Robust Coordinated Attitude Control Of Near Space Vehicle

With the fast development of science and technology in the area of aeronautics and astronautics,as well as the tensing international situation,near space vehicle(NSV)has gradually become the powerful weapon for countries to defend their military position and national security.However,NSV holds the four obvious characteristics such as complex nonlinear,fast time-varying,strong coupling and severe uncertainty,which bring enormous challenge to the design of flight control system.This paper is aimed at relieving the negative impact of strong coupling on the attitude of vehicle,and the robust coordinated attitude control of NSV is studied.The main research contents are listed as follows:Firstly,combined with the existing research achievements and experimental data at home and abroad,typical six-degree-freedom twelve-state nonlinear model of NSV is established.Meanwhile,using the standard atmosphere as the flight environment,the open loop flight characteristics of NSV are studied.On the basis,the coupling characteristics among the attitude system are analyzed from an intuitional view by setting different initial values of the states.Subsequently,in order to satisfy the intense demand of NSV for the robustness of controller,sliding mode control(SMC)is applied to design attitude controller.However,considering the common chattering phenomenon in classical SMC,assembled sliding mode surfaces and unidirectional auxiliary surfaces(UASs)are combined to improve the classical SMC,and the sliding mode controller with UASs is designed.Considering the improved sliding mode controller has the deficiency of low convergence rate,experiment and analysis on accelerating convergence rate is carried out by designing different reaching laws.The time of state errors reaching the sliding mode surfaces is regarded as the performance index.In the end,the completed controller not only can relieve the chattering phenomenon of sliding mode,but also can accelerate the convergence rate of controller.Then,coupling coordinated controller is designed for the attitude system of NSV.The relative mathematical statistics is applied in the design of attitude controller of NSV,respectively,coupling degree and coupling matrix are defined.By closed loop states sampling for the attitude system of NSV and data statistics,the coupling matrices among attitude angles,attitude angle rates,and the deflections of control actuator are obtained,which quantify the coupling characteristics of attitude system.On this basis,effective coupling coordinated controller is designed,which is also combinedwith the improved sliding mode controller successfully,providing the attitude system of NSV with good stability,robustness and coordination.Finally,considering the existing of uncertainties,robust coordinated control of attitude system,as well as the dynamic control allocation of thrust vector is studied.Considering the negative impact of uncertainties on attitude system,nonlinear disturbance observers for the slow loop of attitude and the fast loop of attitude are designed,which can estimate the uncertainties accurately online.On the basis,the robust control laws are designed,which compensate the negative impact of uncertainties.In order to guarantee the validity of control command and improve the efficiency of control actuators,thrust vector control is brought in,and the regular quadratic programming algorithm is extended by also penalizing the actuator rates,and the weight matrices in the cost function are designed by the maximum deflections and maximum deflection rates,which restrains the saturated deflection and improves the dynamics and efficiency of actuators in some degree.Furthermore,the coordination among the control actuators is also enhanced.