Principles of appendage design in robots and animals determining terradynamic performance on flowable ground

Abstract

Natural substrates like sand, soil, leaf litter and snow vary widely in penetration resistance. To search for principles of appendage design in robots and animals that permit high performance on such flowable ground, we developed a ground control technique by which the penetration resistance of a dry granular substrate could be widely and rapidly varied. The approach was embodied in a device consisting of an air fluidized bed trackway in which a gentle upward flow of air through the granular material resulted in a decreased penetration resistance. As the volumetric air flow, Q, increased to the fluidization transition, the penetration resistance decreased to zero. Using a bio-inspired hexapedal robot as a physical model, we systematically studied how locomotor performance (average forward speed, v(x)) varied with ground penetration resistance and robot leg frequency. Average robot speed decreased with increasing Q, and decreased more rapidly for increasing leg frequency, ω. A universal scaling model revealed that the leg penetration ratio (foot pressure relative to penetration force per unit area per depth and leg length) determined v(x) for all ground penetration resistances and robot leg frequencies. To extend our result to include continuous variation of locomotor foot pressure, we used a resistive force theory based terradynamic approach to perform numerical simulations. The terradynamic model successfully predicted locomotor performance for low resistance granular states. Despite variation in morphology and gait, the performance of running lizards, geckos and crabs on flowable ground was also influenced by the leg penetration ratio. In summary, appendage designs which reduce foot pressure can passively maintain minimal leg penetration ratio as the ground weakens, and consequently permits maintenance of effective locomotion over a range of terradynamically challenging surfaces.

The dynamics of legged locomotion in heterogeneous terrain: universality in scattering and sensitivity to initial conditions

“…We used 1 mm diameter poppy seeds as the model fine grains in this study, but this ground stiffness control technique can be applied to a large variety of granular media, with particle diameter ranging from hundreds of microns to a few millimeters. The resistance of the granular substrate created using this method is highly repeatable, and could be varied over a large range, from greater than that of close packed compaction states [12] to a zero resistance state [13]. Also, as previously measured in intrusion tests, the results obtained using these model granular substrates can be applied to more complex natural granular media composed of grains with greater polydispersity and angularity [3].…”

Cockroaches traverse crevices, crawl rapidly in confined spaces, and inspire a soft, legged robot

“…First, an improved understanding of foot contact mechanics is needed to provide insight into the complexities of thrust generation beyond simple coulomb friction-type attachments that include interlocking (14,54,55), adhesion (56)(57)(58), and resistive forces in granular media (7,59). Second, a leg actuation model that more effectively captures dependence of force production on such factors such as leg morphology, lever and transmission systems, and muscle mechanics (21).…”

Thermal properties of an impurity immersed in a granular gas of rough hard spheres

“…Moreover, the activity of this research field has grown enormously over the last decades [4][5][6][7], becoming an important area of study in the field of soft matter physics [8,9]. In particular, the control of granular dynamics has a large impact on a variety of industries, including some high-technology related interesting studies [10][11][12]. It is worth to mention also that one of the most ubiquitous problems in granular dynamics related industry is the selection of mechanically peculiar particles in a set of otherwise identical grains; i.e., granular segregation [13].…”