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A millionth-scale automobile differential can help govern the flight of minuscule aerial robots that could someday 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,” said Pratheev S. Sreetharan, a graduate student in Harvard University’s School of Engineering and Applied Sciences and lead author of a paper on the subject. “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.”

Minuscule aerial robots could someday probe environmental hazards, forest fires, and other places too dangerous for people.

Minuscule aerial robots could someday probe environmental hazards, forest fires, and other places too dangerous for people.

High-performance aerial microrobots could ultimately 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 able to operate in search and rescue operations, agriculture, environmental monitoring, and exploration of hazardous environments.
To fly successfully through unpredictable environments, aerial microrobots – much 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, associate professor of electrical engineering at Harvard Robert J. Wood, recognized 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,” said Wood. “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 even when they removed a significant part of an aerial microrobot’s wing, 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 said 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 said. “We take our inspiration from biology, and from the elegant simplicity that has evolved in so many natural systems.”

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