Path Planning and Guidance for Marine Surface Vessels
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Path planning and guidance for marine surface vesselsis the main topic of this thesis. The subject is of great relevance both for unmanned surface vehicles (USV) and for autopilot systems for manned vessels.A general overview of path planning is given. Dominating concepts from the field of robotics are discussed. An adapted terminology for marine applications is proposed, as there are differences between the fields in the approach to motion control.Path generation from an ordered set of waypointsencompasses the main effort of the thesis. Such a set may be the result of an automatic motion planning algorithm or manual input by a human operator.Starting from the unprocessed piecewise linear path between waypoints, the discussion continues with traditional naval passage planning by circular smoothing, and further progresses to both advanced and until now unpublished methods. Concepts and algorithms are comprehensively presented with extensive use of illustrations, thus providing the reader a solid basis for implementation on own systems.Fermat's spiral is proposed for path generation as it has zero initial curvature and is curvature continuous. It is thus an alternative to the clothoid, both as a smoothing and a transition curve. Computing a Fermat's spiral subpath is shown to be significantly less computational intensive than the corresponding clothoid. However, the curvature is not linear and a parametric singularity appears at the transition between the spiral and the straight line. This is unproblematic for path-following applications but may complicate trajectory generation and tracking.Numerical computation of the clothoid is investigated and a reasonable approach for handling the integration is presented.Moreover, an extensive presentation of the classic Dubins path is given. By including intermediate waypoints, the study goes beyond common presentations, which are usually limited to a starting and an ending point.Pythagorean-hodograph cubics are shown to be a close to equivalent replacement for the Pythagorean-hodograph quintics. One unique solution of the cubic is shown to be sufficient, thus avoiding the significant computational requirements, including integration over four candidate curves, of the quintic variant.Even if presented in a marine setting and notation, the concepts discussed above are transferable, applicable and of value for other fields, such as wheeled robots, as well.A brief introduction to popular motion planning approaches,including visibility graphs, Voronoi diagrams and cell decomposition, is also given. The concepts are directly transferred to applied examples for marine surface applications.Surface vessel model simulationswith general path-following guidance are conducted in order to verify the applicability of the path generation results. Amongst the findings, is the observation that even if valid for wheeled vehicles and popular for aircraft, the turning circle is not necessarily a directly applicable concept for marine vessels.