Rapid Wheeled Locomotion in Dry Granular Media

Wheeled locomotion can be an energy efficient means of traversing ground, but when applied to soft granular terrain, excessive wheel slippage and sinkage due to overly fast wheel actuations can entrap vehicles. We examine the scenarios of a rapidly turning wheel in dry granular media using our lab’s “robophysics” principles by developing an automated experimental apparatus based on terramechanics testbeds. This setup allows us to compare wheeled locomotion experiments to rheological models of intruder-substrate interaction, such as Resistive Force Theory and granular plasticity, in high-speed regimes where rapid shearing can cause changes in the rheological behavior. For a rigid wheel with circumferential protrusions (grousers), we observe that increasing wheel rotational speed leads to sudden increased slippage and sinkage into granular media at a critical rotation speed, after which horizontal speed cannot increase. This phenomenon appears to enforce a kind of speed limit in locomotion through granular media, mediated by the rheological properties of the medium itself.