Energy-Optimized Path Planning for Autonomous Ferries

The topic of energy-optimized path planning using pseudospectral optimal control is considered. An optimal control problem (OCP) is formulated to produce an energy-optimized path between two points among static obstacles. A nonlinear 3-degree-of-freedom underactuated ship model is considered in the OCP with an energy-based cost function. The ship model is affected by external disturbances in the form of ocean currents. The OCP is solved using a pseudospectral method, which has proven successful in real-world applications. Additionally, the method is not as sensitive to dimensionality compared to Hamilton-Jacobi-Bellman methods. Position and speed information from the OCP solution is used in a closed-loop simulation with a guidance controller to show feasibility of the path. The simulation results show that the path is feasible, and that including ocean-current information in the path planning significantly reduces energy consumption. The paper also gives a short overview and classification of alternative path-planning methods. Finally, a proposal for how the path planner fits in a complete motion-control architecture is provided.