Nonlinear Cascade Control for a New Coaxial Tilt-rotor UAV

This paper proposes a nonlinear control strategy for a newly-designed coaxial tilt-rotor (CTR) unmanned aerial vehicle (UAV), which is a special class of tilt-rotor (TR) UAVs with two pairs of coaxial rotors, two servos, and a rear rotor. The CTRUAV is an underactuated system, and the controller is designed in cascade form. The proposed controller includes two sub-controllers: an inner-loop attitude controller and an outer-loop velocity controller. Each sub-controller is proposed by using a backstepping-like feedback linearization method to control and stabilize the CTRUAV. The developed control strategy can realize the motion control for the CTRUAV. The asymptotic stability of the resulting closed-loop system is analyzed by the Lyapunov method. Finally, simulations and real flight tests are performed to validate the effectiveness of the proposed control system.