Hybrid position/force control for biped robot stabilization with integrated center of mass dynamics

Traversing uneven terrain with unexpected changes in ground height still poses a major challenge to walking stabilization of humanoid robots. A common approach to balance a biped in such situations is the control of the ground reaction forces at the feet. However, the dynamics of the center of mass is not considered in existing solutions for this direct force control scheme. In this work, we present a force control method to realize contact forces by accelerating the center of mass, which is directly integrated into our hybrid position/force control scheme. For this, we first introduce an analytical formulation for a contact model in task-space. We evaluate the performance of our approach in simulation and real-world experiments with our humanoid robot LOLA. The integration of center of mass dynamics shows great reduction of upper-body inclination angles for a late contact experiment with 5.5 cm change in ground height. We found that by using the system's center of mass dynamics in the force controller, undesired movements along the under-actuated degrees of freedom can be compensated effectively. We consider our approach a starting point for the development of more sophisticated direct force control concepts for humanoid robots.