  The mechanical simplicity, hover capabilities, and high agility of quadrotors
lead to a fast adaption in the industry for inspection, exploration, and urban
aerial mobility. On the other hand, the unstable and underactuated dynamics of
quadrotors render them highly susceptible to system faults, especially rotor
failures. In this work, we propose a fault-tolerant controller using nonlinear
model predictive control (NMPC) to stabilize and control a quadrotor subjected
to the complete failure of a single rotor. Differently from existing works,
which either rely on linear assumptions or resort to cascaded structures
neglecting input constraints in the outer-loop, our method leverages full
nonlinear dynamics of the damaged quadrotor and considers the thrust constraint
of each rotor. Hence, this method could effectively perform upset recovery from
extreme initial conditions. Extensive simulations and real-world experiments
are conducted for validation, which demonstrates that the proposed NMPC method
can effectively recover the damaged quadrotor even if the failure occurs during
aggressive maneuvers, such as flipping and tracking agile trajectories.
