Development of Reaction Wheel Disturbances General Model and Validation with Experimental Data

Precision spacecrafts require high levels of pointing stability. Small levels of vibration can cause a significant reduction in image quality. There are many possible disturbance sources on spacecraft (mechanical systems or sensors), but the reaction wheel assembly (RWA) is anticipated to be the largest. Therefore, accurate models of reaction wheel disturbances are necessary to predict their effect on the spacecraft performance and develop methods to control the undesired vibration. In this paper, two types of reaction wheel disturbance models is presented. The first is a steady-state empirical model that was originally created based on a prototype RWA hard-mounted test data. The model assumes that the disturbances consist of discrete harmonics of the wheel speed with amplitudes proportional to the wheel speed squared. Experimental data obtained from RWA designed and manufactured by Aghalari and et al. are used to illustrate the empirical modeling process and provide model validation. The model captures the harmonic disturbances of the wheel quite well, but does not include interactions between the harmonics and the structural modes of the wheel which result in large disturbance amplifications at some wheel speeds. Therefore the second model, a nonlinear analytical model, is created using energy methods to capture the internal flexibilities and fundamental harmonic of an unbalanced wheel. Then the analytical model has been extended to capture all the wheel harmonics as well as the disturbance amplifications that occur due to excitation of the structural wheel modes by the harmonics. Finally experimental data obtained from hard-mounted test of RWA is used to determine the model parameters for both types of models and a comparison between the models and data is presented.