Robust Discrete-time Flight Control Of UAV With External Disturbances

On the basis of the excellent ability of unmanned aerial vehicles(UAVs)to work in remote areas or dangerous situations,UAVs have been widely utilized for many civil and military applications.However,the structure of UAVs is complex and the flight environment is often variable.The stability of the flight control(FC)of UAVs is easily affected by system uncertainties and external disturbances.In addition,for the actual UAVs,the rudder deflection angle of UAVs is limited to a given range.If the effect of the actuator saturation is not considered in the designed UAVs control,when the rudder deflection angle reaches the limit boundary,it will affect the control performance of UAVs system,and even destroy the stability of UAVs system.Furthermore,with the widespread use of digital computers and microprocessors in control applications,the research on discrete-time(DT)control methods has received increasing attention.Therefore,in view of the above mentioned problems,this dissertation studies the discrete-time flight control(DTFC)methods for the fixed-wing UAV system under external wind disturbances.The main works are given as follows:Firstly,on the basis of the previous research works,the continuous-time(CT)fixed-wing UAV attitude dynamics model under external wind disturbances is given,and the Euler approximation method is used.The fixed-wing UAV attitude dynamics model with CT form is transformed into an approximate DT one.A discrete-time neural network(DTNN)control scheme based on discrete-time disturbance observer(DTDO)is proposed for the DT fixed-wing UAV attitude dynamics model with external disturbances and system uncertainties.The scheme combines the DTDO,the neural network(NN)approximation method,and the backstepping control(BC)technique to realize the DT tracking control of the fixed-wing UAV,and the robustness of the resulted UAV system is also enhanced.The feasibility of the DTFC scheme is demonstrated by the numerical simulation of the fixed wing UAV attitude dynamics system.Secondly,since the actuator is prone to rendering saturation,the input saturation problem is further considered in the design of the robust DTFC scheme,and a DT robust anti-saturation control method based on the auxiliary system and the nonlinear DTDO is proposed to achieve the stable FC of the fixed-wing UAV with external wind disturbances,the input saturation and system uncertainties.On the basis of the BC technique,the stability of closed-loop system is proven by using Lyapunov stability theory.The simulation results of the uncertain fixed-wing UAV attitude dynamics model show that the DTNN-based control scheme is effective.Thirdly,for the trajectory control model under external wind disturbances and the uncertain DT fixed-wing UAV attitude nonlinear model under wind disturbances and input saturation,a DT robust control scheme is proposed for the uncertain fixed-wing UAV system based on an auxiliary system and discrete sliding mode disturbance observer(SMDO).The scheme introduces an auxiliary system to suppress or eliminate the adverse effects of input saturation for the closed-loop FC system of the fixed-wing UAV,and the NN approximation is used to deal with the uncertainties in the fixed-wing UAV system,and a NN-based discrete SMDO is employed to compensate for the negative influence of external disturbances,and a DT robust controller is designed by utilizing the BC technique.The numerical simulation results verify that the FC performance is well under the proposed NN-based DT control method.Fourthly,the system uncertainties and the problem of prescribed performance are considered in the design of discrete-time fractional-order(DTFO)control scheme for the fixed-wing UAV attitude system with external disturbances,and the control problems of steady state and transient performance of the fixed-wing UAV attitude closed-loop system are comprehensively studied,and then combining the prescribed performance control method,the NN-based DTDO and the discrete-time fractional-order calculus(DTFOC)theory,a DTFO tracking controller based on the DTDO and the NN was designed by using the BC technique.On the basis of the designed DTFO tracking controller,the FC of the fixed-wing UAV system in the presence of external disturbances and system uncertainties is realized.The simulation results show that the designed DTFO tracking controller can ensure that the fixed-wing UAV closed-loop system satisfies the steady-state and transient performance,and the designed DTDO can guarantee that the disturbance estimation errors are bounded.Finally,aiming at the DTFO control problem of the fixed-wing UAV attitude system with external wind disturbances,a DTFO control method is proposed for the fixed-wing UAV system.In this method,the DTFO control method based on DTDO is designed by using BC technique,and then the DTFO tracking control of the fixed-wing UAV system is realized.Simulation results show the effectiveness of the proposed DTFO control scheme based on DTDO.