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how to design an unbiased resonator

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hello, I'm relatively new to comsol. I use version 4.2a.
I want to design a mems resonator similar to the example model in the model library - mems.
Here the example biased_resonator_2d_freq is interesting for me.

To understand how comsol uses the parameter internally, I wanted to try an unbiased resonator, which is just actuated by an AC voltage to see the unbiased displacement of the resonator vs. the frequency in a sweep around the resonance frequency (instead of like in the example a actuation AC voltage + a DC bias).

In the example the study prestressed analysis, frequency domain is used, which contains a stationary part and a frequency domain part.
As far as I understand it, the DC bias is represented by the electric potential in electromechanics - electric potential 3 and the AC actuation by the electric potential in electromechanics - electric potential 3 - harmonic perturbation 1.
But if I just want the AC actuation and set the DC bias to 0 an error occurs.

Can I instead use only a frequency domain study for analyzing the unbiased case. But here I have only the option of the electric potential in electromechanics - electric potential 3 (which represented the DC voltage), The harmonic perturbation does not have any effect on the result (as far as I can say). Do I have to use the effective voltage of the AC actuation for the electric potential or is this model also not suited to solve the unbiased resonator?

How can I simulate the problem instead?

I hope somebody can give me a tip how to solve it.

2 Replies Last Post Aug 12, 2012, 1:31 p.m. EDT
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Hello Stefan Wagner

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Posted: 1 decade ago Aug 12, 2012, 6:42 a.m. EDT
struggling with the same principles
struggling with the same principles

Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)

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Posted: 1 decade ago Aug 12, 2012, 1:31 p.m. EDT
Hi

if you have an unbiased model, with a continuous AC excitation of sinus type, it's enough to run a simple frequency domain solver either with one frequency, or for several as a sweep (range(...)), It is when you add a bias that you ned to use special cases to "stress" your model first, then use that point as a linearisation point for next frequency domain solving. COMSOL has added a few special solver cases, that looks after this for certain physics, check your do (v4.3 in particular)

--
Good luck
Ivar
Hi if you have an unbiased model, with a continuous AC excitation of sinus type, it's enough to run a simple frequency domain solver either with one frequency, or for several as a sweep (range(...)), It is when you add a bias that you ned to use special cases to "stress" your model first, then use that point as a linearisation point for next frequency domain solving. COMSOL has added a few special solver cases, that looks after this for certain physics, check your do (v4.3 in particular) -- Good luck Ivar

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