How does modalfit manages 3D FRFs?

Hi,

I'm using modalfit to get the mode shapes of a structure from experimental measurements. The FRF dimensions are p-by-m-by-n. In my case:

p=15361 : 15361 frequency bins.

m=36 : excitation signals (21 hammer impacts in Z axis + 15 hammer impacts in Y axis)

n=2 : response signal (fixed accelerometer in Y axis + fixed accelerometer in Z axis)

I'm observing an issue with how modalfit manages the 3D FRF. According to the documentation, it should use all dimensions to form the mode shape. However, what I see is that it seems to use only the first response signal (accelerometer in Y). In the image below, it is represented by the yellow line being over the blue line.

If I flip the dimensions of the FRF by using FRF(:,:,2:-1:1) to do the modal fit, I can see that it still only uses the first response signal (which is now accelerometer in Z). In the image below, it is represented by the purple line being over the orange line.

If modalfit was using both response signals, shouldn't it ouput the same mode shape no matter the order of the response signals? If so, orange and blue lines should be one on top of the other.

Thanks!

Answer 1

[Ritam]

The documentation indicates that in modalfit, m corresponds to the number of responses and n corresponds to the number of excitations. If a different convention was used, could you please verify whether you are obtaining the expected results when following the denominations as specified in the documentation?

If the issue persists after confirming this, I recommend reaching out to MathWorks Technical Support for further assistance.