This paper investigates the trajectory-tracking control problem of quadrotor unmanned aerial vehicles (UAVs) subject to unknown external disturbances. A nonsingular fixed-time sliding mode control scheme integrated with an adaptive predefined-time disturbance observer is proposed to achieve fast and robust tracking performance. The proposed observer enables accurate disturbance estimation within a predefined time without requiring prior knowledge of disturbance bounds, while a novel nonsingular fixed-time sliding surface guarantees convergence with an explicit time bound independent of initial conditions and avoids singularity issues. By combining disturbance compensation with the nonsingular sliding surface, a fixed-time sliding mode controller is developed to ensure high-precision trajectory-tracking and smooth control performance. Simulation results demonstrate that the proposed method achieves fast fixed-time convergence, strong robustness against disturbances, and reduced chattering, highlighting its effectiveness for practical quadrotor UAV applications.
Zhai et al. (Wed,) studied this question.