WiFi Sensor Node With High Sensitivity and Linearity Based on a Quarter-Wavelength Resonator for Measuring Crack Width

In this work, a WiFi sensor node is proposed for measuring crack width. The proposed sensor node consists of a quarter wavelength, a movable patch, a power unit, a reflectometer for measuring the reflection coefficient, a processing unit, and a WiFi transceiver module. The sensor was simulated, measured with a vector network analyzer (VNA), and a wireless sensor network (WSN) based on WiFi technology. The sensor sensitivity is 0.125 GHz/mm in simulation, 0.109 GHz/mm with the VNA, and 0.116 GHz/mm with the WSN. The sensor resolution measured in the experiment with the WSN is 0.1 mm. Additionally, the fitting curves representing the data obtained from the sensor have a coefficient of determination of 0.995, 0.997, and 0.998 for the simulation, VNA, and WSN, respectively. Therefore, the sensor node has an initial resonance frequency of 2.733 GHz, a sensitivity of 0.116 GHz/mm, and a coefficient of determination of 0.998 measured within a measurement range from 0 to 5 mm. Thus, the proposed sensor has high sensitivity and linearity, as well as low distortion within the mentioned measurement range.