Experimental investigations on the conditions where radial expansion benefits soil penetration for plant root-inspired diggers

Pre-print, accepted version of the paper published on the conference proceedings 2024 IEEE International Workshop on Metrology for Agriculture and Forestry (MetroAgriFor) DOI: https://ieeexplore.ieee.org/abstract/document/10948747 Abstract: Bioinspired robotics has gained a growing interest in soil exploration applications in recent years. In this context, plant roots represent an important model due to their ability to efficiently move under high-stress conditions thanks to their indeterminate growth from the tip, radial expansion, circumnutation movements, and mucus production. This study investigates how an artificial probe that imitates plant root radial expansion at its tip performs during soil penetration. Performances in penetration pressure obtained by a probe that combines axial movement and radial expansion are compared to those of a probe moving by axial movement only. Penetration tests are performed in chambers filled with cohesionless granular medium (i.e., Hostun sand) at different initial densities to verify how the radial expansion effect varies with changing soil compactness levels. Experimental results suggest important advantages in including radial expansion in the design of artificial systems exploring dense media since it implies lower actuation system specifications than what is required by a system that does not perform radial expansion. This study provides important guidelines for the design of performant underground exploratory robotic systems given different soil conditions.