Passive Walking with Knees

Passive dynamic walking, a phenomenon originally described for bipeds having straight legs, also works with knees. Thus given only a downhill elope aa a source of energy, a humanlike pair of legs will settle into a natural gait generated by passive interaction of gravity and inertia. No muscular input is required. The physice is much the same as in straightlegged walking, but the knee-jointed form has two advantages. First, it offers a simple solution to the problem of foot clearance during the recovery phase. Second, in some cases it is more stable. Straight legs vs knees Walking can be a natural mode of a simple mechanical d e vice; it does not require the continuous forcing which is used almost invariably in robot design. This is the essence of passive dynamic walking, which I reported at the 1989 Robotics & Automation Conference WcGeer 891. Similar ideas have recently been developed for hopping and running as well [Thompson 891, WcGeer Sob] . The simplest demonstration is provided by a mechanism consisting only of two rigid legs hinged at the hip. Such a device will walk all by itself on a downhill slope. The same mechanism can also be made to walk on the level and uphill by “pumping” the passive motion, and the gait can be modulated to vary footfalls from one step to the next. Thus the passive walking effect can serve as a foundation for dextrous and efficient legged locomotion. Here we add knees to the catalogue of design options. Our motivation is twofold. First, knees make the gait more anthropomorphic, which is aesthetically pleasing and also useful for study of walking in nature. Second, knees solve the problem of toe stubbing in the so-called recovery phase, when the free leg is brought forward in preparation for the next step. Our straight-legged biped (described at the 1989 conference) had to clear its feet actively during recovery, using small retraction motors in each leg; these substantially complicated an otherwise passive design, and moreover used a lot of energy. The machine needed only about 0.23 per step (obtained by going downhill) to keep the passive cycle going; meanwhile to motors consumed 35 per step! By comparison foot clearance by knee flexure offers a solution that is passive, efficient, and reliable. CH2876-1/90/000()/1640$01.00