Rotation control of a variable-capacitance electrostatic motor for space equivalence principle tests with rotating masses

Abstract

Variable-capacitance electrostatic motors are ideal for driving the test mass in ultra-low-noise electrostatic accelerometers. Such devices are essential for testing the new equivalence principle (NEP) with rotating extended masses. However, as the air-film damping is greatly reduced by placing the sensor core assembly in a high-vacuum housing, this synchronous motor may easily fall out of step and suffer spin-up failures with traditional open-loop excitation. In this study, a synchronous electronic phase commutation scheme is proposed by sensing the three-phase position change of the rotor poles and activating the stator electrodes in careful correlation with the instantaneous rotor position. Experiments on a ground-test NEP instrument prototype show that the proposed closed-loop excitation scheme can spin-up the rotor synchronously and maintain stable constant-speed operation of this macroscale variable capacitance motor operated in a high-vacuum environment. This rotation control method is also applicable to the synchronous operation of micromachined variable-capacitance electrostatic motors.