Design and Implementation of Thunniform Robotic Fish With Variable Body Stiffness

Abstract

In nature, undulatory swimmers consume minimum energy by adjusting their body's natural frequency to match the tail-beat frequency. Inspired by this, we study the dynamic model of fish body by considering the body shape and the fluid interactions, and develop a soft thunniform robotic fish. The experimental results show that when the driving frequency is close to the natural frequency, the speed of robotic fish reaches the maximum value, approximately 0.25 body length per second. Moreover, an empty space with a different air pressure is embedded into the body of robotic fish to vary its stiffness, and the natural frequency can be adjusted from 2.0 to 2.8 Hz by modulating the air pressure. The results of robotic fish with variable stiffness show that the forward speed and acceleration are increased with natural frequency, and a smaller stiffness of caudal fin contributes to a better propulsive performance. We also predict that the peak speed and acceleration can be obtained simultaneously by adjusting the stiffness of both fish body and caudal fin properly. These results reveal that the swimming abilities of robotic fish are closely related to the driving frequency and the stiffness property.

Tail-stiffness optimization for a flexible robotic fish

“…In the study by Xie et al [18], the performance of a caudal fin with medium stiffness was the best, and a similar conclusion was reached in [19]. In [20], the stiffness of the robotic fish was adjusted by varying the air pressure on the fish body so that the natural frequency could range from 2.0 to 2.8 Hz. The robotic fish developed by Li et al [21] contained an active joint and a passive joint composed of a torsion spring.…”

“…Compared with a rigid fin, a robotic fish propelled by a flexible caudal fin can perform the undulation curve of natural fish better. However, the kinematics and control of a fully flexible fin are more complex [18,20], and changing the stiffness requires making a new tail. Several studies have used passive flexible joints to connect rigid fins to realize propulsion [23,24].…”

New structure of pneumatic networks actuators for soft robotics

“…The materials used for soft robot fabrication include polymers [2, 3], elastomers [4], hydrogels [5, 6], and granules [7]. The model of actuation is another key factor for a soft robot, such as pneumatic [8, 9], electrical [10], or chemical [11].…”