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There is currently an increased interest in the use of long
range/long durration Autonomous Undersea Vehicles (A.U.V.'s) for
oceanographic observation, military surveillance and commercial search
missions. Existing A.U.V.'s are relatively small vehicles for three
reasons; low cost (fully autonomous vehicles have a significant
probability of being lost), ease of deployment (to allow operations
from conventional ships), and safety (to minimize the danger to manned
ships and installations). They are powered by small rotary propellers
driven by electric motors. The propellers typically operate at fairly
low efficieancies and suffer from serious lag times in transient
response. The space required for the batteries often approaches 70% of
the hull volume. These problems lead to short mission times,
restricted payloads, and control problems.
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 mini submarines with similar
attributes, MIT engineers have developed the first robotic version of
Nature's piscine wonder. Three summers ago 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 fundamental objective of the RoboTuna's control system is to
produce a dynamic body motion that can realistically recreate the type
of flow field that exist about and behind a swimming biological
tuna.
This is a difficult task for a number of reasons. The actual
dynamic shape of a swimming tuna has never been accurately measured,
so there is no well defined target to aim for. In any case, the
RoboTuna's body is only a low-order copy of a vastly richer biological
system, and so it's not clear that attempting to precisely reproduce
the biological motion is either practical or desirable. On the other
hand attempting to derive the proper motion by purely analytical means
is arrested by two problems. The dynamic interaction of the RoboTuna's
undulating body with the passing fluid is (currently) too complex to
accurately model. And the hyper-redundant planar kinematic chain
nature of the body itself creates a situation is which there are an
infinity of possible solutions as to how to move the body from one
kinematic position to the next.
In spite of these difficulties and effective body motion controller
was developed. With this controller fully operational, the fundamental
questions of: Can flow past an undulation body propelled by an
oscillation foil be "tuned" such that the body's drag is reduced and
its thrust is enhanced in a benificial way? What are the parameters
which control this tuning? What is the maximum benificial way? What
are the parameters which control this tuning? What is the maximum
benefit that can be achieved? and Can a man-made (non-biological)
system successfully exploit this phenomenon? could be fully
experimentally explored.
"The "robotuna" project began about six years ago with the overall
goal of developing a better propulsion system for A.U.V.'s," said
Michael S. Triantafyllou, a professor in the Department of Ocean
Engineering who is leading the research team. 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.
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