Fish Fins Inspire
Agile Robots


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The robotic fish will be able to monitor and explore the ocean, once developed. Image: Drexel University

"Fish fins are quite amazing devices being moved through water — and they are definitely in the realm of mechanical engineers," says James Tangorra, assistant professor of mechanical engineering and mechanics at Drexel University.

Tangorra leads the College of Engineering's Lab for Biological Systems Analysis, which has a three-year grant from the Office of Naval Research. His team is studying the diverse functions of rayed fish fins, through behavioral observations, as well as robotic and mathematical models.

Eventually, the U.S. Navy may be able to adapt Tangorra's team's findings into sophisticated, fully automated robotic devices for defense, security, and underwater surveillance. "The Navy wants to learn how to create highly agile underwater vehicles. For instance, a swimming robot-fish could be used for harbor defense, as a sensor platform for swimmers, to explore in the ocean, for monitoring reefs–all sorts of possibilities," Tangorra explains.

Understanding Complex Interactions

"As engineers, we can build experimental devices to investigate biological hypotheses. We're comfortable building computer models, and the robot system let's us not just simulate, but also recreate, the world," he says. The mechanical systems they're studying are those that actually propel the fish. "We're looking at what makes them move so well, how the bones bend, and how the muscles act," says Tangorra, who describes fish fins as "having thin bones that are covered in a webbing as fine as Saran wrap."

Professor James Tangorra. Image: Drexel University

His team's model is a bluegill sunfish, selected because it uses multiple fins for different functions as it moves through water. "As engineers, we want to create robots that are self-directed and robust," says Tangorra. That involves understanding the complex interactions among the subsystems involved in swimming. Drexel's research includes biological systems, because they are almost completely autonomous. "What we're seeking are the fundamental principles that govern biological systems which we can then adapt for an engineered system," he informs.

To learn those sophisticated principles, Tangorra recognizes the importance of studying a range of animals in order to identify those principles that apply broadly. The Drexel team is collaborating with researchers at Harvard University and the University of Chicago to study the bluegill, specifically.

The team’s model for the robot is a bluegill sunfish because of its thin bones and intricate webbing.

Research-Driven Goals

Tangorra is careful to specify the goals of his project, which is research-driven rather than mission-driven. "We're not trying to make a fish robot that just swims. Instead, we're building these fish robots as a tool to help investigate biological systems. Fish are so much more agile than anything humans have ever created," he says. Tangorra's research explores exactly how the mechanics interact with a fish's control and sensory systems. "Our robotic fish won't be just a robot driving its fins. Our control system will be based off the motor neurons of fish as well as their sensory and vestibular systems."

The team continually and patiently moves its robotics system to the next level. Their focus is on gaining a broader perspective of how fins sense and propel. "Once we learn how to manufacture at the molecular level, we'll be able to recreate a fish. We'll continue studying and learning using the tools we currently have. As our tools get better, we'll move to the next level of study," Tangorra explains. "We're using the fins as a lab to show us the basics of the fluid mechanics that fish use. If the Navy requests it, "We can use what we've learned and apply it to a system that would be useful for certain Navy missions," he anticipates.

"Most robotic fish that have been developed to date have not included movable fins with fish-like fin rays that can be controlled. But a great deal of fish swimming performance and maneuverability is due to their ability to change fin shape and control fin movement. So the robotic fish being developed in the laboratory of Prof. James Tangorra represents a tremendous advance over current devices," says George V. Lauder, professor of organismic and evolutionary biology at Harvard University's Museum of Comparative Zoology.

His Drexel collaborator is certainly dedicated to pursuing the ongoing development of a robotic fish. "I've been working on this for seven years, and I hope to be working on it for another seventy," says Tangorra.

Carol Milano is an independent writer.

What we’re seeking are the fundamental principles that govern biological systems which we can then adapt for an engineered system.

James Tangorra, assistant professor of mechanical engineering and mechanics at Drexel University

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October 2012

by Carol Milano, ASME.org