Mechanical engineer Raphael Zufferey's lab at MIT contains a giant tank filled with bright turquoise water, an array of fans that can whip up a powerful wind, and small flying robots perched everywhere you look.
It's the robots that are the stars of the show here and they're inspired by diving seabirds like the Atlantic puffin, which uses its wings to both fly and swim.
"These puffins solve this really challenging task of moving in air, in water despite the huge difference in density," says Zufferey.
He and his colleagues wanted to see if they could build a bird-sized robot that could also move through both mediums and transition between them. It's something no one had ever done before.
In a paper published Thursday in the journal Science, they describe the engineering of just such an aerial-aquatic robot. It weighs about half a pound and its wingspan measures not quite three feet, tip to tip.
"This is a beautiful robot," says Glenna Clifton, an animal movement biologist at the University of Portland in Oregon who collaborates with roboticists but wasn't involved in this research project. She says the robot offers insights into what makes the flight biology of diving birds unique.
It also has many potential applications including observing the coastal ocean and monitoring something like a remote coral reef. The robot could fly to the reef — or something else like a pod of whales or an algal bloom — and then sample the water and collect data.
Such bio-inspired robots are fertile ground to learn about both nature and engineering. "The biology inspires the robotics," says Clifton, "but then also the robotics are used to understand the biology."
Designing a novel robot
Creating this robot took two years. "Thinking of a wing that could operate in both [air and water] somewhat efficiently seems implausible," Zufferey recalls thinking.
But he and his colleagues were undeterred. They based the robot's overall body plan on a diving bird, but made a couple of key departures.
First, they decided not to include any legs because in robotics, legs are tricky to build, control, and achieve the desired movement in the robot. "Instead, we thought, 'can we go from the water straight to the air simply with the wings themselves?'" says Zufferey.
Second, the research team decided against making those wings foldable as they are in many diving birds. That would have been too complex, Zufferey says. "You need to add joints, you need to add motors. So instead we rely on wing flexibility."
He holds up the final robot. It's elegant. The central body, which houses the motor and battery is completely open, meaning its electronic guts are visible.
"So water floods the whole system here," explains Zufferey. "You have to waterproof, individually, every single component." Such an approach allows the robot to be both light enough to fly easily through the air and also neutrally buoyant, meaning it won't float to the surface or sink to the bottom. It just stays put in the water.
The robot has a tail to help it fly. The wings are made from a translucent nylon fabric reinforced with carbon fiber struts. Zufferey holds the body of the robot while its wings flap up and down crisply and quickly. "You can really feel the forces," he says.
The robot flaps five to six times a second to maintain flight. To leave the water and propel itself into the air, however, it must move its wings ten times a second to generate sufficient speed and thrust.
Most diving birds can't generate that kind of power with their wings alone, which is why they take off by using their legs to run along the water's surface. (The kingfisher is an exception but it is an especially light bird, says Zufferey.)
"A monumental step"
Zufferey calls up a video that he and his colleagues filmed at Lake Geneva in Switzerland. The Alps rise up in the distance and the water's surface is placid.
There's the slightest of ripples before the robot bursts out of the water and into the air — all in less than a second. It actually sounds like a bird taking flight.
The researchers computed the optimal launch angles and wing size. And they estimate that on a single charge, the robot could fly for not quite four miles or swim for a bit more than a mile, "which is longer than the running and swimming portion of a sprint triathlon," observes Clifton.
She was impressed by the robot overall. "It is light and powerful and a monumental step in the performance at both swimming, flying, and transitioning between the two," she says.
Down the road, Zufferey is excited about using this kind of robot for a range of applications, including monitoring harmful algal blooms, fish stocks, and coastal erosion. He plans to equip the device with a handful of onboard sensors to enable such data gathering.
In addition, Zufferey and his team are continuing to refine and improve their aerial-aquatic robots — honed by experimentation but still inspired by the natural world. "You see that it has already been done in biology," he says.
"So that gives you hope as a robotics researcher. It tells you that it should be possible."
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