Manipulation of center of pressure for bipedal locomotion by passive twisting of viscoelastic trunk joint and asymmetrical arm swinging

To implement successful bipedal locomotion in a robot, its center of pressure (CoP) is placed on a supporting area. To achieve locomotion, many studies have focused on the lower body. Given that the human upper body has a large mass and its behavior influences locomotion even in the case of the robot, this study investigates the effect of the upper body, which contains moving arms and a twisting trunk, on CoP. The dynamics is analyzed using a simple model that has a passive viscoelastic trunk joint around the vertical axis and arms that oscillates back and forth. From the derivations of CoP and trunk joint trajectory, three important findings are made: (i) the CoP oscillates along the lateral direction only when the arm swings in an anterior-posterior asymmetric manner, (ii) the trajectories of CoP and trunk joint are in anti-phase, (iii) the phase of CoP along the lateral direction is influenced by the swinging cycle and viscoelasticity of the trunk joint, the mechanical elements of the upper body. An experiment using a robot verified the first and second findings, and simulation verified the last finding. The last finding will contribute to making a feedback controller that converges to the desired phase, which is an important factor for successful bipedal locomotion.