University of Massachusetts Amherst

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Undergrad Builds Robotic Fish

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 and faster than any manmade vehicle. In order to study this amazing reflex action, senior mechanical engineering student Chengcheng “Charlie” Feng used his summer research in the Research Experience for Undergraduates program to build a robotic fish, which can accurately mimic the escape mechanism of a pike. Feng built the fish under the direction of his faculty mentor, Dr. Yahya Modarres-Sadeghi of the Mechanical and Industrial Engineering Department.

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 Feng isn’t trying for that kind of all-out acceleration with his robotic model. Instead, Feng's robot fish is helping Dr. Modarres-Sadeghi in his research to understand the essential physics behind such fast starting.

“The objective for this design is to build a platform to perform different tests on the fast start of a mechanical fish,” explains Feng, “and ultimately to understand the mechanisms in fluid dynamics that allow for such a high acceleration in pike.”

Once the physics is understood, that knowledge can be applied to everything from the evolution of fish – for instance, the changes in structure that have allowed the pike to achieve faster escape acceleration over the ages – to evasive maneuvers for submarines.

Feng did such a superb job on his REU summer research that his mentor took the almost unprecedented step of allowing him, as an undergraduate, to present his research at the American Physical Society – Division of Fluid Dynamics (APS DFD) 63rd Annual Meeting in Long Beach, California, from November 21-23, 2010.

 “The APS DFD is a very popular conference,” says Dr. Modarres-Sadeghi, “and almost everyone in the field attends it. Normally we do not have undergraduate students present anything there, but as Charlie had done an excellent job, I thought he would be able to do it, and I was right.”

The body of the fish is constructed of a spring steel spine and aluminum ribs, which have the same profile as a live fish. This fish is capable of bending to an angle of 90 degrees between its head and tail. Movement is governed by servos embedded in the head and controlled by a computer. The servos pull on cables attached to certain ribs, bending the fish predictably. An accelerometer is positioned at the center of the mass in order to measure acceleration during a fast start.

Feng’s mechanical fish proved to be a more highly evolved species than an earlier robotic fish built by Modarres-Sadeghi for a research team at the Massachusetts Institute of Technology. Feng’s mechanical fish, in effect, can perform a full jackknife, bending its metal body to a 90-degree angle and then flicking it straight again like the proverbial greased lightning.

“An actual pike bends its body into a C-shaped curve and then uncoils very quickly to send a traveling wave along its body in order to achieve locomotion,” explains Feng. “We have designed a mechanical fish whose motion is accurately controlled by servo motors to emulate the fast-start by being bent to a C-shape from its original straight position, and then back to its straight position.”

Unlike Feng’s model, the first-generation pike built by Modarres-Sadeghi started its escape mechanism from the C-shape posture and then uncorked its body to the straight position, which produced 4 Gs of acceleration. That feat was considered so significant that it was covered on the November 2, 2010, website of the popular British magazine, New Scientist.

“Our new design adds an additional motion to the sequence by first bending from its straight position into a C-shape curve, and then back to its straight position, resulting in higher efficiency compared with the previous version,” says Feng. “Furthermore, this new mechanical fish is designed to be adjustable in swimming pattern, tail shape, tail rigidity, and body rigidity, making it possible to study the influence of all of these parameters on the fast-start performance.” (February 2011)