Retraction Mechanism of Soft Torus Robot with a Hydrostatic Skeleton

Soft robots have attracted much attention in recent years owing to their high adaptability. Long articulated soft robots enable diverse operations, and tip-extending robots that navigate their environment through growth are highly effective in robotic search applications. Robots that extend from the tip can lengthen their body without friction from the environment. However, the flexibility of the thin membrane inhibits the retraction motion of the tip due to buckling. Two methods have been proposed to resolve this issue; increasing the pressure of the internal fluid to reinforce rigidity, and mounting an actuator at the tip. The disadvantage of the former is that the increase is limited by the membrane pressure resistance, while the second method leads to robot complexity. In this letter, we present a tip-retraction mechanism with a hydrostatic skeleton that can prevent buckling and takes advantage of the friction from the external environment. Water is used as the internal fluid to increase ground pressure with the environment, which is different from the conventional methods that use pneumatic. We explore the failure pattern of the retraction motion and propose solutions by using a hydrostatic skeleton robot. Additionally, we develop a prototype robot that successfully retracts by using the proposed methodology. Our solution can contribute to the advancement of mechanical design in the soft robotics field with applications to soft snakes and manipulators.