Tiny automobile differential governs flight of aerial robots

It is envisaged that such devices could be used to investigate environmental hazards, forest fires, and other places too dangerous for people.

Their new approach is the first to passively balance the aerodynamic forces encountered by these flying devices, letting their wings flap asymmetrically in response to gusts of wind, wing damage, and other impediments likely to be experienced in ‘real-word’ scenarios.

’The drivetrain for an aerial microrobot shares many characteristics with a two-wheel-drive automobile,’ said 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 (Passive Aeromechanical Regulation of Imbalanced Torques) differential generates torques up to 10 million times smaller than in a car, is 5mm long, and weighs about one-hundredth of a gram, a millionth the mass of an automobile differential.’

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 — have to negotiate conditions that change constantly in the same way. Insects, for example, usually accomplish this by flapping their wings in unison, a process whose kinematic and aerodynamic basis remains poorly understood.

Sreetharan and Harvard engineering professor Robert J Wood recognised 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,’ said 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 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 and lightweight as possible.

The high-performance aerial microrobots are described by the Harvard scientists in the Journal of Mechanical Design.