Nebula: A Flexible, Solid-State Swimming Robot Enabled by HASEL Actuators

Abstract

Abstract After millions of years of evolution, ocean creatures have mastered the underwater environment. In human interactions with the underwater world, unmanned underwater vehicles (UUVs) are needed for applications where human divers are impractical or at great risk such as exploration of remote locations or the deep ocean. Current propeller-driven UUVs stand to gain greater efficiency and maneuverability by mimicking underwater creatures. Biological fish outperform the state-of-the-art bio-inspired UUVs, but it has been shown in hydrofoil literature that tailoring chordwise flexibility can improve swimming performance across several metrics. In this paper we aim to integrate chordwise flexibility and integrated soft actuators to demonstrate a soft robotic propulsor which better mimics the structure and capability of fish. Inspired by cartilaginous body-and-caudal-fin (BCF) fishes like sharks, we built a soft robotic fish from compliant components with embedded smart actuators. The robot’s body is made of a 3D printed rubber skeleton and electro-hydraulic contracting-pouch actuators embedded in compliant silicone. The actuators used were hydraulically-amplified, self-healing electrostatic (HASEL) artificial muscles placed in an antagonistic configuration across the spine to produce lateral body contractions. We tailored the stiffness distribution of the body to passively induce BCF swimming kinematics at the caudal fin in response to actuator contractions in the main body. In this paper, the design and fabrication of the soft robot is described and its underwater structural dynamics are characterized with experimental testing in quiescent water. Full-body high speed videos of the robotic fish were taken at varying actuator input voltage and frequency to quantify its dynamic response. We show that the robot’s compliant body deforms like a biological fish and is capable of tail-beat speeds up to 73 mm/s. This flexible, joint-less design with embedded actuation is a step toward fieldable, lifelike bio-inspired UUVs for the future of underwater robotics.