Mole crab-inspired vertical self-burrowing

Abstract

We present EMBUR—EMerita BUrrowing Robot—the first legged robot inspired by the Pacific mole crab, Emerita analoga, capable of burrowing vertically downward. We choose Emerita analoga as a model organism for its rapid downward burrowing behaviors, as it is four times as fast as the most rapid bivalve mollusk. Vertical burrowing in granular media is a challenging endeavor due to the tendency for the media to create upwards resistive forces on an intruder, even during purely horizontal motions. Our robot is capable of vertically burrowing its body in granular substrate primarily through excavation using two leg pairs, which are functionally analogous to groupings of leg pairs of the mole crab. We implement a novel leg mechanism with a sweeping trajectory, using compliant fabric to enable an anisotropic force response. The maximum resistive force during the power stroke is 6.4 times that of the return stroke. We compare robot body pitch and spatial trajectories with results from biomechanical studies of the mole crabs. We characterize the sensitivity of the robot to initial depth, body pitch and leg pose, and propose bounds on initial conditions which predict various burrowing failure modes. Parametric studies utilizing Granular Resistive Force Theory inform our understanding of robot behavior in response to leg phasing and orientation. Not only does this robotic platform represent the first robophysical model of vertical mole crab-inspired burrowing, it is also one of the first legged, primarily excavative small-scale burrowing agents.

Fundamentals of burrowing in soft animals and robots

“…This burrowing mechanism is limited to the surface layer of sand; mole crabs generally burrow only deep enough to cover themselves but leave feeding appendages exposed. This is similar to the deepest depths achieved by a mole-crab inspired robot with counter-rotating legs in beads ( Treers et al, 2022 ). Authors suggest that achieving deeper depths may be possible, even in natural soils, with a combination of higher torque actuators, additional fluidization strategies, or local inertial effects.…”

“…Fluidization also is used by rapidly burrowing mole crabs that live in wave-swept sandy beaches; they use rapid movements of their legs to push grains backwards as they quickly burrow down into the sand ( Trueman, 1970 ). Mole crabs have inspired robotic solutions that excavate with legs ( Russell, 2011 ; Teers et al, 2022 ). This burrowing mechanism is limited to the surface layer of sand; mole crabs generally burrow only deep enough to cover themselves but leave feeding appendages exposed.…”

Maneuvering on non-Newtonian fluidic terrain: a survey of animal and bio-inspired robot locomotion techniques on soft yielding grounds

“…For this purpose, they use four pairs of legs to scrape the material off the ground. During the digging motion, legs are successively extended or retracted, hence creating motion anisotropy and enabling the body to burrow ( Trueman, 1970 ; Faulkes and Paul, 1997 ; Treers et al., 2022 ). This animal is also suspected of using fluidization, which will be described later.…”

“…The arms, using an anisotropic motion, are perpendicular to the material while progressing and parallel to the body during the swing phase. In Treers et al. (2022) , a mole crab-inspired robot was built, able to dig itself through the sand by statically moving the sand from below to the top.…”

Efficient reciprocating burrowing with anisotropic origami feet

“…These various types of movements have inspired the development of robotic systems, including robotic snakes ( Marvi et al, 2014 ) with sidewinding and flipper-driven ( Mazouchova et al, 2013 ) walking on the surface of the granular media. Underneath the surface of granular media, self-burrowing of a mole crab-inspired robot with legs ( Treers et al, 2022 ), burrowing with underactuated appendages resulting in an asymmetric profile between power and return strokes ( Chopra et al, 2023 ), and growing with a root-like robot ( Naclerio et al, 2021 ) all show successful burrowing. More reviews on bioinspired robotic burrowers can be found in ( Wei et al, 2021 ).…”