Research On Control Method Of Triaxial Tilting Rotor UAV

In recent years,rotor unmanned aerial vehicles(UAVs)have attracted widespread attention due to their unique advantages and wide range of application scenarios.The thrust direction of the rotor of the traditional multi-rotor UAVs is always perpendicular to the horizontal plane of the fuselage and cannot be changed.This makes them all underactuated systems,and the fuselage motion potential is greatly limited.Therefore,this thesis proposes a new structure of triaxial tilting rotor UAV.The triaxial rotor UAV uses a coaxial double-rotor design.At the same time,each group of rotors has two degrees of freedom of tilting,so that it becomes a redundant overactuated system with nine degrees of freedom.The triaxial rotor UAV can decouple the motion of the fuselage into a translational mode and a rotational mode,thereby completing flight actions that cannot be achieved by an underactuated system,such as constantly changing attitude when hovering at a fixed point or tracking spatial trajectories while maintaining horizontal attitude.The main contribution and innovative results of this thesis are summarized as follows:(1)A new type of triaxial tilting rotor UAV is proposed.This thesis designs each axis as a coaxial double-rotor based on the traditional tri-rotor to solve the problem of positive and negative torque offset.At the same time,in order to solve the underactuated problem of traditional rotor UAV systems,this thesis makes each group of rotors tilt in two directions,turning the fuselage into a nine-degree-of-freedom redundant overactuated system.This thesis introduces the fuselage structure and flight modes in detail,and establishes the model based on the dynamic equations and kinematic relationships.(2)Under the constraints of small attitude angles,for the six-degree-of-freedom simplified model which means each group of rotors with only one degree of freedom of tilting,this thesis uses the backstepping method to design the overall control law of the system,and uses a nonlinear mapping relationship to solve the control allocation problem.The simulation results prove the effectiveness of the backstepping method and control allocation method under this control framework,and the control effect is better than the traditional PID method.(3)Aiming at the nine-degree-of-freedom completed model with two tilting degrees of freedom for each group of rotors,this thesis uses the feedback linearization framework to combine PID control methods and sliding mode control methods to design the overall control law of the system,and integrates the disturbance observer into the control frame to enhance the anti-disturbance ability of the system.At the same time,this thesis proposes a reversible nonlinear mapping relationship combined with a pseudo-inverse control allocation method to solve the model’s unique redundant non-linear control allocation problem.This thesis then further considers the case where the thrust sum of the rotor is used as the control allocation optimization function,and gives the necessary conditions for obtaining the optimal solution in this case.Finally,this thesis gives a more complete method for judging the actuated characteristics of a general tiltable multi-rotor UAV system.By comparing the simulation results,we analyze the control effects of several control methods,and verify the effectiveness of our proposed control framework and nonlinear disturbance observer.(4)The differential flatness of this overactuated UAV system is verified,the control allocation results obtained in Chapter 4 are applied and two cost functions of trajectory smoothness and energy consumption are designed based on the differential flatness theory to generate and optimize the trajectory.Finally,this thesis compares the results of the two trajectory generation methods:the trajectory generated by the former is better in smoothness,while the trajectory generated by the latter consumes less energy.