  This works presents the theoretical analysis and experimental observations of
bidirectional motion of a millimeter-scale bristle robot (milli-bristle-bot)
with an on-board piezoelectric actuator. First, the theory of the motion, based
on the dry-friction model, is developed and the frequency regions of the
forward and backward motion, along with resonant frequencies of the system are
predicted. Secondly, milli-bristle-bots with two different bristle tilt angles
are fabricated, and their bidirectional motions are experimentally
investigated. The dependency of the robot speed on the actuation frequency is
studied,which reveals two distinct frequency regions for the forward and
backward motions that well matches our theoretical predictions. Furthermore,
the dependencies of the resonance frequency and robot speed on the bristle tilt
angle are experimentally studied and tied to the theoretical model. This work
marks the first demonstration of bidirectional motion at millimeter-scales,
achieved for bristle-bots with a single on-board actuator.
