A Study of the Relationship between a Mechanical Coupling Metric and Gait Characteristics for an Ankle-Actuated Biped Robot

Abstract

This paper presents a study of the relationship between the magnitude of coupling between the actuated and unactuated degrees of freedom for an ankle-actuated biped robot and the robustness and cost of transport of the gait. It extends prior results that considered only the instantaneous coupling at individual poses to analyze the cumulative effects of coupling over entire trajectories. The coupling metric is general in that it can be computed directly from the Lagrangian of the system. By using a two-link biped model, a family of ankle-actuated candidate gaits defined by fourth-order Bezier polynomials is generated, and the feasibility of the gaits is verified in the sense that the unilateral constraint of ground reaction force is satisfied. The correlation between the coupling metric and the maximum magnitude of disturbance that can be rejected is significant. It indicates that robust gaits tend to have small coupling under zero disturbance so that the “reserve” coupling may be utilized to reject the disturbance. Moreover, gaits with smaller cost of transport under zero disturbances have smaller coupling and therefore should be more robust.