Robust Trajectory Tracking of Quadrotor via Disturbance Observer-Based Fractional-Order Super Twisting SMC

This paper addresses the robust trajectory tracking issue for an uncertain nonlinear 6-degree-of-freedom (DOF) quadrotor subjected to variations in wind velocity and drag. Based on the Newton-Euler equations, a nonlinear model is developed for the quadrotor. A Fractional-Order (FO) Proportional–Integral–Derivative Super Twisting Sliding-Mode Control (PIDSTSMC) is proposed for the quadrotor system to improve the tracking performance for both position and attitude trajectories during disturbances. The proposed fractional-order FO control strategy achieves rapid convergence, high accuracy, and strong robustness. A nonlinear disturbance observer-based control (DOBC) algorithm is implemented with the proposed controller to estimate wind effects and the drag coefficient, and it compensates for these effects. Finally, to verify the robustness and capability of the proposed controller, simulations are conducted on the quadrotor and compared with the existing controllers under different conditions. The results indicate that the proposed controller effectively improves the transient response, reducing settling time (in the z-axis/altitude) by 10% and 18.18% under steady wind effect and by 20% and 27.4% under random wind effect conditions. The stability of the closed-loop system is achieved using Lyapunov's stability criteria.