Passive dynamic autonomous control (PDAC) for bipedal walking has excellent efficiency. PDAC, however, can control walking direction only in the previous research, and not be used for 3-D walking currently. We then propose a 3-D walking method of PDAC by using a walking speed controller based on a swing-leg motion. The reference walking speed to reach the goal is achieved by adjusting the touchdown position and speed of the swing leg. As a result, the 3-D walking is achieved with the high energy efficiency, although the footstep is hardly controllable.
