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first Eigenmode not found

Posted 9 août 2020, 10:03 UTC−4 MEMS & Piezoelectric Devices, Mesh, Structural Mechanics 6 Replies

Enza P.
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Hi everyone, I have problem in evaluating the eigenfrequency of a sinmple cantilever (one side fixed). I suppose to have problem in building a good mesh, the beam is composed of different layer with differend thikness (min thick: 0.2um, max thick:50um). When I use the eigenfrequency mode, the solution doesn t show the first eighenmode, which I evaluated experimenatlly. Can anyone help me to solve this issue? I can shear the file ... Thank you very much

6 Replies Last Post 23 août 2020, 12:33 UTC−4
Henrik Sönnerlind COMSOL Employee

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Posted: 4 years ago 10 août 2020, 03:56 UTC−4

One possibility is that the input Search for eigenfrequencies around has a value that is far from the expected values.

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Henrik Sönnerlind
COMSOL
One possibility is that the input *Search for eigenfrequencies around* has a value that is far from the expected values.
Enza P.
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Posted: 4 years ago 10 août 2020, 12:02 UTC−4

Thank you very much Henrik for your kind reply. I inserted the "eigenfrequencies around value" that I expected because i experimentally evaluated. But it seems to doesn t work. does it is possible, according your experience, that itis caused by the bad mesh? Or there is something I miss? Thank you very much for the help.

Thank you very much Henrik for your kind reply. I inserted the "eigenfrequencies around value" that I expected because i experimentally evaluated. But it seems to doesn t work. does it is possible, according your experience, that itis caused by the bad mesh? Or there is something I miss? Thank you very much for the help.
Henrik Sönnerlind COMSOL Employee

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Posted: 4 years ago 11 août 2020, 17:56 UTC−4

Usually, eigenvalues and corresponding modes (at least the lower ones) are not that sensitive to the mesh, so I doubt that the mesh quality is the culprit here. Can there be a difference betwen the real world boundary conditions, and those in the FE model?

Have you measured more than one natural frequnecy, and if so, do the others match?

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Henrik Sönnerlind
COMSOL
Usually, eigenvalues and corresponding modes (at least the lower ones) are not that sensitive to the mesh, so I doubt that the mesh quality is the culprit here. Can there be a difference betwen the real world boundary conditions, and those in the FE model? Have you measured more than one natural frequnecy, and if so, do the others match?
Enza P.
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Posted: 4 years ago 23 août 2020, 12:00 UTC−4

Hi Henrik and thank you very much for your replay. Unfortunatelly, I have experimentally evaluated only the first one because I was interested in the "lower" frequency response of the sensor and, actually I am can not do further measurements. Probably you are right, the èroblem is in the boundary conditions. Actually, the structure is very simple, it is a cantilever with a fixed end and the rest free to move. There is a piezo layer betwen two other layer to constitute the sandwich structure, I did not used particular boundary constrain. Can you have some suggestions? Thank you very much again.

Hi Henrik and thank you very much for your replay. Unfortunatelly, I have experimentally evaluated only the first one because I was interested in the "lower" frequency response of the sensor and, actually I am can not do further measurements. Probably you are right, the èroblem is in the boundary conditions. Actually, the structure is very simple, it is a cantilever with a fixed end and the rest free to move. There is a piezo layer betwen two other layer to constitute the sandwich structure, I did not used particular boundary constrain. Can you have some suggestions? Thank you very much again.
Dave Greve Certified Consultant
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Posted: 4 years ago 23 août 2020, 12:30 UTC−4

Without getting into details- that means, details about the experiment and looking at the .mph file...it is impossible to do more than make guesses. Probably a long talk with someone more experienced...who will ask lots of questions about what and how you are doing things...will eventually lead to a solution.

But...having done a lot of experiments myself, some on similar structures...I can say the following: When experiment and theory do not agree, one or both are wrong.

Without getting into details- that means, details about the experiment and looking at the .mph file...it is impossible to do more than make guesses. Probably a long talk with someone more experienced...who will ask lots of questions about what and how you are doing things...will eventually lead to a solution. But...having done a lot of experiments myself, some on similar structures...I can say the following: When experiment and theory do not agree, one or both are wrong.
Enza P.
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Posted: 4 years ago 23 août 2020, 12:33 UTC−4

Yes, i agree with you Dave and, unfortunatelly I really think that I am wrong in the FE simulation!!!

Yes, i agree with you Dave and, unfortunatelly I really think that I am wrong in the FE simulation!!!

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