Robust path-following control of a quadrotor via fixed-time disturbance-estimation-based sliding mode strategy

In this study, we have developed a novel robust fixed-time control framework that integrates a fractional-order sliding mode control with a disturbance observer for a quadrotor UAV. The overall system dynamics are represented in a cascaded structure with two major loops: position and orientation (attitude). To facilitate the design process of the controller, two fixed-time SMC schemes combined with two fixed-time disturbance estimators for approximating unknown external perturbations are utilized, for position and attitude subsystems. The control loop responsible for convergence of the orientation of the UAV to the desired reference attitude in a fixed time which is comprehensively independent of the initial conditions is called the inner, while the outer loop aims to guarantee reference path following of the position states of the quadrotor, respectively. Accordingly, the fixed-time observer concurrently estimates the external disturbances via accelerometer measurements, and these estimations are used in the controller to reduce overshoots and improve path following. The findings from simulation studies substantiate the efficacy of the designed control structure in disturbance compensation and attitude stabilization. Moreover, compared with a finite-time fuzzy terminal SMC in which the upper bound of settling time relies on the initial conditions of position and orientation, our proposed control structure makes the error trajectories converge faster, and the settling time merely depends on the control gains.