Characterization and Correlation of Robotic Snake Scale Friction and Locomotion Speed

Abstract

Snake robots are inspired by the ability of biological snakes to move over rock, grass, leaves, soil, up trees, along pavement and more. Their ability to move in multiple distinct environments is due to their legless locomotion strategy, which combines distinct gaits with a skin that exhibits frictional anisotropy. Designing soft robotic snakes with similar capabilities requires an understanding of how this underlying frictional anisotropy should be created in engineered systems, and how variances in the frictional anisotropy ratio affect locomotion speed and direction on different surfaces. While forward and backward frictional ratios have been characterized for previous scale designs, lateral friction and the associated ratios are often overlooked. In this paper, our contributions include: (i) the development of a novel articulated pseudo-skin design that is modular, easy to construct and has removable or replaceable scales; (ii) experimental measurement of the frictional characteristics of otherwise-identical scales at varying angles of attack (15°, 25°, 35°, 45°) on different surfaces of interest (grass, bark, smooth surface, carpet);(iii) separate measurements of locomotion speed for each angle and surface. Consequently, while we observed some consistent trends between frictional coefficients and scale angle, aligning with literature and intuition, we were not able to consistently identify expected correlations between frictional ratios and locomotion speed. We conclude that either frictional ratios alone are not sufficient to predict the observed speed of a snake robot, or that specific measurement approaches are required to accurately capture these ratios.

Figures (7)

• Figure 1: The snake robot ((a) top view; (b) side view; (c) scales) used herein has an articulated backbone and is actuated by air-driven McKibbens. The modular scale system is attached to the ribs extending from each vertebra.
• Figure 2: Vertebra, ribs, and scale structure forms one segment of the robot (a). Primary, secondary, and tertiary units assembled as one scale structure, and multiple scale structures connected through interlocking mechanism (b). Right-side actuation, neutral (no-actuation), left-side actuation scenarios and air circulation scheme inside actuators (c). Assembly details (d).
• Figure 3: Capping procedure for McKibben actuators: the cap is bonded to the sheath using a 2-part resin, requiring a mold to both contain the resin and protect the sheath. The sheath is inserted through the mold, and then the cap is inserted, before pulling the assembly down into the occlusion mold. Resin is then added and left to cure prior to demolding.
• Figure 4: Experimental test setups for measuring (a) forward and backward, and (b) lateral friction. The displacement was transmitted to the robot via a pulley mechanism, with reaction forces recorded using a 100 N load cell.
• Figure 5: Boxplots illustrate $\mu_{f}$ (red), $\mu_{b}$ (orange), and $\mu_{l}$ (blue) distributions for each scale angle (15$^\circ$, 25$^\circ$, 35$^\circ$, 45$^\circ$) - surface (carpet, bark, smooth surface, grass) pair.