Structure tracing with a ranging system using a sheet laser beam

Autonomous Underwater Vehicles (AUVs) are suitable for condition survey of artificial structures such as pillars and caissons in harbors. The authors have developed a method to investigate the structures using AUVs. The method has been successfully demonstrated in tank tests and sea trials as reported in the previous papers. Since the ranging system in this method used an array of pinpoint lasers, it is oversensitive to noise, for example small obstacles and floating particles. This paper describes a method to trace the structure\´s surface using a sheet laser beam that overcomes the difficulties previously encountered. This ranging system determines the continuous shape of the target objects over a wide area by light-sectioning. The vehicle navigates referencing the principal shape of the structure, tracing its surface while taking video images. The method is implemented using testbed AUV "Tri-Dog 1" and verified by tank tests. It is proved that the new method is more robust against noise and trivial objects than the previous method that used a pinpoint laser ranging system. This ranging system can also be used for 3D mapping of the surface and bottom structure.