A Full Disturbance Model for Reaction Wheels

Reaction wheels are rotating devices used for the attitude control of spacecraft. However, reaction wheels also generate undesired disturbances in the form of vibrations, which may have an adverse effect on the pointing accuracy and stability of spacecraft (optical) payloads. A disturbance model for reaction wheels was developed at Moog Bradford by combining empirical and theoretical models. The empirical data is obtained from a highly accurate reaction wheel characterization test setup from the European Space Agency and includes disturbance signals of ball bearings transmitted through the structures of the reaction wheel assembly. The theoretical model is derived from the equation of motion of a rigid rotor and a disc supported by two ball bearings including static, dynamic unbalances, structural modes and gyroscopic effects of the wheel rotor. To fully model the disturbance signature of the wheel, the bearing stiffness is formulated as a function of ball pass frequency and the flexibility of the supporting structural items like the reaction wheels housing are included. Finally, the empirical model is added into the theoretical model as excitations to form a full disturbance model for reaction wheels. The resulting combined model is then validated by tests on different types of Moog Bradford reaction wheels. The validated disturbance model is used to evaluate the pointing performance of spacecraft as well as to predict micro-disturbance performance for future reaction wheel designs.