# No variation in capacitance with body load in comsol

## Main Question or Discussion Point

Hi Friends,
I am trying to find variation in capacitance between two plates with applied body load. I defined an air box around the plates. Now I'm trying with electromechanics physics for applying body load and finding capacitance. I defined the two plates as linear elastic material. Applied body load to the moving plate. I applied 1V to moving plate and fixed plate is grounded. I also gave parametric sweep for body load. But capacitance is not varying with body load.
Thank You,

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Tom.G
What is this "body load" of which you speak? There are only three things that determine capcitance: dielectric constant of whatever is between the plates, the area of the plates, the distance between the plates. For dimensions in inches the formula is:

C= 0.224*(K*A/D)

K dielectric constant
A plate area in square inches
D thickness of dielectric in inches

Which of the three are you changing?
Since the result will be very small, will it show up in the program output?

Hi,
Body load is the force which deforms the top plate of the capacitor and the bottom plate is fixed. So the changing parameter in my case is the gap between the plates. The device is a MEMS device and the deformation is in nanometers. The end result (capacitance) would be in picofarads which isn't a problem since Comsol can handle such small outputs. My problem is that capacitance is not varying with the varying displacement. I get the same capacitance for different body load (force).

Tom.G
Is that nanometer displacement a small fraction of the nominal distance? If the unloaded distance is in the micron range then a nanometer displacement would change the capacitance by 1 part per thousand, or femtoFarads change hidden in a picoFarads baseline.

Edit: Also if the top plate is a diaphram only the center will deform that much.

Displacement is in nanometers, the gap between the plates is 2um. I tried to get result in fF also, but the value of capacitance remains constant.

Yes, Its a diaphram. So i defined an average operator.

Tom.G
Hmmm... sounds like you're down to a debugging problem.

Two things that come to mind:
Can you add to the simulation a calculated known capacitor in parallel w/ the sensor? Then try different values of the 'known' cap to check the program output.
Have someone doublecheck your existing model to make sure you didn't slip a decimal or get a sign wrong.

I've never used COMSOL so I can't be much help there. Hopefully someone with COMSOL experience will jump in & help.

Thanks a lot for your help