Droplet: Towards Autonomous Underwater Assembly of Modular Structures

This paper presents a first low-cost autonomous robotic system for underwater assembly of mortarless structures. The long-term goal is to enable the construction of large-scale underwater structures, such as retaining walls and artificial reefs. The approach follows the principle of co-design; the 2-DOF manipulator and blocks are designed to complement the localization and control strategies. The blocks and gripper are designed with a connector geometry that removes error during pickup of blocks and drop assembly. This error correction feature allows a simplification of localization and control, which are based on fiducial markers on custom platforms. We developed the proposed system on a low-cost heavily modified BlueROV2 autonomous vehicle — which we call Droplet — with a two-degree of freedom hand that can open and close a gripper and rotate over the yaw. We performed extensive experiments in the pool to evaluate each component and the system as a whole. Results showed a 100% success rate in dropping blocks in the presence of some localization and control errors and the assembly of several different 3D structures composed of up to eight blocks.