Researchers at Northwestern Univ. have created a robotic fish that
can move from swimming forward and backward to swimming vertically almost
instantaneously by using a sophisticated, ribbon-like fin.
The robot—created
after observing and creating computer simulations of the black ghost knifefish—could
pave the way for nimble robots that could perform underwater recovery
operations or long-term monitoring of coral reefs.
Led by Malcolm MacIver,
associate professor of mechanical and biomedical engineering at Northwestern’s McCormick
School of Engineering and Applied Science, the team’s results are published in
the Journal of the Royal Society
Interface.
The black ghost
knifefish, which works at night in rivers of the Amazon basin, hunts for prey
using a weak electric field around its entire body and moves both forward and
backward using a ribbon-like fin on the underside of its body.
MacIver, a robotics
expert who served as a scientific consultant for “Tron: Legacy” and is science
advisor for the television series “Caprica,” has studied the knifefish for
years. Working with Neelesh Patankar, associate professor of mechanical
engineering and co-author of the paper, he has created mechanical models of the
fish in hopes of better understanding how the nervous system sends messages
throughout the body to make it move.
Planning for the
robot—called GhostBot—began when graduate student Oscar Curet, a co-author of
the paper, observed a knifefish suddenly moving vertically in a tank in MacIver’s
lab.
“We had only
tracked it horizontally before,” said MacIver, a recent recipient of the
prestigious Presidential Early Career Award for Scientists and Engineers. “We
thought, ‘How could it be doing this?’”
The Incredible Robot Fish from Northwestern News on Vimeo.
Further
observations revealed that while the fish only uses one traveling wave along
the fin during horizontal motion (forward or backward depending on the
direction on the wave), while moving vertically it uses two waves. One of these
moves from head to tail, and the other moves tail to head. The two waves
collide and stop at the center of the fin.
The team then
created a computer simulation that showed that when these “inward
counterpropagating waves” are generated by the fin, horizontal thrust is
canceled and the fluid motion generated by the two waves is funneled into a
downward jet from the center of the fin, pushing the body up. The flow
structure looks like a mushroom cloud with an inverted jet.
“It’s interesting
because you’re getting force coming off the animal in a completely unexpected
direction that allows it to do acrobatics that, given its lifestyle of hunting
and maneuvering among tree roots, makes a huge amount of sense,” MacIver said.
The group then
hired Kinea Design, a design firm founded by Northwestern faculty that
specializes in human interactive mechatronics, and worked closely with its
co-founder, Michael Peshkin, professor of mechanical engineering, to design and
build a robot. The company fashioned a forearm-length waterproof robot with 32
motors giving independent control of the 32 artificial fin rays of the
lycra-covered artificial fin. (That means the robot has 32 degrees of freedom.
In comparison, industrial robot arms typically have less than 10.) Seven months
and $200,000 later, the GhostBot came to life.
The group took the
robot to Harvard Univ. to test it in a flow tunnel in the
lab of George V. Lauder, professor of ichthyology and co-author of the paper.
The team measured the flow around the robotic fish by placing reflective
particles in the water, then shining a laser sheet into the water. That allowed
them to track the flow of the water by watching the particles, and the test
showed the water flowing around the biomimetic robot just as computer
simulations predicted it would.
“It worked
perfectly the first time,” MacIver said. “We high-fived. We had the robot in
the real world being pushed by real forces.”
The robot is also
outfitted with an electrosensory system that works similar to the knifefish’s,
and MacIver and his team hope to next improve the robot so it can autonomously
use its sensory signals to detect an object and then use its mechanical system
to position itself near the object.
Humans excel at
creating high-speed, low-maneuverability technologies, like airplanes and cars,
MacIver said. But studying animals provides a platform for creating low-speed,
high-maneuverability technologies. Potential applications for such a robot
include underwater recovery operations, such as plugging a leaking oil pipe, or
long-term monitoring of oceanic environments, such as fragile coral reefs.
While the applied work on the robot moves ahead in the lab, the group is
pursuing basic science questions as well. “The robot is a tool for uncovering
the extremely complicated story of how to coordinate movement in animals,”
MacIver said. “By simulating and then performing the motions of the fish, we’re
getting insight into the mechanical basis of the remarkable agility of a very
acrobatic, non-visual fish. The next step is to take the sensory work and unite
the two.”
