Does Bistability Improve Swimming Performance in Robotic Fish?

A bistable mechanism has two stable states with energy input required to move from one stable state to another. This energy barrier allows for energy storage and release which can be used to improve systems characteristics. Bistability has been used to increase the frequency range over which a kinetic energy harvester is effective, and it has been proposed that bistability can increase the efficiency of biomimetic swimming robots. However, experiments involving bistable swimming robots have typically used bistability as a means of overcoming limitations inherent to soft actuators, rather than to increase overall performance. This article implements bistability into a swimming robotic and compares performance with and without bistable action. The static thrust generation and power consumption for bistable and nonbistable configurations for five different tail morphologies are compared. Bistability is generally found to increase the system efficiency, particularly at lower frequencies where increases are observed up to 250%. The untethered swimming speed of the robot in open water is also found to increase by approximately 30%. The results show that bistability can offer direct performance benefits for biomimetic swimming, but that the bistable transmission must be well tuned to the dynamics of the rest of the system. This study asks whether bistability is a fundamentally useful feature of swimming mechanisms, or simply a tool to overcome actuation limitations. It quantifies the effects of introducing bistability into a simple bioinspired propulsion system with variable morphology. The results show efficiency gains up to 250% and speed increases of 30%, but also a need for mechanical tuning.image (c) 2024 WILEY-VCH GmbH