Kinematic Analysis of Crab Locomotion for the Development of an Underwater Drone with Swimming Legs

Abstract

This study attempted to elucidate the locomotion mechanism of a crab, with the final goal of developing a bio-inspired underwater drone with a swimming leg. For the analysis of swimming behaviour, we defined the rotation angles around the root of the swimming legs and conducted detailed motion analysis using images captured by an underwater camera. Results showed that the trajectory of toes during the reciprocating rotational motion forms a figure-eight pattern, similar to insects' flapping flight. The time courses of the angle of attack indicated that the downward stroke motion predominantly governs propulsion. Comparing the independent movements of the left and right legs suggested a potential control strategy for turning swimming, offering design guidelines for new underwater drone development. The characteristics of the calculated Reynolds number and the leg kinematics suggest that crabs may adeptly utilize highly unsteady fluid phenomena, such as vortices, in their swimming.