18, 1998 Design News
ROBOTICS AND AUTOMATION
Robots mimic Mother Nature
Biological modeling and
technology integration take robotics 'where no man has
Rick DeMeis, Associate Editor
Mention robots and most people will think of machines
from robust one-dimensional industrial units on the
factory floor all the way to the personable piece of
machinery in the classic TV series "Lost in Space."
But today, many robotic technologies are being highly
integrated--many on what is termed the micro level,
leading to autonomous devices measured in a few inches
or feet. And many of these devices are intelligent machines
patterned to look and swim like fish or fly from the
palm of your hand. Such machines could survey and monitor
water and air quality and wildlife, or serve as the
unobtrusive eyes and ears of military forces.
Once you scale systems way down, integration becomes
critical as weight, volume, and power become limited.
Often, traditional large-system characteristics, particularly
in fluid dynamics, are modified "in the small."
Skin friction goes up for inch-sized fish or bird-sized
aircraft, and laminar flows predominate.
But what engineering tricks can we learn from trying
to mimic nature's systems? Quite a lot, according to
Jamie Anderson, principal investigator for the Charles
Stark Draper Laboratory's (Cambridge, MA) robotic fish
prototype. "Fish can corner, zigzag, approach something
very closely, and then rapidly maneuver away."
And to investigate such versatility and potential applications,
Draper is running a proof-of-concept test bed, the Vorticity
Control Unmanned Undersea Vehicle (VCUUV), which, for
obvious reasons, is called the robot fish or robot tuna.
Go fish! Once the preliminaries of
piscine propulsion have been plumbed by the prototype
robotic fish, it offers many practical possibilities.
Obvious is its use as a scientific exploration platform.
Well suited as a drone doing repetitive tasks, such
a vehicle can survey fish stocks or do environmental
monitoring. The latter could be accomplished in waves,
turbulence, and turning rivers of various depths, even
to the point of entering outfall pipes.
Military applications include a reconnaissance platform,
mine detection and countermeasures, and locating quiet
submarines. Right now military commanders are reluctant
to commit to autonomous underwater systems because of
limited onboard battery propulsion capacity, notes Anderson.
"A change in the propulsion paradigm, along with
new levels of energy capacity, may see systems that
could go miles up a river and sit in a pipe to monitor
a potential chemical weapons plant."
Smart skin materials may allow a more fish-like lateral
line of sensors along the sides of a robot vehicle as
well as propulsion movements. For the latter, piezo
elements or developments of polymer/metal artificial
muscle tissue from recent medical advancements, may
see use. And finally, highlighting the critical importance
of high propulsion energy density in underwater applications,
Anderson looks for exotic battery materials with good
form factors for packaging, and perhaps small fuel cells,
as always welcome.
Fish tale. "Vorticity control
is the magic that fish do compared to conventional propulsion,"
notes Anderson. Their handling of this rotational energy
for propulsion using the tail, which is highly integrated
with a low drag body, gives an energetically beneficial
result--so much so that some researchers estimate fish
require only about half the power to achieve the same
speeds as a propeller-driven vehicle. Scientists hope
to use the tuna to put hard numbers to such judgements.
According to Anderson's previous research at MIT, a
fish wake has natural instabilities. "In the past,
an unsteady wake was considered a 'bad thing,' "
she notes. "It has a natural instability at the
frequency that the fish drives it--efficiently transferring
momentum to the wake with minimum input energy."
Fish can turn within their body length while conventional
vehicles require several lengths. Anderson has first-hand
experience in studying fish motion gained by observing
tuna during tank dives as a volunteer at the New England
Aquarium. "In the fish we have propulsion and steering
in one device. It takes up more of the hull than a propeller,
but it adds a lot more [turning performance and energy
The robot tuna will directly fathom propulsion parameters
by measuring fluid forces on an unconstrained swimming
vehicle. The prototype program is based on tow-tank
work done with smaller robots at MIT. "We're standing
on their shoulders," adds Anderson. The proof-of-concept
free swimmer is eight feet long and weighs 300 lb. "There's
a lot to float in the boat," she notes, "and
to advance the technology, we needed autonomy, which
drives up the size in order to accommodate the necessary
batteries, hydraulics, and computer power." The
vehicle will run for at least three hours at four knots.
Tune a fish. Anderson says they saved
time and money by reverse engineering one of Mother
Nature's working systems: a yellow-fin tuna. "The
swimming movements of the tuna are very localized to
the tail," she adds. Their amplitude is 10-15%
of the body length and only in the very end portion
of the fish. In the robot, "The whole forward package
is like a big bus, you can put all your stuff in there.
Designers don't like to have a vehicle moving from side
to side like eels or other fishes."
To gauge a tuna's shape, an ungutted 40-inch, 34-lb
tuna was shipped on ice to Draper. "We studied
the fins and how they were attached, particularly the
caudal fin area, near the tail, where the actual pivot
point was," she says. A plaster cast of the fish
served as the mold for a rigid urethane replica. This
plastic fish was digitized and the data set given to
Goetz Marine Technology (Bristol, RI), a builder of
composite components. It constructed the robot's pressure
hull from carbon/epoxy. The fish's nose is a soft urethane
bumper, but in the future this could contain sonar.
In configuring the robot, Anderson cites deciding the
ratio of "hull to wiggly tail. Right now one half
is forward pressure hull." The tail deflects "a
lot farther than a real tuna tail. We really wanted
to push the envelope for maneuvering," she says.
The tunabot is designed to bat-turn 180 degrees in one
body length. Engineers will study "exactly how
it moves as we wiggle it--observing the input/output
relationships between how you wiggle and where you end
up--for transfer functions and system performance,"
To approximate the fish's tail, with its many tens
of vertebrae, the robot's tail, which fills with water,
has four articulated links, each hydraulically driven.
Investigators will study and optimize swimming with
the arrangement, right down to driving just one of the
highly flexible segments. The composite structure, with
aluminum fittings and members, has flexible beams and
ribs, as in the fish, including the tail links. About
80 lbs of lead facilitate trimming and submerging. Laminated
fiberglass scales between the gaps in the tail support
a flexible neoprene skin with a nylon outer layer. The
robot's pectoral (side) fins are somewhat larger planes
for low-speed diving and climbing. Dorsal and ventral
fins act as keels for straight-line stability.
The robot tuna was fabricated in 16 months, after a
4 month preliminary design period. Because time and
money were limited, commercial, off-the-shelf (COTS)
component use was mandatory (see For More Information
box). Exceptions were the Draper designed and made hydraulic
manifold block, and the eight hydraulic tail actuators,
which were fitted with MicroStrain (Burlington, VT)
external position sensors. The Draper team conducted
an adroit eight-week design and fabrication effort on
these actuators--because the only qualified commercial
vendor failed to deliver ordered actuators, with internal
sensors, after 20 weeks.
The brain of the animated tuna is "a 486 processor
with a gigabyte hard disk to store data," Anderson
adds, along with other distributed computing and sensing
processors, such as for actuator control. "Networked
sensors talking to each other are another key feature."
Safety systems allow manual, remote, or automatic recovery.
The computer shuts down operations if sensors detect
a leak, or overheating, or there is no systems response.
Components that would allow advanced uses mentioned
previously might include a "quick-look" GPS
satellite navigation system for pop-up position fixes.
Anderson cites a wish list, including a high-bandwidth
acoustic modem for reliable communication in many environments.
"Some modems are available today, but only in limited
applications and at significant cost." Multipath
signals in shallows, caused by many surface reflections,
are the gremlins here. And although optical based communication
and sensing is being looked at, Anderson notes, "Whales
are capable of talking across oceans, rather than looking
at each other nearby."
"There are so many possibilities in robotics and
autonomy," Anderson concludes. The VCUUV is a propulsion
and maneuvering platform and "guinea pig for demonstrations
of new sensors and advances in autonomy. The goal is
a user-friendly, intelligent system so someone can plug
into the fish's computer and test using real hardware."
Mad about MAVs. Just as mechanical
fish gotta swim, electronic birds gotta fly. This May
in Florida, the International Society of Structural
and Multidisciplinary Optimization (ISSMO) and the University
of Florida (Gainesville) Department of Aerospace Engineering
are having their second annual Micro Aerial Vehicle
(MAV) Competition (For details, http://www.aero.ufl.edu/~issmo).
MAVs are substantially smaller than current remotely
piloted aircraft, with efforts aimed at producing vehicles
on the order of six inches in wingsp