Research On Robust Sliding Mode Nonlinear Flight Control For Fly Wing UAV

With unique advantage in terms of aerodynamic efficiency, stealth performance, load distribution and endurance of flight, flying wing UAV is the ideal layout pattern of long-endurance stealth bomber, unmanned combat aircraft, high-altitude long-endurance unmanned aircraft. The characteristics of flying wing UAV dynamic model are nonlinear, strong coupling, fast time-varying and multiple input multiple output. By eliminating the horizontal tail and vertical tail, flying wing UAV have many deficiencies in stability, such as weak longitudinal stability, neutral heading stability, strong lateral and directional coupling. Flying wing UAV are mainly used for transport, bombing and reconnaissance missions at high subsonic, attitude and flight path require precise control. Therefore, the study of advanced flight control system with good robustness is the key technology of flying wing UAV development. Aiming at this problem, in this paper, traditional sliding mode control and second order sliding mode control are main control methods, combined with adaptive control, fractional theory, prescribed performance, backstepping control, robust control and active disturbance rejection control, the attitude and trajectory control issues of flying wing UAV with unknown complex disturbances are in-depth research. The main research results are as follows:1.Six degrees of freedom flying wing UAV nonlinear dynamics model under the influence of wind field is established including ground effect and control surfaces additional aerodynamic. And in-depth analyze the effectors characteristic of flying wing UAV.2.For the attitude control problem with aerodynamic parameter uncertainty, an adaptive second order terminal sliding mode attitude control scheme is proposed. An adaptive algorithm is used to adjust switch gain in order to suppress the influence of complex disturbances. By second order terminal sliding mode control, the sign function hidden in the integral term to obtain a continuous control input, effectively inhibited the buffeting of control system.3.Surrounding the attitude dynamic equations of flying wing UAV with unmodeled dynamics and external disturbances, combined with fractional theory and sliding mode variable structure control theory, fractional integral sliding mode attitude control scheme based on super twisting second order sliding mode disturbance observer and double power secondorder sliding mode reaching law is proposed. With the characteristic of fractional integral gradually forgetting past, fractional integral sliding surface is designed. The integral weight of initial error will become increasingly smaller. Compared to conventional integral sliding surface, fractional integral sliding surface can significantly reduce attitude overshoot. In order to solve the long convergence time and serious buffeting problems of traditional reaching law, a double power second order sliding mode reaching law with characteristic of finite time convergence is designed. The neighborhood range of system state convergence to the sliding surface is proved by Lyapunov function analysis with complex disturbances. Super twisting second order sliding mode disturbance observer is designed to estimate and compensate complex disturbances in control system. The simulation results show the effectiveness of the proposed method.4.In order to make reasonable arrangements of the attitude tracking convergence process, overshoot and steady-state tracking error, Combined with prescribed performance theory and backstepping sliding mode control, a prescribed performance backstepping sliding mode attitude tracking control scheme based on fast super twisting second order sliding mode disturbance observer is proposed. Attitude control system equivalent error model is established by using prescribed performance theory. In order to solve the problem of slow convergence speed of super twisting algorithm when system state is relatively far away from equilibrium state, fast super twisting algorithm is proposed to improve the convergence properties of super twisting algorithm. The algorithm is applied to flying wing UAV attitude control system, fast super twisting disturbance observer is designed to estimate and compensate for disturbances in backstepping control subsystem. Numerical simulation results show that flying wing UAV attitude overshoot, steady-state tracking precision and convergence rate are to meet pre-set transient and steady state performance requirements.5. For the longitudinal landing control problem of flying wing UAV under the influence of wind field and ground effect, a backstepping L2 gain robust control scheme based on extended state observer and tracking differentiator is proposed. Tracking differentiator is introduced in order to calculate the derivative of virtual control law which is very difficult to evaluate in the traditional backstepping control. To suppress the influence of interference on the performance of the control system, extended state observer are designed to estimate and compensate for disturbances in backstepping control subsystem. To further increase the robustness of the control system, L2 gain robust item is designed to suppress the impact of disturbance estimate error of disturbance observer and differential estimate error of tracking differentiator. It makesthe system satisfy dissipative inequality, and the whole control system has the L2 gain which is less than or equal to the prescribed positive const. In the presence of ground effect and wind interference, the altitude and airspeed control commands is tracked, vertical landing speed is within the allowable range. The simulation results show the effectiveness of the proposed method.6.For the landing control problem of flying wing UAV with ground effect and wind interference, anti-disturbance sliding mode control scheme based on extended state observer is proposed. Separated by time scale, the landing control system is separately designed by divided into four parts: position loop, velocity flight path angle loop, attitude angle loop and attitude angle velocity loop. To suppress the influence of ground effect and wind interference, extended state observer are designed to estimate and compensate for complex disturbances in control system. Sliding reaching law is designed based on fal function. The simulation results show that flying wing UAV has good landing control performance with ground effect and wind interference.