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.

A digital control application for the frequency stabilization of optical frequency standards is presented. The standard employed is a monolithic neodymium in yttrium aluminum garnet laser locked to an iodine frequency reference. Traditionally, laser frequency is locked to the reference through analogue loops, which may require a lengthy set-up and a manual search for the reference to which to lock the source. Digital control avoids these steps and provides complete and robust instrument automation. The theory and results of a Digital Control Unit (DCU) designed to improve and facilitate stability set-up and performance are presented. The DCU elaborates frequency error signal between laser and reference at 10 kHz so as to co-ordinate a set of three frequency actuators (temperature servo, piezo-electric ceramics, acousto-optic modulator) capable of compensating frequency drifts below 1 part per 1012 under normal environmental conditions.

The digital control of an interferometric force balance, 'Nanobalance', for on ground qualification of space micro-thrusters aiming at sub-micronewton accuracy is presented. Low noise thrusters will be essential for upcoming 'drag-free' space missions like GOCE, SMART-2 and LISA. Nanobalance departs from traditional thrust measuring concepts to exploit sensitivity of Fabry-Perot interferometers to micro displacements. The paper focuses on the digital control, driving the interferometer laser frequency to track the micro displacements caused by the micro-thruster under test to a suspended optical cavity. The digital control, designed and tested in parallel to instrument construction, is now currently employed for instrument calibration and tests. The paper extends and updates an introductory report presented to past ACC.

An interferometric force balance (Nanobalance) for space micro-thruster qualification is presented. A new generation of space micro-thrusters is under development aiming at thrust resolution and accuracy below one micronewton. Nanobalance has been conceived and developed for ground calibration of thrust time profile and noise with sub-micronewton accuracy.

The digital control design of an interferometric balance for fine space thruster calibration is presented. The problem was suggested by new micro-thrusters generation aiming at thrusts with sub-micro-Newton resolution. The solution, the Nanobalance instrument, under development. will be employed for on ground qualifying thrust time profile and noise with sub-micro-Newton accuracy. The digital control has been designed and tested in parallel to instrument construction. through a fine numerical simulator of the instrument itself and of the environment disturbances, developed within a proprietary control design environment, ControLab. Severe simulation tests have shown that requirements are met. Next step will be Digital Control Unit interfacing to Nanobalance instrument and testing.