"Real fish follow a robotic one"

My theory on Structural Design and Control System of a Caudal Fin Robotic Fish.

For nearly 200 years, engineers have been optimizing one way of moving through water, essentially using a propeller. "Fish are very efficient," explained Xiaobo Tan, an assistant professor of electrical and computer engineering. "They can perform very efficient locomotion and maneuvering in the water." 

Robotic fish - perhaps schools of them operating autonomously for months - could give researchers far more precise data on aquatic conditions, deepening our knowledge of critical water supplies and habitats.

There are two types of crescent-shaped caudal fin propulsion: one is the left-right motion of body and caudal fin, such as tuna swordfish and shark; another is the up and down movement of body and caudal, such as whales and dolphins. Both of them have the same principle. 

Swimming models of fish are various depended on the types and species of the fish i.e. Eel swims by waving the whole body (anguilliform locomotion), Tuna waves the tail peduncle (thunniform locomotion), Salmon uses both tail and rear body (carangiform locomotion). Furthermore, influenced by the fluid environment, robotic fish’s propulsion is concerned with hydrodynamic and is hard to establish purely analytical methods. This means that exact mathematic models are difficult to be applied to specify the whole methodology by plain system. The system architecture of our robotic fish is based on the study of the natural movement of Carp Fish.  The robotic fish consists of modulating joint that propels the body by oscillating the tail peduncle and pectoral fins.

During the designing process, the swing-translational movement should be considered firstly. The design should refer to following parameters: 

1) Maximum of tail fin attacking water angle αmax,           15°< αmax <25°； 

2) Phase of tail fin swing – translational ψ, ψ=90° 

3) Amplitude of translational movement of caudal fin swing axis H, H= (0.75~0.1) LB; 

4) Maximum swing of posterior margin of caudal fin swing, ATmax ≈ 0.1 LB 

In designing process, the designing prototype of machine fish is tuna, which is composed of joints: caudal fin, caudal peduncle and pectoral fin. Fish body is designed to pituitary-shaped, its vertical axis and the vertical axis the ratio is 4 or so, and the later body is convergent into a tail. The caudal fin is designed into crescent-shaped. The tail is composed of two DC motor-driven linkage joints to achieve straight swimming and turning movement; the rising and diving movement is achieved by a stepper motor as pectoral fin.

Tuna, sharks have very low aspect ratio of the crescent-shaped tail fins, like an intermediate angle hydrofoil with an arc-shaped leading edge and sharp trailing edge. The profile of tail fin is low resistance shape and obtuse front edge, which can produce a lot of front-end suction, which can provide a large part of the propulsion. In order to better mimic the tail fins of fish, we designed the former half of the tail fin is aluminum, rigidity is good, is conducive to swing tail fin;  The latter half is silica gel, flexibility is good. This design improves propulsive efficiency and advance the speed of fish. The figure shows a shark-shaped caudal fin with large aspect ratio. The advancement model of carangid crescent-shaped caudal fin of fish’s body has greater stiffness, which the amplitude is mainly concentrated in the body after 1/3 part. The bionics design of tail is the most important section in fish designing. As shown in Fig, the unique single-motor drive linkage of the two joints of the caudal peduncle mechanical design will be able to simulate a very good tuna tail posture.

Under high-speed swimming, the pectoral fin as the elevator controls the way of up and dive, which is able to achieve sensitive dynamic response speed and efficient performance. Pectoral fin is usually arranged near fish head, paired-like, and adjustable angle of attack. Reference to the shark's pectoral fin proportion of the total body size and shape, machine fish has a single degree of freedom of the wing pectoral fin. As shown in Fig, by a stepper motor-driven rotating rod with the angle of rotation of the two pectoral fins, pectoral fins are made of thin aluminum sheet material, which have a stronger rigidity and a small gravity.

I performed two different sets of simulations to investigate thrust and drag of the robot. I did one set with constant C value and second with varying C value. As it is difficult to distinguish between drag of the robot and trust produced by the tail, I calculate the sum which we call traction. Traction is the integral of pressure and shear pressure over the body of the fish robot. As the robot swims to the negative x direction the smaller the traction the bigger the thrust is.

With constant C1 =0.372 we investigated the difference between continuous (N=30) and simplified (N=3) tail. The results show us that there is a difference between these two models. The continuous body modeled with 30 joints has greater average thrust and the thrust varies less. 

Swimming Velocity depends on the frequency in direct relationship. In the experiments, the maximum traverse magnitude is set to be 30° and 40° for the rear body part (moving part) and the tail peduncle respectively. Then, four experiments are test by adjusting the swimming amplitude (oscillation frequency) which set to be the proportional of the maximum traverse magnitude (Ka=0.2, 0.4, 0.6, 0.8). The speed of the robotic fish is direct depends on the swimming amplitude.

In accordance with crescent-shaped tail fin fish and on the basis of bionic research, the idea of bio-mimetic robot fish body is proposed, and i design a single-motor drive linkage of the two joints to promote bio-mimetic robot fish tail fin, as shown in Figure, with the stepper motor of pectoral as a fish elevator to achieve the machine fish upping and diving.

The Research paper got published in International Journal of Advanced Mechatronics and Robotics. Volume : 4 Issue No. :1(2012)  ISSN: 0975-6108

And The Short paper was published in International Conference on Mechanical, Automobile and Robotics Engineering (ICMAR'2012) Penang. Malaysia. Feb. 11-12, 2012 ISBN: 978-81-922428-5-9

For more details you may please visit the link. 

Journal Link: http://serialsjournals.com/articles.php?volumesno_id=379&journals_id=225&volumes_id=209

Conference Paper Link:  http://psrcentre.org