Online Gait Transitions and Disturbance Recovery for Legged Robots via the Feasible Impulse Set

Gaits in legged robots are often hand tuned and time based, either explicitly or through an internal clock, for instance, in the form of central pattern generators. This strategy requires trial and error to identify leg timings, which may not be suitable in challenging terrains. In this letter, we introduce new concepts to quantify leg capabilities for online gait emergence and adaptation, without fixed timings or predefined foothold sequences. Specifically, we introduce the Feasible Impulse Set, a notion that extends aspects of the classical wrench cone to include a prediction horizon into the future. By considering the impulses that can be delivered by the legs, quantified notions of leg utility are proposed for coordinating adaptive lift-off and touch-down of stance legs. The proposed methods provide push recovery and emergent gait transitions with speed. These advances are validated in experiments with the MIT Cheetah 3 robot, where the framework is shown to automatically coordinate aperiodic behaviors on a partially moving walkway.