Embedding active force control within the compliant hybrid zero dynamics to achieve stable, fast running on MABEL

A mathematical formalism for designing running gaits in bipedal robots with compliance is introduced and subsequently validated experimentally on MABEL, a planar biped that contains springs in its drivetrain. The methods of virtual constraints and hybrid zero dynamics are used to design a time-invariant feedback controller that respects the natural compliance of the open-loop system. In addition, it also enables active force control within the compliant hybrid zero dynamics allowing withinstride adjustments of the effective stance leg stiffness. The proposed control strategy was implemented on MABEL and resulted in a kneed-biped running record of 3.06 m/s (10.9 kph or 6.8 mph).