Why is the Comb Drive force too low at ANSYS 8.1 in the example contained in
Chapter 14 of the ElectroMagnetic Analysis Guide  PMethod 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 Pelements 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. 

