A novel propulsion mechanism using a fin with a variable-effective-length spring

Since the propulsion mechanism in fluid using an elastic fin, such as the caudal fin or the pectoral fin of fish, is effective. However, the optimum elasticity of the fin is not constant and changes with the movement task and environment. It is very difficult to exchange fins of different stiffness while the robot is swimming. Thus, we attempt to develop a variable-stiffness fin with a variable-effective-length spring. The apparent stiffness of this spring can be changed dynamically. The present paper describes the thrust force, and flow velocity corresponding to the self propelled speed of the fin in a uniform flow. And we compared the thrust efficiency in a uniform flow with the evaluating value of the thrust efficiency in a no flow. Furthermore, we developed a flow visualization system and discussed the flow-field around the fin in a uniform flow.