Robust optimal transition maneuvers control for tail-sitter unmanned aerial vehicles

Optimal transition has been widely studied for tail-sitter unmanned aerial vehicles (UAVs). However, since the derived optimal control can only be applied in an open-loop form, unmodeled dynamics and external disturbances cannot be restrained. Aiming at this problem, this article presents a method to obtain optimal transition maneuvers in the presence of these uncertainties. The approach includes two steps: nonlinear trajectory optimization and robust trajectory tracking. For the trajectory optimization, the minimum altitude variation front and back transition trajectories are derived. Different from existing optimal transition studies, considerable actuator margins are reserved in this step. Thus the tail-sitter possesses control efforts to perform closed-loop corrections for uncertainties. For the trajectory tracking, a robust controller in the form of feedforward plus feedback is proposed. The feedforward term directly employs the derived optimal control to improve the dynamic response of the system. The feedback term adopts linear control and acceleration-based compensator to place desired poles and restrain all uncertainties. To the authors' knowledge, this article is the first time to study the tail-sitter robust transition control method considering the transition optimality and actuator margins. It is proved that the tracking errors converge into a specified neighborhood of the origin in a finite time. Simulation studies corroborate the performance of the optimal transition trajectories and the robust trajectory tracking controller.