Combined Depth Control Strategy for Low-Speed and Long-Range Autonomous Underwater Vehicles

Abstract

Autonomous underwater vehicles (AUVs) are increasingly being applied to highly detailed survey and inspection tasks over large ocean regions. These vehicles are required to have underwater hovering and low-speed cruising capabilities, and energy-saving property to enable long-range missions. To this end, a combined depth control strategy is proposed in which an on-off type variable ballast system (VBS) is adopted for satisfactory hovering or fast descending/ascending without propulsion to reach the designated cruising depth, whereas the bow and stern fins act as the actuator to maintain the cruising depth for more energy saving. A hierarchical architecture-based VBS controller, which comprises a ballast water mass planner and an on-off mass flowrate controller, is developed to assure good hovering performance of the on-off type VBS. Both numerical studies and basin tests are conducted on a middle-sized AUV to verify the feasibility and validity of this depth control strategy.