Powered “Passive” Dynamic Walking

We demonstrate that a simple open-loop actuation/control scheme is all that is required to produce stable, powered, human-like walking motions in a set of roughly human-like legs. This work builds off of the passive dynamic models pioneered by McGeer (1989) and later studied by Garcia (1997). Our model is constrained to move in the sagital plane. It has two identical rigid, straight legs with arbitrary mass distribution, that are hinged to each other at the hip. It has two rigid, round massless feet, one hinged to the base of each leg. The floor is flat, level and rigid. The feet have plastic (no-slip, nobounce) collisions with the floor at ‘heelstrike’. The energy required to balance that lossed at heelstrike is added through part of the stride by an extension of the stance foot. This extension is powered by a permanent magnet DC motor acting under a constant voltage. When the motor is not activated the feet are held at a prescribed angle by a mechanical stop and the robot moves only under the influence of gravity. This model exhibits stable walking over a large range of control and actuation parameters. It shows behaviors similar to those of the passive walkers previously studied. Stable and unstable gait limit cycles and period doubling, for a variety of structural, physical and control/actuation parameters, have been observed.