Hovering control of an underwater robot with tilting thrusters using the decomposition and compensation method based on a redundant actuation model

Six-degree-of-freedom (6-DOF) hovering control is important for underwater robots to perform various tasks. Our previous underwater robot study, which used tilting thrusters, could not control 6-DOF motion simultaneously owing to several mechanical and control problems. In this study, we developed a new robot with tilting thrusters and improved 6-DOF hovering performance. The maneuverability of the robot was evaluated by analyzing the force and moment of the thrust vector. Based on this, a redundant tilting mechanism without constraints was designed to solve structural problems. A proportional–integral–derivative (PID)-based control design using the decomposition and compensation method (PID-DC) that is appropriate for this mechanism, was derived. The decomposition method was used to overcome the nonlinearity of the thrust vector caused by the tilting mechanism, and the null-space projection technique was applied to minimize the thrust force and avoid the boundary of the tilting angle. A compensator based on the empirical model of the tilting thruster transferred the control input to the system with regulation. Simulation and experimental results verified the validity of the controller for the 6-DOF hovering motion of the robot, and the hovering performance was significantly improved. Furthermore, the stability of the hovering performance under tidal currents was demonstrated through disturbance experiments.