Robotic Fish Design and Underwater Pollution Control

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Water pollution is a rampant worldwide predicament today. Scientists think about 8 million metric tons of plastic waste enter the oceans each year. That’s the weight of about 90 airplanes. Aside from plastic waste, chemicals from factories and waste products from humans pollute the planet’s oceans. Underwater species and animals are suffering as a result.

Efforts are being made to curb underwater pollution. Now, there may soon be a new type of paraphyletic aquatic life in the world’s waters – a robotic fish used to track underwater pollution levels and underwater life. Currently, there are many underwater vehicles used to track the movement of submarines and to clear waterways, but their size and shape don’t exactly blend in with aquatic life.

Autonomous underwater vehicles are also limited by their maneuverability and the level of noise created by these tracking systems. When considering locomotion of small fish, these aquatic species can move effectively in an aqueous environment due to their small size, make better use of the water, and make less noise.

The lack of noise in a large aquatic environment will also be a survival technique for small fish to help prevent them from being tracked by larger prey. Monitoring pollution levels isn’t the only reason why the development of robotic fish technology is important, as they also allow one to observe feeding habits and behavioral patterns to help understand a given species and to use this information in efforts to conserve the environment.

Basic Biomechanics of a Robotic Fish

The structural frame of a fish is divided up into specific sections based on function. To begin with, we can look at the caudal fin. Zhao et al and a team of researchers at the Institute of Automation, Chinese Academy of Sciences, Beijing, demonstrate a novel prototype of robotic fish.

The caudal fin, also known as the tail fin, propels the fish and allows it to move through water freely. The team has used a tail that moves up and down to help push the fish in a forward direction using a direct current motor. There are also rubber tubes to this prototype that make up the frame of the fish’s body (figure 1).

Figure 1. The basic structure of a robotic fish. Source: Zhao C, et al. Design and control of biomimetic robotic fish FAC-I. Published in Kato, N., Kamimura, S. (2008). Bio-Mechanisms of Swimming, and Flying: Fluid Dynamics, Biomimetic Robots, and Sport Science. Japan: Springer Science and Business Media.

The caudal fin is only part of the structure of a fish. Fins are also important for a fish to be able to maintain balance in the water. In a protocol design, Zhao and the research team designed the front end of a fish by creating two pectoral fins that can rotate freely and encompass three degrees of freedom with both fins responding to a step motor.

There are three motors to help maneuver the robotic fish: motor 1 is attached to a Cardan shaft (a propeller shaft that transmits torque and rotation), and motors 2 and 3 are also attached to a separate Cardan shaft at the opposite end of the structure. Motors 1 and 3 are responsible for moving back and forth to create a left and right movement in the pectoral fin. The pectoral fins begin to roll to change the positioning of the robot fish in the water.

The application of just one robotic fish in a dynamic underwater environment is not going to be able to capture the range of movement of aquatic life and monitor behavior, or even provide sufficient data on oceanic pollution. It is therefore important to use a multi-robotic fish system that can provide large amounts of data at any one time. Shao et al have created a hardware platform that can support the application of a multi-robotic fish system in monitoring aquatic life. This team has divided the hardware platform into four systems: robotic fish, image capturing, decision making and control, and wireless communication.

The image capturing system comprises of an overhead camera that gathers data on the surrounding underwater environment. These images are then transferred to a decision-maker as input signals which in response generates control commands wirelessly, to be interpreted by the multi-robotic fish system.

SHOAL Project

Recent press has highlighted several research projects currently looking at the application of robotic fish in the management of underwater pollution. A popular example is the SHOAL project which is currently taking the media by storm and is a project managed by the BMT group. The group has been able to engineer a multi-robotic fish system that can work in synchronicity to track pollution in ports.

The interesting functional aspect of these fish is their ability to test water for pollutants in-situ. Each fish is built with chemical sensors that are designed to trace the source of the pollution and are able to do this effectively using ultrasonics. The robotic fish can move underwater for eight hours without any assistance from humans. Professor Huosheng Hu and his team at the School of Computer Science and Electronic Engineering at the University of Essex are working on building up to five robotic fish that are set to be tested in the port area of Gijon in Northern Spain. The video below describes the radical idea of robotic fish and the benefit of using this technology in monitoring pollution levels in the water.

One of the major challenges with robotic fish concerns their power supply – such robotic fish need to be designed so that they are not tethered with wires so that they can move freely in deep water.  Therefore, efficiency and how well this system can use a power supply will be a hot topic for advancing this research area. Furthermore, robotic fish may have been studied extensively in a lab setting but being able to monitor how this system interacts and responds to stimuli underwater will be challenging.

The SHOAL project was a European Research Project under the Seventh Framework Programme for ICT. The robotic fish was designed to swim in ports and coastal waters, navigating with communications systems to let them share information between themselves and the port. They also have chemical sensors to sense pollutants.

A trial of the system was launched at the Port of Gijon, where three robotic fish worked to monitor for pollutants. The robots were able to detect and measure water quality factors like dissolved oxygen and salinity.

The project has successfully shown that it’s possible to monitor pollution in real-time and to also determine sources of water pollution.

Sources and Further Reading

• Mazumder, S.K. (2011). Wireless Networking Based Control. USA, New York: Springer Science and Business Media, LLC.
• Hailes, S. (2010). Sensors Systems and Software: First International ICST Conference, S-CUBE. Pisa, Italy, September 2009. Revised Selected Papers. USA, New York: Springer Science and Business Media.
• Zhao C, et al. Design and control of biomimetic robotic fish FAC-I. Published in Kato, N., Kamimura, S. (2008). Published in Bio-Mechanisms of Swimming and Flying: Fluid Dynamics, Biomimetic Robots and Sport Science. Japan: Springer Science and Business Media.
• Shao, J, et al. Development of the Multiple Robot Fish Cooperation System. Published in: Ali, M. Advances in Applied Artificial Intelligence: 19th International Conference on Industrial Engineering and Other Applications of Applied Intelligent Systems, IEA/AIE 2006.
• The BMT Group

This article was updated on the 3^rd September, 2019.