Disturbance Observer Based Hovering Control of Quadrotor Tail-Sitter VTOL UAVs Using H∞ Synthesis

Hybrid VTOL UAVs such as tail-sitters allow several keymaneuvers such as vertical takeoff, landing, and hovering while at the same time maintaining superior aerodynamic efficiency at level flight. However, the large wing area of a tail-sitter at hovering makes it rather sensitive to the cross wind. In this letter, we present a disturbance observer (DOB) based control method to improve its hovering accuracy in presence of external disturbances such as cross wind. The presented DOB, operating on top of the aircraft baseline position controller, is designed in frequency domain using H-infinity synthesis techniques and is guaranteed to stabilize the closed loop system with robustness to model uncertainties. In addition, it admits nonminimum phase system model, as the case of our tail-sitter platform, and does not require much hand tuning work on the Q-filter. Comparison study with existing UAV disturbance rejection method is conducted under different wind disturbances. Results show that the presented DOB control technique can effectively estimate various types of wind disturbances and lead to improved hovering accuracy of the tail-sitter UAV.