Almost traditional researches about legged robot's walking apply Zero Moment Point (ZMP) as the reference of motion control to avoid falling. And in order to control the ZMP based legged robots precisely, joints of the robots are generally composed by powerful and precise DC-motors with high reduction ratio, such as industrial robots. However these types of robots have been pointed out as their low-efficient walking motion, so more efficient legged walking robot and its control are needed. In such a background, we have tried to develop Passive Dynamic Walking robot design procedure with the aim of optimized design between the hardware structure and control. In this paper we propose a design procedure which applies the adaptive function embedded in Passive Dynamic Walking, and also investigate its effectiveness. Then we try to extend the legged robot design to limit cycle walking control.

[abstFig src='/00290003/03.jpg' width='300' text='Schematic of the proposed design method' ] This article proposes a design method of legged walking robot hardware capable of performing passive dynamic walking with its desirable characteristics. Passive dynamic walking has a relatively good energy efficiency, and is said to be similar to the walking style of animals. However, most legged robot hardware capable of passive dynamic walking is designed through trial and error on the basis of experience. One of the major problems of designing through trial and error is the difficulty of verifying walking for the legged robot hardware that has many degree of freedom. It is relatively easy to determine the initial condition for compass-type robot hardware. However, it often takes long time to determine the appropriate initial conditions and slope angles for complicated robots such as legged robots with knees. We proposed and verified a method to design a legged robot with knees that has a desired leg length and leg mass from a compass-type legged robot. In this article, we propose a method to design a passive dynamic walker that has a desired leg angle, step length, leg mass, etc., and verify the resulting design. More specifically, the physical parameters, such as the leg length, leg mass, and joint friction, are defined as “physical parameters” and the parameters acquired as the result of walking, such as the leg angle, step length, and walking cycle, are defined as “variable parameters.” By observing variable parameters while the robot is walking and by changing the physical parameters according to the observed variable parameters, the variable parameters are indirectly changed to desired values.

It is well known that passive dynamic walking shows chaotic behavior owing to changes in the environment. In addition, when the environment changes continuously during walking, passive dynamic walking shows “adaptive behaviors” in which the stride angle changes itself in an attempt to keep walking. These behaviors are very interesting and useful for the legged robot design. However, the studies on passive dynamic walking are preceded only by numerical simulations. For this reason, it is very important to confirm, by actual experiments, whether these characteristic behaviors appear. In this paper, we verify the existence of these behaviors by several actual experiments.

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