Development of empirical and analytical reaction wheel disturbance models

Abstract

Accurate disturbance models are necessary to predict the effects of vibrations on the performance of precision space-based telescopes, such as the Space Interferometry Mission (SIM). There are many possible disturbance sources on such a spacecraft, but the reaction wheel assembly (RWA) is anticipated to be the largest. The following paper presents two types of reaction wheel disturbance models: a steady-state empirical model that assumes the disturbances consist of discrete harmonics of the wheel speed, and a non-linear analytical model that physically captures the internal flexibilities and fundamental harmonic of an unbalanced wheel. A MATLAB toolbox is designed to facilitate the extraction of the empirical model parameters from steady-state RWA disturbance data. Experimental data obtained from tests of two off-the-shelf RWA’s manufactured by Ithaca Space Systems are used to illustrate the modeling process and validate the model. The empirical model captures the tonal quality of RWA disturbances but does not include the amplification of disturbances caused by the structural wheel modes. Therefore, an analytical model is developed using energy methods, and preliminary results of on-going research show that interactions between the flexible wheel modes and the harmonic disturbances are captured. In addition, the empirical model parameters are used to incorporate harmonics other than the fundamental to the analytical model.