Fixed-time composite observer-based adaptive recursive sliding mode control for tiltrotor UAV attitude tracking

Abstract An adaptive recursive sliding mode control (ARSMC) scheme based on a fixed-time composite observer (FTO) is introduced in this paper, to address challenges in tiltrotor unmanned aerial vehicle (TUAV) attitude control. As an electric vertical take-off and landing (eVTOL) aircraft, the unique structure of TUAV leads to significant aerodynamic disturbances, particularly during transition modes, causing flight instability. To enhance control robustness, a fast-converging FTO is designed to estimate the system states, model uncertainties and external disturbances, achieving fixed-time convergence. The FTO comprises a fixed-time state observer (FTSO) based on homogeneous function and a fixed-time disturbance observer (FTDO). To accomplish rapid and accuracy attitude tracking during TUAV flight, a recursive sliding mode control (RSMC) is employed. This method utilizes reconstructed information from the FTO, incorporates an adaptive law to update control gains, and ensures finite-time convergence. And the recursive structure reduces the convergence time while reducing chattering phenomena. Ultimately, simulation results demonstrate the effectiveness of the proposed FTO-ARSMC (FAC) strategy in rapidly and robustly controlling the attitude during the transition mode of TUAV.