Recently, the demand for the practical use of service robots has risen significantly because of acceleration of demographic aging, and a humanoid robot is one of the promising form factors of service robots. When a humanoid robot is used by a human in a real environment, the robot needs to be designed by taking into account the various external forces that act on the robot. Thus far, most of the walking humanoid robots have been mainly controlled by the conventional ZMP method to maintain a stable walking posture. However, the conventional ZMP method can not be used to handle the various external forces that act on the upper part of the humanoid robot body. To overcome these problems, in this paper, we propose a novel control method, which we called 3DZMP and pZMP, for a humanoid robot to react to the external force on the upper part of the body. The 3DZMP is defined as the point in three-dimensional space at which the moment about all axes is zero. 3DZMP can prevent the rotation of a humanoid robot. The pZMP is defined as the point corresponding to the orthographic projection of the 3DZMP on a plane. pZMP is used to evaluate the stability of 3DZMP. We implemented the proposed method on a prototype robot and verified that the robot gained the capability to react to external forces that could not be handled by the conventional ZMP method.
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