Faculty of Engineering, School of Aerospace Mechanical and Mechatronic Engineering
Abstract
The attitude control system is one of the most important systems for satellites, which is essential for the satellite's detumbling, pointing, and orbital maneuver. The conventional attitude control system consists of magnetorquers, reaction wheels, and thrusters. Among these actuators, magnetorquers are widely used for satellite detumbling and attitude control, especially for small satellites and CubeSats. It consumes zero propellant compared with thrusters and has a high chance of survival compared with the reaction wheel as it does not contain any moving parts, which makes them last longer in harsh environments.
Conventional magnetorquers utilize air or soft magnetic materials, e.g., iron and alloys, as core, and the magnetic field is generated by feeding the electric current to the wrapped solenoid. Due to the power limit of the small satellites, the magnetic field strength is strictly limited, and the continuous current supply results in massive energy consumption for detumbling and other attitude adjustment missions. The long copper wire of the solenoid will also result in high resistance and generate significant heat. To improve the current design and overcome the proposed drawbacks, a novel electro-permanent magnetorquer has been designed and developed in this thesis as one actuator of the attitude control system. Unlike conventional magnetorquers, the electro-permanent magnetorquer utilizes hard magnetic materials as the core, which can maintain the magnetization when the external magnetic field is removed, to generate the required magnetic field. A special driving circuit is designed to generate the desired dipole moment for the magnetorquer, and the components used for the circuit are carefully selected. The experiments show that the electro-permanent magnetorquer can generate 1.287 Am2 dipole moment in either direction. The magnetorquer works in pulse mode to adjust the dipole moment, and it requires around 0.75 J energy maximum per pulse. A single-axis detumbling experiment has been conducted using only one torque rod on the air-bearing table inside an in-house manufactured Helmholtz cage. The experiment results show that the magnetorquer can detumble the air bearing table with 0.0612 kgm2 moment of inertia from an initial speed of around 27°/s to zero within 800s, and total energy of 82.92 J was consumed for the detumbling experiment. A single torque rod single-axis pointing experiment has been conducted with a sliding mode controller on the same platform. The results show that a single torque rod can achieve the target angle and maintain the error discrepancy within the ±0.4° boundary under a specific system configuration.
A micro air-fed magnetoplasmadynamic thruster has been designed and tested as another attitude control system actuator. The thruster is a miniaturized electric propulsion system based on the conventional full-scale magnetoplasmadynamic thruster that operates at hundreds of kilowatts. The thruster is designed and tested using normal air as the propellant under the pulse operation mode on a calibrated micro-force measurement thruster stand. The experiments revealed that the thruster could generate a 34.534 µNs impulse bit with an average power input of 1.857 ± 0.0679 W and thrust to power ratio of 8.266 µN/W. The specific impulse is calculated to be 2319 s with a thruster efficiency of 9.402%, which is quite competitive compared with other solid-state and liquid-fed pulse-mode thrusters. This paper presents the design and test results for the thruster under a low power level, as well as an analysis of its problems and limitations with corresponding future research and optimization directions noted at the end.
The electro-permanent magnetorquer as a payload of the CUAVA-2 satellite mission has been introduced in this thesis. The design considerations and adjustment based on the requirement of the CUAVA-2 has been introduced in detail. A simple sliding mode controller has been developed to achieve three-axis attitude control using both electro-permanent magnetorquer and the micro air-fed magnetoplasmadynamic thruster. The controller's performance has been tested using MATLAB-based simulation with the experimentally obtained performance parameters and some assumptions. The results show that the smart attitude control system can achieve ±0.005° pointing error discrepancy with the help of both actuators
