  This study has developed a fault-tolerant controller that is able to recover
a quadrotor from arbitrary initial orientations and angular velocities, despite
the complete failure of a rotor. This cascaded control method includes a
position/altitude controller, an almost-global convergence attitude controller,
and a control allocation method based on quadratic programming. As a major
novelty, a constraint of undesirable angular velocity is derived and fused into
the control allocator, which significantly improves the recovery performance.
For validation, we have conducted a set of Monte-Carlo simulation to test the
reliability of the proposed method of recovering the quadrotor from arbitrary
initial attitude/rate conditions. In addition, real-life flight tests have been
performed. The results demonstrate that the post-failure quadrotor can recover
after being casually tossed into the air.
