Development of an indirect counterbalanced pendulum optical-lever thrust balance for micro- to millinewton thrust measurement

Abstract

This paper describes the design and testing of an indirect hanging pendulum thrust balance using a laser-optical-lever principle to provide micro-to millinewton thrust measurement for the development of electric propulsion systems. The design philosophy allows the selection of the total thrust range in order to maximize resolution through a counterbalanced pendulum principle, as well as passive magnetic damping in order to allow relatively rapid transient thrust measurement. The balance was designed for the purpose of hollow cathode microthruster characterization, but could be applied to other electric propulsion devices in the thrust range of micro-to millinewtons. An initial thrust characterization of the T5 hollow cathode is presented.

A 10 nN resolution thrust-stand for micro-propulsion devices

“…Wu et al 26 measured the impingement force from a nitrogen cold-gas thruster at atmospheric and lower pressure and found it to be within a few percent of computed thrust. Grubišić and Gabriel 27 reported a similar method and they obtained the similar relation between the computed thrust and measured impingement force from cold xenon and argon emission. Longmier et al 29 measured thrust from a 5 kW xenon Hall Effect thruster and the results agree within a few percent from that measured directly with an inverted pendulum thruststand.…”

“…[25][26][27][28][29][30][31] The present work describes one such thrust-stand. The method allows for easier adaptation to different types of thrusters by completely separating the thruster from the thrust measuring unit; thereby, removing sensitivity to the thruster mass and interconnects used for control or propellant supply.…”

“…The best thrust resolution reported with this technique is 3 µN using an optical sensor. 27 However, the recorded force is not a direct measurement of thrust and measurements are complicated by unknown coefficients of restitution amongst impacting particles. Accordingly, a prior knowledge of the relation between the impingement force on the plate and the thrust on the thruster is required to determine thrust from the measured impingement force.…”

“…Longmier et al 29 measured thrust from a 5 kW xenon Hall Effect thruster and the results agree within a few percent from that measured directly with an inverted pendulum thruststand. The impingement force can be measured with different techniques, such as pressure tap, 26 optical displacement sensor, 27 or strain guage. 29 To date, this technique has been used for thrust measurement in the range of several mN or even higher.…”

“…The thrust-stand measures the impingement force on a conductive low-mass plate using a commercially sourced micro-machined capacitive force sensor and is suitable for measuring thrust from many different types of microthrusters operating in the µN ranges. Compared with existing implementations, 26,27 the presented approach primarily differs in its use of a homodyne detection scheme. Here, the impinging beam is interrupted, by means dependent on the plume type, at a defined frequency f ref which is greater than low-frequency facility vibrations yet below the natural frequency of the target plate.…”