This research is conducted in the framework of a Master’s thesis within the Marine Engineering department of the Maritime Transport and Technology branch and in joint collaboration with the Robotics branch of the Mechanical Engineering Faculty of Tu Delft.
The precise control of robotic systems in granular underwater environments is essential for applications such as deep-sea mining, sediment sampling, and seabed infrastructure maintenance. In such environments, the interaction between the robot and deformable substrates like sand and clay plays a crucial role in operational efficiency and system stability. This research investigates how fine-tuning joint stiffness in an impedance controller influences penetration depth accuracy, horizontal force distribution, and force consistency along a trajectory mapped using 3D camera point cloud data. Understanding these relationships is critical for optimizing force control strategies in unstructured and dynamic underwater settings.
Experiments were conducted using a KUKA iiwa 7 robotic arm equipped with an impedance controller, following a mapped trajectory over a real sandbed in both dry and submerged conditions. The point cloud data from a 3D camera provided accurate environmental mapping, ensuring precise trajectory tracking. The results indicate a significant correlation between joint stiffness and penetration accuracy: higher stiffness improved depth accuracy and reduced external disturbances but compromised adaptability in cases where the robot encountered hard obstacles. Conversely, lower stiffness increased compliance, allowing for smoother interactions but at the cost of greater sensitivity to force fluctuations.
Fluid damping in submerged conditions was found to reduce penetration error variability, highlighting the stabilizing influence of water on force interactions. The study also revealed that current robotic systems for deep-sea applications differ significantly from the 7-degree-of-freedom (DOF) KUKA arm used in this research. In practical scenarios, deep-sea mining robots typically feature a single actuated DOF (pitch), with other degrees of freedom facilitated by passive flexibility rather than active control. These structural differences influence force distribution and overall system behavior, emphasizing the need for future studies tailored to real-world deep-sea mining configurations.
Future research should extend beyond sand to softer seabed sediments such as clay, which behaves more like a Bingham fluid and exhibits significantly lower shear strength—potentially by a factor of 5 to 10 compared to sand. Additionally, exploring adaptive stiffness strategies that dynamically adjust control parameters in real time could enhance the efficiency and robustness of underwater robotic systems. These advancements would contribute to optimizing force control for precise, adaptable interactions in unstructured marine environments.