A new robophysical model for limbless locomotion reveals the importance of passive dynamics in obstacle navigation.

Limbless animals like snakes and nematode worms move in complex terrain via propagation of body bending waves generated by alternating unilateral muscle activation. A previously studied desert specialist snake relies on passive body buckling, facilitated by unilateral activation, to traverse sparse obstacles. To discover principles by which passive body buckling can simplify control in complex terrain, we developed a robot (8 joints, 45 cm long) that models biological muscle morphology and head-to-tail propagating wave activation patterns. Each joint generates a bend using a pair of string actuators that shortens wires when active and allows lengthening when inactive. We studied the dynamics and locomotion of the model in both wall-collision and hexagonal lattice transit experiments. During wall collisions, the robot passively reoriented within a few undulation cycles and then moved parallel to the wall. In the lattice experiments, the robot often traversed the lattice without becoming jammed; these dynamics were generated by passive buckling and a spontaneous reversal behavior. The locomotor success of the robophysical model without need for sensory feedback suggests the importance of passive mechanisms in limbless organisms and can enhance robot locomotion in complex terrain.