Teaching Robots to Fly Like Birds

Rutgers researchers replace motors with smart materials in an innovative approach to flight

A bird banking in a crosswind doesn't rely on spinning blades. Its wings flex, twist and respond instantly to its environment.

Professor Onur Bilgen (at center) works with Rutgers engineering students on prototypes of a bird-like robot powered by voltage-driven smart materials. (From left) Graduate students Ayhan Ozel, Dario Gosevski, Bezawit Gebre and Batuhan Yildirim display models headed for wind-tunnel tests.

Engineers at Rutgers University have taken a major step toward building bird-like drones that move the same way, flapping their wings like real birds, using electricity-driven materials instead of conventional electromagnetic motors to power them.

In a studypublished in Aerospace Science and Technology, aerospace researchers Xin Shan and Onur Bilgen describe a "solid state" bird-like drone, typically referred to as an ornithopter, whose flexible wings flap and twist without motors, gears or mechanical linkages. Instead, the system relies on the piezoelectric effect, special materials that change shape when voltage is applied.

"We apply electricity to the piezoelectric materials, and they move the surface directly, without extra joints, extra linkages or motors," said Bilgen, an associate professor in the Department of Mechanical and Aerospace Engineering in the Rutgers School of Engineering. "The wing is a composite including a piezoelectric material layer and a carbon-fiber layer. Apply voltage to the piezoelectric layer, and the whole composite flexes."

With their bird-like design, ornithopters offer a level of flexibility that makes such drones well suited for future tasks such as search and rescue, environmental monitoring, inspection of hard-to-reach places, and urban package delivery, where aircraft must navigate around buildings, wires, people, and so much more.

The research team also developed a powerful computer model that connects all the important physics involved in flight at once: wing and body motion, aerodynamics, electrical dynamics, and the control architecture. That allows engineers to test and optimize designs virtually before building physical prototypes, saving time and money while speeding development.

In this computer-generated flight sequence, the ornithopter's wings beat without any gears or motors-powered instead by thin piezoelectric actuators that flex the entire structure.

"We've scientifically demonstrated that this type of ornithopter can be possible when we make certain material assumptions," he said. "We can show the feasibility of designs that are not yet physically possible."

For now, the primary obstacle is the performance of the piezoelectric material.

"Today's piezoelectric materials are not capable enough," Bilgen said. "However, our mathematical model allows us to look into the future with reasonable assumptions."

Bilgen first encountered ornithopters in 2007 while he was a graduate student, but he said his interest deepened in 2013, when he began seriously exploring how flapping-wing flight might be reimagined using smart materials. Various companies have built experimental bird-like drones, but most existing designs rely on motors, gears and conventional actuators to drive wing motion.

Those systems, Bilgen said, struggle to match the performance of natural wings, which flex and respond continuously to changing air.

Bilgen says nature offers powerful lessons for engineers.

"Things that need to move fast must be lightweight," he said. "That's why bird wings are delicate structures, and aircraft wings mimic bird wings."

While birds and insects provide inspiration for the work, Bilgen's goal isn't simple imitation.

"We don't want to just mimic nature," he said. "We want to exceed what nature does."

Advanced simulation tools developed at Rutgers help engineers optimize bird-like drones in software first, accelerating innovation while reducing the need for repeated prototypes.

So far, most prototypes of robotic birds rely on mechanisms that imitate bones and muscles. Bilgen's team is taking a simpler path.

"We want to achieve flapping flight without bone-like structures or muscle-like actuators, flapping in a much simpler way," he said.

Instead of motors acting as muscles, thin strips called Macro Fiber Composites (MFCs) are glued directly on their models onto flexible wings. When electricity flows through them, the wings flap, twist and morph.

"The carbon fiber acts like feathers and bone, and the surface-mounted MFCs act like muscles and nerves," Bilgen explained.

Because the system has no gears or joints, the researchers call it a mechanism-free or solid state ornithopter.

Flapping wings offer advantages that spinning propellers found on conventional drones cannot, especially at small scales. "When flapping wings come in contact with the environment, they're less destructive to themselves and to what they contact," Bilgen said.

The use of piezoelectric materials or other smart materials could also improve renewable energy systems.

"A turbine blade is basically a rotating wing," Bilgen said. "We've been looking at applying piezoelectric materials to turbine blades to see if there are aerodynamic benefits."

By subtly changing blade shape in real time, engineers may be able to influence how air flows across the blade surface. That could lead to more efficient wind turbines, he said.

Explore more of the ways Rutgers research is shaping the future.