Thrust balance

Pendulum thrust balance assembly.

Since 2009, the LET started a R&D project to investigate possible micro newton thrusters for high precise attitude control for future space missions. Within the framework of the project, the development and realization of test facilities for the characterizing, test and development of micro newton thrusters is performed. One of the most challenging parts of the project is the development of a micro newton thrust balance. Right now, the first generation of the thrust balance was developed and integrated. First performance tests were performed and show very good results.

Introduction

For the development of micro newton thrusters and especially for the basic research of micro newton ion thrusters, a direct thrust measurement system is indispensable. A preliminary design study has shown that as micro newton thrust balance a vertical pendulum balance can be used. The thrust measurement works by the interaction between the thruster and the pendulum arm. The applied force or thrust F of the thruster deflects (Dx) the pendulum in reference to the specific spring rate (k) of the pendulum. The relation of these parameter is given by:

F = k ⋅ Δx

The thrust can be calculated using the known spring rate and the measured translation. The balance project plan envisages a three step development. For the first balance generation the development goals are a measurement range from 1µN to 300 µN, thrust sensitivity up to 0.1 µN and a measurement bandwidth from 10Hz down to  10µHz.

Measurement Setup

The measurement setup consists of the pendulum balance, a high resolution optical read out (based on the LET interferometer technology), an electro static comb (ESC) witch allows a highly precise system calibration and the investigated thruster. The whole setup is shown in figure 1.
The pendulum balance consists of two vertical pendula, an active pendulum (measurement pendulum) and a passive pendulum (reference pendulum). This configuration enables a common mode measurement. The whole assembly will be placed in a vacuum chamber. The vacuum chamber is placed on an optical table in order to reduce ground noise. For high precision calibration an ESC will be used.

Results

The test results give an idea of the pendulum behavior and performance. The balance is able to measure thrust down to 10µN in a bandwidth from 1.1Hz to 0.1Hz. The pendulum balance spring rate is close to 30 N/m. The balance oscillates at an Eigen frequency of 1 Hz with an amplitude of 200nm. The common-mode measurement function is inoperable because of the unexpected pendula behavior, according to each other. But in non-common-mode the thrust balance performed good results.
The thrust balance was calibrated with the ESC. The result of the this calibration is presented in figure 2. The translation of the pendulum was measured versus a fixed mirror. The force was continuously applied by the ESC, i.e. the used voltage was varied from 0 V to 2750 V. The variation was performed as linear ramp from a minimal to a maximal voltage and back again. In reference to this results the specific pendulum spring rate was calculated. It is close to 30 N/m.

After completed pendulum characterization and calibration, the balance was used for the first direct thrust measurement of a micro-Newton HEMP-T, shown in figure 1. For the first thrust measurement, the HEMP-T was ignited and driven into a stable operation point (anode -voltage 350 V, mass-flow 1sccm, self consistently anode current: ca. 20mA). From this point, the anode voltage was linear, continuously modulated from 300 to 600 V. After three cycles, the thruster was deactivated to determine the zero point of the measurement. The modulated anode voltage causes a force variation which corresponds to a root function, this is equivalent to the theory. According to figure 3, the thrust varies from  (260 ± 25) µN to  (495 ±  25) µN.

 

First calibration run displaying the applied force (red curve) and the translation of the pendulum (blue curve).
Relative ion acceleration voltage U of ceramics thruster at two different angles.

Conclusion

The test results gave an idea of the pendulum behavior and performance, which is sufficient to measure the actual micro-Newton HEMP-Ts. The developed, integrated and tested balance can be used as baseline for the further thruster development project. Further performance improvements of the pendulum balance seem possible

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