Compared with apodization, the piston-mode technique is a preferable solution to suppress the spurious transverse modes in surface acoustic wave (SAW) devices. This paper extends the technique to the SAW resonator (SAWR) strain sensors to achieve a high performance, that is, conquer some negative influence in the wireless strain sensing system caused by the spurious modes, such as low signal to noise ratio (SNR), high misreading rate and demodulation difficulty. The principle of piston-mode operation was presented and a modified expression was given for primary design of the piston-mode waveguide, which takes into account the anisotropy difference in different electrode regions. SAWRs based on the Al/ST-quartz with the conventional and two common piston-mode configurations were designed, simulated, fabricated and measured. The experiments to evaluate the wireless performance and strain sensitivity of the SAWR strain sensors with different electrode configurations were performed.

Since surface acoustic waves (SAWs) are composed of multiple displacement components, their slowness curves sometimes exhibit a more complex shape rather than elliptical or hyperbolic. As a result, the parabolic approximation of 2-D coupling-of-modes (2-D COM) equations is invalid for analysis of waveguiding in such SAW devices. Thence, this paper develops a new general form of 2-D COM equations based on the basic 2-D wave equations of SAWs which contain two displacement components. The fundamentals of the equations were established and the procedures for determining 2-D COM parameters were presented in detail. The new equations were embedded in the partial differential equation (PDE) mode of COMSOL for the simulation of different types of SAWs, including Rayleigh and shear-horizontal (SH) SAWs on ST-quartz and 42°YX-LiTaO3 (42-LT) substrates, respectively. By comparing the calculated results with 1-D COM, parabolic 2-D COM, extended thin plate model, 3-D finite-element method (FEM) calculated results, and the measured result, the applicability of these equations was verified.

In order to improve the measurement accuracy of surface acoustic wave (SAW) torque sensor, an ultra-high precision SAW torque sensor demodulation method based on multi-objective bat algorithm (MOBA) and root-MUSIC algorithm was proposed. The root-MUSIC algorithm was adopted to construct the polynomial equation of SAW echo signals. The construction of the objective function was based on two principles: the least distance to unit circle and least phase angle to the mean roots. MOBA was used to optimize the objective function to improve the accuracy of the roots. The numerical simulation and actual interrogating experiments were carried out and comparisons with other methods were conducted. The demodulation results verified that the proposed method had a minimum estimation error with standard deviation of 1Hz. The designed demodulation method was used to the SAW torque measurement system. The torque sensing SAW resonator was designed using a FEM software and manufactured with a frequency of 433.945MHz and an unload Q factor of 13370. The SAW resonator was glued on a bearing to measure applied torque. The torque loading and unloading experiments demonstrated that the nonlinearity of the proposed method was 0.1402%, which is the minimum when compared to other methods. The hysteresis was 1.1791%, and the sensitivity was 1.3570kHz/Nm. Numerical simulation and experimental results demonstrated the effectiveness and accuracy of the designed algorithm in SAW torque sensor demodulation system.

SAW strain sensing element is an important part of SAW strain sensor. It is of great significance to design and analyze it. In this paper, the influence of piezoelectric substrate and electrode structure parameters on the performance of SAW strain sensing element is simulated and analyzed, using ST-quartz as piezoelectric material and aluminum as electrode material, a high-performance surface acoustic wave strain sensing element was designed, fabricated and measured. By analyzing the measured results, the strain sensitivity of frequency ( SSF ), strain coefficient of frequency SCF ) and temperature coefficient of frequency ( TCF ) of the SAW strain sensing element are 550.9Hz/ $\mu \varepsilon $ , 1.26 ppm/ $\mu \varepsilon $ and −1.22 ppm/°C respectively. The experimental results meet the design requirements well. This research provides a good guidance for the application of SAW strain sensor.

The echo signal energy of surface acoustic wave (SAW) strain sensor is weak, resulting in low demodulation accuracy. In order to improve the measurement result, a SAW sensor demodulation method by combining hybrid changing area niche genetic algorithm and MUSIC algorithm was proposed. The proposed method was based on the MUSIC algorithm to construct the power spectrum function of SAW echo signal. A hybrid changing area niche genetic algorithm (CANGA) and particle swarm optimization (PSO) algorithm was developed to improve the spectrum peak search accuracy and reduce the amount calculations of MUSIC algorithm. The power spectrum function of the MUSIC algorithm was used as the fitness function of the hybrid algorithm. The echo frequency was estimated by the variable value corresponding to the optimal solution. The proposed hybrid PSO-CANGA has the characteristics of strong global search ability and fast convergence speed to improve the performance of MUSIC algorithm. The numerical comparisons were performed. The results showed that the hybrid PSO-CANGA gives fastest convergence and the highest estimation accuracy compared to original PSO and NGA. In addition, compared with traditional spectrum estimation algorithms, the proposed algorithm had a minimum estimation error with standard deviation of 0.27KHz. The proposed demodulation method was used to SAW strain system. The nonlinearity of the strain measurement was improved, with a nonlinearity of 0.55%. Simulation and experiment results verified the effectiveness of the proposed algorithm for the demodulation of the SAW strain sensor system. It helps to improve the accuracy of SAW strain measurement.

The finite element method (FEM) has been applied to extract the coupling-of-modes (COM) parameters of surface acoustic wave (SAW) devices for a long time. It always involves calculating the dispersive curves or harmonic admittance, which makes the extraction process and results sophisticated, time-consuming and inaccurate. Therefore, a simple method is proposed to extract all COM parameters of the SAW devices rapidly and accurately in this paper. It is based on the FEM and combines the stationary analysis with modal analysis. We have described in detail the basic principles and procedures of the proposed method, and made a comprehensive comparison between the proposed method and the other two existing methods. We have also examined the proposed method by extracting COM parameters of some common SAW substrate, and compared our extracted results with those reported in the other literatures. Results show that our proposed method holds higher accuracy and more efficiency (~s order) than the others (~h order). Moreover, our extracted COM parameters are in an excellent agreement with those reported in the other literatures.