I. Introduction
The idea of passive dynamic walking, which is powered by gravity only, was pioneered by McGeer more than a decade ago [21]. Since then, passive walking has received considerable attention in the robotic and control communities[11] [19], [25]. Existence and stability properties of symmetric and asymmetric gaits for compass-like robots are demonstrated under several active control schemes[12] [20], [24]. With a small active power source substituting for gravity, robots based on passive-dynamics consume less energy, yet walk more naturally on level ground[9].