Control allocation for fault tolerant control of a VTOL octorotor

For the fault tolerant control of an eight-rotor VTOL Unmanned Air Vehicle (UAV), a control allocation scheme is proposed. The eight-rotor configuration provides actuator redundancy to ensure safe operation under rotor/motor failures. A PD controller is used to generate total thrust and moment demands. A cascade inverse method of control allocation is proposed to allocate the controller commands to the actuators whilst ensuring that actuator saturation does not occur. If the vehicle is subjected to rotor failures, the scheme re-allocates the commands to maintain the vehicle stability and performance. Until actuator saturation occurs the response of the vehicle is the same when operating with all motors or fewer. The response of the vehicle to several combinations of complete actuator failures is investigated by simulation and it is shown that the proposed method is able to maintain control after failure of up to four actuators. The controller is invarient and the vehicle response to commands is identical until motor saturation occurs.