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A robot fish is a kind of bionic robot, which has the shape and locomotion of a living fish.

Introduction
More than 400 articles about robot fish have been published since the Massachusetts Institute of Technology reported on the first robot fish in 1989. These reports indicate that about 40 different types of locomotive robot fish have been built, as well as about 30 different designs that can only flip and drift in the water. Fish swimming inspired the engineers and scientists who make the robot fish. They design and build the robot fish based on fish flapping. Most robot fish are designed to use the body and the caudal fin for movement, and are called BCF robot fish. BCF robot fish can be further divide into three categories which are the Single Joint (SJ), the Multi-Joint (MJ), and the smart material-based design. One of the most important points of researching and developing robot fish is their control and navigation. Moreover, robot fish can "communicate" with the environment, scroll through a particular path, and flap by controlled.

Design
The basic biomimetic robotic fish is made up of three parts: a streamlined head, a body, and a tail. The head is often made of a rigid plastic material (i.e. fiberglass) and accommodates all control units including a wireless communication module, batteries, and a signal processor. Some robot fishes have pectoral fins fixed on both sides of the body to ensure swimming stability. The body may be made up of many joints which are connected by servomotors. The servomotors can control the rotational angle of joints. Finally, caudal fin connects with the joints and is driven by a motor. The oscillating caudal fin can provide power to moving.


 * Design Inspiration

When studying robot fish locomotion, it is an inevitable aspect to study fish dynamic property. Researchers involve a diversity of structures and various behaviors from fish to design robot fish. The first robot fish (robot tuna) has same construction and shape with tuna. From past experiences, people often focus on functional design. For instance, designers attempt to create robots with flexible bodies (like real fish) that can exhibit undulatory motion. This kind of robot fish body enables the fish to swim as live fish which can deal with the complicated environment. To get thrust and maneuvering forces, robot fish control systems can control body and caudal fin to have wave-like motion. In order to control and analyze robot fish motion, researchers study the shape, dynamic models and lateral movements of robotic tail. One of the tail's shape is lunate; some studies show this kind tail can provide fast swimming and high-efficiency. The posterior tail is important because it creates thrust force. For real fish, its powerful muscles can generate lateral movements for locomotion. However, the anterior part (the head) remains in a relatively motionless state. Thus, researchers have focused on tail kinematics when building modeling robot fish motion.

Slender-body theory is used often study robot fish locomotion. The mean rate of work of the lateral movements is equal to the sum of the mean rate of work available for producing the mean thrust and the rate of shedding of kinetic energy of lateral fluid motions. The mean thrust can be calculated entirely from the following conditions at the trailing edge of the caudal fin: displacement and swimming speed.

Realistic propulsion systems can help improve robot fish autonomous maneuvering and show better locomotor performance. To achieve this goal, people can apply a diverse array of fins on robot fish. For example, by including pectoral fins, robot fish can perform force vectoring and do complex swimming behaviors instead of forward swimming only.

Real fish rely on various kinds of fins to accomplish propulsion. Thus, to achieve fast and maneuverable propulsion, robot fish also need multiple control surfaces. The propulsive performance is related to the position, mobility, and hydrodynamic characteristics of the control surfaces.
 * Control

The key for multilink robotic fish is how to simplify the mechanism and generate reasonable control data. Designers should consider some important factors, including lateral body motions, kinematic and anatomical data. When people mimic a BCF-type robot fish, the link-based body wave must provide fishlike motions. This kind of body wave-based swimming control should be properly discretized and parameterized for a specific swimming gait. It is hard to ensure swimming stability gait and transiting smoothly between two different gaits.

A central neural system known as a "Central pattern generator" (CPGs) can govern multilink robotic fish locomotion. The CPG located in every segment and can connect and stimulate contracting or stretching muscles. Cerebrum (controller) in the head can control supraspinal signal inputs to startup, stop and turn. After the systems formed a steady locomotion, there are not anymore signal from high controller and the CPGs can produce and modulate locomotion patterns. Neural networks are also used to control robot fish. There are several key points in the design of bionic neural network. Firstly, the bionic propeller just adopts one servomotor to drive a joint while the fish has two group muscles in each joint. We can design one CPG in each segment to control the corresponding joint. Secondly, a discrete computational model is adopted to simulate the continuous biological tissues. Lastly, the connection lag time between neurons always exists and determines the intersegmental phase lag. The lag time function in the computational model is necessary.

Used of Robotic Fish
Achieving a consistent response is a challenge in animal behavioral studies when live stimuli are used as independent variables. To overcome this challenge, robots can be used as consistent stimuli for testing hypotheses while avoiding large animal training and use. The robots as controllable machines can be made to "look, sound, or even smell" like animals. We can obtain a better perception of animal behavior because robots can get a steady response in a set of repeatable actions. Moreover, with various field deployments and a greater degree of independence, robots hold the promise of assisting behavioral studies in the wild.
 * Studying Fish Behavior

Robot fish is also used in various fields. Toy robot fish is the most common robot toys on the market. This robot fish is usually not used as a study, but for entertainment. The structure of this robot fish is simple and more inexpensive. They are usually divided into two types: automatic cruise and control movement. The simplest fish toy consists of a soft body (MJ), motor (tail) and head (basic electric control element). They use the battery to provide energy for the motor to produce propulsion and use the remote control systems to control body bending to achieve the purpose of steering. In contrast, the complexity of toys and robot fish with the purpose of research is almost the same. They are not only fully automated, but can also simulate fish behavior. Such as an expensive robot fish toy can be perceived by the changes in the water environment to make a decision of behavior automatically. If you suddenly put in the foreign matter in the water, this robot fish will produce a disordered and fast moving, as if the real fish was shocked. Robot fish record behavior in advance, they will make the corresponding treatment (disordered and fast moving), when faced with a similar situation.
 * Robot Fish Toy

Military defense and marine protection are important research topics, more and more complicated missions require high-performance Autonomous underwater vehicle (AUVs). AUVs need fast propulsion and multidirectional maneuverability. Robotic fish is more competent than current AUVs propelled by motion because the fish is a paradigm of bio-inspired AUV. Like fish, robot fish can operate in more complex environments. They can not only do underwater exploration and find natural species but also can achieve the purpose of salvage or setting up underwater facilities. In dealing with the malicious environment, robot fish also have better performance. For example, in the coral zone, soft robotic fish can better cope with the environment. Unlike existing AUVs which are non-flexible, robot fish can shuttle on narrow caves and tunnels.
 * Application on AUV

Besides from their vast potentials for research, robotic fish also show many opportunities for reaching out to students and the public. Bio-inspired robots are valuable and effective and can attract students to those areas of science, technology, engineering and math. Robotic fish have been applied as auxiliary educational tools all over the world. For instance, thousands of youth attracted the carp-like robots during their exhibits at London Aquarium. People have presented various kinds of robotic fish at many outreach programs, including the first and the second USA Science and Engineering Festivals, in 2010 and 2012, respectively. At these events, the visitors got a chance to not only see the robotic fish in action but also interacted with the lab members to understand the technology and its applications.
 * Education

Example of Robotic Fish
The [[Robo Tuna[[ is a robotic fish which has the shape and function of the real tuna fish which designed and built by a team of scientists at the Massachusetts Institute of Technology (MIT) in the US. It has a complicated system of stainless-steel cables and pulleys, which act as muscles and tendons. The envelope composed of a flexible layer of foam covered with Lycra to advance the flexibility and smoothness the tuna skin. It controlled by six powerful servomotors at two horse-power each, that power through the body. It can adjust its motions in real-time because some force sensors which positioned on the side of the ribs present continuous feedback to the robot.
 * Robot tuna

Robot Pike is the world's first free-swimming robot fish which designed and built by a team of scientists at the MIT in the US. It is not autonomous free-swimming, it directed by the human. It is the computer which interprets the orders and returns the signals appropriate to each engine. It has a skin composed of silicone rubber and a spring-wound fiberglass exoskeleton which makes the robot fish flexible. It can accelerate at a rate of eight to twelve m/s in the water, but it cannot avoid obstructions because it does not equip with sensors.
 * Robot Pike

The Essex Robotic Fish is a robot fish built by scientists at Essex University. It can be autonomous swim like a real fish, and achieve different types of displacement. It has four computers, five motors and over ten sensors onboard. It can swim around a tank and avoid objects. It also will adapt to the uncertain and unpredictable environment in the future. It will have a broad range of future uses, including seabed exploration, detection of leaks in oil pipelines, sea life exploration, and even spying.
 * Essex Robotic Fish

The Jessiko is an underwater robot which produced by French start-up Robotswim in the Paris area. It only 22 centimeters’ length which is one of the smallest robotic fishes in the world. It is very easy to control, can go backward, shift colors, and imitate living fish performance. Because of these function, it can share emotions and even interact with people. It applied artificial intelligence and communication potential uses, so it can swim in more than ten fish to create exciting swimming choreography and light effects. It uses fins to move through the water. It approved that the probability of a small robot fish can swim autonomously for hours.
 * Jessiko by Robotswim

The Robotic Fish SPC-03 is a robot fish with SPC-03 which designed by Chinese Academy of Sciences (CASIA) in China. It can dimension 1,23-meter length in the water. It is steady, very handy, and is controlled remotely by technicians. It can work 2 to 3 hours in immersion at the maximum speed of 4 km/h controlled by the human. It can take and transfer photos, the cartography of the underwater funds, and the transport of small objects.
 * Robotic Fish SPC-03

The Robotic Koi developed by Ryomei Engineering of Hiroshima in Japan. It is 80 centimeters and 12 kg. It can be used to control the oxygen concentration in the water because it has sensors on its mouth. It also can gather information about the species by swimming among them by remote managed to supervise the health of fish. It can equip camera, so it can record of examining the resources present in the depths. It could also be used to survey the possible damage to the bridges and oil platforms underwater.
 * Robotic Koi