A robot that can move through both water and air is exponentially more useful than a robot restricted to one or the other. Yet, it's not an easy feat. In nature, there are very few animals that can both fly and swim, because the types of propulsion needed for flight and swimming are very different.
Harvard's RoboBee, a tiny flying robot the size of a house fly, has now been adapted so it can swim as well as fly. It is, the researchers say, the first insect-sized robot to be able to do so.
The reason it's difficult to design a robot that does both is because moving through water and moving through the air require different designs.
To move through the air, the robot needs to be able to generate vertical lift stronger than the downward pull of gravity. On the other hand, to move through the water, the robot needs to have a smaller surface area and to be able to generate rear thrust to counter the water's drag. For this reason, birds like the penguin are sleek and bullet-shaped when diving.
To design the RoboBee to swim as well as fly, the team of engineers at the Harvard Johnson A. Paulson School of Engineering and Applied Science Microbiotics Lab looked to the puffin, one of the few birds that uses its wings to swim in much the same way as it uses them to fly.
"Through various theoretical, computational and experimental studies, we found that the mechanics of flapping propulsion are actually very similar in air and in water," said graduate student and first author Kevin Chen. "In both cases, the wing is moving back and forth. The only difference is the speed at which the wing flaps."
The RoboBee itself posed several challenges. It's extraordinarily small and lightweight, smaller than a paper clip. While generating lift of any sort, it's far too light to break the surface tension of the water. The solution is a little clumsy, but it works: The RoboBee stops flapping and lets gravity do the work, falling into the water before resuming its flaps.
The RoboBee has fragile, wafer-thin wings it flaps at a rate of 120 beats per second. Water, which is almost 1,000 times denser than air, would snap these delicate wings if they continued to beat at flight speed as the robot moved through it. Taking cues from the puffin, the team slowed the speed of the RoboBee's wings to 9 beats per second for moving through the water.
Nothing else needed to be changed. To move through water, the RoboBee simply adjusts the stroke angles of its wings. The mechanisms for doing so remain the same as in the air.
It's still not perfect. The RoboBee still has to be tethered, as it is too small for an on-board power supply. In addition, the RoboBee cannot yet make the transition from water back into the air, as it can't generate enough lift from the water without one of the wings snapping. That's the next step in the research. Once it's figured out, though, the design could be used to help create larger-scale machines that can move through water and air.
"What is really exciting about this research is that our analysis of flapping-wing locomotion is not limited to insect-scaled vehicles," Chen said. "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."