In this paper, a method that makes biped robots track zero-moment-point (ZMP) reference trajectories is proposed. The biped robots have flat rectangular soles with force sensors at each corner. The sensors detect reaction force from the ground. Sensor information is transformed into useful structure, which is defined as "environmental modes." The environmental modes represent contact states between the ground and the soles and are extracted by the information of four force sensors at each corner of the rectangular soles. The environmental modes consist of the following four modes: 1) heaving; 2) rolling; 3) pitching; and 4) twisting. These modes are related to the ZMP, by which walking stability is evaluated. Therefore, command values of the environmental modes can be derived by the ZMP reference trajectory. By tracking the command values, the walking motion becomes more stable. The conventional study on controlling the environmental modes takes only the rolling and pitching modes into account. However, it is not sufficient to stabilize the walking motion, because the heaving mode has been neglected. The heaving mode is also very important to stabilize the walking motion. Therefore, in this paper, we extend the conventional method for considering the heaving mode to track the ZMP reference trajectory. Validity of the proposed method was confirmed by experimental results.
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