The Dynamics of Wheeled Locomotion in Granular Media

We have developed an automated experimental setup for studying the dynamics of wheeled locomotion in dry granular substrates. The apparatus enables systematic tests of quasistatic rheological models of intruder-substrate interaction, such as Resistive Force Theory and Terramechanics, in dynamic regimes where inertial effects can cause deviation from theory and changes in the rheological behavior. For a rigid wheel with circumferential protrusions (grousers), we observe that increased wheel rotational acceleration α leads to increased sinkage into the media during transit, suggesting transient effects cause the granular media to fail under rapid shear. Low α and intermediate terrain pressure can minimize wheel slippage and maintain constant wheel sinking depth d over time. For a given final constant rotational speed ω, increasing the ramp-rate to ω results in increased d and slip (the normalized ratio between translational and rotational speed) relative to a slower ramp, suggesting reduced reaction force when the grains under the wheel are more quickly accelerated from rest. Wheel slip and d also sharply increase above a threshold in ω. These transient effects show that time-independent models are insufficient for capturing high inertia wheeled locomotion.