This biped was invented as a platform for a humanoid robotic
SUMO contest, but it is too lightweight for that purpose. The
robot can walk and turn using just two motors, so the third
could be used for moving the arms.
The biped walks by swinging the weight (the NXT) from side to
side at a rate that is the natural pulsation of the mechanical
system, an inverted pendulum: resonance allows using only
a few energy to get big oscillations.
To keep the legs movement in sync with the oscillation of the
NXT, the swinging motor angle is measured. A touch sensor acts
as limit switch to reset the upper body at program start.
If you add the arms to make it a real combat robot, let me know!