  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.
