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At present, most of bionic robotic fish in BCF mode make use of reciprocating positive and negative rotation of a servo to drive the swing motion of the tail fin. However, the swing frequency of the tail fin of the robotic fish is relatively low and the generated propulsive force is also relatively small due to the influence of the properties of the servo. The other robotic fish which use DC motor or other unidirectional rotation motor to drive the tail fin have to take advantage of complex mechanical structure to realize reciprocating swing motion of the tail fin, even so, the swing amplitude of the tail fin cannot be adjusted directly. This paper presents a novel bionic swing propulsion mechanism which is driven by a stepper motor. The tail fin can swing at a high frequency by controlling the rotate speed in positive rotation of the stepper motor, and correspondingly, the swing amplitude of the tail fin can be adjusted by controlling the stepper motor to reverse rotation. In addition, a steering mechanism is added to this bionic swing propulsion mechanism to realize the steering function for the propulsion mechanism. The presented swing propulsion mechanism can be directly used for a swing mechanism of tail fin of robotic fish, or even can act as a kind of bionic propeller of underwater vehicle to replace the traditional screw propeller. In order to verify the feasibility of this underwater bionic propulsion mechanism, a series of swimming experiment were carried out.
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