Polymer Resonance Could Enable Soft Robotic Wings

A newly discovered resonance property of dielectric elastomers could be exploited to build highly efficient robotic wings.

AsianScientist (Apr. 8, 2015) – A resonance phenomenon of materials known as dielectric elastomers could be used to create robotic flapping wings. The study documenting these findings has been published in Applied Physics Letters.

Soft robotics provides many advantages compared to traditional robotics based on rigid materials, including safer physical human-robot interactions, more efficient and stable locomotion, and adaptive morphologies. Dielectric elastomers, due to their soft and lightweight inherent properties and superior electro-mechanical performances, are considered as a material close to human muscles, attracting wide attention among soft-technology scientists in recent years.

“The dielectric elastomer is a kind of electro-active polymer that can deform if you apply a voltage on it,” explained first author Dr. Zhao Jianwen, an associate professor at the Harbin Institute of Technology.

Made by sandwiching a soft insulating elastomer film between two compliant electrodes, dielectric elastomers can be squeezed and expanded in a plane when a voltage is applied between electrodes. In contrast to actuators based on rigid materials such as silicon, dielectric elastomers can reach a very large extent of stretch, often exceeding 100 percent elongation while not breaking, enabling new possibilities in many fields including soft robotics, tunable optics and cell manipulation.

However, most existing studies on dielectric elastomers use a static or stable voltage to stimulate the joint motion, which makes the joint bend at a fixed angle. Zhao and colleagues were interested to see how an artificial joint made of dielectric elastomers would react to an alternating or 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.”

Zhao said this new phenomenon makes the dielectric elastomer joint a good candidate for creating a soft and lightweight flapping wing for robotic birds, which would be more efficient than bird wings based on electrical motors due to the higher energy conversion efficiency (60 to 90 percent) of the dielectric elastomer.

After experimenting with various parameters such as voltage values, frequencies and the joint mass in the dielectric elastomer joint system, Zhao and colleagues observed a new resonance phenomenon: When the rotational inertia of the joint is large enough or the applied voltage is high enough, the joint can bend up and down like a flapping wing, reaching a bending angle over 90 degrees or what the researchers call negative angles.

“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.

In normal resonance, the joint bends following the voltage frequency, and will reach the largest bending angle when the joint’s inherent frequency is equal to the voltage frequency, Zhao explained. While in nonlinear oscillation, the joint reaches its largest bending angle when the provide voltage frequency is near but smaller than twice the joint’s natural frequency. Meanwhile, the joint amplitude (the bending scope) is also larger than in normal resonance, indicating a larger lift force in the special resonance.

This new phenomenon and the principle, Zhao noted, may open doors for many novel soft devices, such as soft and lightweight robots for circumstances with restricted space and weight requirements or flapping wings of soft robotic birds that can generate a large lift force.

Also, since dielectric elastomers feature high energy density (seventy times higher than conventional electromagnetic actuators) and high-energy conversion efficiency (60 to 90 percent), they could be good candidates for making energy-efficient devices, Zhao said.

The researchers’ next step is to improve the function of the dielectric elastomer rotary joint and refine the fabrication technique to make a real flapping wing.

The article can be found at: Zhao et al. (2015) Phenomena Of Nonlinear Oscillation And Special Resonance Of A Dielectric Elastomer Minimum Energy Structure Rotary Joint.

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Source: American Institute of Physics.
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