A Japanese university researcher developed a small-size vibration-driven generator that measures approximately 2 x 3 x 12mm and can output a power of 1.56mW with a vibration of 357Hz.
The energy density of the generator, which was developed by Toshiyuki Ueno, associate professor at Kanazawa University, is about 22mW/cm3. It is about 20 times higher than the energy density of normal small-size vibration-driven generators, which is about 1mW/cm3.
“The energy density is high enough to replace a button battery,” Ueno said.
He is planning to apply the generator to tire pressure monitoring systems (TPMSes) for automobiles and mobile devices.
As a power generating element, Ueno used a magnetostrictive material called “Galfenol” (Fe81.4Ga18.6), which is an alloy of iron (Fe) and gallium (Ga) and has a large magnetostriction effect. Magnetostriction effect is a phenomenon in which the shape of an object changes due to magnetization. In the case of Galfenol, its magnetostriction effect is about 300ppm (a 1km-long bar would extend by about 300mm).
To generate electricity, Ueno used inverse magnetostriction effect, which is the converse phenomenon of magnetostriction effect. Inverse magnetostriction effect is a phenomenon in which the application of stress changes magnetization. In the case of Galfenol, its magnetic flux density decreases by up to more than 1T when compressional stress is applied. And the change in magnetic flux generates an induced voltage.
Galfenol was developed by the US Naval Research Laboratory (NRL) in 1998. But this is the first time that it has been used for a vibration-driven generator of small size, Ueno said.
Galfenol is a ductile material. Therefore, it can be easily processed by machines, and it does not easily break when warped. Piezoelectric elements, which are normally used for vibration induced power generation, are made of a ceramic, which is a brittle material.
Terbium (Tb), dysprosium (Dy) and Terfenol-D, which is an iron-based super-magnetostrictive material with a magnetostriction effect of about 1,000ppm, have large magnetostriction effects. But they are also brittle materials.
This time, Ueno created a structure that is small but can generate a large amount of electricity by taking advantage of the ductility of Galfenol. Specifically, one ends of two long and thin magnetostrictive elements are fixed, and weights are added to the other ends (parallel beam structure). To output electricity, coils are made by winding thin electric wires around the magnetostrictive elements.
When the weights vibrate up and down, bending deformation occurs to the two magnetostrictive elements. At this point, a compressive force is applied to one element, and a tensile force is applied to the other element.
When the weights are moving up and down, the compressive force and the tensile force are alternately applied to each of the two elements, periodically changing the magnetic flux. This time variation generates induced voltage on the coils, and electric power can be efficiently output.
“Because Galfenol is a ductile material, even the thin rod-shaped structure does not easily break,” Ueno said.
The dimensions of the magnetostrictive element are 1.0 x 0.5 x 10mm, and it is equipped with a magnet that is 2mm in diameter and 2mm in length for bias magnetization.
Moreover, Galfenol features an excellent thermal property. Its Curie temperature is as high as 700°C, and its performance does not change much within the temperature range of -200 to 200°C, Ueno said. Therefore, Galfenol is suited for use in places where change in environment is fast and violent.
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