Stereo Vision System for Autonomous Ship Hull Inspection

The periodical hull inspection represents a necessary task to ensure the maintenance of a vessel since it allows to counteract decay, check for structural damages, and fight the biofouling phenomenon affecting the navigation efficiency. Typically, this task is executed by divers, resulting in a dangerous job for the human operator, or by Remotely Operated Vehicles, driven by highly trained users. Aiming at automating the task and increasing its operational safety, this work proposes a strategy to perform the ship hull inspection using an Autonomous Underwater Vehicle (AUV), equipped with a stereo camera and a proximity sensor, without a prior knowledge of the target shape. At first, the images from the stereo vision system allow to estimate the lateral velocity of the vehicle and its orientation with respect to the hull surface. Then, the proximity measurement, properly projected along the normal axis to the surface of the target, provides a measure of the distance of the AUV from the surveyed structure. Lastly, the robot control system exploits these estimates to perform the mission with a constant lateral velocity, maintaining both a predefined safety distance from the target and the optical axis of the camera orthogonal to the examined surface. The proposed approach has been tested in a simulated environment, performing the investigation of a simplified model of ship hull. The results suggest the feasibility of the strategy: during the simulations, the AUV completes the mission with a full autonomy, safely, obtaining a 3D reconstruction of the surveyed structure.