No variation in capacitance with body load in comsol

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Discussion Overview

The discussion revolves around the variation of capacitance between two plates in a MEMS device when a body load is applied. Participants explore the effects of mechanical deformation on capacitance within the context of COMSOL simulations, focusing on the relationship between body load, plate displacement, and capacitance measurement.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Aswathi M Madhu describes an attempt to measure capacitance variation with an applied body load in a COMSOL simulation, noting that capacitance remains constant despite changes in body load.
  • One participant questions the definition of "body load" and emphasizes that capacitance is determined by the dielectric constant, area of the plates, and distance between them, asking which of these parameters is being altered.
  • Aswathi clarifies that the body load refers to the force causing deformation of the top plate, which changes the gap between the plates, and that the expected capacitance values are in picofarads.
  • Another participant raises the point that a nanometer displacement may be negligible compared to the nominal distance between the plates, suggesting that such small changes might not significantly affect the capacitance measurement.
  • Aswathi confirms the gap between the plates is 2 micrometers and reiterates that capacitance remains constant despite attempts to measure it in femtoFarads.
  • A participant suggests debugging the simulation by adding a known capacitor in parallel to check the output and recommends verifying the model for potential errors.

Areas of Agreement / Disagreement

Participants express differing views on the impact of small displacements on capacitance and whether the simulation setup is correctly configured. There is no consensus on the reasons for the constant capacitance measurement despite varying body loads.

Contextual Notes

Participants note the importance of the scale of displacement relative to the nominal distance between the plates, as well as the potential for measurement issues in the simulation. The discussion highlights the complexity of accurately modeling mechanical effects on capacitance in COMSOL.

Aswathi M
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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.
What changes should I make to get a varying capacitance? Please Help.
Thank You,
Aswathi M Madhu
 
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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)

C in picoFarads, 10 -12 Farads
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).
 
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.
 
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
 

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