Where center of pressure should be controlled in biped upright posture

The stability of the upright posture of the biped system on horizontal ground has been evaluated in terms of the vertical projection of the center of gravity on the ground. Since the point of projection coincides with the center of pressure of the ground reaction force (CoP), so long as static equilibrium is maintained, robust control can be realized if the ground reaction force is directly controlled. Consequently, we modeled the upright standing state of the biped system by a two-link system composed of the foot and the remainder and proposed a method of controlling the ground reaction force based on the output of the ankle joint. The method is equivalent to control of the CoP, but no detailed study of where the center should be controlled has been presented. This point is crucial if the foot is not symmetrical in the anterior–posterior direction, or if the ankle joint is located at a certain height, as in the case of humans. The purpose of this paper is to investigate this problem. Two control methods can be considered, depending on whether the evaluation criterion is defined as the minimum steady output of the ankle joint or as the stability margin. For each of the criteria, we analyze the stability and the steady-state posture. To clarify the criterion by which humans perform control, the CoP was measured for humans in the upright posture. The results suggest that humans are less likely to use the minimum output of the ankle joint as the evaluation criterion. © 2004 Wiley Periodicals, Inc. Syst Comp Jpn, 35(5): 23–31, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/scj.10559