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Research paper : Development of a pressure sensor using a piezoelectric material thin film (M. Akiyama et al.)−178−Synthesiology - English edition Vol.5 No.3 (2012) assembled as shown in Fig. 12. The AlN sensor housing was a screw of external diameter of 10 mm and 1 mm pitch, and this was attached to the engine. The appearance was similar to the commercial sensor (Kistler 6001) installed using the engine mounting adapter.The AlN sensor and the commercial sensor were installed on an engine, and the combustion pressures were measured. The output waveforms of sensors with one, three, and five layers of AlN elements were measured. The output waveform of the sensor with five layers is shown in Fig. 13(a). A waveform similar to the commercial sensor was obtained, and the generated charge was higher. Figure 13(b) shows the dependency of the generated charge on the number of layers. The output of the element was 40 pC with one layer, 140 pC with three layers, and 210 pC with five layers. The generated charge increased linearly as the layers increased, and about the same output was obtained with three layers as the commercial layer. It was determined that the minimum sensitivity as an actual sensor onboard a car was satisfactory if the generated charge was the same level as the commercial sensor. Using this housing, a maximum of 15 layers of the element with thickness of 0.2 mm can be installed, and the output of 4.5 times higher than the commercial sensor can be obtained when 15 layers are used.The two-cycle engines are used in motorcycles and scooters since there are problems of fuel efficiency and exhaust gas. Four-cycle engines are used in mass-produced cars, and evaluation using the commercial four-cycle engine (Robin Engine EY28DS manufactured by Fuji Heavy Industries Ltd.) was conducted. As shown in Fig. 14, the AlN sensor was installed on the cylinder head valve side of the engine using an adapter, the commercial sensor was attached directly on the right side, and the measurements were taken. For this AlN sensor, three layers of the AlN thin film with increased sensitivity achieved by adding Sc was used.[12] Figure 15 shows the outlet waveforms of the AlN sensor and the commercial sensor. The output of the commercial sensor was 52 pC and the output of the AlN sensor was 151 pC, three times the output of the commercial sensor. If the maximum of 15 elements were used, 15 times the output of the commercial sensor can be expected. The slight pressure change at exhaust that could be seen as broad peaks in the commercial sensor could be observed as clear peaks in the AlN sensor. From these results, it can be expected that the AlN sensor may greatly exceed the performance of commercial sensors, and will perform as a practical combustion pressure sensor.5 ConclusionIn this paper, the history of the R&D of combustion pressure sensors was described, where the authors used the thin film piezoelectric material for the first time in the world. In the field of combustion pressure sensors before the ideation of this R&D (before 2003), the thin film piezoelectric material was an unknown field and its usefulness was not recognized. However, since the start of this research, the automotive parts companies and universities of Japan and overseas became interested, and joint research was done with the Japanese automotive parts company and Meiji University. Before this thin film combustion pressure sensor can be installed in the mass-produced cars, there are still several more issues that must be solved such as the demonstration test of environment resistance and durability, stabilization of sensor signals, achievement of high output, simplification of sensor structure, and others. It is still in the “valley of death.” However, through the cooperation with various people, the issues are being overcome one at a time, and we have been able to move forward one step at a time. With the actual use and diffusion of this thin film sensor, the exhaust gas volume of the cars that are running around the world may decrease dramatically, hence it will contribute greatly to the environment and energy fields. Also, the potential is high for application to the engines of vessels, special vehicles, and generators, and the ripple effect can be infinite. AcknowledgementI express my gratitude to the following people: Dr. Yasukata Tsutsui and Dr. Hiroshi Tateyama of AIST who promoted this R&D; Professor Kazuo Tsuchiya of the Meiji University who instructed us on the sensor design and the evaluation method using an engine; people of the automotive parts company with whom we conducted the joint research; Dr. Hiroaki Noma, Dr. Kazuhisa Shobu, Dr. Yasunobu Oishi, Dr. San’yo Takahashi, and Dr. Hirohide Furutani of AIST; and all other people who were involved in this project.Fig. 15 Response waveform of AlN sensor in four-cycle engineCommercial sensor(52 pC)AlN sensor(151 pC)M 50.0 msCH2500 mV500 mV1

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