Normalized Criteria and Comparative Analysis of Legged Stability

Stepping is a vital strategy for legged systems to recover balance while they interact with their environment. This work presents normalized criteria for the analysis of legged system stability based on balanced and steppable regions. These criteria are applied to both a comprehensive region-based analysis of the contact transitions during a normal human step cycle and a comparative analysis of humanoid and human systems. In this work, the steppable, single support balanced, and double support balanced regions are evaluated for a human model with full-order system dynamics and joint actuation limits based on biomechanical models of maximum voluntary joint torques. The normalized regions are also compared with the capturability of an equivalent reduced-order model in order to demonstrate the role of steppability as a general extension of and complementary concept to capturability. In the comparative analysis of the humanoid and human systems, the normalized regions are used to directly compare the capability for balance recovery in both systems. The proposed approaches can be generalized beyond sagittal planar walking to analyze balance stability in any multi-contact scenario where stepping or step-like contact transitions can occur.