Tether-based localisation system for underwater robots

Tank-inspection robotic crawlers often rely on localisation techniques such as dead reckoning to survey walls and surfaces. However, the robot position can drift over time when using these techniques, which therefore require precise calibration. This paper presents a novel tether-based system providing a real-time localisation reference tool to help calibrate and understand prevalent localisation technologies, inside uncluttered tanks. The assessed localisation technologies can subsequently be used in cluttered and complex operational tanks. The developed system relies on a mechanical connection between the tank edge and the robotic crawler and is composed of two distinct units. The first unit is the tether orienting platform clamped onto the tank edge and guiding a tether coming out of a draw-wire position sensor. The second unit is the crawler platform mounted on top of the robotic crawler and holding the tip of the tether, while allowing the robot to move freely. The performance of the system is assessed by determining the accuracy on the 3D position of the robot and the accuracy on its 2D heading, over a 1m x 1m area. The positional accuracy with 90% confidence interval is ±39mm and the heading accuracy with 95% confidence interval is ±4.8°.