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.

Japan has added another soldier to its humanoid robot army following last year's fembot supermodel. The HRP-4 is the latest edition in the state-backed humanoid project. It's leaner, lighter, and can balance itself with yogic ease. Developed by bridge builder Kawada Industries and the National Institute of Advanced Industrial Science and Technology (AIST), the HRP-4 can stand on one leg, track faces and objects, and respond to voice commands. HRP-4 sports a RoboCop look, but it's more C-3PO.