Upper limb compliant strategy exploiting external physical constraints for humanoid fall avoidance

Ensuring humanoid balance stability to prevent falling is one of the most crucial control problems in humanoid robotics and has been extensively studied in the past, resulting in a diverse range of balance recovery schemes. These balancing control methods effectively perform body posture control using three main motion strategies, namely: ankle, ankle-hip and stepping strategies. In this work we present a novel balance strategy which fundamentally differs from the previous methods as its principle is to exploit contacts with the environment to prevent falling rather than only performing body posture control. An uncoupled impedance controller for the upper body of the humanoid robot is combined with a lower body stabilizer, and the balancing capabilities are enhanced by the establishment of additional physical contacts with the environment. The generation of the reactive arm motion and the impedance regulation are discussed in details. Experimental trials with the humanoid robot COMAN, provided with active and passive compliance, demonstrate the potential of the approach and the first step towards the development of more human-like balancing skills using an integrated approach where all body parts (not only legs but also arms and hands) can establish contacts with the surrounding environment so as to ensure a stable and balanced behavior.