Fixed-time filtered backstepping control for quadrotor UAVs with an adaptive fixed-time disturbance observer

This work has proposed a novel robust fixed-time tracking control scheme for quadrotor unmanned aerial vehicles subject to unknown external disturbances and model uncertainties. Accounting for the inherent underactuated dynamics of the quadrotor, a hierarchical control architecture was constructed that decouples position and attitude regulation into two separate control loops, namely, an outer loop dedicated to translational motion and an inner loop responsible for rotational dynamics. The proposed framework integrates a fixed-time backstepping design with a command filtering technique to mitigate the explosion-of-complexity problem that typically arises in conventional backstepping approaches, while simultaneously guaranteeing fixed-time convergence of the tracking errors. To effectively address the effects of model uncertainties and external perturbations acting on the system, an adaptive fixed-time disturbance observer was introduced and integrated into the control architecture. This observer is capable of accurately estimating the unknown disturbances and uncertainties, achieving estimation convergence within a predetermined time that remains independent of the initial conditions of the system. Rigorous stability analysis based on Lyapunov theory was carried out to establish the fixed-time convergence of the tracking errors to a bounded neighborhood of the origin. Comprehensive numerical simulations were performed on a quadrotor system, and the results obtained were compared against several existing control methods. The comparative analysis clearly demonstrated that the proposed controller achieves superior tracking accuracy, faster convergence, and improved disturbance rejection performance compared to the alternate approaches considered.