Homing and docking of autonomous underwater vehicles

Abstract

Autonomus Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs) are becoming increasingly popular in both research and commercial applications. These vehicles are flexible, efficient and increasingly cost-effective, and are used for both simple tasks such as explorating, mapping, inspection of underwater infrastructures or monitoring of marine enviroments, as well as complex tasks such as collaborative tasks requiring the participation of multiple vehicles. However, they are limited by the reliability of their systems and the energy stored in their batteries. Moreover, working autonomously for long periods of time is one of the biggest challenges that these vehicles should face. For this reason, in the last years an important research trend is the development of vehicle docking technology for battery charging and data collection (docking stations), as well as control and guidance strategies for driving vehicles to these docking stations, and navigation and communication systems between vehicles and docking stations that allow them to obtain their relativa position so that the vehicles can be correctly positioned and oriented towards the docking stations. In addition, these vehicles are working in hazardous conditions and enviroments, so it is necessary that in the event of a failure, the vehicles are able to reach a recovery point and avoid compromising their integrity. To this end, the development of fault-tolerant systems to detect, isolate and accommodate the failure is of utmost importance. Motivated by these considerations, the subject of this dissertation focuses on the homing and docking operations and on the fault tolerant control problem to determinate the control actions required to contain a given failure for ROVs and AUVs. Specifically, the objectives are: i) develop a novel guidance sustem and control law for ROVs and AUVs for the homing and docking manouvres using acoustic range and angle measurements of the relative position between a vehicle and a docking station provided by an Ultra-Short BaseLine (USBL) system, with one beacon installed in the vehicle and another in the docking station, ii) define an optimal control law to be executed in the extreme case of an underactuated AUV suffering form thruster failure to drive tha AUV to a recovery point. This takes into account not only the time it takes to reack the recovery point, but also the energy consumption of the vehicle´s batteries. Simulations have been performed to determinate the robustness of the different control situations proposed, both for the homing and docking strategy, as well as for the fault tolerant control in case of thruster failure. The results show how the proposed homing and docking strategy using an USBL system is able to position the vehicle in the proper position and orientation with respect to the docking station entrance, even in the presence of corrents. In addition, it has been demonstrated that the control laws proposed for the fault tolerant problem, in the extreme case of an underactuated AUV, are able to guide the vehicle towards a defined recovery point, taking into account not only the time needed to reach the recovery but also the energy cosumption of the batteries.