Shape-morphing robotic fish

Abstract

Robotic fish have gained attention for their potential applications in underwater exploration, environmental monitoring, and bio-inspired robotics research. These systems aim to replicate the efficient propulsion and maneuverability observed in biological fish. However, current robotic fish designs are limited by their fixed stiffness and inability to dynamically adapt to varying environmental conditions. Traditional solutions for modulating stiffness or morphology often rely on bulky hardware or complex external systems, limiting scalability and versatility. This thesis addresses these limitations through the development of an embedded morphing scheme that integrates actuation, sensing, and shape-locking mechanisms directly into the robot's structure. Utilizing Shape Morphing Modules (SMMs) composed of Shape Memory Polymers (SMPs) and Twisted and Coiled Actuators (TCAs), this scheme enables compact and efficient systems capable of real-time stiffness and shape modulation. Beyond robotic fish, this scheme demonstrates versatility in applications such as adaptive grippers, reconfigurable surfaces, and robotic manipulators requiring dynamic morphing. We first implemented the embedded morphing scheme in a robotic fish with a variable-stiffness tail. The tail's stiffness was adjusted by controlling the curvature of thin plates via TCAs, with the shape locked by SMP ribs. Experimental results revealed improved adaptability for both speed and maneuverability under different conditions, though excessive stiffness caused buckling under high forces, indicating a trade-off between stiffness and structural limits. The embedded morphing scheme was extended to a robotic fish with a morphing body to explore the relationship between body shape and swimming performance. Modified SMMs allowed for dynamic changes in body depth and width. The fish was controlled using a Central Pattern Generator (CPG) model, which enabled precise tuning of swimming parameters such as frequency, amplitude, and phase offset. Testing showed that body shape significantly influenced swimming performance, with a flat-flat configuration yielding higher speeds compared to a medium-large configuration due to reduced drag.