Get a grip: inward dactyl motions improve efficiency of sideways-walking gait for an amphibious crab-like robot

Abstract

Crabs are adept at traversing natural terrains that are challenging for mobile robots. Curved dactyls are a characteristic feature to engage terrain for resisting wave forces in surf zones. Inward gripping motions at the onset of stance could increase stability.Here, we add inward gripping motions to the foot trajectories of walking gaits to determine the energetic costs and speed for our 12 DOF crab-like robot, Sebastian. Specifically, we compared two gaits in which the step size (stance length) was the same, but the swing trajectories were either triangular (to minimize trajectory length) or quadrilateral (in which the leg deliberately oversteps in order to perform a distributed inward grip (DIG). The resulting gripping quadrilateral gait significantly outperformed the non-gripping triangular gait on diverse terrains (hard linoleum, soft mats, and underwater sand), providing between 15% and 34% energy savings. Using this gait eliminates the advantage of spherical end effectors for slip reduction on hard linoleum, which may lead to a better understanding of how to use crab-like morphology for more efficient locomotion. Finally, we subjected the walking robot to lab-generated waves with wave height approximately 166% of the dactyl length. Both gaits enabled the robot to walk undisturbed by the waves. Taken together, these results suggest that impact trajectory will be key for future amphibious robots. Future work can provide deeper understanding of the relationships between dactyls, gaits, and substrates in biology and robots.

Underwater legged robotics: review and perspectives

“…The interaction between ULRs and the sediment is mediated by the feet, however, notwithstanding the great importance of this subsystem, little attention has been dedicated to this topic (figure 4(d)). Researchers from the Crab Lab of the Case Western Reserve University have recently focused on the use of tapered, curved feet (like crab dactyl shape) paired with a distributed inward gripping method, to allow the amphibious hexapod Sebastian (figure 3(B)) to anchor to the sediment and improve its speed and cost of transport on different kinds of sediment and in the presence of waves [66,92]. Similarly, even though never demonstrated on ULRs, the use of microspines [91], can yield excellent anchoring properties, especially on rocks.…”

“…The category of ULRs is very heterogeneous in terms of morphology and size. Prototypes with one [70,71], two [75], four [68,72,89], six [24,84,102,103,107,108], and eight legs [87] were developed, and legs with one [99], two [71,92], three [24,72,109], or more [110] DoFs were employed. Naturally, ULRs with a higher number of DoFs are potentially more versatile and several behaviours can be implemented on the same robot.…”

Optimal planar leg geometry in robots and crabs for idealized rocky terrain

“…A 7.4 V lithium polymer battery powers the motors. A basic gait is described in a companion paper [73] (figure B1 and table B1). We estimate the coefficient of friction between the substrate and the dactyls to be between 0.4 and 0.5 based on [74,75].…”

Probing with Each Step: How a Walking Crab-like Robot Classifies Buried Cylinders in Sand with Hall-Effect Sensors

“…In this paper, we propose the use of a legged crab-like robot with Hall effect-based sensors for UXO and non-UXO discrimination. In previous work, we have shown that hexapod robots with crab-inspired limbs are suitable for navigation in shallow, turbid water [ 11 ], making them suitable platforms for incorporating underwater UXO sensing strategies. We have also shown that Hall-effect magnetometers measuring the field of a magnet embedded in compliant robot limbs can be used to classify the terrain from measurements taken as the robot dynamically interacts with the terrain [ 12 ].…”