Abstract
Momentum wheels are frequently used for the attitude control of satellites. Most existing momentum wheels support their spinning rotor using ball or roller bearings. These mechanical bearings are difficult to lubricate in the vacuum of space; they are prone to wear and so have a limited lifespan that can limit the useful lifespan of the satellite. Jitter in the bearings can generate microvibrations on the satellite that can affect its image quality; the noise level on the satellite platform is becoming increasingly important as the resolution of the cameras on small satellites approaches one metre ground resolution. Replacing the mechanical bearings with active magnetic bearings, where the spinning rotor is suspended by electromagnets potentially offers many benefits. There will be no contact between moving parts, eliminating friction, stiction and wear. Because the rotor’s position is actively controlled it can be controlled precisely generating very low levels of noise. This thesis introduces a new concept - the 3Dwheel. The number of degrees of freedom that can be actively controlled in a magnetic bearing can be chosen. An engineering model of the 3Dwheel, a magnetically levitated momentum wheel with five degrees of freedom actively controlled, that has been designed, built and successfully tested is presented here. This design allows the rotor to be tilted generating a gyroscopic output torque; one 3Dwheel can therefore generate a torque about all three principal axes of the spacecraft. The electromagnets allow the wheel to be tilted with a high rate generating an output torque with a large magnitude and a bandwidth much greater than existing actuators. In this thesis the theory behind magnetism is used to model and investigate the design of a magnetic bearing. From this, the design of the 3Dwheel is presented and explained. This process can be used to help design other magnetically levitated momentum wheel designs. The engineering model of the 3Dwheel was successfully levitated in the laboratory by four different controllers. The position of the rotor can be maintained within three standard deviations of the desired position within 11.6 μm. It has been tilted at a rate of 0.556 rads -1, so generating a torque of 0.68 Nm while spinning at 5000 rpm. This resolution allows the 3Dwheel to have very low output noise levels. The 3Dwheel’s bandwidth has been demonstrated to be two orders of magnitude greater than a conventional momentum wheel's. Various techniques for improving the performance of controllers and allowing stable levitation of the rotor at spin rates exceeding the controller’s bandwidth are presented. Simulations of the 3Dwheel fitted to a small satellite prove that from its demonstrated ability in the laboratory a single 3Dwheel is capable of providing 3-axis attitude control of a small satellite.