A Wire-Driven Dual Elastic Fishtail With Energy Storing and Passive Flexibility

The extraordinary swimming performance of fish benefits from the complex motion of the continuum fishtail, which is difficult for robotic fish to replicate. The common approach is adopting a large number of discrete joints, which introduces some challenges, e.g., the redundant degrees of freedom, the friction loss of adjacent joints, etc. Hence, this article presents a robotic fish equipped with a wire-driven dual elastic fishtail with energy storing and passive flexibility. This fishtail consists of an active elastic fishtail and a passive flexible joint, both of which periodically store energy due to the elastic deformation, benefiting to improving the motor's output stability and swimming performance, respectively. Especially, fishtail automatically adapts to complicated fluid by passive flexibility. Differing from the existing robotic fish, the wire-driven mode is combined with an efficient transmission mechanism to improve transmission efficiency. Using the developed dynamic model, the fishtail's energy storing is analyzed, and fishtail's stiffness is optimized to obtain the expected swing and high swimming performance. Extensive simulations and experiments have been conducted to validate the proposed model, and our robotic fish is capable of a maximum speed of 0.92 m/s, i.e., 1.87 BL/s.