Practical predefined-time control for stable and safe docking of fixed-wing receiver in aerial refueling

The docking phase of probe–drogue aerial refueling demands fast and reliable stabilization of a fixed-wing unmanned receiver under severe flowfield disturbances and strict safety envelopes. This paper proposes an integrated docking control framework that explicitly enforces attitude and actuator constraints while preserving the minimum-airspeed margin required by a lift-dependent fixed-wing receiver, and provides user-assigned design parameters that yield practical predefined-time residual-set guarantees. Based on a 6-DOF nonlinear receiver model, a constrained pitch/yaw attitude controller is developed by combining an asymmetric barrier Lyapunov function (ABLF) with a predefined-time command-filtered backstepping structure. A predefined-time disturbance observer is incorporated to estimate lumped nonlinear couplings and external disturbances, and a continuous saturation compensator is introduced to handle actuator limits, yielding practical predefined-time bounded tracking of the compensated attitude errors within prescribed asymmetric safety bounds. The physical pitch/yaw envelope satisfaction is further ensured through tightened command envelopes and bounded auxiliary states. For the outer loops, predefined-time nonsingular sliding-mode regulators are designed for airspeed and altitude. A thrust-mismatch auxiliary system preserves the affine error structure in the airspeed loop, whereas a software limiter and a physical pitch envelope ensure self-consistency of the altitude-to-pitch command chain under practical implementation. To avoid step-command-induced overshoot and oscillations, nonlinear tracking differentiators generate smooth reference trajectories for altitude and heading, and a bounded lateral-directional following command is constructed to respect maneuverability limits. Rigorous analysis establishes the uniform boundedness of all closed-loop signals and derives explicit residual bounds with tunable time/accuracy tradeoffs. Nonlinear flight simulations under representative docking disturbances with explicitly reported disturbance forms demonstrate that the proposed controller achieves rapid convergence and high-precision tracking of the receiver states. In the reported cases, the compensated attitude errors remain strictly inside the designed safety bounds, and the input commands remain within their admissible ranges, supporting the framework’s robustness and practical applicability for automated aerial refueling docking tasks.