  Snake robots have the potential to maneuver through tightly packed and
complex environments. One challenge in enabling them to do so is the complexity
in determining how to coordinate their many degrees-of-freedom to create
purposeful motion. This is especially true in the types of terrains considered
in this work: environments full of unmodeled features that even the best of
maps would not capture, motivating us to develop closed-loop controls to react
to those features. To accomplish this, this work uses proprioceptive sensing,
mainly the force information measured by the snake robot's joints, to react to
unmodeled terrain. We introduce a biologically-inspired strategy called
directional compliance which modulates the effective stiffness of the robot so
that it conforms to the terrain in some directions and resists in others. We
present a dynamical system that switches between modes of locomotion to handle
situations in which the robot gets wedged or stuck. This approach enables the
snake robot to reliably traverse a planar peg array and an outdoor
three-dimensional pile of rocks.
