Snake robot’s poor 3-D obstacle traversal reveals snake’s better stability mechanisms

Snakes must often move through complex 3-D terrains like rocks and felled trees with large height changes and wide gaps. Despite snake studies on flat ground, granular media, and branches, the physics of snake-like locomotion in complex 3-D terrain is less known. Recently we discovered that snakes can traverse large steps up to 1/3 body length tall and large gaps up to 1/2 body length wide. Here, we developed a snake robot as a physical model to better understand complex 3-D terrain traversal. The snake robot’s ability to deform its body both laterally and vertically enabled it to achieve similar overall body deformations to that of snakes. A one-direction ratchet wheel mechanism enabled anisotropic frictional profile similar to that of snake scales moving on 2-D surfaces. Despite using these animal kinematics and 2-D friction anisotropy, the snake robot still failed to traverse. As it progressed forward to bridge onto large steps or across large gaps, the robot frequently flipped over (over 90% probability). Closer comparison between the animal and robot revealed that snake’s bottom-wide body cross section shape and ability to locally deform each “segment” for better ground contact via active control and body compliance contributed to its better stability over the robot.