Bio-inspired tail oscillation enables robot fast crawling on deformable granular terrains

Abstract

Deformable substrates such as sand and mud present significant challenges for terrestrial robots due to complex robot-terrain interactions. Inspired by mudskippers, amphibious animals that naturally adjust their tail morphology and movement jointly to navigate such environments, we investigate how tail design and control can jointly enhance flipper-driven locomotion on granular media. Using a bio-inspired robot modeled after the mudskipper, we experimentally compared locomotion performance between idle and actively oscillating tail configurations. Tail oscillation increased robot speed by 67% and reduced body drag by 46%. Shear force measurements revealed that this improvement was enabled by tail oscillation fluidizing the substrate, thereby reducing resistance. Additionally, tail morphology strongly influenced the oscillation strategy: designs with larger horizontal surface areas leveraged the oscillation-reduced shear resistance more effectively by limiting insertion depth. Based on these findings, we present a design principle to inform tail action selection based on substrate strength and tail morphology. Our results offer new insights into tail design and control for improving robot locomotion on deformable substrates, with implications for agricultural robotics, search and rescue, and environmental exploration.

Figures (8)

• Figure 1: Locomotion challenges on deformable substrates and biologically-inspired solutions. Deformable substrates can cause catastrophic failures for vehicles (A) and robots (B), including sinkage, slippage, or even getting completely stuck. A flipper-driven locomotor, the mudskipper mudskippertail (C), can utilize its tail to effectively move through sand slopes and muddy terrains (D).
• Figure 2: Experimental setup. (A) A mudskipper-inspired robot as a physical model to systematically test the effect of tail oscillation on locomotion performance in granular media. The two front flippers rotate synchronously in the sagittal plane at a constant angular speed, $\omega$, while the tail oscillates horizontally with amplitude $\alpha$ and frequency $f$. (B) Two tail actions were tested: idle tail and oscillating tail. (C) Various tail morphologies were tested, with the tail height, $l$, kept at 40 mm, and the tail support area ( i.e., the projected surface area perpendicular to the penetration direction), $A$, ranging from 2 cm$^2$ to 24 cm$^2$. (D) Experimental apparatus to assess robot locomotion performance on granular media, with four motion capture cameras tracking the robot's forward and backward movements and a lateral video camera recording the tail-terrain interaction.
• Figure 3: Comparison of average forward speed ($v_x$) of the robot between idle and oscillating tails. Results are shown for tail size $A=$16 cm$^2$.
• Figure 4: Percent improvement in averaged forward speed ($\eta = (v_o - v_i) / v_i$) with an oscillating tail compared to the idle tail, for different tail support surface areas, $A$.
• Figure 5: Shear force experiments. (A) Schematic of the force data collection setup. The robot body is dragged horizontally through the granular media over a total distance of 18 cm with a constant velocity of 2 cm/s, at a fixed insertion depth of 1 cm. The shear resistance force exerted on the robot body, ${f_x}$, was measured using a force sensor. (B) Shear resistance force measured from both the oscillating and idle tail conditions, plotted against the shear distance, $s$. (C) Comparison of the averaged shear force, $\bar{f_x}$, between the idle tail and oscillating tail. $\bar{f_x}$ was computed as the averaged shear resistive force, ${f_x}$, between $s$ = 2.5 cm and $s$ = 17.5 cm.