Model-based motion control for underwater vehicle-manipulator systems with one of the three types of servo subsystems

This paper deals with a motion control scheme for underwater robots equipped with manipulators. In general, for each robot, its manipulator is directly driven by electric motors with position sensors such as encoders, whereas its robot body is propelled by marine thrusters with position sensors such as inertial measurement units. It has been pointed out in the literature that for this type of underwater robots, its robot body control is more challenging than its manipulator control, because the robot body has much larger inertia, and many more inaccurate position sensors and actuators than the manipulator. Therefore, it may be practically difficult to accurately control the motion of the robot body, even if an advanced control scheme with good performance is implemented in the controller of the robot body. In this paper, we develop a model-based motion controller for the manipulator under the condition that the robot body is independently controlled by a motion controller with poor performance. Its features are to design the manipulator controller in consideration of the dynamics of the robot body including the marine thrusters as well as those of the manipulator, and to be applicable to underwater robots equipped with one of the three types of servo systems (i.e., a voltage-controlled, a torque-controlled, and a velocity-controlled servo system). Some stability properties of the control system were theoretically ensured in the stability analysis. Furthermore, these results were supported by those obtained in numerical simulations.