Finite-time Attitude Control Of Satelite With Actuator Fault And Misalignment

To accomplish orbital missions such as Earth imaging, surveillance, and commu-nication, satellites often need to perform attitude tracking maneuvers. This may require the spacecraft to supply the payload with high attitude pointing accuracy and stability. Because on-orbit satellites are under the effect of unknown disturbance and uncertain sys-tem parameters, these issues will deteriorate attitude control performance. Consequently, the design of attitude control system to accomplish highly accurate slewing or pointing maneuvers has attracted considerable academic interest. Additionally, actuator fault is another challenge for the reliability of attitude control system. Although there exists a number of investigations in literature on satellite attitude fault tolerant control design with desirable control performance guaranteed, actuator misalignment is not handled. More-over, most of the existing fault tolerant attitude control is only able to achieve global asymptotical attitude control. It requires infinite time rather than finite time to stabilize the attitude system. As a result, those approves is not desirable for satellites that need to perform fast slewing attitude maneuvering. Based on those results on fault tolerant control and attitude control, this dissertation will present a finite-time control approach to solve the attitude tracking problem in the presence of disturbances, uncertain inertia pa-rameters, and actuator uncertainties. The attitude and the angular velocity tracking error will be stabilized with finite-time convergence. The main contents of this dissertation are presented as follows:Considering all the wildly used actuators in satellite engineering, it summarize that actuator uncertainties incorporate fault and misalignment. Based on the dynamics of ac-tuator, especially, reaction wheel, the reasons why and how faults occur in reaction wheel are investigated. Moreover, the effect of reaction wheel fault on the actual generated torque is analyzed. As a result, summarizing all possible faults of reaction wheel, the mathematical model of its fault is synthesized. Furthermore, the mathematical model of actuator misalignment is also established. The relationship between the actual torque generated by actuators and the total torque acting on the satellite body is analyzed.A terminal sliding mode-based finite time control scheme is developed for the atti-tude tracking maneuver in the presence of uncertain inertia parameters, external distur-bances, and actuator faults. Viewing partial loss of actuator effectiveness fault, distur- bance, and actuator misalignment as a lumped system uncertainty, by using the robust control theory, a terminal sliding mode controller is first proposed. The robustness to the system uncertainty is guaranteed. Then, explicitly taking all possible fault and actuator constraint into consideration, an adaptive terminal sliding mode control law is developed. In this approach, adaptive control technique is applied to estimate the upper bound on the system uncertainty, and the estimated value is then compensated. It is proved by Lyapunov stability theory that, those two controllers guarantee that the closed-loop attitude tracking system is practical finite-time stable. The attitude and the angular velocity tracking error is governed to converge to an small set with arbitrary small radius. Moreover, disturbance rejection control and robust control to uncertain inertia are achieved simultaneously.A compensation control scheme is presented to address the attitude tracking control problem of satellite equipped with reaction wheels. Reaction wheel uncertainties includ-ing fault and misalignment are considered. For the satellite equipped with redundant wheels, a sliding mode controller is designed to achieve globally asymptotical attitude tracking. For a satellite controlled by using three reaction wheels with severe configu-ration misalignment, taking bias fault, loss of effectiveness fault, and slope-varying fault into consideration, an adaptive sliding mode compensation controller is developed. The controller incorporates a compensation effort to accommodate external disturbance, sys-tem uncertainties, and the bias torque introduced by misalignment and fault. The con-troller is able to guarantee that the closed-loop attitude system is finite-time stable. More specifically, those two controllers can achieve finite-time convergence of the attitude and the angular velocity tracking error. Numerical simulation results are also presented to verify the effectiveness of the proposed approach.An uncertainties estimation-based finite-time attitude tracking control strategy is de-veloped to simultaneously tackle with all possible actuator faults and misalignment. In this approach, by viewing external disturbance and actuator uncertainties as a lumped un-certainty, a terminal sliding mode observer is designed to estimate this uncertainty by us-ing the equivalent control approach in sliding mode control. This observer guarantees that the estimation error is finite time stable. By using this precisely estimated information, a non-singular terminal sliding mode controller is synthesized. A compensation effort in-corporated in the controller is used to compensate for the lumped uncertainty. It is proved that the controller governs the attitude tracking error to zero with finite-time convergence. In comparison with the existing methodologies on finite-time control or attitude fault tol- erant control, the main contribution of the proposed approach is that, it has capability to tackle with actuator uncertainties including fault and misalignment simultaneously.