Data error or curve fitting error? 4

[GMarsh]

Hi,

I am trying to curve fit FRFs of a cylindrical casing. When I extract the modal parameters and see the mode shape, I am getting half of the mode shape quite amplified, as shown in attached document. Actually the lobes seen in mode shape should have been equally amplified along the periphery.

Can someone tell it is a mistake with FRF data or error in curve fitting? The FRFs have very good coherence and reciprocity.

Thank you in advance.

Kind regards
Geoff

 

[GMarsh]

hackwsaw - interesting inference. But what you said applies to operational mode shape I guess because we are not considering here excitation due to milling tool. We are looking at simple modal testing using impact hammer. So I think mean force value and varying amplitude may not come in picture. It is just a single impulse of force.

Greg - I am sorry. I don't know what you meant by calculating 'quadrature frequency' at each measurement. I have 144 FRFs. So did you mean to say to find the first fundamental frequency exactly at quadrature for all these and then to group them ?

And by "Q ~D(Phase)/dFreq", I hope you are asking me to calculate Q-factor, again for the first fundamental mode.

And now I am again re-looking at your previous graphs. Can you please tell how you arrived at that 6dB for repeatability ? I mean what reference you took for amplitude dB calculation ? And looking at your phase plot, we can see that the 'new' data quadrature is very good. So I think I will go ahead with new drive point FRFs.

Many thanks Greg, hacksaw for all your useful tips and discussion.

Kind regards
Geoff

 

[GregLocock]

Eyeballing it, 105_new had a mag of about 750, 105_old is 1500, ergo 6 dB.

Quadrature is merely the frequency near 1366 hz where phase = +/-90 degrees. A lot of modal analysis consists of writing little scripts to page through EVERY FRF in the survey looking for clues.

Q is proportional to d(phase)/d(frequency), that is a high Q mode has a rapid change of phase.

Sorry I got my olds and news muddled, 105 old has the right damping but the wrong frequency, 105 new has the right frequency but the 'wrong' damping. If I were analysing by hand I'd use the response at quadrature for this modeshape, and let the 'frequency' slither around between 1365 and 1367.





Cheers

Greg Locock


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