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

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.