Highly accurate motion control system for omni-directional underwater robot

Since an underwater robot can be used as various purposes, fast and accurate motion becomes a key issue because basically overall efficiency of given tasks and quality of jobs highly depend on the low-level control performance. For this, an underwater robot is typically designed to be able to generate omni-directional motion. An omni-directional thruster configuration brings lots of advantages for fast and accurate motion. But, controlling of an omni-directional underwater robot is not easy because 3 or 4 horizontal thrusters have slightly different characteristics and each thruster has nonlinearity with dead-zone. Additionally, navigation sensors such as IMU(inertia measurement unit), DVL(doppler velocity log) provide accurate information only when their geometrical relationships are appropriately considered. In this paper, we briefly explain an underwater robot, yShark and its thruster configuration matrix (TCM). Then, methods to achieve fast and accurate motion for depth, heading, and lateral motion control are proposed. Also, this paper presents some experimental works to show incompleteness of thruster configuration, which shows a gap between theory and practice. The proposed schemes are experimentally confirmed by yShark which has been developed by Korea Institute of Ocean Science and Technology (KIOST).