Bipedal Walking on Constrained Footholds: Momentum Regulation via Vertical COM Control

TL;DR

This paper introduces an online control method enabling underactuated bipedal robots to walk stably on constrained footholds by regulating vertical COM and angular momentum through optimization and model approximation.

Contribution

It proposes a novel online walking synthesis approach that modulates angular momentum via vertical COM control, demonstrated on two different robots for various terrains.

Findings

Stable walking achieved on flat ground, stairs, and stepping stones.

Method effectively modulates angular momentum for constrained footholds.

Demonstrated on AMBER and 3D Cassie robots.

Abstract

This paper presents an online walking synthesis methodology to enable dynamic and stable walking on constrained footholds for underactuated bipedal robots. Our approach modulates the change of angular momentum about the foot-ground contact pivot at discrete impact using pre-impact vertical center of mass (COM) velocity. To this end, we utilize the underactuated Linear Inverted Pendulum (LIP) model for approximating the underactuated walking dynamics to provide the desired post-impact angular momentum for each step. Desired outputs are constructed via online optimization combined with closed-form polynomials and tracked via a quadratic program (QP) based controller. This method is demonstrated on two robots, AMBER and 3D Cassie, for which stable walking behaviors with constrained footholds are realized on flat ground, stairs, and randomly located stepping stones.