Evaluation and reduction of the dynamic coupling between an ROV and manipulator

Current underwater robotic systems typically comprise one or more robotic manipulators mounted on the front of a tethered ROV, equipped with an underwater camera system. In addition an attachment system is required to hold the ROV reasonably static relative to the workpiece. This involves either using using the second manipulator, or a dedicated hydraulically powered arm mechanism with suction feet that fixes itself to the structure being inspected. As the ROV will move due to the manipulator reaction forces and moments, the attachment arm must permit some ROV motion, and therefore has a yaw and pitch capability as well as an extend/retract facility. The design of this arm involves significant mechanical engineering effort. The operator must use the vehicle thrusters and master manipulator to compensate for the ROV motions induced by manipulator movements and sea-current disturbances. The objective of the work described is to remove the need for a mechanical attachment arm by providing the ROV with a hover control system that takes account of the dynamic interactions caused by manipulator motions. These disturbances alter the vehicle´s position and orientation and hence influence the manipulator end-effector position and orientation. This paper outlines the modelling process and control strategy for the coupled control problem, and presents results showing the effect of dynamic coupling for a range of conditions, with and without an ROV hover control system