A. R. Aghalari; M. Iranzad
Volume 6, Issue 1 , April 2013, , Pages 79-91
Abstract
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. ...
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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.
Alireza Aghalari; Javad Tayebi; Ahmad Kalhor
Volume 5, Issue 4 , January 2013, , Pages 61-68
Abstract
Recently, many space missions have been using small satellites, because small satellites are easier and faster to develop and thereby, provide increased launch opportunities. Some of these missions include tasks that required agile maneuvers. In this paper, attitude stability testing of an agile three-degree-freedom ...
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Recently, many space missions have been using small satellites, because small satellites are easier and faster to develop and thereby, provide increased launch opportunities. Some of these missions include tasks that required agile maneuvers. In this paper, attitude stability testing of an agile three-degree-freedom micro-satellite simulator – which is equipped with a pyramid arrangement of single-gimbal control-moment gyros (SGCMGs) – is presented. In the attitude stability testing, the local quadratic regulator (LQR) control strategy is used, which has superiority to other approaches due to its independence of using steering law. This simulator allow to test different control laws by using SGCMGs. In this work, after introducing the actuator and satellite simulator and using the control strategy in the simulator, the attitude stability testing is performed and then, the experimental results are presented and discussed. The results show the attitude stability of the simulator which is exposed to the disturbing toques.
