Performance and Micro-Vibrations in Magnetic Bearings for Space Actuators

The development of new space missions with novel high-performance and very sensitive payloads for Earth observation or scientific missions has imposed considerably tougher requirements in terms of the satellite's pointing accuracy and stability, and thus on the maximum allowed on-board micro-vibrations. The main sources of on-board disturbances are any moving or rotating parts in the satellite, such as cryocoolers, momentum or reaction wheels (RWs) and control moment gyroscopes (CMGs). These actuators generate narrow-band harmonic vibrations dependent on the actuator's speed, which are transmitted and amplified through the satellite's structure and reach the sensitive payload such as high-resolution cameras, mirror structures or telescopes.

A very promising alternative to overcome these limitations is the use of magnetic bearings (MBs), as identified by the European Space Agency (ESA), to levitate the rotor during operation, and thus allow a contact- and friction-less operation with virtually infinite life-time. Furthermore, due to the active control of the position of the rotor it is possible to actively suppress any other rotor vibrations such as exported forces due to rotor residual unbalance, creating a very-low disturbance actuator that can satisfy the needs of future high-performance space missions.

In the present dissertation, a study of the main aspects of magnetic bearings for space applications is undertaken, and more specifically of a promising magnetic bearing reaction wheel configuration: a fully active, Lorentz-type, self-bearing, slotless magnetic bearing and permanent magnet synchronous motor. The main goal of this thesis is to identify the key factors and characteristics of a magnetic bearing system, for its use in reaction wheels for attitude control of satellites, in terms of requirements and performance criteria, and undertake the required analysis and modifications in order to address such aspects.

As a result of requirements from in-orbit conditions and on-ground qualification and testing, the key features of magnetic bearings in reaction wheels are: generated micro-vibrations during operation, magnetic bearing and motor efficiency affecting power consumption and heat dissipation, and system complexity linked to the actuator's failure risk and cost.

Regarding micro-vibration generation it is necessary to study its sources, countermeasures and active suppression control techniques, to materialise the advantages of magnetic bearings and achieve very-low disturbances. Through the micro-vibration characterisation of the studied magnetic bearing system, the main sources of vibrations are identified and several countermeasures are undertaken: highly-symmetric bearing and motor windings reduce cross-couplings and asymmetries in the bearing and motor forces, and a multi-harmonic force rejection control technique is proposed and successfully implemented, achieving a reduction in the generated vibrations of at least one order of magnitude.

In order to limit in-orbit power consumption and guarantee on-ground testing, a high magnetic bearing and motor efficiency should be sough, minimising thermal and power consumption constraints. For this reason an accurate electromagnetic modelling of the studied slotless magnetic bearings and motor, combined with a general optimisation technique allowing the maximisation of the overall machine efficiency, and resulting in important reductions o