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PARITy drivetrain govern the flight of minuscule aerial robots

By Damir Beciri
2 September 2010

harvard-university-parity-drivetrain-1Engineers at Harvard University have created a millionth-scale automobile differential to govern the flight of minuscule aerial robots. 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 lead researcher 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.”

To fly successfully through unpredictable environments, aerial microrobots have to negotiate conditions that change second-by-second. Insects usually accomplish this by flapping their wings in unison, a process where 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 doesn’t have to contain complex electronic feedback loops to precisely control wing position.

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“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 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 and 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.”

Scientists at many institutions around the globe are exploring aerial microrobots as cheap, disposable tools that could be deployed in search and rescue operations, agriculture, environmental monitoring, and exploration of hazardous environments. For more information, read the paper they published in the Journal of Mechanical Design named “Passive Aerodynamic Drag Balancing in a Flapping-Wing Robotic Insect“.

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