  Bristle bots are vibration-driven robots actuated by the motion of an
internal oscillating mass. Vibrations are translated into directed locomotion
due to the alternating friction resistance between robots' bristles and the
substrate during oscillations. Bristle bots are, in general, unidirectional
locomotion systems. In this paper we demonstrate that motion direction of
vertically vibrated bristle systems can be controlled by tuning the frequency
of their oscillatory actuation. We report theoretical and experimental results
obtained by studying an equivalent system, consisting of an inactive robot
placed on a vertically vibrating substrate.
