A Mole-Inspired Scratch-Digging Robot for Granular Media Traversal

This letter proposes a mole-inspired scratch-digging robot to investigate four-limb subsurface locomotion in granular media. The robot integrated a conical head, a rigid torso, a hybrid crank-rocker and crank-slider forelimb that reproduces scratch-digging strokes, and a two-degree-of-freedom (DOF) closed-chain five-bar hindlimb for propulsion. The forelimb was optimized in ADAMS using a sequential quadratic programming algorithm to enlarge the toe-tip excavation envelope, increasing the excavated area by 33.1% and yielding a clearer retraction phase. For the hindlimb, Bézier-parameterized end-point trajectories were adopted, and link lengths were optimized via particle swarm optimization simulation, by which the reachable workspace was expanded by 15.22%. A prototype with an embedded motion controller was built (193 × 96 × 58 mm, 411 g). A traversal distance of 359 mm was achieved in loose granular media, and stable semi-buried progression was demonstrated in a denser lunar-regolith simulant using a sequenced gait.