Research On Flexible Surface Acoustic Wave Temperature Sensing Technology

In this paper, a passive and wirless surface acoustic wave (SAW) temperature sensor based on flexible material is designed, to realize the high precision measurement and monitoring of the surface temperature on the rotating shaft and curved surface. The key technology of flexible surface acoustic wave (SAW) temperature sensing is studied. A method of sputtered ZnO thin film on polyimide flexible substrate is adopted to prepare piezoelectric substrate of the sensor. The feasibility of the design scheme is verified by modeling and simulation, and the structure and performance of the flexible SAW sensor are analyzed by the simulation results.First of all, the paper discusses the theory of the excitation and propagation of surface acoustic wave and the working principle of IDT. Compared with the traditional SAW sensor, the design method of SAW device based on piezoelectric multilayer-film structure and the design theory of single-port resonator are studied to determine the structural parameters of the sensor. The simplified model of flexible SAW sensor is established by using the finite element analysis software COMSOL, to verify the feasibility of the design scheme of flexible SAW devices. Then the characteristic frequency, the characteristics of modality and electrical of this flexible SAW devices are analyzed. What's more, the diagrams of symmetric-mode vibration and anti-symmetric-mode vibration are extracted. Phase velocity of surface acoustic wave spread in the ZnO piezoelectric thin film is calculated by resonance frequency analysing, which is 2440.75m/s. The distribution of electric potential in the ZnO film is obtained, and so was the relationship between the sound radiation admittance and the resonance frequency of resonator. And the effects of the structural parameters of one-port resonator on the performance of the sensor are analyzed. The results shows that if the thickness of ZnO film is half of the SAW wavelength, the surface acoustic wave energy generated by excitation will concentrate on the surface of the substrate, energy degration is very small. Sensor output response is proportional to the logarithm of the IDT, the more number of cross fingers, the smaller loss of the output, the greater the sound aperture of IDT, the higher the efficiency of the acoustic-electric conversion, the greater the power of the conversion output.Finally, the frequency response of the flexible SAW temperature sensor is simulated under different temperature conditions. The simulation results illustrate that the resonant frequency of the sensor can be increased linearly with the increase of temperature, showing good characteristics between frequency and temperature. Besides, the micro manufacturing technology of flexible SAW sensors is studied and its process flow of fabrication is investigated, which providing theoretical basis for the fabrication of flexible SAW temperature sensor.