Design And Performance Research Of Humidity Sensor Based On Microwave Detection Technology

Humidity sensors have been widely used in many fields of social production and daily life,and higher requirements are put forward for various performance parameters of humidity sensors,including sensitivity,accuracy,hysteresis,and linearity.Compared with traditional resistive and capacitive detection solutions,microwave detection has the advantages of high resolution,multiple derivative parameters,and low power consumption.It is expected to further improve the humidity detection characteristics.Research on microwave humidity detection will also provide an important theoretical basis and reference for the detection of microwave material properties.This thesis discussed the theory and implementation process of microwave detection technology in detail.By analyzing the design principles and response characteristics of different types of resonators,the preparation,testing,and analysis of microwave humidity sensors based on the coupled resonators are carried out,and the key parameters and humidity sensing mechanisms of microwave sensors are explained.The main research contents of the thesis are as follows.（1）Two types of inductance–capacitance resonators（LCRs）based on the dual microstrip transmission line model are proposed.Based on the theory of the lumped element model of the transmission line,by switching short-circuit and open-circuit states of the terminal load of the transmission line,two resonators are designed on the basis of the reflection coefficient resonance mode and transmission coefficient zero response,respectively.The working mechanisms of the two resonators are analyzed under inductive coupling and capacitive coupling by means of equivalent circuits.The capacitance and inductance parameters of the resonators are extracted by using high-frequency simulation tools.The inductance and capacitance change curves under the adjustment of the substrate permittivity are used to reveal the electrical principle of microwave detection technology.Simulation results show that such resonators are flexible in design,simple in structure and small in size,which provides an effective and convenient solution for designing microwave sensors based on resonators.（2）A passive filter based on the coupled symmetrical split ring resonator（SSRR）is proposed,which uses a loaded bar（LB）and an embedded S-shaped resonator（ESSR）to perturb the resonant mode in the working frequency range of SSRR.Taking the resonant frequency shift and degenerate mode excitation under magnetic coupling and electrical coupling as examples,the design principles and solutions of microwave sensor detecting frequency based on the electromagnetic coupling disturbance of micro-components are demonstrated.（3）A microwave sensor based on the electric-LC resonator（ELCR）is proposed.On this basis,a cooperative design of circuit analysis and layout simulation is performed.With the help of three-dimensional electromagnetic field simulation tool,the electric field strength of the polyimide（PI）medium coated ELCR is analyzed in a humidity environment.Experimental results show that there are strong horizontal and vertical electric fields between the T-shaped branch resonators,and the interdigital capacitor（IDC）between fingers of the ELCR interdigital electrodes（IDE）branches can improve the quality factor and fill factor of the device.On the basis of ELCR,a polyimide microwave humidity sensor（PMHS）with dual detection parameters is prepared by adopting the film forming process of spin-coating and curing PI on the sensitive area.Test results show that the sensitivity of the sensor’s detection parameters at the first and second frequency points are 83.8 kHz/%RH and 0.117 dB/%RH,respectively,and the linearity reached 0.985 and 0.999,respectively.The detection parameters of the dual frequency point of this device have complementary characteristics of humidity sensitivity in the full humidity range,providing good reference for improving the overall sensitivity of sensors.（4）A sensor based on the T-branch double split ring resonator（TDSRR）is proposed.The source-load coupling is enhanced by loading the T-branch on the double split ring resonator（DSRR）,and new resonant mode along with transmission zero is excited by sensitive perturbation of T-branch.The correctness of the sensitive area design is verified through current distribution simulation.The sensor adopts a nested DSRR coupling structure,by adjusting the coupling structure between the split ring resonator（SRR）and the perturbation component,the detection frequency of the sensor can be flexibly designed.In addition,in-situ polymerization technique is used to prepare different proportions of composite moisture-sensitive films combined with doping research on the polyimide-based BaTiO₃.Test results show that under the condition of normal temperature and pressure,the TDSRR sensor using composite material as humidity-sensing film has a maxium sensitivity of 342 kHz/%RH in the range of 10-90%RH,and a maxium hysteresis of 3.7%RH,the shortest response time and recovery time are as low as 1 s and 7 s respectively,and the sensor shows good stability.This work provides ideas for improving the humidity sensor performance based on the modification of composite humidity-sensitive films.