A self stabilizing underwater sub-surface inspection robot using hydrodynamic ground effect

In this paper we present a unique self stabilizing ellipsoidal robot for inspection of underwater structures using the principles of ground effect. Underwater metal structures - whether it is ship hulls or internals of a boiling water reactor - require subsurface inspection to detect internal cracks, hidden cavities and other structural damage. This is usually done with on-contact ultrasonic sensors, a slow process if the structure is not sufficiently smooth. However, ultrasound can also be used with a precisely controlled gap. Such precision is challenging relying solely on actuators for control. This paper exploits near surface hydrodynamics to self stabilize a body at a precise gap. Specifically we show how boundary layer and venturi effects combine to create a stable, zero force position at a very small distance from the surface - conceptually similar to air bearings sliders on hard disk drives. Below the stable point lift force dominates, while above it Venturi suction prevails, each bringing the body back to equilibrium. Limitations in the restoring force are considered in the stability analysis included in the paper. This self stabilization method opens a whole new door for non-contact subsurface inspection of underwater structures by autonomous vehicles as well as precision distance maintenance in underwater environment. Here we present our initial analysis and preliminary experimental results for the method when used with an ellipsoidal robot.