Research On Strain Sensor Of Concrete Structure Based On CPW Theory

As the main material of most building structures,concrete is often suddenly damaged or even collapsed due to overload,aging,corrosion,natural disasters and other factors during construction or later use,and the impact is catastrophic.In order to prevent accidents,the importance of strain monitoring in important parts of concrete structures is self-evident.So far,there have been a variety of structural strain monitoring methods for evaluating the damage status of concrete structures,such as electrical measurement,optical measurement and piezoelectric method.Although these structural damage monitoring methods have been widely used,there are still some problems such as complex wiring,high price,poor anti-interference ability,poor measurement stability and material properties are not easy to be conformal with concrete structures.Compared with the traditional strain monitoring method,the antenna sensor has the advantages of high sensitivity,low cost,both transceiver signal function,convenient measurement,flexible size design and easy conformal with the structural surface.In recent years,it has been gradually applied to the field of real-time strain monitoring.This paper is supported by the joint fund project(U2006223).Based on the theory of coplanar waveguide(CPW)transmission line,the design goal is simple structure,high sensitivity and strain monitoring in different directions.Combined with a large number of finite element simulation analysis and experimental measurement work,two CPW strain sensors applied to concrete structures are made,which belong to the interdisciplinary work of civil engineering structure strain monitoring and electromagnetic field.(1)According to the transmission line theory,combined with the equivalent circuit diagram of the planar transmission line,the distribution law of voltage and current is given.The linear relationship between the size change of CPW transmission line and the change of each parameter is explained theoretically,which lays a theoretical foundation for the strain sensing of subsequent sensors.(2)According to the relationship between the dielectric constant of the material and the size of the CPW sensor,the suitable substrate is selected according to the actual application environment of the sensor.The size of the CPW sensor is preliminarily determined according to the calculation formula of the size of the CPW transmission line.The preliminary model of the CPW sensor is simulated and analyzed in the CST electromagnetic simulation design software,and the impedance matching is optimized by adjusting the size of the sensor.In the simulation environment,the length of the sensor is manually adjusted to simulate the strain of the structure,and the sensitivity of the CPW sensor is optimized.The CPW strain sensor with three-dimensional feed mechanism is designed,and it is verified that the bending of the sensor has little effect on S-parameters and impedance matching.Through a large number of sensitivity calculation and analysis experience combined with the derivation of sensitivity calculation formula,the relationship between the sensitivity of CPW sensor and the initial value of parameters is obtained,which simplifies the steps of improving the sensitivity of sensor.(3)The characteristic mode analysis(CMA)antenna design method is applied to the sensor design,and a CPW elliptical patch strain sensor that can identify strains in different directions is designed.The Modal Significance(MS)and Characteristic Angle(CA)parameters of the CPW elliptical patch sensor are optimized.The ratio of the long and short axes of the elliptical patch is determined and the variation of MS parameters with the strain of the sensor is simulated.The characteristic impedance of the CPW elliptical patch sensor is optimized by full-wave simulation and the variation of parameters in each direction is summarized.The sensitivity of ten different strain directions is calculated,and it is found that the strain sensitivity of S11 extreme value monitoring is almost sinusoidal with the strain direction.(4)The appropriate PVA fiber content ratio was selected to make the fiber reinforced cement mortar matrix tested by CPW sensor experiment.The experimental platform was first built to test the strain transfer ratio between the sensor and the cement mortar matrix,and then the actual working conditions of the CPW unidirectional strain sensor and the CPW elliptical patch strain sensor were verified by experiments.The experimental results are compared with the simulation results to verify the feasibility of the two sensors in the field of concrete structure health monitoring,which makes it possible to use the strain monitoring sensor designed based on CPW theory for strain monitoring of civil engineering structures.