Research On High-Speed-to-Hovering Back-Transition Control Of Ducted Fan UAV

The ducted fan UAV(DFUAV)is a new kind of vertical take-off and landing UAV.The unique aerodynamic layout makes it integrate the advantages of rotary-wing and fixed-wing UAVs,and it has both hovering and high-speed flight modes.DFUAV can transit from hovering to high-speed flight by continuously pitching down,reaching a high-speed and high-efficiency status.However,during the back-transition process from high speed to hovering,unstable environmental airflow and rudder surface saturation could easily lead to serious oscillation or even divergence of the aircraft states.Therefore,higher requirements are put forward on the stability and robust performance of the controller.This dissertation focuses on the flight control issues of ducted fan UAV during its back-transition process.In this work,the kinematics and dynamics of the DFUAV are modeled first,followed by quantitative analysis of the moment and force acting on the fuselage during the hovering and forward flight process.On these bases,we focused on the control issues during the back-transition process of DFUAV.The flight controller of the DFUAV adopts the method of separating the inner and outer loops:The inner loop controller represents the attitude controller where the three-axis driving torque required for attitude tracking is calculated according to the desired attitude quaternion and related state feedback.An adaptive control method of attitude loop based on a reference model is proposed,which takes into account the uncertainty of modeling and the undetectable disturbance caused by airflow during flight,and compensates the control amount to reduce the attitude tracking error.Adaptively adjustable parameters enable the aircraft to meet the dynamic performance requirements of attitude tracking in different modes.Pseudo-limiting control amount compensation is introduced to reduce the influence of control input saturation on adaptive parameter estimation.The outer loop controller is also called the position loop controller,which is used to calculate the desired pull force and desired attitude quaternion of the aircraft required for trajectory tracking and is used as the reference input of the inner loop controller.To make the backtransition process faster and smoother,a reference trajectory design method is first proposed to meet the requirements of gradual velocity and acceleration during deceleration.The model predictive control method is applied to the outer loop system of the DFUAV,the position tracking error is used as the control performance optimization index,and the limiting constraints of acceleration and acceleration increment are considered to solve the open-loop optimization problem in the finite time domain.To reduce the computational scale of the numerical optimization problem,the outer loop system is simplified to a second-order linear system model,and the uncertainty caused by this is compensated by the adaptive parameter estimation of the predictive model.The theoretical analysis process respectively proved the nominal stability of the above inner and outer loop control methods.Simulation and flight experiments further verified the effectiveness and robustness of the proposed control strategy.