Flight Control Of Tilt Rotor UAV During Transition Mode Based On Sliding Mode Theory

The tilt rotor UAV is a special kind of aircraft.There are rotors mounted on the lateral sides of the aircraft airframe,which can be tilted to provide lift and forward thrust.With the variation of the rotors’position,the aircraft is working in different flight modes:helicopter mode,airplane mode,and the conversion process between the two flight modes,i.e.,the transition mode.The tilt rotor UAV features the ability to hover,vertical take-off and landing,and high-speed long-range cruise.The tilt rotor UAV combines the advantages of helicopter and fixed wing aircraft.However,the special configuration brings its own problems.Especially in the transition mode,there exist complicated aerodynamic characteristics,nonlinearities,strong couplings and control input redundancy.Therefore,it is a difficult and challenging task to design an effective controller for the transition mode of the tilt rotor aircraft.This dissertation focuses on the transition flight control for a small tilt rotor UAV.The main work is as follows:Firstly,the kinematic and dynamics equations of the tilt rotor UAV are established.The control strategy of the UAV in each flight mode is introduced.Aiming at the transition mode,the nonlinear model of the UAV is trimmed at different tilt angle to obtain the proper transition trajectory.In addition,to facilitate the control design,the nonlinear model of the UAV is decoupled into the longitudinal and lateral model.Secondly,due to the maneuvering characteristics of the UAV vary significantly with the change of the tilt angle,a longitudinal control strategy is proposed for the transition mode.The sliding mode controllers are designed in each channel of the UAV to guarantee the states converge to the desired sliding surfaces in finite time and converge to the equilibrium point asymptotically.Extended state observers(ESOs)are provided to enhance the system’s robustness to the uncertainties.Moreover,to cope with the control input redundancy,a control allocation strategy for the longitudinal model of the UAV is proposed.The strategy takes into account the effectivenesses of the aerodynamic control surface and the engine thrust at different tilt angles.Simulations show the effectiveness of the proposed approach.Finally,to improve the convergence rate of the system,a nonsingular terminal sliding mode control method is applied to make the states converge to the reference trajectories in finite time.With the aid of the ESOs,the effect of uncertainties to the system is estimated and compensated.In addition,analogous to the longitudinal control method,the lateral control method and control allocation strategy are proposed.The longitudinal controllers and the lateral controllers are applied to the UAV simultaneously.The simulations show that the system has good control performance and robustness.With or without disturbances,the aircraft can achieve the mode transition safely and smoothly.