OctoBot
Octopus-inspired compliant-body aquatic robot

Underwater robots are becoming increasingly important in industrial and service applications, such as search-and-rescue operations in narrow flooded zones or undersea shipwrecks, industrial inspection or maintenance in tortuous fluid-filled spaces, as well as shallow- or deep-water marine exploration. The scope of such applications can be significantly enhanced by equipping these underwater vehicles, which frequently employ propellers for propulsion, with robotic manipulators, endowed with multi-function capabilities. The possibility of using these manipulators also for propulsion has not been examined in detail, and could be of some interest.

Indeed, marine animals like the octopus, may use their agile arms in various locomotion modes, like crawling or walking on the seabed or for complementing their frequently-employed jet propulsion. Although lacking skeletal support, the octopus arms are highly dexterous and can achieve complex shapes and perform movements, such as reaching and fetching, by activating several groups of arm muscles [Huffard 2006, Sumbre 2001]. During medusoid jetting, octopuses open and close their arms and arm crown in synchrony, like an umbrella, to produce bursts of activity, supplementing jet propulsion. The Abdopus aculeatus occasionally use this mode of locomotion in the wild, during sustained jetting, to chase conspecifics or prey. Similar swimming patterns have also been observed for Octopus vulgaris [Kazakidi 2012]. The kinematics of arm swimming do not appear to have been investigated in detail, although one essential feature, extracted from related data, is that each stroke comprises two phases, one where the arms, initially trailing behind, open by bending outwards relatively slowly ("recovery stroke"), and one where they return fast to their initial position ("power stroke").

Investigations are currently under way to develop dexterous robotic manipulator arms inspired by the morphology and mechanical properties of the octopus arms [Walker 2005, Yekutieli 2005, Laschi 2009, Cianchetti 2009, Kang 2011]. However, the present work focuses on employing a set of such robotic arms for aquatic propulsion, not for manipulation. A preliminary study of a planar robotic swimmer with a pair of rigid arms assumed an axisymmetric arrangement of the octopus arms during arm-swimming motion, and demonstrated some basic aspects of this mode of swimming [Sfakiotakis 2012]. Later studies considered the extension to an 8-arm swimmer [Sfakiotakis 2013a, Sfakiotakis 2013b], equipped also with rigid arms, for investigating the propulsive ability of such a system under various forward and turning gaits. A recently presented robot considered the effect of a silicone web in-between the arms, demonstrating advanced propulsion performance [Sfakiotakis 2014]. Towards this end, computational models have been developed to study propulsion generation for a robotic system with a pair of multi-segment arm-like appendages. A fluid drag model, commonly employed in the robotics literature [Ekeberg 1993,Ijspeert 2001, Sfakiotakis 2006], models the interaction of the segments with the aquatic environment. Detailed computational fluid dynamics (CFD) analysis of such octopus-inspired robotic arms is then used to validate this force model, and to calculate the values for the associated fluid force coefficients [Kazakidi 2014]. Simulations demonstrate the generation of propulsive forces by sculling and undulatory arm movements. Our computational studies are supported by experimental results of an eight-arm robotic prototype in a water tank.

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