Measurement For Solution Concentration And System Implementation Based On Resonance Perturbation Technology

With the continuous development of microwave technology,microwave measurement technology has been widely used in the field of material performance characterization.Its measurement methods can be generally divided into non-resonance method and resonance method.Among them,microwave perturbation technology based on resonance method usually has higher precision and sensitivity.The basic principle of the resonant perturbation technology is to reflect the change of the dielectric properties of the perturbation material through the shift of the resonant frequency of the resonant cavity.The difference in solution concentration will lead to the change of dielectric parameters,and the measurement of solution concentration has a wide range of needs in the food industry,material science,environmental science and other fields.Applying the resonance perturbation technology to the measurement of solution concentration and designing a measurement system with perturbation cavity as the core helps to develop a measurement method for rapid,accurate and real-time monitoring of solution concentration.This article will take polyacrylamide（PAM）solution as the test object to improve the shortcomings of its traditional concentration measurement methods and improve the output efficiency of related production areas.In this paper,the TE₀₁₁ mode cylindrical resonator with the working frequency of7 GHz is designed through the establishment and simulation analysis of high frequency structure model.On the basis of the classical perturbation theory,the deviation of the calculated value when the dielectric constant of the medium changes relatively large in the material perturbation technique is considered,and the influence of the conductivity on the resonant frequency of the resonator is analyzed combined with the shape perturbation technique.Through theoretical derivation and simulation data processing,the correction parameters are obtained,and the perturbation mathematical model suitable for high loss solution medium is established.Nine groups of PAM solutions with concentration gradient of 25 mg/dl and concentration range of 100 mg/dl to 300mg/dl are prepared for experimental verification.Speag coaxial probe is used to measure the relative dielectric constant and conductivity of PAM solution with different concentrations in the required frequency range.Vector network analyzer（VNA）is used to measure the S₁₁ parameters of the resonant cavity under the disturbance state of the solutions of different concentrations,and the corresponding resonant frequency is obtained.The experimental measurement results show that the corresponding relationship between the resonance frequency of the resonant cavity and the solution concentration is consistent with the theoretical calculation.Taking the slope of the experimental result to fit the straight line as the standard,the accuracy of the parameter-corrected perturbation mathematical model is 97.11%,which is 23.43%higher than the traditional theoretical model.The overall relative error of concentration measurement is less than 5.12%,and the average measurement error is 2.62%.Based on the resonant perturbation technique,the structure of the resonant cavity solution concentration measurement system is designed.According to the basic principle of system function realization,relevant performance indicators are determined,and functional modules with appropriate parameters are selected.The ideal frequency offset that the system can distinguish is 12.2 k Hz.The basic construction of the system is completed,and the correctness of the system structure is verified by means of node measurement.The structure of the resonant cavity is further optimized to increase the theoretical sensitivity by more than 10 times to fit the measurement system.The output of DAC is automatically controlled through FPGA,and then the dynamic debugging of the system is completed.The basic functions of signal receiving,transmitting and displaying in the measurement system are realized.