Nanobalance: an automated interferometric balance for micro-thrust measurement.

The paper is concerned with a new instrument, nanobalance, measuring thrust and noise of space micro-thrusters in the range 0-1 mN with submicronewton accuracy. Low-noise micro-thrusters will be essential for upcoming "drag-free" space missions like GOCE, SMART-2, LISA, and Darwin. Nanobalance departs from traditional thrust measuring concepts so as to exploit sensitivity of in-vacuum Fabry-Pérot interferometers to subnanometric displacements. That is achieved by an optical cavity embraced by two equal pendulums suspended at a constant distance, one of which carrying the micro-thruster under test. Any thrust, changing the optical path length of the injected laser beam, is detected as the frequency variation restoring a standing wave in the cavity. The paper presents the measurement principles, the error budget as derived from recent tests as well as instrument automation, which is essential for facilitating instrument setup and adaptation from measurement trial to trial. The authors are responsible for instrument automation and measurement processing.