Robust Control For Near Space Vehicle With Input Saturation

Near space vehicle(NSV)has become one of the most import development aircrafts due to some advantages,such as fast speed(over five Mach),wide flight envelop(flight altitudes from 20 km to 100km)and flexible applicability.However,because of the variable working modes and environment,the NSV system is affected by the system uncertainties and strong external disturbance.In addition,the control surface deflection angular,deflection rate and engine thrust of the NSV are unavoidable to be with the determined bounds in flight,and the actuator constraint problem is often not considered in the designing process for the traditional control method.Once the actuators of the NSV reach the constrained bounds,the performance of the control system will be degraded,and even the closed-loop system stability will be destroyed.Thus,the robust constrained control scheme for the uncertain NSV is developed to solve the uncertainty and actuator constrained problem in this dissertation.The main results are given as follows:First of all,based on the existing research results,the nonlinear model of the NSV in hypersonic mode is derived,and the attitude motion model of the NSV will be linearized at the equilibrium point.And,the properties of some typical nonlinear blocks are analysed and combined with the NSV model to obtain the constrained NSV attitude model.Secondly,a disturbance observer based composite nonlinear feedback(CNF)control scheme is proposed for the attitude linear model.The disturbance observer is introduced to the CNF control method to improve the robust performance of the control system,and ensure the system output to track the desired trajectory.Furthermore,a disturbance observer based anti-windup control scheme is developed to overcome the limitations of the CNF method.And considering the fact that the system fault will probably lead to the actuator saturation problem,a fault detection observer based anti-windup robust fault-tolerant controller is designed to ensure the stable flight control for the NSV linear model subject to system faults,input saturation,external disturbance and uncertainties.It is verified by the simulation results that the designed control scheme has well robust control performance.Then,the robust constrained control method is presented for the uncertain NSV attitude nonlinear model.A disturbance observer and auxiliary system based robust constrained control scheme is developed for the NSV attitude nonlinear model with system uncertainties,external disturbance and input saturation.An auxiliary system is constructed to eliminate the effect of input saturation based on the compensation idea.The disturbance observer is developed to estimate the external disturbance,and the robust constrained controller is designed using the backstepping method.Furthermore,the robust fault-tolerant constrained controller is proposed for the NSV attitude model with actuator fault.The effectiveness of the studied control scheme is determined by the simulation study.Next,comparing with the existing research that only the steady-state performance was considered,the transient and steady-state performance of the closed-loop system will be simultaneously considered in the tracking control problem.The prescribed performance method combined with the auxiliary system is introduced to transform the constrained tracking errors into the unconstrained signals.The robust prescribed performance tracking controller is proposed for the NSV attitude model based on the backstepping technology.It is shown by the simulation results that the given transient and steady-state performance of the closed-loop system can be guaranteed under the proposed controller.Furthermore,a disturbance observer based robust tracking control method is designed for the uncertain NSV with input and output constraints,in which the output constrained problem is transformed into constrained tracking error problem,and the boundedness of the tracking error is guaranteed by the prescribed performance method.Meanwhile,the sliding model observer and the auxiliary system are constructed to guarantee the stability of the closed-loop system.The excellent performance of the developed input and output constrained control method is observed by simulation study.Finally,the works of this dissertation are summarized and the further work is also discussed.