WEST LAFAYETTE, Ind. — What can fly like a bird and hover like an insect?

Your friendly neighborhood hummingbirds. If drones had this combination, they would be able to maneuver better through collapsed buildings and other cluttered spaces to find trapped victims.

Purdue University researchers have engineered flying robots that behave like hummingbirds, trained by machine learning algorithms based on various techniques the bird uses naturally every day.

This means that, after learning from a simulation, the robot “knows” how to move around on its own like a hummingbird would, such as discerning when to perform an escape maneuver.

Artificial intelligence, combined with flexible flapping wings, also allows the robot to teach itself new tricks. Even though the robot doesn’t yet have cameras to see, for example, it senses by touching surfaces. Each touch alters an electrical current, which the researchers realized they could track.

“The robot can essentially create a map without seeing its surroundings,” said Xinyan Deng, an associate professor of mechanical engineering at Purdue. “This could be helpful in a situation when the robot might be searching for victims in a dark place — and it means one less sensor to add when we do give the robot the ability to see.”

This work aligns with Purdue’s Giant Leaps celebration, acknowledging the university’s global advancements made in AI, algorithms and automation as part of Purdue’s 150th anniversary. This is one of the four themes of the yearlong celebration’s Ideas Festival, designed to showcase Purdue as an intellectual center solving real-world issues.

The researchers will present their work on May 20 at the 2019 IEEE International Conference on Robotics and Automation in Montreal.

Size limitations lead to hummingbird drones

Drones can’t be made infinitely smaller, due to the way conventional aerodynamics work. They wouldn’t be able to generate enough lift to support their weight.

But hummingbirds don’t use conventional aerodynamics — and their wings are resilient.

“The physics is simply different; the aerodynamics is inherently unsteady, with high angles of attack and high lift,” Deng said. “This makes it possible for smaller, flying animals to exist, and also possible for us to scale down flapping wing robots.”

Researchers have been trying for years to decode hummingbird flight so that robots can fly where larger aircraft can’t. In 2011, AeroVironment built a robotic hummingbird for the U.S. Defense Advanced Research Projects Agency (DARPA) that was heavier than a real one but not as fast. It had helicopter-like flight controls and limited maneuverability, and it required a human to be behind a remote control at all times.

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Learning from Mother Nature

Deng’s group and her collaborators studied hummingbirds themselves for multiple summers in Montana. They documented key hummingbird maneuvers, such as making a rapid 180-degree turn, and translated them to computer algorithms that the robot could learn from when hooked up to a simulation.

Further study on the physics of insects and hummingbirds allowed Purdue researchers to build robots smaller than hummingbirds — and even as small as insects — without compromising the way they fly. The smaller the size, the greater the wing flapping frequency, and the more efficiently they fly, Deng saof.

The robots have 3D-printed bodies, wings made of carbon fiber and laser-cut membranes. The researchers have built one hummingbird robot weighing 12 grams — the weight of the average adult Magnificent hummingbird — and another insect-sized robot weighing 1 gram. The hummingbird robot can lift more than its own weight, up to 27 grams.

More technology, use cases to come

Designing their robots with higher lift gives the researchers more wiggle room to eventually add a battery and sensing technology, such as a camera or GPS. Currently, the robot needs to be tethered to an energy source while it flies, but that won’t be for much longer, the researchers said.

The robots could fly silently just as a real hummingbird does, making them more ideal for covert operations. And they stay steady through turbulence, which the researchers demonstrated by testing the dynamically scaled wings in an oil tank.

The robot requires only two motors and can control each wing independently of the other, which is how flying animals perform highly agile maneuvers in nature.

“An actual hummingbird has multiple groups of muscles to do power and steering strokes, but a robot should be as light as possible, so that you have maximum performance on minimal weight,” Deng said.

Robotic hummingbirds could not only help with search-and-rescue missions, but also allow biologists to more reliably study hummingbirds in their natural environment through the senses of a realistic robot.

“We learned from biology to build the robot, and now biological discoveries can happen with extra help from robots,” Deng said.

Simulations of the technology are available open-source at https://github.com/purdue-biorobotics/flappy.

Early stages of the work, including the Montana hummingbird experiments in collaboration with Bret Tobalske’s group at the University of Montana, were financially supported by the National Science Foundation.