MOFU: Development of a MOrphing Fluffy Unit with Expansion and Contraction Capabilities and Evaluation of the Animacy of Its Movements

Abstract: Robots for therapy and social interaction are often intended to evoke "animacy" in humans. While many robots imitate appearance and joint movements, little attention has been given to whole-body expansion-contraction, volume-changing movements observed in living organisms, and their effect on animacy perception. We developed a mobile robot called "MOFU (Morphing Fluffy Unit)," capable of whole-body expansion-contraction with a single motor and covered with a fluffy exterior. MOFU employs a "Jitterbug" structure, a geometric transformation mechanism that enables smooth volume change in diameter from 210 to 280 mm using one actuator. It is also equipped with a differential two-wheel drive mechanism for locomotion. To evaluate the effect of expansion-contraction movements, we conducted an online survey using videos of MOFU's behavior. Participants rated impressions with the Godspeed Questionnaire Series. First, we compared videos of MOFU in a stationary state with and without expansion-contraction and turning, finding that expansion-contraction significantly increased perceived animacy. Second, we hypothesized that presenting two MOFUs would increase animacy compared with a single robot; however, this was not supported, as no significant difference emerged. Exploratory analyses further compared four dual-robot motion conditions. Third, when expansion-contraction was combined with locomotion, animacy ratings were higher than locomotion alone. These results suggest that volume-changing movements such as expansion and contraction enhance perceived animacy in robots and should be considered an important design element in future robot development aimed at shaping human impressions.