The effect of compliant joint and caudal fin in thrust generation for robotic fish

Fish generates large thrust through an oscillating motion with a fin. It is assumed that the flexibility of a fin affects the thrust generated by the fish. However, detailed investigation on the relationship between the flexibility of the fin and thrust generation is lacking. In this paper, the driving mechanism of a robotic fish is implemented using a compliant joint and caudal fin that is adapted from fish. The effect of the passive mechanism on the thrust generation with changes in the stiffness and frequency is investigated with this driving mechanism. The present research is carried out to understand the relationship among the compliance of the joint and caudal fin, the frequency of oscillating motion and the thrust which is generated by oscillating motion. The thrust is measured using a force transducer while varying the frequency and compliance. The bending angles between the compliant joint and caudal fin are also compared with the changes of the thrust in one cycle. The results show the appropriate stiffness of the compliant mechanism can be found in order to generate maximum thrust under the condition that the other parameters are not varied.