Visual observation of underwater objects by autonomous underwater vehicles

To investigate underwater objects such as piles and caissons in harbors by taking visual images, autonomous under water vehicles (AUVs) have advantages that they have no umbilical cable which may get entangled in the target object and surrounding obstacles. This paper proposes a navigation method for AUVs to observe such objects using a laser range measurement system that consists of a TV camera and laser pointing devices. This ranging system provides precise distance to the target object so that the vehicle can trace the shape of the target object keeping constant distance and attitude. It is assumed that the visibility is adequate for taking visual images of the target. It was implemented in the testbed AUV tri-dog 1, and proved experimentally through tank tests. The vehicle accomplished the observing mission that automatically detects the turn around three pile-objects those are modeled on actual piles in a harbor. This paper also shows the result of sea trials. Based on the proposed method, AUVs will find new practical applications of autonomous observation of underwater objects.