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New Technique Makes Joints Flap Like Wings

The dielectric elastomer used to create the flapping wing. (Photo credit: ianwen Zhao/Harbin Institute of Technology in Weihai, University of California-Los Angeles)
The dielectric elastomer used to create the flapping wing. (Photo credit: ianwen Zhao/Harbin Institute of Technology in Weihai, University of California-Los Angeles)

Researchers have discovered a new resonance phenomenon of dielectric elastomer rotary joints. Others have studied on dielectric elastomers before, but using a stable voltage that made the joint bend at a fixed angle. This project, by researchers from the Harbin Institute of Technology in China and the University of California, Los Angeles (UCLA), uses a periodically changing voltage.

“We found that alternating voltages can make the joint continuously bend at different angles. Especially, when the rotational inertia of the joint or the applied voltage is large enough, the joint can deform to negative angles, in other words, it can bend beyond 90 degrees to 180 degrees, following a principle different from the normal resonance rule,” said Jianwen Zhao, an associate professor of the Department of Mechanical Engineering at the Harbin Institute of Technology.

Zhao said this new phenomenon makes the joint ideal for creating soft, stretchy lightweight wings for robotic birds. Such wings, he said, would be more efficient that those that employ electrical motors due to the higher energy conversion efficient (60 to 90 percent) of the dielectric elastomer.

Dielectric elastomers are made by sandwiching a soft insulating elastomer film between two compliant electrodes. When a voltage is applied between the electrodes, the dielectric elastomers is squeezed. Zhao’s is made of a thin elastic frame and pre-stretched dielectric elastomer films. The film is stretched, then put on a thin elastic frame. The restoring force of the dielectric elastomer film bends the elastic frame, so that it balances at a minimum energy state. When low-current electricity is applied to the dielectric elastomer, the frame flattens and the bending angle decreases. To restrict frame bending to only one axis, two stiffening frames are mounted to the primary frame as rigid non-bending edges, the whole thing then forms a rotary joint. Dynamically changing the voltage can dynamically change the joint angle, which makes dielectric elastomer minimum-energy structures a useful structure for fabricating soft devices, Zhao said.

“The advantage of alternating voltages is that they shift between different values, thus helping us continuously manipulate the joint’s bending angles.” Zhao said.

It also yielded new results. Zhao and his colleagues found that when the joint or voltage is large enough, the joint bends up and down like a flapping wing. “When the joint realizes negative angles, its motion will become more complicated, following a special resonance rule different than the normal one, which we call nonlinear oscillation,” he said.

The researchers still need to refine the fabrication technique to make a real flapping wing.

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