| Summary: | Future spacecraft are expected to achieve highly accurate pointing, fast slewing from large initial condition, and reliable maneuvers in the presence of environmental disturbances, unknown or uncertain inertia parameters, control input saturation, and component failures. Actuators play an important role of generating control torque/force in the desired direction under command from the controller to achieve specific aerospace objectives. However, an abrupt occurrence of an actuator fault/failure could significantly disrupt mission performance or even lead to totally loss of the spacecraft. In order to enhance the reliability and reduce critical failure of spacecraft, fault-tolerant capability should be addressed in the control system of spacecraft. Generally, the available fault-tolerant control (FTC) schemes can be classified into two categories: active FTC and passive FTC. In this thesis, both active and passive FTC approaches are developed for attitude control systems of spacecraft to enhance reliability and safety while achieving high precision attitude maneuver.
Two control allocation (CA) based active FTC schemes are developed for overactuated spacecraft attitude control systems to accommodate actuator faults/failures. The active FTC approach reacts to the system component malfunctions by reconfiguring the controller based on real-time information from a fault detection and diagnosis (FDD) scheme. The proposed two active FTC schemes use the estimated fault information in the on-line CA to redistribute the virtual control efforts into each individual actuator judiciously without reconfiguring the controller. In the first active FTC scheme, taking the effects of FDD imprecision into account, an inertia-free adaptive fault-tolerant attitude tracking controller is developed to handle multiplicative actuator faults/failures. A novel time-varying dead-zone modification is proposed in the design of parameter adaptation to stop the adaptive gain from increasing, and it also improves the robustness of the adaptive law. In the second active FTC scheme, instead of ensuring robustness with respect to imprecise fault estimation in controller design, a novel robust control allocation (RobCA) is proposed to ensure some robustness in CA design such that design complexity of the controller can be reduced. Consequently, a model reference adaptive high-level controller incorporating RobCA is designed to achieve asymptotic attitude tracking irrespective of actuator faults/failures, imprecise fault estimations, external disturbances, and actuator amplitude and rate constraints.
Contrary to CA-based active FTC method, passive FTC method utilizes an unique robust controller to tackle all expected faults. It has a simple structure as neither FDD scheme nor a controller reconfiguration mechanism is needed. This thesis also presents two passive FTC strategies to solve the fault-tolerant attitude control stabilization problem for a rigid spacecraft. Firstly, based on the establishment of an integral-type sliding manifold and a novel simple saturated proportional-derivative control law, an adaptive FTC strategy is developed such that the resultant closed-loop system is capable of tolerating partial loss of control effectiveness fault and additive fault. Furthermore, with consideration of inertia uncertainties, actuator redundancy, and actuator saturation constraints, another finite-time fault-tolerant attitude stabilization scheme is proposed to guarantee that the attitude and angular velocity converge to a neighborhood of origin in finite time despite four types of common actuator faults/failures.
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