Torsional thrust balance for electric propulsion application with electrostatic calibration device

Abstract

A direct measurement torsional thrust balance for electric thrusters has been developed to test pulsed plasma thrusters (PPTs) and other electric thrusters. The current configuration is able to measure impulse bits between 14 and 79 μN • s with a resolution of 0.05 to 0.10 μN • s. The direct measurement technique, in which the thruster is mounted on the thrust balance, is considered the most suitable and accurate for applications with PPTs. The use of flexural pivots with low stiffness makes the balance compact, with a smaller torsional arm. An electrostatic calibration device (ECD) was designed and built to calibrate the thrust balance, simulate the thrust of PPTs, and to allow forthcoming tests of the double discharge pulsed plasma thrusters developed at the Associated Laboratory of Combustion and Propulsion of the Brazilian National Institute for Space Research at the Electric Space Propulsion Laboratory. The maximum impulse bit measurement error achieved was as low as 10%, lower than the average of 12%-15%, typical for this type of thrust balance. The ECD was calibrated using a previously calibrated precision analytic scale. The ECD then can carry this calibration traceability to the thrust balance, providing a higher level of accuracy.

Mechanically Amplified Milli-Newton Thrust Balance for Direct Thrust Measurements of Electric Thrusters for Space Propulsion

“…compliant structure that is able to transduce the low thrust values to a measurable signal. The solution of these opposite requirements generally comes in the form of a pendulum, either simple [10], [11], inverted [12], [13] or torsional [14], [15], [16] that converts thrust into displacement which is then measured. Each of these configurations has its advantages and disadvantages, which are summarized in Tab.…”

In vacuum dynamic and static tests of a thrust balance for electric propulsion with hysteresis analysis and behaviour prediction with transfer function

“…Therefore, a thrust-stand, that is the object of the present effort, was designed to solve this problem, allowing for the actual measurement of the total impulse-bit. The initial development had its outcome previously published [16,17] and the present work shows the continuation of this effort.…”

“…The thrust-stand developed measures impulses from 14 µNs to 79 µNs, with a resolution of 0.05 µNs to 0.10 µNs and an error lower than 10% [16,17]. In that initial effort the instrument was calibrated in atmospheric conditions and external sources of vibration were not considered.…”

A closed-loop torsional micro-thrust measurement system

“…Balanced torsional swing displacement measurement is a prerequisite for closed-loop control. In the angular displacement measurement of this scheme, two laser displacement sensors are used to implement the differential method to make the minimum resolution of angle measurement 0.30 × 10 −6 rad and the minimum micro thrust is identified by combining the torsion wire stiffness measurement value (k = 0.003 25 N•m rad −1 ) to meet the formula (7). Among them, L is 318 mm, and it can be calculated that the minimum micro-thrust recognized is only 0.0095 µN, which is an order of magnitude lower than the electromagnetic calibration force resolution of 0.1 µN, so the angle measurement error can be ignored.…”

“…Since the micro-thrust and gravity directions are perpendicular to each other during testing, the influence of load gravity such as the thruster can be effectively separated, and it has the remarkable characteristics of low stiffness and high sensitivity, and the signal-to-noise ratio and range can be adjusted by balancing the length of the torsion pendulum. The torsional micro-thrust measurement device mainly includes two types of structures: a suspension pendulum [3][4][5] and a pivot pendulum [6][7][8][9]. Compared to the suspension pendulum structure, the pivot pendulum structure has a lower sensitivity with only one sensitive degree of freedom.…”