A Differential-Flatness-Based Approach for Autonomous Helicopter Shipboard Landing

This article presents a differential-flatness-based approach to trajectory planning and tracking for helicopters and demonstrates this strategy for time-optimal autonomous helicopter shipboard landing. For planning and outer-loop tracking control, helicopter dynamics are captured by a simplified nonlinear model that is shown to be differentially flat, which enables the transformation of the original nonlinear system dynamics into an equivalent linear form with endogenous nonlinear constraints. For time-optimal trajectory planning, this equivalent model is used to construct a computationally efficient optimization problem for free end-time end-state problems, where the decision space is that of the trajectories of the flat outputs and their derivatives. While the attitude dynamics are stabilized by an inner-loop controller, for tracking the planned trajectory, a flatness-based outer-loop control law is designed to stabilize the unstable zero dynamics. Furthermore, analytical guarantees that relate inner-loop tracking error to outer-loop robust stability and tracking performance are derived. Simulations are performed on a high-fidelity numerical UH-60 helicopter model underground effect and ship airwake disturbances, together with ship motion profiles collected from a naval environment to validate the proposed approach for realistic landing scenarios.