Undulatory swimming in sand: experimental and simulation studies of a robotic sandfish

Abstract

A previous study of a sand-swimming lizard, the sandfish, revealed that it swims within granular media at speeds up to 0.4 body-lengths/cycle using body undulations (approximately a single period sinusoidal traveling wave) without limb use. Inspired by the organism, we develop a numerical model of a robot swimming in a simulated granular medium to guide the design of a physical device. Both in simulation and experiment the robot swims limblessly subsurface at speeds up to 0.3 body-lengths/cycle and, like the animal, increases its speed by increasing its oscillation frequency. The performance of the robot measured in terms of its wave efficiency η, the ratio of its forward speed to wave speed, is 0.34 ± 0.02, within 8% of the simulation prediction. Both in simulation and experiment, η increases with increasing particle-particle friction but decreases with increasing body-particle friction. On a flat, rigid surface the robot fails to move forward, as expected, due to the frictional isotropy between the interacting surfaces. However, the surface and subsurface performance of the robot on low friction particles are comparable. Our work provides a validated simulation tool and the design of a robot that can move on or within yielding terrestrial substrates.

Environmental interaction influences muscle activation strategy during sand-swimming in the sandfish lizard Scincus scincus

“…A previous study of sandfish showed that η is independent of particle size over an order of magnitude (Maladen et al, 2011b). Also, simulation of sandfish has shown that graingrain friction has a small effect on sandfish η while body-grain friction has a larger effect (Maladen et al, 2011a). Increasing particle-particle friction by 50% caused a 14% increase in η while increasing particle-body friction by 50% caused an 84% decrease in η.…”

“…Korta and colleagues found that mechanosensory input may act to regulate this temporal frequency and allow gait adaptation in response to different surroundings (Korta et al, 2007). Studies of our sandfish models (Maladen et al, 2011a;Maladen et al, 2011b;Maladen et al, 2011c) could be used to determine the extent of muscle power reduction as a result of decreasing frequency with depth for a sandswimmer.…”

Mechanical models of sandfish locomotion reveal principles of high performance subsurface sand-swimming

“…No tapering in height of the body along its length is considered, as doing so results in the model rising as it moves forward owing to drag-induced granular lift. This lift results from the vertical component of the normal force on an inclined surface dragged through a granular medium and is discussed in Ding et al [50] and Maladen et al [51]. We perform simulations for a square tube body shape, and to more closely match the animal we perform simulations using a model tapered in the coronal plane (figure 3a) with width varying linearly from the snout tip to 1/6 bl, and from 3/5 bl to the tail tip.…”

“…For the smooth robot, the animal in 0.3 mm (loosely and closely packed media) [22] and 3 mm particles ( §2), and in model predictions ( § §3 and 4) in simulated 3 mm glass particles, the values of h are all close to 0.5. This suggests that subsurface locomotion in granular media is largely independent of media properties like particle size and density [51].…”

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

“…Locomotion in confined environments offers several challenges for animals (18) that include limitations due to body shape changes (19,20), restricted limb mobility (21), increased body drag, and reduced thrust development (22). Examining the motion repertoire of soft-bodied animals (23), such as annelids (19), insect larvae (24), and molluscs (25), has offered insight into a range of strategies used to move in confined spaces.…”