A Study on Kinematic Pattern of Fish Undulatory Locomotion Using a Robot Fish

Abstract

Recent state-of-art researches on robot fish focus on revealing different swimming mechanisms and developing control methods to imitate the kinematics of the real fish formulated by the so-called Lighthill's theory. However, the reason why robot fish must follow this formula has not been fully studied. In this paper, we adopt a biomimetic untethered robot fish to study the kinematics of fish flapping. The robot fish consists of a wire-driven body and a soft compliant tail, which can perform undulatory motion using one motor. A dynamic model integrated with surrounding fluid is developed to predict the cruising speed, static thrust, dynamic thrust, and yaw stability of the robot fish. Three driving patterns of the motor are experimented to achieve three kinematic patterns of the robot fish, e.g., triangular pattern, sinusoidal pattern, and an over-cambered sinusoidal pattern. Based on the experiment results, it is found that the sinusoidal pattern generated the largest average static thrust and steady cruising speed, while the triangular pattern achieved the best yaw stability. The over-cambered sinusoidal pattern was compromised in both metrics. Moreover, the kinematics study has shown that the body curves of the robot fish were similar to the referenced body curves presented by the formula when using the sinusoidal pattern, especially the major thrust generation area. This research provides a guidance on the kinematic optimization and motor control of the undulatory robot fish.