Robust Energy-optimal Path Following Control for Autonomous Underwater Vehicles in Ocean Currents

Energy efficiency is crucial for autonomous underwater vehicles (AUVs) with limited on-board energy resources. In this paper, we study the energy-optimal control problem for an AUV to follow a planned reference in ocean currents. The objective of reference following in ocean currents can be effectively achieved by the line-of-sight (LOS) guidance-based path following control, which yet does not explicitly address the problem of energy minimization. We propose a method of LOS guidance-based control that incorporates energy minimization while achieving robust path following in the presence of uncertainty in ocean current information. First, the desired heading of a vehicle is obtained based on a LOS guidance law and the optimal surge speed is computed by minimizing the energy consumed for traveling unit distance towards the desired heading in the nominal current. Then, to deal with uncertainty, we analyze the sensitivity of the resulting optimal surge speed to bounded uncertainty around the nominal current and robustify the optimal surge speed by minimizing the maximum potential energy loss due to the uncertainty. The proposed controller tracks the desired heading and the optimal surge speed in a model predictive control framework. The simulation results show that the proposed method can save considerable energy while maintaining a satisfactory path following performance under bounded uncertainty in the nominal current.