Sensor-based Formation Control of Autonomous Underwater Vehicles

We address the subject of formation control of underwater vehicles, using either inter-vehicle relative position measurements or range-only measurements. The work is motivated by the MORPH project, aiming to use formations of vehicles with complementary capabilities to map the underwater environment in challenging conditions. The main problem studied is that of driving a vehicle (follower) to an appropriately defined position with respect to another vehicle (leader), which moves along an unknown trajectory. This control task is divided into two steps. Firstly, a trajectory-tracking controller is designed, so that a follower vehicle can track a specified virtual target. Conditions under which the tracking error remains bounded are derived using tools from nonlinear systems theory. Secondly, we design path-generators that provide the position and velocity of the virtual target to be tracked, for both the relative position measurements and the rangeonly measurements scenarios. These path-generation strategies take into account the formation objectives, typical maneuver geometries and limitations of underwater inter-vehicle communications. Applications envisioned in the scope of the MORPH project are presented and discussed. The implementation of the strategies proposed for formation control with relative position measurements is described, taking explicitly into account the filter architectures required for relative vehicle navigation. Finally, results of simulations and sea trials are shown, with position errors generally below 1 m in MORPH trials in Lisbon and the Azores.