Fast Swimming Robots Based on Elastic Instability

Inspired by a steel hairclip, we exploit in-plane prestress instability to form a compliant bistable flapping fish-tail mechanism. Because of nonlinear drag effects, the rapid snap-through action produces large propulsion forces against water. Using this mechanism, we demonstrate an untethered robotic fish able to achieve a velocity of 2.03 body lengths per second (43.6 cm/s), an improvement of 194% (85.5%) over past compliant fish robots. A pneumatic version of the fish proves the compatibility of the mechanism with smart actuation and that the mechanism is about twice as efficient as the traditional design. We study the mechanism both computationally and experimentally and suggest that elastic instability may offer a path to overcome the speed problem of soft and compliant robots.