Active vibration suppression in a suspended Fabry-Pérot cavity

This paper is concerned with active vibration suppression in a suspended Fabry-Pérot cavity, employed as the sensor in an innovative thrust-stand, called Nanobalance. The Nanobalance aims to exploit the sensitivity of in-vacuum Fabry-Pérot interferometers to sub-nanometric displacements in order to measure thrust (< 1 mN) and noise of space microthrusters with micronewton accuracy. The instrument has been conceived around an in-vacuum optical Fabry-Pérot cavity embraced by two pendulums (the active and the passive) suspended to an athermic spacer. The thruster under test is mounted on the active pendulum and an equal dummy thruster is mounted on the passive one for balancing. The objective of the paper is to suppress the beat motion centered on the mean pendulum natural frequency (10-14 Hz depending on the thruster under test) without affecting the measurement bandwidth (2 Hz) where thrust has to be measured. Beat motion arises because of small pendulum imbalances excited by ground noise. Relevant digital control strategies and experimental results will be presented and discussed.