Hovering Control of an Underwater Robot with Tilting Thrusters Enabling Various Works

Abstract

Most underwater work still depends on human divers despite the development of technique in fisheries and ocean industries. A robotic platform is considered to be a useful instrument to carry out underwater work in an efficient manner. In order to achieve various underwater tasks, a robotic platform must be able to maintain its position and orientation against ocean currents and reaction forces from the manipulators operation. We present a new remotely operated robotic platform with tilting thrusters for various manipulating operations in coastal waters. In the proposed design, four tilting thrusters are equipped on vertexes of a rectangular mainframe obliquely for the reduction of the number of actuators and the enlargement of space efficiency. The four thrusters can be tilted with respect to the mainframe to achieve six degree-of-freedom (DOF) motion. The tilting mechanism causes an increase in the nonlinearity of thrust force vector map. A dynamic model is derived for the underwater vehicle based on hydrodynamic analysis and nonlinear thrust vector mapping. In order to conquer the nonlinearity, the selective switching controller is designed and applied to the robot system. The selective switching PD controller chooses each three DOF linear sub-controller according to the error in real-time, and simulations show feasibility of the control system. The ability of robust hovering control is verified with the experiment in water tank. A standard RR manipulator is attached to the robotic platform and generates reaction forces and moments.