Research On Transition Trajectory Optimization And Robust Control Of Tail-sitter UAVs

The tail-sitter unmanned aerial vehicle(UAV)combines the advantages of traditional fixed-wing UAVs for high-speed cruise ability and rotary-wing UAVs for vertical take-off and landing ability,which leads to broad application prospects in both military and civil fields.Among all flight stages of tail-sitter UAVs,the unique front and back transition processes are most challenging due to wide changes in the pitch angle,flight speed,and so on,which severely limits its application.Aim at this problem,this paper conducts research on the transition flight corridor construction,trajectory optimization,and robust controller design of tail-sitter UAVs.The main work and innovations are listed as follows:(1)To solve the problem that existing equilibrium transition corridors(ETC)do not consider the dynamic changes of the flight speed and attitude of the tail-sitter transition processes,a novel dynamic front transition corridor(DFTC)and a novel dynamic back transition corridor(DBTC)are developed.Based on the active forces/moments in the transition processes,mathematical models which can characterize the “Accelerate and pitch down” features of the front transition and the “Decelerate and pitch up” features of the back transition are developed.The DFTC and DBTC are then derived by numerical iteration.To avoid excessive large altitude climb during front and back transitions,a climb velocity constraint is further added.The effects of model mismatches on the transition corridors are also analyzed.(2)To establish a transition trajectory that can take into account both the safety margin and flight performance,a dynamic corridor-integrated multi-index transition trajectory optimization strategy is proposed.In order to ensure the distance margin between the transition trajectory and boundaries of the transition corridor,a weighted reference line of the dynamic transition corridor is developed.Based on it,a multi-index cost function is defined by combining the weighted reference lines and flight performance requirements.By introducing the sine transform on the basis of the penalty function,the nonlinear constraints of the trajectory optimization problem are eliminated.The approximate optimal solution is obtained by using the quasi-Newton method and sequence optimization technology.Numerical simulation shows that the weighted reference line is a good trade-off between the safety margin and flight performance.The resulting transitional trajectory has a smaller angle of attack and a larger control margin.(3)To ensure the robustness of the transition trajectory when the dynamic model is mismatched,a robust trajectory optimization problem considering coupling uncertainties is studied.By considering the transition initial state uncertainty as well as the propeller thrust coefficients and the wing aerodynamic coefficients uncertainties,the robust trajectory optimization problem is firstly constructed with the expectation and variance of stochastic system states.By employing the Gram-Schmidt transformation to decouple correlated uncertainties,and using polynomial chaos expansion(PCE)for uncertainty quantification(UQ),the stochastic robust transition trajectory optimization problem is expanded as a higher-dimensional deterministic transition trajectory optimization problem.Monte-Carlo simulation results show that the robustness of the derived transition trajectories is greatly improved.(4)To improve the trajectory tracking performance in the presence of uncertain disturbances,a composite robust transition trajectory tracking control law,which includes a trajectory optimization derived feedforward term,a closed-loop LQR feedback term,and an angular acceleration-based attitude disturbance compensation term,is designed.Based on the dynamic characteristics of the long-and short-period modes of the transition process,the weight matrix tuning rule of the LQR control is discussed.A closed-loop commands scheduling strategy is then established.In order to obtain accurate angular acceleration signals,an acceleration estimation model with prior information provided by the nominal moments is constructed.Based on the Kalman filtering,high-quality angular acceleration signals can be obtained.Numerical simulation and flight experiments demonstrate the effectiveness and robustness of the proposed control law.