| Limit Cycle Walkers are bipeds that exhibit a stable cyclic gait without
requiring local controllability at all times during gait.Well-known
example are McGeer’s “Passive Dynamic Walkers”, but the concept
expands to actuated bipeds as involved in this study. Current stateof-
the-art Limit Cycle Walkers excel in being very energy efficient,
but their ability to handle disturbances (i.e. disturbance rejection) is
still limited. A way to improve this ability while maintaining low energy
consumption is the use of ankle actuation, which has so far seen
few applications in this type of walker. In this paper we study the effect
of (1) applying (passive) stiffness in the ankle joint, (2) applying
control in the stance ankle based only on local sensor information
and (3) modulating ankle push-off. For all three strategies the paper
shows how they influence energy use and disturbance rejection
of a simple point mass walking model, a more realistic model and a
physical prototype. We find that applying a passive ankle spring that
results in premature heel rise is energetically optimal and gives an
actuation pattern that largely resembles that of humans. Local stance
ankle control and ankle push-off modulation can improve the disturbance
rejection of a Limit Cycle Walker by at least 60%, without increasing
its energy use. These findings are substantiated by showing
that our prototype is able to handle large disturbances such as a stepdown
of 5% of its leg length, while walking efficiently at a mechanical
cost of transport of 0.09. |
