Reactive collision avoidance for ASVs based on control barrier functions

A reactive collision avoidance method for autonomous surface vehicles based on control barrier functions (CBFs) is proposed. An encounter between the ownship (the vessel that we control) and a target ship is classified, in accordance with the International Regulations for Preventing Collisions at Sea (COLREGs), to be either a head-on, overtake, give-way, stand-on or a safe situation with respect to the ownship. Subsequently, a spatial region is assigned to the target ship based on the classification, and this region is used to define a collision-free set. Based on this, a CBF is formulated to ensure forward invariance of the collision-free set. This CBF can then be applied as an inequality constraint to any guidance, navigation and control system with an optimization-based trajectory tracking or thrust allocation system. The method is verified through simulations and is seen to handle head-on, overtaking and crossing situations with both give-way and stand-on duty in compliance with COLREGs rules 13-15 and 17.