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How to calculate the differential capacitors using 3D comb drive model
Posted Feb 22, 2011, 7:36 a.m. EST MEMS & Nanotechnology, MEMS & Piezoelectric Devices Version 4.2a 13 Replies
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I'm a beginner for using COMSOL3.5 software.
What I'm doing is similar with the comb drive 3d in model library.
The difference is in my model, for the moving mass, it has both sides fingers. So there are totally two fixed part at each side of the proof moving mass.
With one side of the capacitance increased, the other side capacitance is decreasing.
I totally followed the tutorial of "comb drive 3d" to do the simulation, except the model was draw according my own design.
Now the problem is, in the model documentation, after give the Expression: C11_emes, then one capacitance value of this model is calculated by the software.(e.g. 0.021 pF—appears in the message log at the bottom of the screen).
However, in my model, I should get two capacitance values. So what should I do? what kind of the expressions should I give.
By the way, what's mean of C11_emes. I know that "emes" is the model name, but what about C11?
Your help is very important to a beginner.
Thanks a lot.
Regards
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check the doc abut the ports in the acdcug.pdf doc of 3.5 (you can also search for "C11" or lumped parameters, it's all in there)
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Good luck
Ivar
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where can I find the acdcug.pdf doc of 3.5?
Thanks a lot.
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It's part of the 3.5 distribution in the doc folder
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Ivar
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I am using comsol 4.2a. I am trying to run a simulation on capacitor comb drive accelerometer but when i apply a prescribed acceleration in y direction i get an error as ' mod1.emi.uInitY '
Please help.
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what about trying to use a prescribed body load "acceleration*rho"
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Good luck
Ivar
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thank you sir. that really helped.
Also i am using the electromechanics physics for my model. My model is a symmetric model with proof mass and fingers on each side while the mass is anchored by beams.. I have used two terminals with a +ve voltage for the proof mass and sensing fingers and -ve same voltage for the fixed fingers. So that when i apply acceleration or force to the proof mass, the voltage difference produces capacitance. I need to find out the change in capacitance. But i am not sure what command to use.
Awaiting for your reply. thank you in advance.
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then what about trying a frequency domain sweep with a body load as defined above. The thing is that you might need some damping, particularly if you pass some resonances. Not sure how to define something representative, in small MEMS devices its mostlay air flow viscuous damping, that again depends strongly on the environment and pressure.
On the other hand, a capacitance is a geometric affaire, for a given gap surface and epsilon_r you have a given capacitance, then if you apply a static force to deflect the system you will get a new gap value hence a new capacitance value. So when you load your device with an acceleration you will only "measure" the displacements effect on the capacitor, and this is driven by the damping and if you are above (and how much above) or below the fundamental eigenfrequency
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Good luck
Ivar
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Thank you sir. I will definetly look into that.
But i am not adding any additional damping. So i was thinking maybe there was a command to know the change in capacitance, i.e. difference between the initial capacitance and the capacitance after the displacement takes place. I know the formula and relating terms but just need to know how to obtain that as in results.
Thank you
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normally if youhave used the terminal/ports then I believe you have automatically the capacity as output global values in the Results Derived Global variable nodes, check it out. Else you might derive it from current/voltage values if yu are in the time domain
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Ivar
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I couldnt find the capacitance in the desired values in results.
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if you are using the EMI physics you are right, but then if you have defined a Gnd and a Terminal for the voltage in the EMI, then you can get the capacitance as the ratio Derived Values, Global, Terminal Charge over Terminal Voltage
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Good luck
Ivar
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Thank you for all your help sir.
Regards.