammer impact; afterwards I am performing a Fast Fourier Transform of the truncated and windowed hammer and vibrometer signals and I calculate the FRF by dividing the two transformed signals.
Now my problem arises: If I plot the phase of the FRF I get a highly oscillating phase plot - the phase is rapidly jumping between +180° and -180°, and this over a wide frequency range:
If I use unwrap(...) I get the following result, which does not really look better either:
So has anyone an idea? Both signals look quite sensible when plotted over time; but I really have no idea right now what causes these jumps in the phase of the FRF.
Maybe someone can help? Thanks!
2 Comments
TM Med
on 28 Apr 2017
Hello, I have the same problem for a different application of fft2,did you find a solution? thank you in advance for your help.
Greg Dionne
on 28 Apr 2017
Edited: Greg Dionne
on 28 Apr 2017
Have you tried the modalfrf function with the Signal Processing Toolbox?
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