Sony and Japanese Researchers Build Expanding 'Fluffy' Robot to Study Human Perception

Researchers from Japan's University of Electro-Communications and Sony Corporation have developed MOFU, a soft, spherical mobile robot that expands and contracts its entire body to appear more lifelike to human observers, according to a study published in PLOS ONE and first reported by TechXplore.

As robotic systems are increasingly deployed in healthcare, eldercare, and public assistance settings, engineers are focusing on how design and physical motion influence human perception. While earlier human-robot interaction research largely concentrated on jointed limbs or single-axis extension, natural organisms frequently display volumetric changes, such as chest expansion during breathing or animal courtship displays. The research team developed MOFU—short for MOrphing Fluffy Unit—to evaluate whether whole-body volume adjustments alone can elevate a robot's perceived animacy.

Mechanically, the robot relies on a geometric transformation mechanism known as a Jitterbug structure, allowing its spherical frame to contract and expand through a single motor-driven linear actuator. The mechanism alters MOFU's vertical height between approximately 210 millimeters and 280 millimeters. For mobility, the unit utilizes a differential two-wheel drive system capable of both straight translation and turning, powered by an internal battery to eliminate the need for external power cords.

Acoustic performance and visual approachability were central to the design. To minimize mechanical noise during operation, MOFU incorporates gearless direct-drive motors alongside low-friction components. Rather than mimicking a specific biological species, the machine presents an abstract spherical silhouette wrapped in a soft, plush material to present a non-threatening appearance in close-contact environments.

To measure human responses, the research team conducted three online video experiments involving 498 evaluated participants with a average age of 51.69 years. Participants rated the robot using the Japanese translation of the Godspeed Questionnaire Series Animacy subscale, which assesses impressions across six five-point metrics. Because the Jitterbug mechanism inherently generates rotational motion during expansion, the developers programmed the wheel system to counter-rotate in specific trial conditions, isolating volume change from rotation.

In the first experiment evaluating a stationary robot, both whole-body expansion-contraction and left-right rotational movements significantly raised perceived animacy scores compared to a static robot. A direct comparison between volume expansion alone and rotation alone showed no statistically clear difference between the two movement types. A second experiment comparing a single expanding robot against two simultaneous expanding robots showed no distinct shift in animacy scores based on unit count.

A third experiment showed that adding volume expansion and rotational movement to forward locomotion increased animacy scores over simple rolling movement, though the researchers noted that the participant sample for this trial fell short of initial target sizes. Because the study relied on recorded video, participants were unable to evaluate tactile or acoustic characteristics, which the team intends to address in subsequent physical interaction studies.

The authors—including UEC Associate Professor Yoshihiro Nakata, Taisei Mogi, and Sony's Mari Saito—suggest that whole-body volumetric changes could provide an intuitive method for robots to express state and presence in space-constrained facilities like clinics and care homes where low noise and physical safety are critical.