Exploring the role of passive mechanics in limbless locomotors via a novel robophysical snake.

Animals like snakes use traveling body bends to move in terrestrial terrain. Previously, we discovered that passive body buckling, facilitated by unilateral muscle activation, allowed negotiation of sparse obstacles without additional control input in the snake (C. occipitalis) [Schiebel et al., 2019]. To explore the implications of this scheme we developed a robophysical model with passive mechanical flexibility. Most snake robots precisely control the position of each joint with a single actuator. In contrast, the actuation in our robot is modeled after biological snakes, in which pairs of muscles, one on each side of the spine, create body bends by contracting unilaterally. The robot has 8 joints and 16 motors, each joint has two motor-driven pulleys on opposite sides of the body. The pulleys increase curvature by shortening a wire attached to the adjacent joint. Opposite an active motor, the pulley is completely unspooled; pairs of motors can resist forces which would lengthen active wires but not those pushing them shorter, allowing the body to passively buckle during certain obstacle collisions. Preliminary tests demonstrate that a travelling sinusoidal wave of joint angles allows locomotion using wheels (which provide directional friction).