Attitude tracking of 2-DOF Aero System with Singular Terminal Sliding mode controller under affine disturbance

The robust attitude regulation of aero systems (helicopter UAV) is challenged by input-affine disturbances, nonlinear dynamics, and the limitations of conventional controllers, which often result in chattering, slow settling, or steady-state errors. This study proposes a Singular Terminal Sliding Mode Controller (STSMC) for a two-degree-of-freedom (2-DOF) aero system, where a nonlinear singular sliding surface ensures asymptotic stability through proportional error feedback and finite-time convergence via a nonlinear sign function. The novelty of this study lies in embedding a singular term into the control law, which enables the effective mitigation of affine disturbances, suppression of singularity-induced behavior, and generation of smooth, chattering-free control inputs. The Lyapunov-based analysis establishes closed-loop stability, and the controller parameters are tuned using a genetic algorithm to ensure the optimal performance. Extensive evaluations demonstrate the superiority of the STSMC over Sliding Mode Control (SMC), Backstepping Nonlinear Control (BNC), Fuzzy Logic Control (FLC), and Non-Singular Terminal SMC (NSTSMC). For pitch dynamics, STSMC achieves a settling time of 1.4 s (versus 2.5-4 s ), overshoot of 2.5% (versus up to 33%), and steady-state error of 0.06° (versus 0.1-0.3°). For yaw, it achieved 2 s settling time of 2 s, 1.65% overshoot, and 0.08° error. In trajectory tracking, STSMC reduces IAE, ISE, and ITAE by 60-70%, with the lowest ITAE (820-1584) compared to competing approaches (2000-9000), confirming its robustness, efficiency, and real-time suitability.