Robust Adaptive Control For Nonlinear Flight Control Systems Of Near Space Vehicle

Near space vehicle (NSV) has great strategic value both in military and civilian area, which hasboth advantages of traditional aircrafts and spacecrafts. Due to the characteristics of multi-task modesin different phases of flight, NSV possesses some distinct characters, such as serious nonlinearity, fasttime variation, and intense coupling. Therefore, it is a great challenge to design flight control systemsfor NSV. For these problems, some works are studied in this dissertation by using NSV mathematicalmodel of six degrees of freedom and twelve states, such as the flight attitude control, longitudinalflight control and coordinated turn in uncertain environment. The main results of the subject areexhibited as follows:First of all, the6DOF(six degrees of freedom) twelve states equations of NSV are established fordescribing NSV dynamic system, based on the research contributions of the abroad institution and ourlaboratory. In order to solve the problem of NSV attitude control with the multi-variables, strongcoupling and complex nonlinearity, we propose a backstepping scheme for a class of multi-inputmulti-output nonlinear systems. Thus, NSV attitude angle controller and angular velocity controllerare presented by using the proposed method. Simulation results show the desired tracking controlperformance is achieved by the proposed method.Secondly, a backstepping control method based on the disturbance estimation is proposed for theNSV attitude system with uncertainty and external disturbances. Two types of online estimationalgorithms including an adaptive interference estimation algorithm and a second order sliding modedisturbance observer, are given to compensate the unknown disturbances. The good performance ofthis method is demonstrated by both of the theoretical analysis and simulation results.Next, the switched nonlinear systems are used to describe the NSV attitude model of differentstages, when the NSV’s wing sweep angle is variable in large envelop. A smooth backstepping controlmethod is presented for a class of the precisely known switched nonlinear systems, to solve theproblem of controlled variables jumping caused by mode switching. To accurately estimate unknowndisturbances, we design a set of radial basis function neural network disturbance observers, and usesome robust terms to offset the impact on system performance caused by the approximation errors.Then, we derive the structure of uncertain switched nonlinear controller, and analyze the closed-loopstability condition by using the multiple Lyapunov functional method. Based on the above works, anattitude angle controller and an angular velocity controller are designed for the uncertain NSV model. Simulation results show the desired robustness of the proposed method for uncertainty and externaldisturbances.Subsequently, a longitudinal attitude-trajectory coordinate control scheme based on fastconvergence of nonlinear differential disturbance compensator is proposed, aiming at the issue of thelongitudinal coordinate control. According to the characteristics of NSV longitudinal flight, weintroduce NSV longitudinal attitude-trajectory coordinate control equations. A novel non-affinenonlinear approximation method is proposed for non-affine nonlinear flight path dynamic model. Inorder to estimate compound disturbances during the flight phase of longitudinal NSV, we present arapidly convergent nonlinear differentiator and prove that differential estimation residuals converge toan arbitrarily small neighborhood. Based on the above works, a NSV attitude-trajectory coordinatecontroller is designed by using the constrained command filter backstepping method and the dynamicinversion method. Simulation results show that the proposed control strategy can achieveattitude-trajectory coordinate movements.Finally, a robust coordinated-turn control strategy based on second order sliding modedisturbance observer is presented, aiming at the issue of realizing NSV coordinated-turn. According tothe characteristics of NSV turning flight, we establish NSV coordinated-turn control model, in whichthe control of flight path, attitude angle and angular velocity can be considered at the same time.Combining the dynamic inversion method, sliding mode control with regulatory factors and slidingmode disturbance observer, we design a robust controller for the gravity center trajectorycoordinated-turn. Sliding mode control with regulatory factors is used to improve dynamicperformance of flight-path angle and heading angle, and to coordinate the longitudinal and lateralmovement. Simulation results show that this method can effectively implement the uncertain NSVcoordinated-turn control task.