A Torsional Balance that Resolves Sub-micro-Newton Forces

In this article we present our ongoing research on micro-Newton thrust stands. The capability of measuring thrust in the range of micro-Newtons is essential to validate the performance of devices such as colloid thrusters and FEEPs. Although time of flight and energy analysis techniques have been satisfactory to measure the thrust and the specific impulse associated with colloid beams, it is clear that the confidence level in a micro-Newton thruster would increase by the direct thrust measurement provided by a thrust stand. Our approach is based on a torsional balance. Key features include the use of a fiber optic displacement sensor for measuring the angular displacement of the balance’s arm, the use of electrostatic forces to calibrate accurately applied torque and balance’s response, and an external damping mechanism. We measured comfortably thrust values as low as 0.11 μN, which were generated with a single emitter colloid source. A rough measurement of the resolution of this torsional balance yields a value of 0.01 μN, and we expect to lower this figure with further improvements.