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.)−176−Synthesiology - English edition Vol.5 No.3 (2012) frequency responsiveness, since the revolution of the car engine is several thousand rpm (several ten Hz). When the measurements were done in a high temperature environment, it was confirmed that the piezoelectric response of the AlN thin film did not change at all for 54 h at 450 °C (2006).[9] 4 Fabrication of the sensor prototype and its evaluation4.1 Performance evaluation with an engineSince we obtained a prospect of using the AlN thin film element, we fabricated a prototype of the combustion pressure sensor (2007).[10] Figure 7 shows a photograph of the combustion pressure sensor fabricated with the AlN thin film and the schematic diagram of the sensor structure. The pressure receiving part of the sensor had a simple structure where the AlN thin film element was held with a discoid internal electrode to receive the charge with the signal wire. An alumina plate and an alumina tube were used to insulate it from the housing. To investigate the combustion pressure response property of the AlN sensor, single cylinder two-cycle engine (HONDA LEAD 90, HF05) was used. As shown in Fig. 8, the sensor was installed near the apex of the cylinder head to measure the pressure of the combustion chamber. To conduct performance comparison, the combustion pressure sensor (No. 6001) of Kistler Corporation was used because this was most widely used in engine research. The output of the AlN sensor when the engine revolution was set to about 4000 rpm without load was compared to the commercial sensor. The result is shown in Fig. 9. Since the generated charge of the commercial sensor was 140~160 pC, the pressure inside the combustion chamber was 1.1~1.2 MPa. There was almost no external noise in the output wave of the AlN sensor, and almost the same waveform was observed as that of the commercial sensor.To evaluate the durability of the AlN sensor, the change of sensor output when the engine was repeatedly run at about 2000 rpm for 20 min, cooled, and then run for 20 min again is shown in Fig. 10. Since the revolution of the engine could not be kept constant accurately, there were variations in the output value for each measurement, but a fairly constant output was obtained. There was almost no decrease in output after a total of 40 hours, and stable operation continued.The initial objective was to confirm whether the measurement of combustion pressure was possible in a real engine, and to obtain the output waveform close to the commercial sensor. We were able to obtain almost similar response waveform as the commercial sensor and the initial objective was achieved.The commercial sensor is made by finely processing fragile crystal monocrystals and then combining the monocrystal pieces. Since it has a complex structure where the small crystal plates are cut out by orienting the crystal axes and then combining them, high cost and technique are required to fabricate the element. In fact, the commercial sensor used for comparison cost several hundred thousand yen. On the other hand, the AlN sensor developed by the authors has a simple structure where the AlN thin plate fabricated on the metal plate is held with electrodes, and we aim for the price of 10,000 yen or less, as the sensor for mass-produced cars.Fig. 9 Output waveform of sensor during operation of two-stroke engineEngine revolution: about 4000 rpmFig. 10 Dependence of AlN sensor output on engine run time Intermittent run at engine revolution 2000 rpm × 20 minFig. 8 Attachment of sensor to two-cycle engineCommercial sensorAlN sensorSpark plugTime(sec)Generated charge(pC)Commercial sensorAlN sensor0.025005010015020000.02Time(h)Generated charge(pC)403020100050100150

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