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# Modal analysis

Pioneer

Hi,
How we can choose the order of the modal in order to obtain the stabilization diagram ?
Than how can we choose poles.
For example, in this stabilization diagram, I find difficulty to select poles.

Thank you
Asia

6 REPLIES

# Re: Modal analysis

Siemens Legend

The stabilization diagram you provided, there aren't any stable pole "S".  Here I upload a picture to explain what O, F, D, V & S mean on the diagram, you want to select the pole where it has  "S", both frequency, dampling and vectors are stable within the tolerance.

You can find this information by clicking the question mark on the upper right hand corner of the Stabilization minor worksheet.  I would look at the integrity of all the FRFs measurement you acquired, and look at the coherence of each one.  If the coherence is bad, the FRF might not be any good.

Good Luck.

Hong

# Re: Modal analysis

Pioneer

First thank your for your response .
These are all the FRF mesured and the sum of them:

And this is the stabilization diagram:

For the coherence function, they are no saved in my project.

# Re: Modal analysis

Siemens Legend

You should always saved the coherence function in impact testing.  Ideally where there's a resonance(Peak in the the FRF), you would like the coherence close to one; it indicate the relationship between the output(accelerometer) vs input(hammer), when it's close to one that mean the energy input by hammer is excite the structure and responde is contribute by the input, and when the coherence is low, that mean there's energy loss due to some other contraint or could be anti-resonance.

How big is the structure you're measuring?  Can you use more accelerometers?  You might not have enough data(FRFs) to analyze the structure.

In addition what you're already select to display, you should select MIF/Imaginary Sum then there are more options from the drop down menu, these are different type of Mode Indicator Functions(MIF):

again you can find the information by going the question mark on the upper right hand corner in any of the modal analysis workbook you're in.

Hong

# Re: Modal analysis

Creator

Have you considered using the Polymax Curvefitter rather than the Time MDOF curvefitter?

Here is a picture of the difference in the stabilization diagram for the same set of FRFs:

A little easier to interpret the Polymax curvefitter results.

# Re: Modal analysis

Creator

Which kind of structure are you characterizing? Which is the frequency range of interest?

It seems that you have issues with you FRFs, did yout confire the windowing properly? And the pretrigger? The ranges were adjusted? What is the S/N ratio?

If all the previous is correct, it seems that you are getting very low responses (very big structure? ,close to other noise sources (machines operating nearby)??

If you get a good FRF set, you can use Time MDOF without any troubles.

# Re: Modal analysis

Pioneer

Yes,
when I tried with the polymax method, results are better.

Thank you

Asia