Nonlinear control of an underwater vehicle/manipulator with composite dynamics

This paper is devoted to the problem of control design of an underwater vehicle/manipulator (UVM) system composed of a free navigating platform equipped with a robot manipulator. This composite system is driven by actuators and sensors having substantially different bandwidth characteristics due to their nature. Such difference allows for a mathematical setup which can be naturally treated by standard singular perturbation theory. On the basis of this analysis, two control laws are proposed. The first is a simplification of the computed torque control law which only requires partial compensation for the slow-subsystem (vehicle dynamics). Feedback compensation is only needed to overcome the coupling effects from the arm to the basis. The second aims at replacing this partial compensation by a robust nonlinear control that does not depend on the model parameters. The closed-loop performance of this controller is close to that of the model-based compensation. Both control laws are shown to be closed-loop stable in the sense of the perturbation theory. A comparative study between a linear partial derivative (PD) controller, a partial model-based compensation, and the nonlinear robust feedback is presented at the end of this paper.