In recent years, humanoid robots, particularly the walking of humanoid robots has been drawing attention of many researchers. The majority of researches regarding humanoid robot walking uses zero moment point (ZMP), and controls the ZMP to keep the ZMP inside the supporting polygon. In this approach, a humanoid robot and the surrounding environment of the robot are accurately modeled and differential equations are solved. However, the modeling becomes difficult if there is an unknown element. Moreover, since solving differential equation consumes time, it is difficult to perform real-time control.
Another approach does not use the ZMP. For example, one conventional technology makes use of a periodic motion of moveable parts of a robot and adjusts the phase of the periodic motion so that the posture of the robot remains stable (see Japanese Laid-open Patent Publication No. 2005-96068). Here, the movable parts indicate the legs and arms of a robot.
Moreover, Japanese Laid-open Patent Publication No. 2007-175809 discloses a technology that efficiently controls the robot so as to allow the humanoid robot to stably perform various motions while eliminating the need of modeling a humanoid robot or the surrounding environment thereof.
The conventional technologies, however, are not immune from the problems. Specifically, when an external force is applied to the upper body of a walking robot, the robot cannot continue the periodic motion and the conventional technologies cannot control the robot.