Controllable dielectric elastomer actuators for centimeter-scale flexible underwater robot

Traditional rigid underwater vehicles are often bulky and exhibit poor maneuverability, which limits their deployment in complex marine environments. In contrast, bio-inspired soft robotic fish are capable of mimicking efficient aquatic propulsion, while maintaining a compact structure and enhancing interaction safety. They demonstrate significant potential for marine sensing and exploration. Despite these advantages, reliable underwater locomotion control for dielectric elastomer actuators (DEAs) driven systems remains highly challenging because of large nonlinear deformations, time-varying parameters, and strong hydrodynamic damping. As a result, most existing prototypes still operate primarily in open loop. In this study, we develop a centimeter-scale manta ray-inspired robotic fish actuated by underwater-compatible rolled-type DEAs. Systematic underwater experiments are conducted, including speed regulation and turning performance tests. To attain stable and reliable underwater control, an experimentally calibrated voltage-curvature relationship for the rolled-type DEAs is established, and an underwater input-output model for reciprocating actuation is identified. These two factors jointly facilitate the implementation of a fuzzy proportional-integral-derivative (PID) closed-loop controller. Compared with open-loop operation, the closed-loop system improves disturbance rejection and dynamic response, resulting in more consistent swimming performance and maneuvering control. These results offer a practical approach for integrating the fabrication of DEAs, underwater integration, and closed-loop control, and contribute to the advancement of highly maneuverable centimeter-scale soft underwater robots.