When a pike is attacked, the fish escapes by performing a lightning-fast jackknife, which generates a remarkable 25 Gs of acceleration for a tenth of a second – more than three times the acceleration of an Apollo launch. In order to study this amazing reflex action, Dr. Yahya Modarres-Sadeghi of the University of Massachusetts Amherst Mechanical and Industrial Engineering Department has spent the past two years working on two generations of robotic fish, which mimic the escape mechanism of a pike.
“Once you understand the physics behind this fast start, which is what we are studying with our robotic fish,” explains Dr. Modarres-Sadeghi, “then you can apply it in many ways.” Those applications include everything from the evolution of fish to evasive maneuvers for submarines.
The pike is a real escape artist, the Houdini of freshwater fish, reaching an average acceleration of 15 Gs when escaping from predators, and 10Gs when attacking. But Modarres-Sadeghi isn’t trying for that kind of all-out acceleration with his robotic model.
“First of all, our mechanical fish is not being attacked,” he quips about the effects of adrenaline on real escaping fish. “But, seriously, we’re not aiming at going to the maximum possible acceleration. All we want is to emulate the kinematics of the fish as closely as possible and get substantial acceleration so we can study how the pike stores energy and releases it quickly.”
Modarres-Sadeghi and his colleagues produced 4 Gs of acceleration in a first-generation model, a 50-centimeter-long rubberized pike built at the MIT towing tank. That feat was considered so significant that it was covered on the November 2 website of the popular British magazine, New Scientist.
How does the robot work? Running lengthwise inside this first-generation robotic fish was a thin steel beam, which could be bent into a C-shape, or much the same posture as a live fish would assume when under attack, and kept in that position by using a clamping mechanism. When released by an external pneumatic actuator, the potential energy of the bent beam was converted into kinetic energy, causing the model's tail to buggy-whip quite quickly and powerfully.
Modarres-Sadeghi is now testing a second generation mechanical pike that is more evolved than the first. It can make a fast start using internal machinery instead of the clunky clamping mechanism and external actuator of the first generation model.
“We are currently building a fish that can bend its body from straight, to C-shape, and back to straight in much the same way that a live fish does," says Modarres-Sadeghi. "For our new robotic fish, we are using some servo motors placed close to the head. These motors pull strings attached to the tail through the body to bend the fish.”
One direction his mechanical fish might go is answering questions that biologists who study this kind of escape mechanism in pike are asking about the evolution of the fish.
“An example is that we will include separate anal and dorsal fins at various locations to study how this separation can increase the maximum acceleration and the corresponding efficiency,” says Modarres-Sadeghi. He could do similar kinds of experiments to test the shape of the fish as it has evolved.
When Modarres-Sadeghi has perfected the live-action motion of a real fish in his current robot, then he will build a bigger model of the same kind, but with a thicker steel beam within, which will store much more energy when bent, and then generate much more G-force when released. He will be testing that model in a large water tank, which will soon be installed in his lab.
How will he know when his robotic pike is fully evolved? “When everything makes sense,” he says simply. “When it accurately reproduces the physics of a live fish.”
The MIT team also published a scientific journal article about its first generation robotic pike in Bioinspiration & Biomimetics, a piece entitled "A fast-starting mechanical fish that accelerates at 40 m s−2.” Modarres-Sadeghi’s collaborators from MIT were J. Conte, M. N. Watts, F. S. Hover, and M. S. Triantafyllou. (January 2011)