Reactivity to unforeseen disturbances is one of the most crucial characteristics for biped robots to walk robustly in the real world. Nevertheless, conventional walking methods generally have limited capability for generating rapid reactions to disturbances, because in these methods it is necessary to wait until the end of the preplanned time period to proceed to the next phase. In this study, to improve reactivity, we develop an event-based finite-state machine (E-FSM) for walking pattern generation. Reactivity is enhanced by determining the state transition conditions of the E-FSM only with time-independent events based on the present robot state. Moreover, in the E-FSM, the robot can walk robustly even when the center of mass and the swing foot motion are disturbed, by employing the capture point concept combined with a new swing foot position constraint. Finally, we propose to control the walking robot by incorporating the E-FSM with an inverse dynamics-based motion/force controller to achieve compliant behavior. This can provide safe responses to external disturbances. The developed method is verified by experiments on a 12-degrees-of-freedom torque-controlled biped robot while it locomotes under irregular external disturbances applied to the upper body or swing leg.

The actuator bandwidth limitation deteriorates the stability and performance of torque-based robot controllers. Operational space control is especially prone to this problem, since the limited bandwidth of a single actuator can reduce the performance of all related tasks simultaneously. In this article, an intuitive way to penalize low performance actuators is proposed to improve the performance of the operational space controller. The basic concept is to add joint torques only to high performance actuators, when the control bandwidth cannot reach the target level using all actuators. If that is not enough, additional torques are commanded to even higher performance actuators. This procedure can be executed recursively, meaning the controller can achieve almost maximum performance under the actuator bandwidth limitation. The proposed method was experimentally verified using the robot manipulator Franka Emika Panda.