Thrust stand based on a single point load cell for impulse measurements from plasma thrusters

Abstract

We introduce a simple thrust stand for the direct measurement of the millinewton impulses or thrusts delivered by small thrusters intended for in-space electric propulsion. The thruster under test, with a weight below 1.5 kg, is disposed on a horizontal platform and its impulse is measured as an overweight by using a strain gauge cell physically protected from the ambient plasma and vacuum conditions. This system provides ten thrust readings per second with noise peak to peak amplitudes of 0.10-0.18 mN. The calibration procedures to verify its dynamic response to time dependent thrusts in the range of 0-15 mN using control weights as well as its minimum thrust sensitivity δTs = 0.3 mN are discussed. Additionally, its simple conception permits a plain data reduction and analysis of steady state and low frequency thrust transients. This thrust stand was employed under low pressure and plasma ambient conditions to measure the steady impulses delivered by the Alternative Low Power Hybrid Ion Engine (ALPHIE) of 0.4-4.0 mN with absolute errors T = ±0.3 mN. Finally, the experimental results show that a control electric voltage governs the ALPHIE thruster throttle.

Comparison of Submillinewton Thrust Measurements Between a Laser Interferometer and a Load Cell on a Pendulum Balance

“…Since the thruster exhaust faces up, the scale will record an increase in the measured value when the thruster is switched on, which is equal to the thrust. Examples of such thrust stand are given by Conde et al (2020) and Karadag et al (2018). The disadvantage of this method is that the measurement system must be able to measure the weight of the thruster F W , which is typically many orders of magnitude larger than the thrust F T .…”

Impulse Measurement Methods for Space Micro-Propulsion Systems

“…Usually, the resolution is determined according to the measurement noise level, and the resolution can be specified to be twice the noise signal. The noise signal can be characterized by the ratio between amplitude and frequency or power spectral density [5,8,9,25].…”

Determination of the parameters of the microwave ion thruster by the calorimetric method

“…the effect of adhesion of neutral molecules or atoms to ionized particles; gas-dynamic resistance of the movement of ions in a neutral gas medium; the values of the actual value of the accelerating voltage, etc. To determine the thrust, stands based on: load cells, pendulums and torsional scales are used [5][6][7]. Load cell-based stands can give erroneous thrust readings due to the fact that the presence of a plasma medium can lead to parasitic currents in the electrical connections of the sensor.…”