Improved Zero Step Push Recovery with a Unified Reduced Order Model of Standing Balance

Abstract

Standing balance for legged robots can be achieved through regulating the center of pressure (ankle strategy), the angular momentum about the center of mass (hip strategy), and the magnitude of ground reaction force (variable height strategy). Prevalent reduced order models used to model legged robots at most only capture two of these strategies, and the contribution of the three available strategies is unclear. We propose a unified reduced order model that includes all three standing balance strategies and compared push recovery simulations of the unified model against existing balancing models using a nonlinear model predictive controller. We also developed a full body controller for a simple one legged balancing robot that tracked control from the reduced order models. For both the reduced order model and robot simulations, we found that the unified model could recover successfully from the largest pushes and yielded the smallest center of mass excursions. Between the hip and variable height strategies, the hip had the greatest effect on improving performance. Our results suggest that successful implementation of a unified reduced order model on physical robots would enable a simplified controller that takes advantage of available balancing strategies as needed to recover from larger push disturbances than feasible before.