  The robotic fish with high propulsion efficiency and good maneuverability
achieves underwater fishlike propulsion by commonly adopting the motor to drive
the fishtail, causing the significant fluctuations of the motor power due to
the uneven swing speed of the fishtail in one swing cycle. Hence, we propose a
wire-driven robotic fish with a spring-steel-based active-segment elastic
spine. This bionic spine can produce elastic deformation to store energy under
the action of the wire driving and motor for responding to the fluctuations of
the motor power. Further, we analyze the effects of the energy-storing of the
active-segment elastic spine on the smoothness of motor power. Based on the
developed Lagrangian dynamic model and cantilever beam model, the
power-variance-based nonlinear optimization model for the stiffness of the
active-segment elastic spine is established to respond to the sharp
fluctuations of motor power during each fishtail swing cycle. Results validate
that the energy-storing of the active-segment elastic spine plays a vital role
in improving the power fluctuations and maximum frequency of the motor by
adjusting its stiffness reasonably, which is beneficial to achieving high
propulsion and high speed for robotic fish. Compared with the active-segment
rigid spine that is incapable of storing energy, the energy-storing of the
active-segment elastic spine is beneficial to increase the maximum frequency of
the motor and the average thrust of the fishtail by 0.41 Hz, and 0.06 N,
respectively.
