Design Of Measurement System For Solution Concentration Based On Microwave Perturbation Technique

Since Maxwell found phenomenon of electromagnetic in 1864,the theory of electromagnetic become more perfect after a so long time.As a kind of high-frequency electromagnetic wave,microwave attracts more attention especially in the high-frequency era nowadays.Many kinds of applications of microwave draw more researchers'eyes in field of both scientific,such as microwave radar,remote sensing and controlling applied in wireless-communication,Meanwhile there are many applications of measurement instruments of high-frequency and microwave weapons.At the same time microwave detection technology has also been widely used in detecting concentration in chemical industry,humidity detecting in atmospheric science and disease detecting of human body in medical science.The resonant cavity system described can be used to detect liquid concentrations.Firstly,7GHz is and TE₀₁₁ mode are chosen as work frequency and work mode.Based on the perturbation theory,the relationship between the resonant frequency shift and the concentration in the TE₀₁₁ mode is presented.The relationship demonstrates that the effects of parameters such as dielectric constant,resonant frequency and cavity radius on the slope.This relationship can provide a theoretical basis for simulation and experimental and scientific applications.After analysis and calculation,BJ-84 rectangular waveguide is used for energy supply,and energy transfer is accomplished through coupling aperture.In order to prevent the corrosion of the cavity wall,a capillary used for loading solution is put in the center of the cavity where the electromagnetic is very strong.An absorbing layer is provided in the resonator to absorb the disturbing waveform.Four critical parameters of the rectangular waveguide length,the radius of the coupling aperture,the radius of the solution tube and the radius of the absorbing layer are optimized in Ansys HFSS.By comparing the results S₁₁ parameter,the four parameters are selected as 20mm,4mm,2mm and 11mm,respectively.Based on this model,the resonant frequency of the pure water solution is 7.6318GHz.20 groups of different solution from 10mg/dL to200mg/d L uniformly distributed is simulated.The relationship between the resonant frequency shift and the concentration is linear,which accords with the theoretical results.H55 copper is selected as the material to make the cavity.Automatic detection system mainly includes amplitude detection,phase detection and frequency detection module.Amplitude detection mainly detects whether the resonant cavity resonant,phase detection mainly detects the deviation of input frequency and resonant frequency,those two modules are simulated in Multism.Amplitude detection and phase detection can output the digital results 0 and 1,which can be easily received and analyzed by FPGA.The frequency detection module uses a measurement method to measure the frequency offset,and FPGA platform to verify and implement the method.During 1s time gate,the frequency detection module error is1Hz,fully meeting the requirement of frequency offset test of different concentration solution.