Control systems for vehicle-manipulator systems (VMSs) must handle the effects of complex, unmodelled dynamics and disturbances affecting the VMS. These effects are particularly severe in underwater environments, including highly complex hydrodynamics and strong ocean currents. We have thus developed novel control systems that make the VMS robust to such perturbations, i.e. their effect on the motion of the VMS is strongly reduced. We have used mathematical analysis to develop and prove the stability and performance of the proposed control algorithms and have validated the theoretical results through numerical simulations and full-scale experiments.
VMSs with a small and light base compared to the size of the manipulator arm it carries are denoted light-VMSs. There will always be coupling effects between the motion of the manipulator arm and the base of the vehicle, but for VMSs with a large, heavy base, like a work-class ROV, the motion of the arm will only affect the motion of the base slightly. For light-VMSs, on the other hand, there will be a strong coupling. Moreover, all VMSs are redundant systems, as they can always choose between moving their base and arm to position their end-effector. Therefore, we have developed control methods that handle this redundancy and utilize it to achieve several tasks simultaneously while also considering the coupling effects. Task-based methods provide a structured way of building up autonomy in a system, where many tasks can be defined with different priorities attached to them, and the algorithm plans and executes the pertinent VMS motion online. Our methods ensure strict priority between the different tasks, such that low-priority tasks do not impact the execution of high-priority tasks like collision avoidance and other strict safety requirements.
To achieve true autonomy for underwater vehicles, energy autonomy must be achieved. Today, underwater vehicles rely on subsea infrastructure with docking stations or a surface supply vessel to recharge their batteries. It will be revolutionary for our ability to explore, monitor, and care for our oceans if we can develop underwater vehicles that can extract the energy they need from the ocean. In this project, we have investigated whether articulated underwater VMSs, called Articulated Intervention-AUVs (AIAUVs), can harvest energy from the wakes downstream of bluff bodies when currents pass by. We have obtained promising results, showing that energy can be harvested and that there exists a clear optimal position in which the most energy is generated. We have, furthermore, developed control algorithms that enable the AIAUV to locate and stay in this optimal position for energy harvesting.
