Research On Temperature And Strain Coupling Effect Of Surface Acoustic Wave Sensor

Surface acoustic wave?SAW?sensors have a good application prospect due to their advantages of wireless and passive operation,high sensitivity and stable performance.SAW strain sensors have been widely studied and applied in the fields of civil aviation,thermoelectric energy industry and automobile industry.However,there are still some problems.SAW devices are very sensitive to temperature.If SAW strain sensors are operated at high temperature,the strain responses of the SAW strain sensors are influenced by temperature.Thus,the strain responses become inaccurate.This situation limited high temperature applications for the SAW strain sensors.Then,only if the strain responses are extracted from the original responses from SAW strain sensors are the test results of SAW strain sensors accurate and credible.To solve this problem,the temperature and strain coupling effect of langasite?LGS?SAW sensor was studied in this dissertation.In this dissertation,SAW resonators deposited on?0o,138.5o,??LGS cut were fabricated.The frequency-temperature and frequency-strain characteristics of different LGS cuts were studied.The frequency-temperature characteristics of different LGS cuts obeyed quadratic function.The first-and second-order temperature coefficients of?0o,138.5o,26.6o?cut LGS were close to 0,and the temperature coefficients of?0o,138.5o,86.6o?cut LGS were the largest.The strain sensitivities of the LGS SAW resonators decreased with temperature increasing.There was a linear relationship between the strain sensitivity of the SAW resonator on?0o,138.5o,86.6o?LGS cut and temperature.The linear correlation coefficient was 0.996.Because the resonance frequencies of the LGS SAW resonator shifted greatly as temperature increasing,?0o,138.5o,26.6o?LGS cut was selected to fabricate the SAW resonator and the different thicknesses Si O₂ films as temperature compensation layers were deposited by plasma enhanced chemical vapor deposition?PECVD?on the LGS SAW resonators.The frequency-temperature characteristics of the LGS SAW resonators with different thicknesses Si O₂ films were studied.The thickness of Si O₂ films was from 0?m to 1.80?m.The results showed that the temperature compensation effect of Si O₂ films on the LGS SAW resonators was obvious.However,the compensation temperature range decreased as the Si O₂ film thickness increasing.It was difficult to compensate in wide temperature range.The absolute values of the first-and second-temperature coefficients increased as the thickness of Si O₂ film increasing.When the thickness of Si O₂ film was 0.30?m,the first-order temperature coefficient was about 0.According to the frequency-strain characteristics test results of the LGS SAW resonator with 0.90?m Si O₂ film,it was found that the strain sensitivity of the resonator was effectively reduced in the temperature range near the turnover temperature.Based on the strain responses of several LGS SAW resonators at high temperature,the differential measurement was adopted to decouple temperature and strain.Three LGS SAW resonators were bonded to the cantilever beam.The propagation directions of the SAW in the three SAW resonators were parallel,perpendicular and at an orientation of 30oto the applied strain,respectively.The strain sensitivities of the three SAW resonators from room temperature to 250oC were investigated.It was deduced that the strain could be linearly represented by the resonant frequency shifts of the three SAW resonators.In the experiment,the three SAW resonators were adopted to decouple temperature and strain in the temperature range from room temperature to 250oC.The average values of the calculated strains were in agreement with the practical applied strains,though there was a little fluctuation between the calculated and preset strain.The standard deviation was 14.9??and the relative error was 0.25%.The stability and reliability of the method were proved.Finally,based on SAW and BAW modes of one LGS SAW resonator,the method of decoupling temperature and strain was studied by differential measurement.The resonance frequency of BAW mode decreased monotonically as temperature increasing,and the frequency-temperature characteristic obeyed the quadratic function.From room temperature to 250oC,when the temperature was fixed,the strain sensitivities of BAW mode was very close to these of SAW.SAW and BAW modes of the LGS SAW resonator were adopted to decouple temperature and strain.The resonance frequency differences of SAW and BAW modes were only related to temperature.In the experiment,the calculated strains were consistent with practical applied strains.The relative error was 1.7%.Besides,the problem of fluctuation was solved.