Discussion Forum

Hi

COMSOL is a FEM programme as the other so a reasonable mesh density is needed for the theory to work. But as higher order shape elements are used, mostly the results are good even with coarse meshes. But the rule is always to increase the mesh size once to check the model stability, but one can do that "intelligently" by concentrating on the region with high gradients, to better identify them turn OFF the plot smoothing and rendering and look at the raw wire-frames. For a beam I start with a minimum of 2 per thickness (when lazy only 1). You have also the solver refinement meshing feature

For the surrounding air box, the best is to look at the field lines and the gradients, is my surrounding air SHAPE and SIZE influencing the field lines ? if yes increase the size, if no decrease it. You have also the infinite or PML elements to artificially expand your air volume, I use as rule of thumb infinite elements = 1/5 to 1/10 of the air size

I leave COMSOL mostly in default mode (to start with), then I observe the model in a "coarse mode" to decide hoe to refine. Normally the weak mode is there to improve the model, at the cost of some extra computations. But indeed when you look at the weak functions they are very spiky (by design) and do not give a "nice" impression but I find that this approach is far better than the traditional one used in older FEM programmes

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Good luck
Ivar

Thanks Ivar for your reply.

I will perform some mesh refinement studies and post it back here.

Meantime, can you please direct me to some good starting point for using the PML elements .

Also, if somebody is having a few guidelines about the optimum dimensions of air chamber (parameterized in terms of beam dimensions), please let me know.

Thank you.

Dekar.

I am on COMSOL and presently trying to simulate the pull-in instability of electrostatically actuated micro beams in COMSOL MEMS module. I need further insights into the following topics pertaining to this problem:

Has anyone benchmarked some typical beam problem so that one can have some idea about what is the optimum number of elements; say along beam-length, width, thickness? I have followed the tutorial problem in COMSOL and the results do match with the literature, but just checking if someone has already tried this out.

How do we decide the optimum dimensions of the surrounding box (air chamber) in which the electrostatic problem is solved. I want model damping pull in parameters like pull in voltage and displacement. My thinking is surrounding box (air chamber) in which the actual beam is placed will be filled with some viscous medium. How can I do that or help me if any other way is possible.

Hi Rakesh,

Here's a MEMS cantilever electrostatic simulation that I did. I don't know if COMSOL calculates pull-in since if you apply a high enough voltage, the cantilever displaces past the electrode.

Hi Rakesh,

Here's a MEMS cantilever electrostatic simulation that I did. I don't know if COMSOL calculates pull-in since if you apply a high enough voltage, the cantilever displaces past the electrode.

Hi Wnag,
Thanq for ur reply. I could not able open the model which u have send me, it is giving error the file can't be opened.Can u plz send me the proper one.

Hi Rakesh,

I'm sorry, this is a 4.1 version and I don't have a 3.5a version.

Sorry,
Andrew Wang

Hi,
I have tried with 4.1 also but I could not able to open it..can u plz guide me how I can model damping.

Hi,
I have tried with 4.1 also but I could not able to open it..can u plz guide me how I can model damping.

Hi Rakesh,

Did you update your comsol?

Andrew

Hi,
I am using 3.5 only. From starting I've been using 3.5 only. Still its working.

hii Rakesh, I would like to see that file. I'm working on modal analysis of cantilevers . would u please post the pdf format of your results?

Do we have a tutorial describing how to make an electrostatically actuated beam

I am very new to comsol

Sir i need comsol pull in simulation. Can you direct me to that?