Study On Humidity And Moisture-content Measurements Based On The Microwave Detection Technology

With the features of high sensitivity,multiple parameters,strong penetrability and moisture sensitivity,the microwave moisture measurement technology shows unique advantages in the application fields of environment humidity sensing and moisture content determination.However,some key scientific and technical issues still need in-depth studies,including the cooperative operating mechanism between microwave devices and humidity sensing materials,the hybrid modeling between microwave signals and measured materials,and the frequency regulation and structure design for the microwave moisture determination.This thesis focuses on the sensing mechanism of microwave detection modules and the perturbation model of microwave moisture measurements.Two kinds of microwave detection structures,namely the coupled microstrip resonator and the slot patch antenna,are designed for environment humidity detection and material moisture determination,respectively.Dielectric properties of the sensitive material and the tested material are analyzed under the changing moisture condition,and the electromagnetic perturbation model is established.The microwave multi-parameter selection scheme is studied,followed by the precise characterization of the moisture based on the multi-variable linear regression algorithm.The main work of this thesis is summarized as follows.（1）Microwave moisture sensors operate on the relationship between the detection marker and the environmental or material moisture.The design scheme of the microwave detection unit（resonator type and microstrip antenna type）with multiple resonant modes is proposed,where the resonance is used as the detection marker.The generating mechanism of detection markers is the key to designing microwave sensing structures.The increasing number of detection marks provides more detection bases for humidity and moisture sensing.The high-intensity electromagnetic coupling field and radiation field are constructed by the full-wave electromagnetic simulation to obtain the sensitive area.It can be found that the resonant-mode number increases with the coupling path in regard to the resonator type detection unit,and the resonant-mode number increases with the slot number in regard to the microstrip antenna type detection unit.Besides,the evaluation indexes of microwave moisture sensing performance are summarized,providing effective guidance for sensor development and testing.（2）Current researches on the microwave humidity detection lack the exploration of the cooperative operating mechanism between detection units and sensitive films.It is necessary to clarify the electromagnetic perturbation mechanism of the sensing material to improve the sensitivity,detection range and other detection performance effectively.A novel double split-ring resonator（DSRR）based on interdigital capacitors（IDCs）coupling is proposed.The IDCs structure enhances the capacitive coupling between split gaps,expanding the sensitive area from the split ring gaps to the whole IDCs.The DSRR structure has dual resonances,whose response under different humidity levels presents the same relationship.Sensitive materials,including polymer（polyimide,PI）,polymer doped metal oxide（PI/Ti O₂）and polymer doped metal（PI/Ag organic complex solution）are coated onto the sensitive area respectively,in order to study the cooperative operating mechanism between the novel DSRR and different types of sensing films.The results show that the doping of Ti O₂and Ag causes higher detection sensitivity（up to 880 k Hz/%RH and 0.407 d B/%RH based on the resonant frequency and return loss）,as well as shorter response and recovery time（about 10 s）.（3）Based on the microwave moisture sensing mechanism,the slot patch antenna is used in the rice moisture measurement.Six rice samples with the moisture content of 10%～20%are prepared by the water conditioning method for the moisture test.The bulk-density effect of rice samples and the height effect of the slot patch antenna on microwave characteristics are discussed.By far,most studies only use one certain microwave characterization parameter to establish the corresponding relationship with the material moisture.Several parameters,such as resonance frequency,return loss and phase response,are extracted to ensure the utilization of multiple microwave parameters.A moisture prediction model is proposed based on the multi-variable linear regression algorithm.Besides,the compatible technology of microwave sensors and flexible substrates is also studied.The flexible patch antenna is fabricated using the screen-printing technology,and the moisture of rice in the small grain storage barrel is measured successfully.Although the mean square error（MSE）of the rice moisture calculation model with regard to the flexible patch antenna is 1.23,higher than that of the rigid patch antenna,the model can still be considered to have a relatively accurate prediction ability.The multi-variable linear regression characterization model defines the measurement system with multiple derived microwave parameters,which develops a new characterization way by selecting and restructuring microwave parameters.Compared with the univariate linear regression representation model,the multivariate linear regression representation model provides higher accuracy in moisture prediction.