Reaction Wheel Performance Characterisation and Assessment of Electromagnetic Interactions with Magnetic Torquers

Abstract: Having an in-depth knowledge on the performance characteristics of space mechanisms in flight operation, with special attention to nominal vs. anomalous performance, is vital for mission success. On many unmanned spacecraft for Earth observation missions, reaction wheel assemblies are used in combination with magnetic torquers for their attitude control. Understanding the magnitude of potential electromagnetic interactions between both types of attitude control actuators is of particular interest for large spacecraft as they are usually equipped with strong magnetic torquers. In this frame, experimental investigations have been performed on simplified test set-ups with flight representative reaction wheel assemblies operated in external homogeneous magnetic fields as well as in close vicinity of magnetic torquers which create inhomogeneous magnetic fields. The test results have been successfully correlated with computer-based simulation output obtained from models with different levels of complexity. The impact of critical parameters like the location of magnetic torquers relative to reaction wheels and their material properties such as electrical conductivity and magnetic permeability have been particularly studied. It has been found that magnetic torquers pointing orthogonal to the reaction wheel spin axis cause the highest influence on the reaction wheel's performance characteristics. The material choice for the flywheel rotor, being either ferromagnetic or paramagnetic, has a strong influence when exposing the reaction wheel assembly to external magnetic fields. In general, the increase of loss torque noticed with all reaction wheels tested has been caused by eddy current effects. In this frame, the impact of using ferromagnetic materials has been surprisingly strong. Specifically, the local distortions and guidance of the magnetic field due to ferromagnetism has a highly amplifying effect on eddy currents. However, interestingly it has also been found that the impact of material choice is much more severe when considering homogeneous magnetic fields and strong magnetic torquers while being less important with relatively small magnetic torquers. The main reasons for this finding have been compensating effects of ferromagnetic vs. highly conductive materials.