1. Introduction
With the development of aerospace technology, the space mission requirements for propulsion systems are becoming more rigorous [
1,
2,
3,
4,
5]. One of the drag-free satellite platforms represented by space gravitational wave detection requires the use of thrusters to generate micro-newton thrust to counteract non-conservative forces such as solar radiation light pressure in real time, making the test mass less than 10
−9 m/Hz
1/2 relative to the spacecraft displacement control accuracy to ensure sufficient accuracy of scientific measurements. Therefore, high-performance indicators such as 0.1 μN/Hz
1/2 thrust noise and 50 ms response time are proposed for the propulsion system (see
Table 1) [
6].
In order to meet the requirements of micro-propulsion technology for space gravitational wave detection missions, various countries have conducted research on micro-newton propulsion systems. LISA Pathfinder carries both cold gas thrusters and colloidal thrusters as its propulsion system, but the thrust noise is only suppressed to below 0.1 μN/Hz
1/2 in the frequency band of 1 mHz to 30 mHz, and the step response time of its upper and lower output limits of 5 to 30 μN is as long as 170 s [
7]. The full-size NG-µHEMPT and miniaturized NG-µHEMPT developed by TU Dresden achieve thrust output ranges of 6 to 4800 µN and 29 to 86 µN, respectively [
8]; the Field Emission Electric Propulsion developed by Austria meets the requirements for thrust range, resolving power, and noise level, but the application needs to be improved in terms of controllability and lifetime [
9]. In-orbit validation of Taiji-1 with RF ion thruster has achieved 1.5~60 μN thrust adjustment range, 0.5 μN resolution, 0.2 μN/Hz
1/2 or less thrust noise, 50 ms response time, and greater than 60 s specific impulse [
10]; ECRIT-µ1, designed by Shizuoka University in collaboration with the University of Tokyo, can achieve 20~316 µN thrust output range and specific impulse of up to 1500 s [
11]. It can be seen that the research on thrusters for gravitational wave detection missions has been abundant in various countries, but there is still a gap between the achievable performance and the target requirements.
The microwave ion thruster has many advantages such as high specific impulse, wide range of thrust, high accuracy of thrust estimation, fast adjustment rate, etc. [
12]. Its schematic diagram is shown in
Figure 1, mainly composed of magnetic circuit, antenna, cavity and double gate system, etc. When the microwave ion thruster works, the microwave in the GHz band is fed into the ionization chamber through the antenna. Then, the electrons are heated based on the electron cyclotron resonance principle, the heated energetic electrons collide with the working gas in the discharge chamber to cause ionization, and the generated ions are accelerated and ejected through the gate system to generate propulsion. Since the ionization and acceleration processes of the microwave ion thruster are separated, the thrust can be accurately estimated by the beam parameters.
In the space gravitational wave detection mission requirements, Northwestern Polytechnical University conducted an experimental study on the open-loop control of the 2 cm microwave ion thruster. Under the conditions of flow rate 0.1 sccm, microwave power 0.5~2.0 W, and acceleration voltage 150~1850 V, the thrust can be continuously adjusted in the range of 1.38~139.18 µN [
13]. The microwave ion thruster μ1, designed for the DECIGO pathfinder mission, suppresses the thrust noise in the 0.1~1 Hz band to less than 0.2 μN/Hz
1/2 by means of closed-loop feedback control of the input microwave power [
14], which effectively suppresses the low-frequency disturbances but still requires more precise feedback control means.
Because the microwave ion thruster has the ability to use electrical parameters for accurate thrust estimation and can achieve closed-loop feedback control of the thruster, it can significantly improve the thruster response speed, thrust noise, stability, and other indicators, so there is a high prospect for future gravitational wave detection.
In order to achieve the high requirements of the gravitational wave detection mission for the micro-newton thruster, the control strategy and performance optimization of the microwave ion thruster are investigated in this paper. In the microwave ion thruster system, the microwave source has the advantages of high resolution and fast response adjustment compared with the voltage source and storage unit, so it is used as the main control means of the thruster. Compared with the analog control system, the digital control system has higher flexibility, and the controller parameters can be modified by telemetry and remote control to improve the adaptability in a deep space environment [
15], so the thruster control scheme is developed in this paper under the framework of digital feedback control. Because of the high requirements of gravitational wave detection on the thruster noise, the overlapping phenomenon generated by the digital control process directly affects the final control accuracy [
16]; therefore, this paper uses various filters to optimize the control system performance and reduce the impact of system noise on the system performance.
The main work of this paper is as follows:
Proposed a microwave feedback control strategy for ion thruster, built a digital control system, and analyzed the effects of quantization noise and aliasing effect on the output performance of the thruster.
A microwave feedback-based controller is designed to achieve accurate and stable thrust output under the premise of low bandwidth. In addition, the fourth-order Butterworth low-pass filter circuit, FIR, and other filters are designed to anti-alias and process noise reduction of the acquired signal, and then the control system design is optimized.
Based on the ground vacuum experimental platform, the proposed thruster control scheme is experimentally verified to achieve 0.1 μN resolution, 20 ms response speed, and ≤0.05 μN/Hz1/2 (10−3~1 Hz) thrust noise, which is better than the requirements for the micro-propulsion system proposed by the gravitational wave detection mission.