I. Introduction
Human–robot interaction (HRI) [1] has emerged as a research field that is dedicated to understanding, designing, and evaluating robotic systems for use by or with humans. While multimodal channels and sensor fusion allow for more robust communications between them, study on how to improve the natural HRI is still undergoing. The result on human acceptance of the robot [26] can be deemed as a preliminary pioneer that may be helpful to developing natural communication interfaces. After shifting from a robot-centered point of view, more and more researchers have been drawn to study the interactions and reciprocal effects between robots and humans. A new generation of robots is expected to adapt to users, to interact with them naturally, and to participate in their daily lives. One important perspective is that robots should interact with humans in their most natural ways.