On the stability of bipedal walking

Stability of bipedal locomotion is analyzed using a model of a planar biped written in the framework of systems with unilateral constraints. Based on this model, two different stable walking gaits are derived: one which fulfills the widely used criterion of the Zero Moment Point (ZMP) and another one violating this criterion. Both gaits are determined using systematic model-based designs. The model and the two gaits are used in simulations to illustrate conservatisms of two commonly used methods for stability analysis of bipedal walking: the ZMP criterion and Poincare return map method. We show that none of these two methods can give us a general qualification of bipedal walking stability