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Since the propulsion mechanism using elastic fins, such as the caudal fin and pectoral fin of fish, is effective in fluid, many studies on bioinspired elastic fins for propulsion in water and on the development of fish-type robots with elastic fins have been carried out. The optimum elasticity of the fin is not constant and varies according to the movement task and environment, such as swimming speed and oscillating frequency. However, it is very difficult to exchange fins of different stiffnesses while moving. Thus, we aimed to develop a variable-stiffness fin of which stiffness can be changed dynamically. As the one such variable-stiffness fin, we have developed a fin with a variable-effective-length spring. The effective length of a plate spring is changed by adjusting the length of the rigid plate that supports the plate spring. Apparent stiffness is changed by varying the effective length. In this paper, we have described the structure of the propulsion mechanism in fluid using a fin with a variable-effective-length spring, and the thrust force characteristics in water. Furthermore, we have discussed the optimum effective length for providing the maximum thrust force and the effect of the dynamic change of effective length on thrust force. |
