Abstract
This paper presents a three-layered hybrid collision avoidance (COLAV) system for autonomous surface vehicles, compliant with rules 8 and 13–17 of the International Regulations for Preventing Collisions at Sea (COLREGs). The COLAV system consists of a high-level planner producing an energy-optimized trajectory, a model-predictive-control-based mid-level COLAV algorithm considering moving obstacles and the COLREGs, and the branching-course model predictive control algorithm for short-term COLAV handling emergency situations in accordance with the COLREGs. Previously developed algorithms by the authors are used for the high-level planner and short-term COLAV, while we in this paper further develop the mid-level algorithm to make it comply with COLREGs rules 13–17. This includes developing a state machine for classifying obstacle vessels using a combination of the geometrical situation, the distance and time to the closest point of approach (CPA) and a new CPA-like measure. The performance of the hybrid COLAV system is tested through numerical simulations for three scenarios representing a range of different challenges, including multi-obstacle situations with multiple simultaneously active COLREGs rules, and also obstacles ignoring the COLREGs. The COLAV system avoids collision in all the scenarios, and follows the energy-optimized trajectory when the obstacles do not interfere with it.
Highlights
Motivated by the potential for reduced costs and increased safety, the maritime industry is rapidly moving toward autonomous operations
We demonstrate the three-layered hybrid collision avoidance (COLAV) shown in Figure 1 by combining and extending the COLAV algorithms developed in Eriksen and Breivik (2017b, 2019), Bitar et al (2019a,b), Eriksen et al (2019)
For the short-term layer, the branching-course model predictive control (BC-MPC) algorithm is used, which is a sample-based MPC algorithm intended for short-term autonomous surface vehicle (ASV) COLAV
Summary
Motivated by the potential for reduced costs and increased safety, the maritime industry is rapidly moving toward autonomous operations. In addition to generating collision-free maneuvers, a COLAV system must adhere to the “rules of the road” of the oceans, i.e., the COLREGs (Cockcroft and Lameijer, 2004) These rules are written for human ship operators and include qualitative requirements on how to perform safe and readily observable maneuvers. A number of COLAV approaches considering the COLREGs have been proposed in the past This includes algorithms using simulation-based model predictive control (Hagen et al, 2018), velocity obstacles (Kuwata et al, 2014), rule-based repairing A* (Campbell et al, 2014), and interval programming (Benjamin et al, 2006). The middle layer avoids moving obstacles, while the bottom layer implements safety functions for handling cases where the two other layers fail This approach does, not consider the COLREGs. Figure 1 shows a three-layered hybrid COLAV system for an autonomous surface vehicle (ASV).
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