Electrostatic Displacement Control Compensates for Spring

The figure schematically illustrates part of an accelerometer in which a feedback control subsystem applies voltages to generate electrostatic forces to minimize the displacement of a proof mass suspended on a spring. Older systems of this type provide active control of the displacements of proof masses but do not provide any compensation for spring stiffnesses; as a result, low-frequency responses are characterized by displacement errors proportional to mechanical resonance frequencies. The present system differs from older systems in that it operates with a combination of voltages chosen to produce not only active control of the displacement of the proof mass but also passive compensation for the stiffness of the spring. Consequently, in the present system, the effective spring stiffness and resonance frequency are reduced to near zero, and the low-frequency position error is greatly reduced.