Tuna-like robot may pave the way for more agile and efficient underwater robots and drones
Robots that can swim underwater are nothing new. For example, in early 2021 Carnegie Mellon equipped its famous snake robot with turbines and thrusters to give it water sports capabilities. But few can do this with the elegance, speed and ease of a real fish. And it's not that scientists have avoided trying to build a robot capable of doing exactly that.
Underwater robots are useful for exploring deep-sea objects, from sunken ships to new forms of life. However, they have some limitations. For example, they tend to move at a single speed, and operators most often have to drop them into the water near the desired location. Diver propulsion vehicles, mainly underwater scooters, are fast, but they cannot get close to something that requires closer study.
What is really needed is an underwater robot that can swim to a distant location and then slow down for a closer look. But this is more complicated than it might seem at first glance.
Researchers from the University of Virginia figured out how to create a robotic vehicle that does all of the above by mimicking the stiffness of a fish's tail, and discovered and applied a beautifully simple mathematical formula. As a result, they built a robot that can speed up and slow down almost as well as a real fish. They published their work in Science Robotics.
The mystery of fish solved
“The stiffness of a single tail is like having a single gear on a bicycle. You would only be efficient at one speed. It would be like riding a fixed-gear bike around San Francisco, and you would be exhausted after a few blocks”
Quinn and his research colleague Qiang Zhong hypothesized that fish change the stiffness of their tails to meet their often-changing need for speed. Unfortunately, this is hard to confirm or measure while a fish is swimming, so the stiffness needed for efficient movement at different speeds was unknown. The researchers combined hydrodynamics and biomechanics to develop a model that could provide an answer and, as Zhong says, "uncover a long-standing mystery about how tail stiffness affects swimming efficiency".
The solution to the puzzle turned out to be unexpectedly elegant. "Amazing. A simple result came out of all the math: stiffness should increase with the square of swimming speed," said Quinn.
Tuna passes the test
To test their conclusions, the scientists built a tuna-like underwater robot with a programmable artificial tendon, which could be used to adjust the stiffness of its tail for different speeds as it moved through a test water channel.
“Our robot was suddenly able to swim across a wider range of speeds, using almost half as much energy as the same robot with a fixed-stiffness tail. The improvement was really remarkable”
Zhong credits their interdisciplinary approach: "We are not only focused on theoretical analysis, but also offer practical guidance on tuning stiffness." The researchers' proposed adjustable-stiffness strategy proved effective in realistic swimming missions, when the robot fish achieved high speed and high swimming efficiency at the same time.
Having demonstrated their theory with the adjustable tuna, the scientists are interested in transplanting the tail technology to aquatic robots ranging in size from tadpoles to dolphins. Quinn and Zhong are currently developing a wave-like robot based on rays.
"Fish have high swimming efficiency across a wide range of speeds. The key to this achievement is their flexibility, but in terms of performance even flexible robotic fish cannot track real fish. <…> We developed a model that explains how and why tuning stiffness affects performance. We show that for maximum efficiency, muscle tension should scale with the square of swimming speed, suggesting a simple tuning strategy for fish-like robots. Stiffness adjustment can double swimming efficiency at frequencies and speeds similar to tuna (from 0 to 6 hertz, from 0 to 2 body lengths per second). Energy savings increase with frequency, suggesting that high-frequency fish-like robots benefit most from stiffness tuning," says the scientific paper.
“I don't think we'll run out of projects any time soon. Every aquatic animal we've looked at has given us new ideas on how to build better swimming robots. And there are still many fish in the sea”
One day, the researchers' work may lead to a whole class of drones that can quickly travel to a remote location and then slow down to explore the area. Similar to the aquatic snake robot, this is something the US Army could use to inspect its ships for damage.
