  The ability of legged systems to traverse highly-constrained environments
depends by and large on the performance of their motion and balance
controllers. This paper presents a controller that excels in a scenario that
most state-of-the-art balance controllers have not yet addressed: line walking,
or walking on nearly null support regions. Our approach uses a low-dimensional
virtual model (2-DoF) to generate balancing actions through a previously
derived four-term balance controller and transforms them to the robot through a
derived kinematic mapping. The capabilities of this controller are tested in
simulation, where we show the 90kg quadruped robot HyQ crossing a bridge of
only 6 cm width (compared to its 4 cm diameter spherical foot), by balancing on
two feet at any time while moving along a line. Lastly, we present our
preliminary experimental results showing HyQ balancing on two legs while being
disturbed.
