Experimental study of soil penetration strategies for earthworm-like robots

Earthworm-like robots represent a promising alternative to conventional soil investigation tools currently used in geotechnics. Their limited invasiveness and ability to navigate in three-dimensions underground are highly desirable properties. Despite the multitude of existing strategies developed for exploring underground soils, the energy improvement is difficult to compare due to various soil conditions and robot tip design. In this study, we present an experimental comparison between burrowing strategies bio-inspired by the motion of earthworms in a fine, dry sand. Results showed that a high aspect ratio (AR) of the tip during burrowing, compared to smaller aspects ratios reduces energy consumption. An AR of 4 reduced the energy required to reach a depth of 200 mm by 65%, with respect to the AR of 1. Additionally, asymmetric geometries did not improve the penetration energy during vertical burrowing while led to energy reduction of nearly 40% for horizontal penetration. Active strategy by vacuuming showed a high reduction in penetration energy by nearly 70% at 200 mm, while fluidization strategy using low pressurized air provided limited advantages. The results highlight the importance of considering both bio-inspired passive and active strategies in the design of burrowing robots to exploit their potential and reach deep soil via three-dimensional motion. This approach should be coupled with online soil characterization tools to tune the strategy by need.