Research On Robust Attitude Tracking Control For Flexible Spacecraft With Input Saturation;research On Robust Attitude Tracking Control For Flexible Spacecraft With Input Saturation

In practical space missions, note that during operation the mass properties of thespacecraft may be uncertain or may change due to onboard payload motion, rotation offlexible appendages such as solar arrays, or fuel consumptions, and an orbital spacecraftis also affected by external disturbances, such as solar radiation, aerodynamic drags, etc.All these issues lead to the uncertain coupling nonlinear attitude system with multi-inputand multi-output. Moreover, due to physical limitation, momentum exchange devices orthrusters as actuator for the spacecraft attitude control plant fail to render infinite controltorque and thus the actuator outputs are constantly bounded or constrained. Once the ac-tuator reaches its input limit, the efforts to further increase the actuator output would notresult in any variation in the output, and then this usually deteriorates the system perfor-mance and even results in system instability. Consequently, it is very desirable to takeinput saturation into account during the attitude controller design. In this thesis, dynamicmodeling of spacecraft with ?exible appendages and attitude tracking control are deeplystudied, which is funded by the National Natural Science Foundation of China(ProjectNumber: 60774062) and the Research Fund of the Doctoral Program of Higher Educa-tion of China(Project Number: 20070213061), and the main contents of this thesis arepresented as follows:Firstly, flexible spacecraft attitude kinematics described by Euler angle and unitquaternion is presented based upon Euler Theorem, and an approximately analytical dy-namic model of spacecraft is derived using Hamilton's principle with discretization bythe assumed mode method. The obtained mathematical model is then converted to statespace form for the purpose of attitude tracking control design.Secondly, two robust adaptive backstepping attitude tracking control schemes aredeveloped for a moment exchange devices controlled spacecraft based on the linearizedattitude kinematics, in which the unknown inertia matrix, disturbances torques and in-put saturation are considered. The first control strategy combines backstepping techniquewith L₂ gain control. In this approach, a robust controller and an auxiliary input signalerror system are incorporated to treat the input saturation problem. Although ??2 trackingperformance with the desirable attenuation level to disturbances can be achieved with this proposed control scheme, it can only guarantee uniformly ultimately bounded stability ofattitude tracking error. For the purpose of achieving globally asymptotically attitude track-ing control, an adaptive backstepping sliding mode control scheme is then proposed withthe advantages of sliding mode control for its robustness to system unmodeled dynamic.Furthermore, the benefits of these two control approaches are analytically authenticatedand also validated via simulation study.Moreover, a theoretical framework for attitude tracking control using quaternionmeasurements only is developed and applied to the flexible spacecraft with external dis-turbance, parameter uncertainties and control input constraint taken into account simul-taneously. In contrast to the most existing approaches, the proposed controller does notrequire the knowledge of body angular velocity and the flexible modal vibrations as well.By employing neural network approximate technique, the problem of unknown systemdynamics is explicitly addressed in the control design. In the closed-loop systems, it isshown that uniform ultimate boundedness of all signals is guaranteed, and the attitudecan track the reference trajectories as close as possible. Moreover, the derived adaptiveattitude tracking controller can ensure that the controller rigorously enforces actuator-magnitude saturation constraints. Numerical simulation results are also presented whichnot only highlights the ensuring closed-loop performance benefits from the control lawderived here but also illustrates its robustness in face of external disturbances and systemuncertainties.Last but not least, the details of system responses are numerically simulated in anorbital flexible spacecraft in conjunction with the above proposed three novel attitudetracking control laws. It is shown that parametric uncertainty and unmodeled dynamicof the attitude system, external disturbances and even input saturation can be explicitlyaddressed by our proposed controllers with perfect attitude tracking performance guar-anteed. Moreover, theirs benefits of theoretical and engineering practice are also high-lighted.