Fault Tolerant Attitude Control Of Flexible Spacecraft With Control Input Constraint

Accurate and reliable control is one of the most important problems in spacecraft design. Although the missions of space vehicles and their attitude requirements vary greatly, high pointing accuracy and fault tolerance are important parts of the overall design problem for the spacecraft control system. However, the orbiting attitude slewing or tracking operation will introduce certain levels of vibration to flexible appendages, which will deteriorate its pointing performance. Dynamics of spacecraft are time varying and highly nonlinear, and they are affected by various disturbances coming from the environment and knowledge about system parameters such as the inertia matrix, which are usually not well known. Moreover, due to physical limitation, momentum exchange devices or thrusters as actuator for the spacecraft attitude control plant fail to render infinite control torque and thus the actuator outputs are constantly bounded or constrained. Once the actuator reaches its input limit, the efforts to further increase the actuator output would not result in any variation in the output, and then this usually deteriorates the system performance and even results in system instability. In addition, actuators may fail during system operation, and the actuator failures are often uncertain in the sense that it is not known when, how much, and how many actuators fail. All these in a realistic environment create considerable difficulty in the design of attitude control system for adequate performance and stability, especially, when all these issues are treated simultaneously. In this dissertation, dynamic modeling, attitude control with control input constraint, robust fault tolerant attitude control and vibration control of spacecraft with flexible appendages are deeply studied, which is funded by the National Natural Science Foundation of China and the Research Fund of the Doctoral Program of Higher Education of China. The main contents of this dissertation are presented as follows:Flexible spacecraft attitude kinematics described by Euler angle and unit quaternion is presented based upon Euler Theorem, and with assumption of small elastic displacements, an approximately analytical dynamic model of spacecraft is derived using Lagrange's principle. For the purpose of control law design, a reduced-order model of flexible spacecraft is then developed.Neural network (NN) based robust controller for rotation maneuver is considered for an orbiting flexible spacecraft, explicitly taking into account the actuator saturation, uncertainties and external disturbances. The actuator saturation is assumed to be unknown and treated as the system input disturbance. With the universal approximating property and learning capability of NN, the NN-based saturation compensator is design and inserted into a feed-forward to compensate the saturation nonlinearity. Then a variable structure controller, which only uses the output information, is designed to reject the disturbance, deal with uncertainty and ensure that the system trajectories globally uniformly ultimately bounded. In addition, a modified adaptation control law is presented for the upper bound on the uncertainty to improve the adaptive performances such that a new controller is designed which can guarantee the boundedness of the estimated gains when the boundary layer technique is employed. To study the effectiveness of the corresponding control scheme, the traditional methods are also developed for the control system. Both analytical and numerical results are presented to show the theoretical and practical merit of this approach.Time-delay-control (TDC) based adaptive variable structure control (AVSC) system is designed for spacecraft attitude maneuvers with redundant actuators in the presence of actuator failures, parametric uncertainties and external disturbances. More specifically, this proposed scheme combined the TDC and AVSC design technique such that this design does not require the knowledge of the boundedness of the considered uncertainties/disturbances, and only one parameters are required to be updated in the adaptive loop; with the TDC, the unknown actuator faults are estimated by using one-step previous state information and canceled out by the estimated values such that it does not need a fault detection and isolation mechanism. Lyapunov stability analysis shows that the resulting closed-loop system is proven to be stable and the effect of the external disturbances and possible uncertainties on the output can be attenuated by appropriately choosing the design parameters. Furthermore, the benefits of the control approach is analytically authenticated and also validated via simulation study.For further suppressing the flexible vibration, the inner-control loop uses the piezoceramics as sensors and actuators to actively suppress certain flexible modes by designing strain rate feedback (SRF) compensators. An attractive feature of the method is that the vibration reduction and attitude control are achieved separately in the two separate feedback loops, allowing the pointing requirements and simultaneous vibrations suppression to be satisfied independently of one another. Then, for the outer-loop, a variable structure based fault tolerant attitude control system is further investigated for an orbiting three-axis stabilized flexible spacecraft with redundant thrusters, in which the thruster failures, control input saturation and external disturbances are explicitly taken into account simultaneously. In addition, by explicitly considering the saturation magnitude of the available control input of thruster, a straightforward relationship between this magnitude and those of the desired trajectories and disturbances even with continuous control is established. Lyapunov stability analysis shows that the resulting closed-loop system is proven to be stable and the effect of the external disturbances and faults can be attenuated by appropriately choosing the design parameters. Numerical examples are also presented to demonstrate that the control algorithms developed are not only robust against external disturbances, but also able to accommodate thruster failures under limited saturation value.