Relaxed Control Barrier Function Based Control for Closest Approach by Underactuated USVs

The use of automatic safety-critical control for uncrewed surface vessel (USV) survey, inspection and intervention can provide a computationally lightweight controller which guarantees that a minimum safe standoff distance to a target of interest is always maintained. We propose a trajectory tracking safety-critical controller for the closest safe approach of an underactuated USV with nonholonomic dynamic (acceleration) motion constraints to a target. A backstepping-based control law is designed using a relaxed control barrier function and an analytical convex optimization method. The stability of the controller is proven. Simulations of a USV approaching both stationary and moving targets are used to demonstrate implementation of the method. The performance of the proposed controller is compared with that of a nonlinear model predictive control (MPC) controller in simulation. The simulation results demonstrate that, while the tracking error of the proposed controller is higher than that of an MPC controller, it requires lower computational resources, suggesting it is a good candidate for use on small USVs with low computational power.