Proposes using two under-actuated robotic systems as two legs of a walking machine. Since the ankle joints of both legs have no actuation, the walking machine remains upright only when it is balanced via a computer-controller. The design of a stabilizing controller for one under-actuated leg, using a μ-synthesis approach, is explained in detail. The system at the equilibrium can be represented by a non-minimum phase system. The right half plane zero severely limits the achievable closed-loop bandwidth and robustness in modeling errors.