  In this paper, we present a full attitude control of an efficient quadrotor
tail-sitter VTOL UAV with flexible modes. This control system is working in all
flight modes without any control surfaces but motor differential thrusts. This
paper concentrates on the design of the attitude controller and the altitude
controller. For the attitude control, the controller's parameters and filters
are optimized based on the frequency response model which is identified from
the sweep experiment. As a result, the effect of system flexible modes is
easily compensated in frequency-domain by using a notch filter, and the
resulting attitude loop shows superior tracking performance and robustness. In
the coordinated flight condition, the altitude controller is structured as the
feedforward-feedback parallel controller. The feedforward thrust command is
calculated based on the current speed and the pitch angle. Tests in hovering,
forward accelerating and forward decelerating flights have been conducted to
verify the proposed control system.
