Horizontal Burrowing in Granular Media by Breaking Symmetries

Self-burrowing robots are a class of robot that can move underground. These robots can find applications in geotechnical site investigation, sensor deployment, save and rescue, and construction. Breaking the symmetry is believed as the key to burrowing in the granular medium. In this study, we proposed, fabricated, and tested burrowing robots with a minimalistic and modular design. The robot mainly consists of a tip (flat, cone, or auger), and a pair of cylindrical parts. The robot can achieve extension-contraction with the utilization of a linear actuator and have options for tip rotation with an embedded gear motor. With the flat tip, the robot was almost symmetric and had reciprocal kinematics, so it could hardly move. With a cone tip, the robot was no longer symmetrical, and it could move horizontally through cyclic extension-contraction. Introducing cone tip rotation during the extension period breaks the symmetric kinematics and could increase the burrowing speed of the robot. The burrowing speed was the highest for the robot with an auger tip that rotated during the extension period. All the asymmetries contributed to the difference in resistance during the robot movement. A generic load-displacement-based model was proposed to quantify the effect of asymmetries and predict the movement of developed burrowing robots.