Adaptive fixed-time observer and command-filtered control of an uncertain 2-DOF helicopter system

In this paper, a novel fixed-time control framework is developed for a 2-DOF helicopter system subject to nonlinear dynamics and external disturbances. First, a fixed-time command filter with a compensation mechanism is designed to overcome the ‘explosion of complexity’ problem commonly encountered in backstepping-based control schemes, while simultaneously mitigating the filtering errors. Then, to address the challenge of unmeasurable states and unknown lumped disturbances, a fixed-time observer is constructed to provide accurate estimations and disturbance compensation. On this basis, a fixed-time controller is further designed to guarantee that the tracking error converges to a small neighbourhood of the origin within a pre-defined fixed time. Rigorous Lyapunov-based analysis shows that all signals in the closed-loop system remain bounded and that the proposed scheme achieves fast convergence independent of the initial conditions. Finally, simulation and experimental studies on a 2-DOF helicopter platform demonstrate the effectiveness and superiority of the proposed method.