Robust Trajectory Tracking of a Quadrotor UAV Using Backstepping Nonsingular Terminal Sliding Mode Control

Abstract

In this paper, a novel robust flight control strategy, named Backstepping Nonsingular Terminal Sliding Mode Control (BNTSMC), is proposed for a six-degree-of-freedom (6DOF) quadrotor Unmanned Aerial Vehicle (UAV) subjected to unmodeled dynamics, parametric uncertainties, and time-varying external disturbances. Quadrotor systems exhibit highly nonlinear, strongly coupled, and underactuated characteristics, creating significant challenges for high-precision trajectory tracking. To systematically address the cascaded structure in which translational and rotational dynamics are hierarchically coupled, a dual-loop control architecture is developed. A unified hybrid BNTSMC strategy is developed for both the inner and outer loops of the system. Specifically, the recursive design of BNTSMC is applied to the outer loop for rapid position tracking, whereas the inner loop employs it for attitude stabilization, thereby guaranteeing singularity-free operation and finite-time convergence. To suppress the unwanted chattering phenomenon common in conventional Sliding Mode Control (SMC) without sacrificing the controllers inherent robustness, a continuous saturation-based reaching law is integrated into the control design. Using Lyapunov stability analysis, finite-time convergence to the sliding surface is established for the discontinuous law, and practical finite-time stability, i.e., finite-time convergence to a boundary layer around the sliding surface, is established for the saturation-based implementation. Comprehensive numerical simulations are conducted under time-varying wind gust disturbances to evaluate the performance of the proposed BNTSMC against conventional SMC and nonsingular terminal SMC (NTSMC). The quantitative comparative analysis demonstrates that the proposed hybrid architecture achieves superior tracking precision, faster error convergence, minimal overshoot, and excellent disturbance rejection capabilities with a bounded and chatter-free control effort.