A coating thickness measurement method based on swept frequency eddy current

Abstract

The present invention discloses a coating thickness measurement method based on swept-frequency eddy current. First, a plurality of plastic sheets of different standard thicknesses are manufactured to replace the non-conductive coating layer. Then, the plastic sheets are sequentially placed on a metal substrate. Then, based on a swept-frequency eddy current thickness detection platform, sufficient eddy current data within a specific frequency band is collected. The eddy current data is fitted by MATLAB to obtain a relationship between the coating thickness and the minimum value of the change in the imaginary part of the impedance. Finally, when actually measuring the coating thickness, it is only necessary for the detection platform to measure the eddy current data of the test piece, thereby calculating the minimum value of the change in the imaginary part of the impedance, and converting the coating thickness into the relationship through the relationship.

Description

Technical Field

[0001] The present invention belongs to the technical field of eddy current non-destructive thickness measurement, and more specifically, relates to a coating thickness measurement method based on swept-frequency eddy current. Background Art

[0002] When the metal surface is corroded or worn, it will cause equipment damage, pipeline leakage, product contamination, and even lead to serious accidents such as combustion or explosion, as well as serious waste of resources and energy, causing huge losses to the national economy.

[0003] Coating technology is a technique that imparts specific properties to the surfaces of materials and their components, enhancing their usefulness and extending the lifespan of components. Uniform coating thickness is a prerequisite for materials to possess specific protective properties. Coating thickness and coating uniformity directly impact the coating's lifespan, bond strength, stress conditions, material consumption, and substrate temperature distribution. Therefore, testing and evaluating coating thickness is of paramount importance.

[0004] Conventional single-frequency eddy current technology struggles to eliminate interfering factors such as the substrate's conductivity and shape during coating thickness testing. Swept-frequency eddy current testing, a method for testing conductive specimens by collecting sufficient eddy current data within a specific frequency band, outperforms conventional eddy current testing in analyzing multiple parameters. This method, based on swept-frequency eddy current testing technology, aims to develop a thickness detection module for non-conductive coatings on conductive substrates. By leveraging the advantages of swept-frequency eddy current technology for multi-parameter analysis, it eliminates the influence of substrate material conductivity and thickness during thickness testing, thereby improving measurement accuracy and environmental adaptability. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for measuring the thickness of a coating layer based on a swept frequency eddy current to measure the thickness of coating layers on different substrates.

[0006] To achieve the above-mentioned object of the invention, the present invention provides a coating thickness measurement method based on swept frequency eddy current, which is characterized by comprising the following steps:

[0007] (1) Make a metal matrix; make different standard thicknesses d1, d2, ..., d n , n plastic sheets replace the non-conductive coating layer;

[0008] (2) Place plastic sheets of different standard thicknesses on the metal substrate in sequence, and then place the metal substrate on a horizontal table;

[0009] (3) Fit the eddy current coil at the front end of the eddy current probe vertically to the plastic sheet;

[0010] (4) Set the signal generator to sweep mode, set the sweep frequency band, and the sweep step size to λ;

[0011] (5) The signal generator scans the plastic sheet d on the substrate with a step length λ within the frequency band. i Scan and then record the imaginary part of the impedance of the eddy current coil at each frequency;

[0012] (6) Difference the imaginary part of impedance at two adjacent frequencies to obtain the change in imaginary part of impedance. Then find the minimum value of the change in imaginary part of impedance and record it as Δl i ;

[0013] (7) Replace the plastic sheet and repeat steps (5) and (6) to find the minimum value of the change in the imaginary part of the impedance corresponding to the plastic sheets of different standard thicknesses, which is recorded as Δl1, Δl2, ..., Δl i ,…,Δl n ;

[0014] (8) The thickness values ​​d1, d2, ..., d of plastic sheets with different standard thicknesses are n , and the corresponding minimum value of the change in the imaginary part of the impedance Δl1, Δl2, ..., Δl i ,…,Δl n Input the data into the fitting tool MATLAB, and use MATLAB to fit the fourth-order polynomial between the coating thickness D and the minimum value l of the imaginary part change of the impedance;

[0015] D=p1l 4 +p2l 3 +p3l 2 +p4l+p5

[0016] Among them, p1~p5 are fitting coefficients;

[0017] (9) Place the test piece with the coating layer on the substrate, and then follow steps (3) to (6) to find the minimum value Δl of the change in the imaginary part of the impedance;

[0018] (10) Let Δl = l and substitute it into the fitting formula in step (8) to calculate the coating thickness D of the test piece.

[0019] The object of the invention of the present invention is achieved like this:

[0020] The present invention is based on a coating thickness measurement method based on swept-frequency eddy current. First, multiple plastic sheets of different standard thicknesses are made to replace the non-conductive coating layer. Then, the plastic sheets are placed on the metal substrate in sequence. Then, based on a swept-frequency eddy current thickness detection platform, sufficient eddy current data within a specific frequency band is collected. The eddy current data is fitted by MATLAB to obtain a relationship between the coating thickness and the minimum value of the change in the imaginary part of the impedance. Finally, when actually measuring the coating thickness, only the detection platform needs to measure the eddy current data of the test piece, thereby calculating the minimum value of the change in the imaginary part of the impedance, and converting the coating thickness through the relationship.

[0021] At the same time, the coating thickness measurement method based on swept frequency eddy current of the present invention also has the following beneficial effects:

[0022] (1) By taking the minimum change of the imaginary impedance of the eddy current coil as the electromagnetic insensitivity of the coating thickness measurement, the influence of the conductivity and thickness of the substrate material during the thickness detection process is eliminated, and a main curve of the coating thickness measurement that is independent of the substrate is obtained, thereby improving the measurement accuracy and the environmental adaptability.

[0023] (2) Utilizing the advantages of multi-parameter analysis of swept frequency eddy current technology, compared with conventional single-frequency eddy current technology, it is a method to collect enough eddy current data within a specific frequency band to detect conductive specimens, which can eliminate interference factors such as the conductivity and shape of the substrate.

[0024] (3) The method of collecting enough eddy current data in a specific frequency band to detect the conductive specimen is adopted. The swept frequency signal has rich spectral information and its performance in analyzing multiple parameters is better than that of conventional eddy current detection technology.

[0025] (4) By obtaining the impedance information of the eddy current coil at different sweep frequencies, the parameters of the conductive test piece are analyzed to obtain the characteristic information of the test piece. Finally, the analysis found that the lowest point in the impedance change response curve can be used as an electromagnetic insensitive characteristic point to eliminate the interference of the substrate. Therefore, the present invention can measure the thickness of the coating layer on different substrates. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a flow chart of the coating thickness measurement method based on swept frequency eddy current of the present invention;

[0027] Figure 2 This is a schematic diagram of a platform for thickness testing based on frequency sweeping eddy current;

[0028] Figure 3 is a schematic diagram of a metal substrate and a plastic sheet;

[0029] Figure 4 is the coating thickness measurement curve. DETAILED DESCRIPTION

[0030] The following describes the specific embodiments of the present invention in conjunction with the accompanying drawings so that those skilled in the art can better understand the present invention. It should be noted that in the following description, when detailed descriptions of known functions and designs may dilute the main content of the present invention, such descriptions will be omitted here.

[0031] Example

[0032] Figure 1 It is a flow chart of the coating thickness measurement method based on swept frequency eddy current of the present invention.

[0033] In this example, we first describe the platform for testing thickness using a swept-frequency eddy current thickness test. As shown in Figure 2, the main instruments used in the test are a HIOKI IM3570 impedance analyzer, a signal generator, a DC power supply, and an oscilloscope. The DC power supply is set to 12V and connected to the swept-frequency eddy current thickness test hardware module. The signal generator is set to a swept-frequency signal with a frequency range of 10kHz to 510kHz, with a step size of 50kHz. The impedance analyzer is connected to an eddy current coil to measure the imaginary impedance.

[0034] Next, we will describe in detail a coating thickness measurement method based on swept frequency eddy current according to the present invention in conjunction with a test platform. Figure 1 As shown, the specific steps include:

[0035] S1. Making a metal substrate;

[0036] In this embodiment, the shape of the metal substrate is rectangular, and the material of the metal substrate can be a conductive metal with any conductivity, as shown in 3(a), such as aluminum, copper, 304 stainless steel and titanium alloy; in this embodiment, an aluminum substrate with a conductivity of 27MS / m is selected for the experiment.

[0037] like Figure 3 As shown in (b), six plastic sheets with different standard thicknesses are made to replace the non-conductive coating layer. The thicknesses of the plastic sheets are 250 μm, 798 μm, 1473 μm, 2002 μm, 2518 μm and 3032 μm respectively.

[0038] S2. Place plastic sheets of different standard thicknesses on the metal substrate in sequence, and then place the metal substrate on a horizontal table;

[0039] S3. Place the eddy current coil at the front end of the eddy current probe vertically on the plastic sheet, as shown in the following example. Figure 2 As shown;

[0040] In this embodiment, the larger the outer diameter of the eddy current coil, the larger the linear range of the eddy current probe. Considering the subsequent inner diameter design and the limitation of the inductance value, an eddy current coil with an outer diameter of 12mm is selected here. When designing the inner diameter, the ratio s of the inner and outer diameters will affect the Q value of the eddy current coil. Therefore, when manufacturing the eddy current coil, its Q value satisfies:

[0041]

[0042] Where f is the frequency of the current flowing through the coil, σ is the coil conductivity, μ0 is the vacuum magnetic permeability, φ is the outer diameter of the coil, and d is the coil diameter. w is the coil diameter, s is the ratio of the inner and outer diameters of the eddy current coil, and C(s) is the Q factor of the coil.

[0043] In order to obtain the largest possible Q value, s is guaranteed to be between 0 and 0.3, so the inner diameter is selected within the range of 3-5 mm.

[0044] In addition, in order to maximize the energy utilization of the eddy current probe and ensure that the eddy current probe works at a higher efficiency, the thickness of the eddy current coil should be designed to meet the ratio of thickness to outer diameter should be less than 0.1.

[0045] S4. Set the signal generator to sweep mode, and set the sweep frequency band and the sweep step size to λ;

[0046] S5. When the excitation signal generated by the excitation signal source is input to the eddy current coil, the signal generator scans the plastic sheet d on the substrate with a step size λ within the frequency band. i Scan and then record the imaginary part of the impedance of the eddy current coil at each frequency;

[0047] S6. Difference the imaginary impedance values ​​at two adjacent frequencies to obtain the change in the imaginary impedance. Then find the minimum value of the imaginary impedance change and record it as Δl. i ;

[0048] S7, replace the plastic sheet, repeat steps S5 and S6, find the minimum value of the change in the imaginary part of the impedance (i.e., the electromagnetic insensitivity) corresponding to the plastic sheets of different standard thicknesses, and record it as Δl1, Δl2, ..., Δl i ,…,Δl6, as shown in Table 1;

[0049] Table 1 shows the electromagnetic insensitivity corresponding to different coating thicknesses on the aluminum substrate;

[0050] Coating thickness 0 250 798 1473 2002 2518 3032 Electromagnetic insensitivity 196.21 209.94 231.78 253.79 259.78 263.48 266.56

[0051] Table 1

[0052] S8, the thickness values ​​d1, d2, ..., d6 of plastic sheets of different standard thicknesses and the corresponding minimum values ​​of the imaginary part changes of the impedance Δl1, Δl2, ..., Δl i , ..., Δl6 is input into the fitting tool MATLAB, and a fourth-order polynomial between the coating thickness D and the minimum value l of the imaginary part change of the impedance is fitted by MATLAB;

[0053] D=p1l 4 +p2l 3 +p3l 2 +p4l+p5

[0054] Among them, p1~p5 are fitting coefficients, and their values ​​are shown in Table 2;

[0055] Table 2 shows the fitting coefficient values;

[0056] Parameter name <![CDATA[p1]]> <![CDATA[p2]]> <![CDATA[p3]]> <![CDATA[p4]]> <![CDATA[p5]]> Parameter value 6.48e-4 -0.5795 193.7 -2.868e+4 1.587e+6

[0057] Table 2

[0058] In this embodiment, by fitting the data measured on plastic sheets of different standard thicknesses, a coating thickness measurement curve that is independent of the substrate is obtained, as shown in Figure 4. Based on this curve, the coating thickness can be obtained when the electromagnetic insensitivity is measured, thereby realizing the coating thickness detection without affecting the substrate.

[0059] S9, placing the test piece with the coating layer on the substrate, and then following steps (3) to (6), find the minimum value Δl of the change in the imaginary part of the impedance;

[0060] S10 , let Δl=l, and substitute into the fitting formula in step S8 to calculate the coating layer thickness D of the test piece.

[0061] Although the above describes the illustrative specific embodiments of the present invention to facilitate understanding of the present invention by those skilled in the art, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concepts of the present invention are protected.

Claims

1. A coating thickness measurement method based on swept frequency eddy current, characterized in that: The following steps are involved: (1) Make a metal matrix; make different standard thicknesses d1, d2, ..., d n , n plastic sheets replace the non-conductive coating layer; (2) Place plastic sheets of different standard thicknesses on the metal substrate in sequence, and then place the metal substrate on a horizontal table; (3) Fit the eddy current coil at the front end of the eddy current probe vertically to the plastic sheet; (4) Set the signal generator to sweep mode, set the sweep frequency band, and the sweep step size to λ; (5) When the excitation signal generated by the excitation signal source is input to the eddy current coil, the signal generator scans the plastic sheet d on the substrate with a step size λ within the frequency band. i Scan and then record the imaginary part of the impedance of the eddy current coil at each frequency; (6) Difference the imaginary part of impedance at two adjacent frequencies to obtain the change in imaginary part of impedance. Then find the minimum value of the change in imaginary part of impedance and record it as Δl i ; (7) Replace the plastic sheet and repeat steps (5) and (6) to find the minimum value of the change in the imaginary part of the impedance corresponding to the plastic sheets of different standard thicknesses, which is recorded as Δl1, Δl2, ..., Δl i ,…,Δl n ; (8) The thickness values ​​d1, d2, ..., d of plastic sheets with different standard thicknesses are n , and the corresponding minimum value of the change in the imaginary part of the impedance Δl1, Δl2, ..., Δl i ,…,Δl n Input the data into the fitting tool MATLAB, and use MATLAB to fit the fourth-order polynomial between the coating thickness D and the minimum value l of the imaginary part change of the impedance; <h2 style=";text-align:left;direction:ltr">D=p1l<h2 style=";text-align:left;direction:ltr"> 4 <h2 style=";text-align:left;direction:ltr"> +p2l<h2 style=";text-align:left;direction:ltr"> 3 <h2 style=";text-align:left;direction:ltr"> +p3l<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> +p4l+p5 Among them, p1~p5 are fitting coefficients; (9) Place the test piece with the coating layer on the substrate, and then follow steps (3) to (6) to find the minimum value Δl of the change in the imaginary part of the impedance; (10) Let Δl = l and substitute it into the fitting formula in step (8) to calculate the coating thickness D of the test piece.

2. The coating thickness measurement method based on swept frequency eddy current according to claim 1, characterized in that: The quality factor value of the eddy current coil satisfies: Where f is the frequency of the current flowing through the coil, σ is the coil conductivity, μ0 is the vacuum magnetic permeability, φ is the outer diameter of the coil, and d is the coil diameter. w is the coil diameter, s is the ratio of the inner and outer diameters of the eddy current coil, and C(s) is the Q factor of the coil.

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