Impact-Aware Multi-Contact Balance Criteria

TL;DR

This paper introduces impact-aware balance criteria for humanoid robots, enabling controlled impacts while maintaining stability through a novel zero-step capture region and impact dynamics analysis.

Contribution

It proposes the zero-step capture region for non-coplanar contacts and develops a method to compute post-impact CoM velocities considering impact dynamics.

Findings

Validated with push-recovery experiments on HRP-4

Defined maximum contact velocities for various stances in simulation

First approach to intentional impacts on non-coplanar contacts in humanoids

Abstract

Intentionally applying impacts while maintaining balance is challenging for legged robots. This study originated from observing experimental data of the humanoid robot HRP-4 intentionally hitting a wall with its right arm while standing on two feet. Strangely, violating the usual zero moment point balance criteria did not systematically result in a fall. To investigate this phenomenon, we propose the zero-step capture region for non-coplanar contacts, defined as the center of mass (CoM) velocity area, and validated it with push-recovery experiments employing the HRP-4 balancing on two non-coplanar contacts. To further enable on-purpose impacts, we compute the set of candidate post-impact CoM velocities accounting for frictional-impact dynamics in three dimensions, and restrict the entire set within the CoM velocity area to maintain balance with the sustained contacts during and after impacts. We illustrate the maximum contact velocity for various HRP-4 stances in simulation, indicating potential for integration into other task-space whole-body controllers or planners. This study is the first to address the challenging problem of applying an intentional impact with a kinematic-controlled humanoid robot on non-coplanar contacts.