Walk-Burrow-Tug: Legged Anchoring Analysis Using RFT-Based Granular Limit Surfaces

We develop a new resistive force theory based granular limit surface (RFT-GLS) method to predict and guide behaviors of forceful ground robots. As a case study, we harness a small mobile robotic system – MiniRQuad (296 g) – to ‘walk-burrow-tug;’ it actively exploits ground anchoring by burrowing its legs to tug loads. RFT-GLS informs the selection of efficient strategies to transport sleds with varying masses. The granular limit surface (GLS), a wrench boundary that separates stationary and kinetic behavior, is computed using 3D resistive force theory (RFT) for a given body and set of motion twists. This limit surface is then used to predict the quasi-static trajectory of the robot when it fails to withstand an external load. We find that the RFT-GLS enables accurate force and motion predictions in laboratory tests. For control applications, a pre-composed state space map of the twist-wrench pairs enables computationally efficient simulations to improve robotic anchoring strategies.