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Model ID: 450

Computing Capacitance in a 3D Comb Drive

Capacitive comb drives are commonly used both as actuators and position sensors. The following model focuses on position measurement. Many approaches exist for positional measurements with MEMS devices, of which capacitance measurement is the most widely used, particularly in micro-accelerometers.

In a pair of interdigitated combs the fingers of one comb do not touch those of the other comb, but slide in and out relative to each other with a variable overlap. You typically can get a rough idea of a drive’s capacitance by approximating the 3D structure as multiple parallel-plate capacitors. Here a quick 3D analysis of the electrostatic field gives a capacitance value that you can combine with the parallel-plate theory to provide considerable design information. Additionally, you can use the capacitance in reduced-order electrical circuit models to simulate the electrical system as a whole.

Static comb drives are relatively easy to set up in COMSOL Multiphysics. This model illustrates how to approach such through creating a geometry, using the array and extrusion tools, and computing capacitances using the Electrostatics application mode’s port boundary condition.

Electric potential on the surfaces of a comb drive

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