Designing novel carangiform fish robots with undulating hair clip mechanisms

Bi- and multistable mechanisms have gained significant attention in the field of soft and compliant robotics due to their distinctive mechanical and dynamic properties, which enable complex motion, adaptability, and energy-efficient actuation for various applications. This study investigates the Hair Clip Mechanism (HCM), defined as a kinked ribbon with its two extremities pinned together, analogous to a snap hair clip. While previous studies have explored similar concepts, they have not specifically investigated the snapping behavior of hair clip-like mechanisms as robotic actuators, leaving a critical gap in the understanding and application of this unique bistable structure. This paper systematically analyzes the static and dynamic properties of a generalized HCM, validates the theoretical models through experimentation, and demonstrates its application in the design of two distinct versions of carangiform fish robots. The first, a tethered pneumatic fish robot, achieves a swimming speed of 1.40 body lengths per second (BL/s) or 26.54 cm/s, approximately twice the velocity of a conventionally designed counterpart. The second, an untethered motor-driven HCM-based fish robot, attains a speed of 2.03 BL/s or 42.6 cm/s, thrust of 245.66 mN, cost of transport (CoT) of 5.14, energy efficiency of 3.89%, and thrust-to-power ratio of 79.46 mN/W at 3 Hz undulation. These metrics position it among the fastest soft robotic swimmers reported. Notably, its performance is 40% (in BL/s) higher than that of the previously documented record-holding design. These findings highlight the potential of HCM-based structures to enhance the adaptability and performance of soft robotic systems, paving the way for innovative applications in bio-inspired locomotion and compliant mechanism design.