Sidewinding with minimal slip: Snake and robot ascent of sandy slopes

Abstract

Limbless organisms like snakes can navigate nearly all terrain. In particular, desert-dwelling sidewinder rattlesnakes (C. cerastes) operate effectively on inclined granular media (like sand dunes) that induce failure in field-tested limbless robots through slipping and pitching. Our laboratory experiments reveal that as granular incline angle increases, sidewinder rattlesnakes increase the length of their body in contact with the sand. Implementing this strategy in a physical robot model of the snake enables the device to ascend sandy slopes close to the angle of maximum slope stability. Plate drag experiments demonstrate that granular yield stresses decrease with increasing incline angle. Together these three approaches demonstrate how sidewinding 1 arXiv:1410.2945v1 [physics.bio-ph] 11 Oct 2014 with contact-length control mitigates failure on granular media.The majority of terrestrial mobile robots are restricted to laboratory environments, in part because such robots are designed to roll on hard flat surfaces. It is difficult to systematically improve such terrestrial robots because we lack understanding of the physics of interaction with complex natural substrates like sand, dirt and tree bark. We are thus limited in our ability to computationally explore designs for potential all-terrain vehicles; in contrast, many of the recent developments in aerial and aquatic vehicles have been enabled by sophisticated computationaldynamics tools that allow such systems to be designed in silico (1).Compared with human-made devices, organisms such as snakes, lizards, and insects move effectively in nearly all natural environments. In recent years, scientists and engineers have sought to systematically discover biological principles of movement and implement these in robots (2). This "bioinspired robotics" approach (3) has proved fruitful to design laboratory robots with new capabilities (new gaits, morphologies, control schemes) including rapid running (2, 4), slithering (5), flying (6), and swimming in sand (7). Fewer studies have transferred biological principles into robust field-ready devices (4, 8) capable of operating in, and interacting with, natural terrain.Limbless locomotors like snakes are excellent systems to study to advance real-world allterrain mobility. Snakes are masters of most terrains: they can move rapidly on land (9, 10) and through water (11), burrow and swim through sand and soil (12), slither through tiny spaces (13), climb complex surfaces (14), and even glide through the air (15). Relative to legged locomotion, limbless locomotion is less studied, and thus broad principles which govern multi-environment movement are lacking. Recently developed limbless robotic platforms (5), based generally on the snake body plan, are appealing for multi-functional robotics study because they are also capable of a variety of modes of locomotion. These robots can traverse confined spaces, climb trees and pipes, and potentially dive through loose material. However, 2 the gaits that carry these robots across fir...

Modulation of orthogonal body waves enables high maneuverability in sidewinding locomotion

“…1C) (9). Using this scheme, we showed that sidewinders ascend granular inclines by modulating the vertical wave amplitude to control contact length, and implementing this strategy in a snake robot allowed the device to ascend similar inclines (9).…”

“…1B), thereby enabling minimal slip at other segments and potentially contributing to a low cost of transport (10). This makes sidewinding particularly attractive to explore aspects of multiplane undulation because lifting perpendicular to the plane of the main axial wave is clearly observable and fundamental to this mode of locomotion (5); failure to lift results in locomotor failure in other vipers (9). In addition, field observations show that sidewinders are remarkably maneuverable, capable of rapidly making large direction changes to elude capture.…”

“…model" (9) revealed that despite the anatomical complexity associated with hundreds of body elements and thousands of muscles (11) biological sidewinding could be simply modeled as a combination of two axial waves: horizontal and vertical axial waves with identical spatial and temporal frequency but different amplitudes offset by a phase difference ðϕÞ of ±90° (Fig. 1C) (9).…”

“…A peculiar gait called sidewinding provides an excellent example of the importance of multiplane wave movement in certain snakes (5,9). During sidewinding alternating sections of the body are cyclically lifted from the substrate, moved forward via lateral axial waves, and placed into static contact with the substrate at a new location ( Fig.…”

Evidence of a molecular boundary lubricant at snakeskin surfaces

“…The peculiar locomotion of snakes has inspired engineers for years. For example, robotics researchers are trying to mimic the snake's capability to negotiate a range of challenging terrain types [6]. Research conducted over the last century reveals functional adaptation of both the inner architecture (micron scale) and surface morphology (nanometre scale) of snake skin.…”