Reinforcement Learning of Stable Trajectory for Quasi-Passive Dynamic Walking of an Unstable Biped Robot

Abstract

Biped walking is one of the major research targets in recent humanoid robotics, and many researchers are now interested in Passive Dynamic Walking (PDW) [McGeer (1990)] rather than that by the conventional Zero Moment Point (ZMP) criterion [Vukobratovic (1972)]. The ZMP criterion is usually used for planning a desired trajectory to be tracked by a feedback controller, but the continuous control to maintain the trajectory consumes a large amount of energy [Collins, et al. (2005)]. On the other hand, PDW enables completely unactuated walking on a gentle downslope, but PDW is generally sensitive to the robot's initial posture, speed, and disturbances incurred when a foot touches the ground. To overcome this sensitivity problem, ``Quasi-PDW'' [Wisse & Frankenhuyzen (2003); Sugimoto & Osuka (2003); Takuma, et al. (2004)] methods, in which some actuators are activated supplementarily to handle disturbances, have been proposed. Because Quasi-PDW is a modification of the PDW, this control method consumes much less power than control methods based on the ZMP criterion. In the previous studies of Quasi-PDW, however, parameters of an actuator had to be tuned based on try-and-error by a designer or on