  An approximate analytical formula for the capacitance of a non-parallel plate
capacitor with small values of inclination angles and distance separations of
the plates is presented. Most applications involving position sensing that are
employed in precision experiments and micromechanical devices often use a
parallel plate approximation for modeling the capacitance. The analytical
approximation presented here is based on the more general, but hard to
implement formalism for small angles, and that may be used directly to compute
the forces acting on the proof masses by taking the derivative of the
capacitance.
  Effects of varying the inclination angle, plate dimension and separation on
the value of the capacitance per unit length are analyzed in detail. We use the
formula derived in this paper to compute the acceleration of the proof mass due
to the presence of sensing electrodes (used for positioning the proof masses)
with an assumed tilt of $\sim 10^{-6}$ rad as applicable to SR-POEM: an
experiment aiming to test the weak equivalence principle - a fundamental
postulate of general relativity.
