Passive Wireless Strain Sensor Based On Lc Resonance Study

Strain sensor can be widely used in buildings, bridges, vehicles, aerospace and other fields with the function of engineering safety supervision, damage detection and damage prevention. However, in some harsh environment, such as high temperature and high pressure environment, flammable and explosive environment, especially in dynamic rotating parts, the application of wired active strain sensor will be greatly limited, and sometimes even not available in these special occasions. In these cases only wireless passive strain sensor can be used. Therefore, it is important to study wireless passive strain sensors.In this work, a LC resonant circuit wireless passive strain sensor consisted of a plane spiral inductor and an interdigital capacitor was investigated. Firstly, the theoretical analysis and design of the planar spiral inductors and planar interdigital capacitor were studied. And then the approximate formulas were obtained. Secondly, three different sensors with high Q value and high planar interdigital capacitor electrode geometry size ratio η were designed. The Q values of the three different sensors were21,20.7and15.4, respectively. The η values of the designed three sensors were0.75,0.8and0.67, respectively. The geometric dimensions of them were30mm,30.2mm and21.6mm. Finally, the LC resonant strain sensor samples with minimum line width of100μm were prepared by the technologies of photoetching and etching. The performances of the sensors were tested by strain cantilever system and vector network analyzer. The results show that the resonance frequency of the samples decreases with the increase of tension strains. The response sensitivity along to the direction of the capacitor electrode of the three sensors is0.641kHz/ppm,0.677kHz/ppm and0.596kHz/ppm. The Q values of the three different sensors are22.5,20.9and16.3, respectively. The maximum detection range of the sensor is about4.5cm.Ferrite magnetic materials have been used to overcome the problems that the wireless passive strain sensor based on LC resonance can not be applied in metal environment. The ferrite magnetic material can weaken the eddy current losses in the metal surface by the detection coil antenna magnetic field. The magnetic field lines of detection coil antenna are more concentrated by using ferrite magnetic material. The results show that, when the thickness of the ferrite magnetic material between metal and sensor is1.5mm, the LC resonant strain sensor can obviously be detected.