Fixed-time adaptive fault-tolerant control of a multi-mode VTOL UAV with variable prescribed performance boundaries under random disturbances

The multi-mode vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs) are susceptible to random disturbances during transition flight, which may result in a deterioration of flight stability and control accuracy. This article develops a fixed-time adaptive fault-tolerant control method with variable prescribed performance boundaries to effectively compensate for time-varying disturbances, model uncertainties, and actuator faults. Firstly, a longitudinal dynamics model of the researched multi-mode VTOL UAV is established, and its tracking control error is dynamically constrained through a prescribed performance function (PPF). Secondly, a fixed-time extended state disturbance observer is constructed to evaluate both the external disturbances and the internal time-varying aerodynamic interference caused by the main rotor wake. Then, a novel globally fast convergence terminal sliding surface with a PPF is proposed, based on which a fixed-time fault-tolerant controller with variable prescribed performance boundaries is constructed. In order to maintain the desired tracking precision of the multi-mode VTOL UAV under the condition of actuator faults, an innovative adaptive control framework is further introduced to realize the expected tracking performance within fixed time without requiring prior fault information. Finally, the stability of the developed control strategy is theoretically confirmed, and its superiority is demonstrated through a series of comparative simulation scenarios.