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How to plot normalized amplitude of cantilever vs frequency

Posted 23.04.2011, 11:55 GMT-4 Modeling Tools & Definitions, Parameters, Variables, & Functions, Results & Visualization Version 4.1 3 Replies

dnh37
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Hi,

I want to draw a simple cantilever, find eigenfrequencies, and plot the relative amplitude of the end of the cantilever versus eigenfrequency.

I use comsol 4.1, "solid" module, eigenfreq analysis. I draw a cantilever 100um x 10um x 1um, fix one end, and set it to normalize the mode amplitudes to each other (by setting Study 1 > Eigenvalue Solver 1 > "Output" setting --> set Scaling of eigenvectors to Mass matrix).

Next I set up a Model coupling > "Maximum" and call it maxop1. I select the whole domain.

Then I add a 1D Plot Group to the Results, and type Expression = maxop1(w) to find the maximum amplitude in the z-direction. However then I have to select a given point, and it gives weird results. The amplitudes don't look right, and also even though I tell it to plot eigenfrequency on the x-axis (by going Results > 1D Plot Group > Point Graph > X-axis Data and selecting Parameter = Frequency spectrum), it just plots 0 to 0.5 on the x-axis (should be in the range of MHz not the range of 0.5-ish) and plots them evenly spaced out (eigenfrequencies are not evenly spaced).

Any ideas?

Thanks,

3 Replies Last Post 25.04.2011, 00:25 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 23.04.2011, 12:47 GMT-4
Hi

A few things, normally an eigenfrequency is not good for amplitude evaluation (at most relative values then).
This because the damping is not considered

If you normalise with respect to the mass matrix, it's worth to plot the square of the relative masses, divided by the mobile mass, and this for u,v,w, in a table. The values you will see are the relative energy in the different modes, normally if you have chosen enough modes you should add up to "1"

This is how I do it v4.1.0.185 ;)

--
Good luck
Ivar
Hi A few things, normally an eigenfrequency is not good for amplitude evaluation (at most relative values then). This because the damping is not considered If you normalise with respect to the mass matrix, it's worth to plot the square of the relative masses, divided by the mobile mass, and this for u,v,w, in a table. The values you will see are the relative energy in the different modes, normally if you have chosen enough modes you should add up to "1" This is how I do it v4.1.0.185 ;) -- Good luck Ivar

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dnh37
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Posted: 1 decade ago 25.04.2011, 00:24 GMT-4
Thanks for the suggestions about tabulating data and making sure they add up to 1,

However I'm still fuzzy on the details of actually how to DO it... i.e. to make the model coupling work, or do it in a different way.

Basically my current state is: I have an eigenfreq simulation of a cantilever
The state I want to be in is: I'm looking to make a scatter plot of the relative amplitudes of the end of the beam (or of the max amplitude in the beam, it doesn't matter much at this point) on the y-axis versus frequency on the x-axis..

Any ideas?

Thanks,
Thanks for the suggestions about tabulating data and making sure they add up to 1, However I'm still fuzzy on the details of actually how to DO it... i.e. to make the model coupling work, or do it in a different way. Basically my current state is: I have an eigenfreq simulation of a cantilever The state I want to be in is: I'm looking to make a scatter plot of the relative amplitudes of the end of the beam (or of the max amplitude in the beam, it doesn't matter much at this point) on the y-axis versus frequency on the x-axis.. Any ideas? Thanks,
dnh37
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Posted: 1 decade ago 25.04.2011, 00:25 GMT-4
Oh wait, I didn't notice you attached a file. Perhaps you showed me in the file already! I'll take a look tomorrow morning when I'm at a computer with COMSOL.

Thanks
Oh wait, I didn't notice you attached a file. Perhaps you showed me in the file already! I'll take a look tomorrow morning when I'm at a computer with COMSOL. Thanks

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