High maneuverability and energy efficiency are crucial for underwater robots to perform tasks in engineering practice. Natural evolution empowers aquatic species with skills of agile and efficient swimming, which can be deliberately employed for better robotic swimmers. A critical issue for efficient robotic swimmers is the design and control of an appropriate propulsion system. This study, therefore, presents a completely different realization of a highly flexible and controllable bistable nonlinear mechanism as a “fishtail.” The mechanism combines an elastic spine and a lightweight parallel linkage mechanism. Through active control of the endpoint of the elastic spine, the compliant tail can be empowered with exceptional controllability and tunable bistability for a much more efficient and also the first-ever accurately controlled bistable elastic propulsion system. Experimental results demonstrate that the new bistable fishtail can achieve a faster speed of its size (up to an average speed of 0.8 m·s−1) with an associated higher energy efficiency (corresponding cost of transport as low as 9 J·m−1·kg−1), and greater maneuverability (with an average turning speed of up to 107°/s at a much smaller turning radius of 0.31 body length). This study will definitely provide an efficient controllable and feasible approach to the design of nonlinear compliant propulsion systems for underwater vehicles by exploring nonlinear dynamics.
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