Survey of non-facial/non-verbal affective expressions for appearance-constrained robots

Non-facial and non-verbal methods of affective expression are essential for naturalistic social interaction in robots that are designed to be functional and lack expressive faces (appearance-constrained) such as those used in search and rescue, law enforcement, and military applications. This correspondence identifies five main methods of non-facial and non-verbal affective expression (body movement, posture, orientation, color, and sound), and ranks their effectiveness for appearance-constrained robots operating within the intimate, personal, and social proximity zones of a human corresponding to interagent distances of approximately 3 in or less. This distance is significant because it encompasses the most common human social interaction distances, the exception being the public distance zone used for formal presentations. The correspondence complements prior, broad surveys of affective expression by reviewing the psychology, computer science, and robotics literature specifically relating the impact of social interaction in non-anthropomorphic and appearance-constrained robots, and summarizing robotic implementations that utilize non-facial and non-verbal methods of affective expression as their primary means of expression. The literature is distilled into a set of prescriptive recommendations of the appropriate affective expression methods for each of the three proximity zones of interest. These recommendations serve as design guidelines for retroactively adding affective expression through software to a robot without physical modifications or designing a new robot.