Vol.5 No.3 2012

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Research paper : Development of a pressure sensor using a piezoelectric material thin film (M. Akiyama et al.)−177−Synthesiology - English edition Vol.5 No.3 (2012) 4.2 Downscaling for mounting in automobilesAs the research progressed, we faced the problem that the charge amp that were used in the lab could not be installed in mass-produced cars due to difference in power source, severe use condition, price, and others. Therefore, an amp that was already installed in the car for a different purpose was used, but the sensor output several times higher was necessary to use that amp. The recent high-performance engines have become increasingly complex in structure due to the employment of the high-pressure fuel injection system and increased number of valves, and there was no space to mount the combustion pressure sensor with a large housing as used in the above experiment. Therefore, we had to downscale the housing. In case of the piezoelectric material, since the generated charge was proportional to the pressure receiving surface area, it was necessary to increase the pressure receiving surface to increase the sensor output. However, when the pressure receiving surface was increased, the size of the sensor element increased, and this in turn, meant it could not be installed on the engine.The authors considered the ways to decrease the volume while increasing the sensor output, and devised a way to decrease the volume by increasing the surface area of the sensor element, by stacking the sensor element vertically. However, to realize such laminated structure, the lamination had to be done without having the thin film surface or the substrate contacting the housing. Therefore, the authors devised the element structure shown in Fig. 11.[11] In this structure, a hole was punched in the center of the circular metal substrate, AlN was formed on the entire surface except that part, signal wires were passed through the central hole, and the elements where the AlN surface was covered with copper foil were laminated. Since AlN is an insulator, if the signal wire is insulated from the housing, the substrate side of the AlN thin film and the surface will not come in contact. The commercial sensor and the AlN sensor are compared in Fig. 12. The exterior diameter of the AlN sensor was 4.6 mm, and the thickness of the sensor element was 0.2 mm and AlN was formed at about 3 m thickness on both sides of the metal substrate. This was wrapped with a copper foil electrode of 10 m to fabricate the element shown in Fig. 11, and was Fig. 12 Appearance of fabricated AlN sensor prototypeFig. 11 Diagram and photograph of laminated structure of AlN thin film sensor elementFig. 13 (a) Response waveform of AlN sensor, and (b) dependence of generated charge amount on number of sensor elementsFig. 14 Appearance of four-cycle engine with sensors attached(a) (b) AlN sensor (5 elements, 210 pC)Commercial sensor levelNumber of sensor element layersGenerated charge amount (pC)6543210250200150100500Commercial sensor (140 pC)1 mmAlN thin filmStainless substrateCopper foil electrodeInsulating tubeInsulating plateElectrode with signal wireHousingPush rodCommercial sensorAlN sensor15141312cmSpark plugCommercial sensorAlN sensor

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