Static Orientation Control of AUVs - Conceptual Design and Control Architectures

The possible applications of Intervention AUVs, these are Autonomous Underwater Vehicles capable of interacting with their environment, cover a large variety of scenarios in various surroundings. To cope with a preferably large variety of different tasks, it appears to be beneficial to be able to orientate the vehicle and its usually limited working space to the point of interest, such that manipulators, sensors and lights are aligned to the best advantage. In most cases, underwater vehicles are designed to be hydrostatically stable, i.e. the vehicle's center of gravity is positioned below its center of buoyancy. Thus, if the vehicle gets disturbed in pitch or roll, this results in restoring forces acting back into the direction of the original stable orientation. However, to orientate the vehicle arbitrarily, these restoring forces have to be overcome, which requires for high thruster power and therefore is energy-inefficient. Another design approach is to let the center of gravity coincide with the center of buoyancy. On the one hand, this reduces necessary thruster power significantly, but on the other hand there are no hydrostatic restoring forces anymore that counteract disturbances. A different approach that allows for both energy-efficiency and hydrostatic stability is to actively shift center of gravity and/or center of buoyancy, as seen in e.g. underwater gliders. This can be realized by e.g. shifting mass like batteries or utilizing a buoyancy engine. When a change in pitch or roll is desired, the demanded orientation is simply set as the new stable one by shifting mass and/or volume. The subject of this thesis is the examination of feasibility and benefits of such a system as well as conceptual design and the development of control architectures for hydrostatic orientation control. However, current simulation results show little effect of hydrostatic stability on closed-loop orientation control, questioning the necessity of hydrostatic stability for AUVs that are fully actuated in all six DOF. The aim of this talk is to present intermediate results of the thesis and to discuss future work like experiments for validation.