  Much of the Earth and many surfaces of extraterrestrial bodies are composed
of in-cohesive particle matter. Locomoting on granular terrain is challenging
for common robotic devices, either wheeled or legged. In this work, we discover
a robust alternative locomotion mechanism on granular media -- generating
movement via self-vibration. To demonstrate the effectiveness of this
locomotion mechanism, we develop a cube-shaped robot with an embedded vibratory
motor and conduct systematic experiments on diverse granular terrains of
various particle properties. We investigate how locomotion changes as a
function of vibration frequency/intensity on granular terrains. Compared to
hard surfaces, we find such a vibratory locomotion mechanism enables the robot
to move faster, and more stable on granular surfaces, facilitated by the
interaction between the body and surrounding granules. The simplicity in
structural design and controls of this robotic system indicates that vibratory
locomotion can be a valuable alternative way to produce robust locomotion on
granular terrains. We further demonstrate that such cube-shape robots can be
used as modular units for morphologically structured vibratory robots with
capabilities of maneuverable forward and turning motions, showing potential
practical scenarios for robotic systems.
