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Friday, March 24, 2023

A New Robotic Fly Dips and Dives Like the Real Thing

Respect where respect is due: we humans may be mighty, but there’s still a foe that regularly dodges our best efforts to kill it: the fruit fly. Over millennia of evolution, fruit flies have adapted to burn their pursuers with enviable agility.

Now researchers have built a robotic doppelganger that can twist and bank with astonishing speed. With two pairs of wings beating 17 times a second, it has a wingspan of over a foot and weighs just an ounce. Called DelFly, it can hit speeds of 15 miles an hour and switch directions in an instant, just like the real thing. This new robot could be a boon both for the understanding of the dynamics of insect flight, and for developing flying machines unlike anything you’ve ever seen before.

Like a real fruit fly, DelFly lacks a tail, which aircraft rely on for stability when cutting through the air. The downside, though, is that a tail compromises maneuverability, by introducing a lot of drag if you’re banking and the air is no longer flowing nicely over the body. That’s fine (and in fact welcome) for a 747, but if you want to turn like a fly, you gotta go tailless.

Essential to a fruit fly’s flight are small structures behind the wings called halteres. These act as gyroscopes, feeding the critter information about its orientation. Think of them like our inner ear, only on the outside. Without halteres, flies are pretty much grounded.

The tailless DelFly (which is more or less a fancy stick with motors and a battery that orients vertically to hover, which it can do for 5 minutes on a charge) replicates this control system with its own gyroscopes, which continually help the fly achieve balance in three dimensions. In this sense it’s autonomous to a degree, though the researchers remotely pilot it around.

To fly forward, DelFly’s motors tilt the two pairs of wings forward, like a helicopter. Tilting the wings back sends the fly in reverse. Run the motors on the wings themselves and the pairs clap, providing thrust. (The wings are made of mylar, by the way, the stuff you’d find in space blankets, minus the shininess.)

“We do not copy exactly the way fruit flies move their wings, but we take the main ideas,” says Matěj Karásek, lead author on a new paper in Science that describes the robot. Sideways motion, for example, comes from increasing the flapping frequency on either the right or left wing. A fruit fly achieves the same effect by changing the amplitude of its flapping motion.

Take a look at the flip maneuver above. The operators command DelFly’s right pair of wings to flap rapidly, flipping the robot over on itself. Toward the end of the twist, the left pair of wings ramp back up to slow the roll and bring the robot back to a hover.

You can see just how much power the wings are getting in the animation below as the robot does a different turning maneuver. Yellow is more flapping, purple is less. Notice how the left wings ramp up to bank the robot to the right. Shortly afterward, the right wings kick back in to level out the robot so it doesn’t crater itself.

The robot can also rotate around its axis, using a motor that tugs at the base of the wing roots. If the motor is pushing one pair of wings slightly backward, it’s at the same time pushing the other forward. “What happens is the forces produced by those two wings, they get tilted, but in opposite directions,” says Karásek. Taken together, these wing controls produce a range of maneuvers to rival that of the fruit fly.

Even though DelFly is built nothing like a biological fly—motors instead of muscles, four wings instead of two—the researchers at Delft University of Technology and Wageningen University and Research, both in the Netherlands, could get it to act like a biological fly. “That's what I think is really important about this,” says Nick Gravish, who studies robotics and biology at UC San Diego. “They're not mimicking the exact shape of the flies, but they're mimicking enough of the physics of the way the fly is moving that they can still observe these interesting phenomena.”

That could be key for understanding the dynamics of insect flight. Sure, scientists can watch the acrobatics of fruit flies using high-speed videography, but what’s been missing is how the critter controls these movements. With DelFly, researchers can program the robot’s brain to make certain maneuvers, and may thus be able to discern in finer detail how fruit flies manage to be so dang agile. “The robot really opens new potential in insect and animal flight research because suddenly you can see indirectly into the brains of animals,” says Karásek.

This is not the first robotic insect, but it’s certainly the most agile, in large part because it’s untethered. The RoboBee from Harvard’s Wyss Institute is actually insect-sized, with a wingspan of just over an inch, but you have to hook it up to a power source.

So why bother with insects at all? A typical drone’s glaring weakness is that if a foreign object other than air molecules gets into its rotors, it’s going down. DelFly, on the other hand, might be able to bounce off things and survive—not to mention avoid those obstacles in the first place. Plus, as the technology improves, robotic flies will shrink far smaller than quadrotors, allowing roboticists to fly them in swarms.

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