Improving Swimming Performance in Soft Robotic Fish with Distributed Muscles and Embedded Kinematic Sensing

TL;DR

This work addresses the challenge of achieving biology-like swimming performance in unmanned underwater vehicles by integrating distributed muscles and embedded kinematic sensing into a soft robotic fish. The authors design a slender soft swimmer with three axial muscle groups of HASEL actuators and ten midline strain gauges, optimizing geometry via a parameter study and a validated kinematic model. Experimental results show that simple in-phase actuation yields thrust near the resonances, while introducing an axial phase offset substantially increases thrust (up to $7.2$ mN, a $44\%$ gain) and traveling-wave content (Ti up to $0.53$), with near-equal thrust at the second resonance using only about $25\%$ of the tail displacement. These findings highlight the value of distributed actuation and rich onboard sensing for sensorimotor control, providing a platform for future feedback control and embodied intelligence in soft UUVs.

Abstract

Bio-inspired underwater vehicles could yield improved efficiency, maneuverability, and environmental compatibility over conventional propeller-driven underwater vehicles. However, to realize the swimming performance of biology, there is a need for soft robotic swimmers with both distributed muscles and kinematic feedback. This study presents the design and swimming performance of a soft robotic fish with independently controllable muscles and embedded kinematic sensing distributed along the body. The soft swimming robot consists of an interior flexible spine, three axially distributed sets of HASEL artificial muscles, embedded strain gauges, a streamlined silicone body, and off-board electronics. In a fixed configuration, the soft robot generates a maximum thrust of 7.9 mN when excited near its first resonant frequency (2 Hz) with synchronized antagonistic actuation of all muscles. When excited near its second resonant frequency (8 Hz), synchronized muscle actuation generates 5.0 mN of thrust. By introducing a sequential phase offset into the muscle actuation, the thrust at the second resonant frequency increases to 7.2 mN, a 44% increase from simple antagonistic activation. The sequential muscle activation improves the thrust by increasing 1) the tail-beat velocity and 2) traveling wave content in the swimming kinematics by four times. Further, the second resonant frequency (8 Hz) generates nearly as much thrust as the first resonance (2 Hz) while requiring only $\approx25$% of the tail displacement, indicating that higher resonant frequencies have benefits for swimming in confined environments where a smaller kinematic envelope is necessary. These results demonstrate the performance benefits of independently controllable muscles and distributed kinematic sensing, and this type of soft robotic swimmer provides a platform to address the open challenge of sensorimotor control.

Figures (5)

• Figure 1: Fabrication process. (a) Exploded CAD rendering displaying internal structure and major dimensions, omitting wiring and mounting hardware. (b) Before silicone molding showing internal skeleton and HASEL locations. Left-side mounted strain gauges are even numbered, right-side are odd numbered. All are attached along the robot's midline. (c) Complete robot submerged underwater.
• Figure 2: Experimental setup with labeled equipment.
• Figure 3: Frequency response of the tail-tip velocity (normalized by input voltage) for each HASEL group.
• Figure 4: Swimming performance for in-phase antagonistic actuation. Each response (a) 2.05 Hz and (b) 8.05 Hz are broken into three parts: Muscle excitation waveform (i), time varying thrust (ii), and spatially varying swept envelope (iii) for in-phase cases. The window for the muscle excitation waveform is trimmed to show one second of oscillation. The time varying thrust plot's window is over the length of an entire trial. The swept envelope is shown over one period of oscillation with 16 linearly spaced strobe lines.
• Figure 5: Swimming performance for out-of-phase case: 8.05 Hz, $180\degree$ paired offset. Muscle excitation waveform (i), time varying thrust (ii), spatially varying swept envelope (iii). The deflection strobe lines are 9.1 ms apart.