Consider the fish: highly maneuverable and an effortless swimmer, this animal 160 million years in the making is superbly adapted to its watery environs. Now, in work that could lead to miniature submarines with similar attributes, MIT engineers have developed the first robotic version of nature's piscine wonder.
In mid July the researchers' creation, patterned after a bluefin tuna, took its maiden swim down the MIT testing tank. That swim and others since have been flawless, reinforcing the engineers' belief that the Lycra-sheathed robot could become an important tool toward understanding the physics of swimming and more.
The "robotuna" project began about three years ago with the overall goal of developing a better propulsion system for autonomous underwater vehicles, or AUVs, said Michael S. Triantafyllou, a professor of ocean engineering who is leading the research team.
AUVs are small robotic submarines that have great potential for mapping the ocean floor, finding the sources of underwater pollution and other tasks. Yet currently they can stay underwater for only limited amounts of time, restricting their usefulness. "You simply can't put enough batteries on board for long-term missions like exploring the mid-Atlantic ridge for a couple of months," said David S. Barrett, a graduate student in ocean engineering who is developing the robotic fish for his doctoral thesis.
The solution lies in better batteries or a better propulsion system, Mr. Barrett said. "And since no one wants to put a nuclear device on an autonomous vehicle, we're going the propulsion route." Fish have the most efficient propulsion system around, so the researchers decided to copy Mother Nature.
The result of their efforts, which included hours of watching tuna at the New England Aquarium and combing the literature on how fish swim, is Charlie the Testing Tank Tuna. About four feet long, Charlie was designed to resemble the real thing as closely as possible. Hence the robot's 2,843 parts include some 40 ribs, a set of tendons, a segmented backbone with vertebrae and, of course, its Lycra skin. The robot swims down the tank propelled by a tail that gently swishes back and forth as its flexible body follows suit.
Charlie is the first of a series of "robofish" that the engineers plan to build, each with successively greater abilities. For example, the current fish is attached to a structure that guides it down the tank and contains all of its electronics. But the "robopike" that the researchers plan to build next will probably be connected only to a tether, allowing the creature to make hairpin turns.
The ultimate goal? A fully autonomous fish that "we could throw into Boston Harbor, tell to go somewhere, and have it come back," said Mr. Barrett. The engineers are hopeful that such a fish could be a reality in about five years.
For now, the researchers are concentrating on getting the robot to swim as efficiently as possible in a straight line. "We hope to find out if [our system] is or is not better than conventional propulsors," Mr. Barrett said.
Although a new, highly efficient propulsion system for AUVs is the main goal of the robotuna project, Mr. Barrett stressed that the robot is a "multi-mission experimental unit" with a number of other scientific thrusts.
For example, the researchers are learning more about the fundamental fluid mechanics of swimming. Further, the robot is a test bed "to let us check out a variety of controllers and sensors for systems like this," he said.
In a specific example, the researchers are planning to "evolve" a computer control system that will make the robot swim most efficiently. Using the computer, they'll create models of different control systems. Each system will be represented by a chromosome-like data structure whose "genes" dictate how the robot moves.
The researchers plan to test each control system through Charlie using a genetic algorithm to determine which will make the robot swim most efficiently. Just like in natural selection, then, "the top 60 percent will get to have virtual offspring," said Mr. Barrett, recombining the "genes" to produce even more efficient control systems (the bottom 40 percent effectively become extinct).
"So the overall efficiency gets exponentially better and better, and we end up with an arrangement of control parameters that make for the most efficient run," he said.
Key to the project are several students. They are: Pehr Anderson, a junior in electrical engineering and computer science (EECS) who is developing sensors for the robot; John Kumph, a senior in mechanical engineering who oversees robot control; Gillian Lee, a senior in EECS who is developing the genetic algorithm; Elise Martin, a junior in mechanical engineering who is developing the skin of the robot; Scott Miller, a graduate student in mechanical engineering who is working on the hydrodynamics of the tail fin, and Owen Wessling, a senior in EECS who is developing the data acquisition system.
The work is funded by the Advanced Research Projects Agency, the Office of Naval Research, the MIT Sea Grant College Program, the Woods Hole Oceanographic Institution, and MIT's Undergraduate Research Opportunities Program.
A version of this article appeared in the September 21, 1994 issue of MIT Tech Talk (Volume 39, Number 5).
