MOFU: Development of a MOrphing Fluffy Unit with expansion and contraction capabilities and evaluation of the animacy of its movements

Robots designed for therapy and social interaction are often intended to evoke a sense of “animacy” in humans. While many robots have been developed to imitate life-like appearance and joint movements, relatively little attention has been given to the effect of whole-body expansion and contraction—volume-changing movements commonly observed in living organisms—on perceived animacy. In this study, we developed MOFU (MOrphing Fluffy Unit), a mobile robot capable of whole-body expansion and contraction using a single motor enclosed within a fluffy exterior. MOFU employs a “Jitterbug” structure, a geometric transformation mechanism that enables smooth volume changes using a single actuator, with the robot height varying from approximately 210 mm to 280 mm. Additionally, it is equipped with a differential two-wheel drive mechanism for locomotion. We conducted an online survey using videos of MOFU’s behavior to evaluate the effect of expansion–contraction movements on animacy perception. Participants rated their impressions using the Godspeed Questionnaire Series. First, we compared participants’ ratings across videos of MOFU in stationary states, with and without expansion–contraction, and separately, with and without rotational motion. Both the expansion–contraction and rotational movements independently increased perceived animacy. Second, we hypothesized that presenting two MOFUs simultaneously would enhance perceived animacy further; however, this prediction was not supported, as no significant difference was observed. Exploratory analyses were also performed across the four dual-robot motion conditions. Third, when rotational and expansion–contraction motions were combined with locomotion, animacy ratings were higher than for locomotion alone. These results suggest that volume-changing movements, such as expansion and contraction, enhance the perception of animacy in robots. Consequently, incorporating physical volume change may be an important design element in the development of future robots aimed at shaping human impressions.