A robotic fish tail and an elegant math ratio could inform design of
next-gen underwater drones
Secrets of highly efficient swimming at varying speeds

Date:
August 11, 2021
Source:
University of Virginia School of Engineering and Applied Science
Summary:
Researchers built a fishlike robot that uses a programmable
artificial tendon to tune its own tail stiffness while swimming
in a water channel.

The results were impressive: The robot could swim over a wider
range of speeds while using almost half as much energy as the same
robot with a fixed-stiffness tail.



FULL STORY ========================================================================== Underwater vehicles are typically designed for one cruise speed, and
they're often inefficient at other speeds. The technology is rudimentary compared to the way fish swim well, fast or slow.


==========================================================================
What if you want your underwater vehicle to travel fast through miles
of ocean, then slow down to map a narrow coral reef, or speed to the
site of an oil spill then throttle back to take careful measurements?
Dan Quinn, an assistant professor at the University of Virginia School
of Engineering and Applied Science, and his colleague, recent UVA
Ph.D. graduate and postdoctoral researcher Qiang Zhong, discovered
a key strategy for enabling these kinds of multispeed missions. They
have demonstrated a simple way to implement this strategy in robots,
which could ultimately inform underwater vehicle design. Their work was recently published in Science Robotics.

When designing swimming robots, a question that keeps coming up for
researchers is how stiff the piece that propels the robots through the
water should be made. It's a hard question, because the same stiffness
that works well in some situations can fail miserably in others.

"Having one tail stiffness is like having one gear ratio on a bike,"
said Quinn, who holds joint appointments in mechanical and aerospace engineering and electrical and computer engineering. "You'd only be
efficient at one speed. It would be like biking through San Francisco
with a fixed-gear bike; you'd be exhausted after just a few blocks."
It is likely that fish solve this problem by tuning their stiffness
in real- time: They dial in different levels of stiffness depending on
the situation.

The trouble is, there's no known way to measure the stiffness of a
swimming fish, so it's hard to know if and how fish are doing this. Quinn
and Zhong solved this by combining fluid dynamics and biomechanics to
derive a model for how and why tail stiffness should be tuned.

"Surprisingly," Quinn said, "a simple result came out of all the math: Stiffness should increase with swimming speed squared.

"To test our theory, we built a fishlike robot that uses a programmable artificial tendon to tune its own tail stiffness while swimming in a water channel. What happened is that suddenly our robot could swim over a wider
range of speeds while using almost half as much energy as the same robot
with a fixed-stiffness tail. The improvement was really quite remarkable."
"Our work is the first that combines biomechanics, fluid dynamics, and
robotics to comprehensively study tail stiffness, which helps to uncover
the long- existing mystery about how tail stiffness affects swimming performance," Zhong said. "What is even more fantastic is that we are
not just focused on theory analysis, but also on proposing a practical
guide for tunable stiffness. Our proposed tunable stiffness strategy
has proved effective in realistic swimming missions, where a robot
fish achieved high speed and high efficiency swimming simultaneously."
Now that the team has modeled the benefits of tunable stiffness, they
will extend their model to other kinds of swimming. The first robot was designed like a tuna; now the team is thinking about how they could scale
up to dolphins or down to tadpoles. They're also building a robot that
emulates the undulatory motions of stingrays.

"I don't think we'll run out of projects anytime soon. Every aquatic
animal we've looked at has given us new ideas about how to build better swimming robots. And there are plenty more fish in the sea," Quinn said.

========================================================================== Story Source: Materials provided by University_of_Virginia_School_of_Engineering_and_Applied Science. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Q. Zhong, J. Zhu, F. E. Fish, S. J. Kerr, A. M. Downs,
H. Bart-Smith, D.

B. Quinn. Tunable stiffness enables fast and efficient swimming in
fish- like robots. Science Robotics, 2021; 6 (57): eabe4088 DOI:
10.1126/ scirobotics.abe4088 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/08/210811162830.htm

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