Research On Key Technologies Of Flight Control System Of A Tilt Rotor UAV

The tilt-rotor unmanned aerial vehicle（UAV）is a unique kind of aircraft,which combines the advantages of rotary-wing UAVs and fixed-wing airplanes,for example,the ability of vertical take-off and landing,a high cruising speed,a long endurance and a large load capacity.Hence,it has broad application prospects in both military and civil fields.The flight control system is the core part of the tilt-rotor UAV system,and its performance directly affects the flight quality and flight safety of the aircraft.On the basis of summarizing the relevant technical research results and research status at home and abroad,this paper proposes a new tilt-rotor UAV structure,and researches on its overall layout,system modeling,control law design,flight simulation and flight test.The main content includes the following aspects:First,by summarizing the relevant domestic and foreign references and the structural characteristics of typical tilt-rotor UAVs,a new type of tilt-rotor UAV is designed.Its configuration characteristics and working principles are introduced,and its structure and aerodynamic parameters are given.Second,using the knowledge of rigid-body dynamics,the model of the entire tilt-rotor UAV is built,and the six degrees of freedom nonlinear model is obtained by coordinate transformation.Then,the mechanical models of each subsystem of the tilt-rotor UAV are calculated respectively.The nonlinear model of the aircraft is linearized by using the small perturbation principle,and the state space equation of each flight mode is obtained.Finally,the model parameter identification test of the rotor lift system and the Computational Fluid Dynamics（CFD）simulation test of the fuselage/wing aerodynamic parameter identification are designed to obtain the model parameters,which lays the foundation for the design of subsequent flight control laws.Third,for the helicopter model of the tilt rotor unmanned,the flight control law based on the combination of robust servo linear quadratic regulator（LQR）control and classic PID control is designed.The simulation results show that compared with the traditional cascade PID control method,the rise time of the two is basically the same,but the control system based on the robust servo LQR has a more gentle response and a smaller overshoot,and greatly suppresses the phenomenon of a large angular rate caused by the sudden input command changes in the initial response of the system.Thus,the overload requirements of the UAV body can be reduced.Fourth,the state observer is applied to the design of the flight control system of the tilt-rotor UAV to estimate the flight states of the UAV and the real-time action of the external disturbance.Taking the roll channel of the helicopter mode as an example,a flight control law based on the extended state observer is designed.The final input of the controlled object is obtained by disturbance compensation for the output of the robust servo LQR controller.The simulation results show that the designed expanded state observer can well estimate the air convection disturbance in the helicopter mode.After it is introduced into the robust servo LQR control,the disturbance is well suppressed and the anti-disturbance ability of the system is improved.Finally,the effectiveness of the flight control system is verified by the hover test.Fifth,in view of the parameter uncertainty and the susceptibility to environmental disturbance in the fixed wing mode and the transition mode,a guaranteed performance optimal state feedback control law based on theH₂/H_∞control is designed.In the form of linear matrix inequality,the conditions for the controller design of the flight control system are given,and the stability analysis and proof of the controlled system are derived and proved.The simulation experiments show that the proposed method has a good robustness to the control system with uncertainties.Moreover,it can effectively suppress the external interference of the control system,and improve the dynamic performance and robustness performance of the controlled system.Finally,a multi-model adaptive robust mixed control law is designed for the altitude maintenance and attitude tracking control of the transition mode of the tilt-rotor UAV.Due to the characteristics of rapid structural changes and large differences in model characteristics of the transition mode,the entire transition flight process is divided into several working spaces.For each space,a linear model is selected,and a corresponding robust controller is designed.In order to solve the problems of state jump and instability during the direct switching of the controllers,a strategy for softening the controller was proposed.By selecting a bump function,the controllers are weighted adaptively to achieve smooth switching among the controllers.Numerical simulation results show that the control strategy that combines multi-model adaptive method and robust control not only realizes the smooth flight of the tilt-rotor UAV in the transition phase,but also ensures the good tracking ability and robustness of the system.