3D limit cycle walking of musculoskeletal humanoid robot with flat feet

Most of traditional biped walkers based on passive dynamic walking have arc feet and locked ankle joints. In this paper, we propose the method to substitute the arc feet with flat feet. We hypothesize that the shape of the arc feet corresponds to a circular roll-over shape (ROS), which is a shape of a trajectory of center of pressure in the shank-fixed frame. Firstly, we show that ankle joints driven by flexible muscles antagonistically can generate a circular ROS by simple simulation and real robot experiments. Radius of the ROS can be controlled by tension of the muscles. Then, we demonstrate stable 3D limit cycle walking by a biped robot with flat feet using the proposed ROS controlling method. We also investigate its behavior and stability when extra load is added to the robot and verified that the stability of the robot is maintained by keeping the ROS. The results suggest that the ROS can be a stability measure for limit cycle walkers to realize adaptive walking.