Vibration Frequency Response

[sarclee]

Hi, I would like to ask if I should expect to see the frequency response same as PSD input for random vibration when the subject is stiff enough >500Hz?

I create a plate and run a modal analysis to get 700Hz+++

Then I run a PSD profile as below...

I thought I would get the output same as input as in grms around 10+ but some points are very high~~~ Why is that happening?
However, I do notice the trend of the frequency response is similar to the PSD input...

 

[GregLocock]

I can't really understand what you are saying, but it looks as though you have resonances above 500 Hz

Cheers

Greg Locock


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[driftLimiter]

Yea its not clear what your model is, the constraints. Resonances can occur at high frequency due to harmonic frequency excitation. If you model this many degrees of freedom then do the modal analysis for a high number of modes you will always find that the response is greater than the excitation (amplification). The closer you get to infinitely rigid theoretically the less amplification you would expect. But like i said for many degrees of freedom and many many modes, the model is always going to exhibit relative flexibility compared to infinitely rigid.

 

[sarclee]

Sorry... my question does not clear enough.

Assuming I have a base plate as shown in the picture and I put it on the shaker to do vibration.

Some of our colleagues expect to see the vibration energy not very far from the input. The PSD input is 11 GRMS but I measure some locations (opposite to the constraint of the base plate) show very high amplification, up to 100 GRMS~~~

So, I was thinking if the base plate is designed to be very rigid (make the frequency up to 1000 Hz, usually we design the test jig ~1000Hz), the grms would be lower?

 

[GregLocock]

Yes, because you'll get rid of the resonances. The bad news is that the Q of welded structures is often much greater than 10, but your bolted joints will bring that down.

Cheers

Greg Locock


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