Bio-inspired Helical Burrowing Robots in Granular Media: experimental observations

Erodium seed awns are moisture sensitive. They coil and uncoil in response to the humidity change in the environment. This coiling-uncoiling motion drives a seed to bury itself like a corkscrew. We present a seed-awn inspired self-burrowing robot which is driven by two augers and stabilized by a fin structure in between. Its burrowing behavior was investigated in the laboratory. Different combinations of auger geometry (handedness, pitch) and control strategies (rotational speed and direction) resulted in different burrowing speeds. To unify the observed burrowing behaviors, the propulsive and resistive forces were examined using additional thrusting and dragging tests. It is observed that rotation of an auger not only reduces the resistive force, but also generates a propulsive force, which depends on the auger geometry, the rotational speed and direction. The burrowing behavior of the robot can thus be predicted from the geometry-rotation-force relations.