Characterizing Mechanical Properties of Natural Deformable Substrates with a Direct-Drive Robotic Leg*

Deformable substrates cover the majority of Earth and planetary surfaces. The mechanical strength and rheological properties of these substrates play a pivotal role in determining traversability, hillslope stability, and planetary exploration success. However, it is often difficult to accurately and rapidly assess substrate strength and rheology in situ. In this study, we created a direct-drive (i.e., gearless) robotic leg with the ability to accurately sense contact forces and measure terrain characteristics, without the need for additional sensor payload. We show from lab and field measurements that the robotic leg could discern subtle variations in mechanics quantities such as penetration resistance, yield stress, brittleness, and resilience. The ability to detect these variations enabled characterizing and understanding the distinct rheological behaviors exhibited by different types of natural terrain materials. Our study highlights the potential for legged robots to use their locomotive limbs as novel terrain sensors, and represents a key step towards the development of terrain-aware legged robots that can map terrain properties by walking.