Exploring the influence of diameter-to-length ratios on soft robot performance in soil environments

Soft robotics holds large promises for precision agriculture, as it has the potential to overcome challenges associated with subsoil exploration, including precision maneuvering, and minimizing soil disturbance. While prior studies have evaluated soft robots in tubes, glass beads, or sand, we perform empirical measurements and validations in real soil environments and show how the diameter-to-length (D/L) ratio affects the penetration performance, taking inspiration from earthworms' D/L ratios. Through a series of experiments, this study investigated the locomotion capabilities and the behavior of various robot designs when submerged within soil. We fabricated five pneumatically-actuated soft robots with different D/L ratios, measured their vertical burrowing depth, their axial force generation, and characterized the total drag force versus depth. We introduced a capability ratio comparing available axial force to experienced total drag at depth, giving an estimate of the theoretical depth that each design could reach before stalling. We evaluated buckling forces using Euler limits under mixed end conditions representing soil confinement and available lateral support. Results show that designs with D/L ratios similar to that of an earthworm exhibit lower drag forces, but D/L is not the only parameter influencing performance. The largest-diameter design generated the highest axial force but underperformed due to buckling, while the mid-range geometry achieved the greatest depth through a balanced combination of axial force generation, drag, and stability. These findings highlight the importance of tailored design parameters for efficient subsoil exploration, the relevance of mimicking natural proportions, and demonstrate that force-based metrics can provide practical performance estimates for soft burrowing robots.