Mode-Reactive Template-Based Control in Planar Legged Robots

Abstract

Translating the center of mass (CoM) while fixing the orientation of a rigid body supported by relatively massless, actuated limbs is a common problem setting in legged robotics. This paper proposes a hierarchical approach to such maneuvers that decouples CoM task planning from body orientation control in sagittal-plane models, thereby exposing a well-studied and computationally effective low dimensional dynamical system that can be used for CoM task planning. The resulting algorithms directly address the control authority (degree of underactuation) available at a given contact mode, enabling a-priori plans with these intuitive, robust pendular dynamics to be formally embedded at the (virtual) CoM of planar floating-torso models, while focusing high gain posture stabilizing feedback upon the body orientation. A series of numerical and empirical examples address single- and multi-legged leaping — transitional maneuvers where only a single brief stance mode is available to load energy into the CoM and guide its direction. We compare our hierarchical method to a model-based model-predictive controller in one of the tasks, demonstrating similar performance with a significantly smaller computational footprint.