A compound pendulum for thrust measurement of micro-Newton thruster.

A thrust stand is developed for testing micro-Newton level thrusters on the ground. The stand is composed of a compound pendulum that is symmetrically suspended by two thin beryllium copper strips, and it is precisely calibrated by gravity. The stiffness of the stand can be adjusted in 3 orders of magnitude by a counterweight. When the stiffness is larger than 1 Nm/rad, the stand demonstrates a fast response to thrust. The measured range of the stand reaches 1000 μN, and the noise is less than 0.1 μN/Hz within 1 mHz-1 Hz. To calibrate the resolution of the stand, an electrostatic force is applied to the stand with an actuator. The equivalent thrust is determined to be 0.09 μN with a standard uncertainty of 0.02 μN. Using the stand, a micro-Newton colloid thruster is tested. The output of the colloid thruster changes with the applied voltage as 0.015(1) μN/V. When changing the voltage by 50 V, the change in thrust is measured to be 0.7 μN with a standard uncertainty of 0.1 μN.