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The World Discovers Modarres-Sadeghi’s Robotic Fish

A robotic fish, built by Dr. Yahya Modarres-Sadeghi of our Mechanical and Industrial Engineering Department, was featured in the New Scientist, inspired by an article in the journal Bioinspiration & Biomimetics and entitled "A fast-starting mechanical fish that accelerates at 40 m s−2." "We have built a simple mechanical system to emulate the fast-start performance of fish," as the Bioinspiration & Biomimetics article explained. "The system consists of a thin metal beam covered by a urethane rubber, the fish body, and an appropriately shaped tail."

The New Scientist article, reproduced below, explained the purpose of the robotic fish this way:

As a spectacle it doesn't amount to much. In fact, it's gone in a flash. But that is the point of a mechanical fish that emulates the high acceleration of a real-life pike.

Underwater robots that mimic fish are usually designed for efficient movement at constant speed. Yahya Modarres-Sadeghi at the University of Massachusetts in Amherst and colleagues at MIT instead aimed to build one to move off as fast as possible from a stationary start.

A startled pike can accelerate at about 15 g for a tenth of a second. A robot with that kind of acceleration could be well suited to covert operations or navigating turbulent water.

The team built a 50-centimetre-long rubber fish modeled on a member of the pike family called the chain pickerel. Running lengthwise inside was a metal strip bent into a C-shape and kept that way by a clamping mechanism. When released by an external pneumatic actuator, the potential energy of the bent strip is converted into kinetic energy, causing the model's tail to whip one way and then the other.

Under water this resulted in an acceleration of just over 4 g, eight times that achieved by previous robotic fish.

"The beauty of our fish is that it is very simple," says Modarres-Sadeghi.

The team is now designing a mechanical fish that can make such a fast start using internal machinery instead of the clunky clamping mechanism and external actuator. "For our new robotic fish, we are using some servo motors placed close to the head. These pull strings attached to the tail through the body," says Modarres-Sadeghi.

"There could be applications in the field of autonomous underwater vehicles for this mechanical fish, but currently we are focusing on understanding the fundamental physics," he says.

New Scientist magazine was launched in 1956 "for all those men and women who are interested in scientific discovery, and in its industrial, commercial, and social consequences". New Scientist reports, explores, and interprets the results of human endeavor set in the context of society and culture.

The Bioinspiration & Biomimetics article was published by Modarres-Sadeghi and four collaborators from the Massachusetts Institute of Technology: lead author J. Conte, M. N. Watts, F. S. Hover, and M. S. Triantafyllou.

Here is the abstract: We have built a simple mechanical system to emulate the fast-start performance of fish. The system consists of a thin metal beam covered by a urethane rubber, the fish body and an appropriately shaped tail. The body form of the mechanical fish was modeled after a pike species and selected because it is a widely-studied fast-start specialist. The mechanical fish was held in curvature and hung in water by two restraining lines, which were simultaneously released by a pneumatic cutting mechanism. The potential energy in the beam was transferred into the fluid, thereby accelerating the fish. We measured the resulting acceleration, and calculated the efficiency of propulsion for the mechanical fish model, defined as the ratio of the final kinetic energy of the fish and the initially stored potential energy in the body beam. We also ran a series of flow visualization tests to observe the resulting flow patterns. The maximum start-up acceleration was measured to be around 40 m s−2, with the maximum final velocity around 1.2 m s−1. The form of the measured acceleration signal as function of time is quite similar to that of type I fast-start motions studied by Harper and Blake (1991 J. Exp. Biol. 155 175–92). The hydrodynamic efficiency of the fish was found to be around 10%. Flow visualization of the mechanical fast-start wake was also analyzed, showing that the acceleration peaks are associated with the shedding of two vortex rings in near-lateral directions. (November 2010)