How do I fix this modalfrf plotting error?

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Noah Johnson 2020-10-6

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https://ww2.mathworks.cn/matlabcentral/answers/606346-how-do-i-fix-this-modalfrf-plotting-error

评论： Noah Johnson 2020-10-20

I am performing an experimental modal analysis on a simple beam in a free-free boundary condition. I have collected roving hammer excitation data. 3 impact locations with 3 impacts per location. The data is collected in segments of exactly 10s with a sampling rate of 10kHz. i.e. 3 impacts per location resulting in a single matrix for each location as seen here. These are the acceleration values.

When using modalfrf, I set the window to 10*Fs, which represents the length of one 10 second impact. This is the same way it is done in the modalfrf example seen here. It is under the section "Roving Hammer Excitation"

https://www.mathworks.com/help/signal/ug/modal-analysis-of-a-simulated-system-and-a-wind-turbine-blade.html

However, with a 10*Fs length window, I get the following result:

If I arbitrarily change my window to 5*Fs (a 5 second window even though my data is in a 10s window), the function works as expected, but these coherence values are not reliable since the window is incorrect.