Improving Grip Stability Using Passive Compliant Microspine Arrays for Soft Robots in Unstructured Terrain

TL;DR

This work tackles the grip stability challenge of soft robots operating on unstructured terrain by introducing passive, compliant microspine arrays integrated into motor-tendon actuated soft robots. A single-material compliant microspine mechanism enables a two-row stacked array whose soft-compliant anchoring to a silicone limb allows independent spine engagement controlled by one actuator, reducing control complexity. Through field tests on concrete, brick, sand, and forest floor, the approach demonstrates substantial gains in planar displacement and grip stability compared with a baseline soft robot lacking microspines. The results indicate improved terrain traversability for mobile soft robots and point to future work on optimizing array configurations and extending generalizability to other prototypes.

Abstract

Microspine grippers are small spines commonly found on insect legs that reinforce surface interaction by engaging with asperities to increase shear force and traction. An array of such microspines, when integrated into the limbs or undercarriage of a robot, can provide the ability to maneuver uneven terrains, traverse inclines, and even climb walls. Conformability and adaptability of soft robots makes them ideal candidates for these applications involving traversal of complex, unstructured terrains. However, there remains a real-life realization gap for soft locomotors pertaining to their transition from controlled lab environment to the field by improving grip stability through effective integration of microspines. We propose a passive, compliant microspine stacked array design to enhance the locomotion capabilities of mobile soft robots, in our case, ones that are motor tendon actuated. We offer a standardized microspine array integration method with effective soft-compliant stiffness integration, and reduced complexity resulting from a single actuator passively controlling them. The presented design utilizes a two-row, stacked microspine array configuration that offers additional gripping capabilities on extremely steep/irregular surfaces from the top row while not hindering the effectiveness of the more frequently active bottom row. We explore different configurations of the microspine array to account for changing surface topologies and enable independent, adaptable gripping of asperities per microspine. Field test experiments are conducted on various rough surfaces including concrete, brick, compact sand, and tree roots with three robots consisting of a baseline without microspines compared against two robots with different combinations of microspine arrays. Tracking results indicate that the inclusion of microspine arrays increases planar displacement on average by 15 and 8 times.

Figures (12)

• Figure 1: Surface adaptability vs. design complexity with various microspine robots asbeck_scaling_2006wang_palm_2016parness_lemur_2017liu_novel_2019hu_inchworm-inspired_2019Daltorio_Wei_Horchler_Southard_Wile_Quinn_Gorb_Ritzmann_2009Spenko_Haynes_Saunders_Cutkosky_Rizzi_Full_Koditschek_2008 compared against one of the presented designs in this work, 1ML.
• Figure 2: Conformable nature of soft limbs enable traversal over large tree root present on forest floor surface.
• Figure 3: Compliant microspine mechanism design. a) A hinge joint enables passive compliance. b) Holes embedded on the righthand side facilitate mechanical integration into the soft limb. c) A rigid microspine is inserted in a center channel matching the spine topology set halfway into the mechanism with contact angle of $\alpha=45\degree$
• Figure 4: Two-row, stacked array configuration. a) Close-up of the microspines gripping onto a non-uniform rock. b) where all six of the microspines on the bottom row are engaged (green) while the top row remains disengaged (red). c) Close-up of the microspines gripping onto a steeper rock. d) where two of the bottom row and three of the top row microspines are engaged with the surface.
• Figure 5: Modular ends of the mold enable soft-rigid integration. a) A baseline robot mold. b) A modified mold that allows different configurations of microspine arrays per limb. c) Microspine compatible end mold with holes for a two-row stacked microspine array configuration. d) End mold with integrated microspine mechanisms and ready for casting.