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Displacement amplitude frequency response
Posted 16.11.2010, 10:50 GMT-5 3 Replies
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Hi,
I was trying to obtain the frequency response plot for a simple cantilever. I wanted to observe the variation in displacement with frequency (similar to transfer function frequency response in control systems).
How can I get such a plot from COMSOL?
I was trying to obtain the frequency response plot for a simple cantilever. I wanted to observe the variation in displacement with frequency (similar to transfer function frequency response in control systems).
How can I get such a plot from COMSOL?
3 Replies Last Post 14.11.2014, 13:55 GMT-5
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Posted:
1 decade ago
Hi
I believe there is an example in 3.5, so there should be one (or it will come) in v4 too.
I would suggest to set up your cantelever, try 2D first it's quicker to solve and debug, fix it at one end, apply a boundary load () i.e. 1[N) at the other end, and performa frequency sweep (harmonic analysis), perhaps start with an eigenfrequency to define a start and stop frequency for the frequency sweep, such to includea few modes.
Then you plot the amplitude of a point, or an average of the tip area versus frequency. Use a log scale., you might also normalise by your excitation force amplitude (if it's 1[N], its not really required;)
Check the real and imaginary part of the displacement, this allows you to extract the phase too, and you can get a full Bodee plot
The main issue is the noramlisation, as the amplitude will essentially depend on the damping you introduce (or not). COMSOL has some numerical damping dependent on the physics and solver settings, so normally the amplitudes remain finite, but can be rather big and unaturel compared to the finite size of your canteliever.
--
Good luck
Ivar
I believe there is an example in 3.5, so there should be one (or it will come) in v4 too.
I would suggest to set up your cantelever, try 2D first it's quicker to solve and debug, fix it at one end, apply a boundary load () i.e. 1[N) at the other end, and performa frequency sweep (harmonic analysis), perhaps start with an eigenfrequency to define a start and stop frequency for the frequency sweep, such to includea few modes.
Then you plot the amplitude of a point, or an average of the tip area versus frequency. Use a log scale., you might also normalise by your excitation force amplitude (if it's 1[N], its not really required;)
Check the real and imaginary part of the displacement, this allows you to extract the phase too, and you can get a full Bodee plot
The main issue is the noramlisation, as the amplitude will essentially depend on the damping you introduce (or not). COMSOL has some numerical damping dependent on the physics and solver settings, so normally the amplitudes remain finite, but can be rather big and unaturel compared to the finite size of your canteliever.
--
Good luck
Ivar
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Posted:
1 decade ago
Thank you, Ivar.
Did as you suggested and got a frequency response!
Will try out the Bode plot next as your advice. Did not give damping, wil try that too.
Will revert back again in case of problems!
Did as you suggested and got a frequency response!
Will try out the Bode plot next as your advice. Did not give damping, wil try that too.
Will revert back again in case of problems!
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Posted:
1 decade ago
Hi, Ivar,
I think I came across the issue you described below. I also got abnormal larger displacement values.
See my post here:
www.comsol.com/community/forums/general/thread/49337/#p127993
I assume the damping is introduced by the material property, but seems there is something wrong.
Is there any solution to such issue?
Tahnks.
I think I came across the issue you described below. I also got abnormal larger displacement values.
See my post here:
www.comsol.com/community/forums/general/thread/49337/#p127993
I assume the damping is introduced by the material property, but seems there is something wrong.
Is there any solution to such issue?
Tahnks.
The main issue is the noramlisation, as the amplitude will essentially depend on the damping you introduce (or not). COMSOL has some numerical damping dependent on the physics and solver settings, so normally the amplitudes remain finite, but can be rather big and unaturel compared to the finite size of your canteliever.
--
Good luck
Ivar