Design and fabrication of a full elastic sub-micron-Newton scale thrust measurement system for plasma micro thrusters

In this work, a force measurement system is proposed to measure the thrust of plasma microthruster with thrust magnitude ranging from sub-micro-Newtons to hundreds micro-Newtons. The thrust measurement system uses an elastic torsional pendulum structure with a capacitance sensor to measure the displacement, which can reflect the position change caused by the applied force perpendicular to the pendulum axis. In the open-loop mode, the steady-state thrust or the impulse of the plasma micro-thruster can be obtained from the swing of the pendulum, and in the closed-loop mode the steady-state thrust can be obtained from the feedback force that keeps the pendulum at a specific position. The thrust respond of the system was calibrated using an electrostatic weak force generation device. Experimental results show that the system can measure a thrust range from 0 to 200 μN in both open-loop mode and closed-loop mode with a thrust resolution of 0.1 μN, and the system can response to a pulse bit at the magnitude of 0.1 mN s generated by a micro cathode arc thruster. The background noise of the closed-loop mode is lower than that of the open-loop mode, both less than 0.1 mN Hz / in the range of 10 mHz to 5 Hz.