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Insect-sized robot is capable of flying and swimming

23 October 2015

Harvard engineers have taken their cue from puffins to develop what they believe is the first insect-sized robot that is capable of flying and swimming.

RoboBee in swimming mode (courtesy Harvard SEAS)

In 1939, a Russian engineer proposed a 'flying submarine' - a vehicle that can seamlessly transition from air to water and back again. While it may sound like something out of a James Bond film, engineers have been trying to design functional aerial-aquatic vehicles for decades with little success. Now, engineers may be one step closer to the elusive flying submarine.

The biggest challenge is conflicting design requirements: aerial vehicles require large airfoils like wings or sails to generate lift while underwater vehicles need to minimize surface area to reduce drag.

To solve this, engineers at the Harvard School of Engineering and Applied Science (SEAS) took a clue from puffins. The birds with flamboyant beaks are one of nature’s most adept hybrid vehicles, employing similar flapping motions to propel themselves through air as through water.

“Through various theoretical, computational and experimental studies, we found that the mechanics of flapping propulsion are actually very similar in air and in water,” says Kevin Chen, a graduate student in the Harvard Microrobotics Lab at SEAS. “In both cases, the wing is moving back and forth. The only difference is the speed at which the wing flaps.”

The Harvard RoboBee, designed in Professor Robert Wood’s lab at SEAS, is a microrobot, smaller than a paperclip, that flies and hovers like an insect, flapping its tiny, nearly invisible wings 120 times per second. In order to make the transition from air to water, the team first had to solve the problem of surface tension.

The RoboBee is so small and lightweight that it cannot break the surface tension of water. To overcome this hurdle, the RoboBee hovers over the water at an angle, momentarily switches off its wings, and crashes into the water in order to sink. The team then had to cope with the effects of water's denser environment

In order to do this, the team lowered the wing speed from 120 flaps per second to nine but kept the flapping mechanisms and hinge design the same. A swimming RoboBee changes its direction by adjusting the stroke angle of the wings, the same way it does in air. Like a flying version, it is still tethered to a power source. The team prevented the RoboBee from shorting by using deionized water and coating the electrical connections with glue.

While this RoboBee can move seamlessly from air to water, it cannot yet transition from water to air because it can’t generate enough lift without snapping one of its wings.  Solving that design challenge is the next phase of the research.

“What is really exciting about this research is that our analysis of flapping-wing locomotion is not limited to insect-scaled vehicles,” says Chen. “From millimetre-scaled insects to metre-scaled fishes and birds, flapping locomotion spans a range of sizes. This strategy has the potential to be adapted to larger aerial-aquatic robotic designs.”

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