Like a fish to water

Inspired by the efficient swimming motion of fish, researchers are building a mechanical fin that could forever change the way robotic submarines are made

Propeller-driven submarines, or autonomous underwater vehicles (AUVs), currently perform a variety of functions, from mapping the ocean floor to surveying shipwrecks.

But a US-based team at Massachusetts Institute of Technology (MIT) hopes to create a more manoeuvrable, propeller-less underwater robot by mimicking the action of the bluegill sunfish. The robot could be used for military tasks such as sweeping mines and inspecting harbours.

“If we could produce AUVs that can hover, turn, store energy, and do all the things a fish does, they’ll be much better than the remotely operated vehicles we have now,” said James Tangorra, an MIT postdoctoral associate working on the project.

The researchers chose to copy the bluegill sunfish because of its distinctive swimming motion, which results in a constant forward thrust with no backward drag.

Tangorra and his team have built several prototypes of a mechanical fin that successfully mimics the sunfish fin. They reported the successful testing of their most recent fin, which is made of a cutting-edge polymer that conducts electricity, in the June issue of the Bioinspiration & Biomimetics journal.

Learning from nature

Work at George Washington University by Dr Meliha Bozkurttas was crucial to the fins built at MIT, said Tangorra. Bozkurttas broke down the fin movement of the sunfish into 19 components and analysed which ones are critical to achieving the fish’s powerful forward thrust.

“We don’t want to replicate exactly what nature does,” said Tangorra. “We want to figure out what parts are important for propulsion and copy those.

”The fin is able to replicate two motions that the researchers identified as critical to the propulsion of the sunfish fin: the forward sweep of the fins and the simultaneous cupping of the upper and lower edges of the fin.

Swimming with currents

The latest fin is made of a thin, flexible polymer material that conducts electricity.

When an electric current is run across the base of the fin, it sweeps forward, just like a sunfish fin. By changing the direction of the electric current, the researchers can make the fin curl forward at the upper and lower edges.

The researchers’ new approach, using the polymer, could eliminate the need for electric motors.

“This gives us the potential to build machines or robots in a manner closer to how nature creates things,” said Tangorra.

In future research, the team plans to look at interactions between different fins and between fins and the body. That will help in adapting nature’s principles to designing robotic vehicles, Tangorra said.

“To be appropriate for AUVs, you can’t just look at these as propeller replacements,” he added.

Right: Professor Ian Hunter (standing), and James Tangorra in MIT’s Bioinstrumentation Lab are working to create a robotic version of the bluegill sunfish’s fin. Such a robotic fin, like the one Tangorra is holding, could improve the manoeuvrability of autonomous underwater vehicles. Left: A bluegill sunfish swims in a laboratory tank near a prototype of a robotic fin designed with the fish’s fin as a guide