Modeling and Control of Remotely Operated Underwater Vehicles

Remotely Operated Underwater Vehicles (ROUVs, or just ROVs) belong to the larger group of Unmanned Underwater Vehicles (UUVs), submersible vehicles that operate underwater without the presence of a man on board, as they are remotely driven. Among the many different kinds of UUVs, ROVs are wire-guided by a human pilot which makes decisions and controls the vehicle. ROVs play an important role in a number of underwater missions for marine science, oil and gas extraction, exploration and salvage. For their various and important applications, in recent years there has been a significant increase in ROV’s demand from industry, which has stimulated increasing interest from researchers. Indeed, the marine environment, characterized by non-linear hydrodynamic effects and unknown disturbances, such as waves and currents, together with the uncertainty about the parameters and the lack of a precise model, make the ROV a complex system to identify and control. In this thesis work, a control system has been designed and developed to be implemented on EVA, an ROV prototype developed by PoliTOcean, a Student Team of Politecnico di Torino, active in underwater robotic field. First, a 6 degree- of-freedom (6-DOF) mathematical model of the prototype has been designed. Then, a model-based control system has been developed and shown to work in simulations. Two different control techniques (PID and LQI/LQR), which are widely used in underwater applications, have been proposed and compared. The comparison showed that regarding the speed controller, the LQI allows to reach the settling time faster than the PID controllers. However, PIDs, especially for angular velocities, stem better the oscillations. As for the position controller, the LQR performs better than the PIDs, if the saturation of the motors is avoided. Otherwise, the PIDs are able to better manage the non-linearities introduced by saturation, allowing faster settling times.