This article presents a framework for systematic, stable and passive, dynamical balancing and locomotion control of flexible-joint bipedal robots. In order to achieve stability/passivity of the full flexible-joint, floating-base model with contacts, several novel control designs are proposed, whose ability to guarantee regulation and tracking stability is mathematically and practically demonstrated. The proposed designs enable usage of full-state feedback terms, thereby increasing both link tracking and oscillation suppression performance. These constitute the only control schemes reported in the literature, which are capable of asymptotically stabilizing flexible-joint, floating-base systems with contacts, during trajectory-tracking tasks. Moreover, a novel linear quadratic regulator (LQR) tuning approach is proposed, which permits the creation of models characterized by distinct kinetic chain and impedance combinations. Stable switching between these gain sets is guaranteed, as it is demonstrated that the proposed controllers enable unconstrained and stable, variable impedance control (VIC). The proposed control methods are corroborated through practical, balancing and locomotion experiments on the COmpliant huMANoid, as well as via dynamical simulations; these results demonstrate stability maintenance during tracking and VIC tasks. The ability to stably modulate a legged robot's active impedances could enable closer replication of biologically inspired behaviors.
