Dual Modality Inspection Technique Based On Capacitive Coupling Effect Of Planar Coil

“Insulator-conductor” hybrid structures are widely used in industry in recent years.Due to the two distinct types of materials,the comprehensive Non-destructive Evaluation(NDE)of such hybrid is often achieved by complementary NDE techniques,which may cause efficiency and cost issues.This paper proposes a composite non-destructive testing technology based on the capacitive coupling effect of planar coils,which combines inductive and capacitive nondestructive evaluation technique.The spiral coil printed on the PCB is used as a dual-mode sensor.The technique uses the inductance and the stray capacitance of the coil as well as the cable capacitance in the detection system.The technique changes the contribution of capacitance and eddy current effects through frequency switching,offering a simple and efficient way to image and discriminate the possible defects in both layer of the hybrid,realizing the detection,location and identification of defects in hybrid structures.Theoretical analysis,Finite Element method simulations and experimental research are applied.A theoretical model of the dual-mode detection technology based on the capacitive coupling effect of the planar coil has been built,the equivalent circuit analysis of the detection system has been completed,the distribution law of the detection field of the dual-mode sensor with frequency and sample changes has been studied,an experimental system to implement the technique has been built,and its feasibility to detect the integrity of the hybrid structure has been verified.The parameters that affect the detection performance and the optimized design of the sensor have been studied.The thesis mainly carries out the research from the following four aspects:(1)Theoretical study of capacitance-eddy current dual-mode detection based on the capacitive coupling effect of planar coilsThe technical basis of capacitance detection and eddy current detection alone is introduced;the theoretical principle of combining capacitance detection and eddy current detection based on the capacitive coupling effect of the planar coil is introduced;the source of the capacitive effect of the planar coil is analyzed;The equivalent circuit of the mode detection system is analyzed;the complex impedance formula of the dual mode detection system in different application places is derived.The principle that the measured object parameters change the total impedance of the dual-mode detection system is analyzed.(2)Simulation analysis of the capacitive-eddy current dual-mode detection model based on the capacitive coupling effect of the planar coilTaking the single-coil capacitance-eddy current dual-mode sensor as the research object,using the finite element software COMSOL Multiphysics,a dual-mode sensor simulation model is established,and the detection field distribution of the electric and magnetic field of the singlecoil dual-mode sensor is explored.The influence of frequency changes,different types of test blocks and the existence of defects on the detection field are studied.(3)Dual-mode detection system development and technical feasibility experimental researchA set of capacitance-eddy current dual-mode detection system based on the capacitive coupling effect of planar coils has been developed and built.The single-coil dual-mode sensor probe is used to test three types of typical test blocks of “glass fiber reinforced plastic”,“glass reinforced plastic-metal hybrid structure”,and "carbon fiber-metal" hybrid structure.The surface and hidden defects in the test block were detected,and the frequency response characteristics of the complex impedance parameters were obtained when the dual-mode sensor probe was located above the test block.According to the impedance response curve change rule under the multi-frequency detection of the detection system,the defect was detected and identified.(4)Research on the parameters affecting the detection performance of the dual-mode detection sensorBased on the dual-mode sensor design principle of the planar coil capacitive coupling effect,three different dual-mode sensors are designed: single-coil dual-mode sensor,dual-coil dual-mode sensor,wire-loop composite dual-mode sensor.The number of turns,coil spacing,inter-turn spacing,excitation frequency and other parameters of each sensor were changed,and the typical test block was tested with a dual-mode test system,the test results were compared,and the parameters affecting the test performance were su mmarized.This provides ideas for future probe optimization design.