MS thesis abstract - Masterson, Rebecca
| Author: | Masterson, Rebecca |
| Degree: | Masters of Science |
| SERC #: | 4-99 |
| File type: | PDF, 10186 kB |
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Development and Validation of Empirical and Analytical Reaction Wheel Disturbance Models
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) and the Next-Generation Space Telescope (NGST). There are many possible disturbance sources on such a spacecraft, but the reaction wheel assembly (RWA) is anticipated to be the largest. This thesis presents three types of reaction wheel disturbance models. The first is a steady-state empirical model that was originally created based on RWA vibration data from the Hubble Space Telescope (HST) wheels. The model assumes that the disturbances consist of discrete harmonics of the wheel speed with amplitudes proportional to the wheel speed squared. The empirical model is extended for application to any wheel through the development of a MATLAB toolbox that extracts the model parameters from steady-state RWA data. Experimental data obtained from wheels manufactured by Ithaco Space Systems 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, an analytical model, is created using principles from rotor dynamics to model the structural wheel modes. The model is developed with energy methods and captures the internal flexibilities and fundamental harmonic of an imbalanced wheel. A parameter plotting methodology is developed to extract the analytical model parameters from steady-state RWA vibration data. Data from an Ithaco E type wheel are used to illustrate the parameter matching process and validate the analytical model. It is shown that this model provides a much closer prediction to the true nature of RWA disturbances than the empirical model. Finally, an extended model, which combines features of both the empirical and analytical models, is introduced. This model captures all the wheel harmonics as well as the disturbance amplifications that occur due to excitation of the structural wheel modes by the harmonics. In addition, preliminary analyses that explore the dynamic coupling between RWA and spacecraft are presented and a plan for laboratory testing to gain insight into the effects of coupling and provide disturbance model validation is outlined.