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, , 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, ) varied with ground penetration resistance and robot leg frequency. Average robot speed decreased with increasing , and decreased more rapidly for increasing leg frequency, . 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 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.
Highlights
Legged locomotor performance depends sensitively on substrate properties, locomotor morphology and gait
We develop a universal scaling model that successfully captures the kinematics of legged robot locomotion performance for low resistance granular states, and we show that our model can be further extended to explain locomotor performance of animals with more complex morphologies and gaits
We characterize the effect of leg frequency and foot size on locomotor performance, and develop a theoretical model that captures normalized locomotor speed on low resistance ground regardless of variation in morphology and gait
Summary
Legged locomotor performance depends sensitively on substrate properties, locomotor morphology and gait. Deformable substrates like loose sand, new snow, mud, and leaf litter can be challenging due to their low penetration resistance, which we define here as the vertical ground resistance force per depth during intrusion During interaction, such substrates can yield and flow, producing complex and dynamic interactions that can result in poor locomotor performance. Scoparia and C. draconoides in their natural habitat substrates, dune and wash sand, and investigated whether habitat distribution and the presence of toe fringes contributed to performance differences These studies provide a better understanding of how morphology and kinematics can contribute to locomotor performance on flowable ground, but the substrate resistances were not systematically varied. Locomotor responses to low resistance flowable substrates were not explored
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