Path Following of Underactuated Marine Vessels in the Presence of Ocean Currents
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The use of marine vessels, especially underwater vehicles, is rapidly increasing. Autonomous marine vehicles are a huge focus area within the oil and gas industry, and can also be utilized for scientific, environmental and military use. There are still many challenges related to making such marine vessels autonomous. A basic task of an autonomous marine vessel is to follow a general path in the presence of unknown ocean currents. This is the key challenge addressed in this thesis.In this thesis, two theorems that ensure path following given that certain assumptionsare satisfied are presented. Theorem 1 applies to surface vessels and Theorem 2 to underwater vehicles. The developed theorems are based on the work of Even Børhaug regarding path following of space curves when no ocean currents are present, and introduce a virtual Serret-Frenet reference frame that is anchored in and propagates along the desired path. The theorems describe a closed-loop system with an ocean current observer, a guidance law, a controller and an update law to drive the Serret-Frenet frame alone the path.The developed theorems have been implemented and simulated using a model for a supply surface ship and the Hugin AUV by Kongsberg Maritime AS for several different desired paths. In all simulations, the marine craft is able to converge to and track the desired path. Future work includes expanding simulations to include more realistic elements such as state observers and measurement noise, as well as testing the developed theorems on an actual ship/AUV.The preliminary goal of this thesis was to develop a method to ensure path followingin the presence of unknown ocean currents. This has been successfully done,and this thesis will be the basis for an article that will be submitted to the 2014American Control Conference.