Nonlinear control of quadrotor UAV under rotor failure for robust trajectory tracking.

We present a trajectory tracking control law for a quadrotor unmanned aerial vehicle (UAV) for handling the complete failure of a single rotor. The control design problem considers the reduced state space, which excludes the angular velocity and orientation about the vertical body axis. The proposed controller enables the quadrotor to track the orientation of this axis and consequently any prescribed position trajectory using only three rotors. The control design is carried out in two stages. First, to track the reduced attitude dynamics, a geometric controller with two input torques is designed on the Lie group SO(3). This controller is then extended to SE(3) by designing a saturation-based feedback law to track the center of mass position with bounded thrust. The control law for the complete dynamics achieves an almost global exponential tracking. The novelty of geometric control design lies in its ability to effectively execute aggressive global maneuvers despite the complete loss of a rotor. Numerical simulations on a conventional quadrotor model, carried out for complete trajectory tracking, together with preliminary experimental tests on a real platform under a hover scenario with sudden motor failure, are presented to demonstrate the practical applicability of the proposed control design.