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Robotic fish gotta swim, too.

There are a lot of fish in the sea, but none quite like Charlie the Tuna. Charlie is a 52-inch, 2843-part robotic tuna with an advanced propulsion system that any real-life, fin-and-bones fish would be proud to possess.

Charlie was designed by a team of graduate students and researchers at the Massachusetts Institute of Technology in Cambridge, Mass., to help them study the fundamental fluid mechanics of how fish swim and to evaluate control circuits and sensors for ocean-based autonomous-vehicle applications. The MIT group plans to put Charlie to work exploring the deep blue sea for months at a time, mapping the ocean floor and detecting underwater pollution.

Charlie, modeled after a bluefin tuna that can swim as fast as 40 knots (although Charlie can get up to only 4), has an articulating aluminum backbone, vacuum-formed polystyrene ribs, and reticulated-foam tissue, all wrapped in Lycra skin. Equipped with actuators, bearings, and electronic circuits, he "swims" under microprocessor control by flexing his body and flapping his tail, just like a real fish.

MIT research professor Michael Triantafyllou developed the idea behind Charlie three years ago while studying the effects of vortex-induced vibrations on cylindrical cables. He noticed that the vortices were similar to the patterns shed by fish in the ocean and realized that these vortices could prevent a low-pressure zone, which normally creates drag, from forming behind an underwater robot.

Conventional autonomous undersea vehicles, with their streamlined fore-and-aft geometries, are driven by either a single propeller or two counter-rotating propellers. These vehicles are limited, however, by their battery power, which allows missions that last only a few weeks.

Charlie; on the other hand, is driven by a propulsion system that uses an oscillating tail. The tail doubles the system's efficiency and, when combined with a flexible structure like Charlie's, will likely improve the system's efficiency even further. "We increased the mission time by converting the battery's energy into forward propulsion more effectively," said David Barrett, a mechanical engineer and graduate student at MIT. "An oscillating tail and flexible body is a better transmission for the electrical energy stored in a battery."

A fully developed robot like Charlie may be a practical alternative to traditional underwater navigation systems. Using submarines to explore sea depths, for example, is expensive and sometimes dangerous.

Charlie relies on a multiprocessor control system, which consists of microprocessors in the robot, in a control center, and in a guidance-and-support unit. The support unit has a carriage that moves on a horizontal rail directly above the water. Charlie is suspended from the carriage and moves underwater on load cells that measure thrust. The carriage automatically increases or decreases thrust to sustain the desired rpms required for a given speed. It does this through a closed-loop system in which the load cells continuously feed data to the control microprocessors.

The microprocessors are networked to orchestrate the movements of the robot. They control both the speed and torque of 2-horsepower brushless dc servo motors, which pivot Charlie's parts on stainless-steel bearings. Charlie actually uses only a tenth of each motor's rated power. The MIT engineering team is now focusing on getting Charlie to swim in a fairly straight line.

The MIT team has plans for a school of future fish prototypes. One design now on the drawing board is a fully autonomous 15-foot-long robot that will use electronically controlled hydraulic pumps, valves, and motors instead of electric actuators. The researchers decided on a hydraulic system because it generates greater force for a given weight and volume than an electrically driven one. This hydraulically powered robot will not have a guidance and support unit but instead will be controlled from a distance with sonar and a camera-based vision system. The MIT researchers plan to launch the 15-foot model in Boston Harbor by the end of the century.

The researchers are also planning to build a 2-foot-long robot optimized for speed and maneuverability. Brush dc micromotors in a closed-loop servo system will drive the robot. Microsystems engineers at MIT have developed electrostatically and piezoelectrically driven miniature motors that could be used in this and other fish robots. They even plan to build a 1-inch-long guppy-size version that makes use of these tiny motors. Whatever their configurations, Charlie and his mechanical friends aren't likely to fall for any hook, line, or sinker.
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Title Annotation:robotic tuna designed by the students Massachusetts Institute of Technology
Author:O'Connor, Leo
Publication:Mechanical Engineering-CIME
Date:Jan 1, 1995
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