Robust Sliding Mode Control Technology For Near Space Vehicles

Since NSV has high maneuverability, larger flight envelope, rapid flight speed and time-varying aerodynamic characteristics, the efficient control technologies are significant to improve its safety and reliability. However, the study of such aircraft is quite difficult. First of all, the NSV system has serious nonlinear characteristics and there is strong coupling relationship among different channels. At the same time, the aerodynamic parameters and the state variables always interact with each other which will further increase the control design difficulty for the NSV. Secondly, if the controller is require to be strong robust stability and high control accuracy, it is necessary to fully consider the parameter uncertainties and the external disturbances in the flight control design stage of the NSV. Although many important parameters are available from prototype testing and simulated flight conditions, the uncertainties of real flight environment can not be entirely taken into account. In addition, the tracking control performance of the control system should be improved for the NSV to meet the various task requirements. This dissertation studies the model of NSV and its robust attitude control is investigated by considering external disturbance, parameter uncertainty, input saturation and actuator failure. The main works are given as follows:Firstly, according to references, the twelve-state kinematic and six-degree-of-freedom equations of NSV are established. In order to facilitate the further research, the attitude model equations of NSV are transformed into the form of affine nonlinear equations based on singularly perturbed theory and time-scale separation principle.Then, to deal with the uncertain disturbances of NSVs, an adaptive backstepping sliding mode control scheme based on terminal sliding mode disturbance observer(TSMDO) is developed. The TSMDO can estimate the unknown external disturbances and the modeling uncertainties within the finite time and the first order sliding mode differentiator is employed to avoid differential expansion problems in the backstepping control design. The rigorous stability of the closed-loop system is proved with Lyapunov method. Simulation results illustrate that the proposed control scheme based on TSMDO can guarantee the satisfactory tracking performance of the multi-input and multi-output(MIMO) attitude motion for the NSV under the external unknown disturbance.Thirdly, a boundary layer adaptive sliding mode control is derived for a class of MIMO nonlinear uncertain systems based on nonliner disturbance observer(NDO) and the developed adaptive sliding mode control is applied to achieve high-precision attitude control for NSVs. Considering the system uncertainty and the unknown upper bound of the external disturbance, the boundary layer adaptive sliding mode controller based on disturbance observer is designed. The developed boundary layer adaptive sliding mode control scheme can eliminate the chattering phenomenon in the traditional sliding mode control and forces the tracking error to approach zero. The rigorous stability of the closed-loop system is proved using the Lyapunov method. Finally, simulation results illustrate that the proposed control scheme can achieve control performance well for the attitude control of the near space vehicle with the unknown disturbance and the system uncertainty.Following, an adaptive backstepping dynamic surface control(DSC) scheme is proposed for the MIMO attitude motion of the NSV system with the external disturbance and the input saturation. The external disturbance and the system uncertainty are efficiently tackled using a Nussbaum disturbance observer, and the adaptive dy-namic surface control is constructed by combining with filters to handle the problem of “explosion of complexity” inherent in the conventional backstepping method. For handle the input saturation, the auxiliary systems are designed to compensate the effect of the saturation in the dynamic surface control design. It is proved that the proposed control scheme can guarantee that all the signals are semi-globally uniformly bounded in the closed-loop system and the errors of the disturbance observer and the tracking errors converge to a small neighborhood of origin. Simulation results of illustrate that the proposed control scheme can achieve satisfactory tracking performance with the input saturation and the external disturbance.Finally, a fault tolerant sliding mode control scheme is derived for NSVs with actuator faults, unknown external disturbance, system uncertainty and input saturation based on disturbance observer and neural networks. The nonlinear disturbance observer(NDO) is designed to eliminate the effect of external disturbance and system uncertainty. To tackle input saturation, the known bound of saturation is used to design the control law such that the control input is within the bounded range. To deal with actuator fault, the radial basis function neural networks(RBFNNs) are employed. Lyapunov stability analysis shows that the closed-loop system is stable and all closed-loop signals are uniformly ultimately bounded. Simulation results demonstrate the effectiveness of the developed fault tolerant control scheme, when actuator faults, unknown external disturbance, system uncertainty and input saturation appear simultaneously.