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Eigenfrequency Analysis

Posted 2023年7月25日 GMT-4 06:46 MEMS & Piezoelectric Devices Version 5.6 3 Replies

George Kalliarekos
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Hello, i want to model a cantilever beam piezoelectric bimorph with 2 blocks of PZT-5A and a middle block of stainless steel.When i compute the analysis the 6 eigenfrequencies are correct but the expression of the displacement is displacement magnitude and not total displacement like other studies i have seen and the values are way wrong. Any suggestions?

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3 Replies Last Post 2023年7月26日 GMT-4 08:51
Mark Cops COMSOL Employee

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Posted: 9 months ago 2023年7月25日 GMT-4 08:42

Hi George,

An eigenfrequency analysis solves for the natural frequencies and mode shapes. The mode shape is a vibration pattern (it can be scaled or normalized and still the same mode shape is maintained). The mode shapes are not meant to be interpreted as physical displacement amplitudes.

To determine physical displacement amplitudes, you need to solve a frequency domain study with an actual load condition applied.

Best, -Mark

Hi George, An eigenfrequency analysis solves for the natural frequencies and mode shapes. The mode shape is a vibration pattern (it can be scaled or normalized and still the same mode shape is maintained). The mode shapes are not meant to be interpreted as physical displacement amplitudes. To determine physical displacement amplitudes, you need to solve a frequency domain study with an actual load condition applied. Best, -Mark
George Kalliarekos
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Posted: 9 months ago 2023年7月26日 GMT-4 04:29

Thank you Mark for your response. My problem is at the default plote of Mode Shape the eigenfrequencies are correct and the mode shapes are correct but it shows at displacement magnitude and not at total displacement and they are completely wrong .I ran some models from the Comsol Video Gallery and the results are way off(my model 1.02 x 10^-4 mm and the correct was 3214 mm). My thoughts are than some options are wrong.

Thank you again.

Thank you Mark for your response. My problem is at the default plote of Mode Shape the eigenfrequencies are correct and the mode shapes are correct but it shows at displacement magnitude and not at total displacement and they are completely wrong .I ran some models from the Comsol Video Gallery and the results are way off(my model 1.02 x 10^-4 mm and the correct was 3214 mm). My thoughts are than some options are wrong. Thank you again.
Jeff Hiller COMSOL Employee

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Posted: 9 months ago 2023年7月26日 GMT-4 08:51
Updated: 9 months ago 2023年7月26日 GMT-4 08:58

Hello George,

To rephrase what Mark said earlier, the magnitude of an eigenmode is arbitrary - it has no meaning. You can multiply an eigenmode by any number and it's still the same eigenmode for the same eigenfrequency.

Best,

Jeff

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Jeff Hiller
Hello George, To rephrase what Mark said earlier, the magnitude of an eigenmode is arbitrary - it has no meaning. You can multiply an eigenmode by any number and it's still the same eigenmode for the same eigenfrequency. Best, Jeff

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