Enlarging Attractor Basin of a Passive Dynamic Walker by Exploiting Cooperative Phenomena

Since the pioneering work of McGeer in the late 1980s, the concept of passive dynamic walking (hereafter PDW) has been attracting lots of attention in the field of legged robots 1) A significant feature of PDW robots is that they have no control system; they merely exploit their intrinsic mechanical dynamics carefully designed, and show surprisingly natural and human-like walking on a gentle slope without any actuation except for gravity. Due to this, PDW robots are expected to largely contribute to the realization of energyefficient humanoid bipeds, which has been totally difficult with the embodiment of current biped robots. As observed in many nonlinear dynamical systems, it is widely recognized that there exists entrainment phenomena in PDW robots 2) : they continue stable locomotion as long as their state is within the basin of attraction. Recently, however, Schwab et al. have pointed out that the basin of attraction of PDW robots are significantly small. Furthermore, they have clarified that this basin of attraction becomes smaller as the gradient of slope increases; consequently, this will disappear when the gradient exceeds a critical value .