CAMBRIDGE, Mass., Sept. 2, 2010 -- Engineers at Harvard
University have created a millionth-scale automobile differential
to govern the flight of minuscule aerial robots that could someday
be used to probe environmental hazards, forest fires, and other
places too perilous for people.
Their new approach is the first to passively balance the
aerodynamic forces encountered by these miniature flying devices,
letting their wings flap asymmetrically in response to gusts of
wind, wing damage, and other real-world impediments.
"The drivetrain for an aerial microrobot shares many
characteristics with a two-wheel-drive automobile," says lead
author Pratheev S. Sreetharan, a graduate student in Harvard's
School of Engineering and Applied Sciences. "Both deliver power
from a single source to a pair of wheels or wings. But our PARITy
differential generates torques up to 10 million times smaller than
in a car, is 5 millimeters long, and weighs about one-hundredth of
a gram -- a millionth the mass of an automobile differential."
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High-performance aerial microrobots, such as those the Harvard
scientists describe in the Journal of Mechanical Design,
could ultimately be used to investigate areas deemed too dangerous
for people. Scientists at institutions including the University of
California, Berkeley, University of Delaware, University of Tokyo,
and Delft University of Technology in the Netherlands are exploring
aerial microrobots as cheap, disposable tools that might someday be
deployed in search and rescue operations, agriculture,
environmental monitoring, and exploration of hazardous
environments.
To fly successfully through unpredictable environments, aerial
microrobots -- like insects, nature's nimblest fliers -- have to
negotiate conditions that change second-by-second. Insects usually
accomplish this by flapping their wings in unison, a process whose
kinematic and aerodynamic basis remains poorly understood.
Sreetharan and his co-author, Harvard engineering professor
Robert J. Wood, recognized that an aerial microrobot based on an
insect need not contain complex electronic feedback loops to
precisely control wing position.
"We're not interested so much in the position of the wings as
the torque they generate," says Wood, an associate professor of
electrical engineering at Harvard. "Our design uses 'mechanical
intelligence' to determine the correct wing speed and amplitude to
balance the other forces affecting the robot. It can slow down or
speed up automatically to correct imbalances."
Sreetharan and Wood found that even when a significant part of
an aerial microrobot's wing was removed, the self-correction
engendered by their PARITy (Passive Aeromechanical Regulation of
Imbalanced Torques) drivetrain allowed the device to remain
balanced in flight. Smaller wings simply flapped harder to keep up
with the torque generated by an intact wing, reaching speeds of up
to 6,600 beats per minute.
The Harvard engineers say their passive approach to regulating
the forces generated in flight is preferable to a more active
approach involving electronic sensors and computation, which would
add weight and complexity to devices intended to remain as small as
lightweight as possible. Current-generation aerial microrobots are
about the size and weight of many insects, and even make a similar
buzzing sound when flying.
"We suspect that similar passive mechanisms exist in nature, in
actual insects," Sreetharan says. "We take our inspiration from
biology, and from the elegant simplicity that has evolved in so
many natural systems."
SOURCE
