Applications of wireless SAW sensing in the steel industry

Dual-Frequency Interrogation and Hierarchical Evaluation Scheme for SAW Reflective Delay-Line Sensors

The resolution of phase difference ambiguity is critically important for simultaneously achieving wide range and high accuracy sensing with surface acoustic wave (SAW) reflective delay-line sensors. A novel scheme called dual-frequency interrogation and hierarchical evaluation (DFI-HE) is proposed to resolve integer ambiguity of phase difference and obtain precise time delay between reflectors. With DFI-HE, only two reflectors are needed as the sensing elements for SAW reflective delay-line sensors. Additionally, DFI-HE has the advantage of fast ambiguity resolution to enable both a wide range and high accuracy sensing. The basic principle of sensing with DFI-HE is elaborated, and simulations and temperature sensing experiments are carried out to verify the proposed scheme, based on a SAW reflective delay-line temperature sensor with two reflectors fabricated on 128° YX-LiNbO₃ substrate. The experimental results demonstrate that the temperature measurement accuracy reaches ±2 °C in the range of 8 °C-300 °C.

A Three-Dimensional Finite Element Analysis Model for SH-SAW Torque Sensors

In this paper, a three-dimensional finite element analysis (3D-FEA) model for shear horizontal surface acoustic wave (SH-SAW) torque sensors is presented. Torque sensors play a significant role in various fields to ensure a reliable torque transmission in drivelines. Featured with the advantages of high propagation velocity, large Q-value, and good power capacity, SH-SAW based torque sensors are promising but very few studies have been carried out. In order to develop a successful sensor, understanding the characteristics of SH-SAWs produced on piezoelectric substrates and torque sensing modes is indispensable. Therefore, in this study, we first investigated the effect on the generation of waves when different Y-cut quartz substrates are engaged. Thereafter, analyses and comparisons, regarding the effect on the polarized displacement, wave guidance, and wave mode, were conducted for different configurations of wave-guide layer thickness to wavelength ratios (h_layer/λ) and materials. Results show that Y-cut quartz at an angle close to 36° with a gold (Au) layer varying from h_Au/λ = 0.02 to 0.03 thickness could be the most effective configuration for the excitation of SH-SAWs, compared to other combinations using platinum (Pt), titanium (Ti) and silicon dioxide (SiO₂). Finally, based on the FEA SH-SAW torque sensor model configuring with a Y + 36° quartz substrate and 0.025 λ-thick gold layer, the relationship between the applied torque and sensed voltage was examined, which shows a perfect linearity demonstrating the performance of the sensors.

Measuring Torque and Temperature in a Rotating Shaft Using Commercial SAW Sensors

Real-time monitoring of torque in a rotating shaft is not easy to implement with technologies such as optic fiber sensors or strain gages. Surface acoustic wave (SAW) sensors are wireless and passive and can be used to monitor strain in moving parts. Commercial solutions (sensors, antennas and interrogation unit) can easily be purchased from some companies; however, they are not customized and may not meet the specificity of the measurements. In order to evaluate the adequacy of commercial off-the-shelf (COTS) solutions, temperature and strain sensors fabricated by SENSeOR (Besançon, France) were mounted on a load cell. The sensors were calibrated using a thermal chamber and a universal testing machine. The load cell was then assembled together with a steel shaft that rotated at different speeds inside an oven. The commercial antennas were replaced with an RF (radio frequency) coupler and the sensors were interrogated with the commercial interrogation unit. The influence of rotation in the accuracy on the measurements, as well as the adequacy of the sensors structure, was evaluated. It can be concluded that SAW sensors can be used to measure temperature or torque in a rotating environment; however, some customization of the components is required in order to overcome the limitations posed by COTS sensing solutions.

Flexible surface acoustic wave resonators built on disposable plastic film for electronics and lab-on-a-chip applications

Flexible electronics are a very promising technology for various applications. Several types of flexible devices have been developed, but there has been limited research on flexible electromechanical systems (MEMS). Surface acoustic wave (SAW) devices are not only an essential electronic device, but also are the building blocks for sensors and MEMS. Here we report a method of making flexible SAW devices using ZnO nanocrystals deposited on a cheap and bendable plastic film. The flexible SAW devices exhibit two wave modes - the Rayleigh and Lamb waves with resonant frequencies of 198.1 MHz and 447.0 MHz respectively, and signal amplitudes of 18 dB. The flexible devices have a high temperature coefficient of frequency, and are thus useful as sensitive temperature sensors. Moreover, strong acoustic streaming with a velocity of 3.4 cm/s and particle concentration using the SAW have been achieved, demonstrating the great potential for applications in electronics and MEMS.