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## Problems with simulation of Capacitive Sensor

Hi,
I am relatively new user to COMSOL and presently I am working on COMSOL 3.5. I am presently working on Simulation of a capacitive sensor and I have completed the following steps in order to achieve my goal.

There are a few problems with respect to achieving the desired goal and therefore, I would like to request seniors and moderators to help me in this regard.

I have two capacitor plates embedded into a substrate and I am required to calculate the capacitance between the two plates. On top of the substrate, with very little distance of separation (in the order of 0.00x mm), I have a superstrate with a different dielectric material. As the distance between substrate and superstrate increases, capacitance value should reduce.

The steps that I have carried out are as follows:
1. Input geometry -Created three rectangles of width 1 mm and length of 150 um (micrometer) with a distance of separation of 0.5mm
2. Covered the three rectangles R1,R2 and R3 with a box of relative permittivity of 4 ( as I want them to be fabricated on a PCB).
3. Drew an other rectangle for superstrate with a relative permittivity of 1835 (Device under test)
4. Covered the superstrate and sub-strate with a box filed with air as I am interested in seeing the difference of capacitance between superstrate and substrate.
5.Fed the respective permittivity values into the sub-domain conditions.
6.Applied the boundary conditions in such a way that
- the box enclosing the superstrate and substrate is grounded on all four sides.
-the substrate and super strate are grounded on three sides except for the top of the substrate and the bottom of
the superstrate.
- capacitor 1 ( rectangle on the left-R1) is given an electric potential of 1 v with the right side boundary of the rectangle being forced as an input port with a voltage of 1v.
- capacitor 2 (rectangle in the middle-R2) is given as port 2 as I am interested in calculating the capacitance between 1 and 2. The other three sides of R2 are grounded.
- capacitor 1 ( rectangle on the right-R3) is given an electric potential of 1v. ( since I need to short R1 and R3 as they are basically just one capacitance C1.)

I get the capacitance C21 as -17 pF pF. I do not understand why I have a negative capacitance , Also, what is C11?

I think I have made some errors and I would like to request your time to help me with the same.

I try to attach my file but it says file extension error.KIndly help.

Thanks and Regards.

1 Reply Last Post Nov 28, 2011, 6:00 AM EST
Posted: 6 years ago
gentle bounce.

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