Towards a unified understanding of basic notions and terms in humanoid robotics

TLDR

Dynamic balance, zero‑moment point misconceptions, gait stability, unpowered degrees of freedom, and control are key issues in humanoid robotics that the paper addresses. The paper aims to unify basic notions and terms in humanoid robotics, clarifying contradictory interpretations. It defines core concepts such as walk, gait, dynamic balance, stability, ZMP, and posture, and discusses their distinctions and implications. The authors expect the work to clarify terminology and improve understanding of high‑performance humanoid robots.

Abstract

The intention of this paper is to contribute towards a unified understanding of the basic notions and terms in the domain of humanoid robotics, having in mind that the same notions are sometimes interpreted in different ways (some interpretations are contradictory, and some even erroneous). Hence, the first part of the paper is devoted to defining some basic notions, walk and gait being among the first. Then, the paper deals with the notion of dynamic balance and stability, particularly the difference between them, since these essentially different notions are often confused and, rarely, regarded as identical. As dynamic balance is directly related to the notion of zero-moment point (ZMP), it was necessary to touch upon some misunderstandings concerning the ZMP. Gait stability is an especially delicate category, as humanoid locomotion systems have certain specific features that are not possessed by other systems. Namely, because of external disturbances, there may appear unpowered (passive) degrees of freedom that cause loss of dynamic balance. Hence, these unpowered degrees of freedom cannot be overlooked in the stability analysis. As the stability of motion of humanoid robots is inseparably linked with control, it was also necessary to pay due attention to this notion. Finally, the paper ends with a discussion of posture and postural stability with all their specificities. The authors hope that this paper will contribute to a clearer understanding of the basic notions of humanoid robotics, especially concerning robots with high dynamic and control performances.