Stable dynamic walking of a quadruped via phase modulations against small disturbances

It is generally accepted that locomotion in animals is based on a trade-off between energy consumption and stability. However, this trade-off is the result of the interaction between complex mechanical and control systems. To gain insight into that issue, a step-by-step approach is needed. In this study, as a first step to investigate three dimensional quadrupedal walking, we aim at establishing a control system as ldquominimalrdquo as possible, able to realize stable dynamic walk. Using a simple mechanical structure, we realized dynamic walk with a distributed control system, made of four independent leg controllers whose swing and stance phases durations are modulated based on leg loading information. Phase modulations contribute to stabilize the posture in the frontal plane via automatic duty ratios adjustments that tend to compensate perturbations of the body rolling motion. By applying lateral perturbations, we found that the control system withstands well perturbations increasing the rolling motion amplitude, but is sensible to perturbations that suddenly decrease it, as the foreleg on the more loaded side is prevented to swing. Hence, we implemented an ascending coordination mechanism where the transition to swing in a hind leg promotes the same event in the foreleg. The duration of the subsequent foreleg swing phase is reduced to prevent excessive increase of the rolling motion amplitude. The resultant control system, although extremely simple, was able to realize dynamic walk resistant to small disturbances (lateral perturbations and terrain irregularities).