Multifunctional soft robotic fish: using artificial muscles for energy recovery

Abstract Bio-inspired, autonomous underwater vehicles (AUVs) are promising to achieve the agile and efficient locomotion of animals, but like all AUVs their endurance is limited by onboard energy storage. This study demonstrates electrical energy regeneration on a bio-inspired robotic fish using its existing artificial muscles to harvest energy from the environment, providing a path towards energy autonomy with minimal added size and weight. Using a robotic fish with hydraulically amplified, self-healing electrostatic (HASEL) artificial muscles and variable capacitance energy harvesting, we establish the role of frequency, strain, and phase on energy harvesting performance. Through cyclic axial testing of individual muscles, we demonstrate a linear increase in output power with increasing input frequency where the HASELs generated a peak power of 3.2 mW cm -1 at 2 Hz. Further increases in frequency led to a reduction in generated power due to lower strain on the HASEL muscle and thus a decrease in capacitance change. We then experimentally demonstrate energy generation on a robotic fish-like prototype using HASEL muscles, and we visually capture the HASEL's change in capacitance during dynamic bending. Finally, we fully establish the role of frequency, strain, and phase on energy generation using a numerical kinematic model of a soft robotic fish with HASEL muscles. We show that maximum energy harvesting occurs 1) at the natural frequencies of the robotic fish, 2) when the HASEL muscles are spatially located at regions of maximum dynamic curvature, and 3) when the phase of the HASEL priming voltage is π/2 with respect to the stroke of the muscle, maximizing the capacitance change. By establishing the role of frequency, strain, and phase during HASEL energy harvesting, this study provides a framework for energy regeneration on robotic swimmers and flexible ocean wave energy converters with distributed energy converters.