Control of an underactuated remotely operated underwater vehicle

In this paper, a steady state model of a thruster and a general equation of rigid-body motion for an underwater robotic vehicle (URV) is presented. By means of modelling, simulation and experiments, the model parameters have been identified. These are used in the analysis and design of closed-loop stabilizing controllers for two control modes: manual cruise and station keeping. Since the URV under study has fewer actuators than possible degrees of freedom, it is necessary to limit the controllable degrees of freedom. These variables are eventually selected based on the inherent vehicle dynamics. Using the Lyapunov direct method, which has been shown to be appropriate for such non-linear systems, appropriate stabilizing controllers have been designed. The manual cruise mode controller is non-linear and would result in chattering in the thruster outputs, but simulations show that the desired results can be achieved. The station-keeping mode controller has a proportional-integral-derivative (PID) structure and its gain values are designed using a non-linear optimizing approach. Simulation and swimming pool tests for the heave and yaw directions have shown that such a controller is possible.