Undulatory Swimming Performance Explored With a Biorobotic Fish and Measured by Soft Sensors and Particle Image Velocimetry.

Fabian Schwab,Ivan Lunati,Mirko Kovac,Yong-Lae Park,Claudio Mucignat,Fabian Wiesemüller,Ardian Jusufi

doi:10.3389/frobt.2021.791722

Abstract

Due to the difficulty of manipulating muscle activation in live, freely swimming fish, a thorough examination of the body kinematics, propulsive performance, and muscle activity patterns in fish during undulatory swimming motion has not been conducted. We propose to use soft robotic model animals as experimental platforms to address biomechanics questions and acquire understanding into subcarangiform fish swimming behavior. We extend previous research on a bio-inspired soft robotic fish equipped with two pneumatic actuators and soft strain sensors to investigate swimming performance in undulation frequencies between 0.3 and 0.7 Hz and flow rates ranging from 0 to 20 in a recirculating flow tank. We demonstrate the potential of eutectic gallium–indium (eGaIn) sensors to measure the lateral deflection of a robotic fish in real time, a controller that is able to keep a constant undulatory amplitude in varying flow conditions, as well as using Particle Image Velocimetry (PIV) to characterizing swimming performance across a range of flow speeds and give a qualitative measurement of thrust force exerted by the physical platform without the need of externally attached force sensors. A detailed wake structure was then analyzed with Dynamic Mode Decomposition (DMD) to highlight different wave modes present in the robot’s swimming motion and provide insights into the efficiency of the robotic swimmer. In the future, we anticipate 3D-PIV with DMD serving as a global framework for comparing the performance of diverse bio-inspired swimming robots against a variety of swimming animals.

Highlights

Bio-inspired and bio-mimetic research have both grown steadily over the last 2 decades and allowed the development of modern, more life-like robots inspired by natural objects
The proposed soft-robotic system is composed of a robotic fish, consisting of two soft pneumatic actuators that are attached to a flexible panel with stiffness comparable to that of a fish body and equipped with integrated eutectic gallium–indium sensors (Figure 1A)
The relationship of the amplitude, frequency, and water flow speed is investigated by examining the strain sensor reading, the tail location derived from the film, and the flow field visualized by Particle Imaging Velocimetry (PIV)

Summary

Introduction

Bio-inspired and bio-mimetic research have both grown steadily over the last 2 decades and allowed the development of modern, more life-like robots inspired by natural objects. Despite those more sophisticated designs, robots still fall short of the universality and robustness of animal movement and lag behind in important areas such as sensing capabilities and perturbation responses. Geckos run with ease across water (Nirody et al, 2018), crocodiles roll in complex patterns to kill their prey (Fish et al, 2007), and despite continually changing flow conditions and strong locomotor. The advantages of soft robotics are numerous: their ease of construction, inherent safety, and ability to handle fragile objects or move through unstructured terrains are all promising characteristics. Among the most often used modes of actuation are elastomeric actuators, hydrogels, form memory alloys (SMA), and electroactive polymers (EAP)

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