| “Limit Cycle Walking” is a relatively new paradigm for the design
and control of two-legged walking robots. It states that achieving stable
periodic gait is possible without locally stabilizing the walking
trajectory at every instant in time, as is traditionally done in most
walking robots. Well-known examples of Limit Cycle Walkers are the
Passive Dynamic Walkers, but recently there are also many actuated
Limit Cycle Walkers. Limit Cycle Walkers generally use less energy
than other existing bipeds, but thus far they have not been as versatile.
This paper focuses on one aspect of versatility: walking speed.
We study how walking speed can be varied, which way is energetically
beneficial and how walking speed affects a walker’s ability to handle
disturbances (that is, disturbance rejection). The study is performed
using one prototype and one simulation model. The speed of these
two walkers is adapted by changing three parameters: the amount of
ankle push-off, upper body pitch and step length. The study has resulted
in four conclusions. (1) Steady-state speeds between 0.24 and
0.68 m s-1 (for a 0.6 m leg length) were obtained, with loss of stability
determining the lower limit and actuation limits determining the
upper limit. This result shows the applicability of Limit Cycle Walking
for versatile walking machines. (2) For any speed, powering the
gait by leaning the body forward costs less energy than using ankle
push-off. (3) In contrast to the apparent tradeoff between speed
and stability in traditional walking robots, in Limit Cycle Walking
we find that increasing the walking speed, independent of how this
is done, automatically results in an increasing disturbance rejection.
(4) A combination of feedforward actuation adjustment and step-tostep
feedback from walking speed shows that it is possible to change
walking speed in only a few steps and maintain a desired speed when
performing tasks such as carrying loads and walking on slopes. In
particular, this fourth conclusion underlines the applicability of the
concept of Limit Cycle Walking for versatile two-legged walking machines. |
