Design And Implementation Of Coating Thickness Detection Module Based On Sweep Frequency Eddy Current

Coating technology is a technology that gives specific properties to the surface of materials and their components,enhances the value of the material and extends the service life of the component.The thickness of the coating and the uniformity of the coating will have a direct impact on the life cycle of the coating,the bond strength,etc.It is therefore of paramount importance that the thickness of the coating is measured and evaluated.Conventional single-frequency eddy current techniques make it difficult to eliminate interfering factors such as the conductivity and shape of the substrate in coating thickness inspection.The swept frequency eddy current inspection technique is a method that collects enough eddy current data in a specific frequency band to detect conductive specimens and outperforms conventional eddy current inspection techniques in the analysis of multiple parameters.The aim of this paper is to develop a thickness measurement module for non-conductive coatings on conductive substrates based on swept eddy current inspection techniques.The advantages of the swept eddy current technique for multi-parameter analysis are used to eliminate the influence of the conductivity and thickness of the substrate material in the thickness detection process,thereby increasing the measurement accuracy and improving the environmental suitability.The main research elements of this paper are as follows:(1)The working principle and loop equivalent model of eddy current sensors are analysed and the principle of swept eddy current thickness detection based on impedance analysis is explored based on the conventional eddy current detection theoretical model.The basic circuit architecture of coil impedance analysis is discussed and the respective advantages and limitations are given to provide guidance for the design of subsequent hardware modules.(2)Finite element modelling analysis based on the COMSOL Multiphysics platform for the swept frequency eddy current coating thickness detection method.The influence of the substrate conductivity and thickness on the thickness detection of the coating layer is investigated.The feasibility of using the minimum impedance change of the imaginary part of the eddy current coil as the minimum amount of electromagnetic insensitivity to eliminate substrate interference is verified,and a substrate-independent master curve for coating thickness measurement is obtained.The effect of the sweep step on the measurement resolution and the ability to discriminate between substrates is investigated,and an idea for the selection of the sweep step is given.(3)The principle and defects of the impedance separation circuit architecture based on the self-balancing bridge method are analysed,and the three defects in the circuit architecture(circuit stability,influence of the detuning voltage and range sensitivity problems)are calculated,analysed,simulated and optimised,and the phase margin of the optimised circuit is changed from 6.68~°to 49.07~°.The design of the hardware module for sweeping eddy current coating thickness detection,including eddy current coil,power supply module,self-balancing bridge module,etc.,and the selection of key components.The noise analysis of the self-balancing bridge module is carried out,and the noise index of the self-balancing bridge module is verified to meet the thickness measurement accuracy requirements with the help of tina noise analysis function.(4)The circuit schematic drawing and PCB design prototype of the whole module were completed,and the corresponding experimental platform was built to test and analyse the functions of the designed module,and to compare and analyse the thickness measurement of standard plastic sheets on different metal substrates with foreign thickness gauges QNix7500.The maximum relative measurement error of the swept eddy current thickness measurement module designed in this project is 2.8%,which is lower than the 8.13%of foreign instruments and meets the design requirements,illustrating the advantages of the swept eddy current thickness measurement module in eliminating the influence of the substrate and reducing the thickness measurement error.