  This study proposes a uniform passive fault-tolerant control (FTC) method for
a quadcopter that does not rely on fault information subject to one, two
adjacent, two opposite, or three rotors failure. The uniform control implies
that the passive FTC is able to cover the condition from quadcopter fault-free
to rotor failure without controller switching. To achieve the purpose of the
passive FTC, the rotors' fault is modeled as a disturbance acting on the
virtual control of the quadcopter system. The disturbance estimate is used
directly for the passive FTC with rotor failure. To avoid controller switching
between normal control and FTC, a dynamic control allocation is used. In
addition, the closed-loop stability has been analyzed and a virtual control
feedback is adopted to achieve the passive FTC for the quadcopter with two and
three rotor failure. To validate the proposed uniform passive FTC method,
outdoor experiments are performed for the first time, which have demonstrated
that the hovering quadcopter is able to recover from one rotor failure by the
proposed controller and continue to fly even if two adjacent, two opposite, or
three rotors fail, without any rotor fault information and controller
switching.
