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January 15, 2010 1:15pm UTC

Matlab script for effective modal mass extraction

Hi everbody,

as posted frequently in this forum, COMSOL does not provide effective modal masses in eigenfrequency analysis for structural analysis. But since effective masses are a very convenient way to determine critical modes from less critical ones, and since this information is often required for documentation, I wrote a Matlab script that extracts the effective masses from an eigenfrequency solution and exports them to an Excel file.

This is how it works:
1. Copy the attached matlab script to your disk (preferably to a directory that is already in the Matlab include path, e.g. C:\Users\...\Documents\MATLAB on windows).
2. Run COMSOL and do the eigenfrequency analysis there.
3. Save your COMSOL eigenfrequency analysis to an .mph file of your choice.
4. Connect COMSOL to Matlab (you don't need to export the fem object to Matlab though).
5. In Matlab, execute the attached effective_mass_analysis.m script (open/run or if located in the Matlab workspace simply type 'effective_mass_analysis' in the command window).
6. The script asks you which .mph file to open, calculates the effective masses and saves them to a Microsoft Excel spreadsheet (you will be prompted where to save it).

In the Excel file there are four different sheets: 'Total Masses', 'Effective Masses', 'Mass Fraction' and 'Cumulative Mass Fraction'. These results resemble the standard output of other FEM software, such as Ansys that provide effective mass output by default.

Unfortunately, this script does not work for large models since the matrix size that Matlab can handle is limited and it may also fail when more than one application mode is used. I have cross-checked the results with only one reference case from Ansys, so there is no guarantee that the script works correctly with all models. Any comments and suggestions are greatly appreciated.

Cheers,
Lars

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January 18, 2010 9:12am UTC in response to Marc Jouan

Re: Matlab script for effective modal mass extraction

Louvet,

The script extracts the number of indepent variables from the application mode fem.appl(1,i).dim . The structural mechanics module uses 4 independent variables: u, v, w, p , so 4 is correct. Of course, the p-column does not give you effective masses. But the script probably does not provide correct results if you manually change it to 3. So you should leave as is and just ignore the p-column.

The problem with this is: I have not yet found a way to link the entries in the mass matrix and result vector to the specific independent variables. If anyone knows how to do this (even with multiple application modes), I could limit the columns to u, v, w, since these are the only ones of interest.

Anyway, does anyone know if modal mass analysis is foreseen in a future release of the structural mechanics module?

Cheers,
Lars

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January 18, 2010 4:41pm UTC in response to Marc Jouan

Re: Matlab script for effective modal mass extraction

Hi everyone,

I have been fighting (yes it has been a gentle fight) with COMSOL for 3 years to get them add the effective mass, so I expect that it will come (once), your scripts are good for simple cases, that is alread very nice.

Unfortunately I'm running heavy models >500kDoF and I have found no way yet.

If each user insists by COMSOL on the importance of the effective mass for modal analysis, I'm sure they will implement it, as its rather straitght forward (once programmed inside COMSOL)

In any case thanks for pushing this issue to support ;)
Ivar

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May 6, 2010 9:17am UTC in response to Marcin Hurkala

Re: Matlab script for effective modal mass extraction

Marcin,

To be honest, I have no idea what these errors mean. Perhaps you could post your mph file here and give a step-by-step procedure of what you did, so I can try it myself.

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May 6, 2010 9:38am UTC in response to Marcin Hurkala

Re: Matlab script for effective modal mass extraction

Hi

from my understanding there is a problem with the matlab <-> Comsol versions and mixed 32 and/or 64 bit mode, that is what I beleive is to be understood from the first message, then it cacades down

Sorry cannot do more just like that

Ivar

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May 6, 2010 11:50am UTC in response to Marcin Hurkala

Re: Matlab script for effective modal mass extraction

Marcin,

I cannot reproduce the error messages you posted. Using the script on your model produced some other error messages though, that I address to the number of independent variables (4 for smsld, see previous post in this thread by Louvet). Changing the application mode to sld (3 independent variables) seems to solve the problem. However, I am not 100% sure if the result is correct, but the script runs without errors. Attached you find the slightly modified model file and the results output file.

Cheers,
Lars

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May 7, 2010 10:03am UTC in response to Lars Tiedemann

Re: Matlab script for effective modal mass extraction

Hi again

I have made s simple test, you make two identical model in sld (u,v,w) and in in smsld (u,v,w,p) (both eigenfrequency) and run them through the script, I see the same dimension for "fem.sol.u" the solution here so I beleive that even in smsld mode "p" is not solved for and you should use a dimension of "3" for "k" on the smlsd model and then it works.

When I compare a simple steel flex beam 1x0.1x0.01 m in fixed-free mode I get < 1E-6 difference from the smsld and the sld models for all items of the effective mass matrices that you develop in yourmatlab script.

So obviosly we have a correct interpetation issue here on the way COMSOL stores the solution w.r.t. the different variables u,v,w and in particular "p".

Finally "p" is related with pressure and probably then also forces, but as most external forces are ignored in eigenfrequency mode (except if you load the model fist in static, save it and rerun from stored solution) it could be that "fe.sol.u" is not populated for "p"

Could be worth a mail to "support" to get it cleared out, hope only this will be implemented in V4 soon

I have found some more info, but I'm still not sure why the number of depedent variables is not 4 in the development of "fem.sol.u" for "smsld" (but in V4 i noted that "p" vas treated differently from u,v,w, so perhaps not always solved for). There are the commandes flngdof(fem) to get the number of dof (lines of fem.sol.u) ans solsize(fem.sol) gives the number of columns, but I have nothing more

Any comments ?

have fun Comsoling
Ivar

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June 21, 2011 8:30pm UTC in response to dnh37

Re: Matlab script for effective modal mass extraction

Also, the original poster notes: " this script does not work for large models". Not sure the limit, but the filesize I am trying is 20MB

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July 11, 2011 7:39am UTC in response to dnh37

Re: Matlab script for effective modal mass extraction

Comsol V4.x supports modal mass extraction, so you don't need the Matlab workaround presented here anymore. Though it's not easy to find in the documentation, Comsol 4.x scales the Eigenvalue solution vectors to the mass matrix. In effect, you have new dependent variables: the participation factors for each mode. With these, you can deduce the effective modal masses and effective mass fractions.

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