Safety-Critical Guidance of Underactuated Marine Vehicles in Dynamic Environments using Velocity Obstacles ^*

Avoiding collisions is of utmost importance in the control of autonomous marine vehicles. This paper proposes a collision avoidance strategy for marine vehicles with underactuated dynamics, utilizing the framework of Control Barrier Functions (CBFs). Such vehicles are subject to second-order nonholonomic constraints and are required to keep a nonzero forward speed to maintain their maneuverability. These characteristics demand additional considerations compared to holonomic systems and to vehicles capable of stopping and reversing. To address these challenges, we formulate the CBFs based on the velocity obstacle (VO) principle, allowing the vehicle to safely maintain its forward motion while avoiding collisions. The strategy is defined on the guidance level, which is combined with dynamic controllers ensuring tracking of the generated references. We provide safety guarantees for the closed-loop system, and the resulting performance is demonstrated in simulations of an underactuated marine vehicle, displaying adept and secure navigation among multiple dynamic obstacles.