Turning Modes for a Fish Robot

February 3, 2001 by Koichi Hirata

Though real fish turn skillfully using not only tail fin but also pectoral fins or ventral fins, we determined a structure of a fish robot, which turns with only swing of tail fin. As the tail fin is utilized both propulsion and turning, the fish robot gets simple structure and easy control for swimming. In the case of turning with only swing of tail fin, we think that there are three turning modes shown as follows.

Turning Mode A
The figure to the right shows the case of Mode A. The fish robot swings its tail only to one side during a turning. It is considered that this mode is the most fundamental and important turning mode, because the robot can turn with various turning diameter and speed in this mode.

In this turning mode, a head and a body of the fish robot are equivalent to a rudder, and the tail peduncle and the tail fin are equivalent to a screw propeller of the ship. If we attend to the resemblance of these functions, we can analyze this mode and control the robot easily.

Turning Mode B
The figure to the right shows the case of Mode B. At first, the fish robot swims straight, and gets kinetic energy. Next, the fish robot turns its tail to one side, and keeps the posture to the side. Then the fish robot turns by hydrodynamics force. It is considered that this mode gets smaller turning diameter than that of Mode A.

In Mode B, as the stationary posture of the fish robot with the leaning of the tail is regarded as a wing, its friction force and lift force can be estimated. It is possible to analyze the turning performance and control the movement by considering the kinetic energy at the straight propulsion.

Turning Mode C
The figure to the right shows the case of Mode C. The fish robot swings its tail to one side rapidly from stationary state. In this turning mode, inertia force and friction force of the moving tail and a body are changed to the moment of rotation. This mode has excellent characteristics. It is possible to turn from the stationary state, and its turning diameter is the smallest in the whole modes. However, it is difficult to control turning speed and turning angle. Also, in order to get quick turning, it is necessary that the power source for tail swing should have sufficiently high torque.

Though there are the other turning modes combined the above three modes, only these basic turning modes are treated in our study.

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