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--- l2_sw [2018/10/23 10:42] – removed alberto
+++ l2_sw [2023/09/06 11:21] (current) – external edit 127.0.0.1
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+==========Collision Avoidance System==================
+For the ASV to obey the rules-of-the-road during when encountering other manned and/or unmanned vehicles, and perform the correct and predictable actions in haz- ardous situations the CAS needs to be COLREGS compliant. From the Mission Planner the CAS receives a desired course and speed based on a sequence of waypoints. These waypoint could for instance be generated by a human operator in Neptus. Meanwhile, the Sensor System composed of e.g. Radar, AIS, Lidar, etc. continually provides naviga- tional measurements, measured obstacle positions and predicted trajectories. Based on this information the CAS module searches for a COLREGs compliant and collision-free trajectories through a series of simulations with a finite set of offsets to the nominal course. The offset associated with the lowest cost while producing a collision-free and COLREGs compliant trajectory is selected as the new modified course reference and is passed on to the autopilot. If there is no obstacles in the vicinity the desired course and speed from the Mission Planner are sent to the Autopilot, composed of a Heading and a Speed Controller. Consequently, the ASV’s Autopilot and Steering and Propulsion System will apply these (modified) steering and propulsion commands, ensuring a COLREGs compliant trajectory associated with the lowest collision risk as possible. When the original desired course and trajectory no longer contains any possible potential collisions, the ASV would return to the desired nominal path and proceed towards the target destination.
+{{:l2-modules.png?1000|}}
+;#;
+{{:l2-imcflow.png?800|}}
+;#;
+\\
+;#;
+{{:cas2.png?600|}}
+;#;
+The architecture has been simplified whereas the weather scenarios is currently removed as it is not considered during the simulation, and still have not been taken in to account. The collision avoidance and selection of the control behaviour with the minimum hazard associated to it is realized as a optimization problem. A finite number of control behaviours and predicted obstacles’ trajectories are combined into finite scenario hazard minimization problems, which, consequently, are being evaluated over a finite horizon. The hazard associated to a certain scenario resulting from a given control behaviour is evaluated using a simulator, provided in the LSTS toolchain, that make predictions based on the dynamics of the ship, steering and propulsion system, the control behaviour and the current position and velocity. In the center of the hazard evaluation is a cost function performing the evaluation based on the collision risk, collision cost and compliance with the rules of COLREGs for the different scenarios. The control behaviour with the minimum hazard associated is the desired control behaviour. This optimization problem is re-optimized based on updated information at regular intervals, e.g. 5 seconds.
+\\
+CAS is implemented in a way that makes the system COLREGS compliant. Indeed, for the AutoNaut to operate with a risk-reducing and predictable behaviour to the operators and other vessels it is required to adhere to the rules specified by the COLREGs. CAS uses the available information to evaluate COLREGS compliance in a hazardous situation, e.g. encountering an obstacle. COLREGs is a set of regulations and navigation rules to be followed by ships and other marine vessels at sea to prevent collisions between two or more vessels. All marine surface vessels are required to adhere to COLREGs at all times in order to minimize or eliminate the risk of collisions. Consequently, it is necessary for the CAS to also adhere to COLREGs in order to produce predictable maneuvers when encountering other vessels. The COLREGs rules are divided into five parts (A-E), however, the rules covered in Part B - Steering and Sailing is the most relevant and applicable for AutoNaut and this thesis. \\
+To differentiate the COLREGs situation and when to apply actions, four different sectors are defined. The following diagram displays how COLREGs situations are classified.
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+{{:l2-colregs.png?500|}}
+;#;
+Note that the Crossing sectors are overlapping the Head-on section. The reason is that you can still experience a Crossing situation when the obstacle is inside the Head-on section. Consider, for example, a situation where an obstacle is in the head-on section, and moving with a heading of 90°relative to the ASV heading. Hence not moving towards the ASV, so this situation be a crossing situation, but still with a potential collision cost in the head-on section.
+===========Software Overview================
+{{:l2-overall.png?1000|}}
+\\
+===========Messages Flow================
+{{:l2-cas.png?1000|}}