Example of connection to a connector with a built-in amplifier (3.5 mm thick)Sensing area: Φ6 mm17mmOutput terminal35mm20mmThickness: 0.45mm10mmCross-section of a porous sheetAs shown in the cross section, a porous sheet is made of overlapping layers of fibers with air trapped in the gaps between them. Until a few years ago, materials formed in this way were only used as paper.Polarization by corona dischargeApplying ultra-high voltage to the electrodes generates a corona discharge, which induces positive and negative polarization in the material. The porous sheet is now ele-ctre t iz ed, making it a porous piezoelectric sheet.Piezoelectric sheet sensor specificationsThe sensor itself is extremely thin, measuring just 0.45 mm thick. It’s about the size of a one-yen coin and can even be installed on curved surfaces, making it potentially suitable for a wide range of applications. It can also be made to be biocompatible by covering it with PET film. On the other hand, the thickness of the connector part still needs reducing, as shown in the photo. Rion is currently working hard to achieve related technological breakthroughs.sheet is a very thin material with piezo-electric properties, created by polarizing a sheet having many air gaps through a special process. Conventionally, crystals and ferroelectric ceramics are used in sensors for vibration measurement, but they differ greatly in terms of thickness compared to porous piezoelectric sheets, which can be made as thin as about 0.1 mm. The ultra-thin sensors made with this porous piezoelectric sheet are what are called piezoelectric sheet sensors. These sensors can be installed on curved surfaces by making them conform to the shape of the object, like the gaps in a car engine compartment, where previously it had been impossible to install sensors, to measure vibration. Conventional piezo-electric elements can’t do that. This is a revolutionary product that overturns conventional concepts. I believe these sensors will open up whole new possibil-ities for the structural design of various products.”This development project began with the objective of explorative research into a microphone that uses an innovative material. Although porous sheets were known as a material, the project team concluded that it would be difficult to apply them to microphones. However, Okubo maintained a strong interest in this material and continued his research to see if it could somehow be applied to other fields. After much trial and error, he found a way to use the material as a sheet sensor, and proceeded with efforts to commercialize it.“I was able to create such a thin sensor due to my experience in developing microphones used in hearing aids. Such microphones are packed full of technolo-gies for making small sensors. My experi-ence dealing with small, flexible sub-strates and low-current-consumption ICs helped me find the shortcut to realiz-ing these thin sensors.”Adding unique functions to the unique sheets“The porous sheet itself has no electric charge, but by inducing corona discharge and imparting the charge generated into this sheet, we can create a positive and negative polarization for each minute pore in the sheet. The porous sheet con-fining the electric charge in this way becomes a so-called electret, which we call a porous piezoelectric sheet. When-ever there’s a change in external forces applied to this sheet, the state of polariza-tion changes due to the expansion or con-traction of the pores, resulting in the movement of electric charge. This allows mechanical changes to be sensed as elec-trical signals.”The project is now entering a phase in which this revolutionary sensor will be applied to practical applications, as Oku-bo continues to explain:“If the electret of a condenser micro-phone is subjected to pressure—for example, due to contact with a human hand, the polarized electric charge typi-cally dissipates. But we found that as the porous piezoelectric sheet is sandwiched between electrodes, the charge is stable and will not dissipate even if we touch the sheet by hand. The most important requirement of a sensor is that its sensi-tivity stays constant. In other words, this sensor has a unique advantage in that its sensitivity remains constant even when accidentally touched by hand. I also believe that this material has a major advantage in terms of its ease in assembly and its applicability to various struc-tures.”Nevertheless, comments Okubo, there still remains room for improve-ment: for example, developing thinner connectors and ensuring sensor perfor-mance at high temperatures.“We want to continue pursuing research to find new applications and ways to improve this sensor, together with our customers. I’m very much look-ing forward to finding out in which fields of industry this sensor can be put to use in the future.”11
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