Measurement Of Coating Thickness Using Lift-off Point Of Intersection Features From Pulsed Eddy Current Signals

Metal coating is widely used in the surface of mechanical parts because of its advantages of corrosion protection,abrasion protection and oxidation resistance.The thickness of the coating is one of the important indexes to characterize the quality of the coating,so the measurement of the thickness of the coating is very important.Due to its advantages of low cost,no contact,no harm to human body and high detection efficiency,eddy current detection method has become a popular method for the detection of coating thickness in the industry.The traditional eddy current detection method is easily affected by the lift-off noise of the probe,which makes the eddy current detection easy to produce errors.In order to reduce the measurement error,a method for detecting the coating thickness of lift-off point of intersection(LOI)based on Pulsed Eddy Current(PEC)is proposed in this paper,which is independent of the lift-off of the probe.The main research contents are as follows.An analytical model of pulse eddy current detection of plating thickness was established.The analytical solution of the pulsed eddy current field generated by a placed coil in a two-layer conducting structure is studied by using the Fourier series method.In this paper,the harmonic eddy current field excited by a single frequency is studied,and the analytical solution of the harmonic eddy current field of a two-layer conductive structure and the analytical expression of the induced voltage of the detection coil are obtained by using Cheng matrix.On this basis,Fourier decomposition is used to decompose the pulse excitation into the superposition of several different frequency harmonics,and the superposition principle is used to solve the pulse eddy current detection induced voltage signal of the two-layer conductive structure.Finally,the finite element model is used to verify the analytical model.Then,a method to measure the thickness of non-ferromagnetic coating using LOI coordinates is proposed,and the adjustment strategy of detection accuracy is studied.Firstly,the LOI point characteristics of non-ferromagnetic coating thickness were simulated.The results show that the LOI time has better linearity than the LOI amplitude.On this basis,the conductivity of non-ferromagnetic coating and the rise time of pulse excitation were evaluated quantitatively.The simulation and experimental results show that the rising time of pulse excitation can be used to balance the sensitivity and linearity of coating thickness measurement.By adjusting the rise time of pulse excitation,the measurement error can be greatly reduced,so that the maximum error is less than 4.9%.For ferromagnetic conductive coating,the LOI cannot be directly extracted because the original detection signals do not intersect when the plating is different from each other.In this paper,the extraction method of ferromagnetic coating LOI based on quadratic difference was studied.The amplitude of each sampling point of the original detection signal was subtracted from the standard deviation of the amplitude of all sampling points of the detection signal,and the processed detection signal was obtained.Then,the intersection of the processed detection signals was used to obtain LOI.Based on the LOI of ferromagnetic coating,a method for measuring the thickness of ferromagnetic conductive coating was proposed by selecting LOI time as the characteristic value.The influence of three key factors on the accuracy and sensitivity of the measurement method,including the permeability of ferromagnetic coating,the conductivity and the rise time of pulse excitation,was studied by simulation,and then the adjustment strategy of the measurement accuracy was studied.The method proposed in this paper has the advantages of simple operation,good robustness,elimination of lift-off effect and adjustable precision.The work of this paper provides a theoretical basis for measuring the thickness of metal conductive coating based on pulsed eddy current.There are 82 figures,12 tables and 79 references in this paper.