Why is the Comb Drive force too low at ANSYS 8.1 in the example contained in
Chapter 14 of the ElectroMagnetic Analysis Guide - P-Method Electrostatic?
The method used, the Maxwell Surface Tensor, does not capture the total force
well when it is concentrated at a corner as is the case in a comb drive. Mesh refinement
or use of P-elements can only produce about 70% of the total force. At 9.0 a new
method of force calculation will produce exact values for this force. A workaround at 8.1
is to solve for the total charge on the moving part and compute force by the equation F = (1/2)* deltaQ * V.
The "reaction force" in an electrostatic analysis is charge, so the nodal charges can be summed by selecting
the moving part and issuing the command FSUM.