We discuss how to prevent a quadruped walking robot from tumbling over when operating on a slope. In the process of tumbling, the potential energy of the center of gravity goes through a maximum. The larger the difference between the potential energy of the center of gravity of the initial position and that of this maximum, the less the robot tumbles. So this difference can be regarded as the stability margin and a novel gait to obtain the largest stability margin is mentioned here. It is nothing but the intermittent crawl gait whose posture is illustrated in this paper. In addition, the energy stability contour, drawn by connecting equal stability points on the inclined plane, is also explained in this paper and it is a helpful tool for designing standard foot trajectories. An optimal posture on the slope designed in this way results in an inverse trapezoid shape, which means that the upper two legs are located wider than lower two ones. This form worked for the experimental machine, TITAN VII. Furthermore, if the standard trajectory for one direction is combined with another direction trajectory, the quadruped robot can easily change directions, maintaining a sufficient stability margin. This sequence is shown in the last part of this paper.
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