Simulation And Design Of Wireless Passive Impedance-loaded SAW Sensors Based On The FEM/BEM Method

Research on the wireless passive surface acoustic wave (SAW) sensors has received significant attention in recent years for they have many outstanding properties: high resolution (digital signals output), wide application range, mass production, et al. Among them, the impedance-loaded SAW sensors expand the application range more widely. Accurate modeling and simulation is the key point to develop the high performance SAW sensors. This paper concentrates on the accurate modeling and simulation of wireless passive SAW impedance-loaded sensors based on the numerical combined finite element method/Boundary element method (FEM/BEM) to develop a high precision simulation tool for the R&D of SAW sensors.The roundoff errors due to different methods obviously affect the results in the numerical calculation. The numerical analysis on the calculation process is proposed to determine the minimum error method. The calculation program was written to calculate the properties of various substrate materials providing parameters for the sensor design procedure.The periodic FEM/BEM method is utilized to analysis the device response of the various sensor configurations. It can provide simulating parameters for the common phenomenon models (Coupling-of-modes (COM) model, for example) of the SAW sensors. One electrode of interdigital transducer (IDT) can be considered as a period of infinite periodic gratings on the semi-infinite piezo-electric crystals. The periodic Green's function is used to describe the electro-acoustical properties of the piezoelectric substrate, and the finite thickness of electrodes is accounted for by FEM. The equations of system can be solved and the harmonic admittance can be calculated. The program was written and can be used to analysis the frequency dependent property of harmonic admittance and the wave patterns existed in the grating structure.The zero point and pole point in the complex plane were estimated by performing the two-dimensional search and the calculating program was written to estimate the dispersion curves of different sensor structures. The variation of dispersion curves due to the mass loading effect of electrodes were investigated. The results show that the width of stop band depends on the height of electrodes. and the shape of dispersion curves relates to the ratio of height/pitch. The summing-up can help to tune up device performance including the the center frequency. The COM parameters used in the simulating procedure of SAW sensors can be extracted from the dispersive curves of grating structures exactly.The impedance-loaded SAW sensors were simulated using the COM model and the non-periodic FEM/BEM model respectively. A fast algorithm was presented to calculate the bulk wave part of the numerical calculation in the non-periodic FEM/BEM simulation. The admittance matrix of the sensors was estimated. The complete model including the impedance-matching circuit, SAW device, loaded sensors was constructed. The sensing response to the testing environment can be estimated and the simulation results agree well with the test results.The optimization procedure of impedance-loaded SAW sensors was presented based on the models of the sensors. The testing samples utilizing the delay-line structure composed of different types of IDTs were fabricated. The simulated and measured device performance were compared and the results proof the effectiveness of this simulating method. The measured and calculated amplitude variation of the impulse response in time domain shows a resonant characteristic with the change of the loaded capacitive impedance. This phenomenon is successfully explained by using the proposed model. Some high performance sensors with greater amplitude modulation and higher resolution could be designed using this method.