On the contrary, swimming speed using the tuna-type tail fin is higher than that of the pike-type tail fin, in the range of higher frequency. It tends to strong at phase angle, β, of 90 degrees. It is considered that an effect of fluid force (lift force) obtained by the streamlined cross-section tail fin becomes strong with increasing frequency in the case of the tuna-type tail fin. Thus, the tuna-type tail fin is suitable in the range of higher frequency.
In the whole experiments, the maximum swimming speed was about 0.4 m/s at frequency, f, of 3 Hz using the tuna-type tail fin. The prototype fish robot swims 0.7 times of its body length per second. This is not sufficiently high speed, because one of real fishes swims about 2 times of its body length per second [2]. It is caused that friction resistance of the prototype fish robot is large, because a shape of body and a surface of tail peduncle are not suitable for high-speed swimming. Also, it is caused that the maximum frequency is limited low by rotating speed of the servomotors.
3.3 EFFECTS OF PHASE ANGLE
Figure 8 shows experimental result of the relationship between phase angle, β, and swimming speed, V, at frequency, f, of 0.8 Hz, 1.6 Hz or 2.3 Hz. In the case of the pike-type tail fin, the maximum swimming speed is obtained at phase angle, β, of 60 degrees approximately. And, in the case of the tuna-type tail fin, the maximum swimming speed is obtained at larger phase angle than that of the pike-type tail fin. Amplitude of the rear end of tail fin increases with decreasing of phase angle. Thus, it is considered that smaller phase angle obtains strong propulsive force in the case of the pike-type tail fin. On the other hand, it is considered that phase angle of about 90 degrees obtains the strongest propulsive force in the case of the tuna-type tail fin. This result agrees with the well known fact in airfoil theory that the phase angle of 90 degrees obtains the maximum propulsive efficiency.
3.4 DISCUSSION
The experimental results are summarized as following fundamental performance.
(1) The maximum swimming speed of the prototype fish robot is about 0.4 m/s using the tuna-type tail fin.
(2) In the case of the tuna-type tail fin, the range of high frequency is suitable for high-speed swimming. Thrust force becomes strong at phase angle of about 90 degrees.
(3) In the case of the pike-type tail fin, thrust force becomes strong at phase angle of about 60 degrees.
From the experiments, following problems are clarified.
(1) The prototype fish robot uses the float, and swims at shallow depth. It is considered that wave on water surface affects propulsive performance. Thus, it is difficult to compare with characteristics of defferent types of the tail fin exactly.
(2) A shape of tail peduncle and location of joints were determined in view of easy manufacturing and easy adjustment of buoyancy. It is necessary to discuss and determine the shape and the location in detail, when hydrodynamics characteristics like propulsive force are estimated.
(3) In high frequency areas, the sine curve is disturbed considerably by the control signal for changing the operation mode. Thus, motion of the servomotors may not realize the programmed motion. It is necessary to develop a high quality control program and a measuring and control system for motion of the servomotors in the next step.
Fig. 7 Swimming speed as a function of frequency
Fig. 8 Swimming speed as a function of phase angle
