Modulation of Orthogonal Body Waves Enables Versatile and Rapid Maneuverability in Sidewinding Locomotion

Abstract

Sidewinding rattlesnakes (Crotalus cerastes) are exceptionally maneuverable, even on sand. Straight line sidewinding locomotion of the snakes and sidewinding snake robots can be described by an appropriate phasing of horizontal and vertical body waves. We hypothesized that the high maneuverability of these animals emerged from independent control of the two waves and that the robots' maneuverability could be enhanced by mimicking the methods used by the snakes. To test this hypothesis we collected motion capture data of the snakes, and observed two distinct turning methods: "differential turning" and "reversal turning". In differential turning we observed one end of the snake to move further forward per cycle than the other, leading us to posit that the snakes were imposing an amplitude gradient in the horizontal wave as it propagated posteriorly. In reversal turning, the snake rapidly exchanged lifted and grounded body segments, resulting in a large change of direction without significant body rotation. We hypothesized that the snakes shifted the phase of the vertical wave relative to the horizontal wave by pi. We tested these mechanisms in the robot and generated differential and reversal turning on sand as well as hard ground. Further explorations of two−wave mixing parameters revealed a third turning mode, "frequency turning", not observed in biological snakes. These results show how the relative modulation of two component body waves can result in the emergence of complex behaviors, and that high degree of freedom biological and robotic systems can be controlled and maneuvered using this simple control template.