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(Left) The structure of the revolute joint. (Right) The system used to measure the rotation of the joint.
New oscillations in the up and down bending of artificial joints based on dielectric elastomer materials reveal the potential of this material in making soft robots, such as the use of lightweight, energy-efficient machine wings.
Dielectric elastomers are a new type of material that can be used to make soft, lightweight actuators or engines with high energy conversion efficiencies that can withstand a large degree of elastic deformation. Scientists favored it and were keen to use it to make robots, soft robots, adjustable lenses, pneumatic valves -- and, perhaps, flapping machine wings.
In a research article published this week in the American Physical Consortium’s Applied Physics Letters, researchers from Harbin Institute of Technology in Weihai, China, and the University of California, Los Angeles, described one of their discoveries in dielectric elastomers. New resonance phenomenon: Artificial elastomers based on dielectric elastomers can achieve negative-angle bending, that is, they can move up and down, such as the wing of a bird.
"Dielectric elastomers are electrically activated polymers that can be deformed by applying voltage," said Zhao Jianwen, associate professor of the Department of Mechanical and Electronic Engineering at Harbin Institute of Technology. He explained that previous researchers usually used stable voltages to activate the motion of artificial joints to study their motion laws. The stable voltage can only make artificial joints bent at a certain fixed angle, and they want to understand that artificial joints are periodically changing. How the alternating voltage moves.
"We have found that the alternating voltage can make the artificial joint continuously bend at different angles. In particular, when the artificial joint's moment of inertia or applied voltage is large enough, the joint can achieve negative angle bending, that is, more than 90 degrees to 180 degrees. The degree of bending will also follow a special law that differs from conventional vibration."
Professor Zhao said that this new phenomenon makes dielectric elastomeric joints a candidate for making light and soft machine wings. Dielectric elastomer engines have a higher energy conversion rate (60% to 90% energy conversion) than electric engines, so these artificial wings are more efficient than electric wings.
Muscle-like engine
Compared to traditional robots based on hard materials, soft robots have many advantages, such as safer human-machine contact, more efficient and stable movement, and highly adaptive elastic deformation. Dielectric elastomers are considered to be the closest materials to human muscles due to their soft, lightweight interior characteristics and excellent electromechanical properties. They have been widely favored by scientists of soft science and technology in recent years.
A sheet of flexible insulating elastomer is added between the two flexible electrodes to form a dielectric elastomer. When the voltage is applied between two electrodes, the dielectric material is squeezed and deformed to extend to both sides. Unlike actuators based on hard materials such as silicon, dielectric elastomers can be deformed to a large degree, typically stretching twice as long without breaking, for flexible robot technology, tunable optics, and The development of areas such as cell manipulation offers many new possibilities.
The dielectric elastomeric actuator used in Professor Zhao’s experiment is called the “lowest energy state structure of the dielectric elastomer,†which is to stick a stretched dielectric elastomer film in a thin, flexible frame. production. After being pasted, the stretched dielectric material will naturally shrink, and its contraction force will naturally bend the frame, eventually reaching a state of lowest energy balance.
When a few kilovolts of low current is applied to the dielectric elastomer, the frame will stretch as the dielectric material expands, thereby gradually bending the angle. In order to allow the frame to flex flexibly along one axis, the researchers fixed two rigid frames on both sides of the main frame, and the entire system then formed a rotatable artificial joint. Dynamically changing the applied voltage will dynamically change the angle of rotation of the joint. This feature makes the structure an effective structure for making soft devices.
A new vibration phenomenon
In Professor Zhao’s experiment, the researchers used alternating square wave voltages to stimulate the rotation of the artificial joint. The alternating square wave is a kind of voltage that is periodically turned on and off. It is different from previous scientists' "usually used stable voltage to study the motion of artificial joints."
"The advantage of alternating voltage is that it can switch between different voltage values, so that the artificial joint can be continuously turning and bending." Professor Zhao said.
New practices have also brought new results. By experimenting with different parameters such as adjusting the applied voltage value and frequency to change the quality of the artificial joint, Zhao and his colleagues discovered a new vibration phenomenon: when the moment of inertia of the artificial joint or the applied voltage is large To a certain degree, it can achieve a negative angle bend of more than 90 degrees, and fan up and down like a bird's wings.
"When the negative-angle bending is realized, the motion of the artificial joint becomes more complicated and the vibration law followed is also different from the conventional one. We call it a nonlinear oscillation," Zhao said.
In conventional vibrations, the flexural vibration of the joint always follows the frequency of the applied voltage. When the frequency of the voltage is the same as its own frequency, the maximum degree of bending is achieved. In nonlinear oscillations, the joint can reach its maximum bending angle only when the voltage frequency is close to and slightly less than twice the eigenfrequency of the artificial joint. At the same time, in nonlinear oscillations, the amplitude of artificial joint oscillations is also greater than that of conventional oscillations, which implies that artificial joints can generate greater traction forces in this particular oscillation.
Prof. Zhao emphasized that this new phenomenon and its laws provide possibilities for the realization of many new kinds of soft material devices. For example, soft and soft robots that have special requirements for the environment and load-bearing can produce great traction. Machine wings and so on. In addition, this dielectric elastomer material also has a very high energy density (70 times higher than that of general electromagnetic actuators) and high conversion efficiency (60% to 90%), which is also an excellent choice for making various energy-saving devices. Choice, Professor Zhao said.
The next step for the researchers is to improve the function of the joints of dielectric elastomers and to improve production techniques to create a true machine wing.
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