Flexible Langasite-Based Surface Acoustic Wave Strain Sensor for High-Temperature Operation

In various types of surface acoustic wave (SAW) strain sensors, diverse piezoelectric substrates have recently been adopted to improve the operation temperature, strain detection range, and strain sensitivity. In this study, we propose a flexible SAW strain sensor based on a $100 \mu \text{m}$ thin langasite substrate with a good linear strain range of 1– $1800 \mu \varepsilon $ and strain sensitivity of 44.68 Hz/ $\mu \varepsilon $ at room temperature. In addition, the effect of temperature on the strain at temperatures 20 °C–800 °C is measured. The results show that the sensor is affected by temperature at a fixed strain. In fact, varying temperatures or strains cause changes in the measured frequency. Furthermore, in environments wherein both temperature and strain change simultaneously, strain measurements given by changes in frequency are inaccurate. To accurately measure the strain, the interference of temperature should be eliminated. In this study, the intercept and slope are extracted from the curves of the relationship between strain and frequency at different temperatures, and the results are fit. Therefore, the strain value in complex environments can be accurately estimated, with the proposed sensor being able to provide accurate strain sensing with a measurement error within ±5%.