A continuum soft robotic trout with embedded HASEL actuators: design, fabrication, and swimming kinematics

Autonomous underwater vehicles with greater maneuverability, efficiency, and resiliency are needed to meet the challenges of exploring and monitoring the underwater world, so we look to underwater creatures to uncover what makes them such excellent swimmers. Bio-inspired, soft robots can combine the performance of biological swimming with the robustness of soft construction, where the ideal robot has a jointless, flexible body with embedded muscles just like real fish. In this paper, we present a continuously deformable robotic trout with embedded electro-hydraulic hydraulically amplified, self-healing electrostatic artificial muscles, experimentally characterize its swimming kinematics, and report a reduced order numerical model which predicts the robot fish's natural frequencies and mode shapes. We characterized the robot's 3D full body swimming kinematics while submerged in water with digital image correlation. The soft robot undergoes whole body bending in response to internal muscle actuation and yields kinematics comparable to biological trout. Tail beat velocity was measured at the first three observed natural frequencies with a maximum of 69 mm s-1 corresponding to a caudal fin trailing edge displacement of +/- 10 mm. We derive a beam-based fluid structure interaction (FSI) model which predicts swimming kinematics in response to embedded muscle forces and includes the effects of nonlinear vortex and convective forces on the robot's body. The nonlinear FSI model predicted the first three damped natural frequencies within 5% error and mode shapes which correlated with the experimental data. This paper contributes the design, fabrication, and characterization of a solid-state robotic trout featuring whole-body flexibility and embedded actuation through numerical modeling and experimental analysis.