Parapodia inspired soft appendages enable robot propulsion in granular media

Along with the high penetration forces, the characteristic nature of granular media varying between a fluid and a solid poses a challenging environment for locomotion and navigation of robots. Many animals like bristle worms, sea turtles, and mole crickets use asymmetric movements of their soft appendages (parapodia in bristle worms) to efficiently burrow in granular media. Soft robots can undergo large deformations relying on intrinsic compliance but their interaction with granular media is not well understood. Here, we investigate the role of inherent bending stiffness of soft appendages for thrust generation in robots. We design the appendage with passive anisotropic stiffness such that it experiences asymmetric drag from the granular media leading to forward propulsion. We present experimental results where the appendage is oscillated in a controlled granular environment and show that propulsion is highly dependent on the bending stiffness, increasing with the increase in stiffness until it reaches a maximum value and then starts decreasing. We demonstrate the use of these appendages on a functional robot with two appendages for steering and propulsion. Our results advance the understanding of appendage enabled robot burrowing as well as the forces involved in the interaction