A method for detecting the thickness of a coating based on the magnetic induction characteristics of low-frequency magnetic fields

Abstract

The present invention discloses a method for detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field. First, a low-frequency magnetic induction thickness measuring device is constructed, and a low-frequency dynamic magnetic field is used as the excitation source. On this basis, a thickness curve of the standard substrate is established through data dimensionality reduction and multi-cycle averaging based on the standard substrate, and then a calibration relationship with the standard substrate is established based on the substrate to be measured without a coating. Finally, the obtained characteristic quantity is calibrated through the calibration relationship to obtain a calibrated characteristic quantity, and the thickness of the coating on the substrate to be measured is obtained by passing it through the thickness curve. The present invention uses a low-frequency dynamic magnetic field as the excitation source, reduces the correlation between magnetic induction thickness measurement and the measurement substrate, eliminates the influence of substrate hysteresis on the measurement result of the coating thickness. At the same time, the multi-cycle hysteresis loops of the collected data are dimensionally reduced and averaged, which more accurately reflects the thickness of the coating compared with the existing single-point magnetic core signal. Therefore, the present invention improves the measurement accuracy of the magnetic induction thickness measurement method.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnetic induction thickness measurement, and more specifically, relates to a method for detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field. Background Art

[0002] In current industrial manufacturing and inspection, the thickness measurement of coatings on a metal substrate is roughly divided into thickness measurement methods such as magnetic induction thickness measurement, ultrasonic thickness measurement, and optical thickness measurement. Among them, the magnetic induction thickness measurement method is widely used in the measurement scenario of measuring the thickness of non-ferromagnetic coatings on ferromagnetic substrates due to its simple and non-destructive measurement method, fast detection speed, and high detection accuracy.

[0003] The detection principle of magnetic induction thickness measurement is that when a magnetic field approaches a ferromagnetic substrate, the substrate will be magnetized, generating an induced magnetic field, which in turn affects the original magnetic field, causing a change in the overall magnetic field strength at a certain point. By analyzing the change in the magnetic field at this point, the thickness of the measured coating can be obtained.

[0004] However, the existing magnetic induction thickness measurement uses a static magnetic field for measurement, and the influence of substrate hysteresis on the final measurement result is relatively large, resulting in low measurement accuracy. 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 detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field, which is applicable to the measurement scenario of measuring the thickness of non-ferromagnetic coatings on ferromagnetic substrates. By using a low-frequency dynamic magnetic field as the excitation source, the correlation between magnetic induction thickness measurement and the measurement substrate is reduced, the influence of substrate hysteresis on the measurement result of the coating thickness is eliminated, and the measurement accuracy of the magnetic induction thickness measurement method is improved.

[0006] To achieve the above-mentioned invention purpose, the present invention provides a method for detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field. This measurement method is applicable to the thickness measurement of non-magnetic coatings on ferromagnetic substrates, and is characterized by including the following steps:

[0007] (1) Establish a low-frequency magnetic induction thickness measurement device

[0008] The low-frequency magnetic induction thickness measurement device includes a soft ferromagnetic material core, an excitation coil, and a magnetic field measurement sensor;

[0009] The excitation magnetic field is emitted by an excitation coil uniformly and densely wound around a cylindrical soft ferromagnetic material core. The radius of the soft ferromagnetic material core is r, and the wire diameter of the wound excitation coil is The current passing through the excitation coil is a sinusoidal current I(t) that varies with time. A magnetic field measurement sensor is placed closely on top of the excitation coil to measure the magnitude of the magnetic field. The measurement probe consists of a soft ferromagnetic material core, an excitation coil, and a magnetic field sensor. The magnetic field measurement sensor is located at the top of the measurement probe, and the top of the measurement probe faces downwards.

[0010] (2) Establish a thickness curve based on the standard substrate

[0011] 2.1) Place the standard substrate below the top of the measurement probe and closely fit it, and simulate the coating thickness h0 = 0 during the 0th measurement.

[0012] 2.2) For the ith measurement, pass a low-frequency alternating excitation signal of n consecutive cycles into the excitation coil. At the same time, collect the magnetic field intensity induced by the magnetic field measurement sensor to obtain magnetic signals of n cycles.

[0013] 2.3) Remove the magnetic signals of the first cycle and the last cycle to obtain magnetic signals of n - 2 cycles.

[0014] 2.4) The magnetic signals of n - 2 cycles are dimension-reduced with one cycle as a unit to obtain n - 2 dimension-reduced characteristic quantities k i1 , k i2 , k i3 , ……, k i(n-2) ;

[0015] 2.5) The n - 2 dimension-reduced characteristic quantities k i1 , k i2 , k i3 , ……, k i(n-2) are averaged to obtain the average characteristic quantity k i ;

[0016] 2.6) Record the average characteristic quantity k i at this time and the simulated coating thickness h i ;

[0017] 2.7) Let i = i + 1 and perform the next measurement. Move the measurement probe vertically to increase the distance between the measurement probe and the standard substrate, thereby changing the simulated coating thickness h i , and then return to step 2.2). Repeat this m times to obtain two corresponding data sets with m elements {[k0, k1, k2, …… k m , [h0, h1, h2, …… h m};

[0018] 2.8) The data set {[k0, k1, k2, …… k m , [h0, h1, h2, …… hm} With the thickness size h i As the dependent variable h and the characteristic quantity size k i As the independent variable k, through the least squares method, data fitting is performed between the two to obtain the fitting result, that is, the thickness curve h = f(k).

[0019] (3) Construct the calibration relationship from the substrate to be measured to the standard substrate

[0020] 3.1) Place the substrate to be measured without a coating layer under the top of the measurement probe, closely fit it with the substrate to be measured, and simulate the coating layer thickness h r0 = 0 during the 0th measurement. Then, in the manner of steps 2.2)-2.5), obtain the average characteristic quantity k r0 ;

[0021] 3.2) Move the measurement probe to infinity, that is, an environment where the probe is not affected by any magnetic induction, and simulate the coating layer thickness h r∞ of an infinitely thick coating. Then, in the manner of steps 2.2)-2.5), obtain the corresponding average characteristic quantity k r∞ ;

[0022] 3.3) Calculate from the thickness curve h = f(k) the characteristic quantity k0 when the coating layer thickness h is 0 and the characteristic quantity k at infinity ∞ ;

[0023] 3.4) Substitute (k0, k r0 ), (k ∞ , k r∞ ) into the linear model. Among them, k r0 , k r∞ are used as the characteristic quantity k x to be calibrated, and k0, k ∞ are used as the calibrated characteristic quantities to obtain the calibration relationship k s = g(k x ) for use during measurement;

[0024] (4) Measure the coating layer thickness of the substrate to be measured

[0025] 4.1) Place the substrate to be measured with a coating layer under the top of the measurement probe, closely fit it with the coating layer of the substrate to be measured, and then, in the manner of steps 2.2)-2.5), obtain the average characteristic quantity and use this average characteristic quantity as the characteristic quantity k x to be calibrated and substitute it into the calibration relationship k s = g(k x ) to obtain the calibrated characteristic quantity k s ;

[0026] 4.2) Use the obtained calibrated characteristic quantity ks Substitute the characteristic quantity k into the thickness curve h = f(k) to obtain the thickness h of the coating layer on the substrate to be measured x of the size, that is, h x = f(k s ).

[0027] The object of the present invention is achieved as follows.

[0028] The coating layer thickness detection method based on the magnetic induction characteristic of low-frequency magnetic field of the present invention first constructs a low-frequency magnetic induction thickness measurement device, which is used to use a low-frequency alternating current excitation signal to be introduced into the excitation coil and use the low-frequency dynamic magnetic field as the excitation source, and then use a magnetic field measurement sensor to collect the induced magnetic field intensity. On this basis, a thickness curve is established for the standard substrate through data dimensionality reduction and multi-period averaging, and then a calibration relationship with the standard substrate is established based on the substrate to be measured without the coating layer. Finally, the substrate to be measured with the coating layer is placed below the top of the measurement probe and closely attached to the coating layer of the substrate to be measured. The obtained characteristic quantity is calibrated through the calibration relationship to obtain the calibrated characteristic quantity, and the thickness of the coating layer on the substrate to be measured is obtained by introducing it into the thickness curve. The present invention uses the low-frequency dynamic magnetic field as the excitation source, reduces the correlation between magnetic induction thickness measurement and the measurement substrate, eliminates the influence of substrate hysteresis on the measurement result of the coating layer thickness. At the same time, the collected data of multiple-period hysteresis loops are dimensionally reduced and averaged, which more accurately reflects the thickness of the coating layer compared with the existing single-point magnetic core signal. Therefore, the present invention improves the measurement accuracy of the magnetic induction thickness measurement method. Brief Description of the Drawings

[0029] Figure 1 is a flow chart of the coating layer thickness detection method based on the magnetic induction characteristic of low-frequency magnetic field of the present invention;

[0030] Figure 2 is a schematic structural diagram of a specific embodiment of the low-frequency magnetic induction thickness measurement device in the present invention;

[0031] Figure 3 is a schematic diagram for calculating the magnetic flux density on the axis of a single ring;

[0032] Figure 4 is a schematic diagram for calculating the magnetic flux density on the axis of an N-turn coil;

[0033] Figure 5 is a schematic diagram for calculating a uniformly magnetized ferromagnetic disk;

[0034] Figure 6 is a graph of the coating layer thickness of different substrates to be measured;

[0035] Figure 7 is a graph of the absolute error of the coating layer thickness of different substrates to be measured;

[0036] Figure 8 It is the absolute error curve graph of the coating layer thickness of different substrates to be measured. Specific embodiments

[0037] The following describes the specific embodiments of the present invention with reference to the accompanying drawings, so that those skilled in the art can better understand the present invention. It should be particularly noted that in the following description, when the detailed description of known functions and designs may dilute the main content of the present invention, these descriptions will be omitted here.

[0038] Figure 1 It is the flow chart of the coating layer thickness detection method based on the magnetic induction characteristics of low-frequency magnetic fields of the present invention.

[0039] In this embodiment, as Figure 1 shown, the coating layer thickness detection method based on the magnetic induction characteristics of low-frequency magnetic fields of the present invention is applicable to the thickness measurement of non-magnetic coating layers on ferromagnetic substrates, and includes the following steps:

[0040] Step S1: Establish a low-frequency magnetic induction thickness measurement device

[0041] In this embodiment, as Figure 2 shown, the low-frequency magnetic induction thickness measurement device includes a soft ferromagnetic material core 1, an excitation coil 2, and a magnetic field measurement sensor 3, and the three form a measurement probe 5.

[0042] The excitation magnetic field is emitted by the excitation coil 2 uniformly and densely wound on the cylindrical soft ferromagnetic material core 1. The radius of the soft ferromagnetic material core 1 is r, and the wire diameter of the wound excitation coil 2 is The current passing through the excitation coil 2 is a sinusoidal current I(t) that changes with time. The magnetic field measurement sensor 3 is closely attached to the top of the excitation coil 2 to measure the magnetic field magnitude. The soft ferromagnetic material core 1, the excitation coil 2, and the magnetic field sensor 3 form a measurement probe 5. The magnetic field measurement sensor 3 is located at the top of the measurement probe 5, and the top of the measurement probe 5 faces downwards. Below the top of the measurement probe 5 is the measurement substrate 4, and the measurement substrate 4 can be a standard substrate, a substrate to be measured without a coating layer, or a substrate to be measured with a coating layer.

[0043] In this embodiment, the frequency of the sinusoidal current I(t) is 10 Hz, and the amplitude is 5 V.

[0044] Figure 2 In, air is used to replace the coating layer thickness, the thickness is d2, d1 is the packaging thickness of the sensor, and d is the thickness between the excitation coil and the measurement substrate.

[0045] Next, the measurement principle is analyzed:

[0046] As Figure 3As shown, for a single circular ring carrying a current of intensity I, the magnetic flux density at any point p on its axis is:

[0047]

[0048] The final result is:

[0049]

[0050] As Figure 4 shown, for an N-turn coil, since it is wound uniformly and densely, and the wire diameter is very small, the current passing through the coil can be regarded as a cylindrical thin current sheet with a surface current density of j s The specific expression is as follows: s The specific expression is as follows:

[0051]

[0052] The total magnetic field B at any point P can be obtained by integrating dB, that is:

[0053]

[0054] If we let Then the above formula can be simplified to

[0055]

[0056] Finally, the expression for the magnetic field strength at any point on the axis of the excitation coil in the experimental model is

[0057]

[0058] where sinθ1 and sinθ2 are the ratios of the radius a to the lines connecting point P with points z1 and z2, respectively.

[0059] Combined with the experimental model, we can see that the magnetic field strength B measured by the magnetic sensor 传 is composed of the superposition of two magnetic fields, namely the magnetic field B 线 propagated to the magnetic sensor by the primary magnetic field generated by the coil passing through a sinusoidal slow time-varying current I(t) and the magnetic field B 基 of the secondary induced magnetic field generated by the magnetic induction effect of the substrate excited by the coil magnetic field and propagated to the magnetic sensor, that is

[0060] B 传 = B 线 + B 基

[0061] where because during the test, factors such as the current I(t) passing through the coil, the distance d1 between the effective measurement area of the sensor and the coil, and the number of turns of the coil remain unchanged, so B 线During measurement, they are all thickness-independent quantities without thickness information. Since the excitation current I(t) used in this test method has a frequency (10 Hz) that is very low compared to the magnetic field change rate, this test method can be considered a quasi-static process, and any of the above magnetic field calculation formulas for a point can be used. Therefore, B 线 can be considered a known thickness-independent quantity.

[0062] Now, analyze B that includes thickness information 基 as shown Figure 5 , this magnetic field is the magnitude of the magnetic field at a distance d2 from the substrate due to the induced magnetic field generated after the substrate is magnetized. Simplify this model to a ferromagnetic material disk with a radius of a 基 and a thickness of d in air (d << a 基 ), and the entire disk is uniformly magnetized, with the magnetization vector perpendicular to the disk bottom and a magnitude of M. Determine the magnetic flux density vector at any point on the axis perpendicular to the bottom of the disk.

[0063] Since the thickness d of the ferromagnetic disk << radius a 基 , the magnetized current layer on its surface can be equivalently replaced by a current loop with a radius of a 基 , that is

[0064] I m = j ms d = Md

[0065] where j ms is the magnetized current density.

[0066] Assume the current loop is in a vacuum, then the magnetic flux density B at point P on the axis at a distance z from the disk surface can be obtained from the magnetic flux density on the axis of the above single ring

[0067]

[0068] For a linear permeable material, the magnetization vector M can be written as

[0069]

[0070] χ m is a dimensionless constant, which is the magnetic susceptibility of the material and is usually determined by experiments. N v is the atomic concentration of the material, e is the single electron charge, L av is the average moment of inertia of all electrons in the atom, and m e is the electron mass.

[0071] Also, because

[0072]

[0073] B and H can be further expressed as

[0074]

[0075] Furthermore, obtain

[0076] B = μ0(H + M) = μ0(1 + χ m )H = μ0μ r H = μH

[0077] where μ r is the relative magnetic permeability.

[0078] For ferromagnetic materials, they usually exhibit high nonlinearity, permanent magnetization, and hysteresis phenomena, resulting in their μ r not being uniquely determined, being related to the previous magnetization history of the material and the applied magnetic field strength H. Further, it leads to the B(H) function generally being nonlinear and having multiple branches. Therefore, a set of B(H) functions cannot be determined through mathematical derivation, but the B(H) magnetization curve of a certain material can be measured experimentally. Thus, on the premise of knowing the magnetization curve of a certain material, the magnetic field strength at any point on the axis of the secondary magnetization induction magnetic field generated by the base under the excitation of the primary magnetic field in the experimental model can be obtained through

[0079]

[0080] obtained as:

[0081]

[0082] Substituting the above formula into the sensor magnetic field expression gives

[0083]

[0084] In the present invention, the magnitude of B at the sensor 传 can be obtained by the sensor, so that the thickness information can be inversely calculated and derived through the above formula, and finally the thickness measurement is completed.

[0085] Step S2: Establish a thickness curve based on a standard base

[0086] Step S2.1: Place the standard base under the top of the measurement probe and closely fit it with the standard base, and simulate that the coating thickness h0 = 0 during the 0th measurement.

[0087] Step S2.2: For the i-th measurement, pass continuous n-period low-frequency alternating current excitation signals into the excitation coil. At the same time, collect the magnetic field strength induced by the magnetic field measurement sensor to obtain n-period magnetic signals. In this embodiment, n = 10.

[0088] Step S2.3: Remove the magnetic signals of the first cycle and the last cycle to obtain n - 2, that is, 8 cycles of magnetic signals;

[0089] Step S2.4: Perform data dimensionality reduction on the magnetic signals of n - 2 cycles with one cycle as a unit to obtain n - 2 dimensionality-reduced feature quantities k i1 ,k i2 ,k i3 ,……,k i(n-2) ;

[0090] Step S2.5: Average the n - 2 dimensionality-reduced feature quantities k i1 ,k i2 ,k i3 ,……,k i(n-2) to obtain the average feature quantity k i ;

[0091] Step S2.6: Record the average feature quantity k i at this time and the simulated coating thickness h i ;

[0092] Step S2.7: Let i = i + 1, perform the next measurement, move the measurement probe vertically, increase the distance between the measurement probe and the standard substrate, thereby changing the simulated coating thickness h i , and then return to step 2.2), repeat this m times to obtain two datasets with m corresponding elements {[k0, k1, k2, …… k m , [h0, h1, h2, …… h m};

[0093] Step S2.8: Use the thickness size h m in the dataset {[k0, k1, k2, …… k m , [h0, h1, h2, …… h i} as the dependent variable h, and the feature quantity size k i as the independent variable k, and perform data fitting between the two by the least squares method to obtain the fitting result, that is, the thickness curve h = f(k),

[0094] Step S3: Construct the calibration relationship from the substrate to be measured to the standard substrate

[0095] Step S3.1: Place the substrate to be measured without coating under the top of the measurement probe and closely fit it with the substrate to be measured, and simulate the coating thickness h r0 = 0 at the 0th measurement, and then obtain the average feature quantity k r0 ;

[0096] Step S3.2: Move the measuring probe to infinity, i.e., an environment where the probe is not affected by any magnetic induction, and simulate the thickness h of the coating with an infinite thickness r∞ , and then obtain the corresponding average characteristic quantity k in the manner of steps 2.2)-2.5) r∞ ;

[0097] Step S3.3: Calculate from the thickness curve h = f(k) the characteristic quantity k0 when the thickness h of the coating is 0 and the characteristic quantity k at infinity ∞ ;

[0098] Step S3.4: Substitute (k0, k r0 ), (k ∞ , k r∞ ) into the linear model. Among them, k r0 , k r∞ are used as the characteristic quantity k to be calibrated x , and k0, k ∞ are used as the calibrated characteristic quantities, and the calibration relationship k s = g(k x ) is obtained for use during measurement;

[0099] Step S4: Measure the thickness of the coating on the substrate to be measured

[0100] Step S4.1: Place the substrate to be measured with the coating under the top of the measuring probe and closely fit it with the coating on the substrate to be measured. Then, obtain the average characteristic quantity in the manner of steps 2.2)-2.5), and use this average characteristic quantity as the characteristic quantity k to be calibrated x Substitute it into the calibration relationship k s = g(k x ) to obtain the calibrated characteristic quantity k s ;

[0101] Step S4.2: Use the obtained calibrated characteristic quantity k s as the characteristic quantity k and substitute it into the thickness curve h = f(k) to obtain the thickness h of the coating on the substrate to be measured x , that is, h x = f(k s ).

[0102] Measurement data:

[0103] The thickness measurement results on four different substrates to be measured, DT4, DT4A, DT4C, and DT4E, are shown in Table 1:

[0104] Unit: μm

[0105]

[0106]

[0107]

[0108] Table 1

[0109] The coating thickness curves of four different substrates to be measured are as follows Figure 6 shown. It can be seen from Figure 6 that the measured thickness curve is basically consistent with the actual thickness curve.

[0110] The absolute error and relative error of the coating thickness of four different substrates to be measured are as follows Figure 7 and 8 shown. It can be seen from Figure 7 and 8 that when the thickness is very thin, the relative error is relatively large, and when it reaches two levels of 100 μm, it is within 0.5%, indicating that the present invention has high measurement accuracy.

[0111] Although the above-described illustrative specific embodiments of the present invention have been described to facilitate the 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 defined and determined by the appended claims, these changes are obvious, and all inventions made using the concept of the present invention are within the scope of protection.

Claims

1. A method for detecting the thickness of a coating based on the magnetic induction characteristics of low-frequency magnetic fields, which is applicable to measuring the thickness of non-magnetic coatings on ferromagnetic substrates, characterized in that, It includes the following steps: (1) Establish a low-frequency magnetic induction thickness measuring device The low-frequency magnetic induction thickness measuring device includes a soft ferromagnetic material core, an excitation coil, and a magnetic field measurement sensor; The excitation magnetic field is emitted by an excitation coil evenly and densely wound around a cylindrical soft ferromagnetic material core with a radius of r. The wire diameter of the wound excitation coil is The current passing through the excitation coil is a sinusoidal current I(t) that changes with time. A magnetic field measurement sensor is closely attached to the top of the excitation coil to measure the magnetic field strength. The soft ferromagnetic material core, the excitation coil, and the magnetic field sensor form a measurement probe. The magnetic field measurement sensor is located at the top of the measurement probe, and the top of the measurement probe faces downward; (2) Establish a thickness curve based on a standard substrate 2.1) Place the standard substrate below the top of the measurement probe and closely fit it to the standard substrate, and simulate that the thickness h0 of the coating layer during the 0th measurement is 0; 2.2) For the ith measurement, pass a low-frequency alternating current excitation signal of n consecutive cycles into the excitation coil. At the same time, collect the magnetic field intensity induced by the magnetic field measurement sensor to obtain magnetic signals of n cycles; 2.3) Remove the magnetic signals of the first cycle and the last cycle to obtain magnetic signals of n - 2 cycles; 2.4), the magnetic signals of n - 2 cycles are reduced in dimension with one cycle as a unit to obtain n - 2 dimensionality-reduced characteristic quantities k i1 , k i2 , k i3 ,......, k i(n-2) ; 2.5), n - 2 dimensionality-reduced feature quantities k i1 , k i2 , k i3 ,......, k i(n-2) are averaged to obtain the average feature quantity k i ; 2.6), record the average feature quantity k at this time i and the simulated coating thickness h i ; 2.7), i = i + 1, conduct the next measurement, move the measurement probe in the vertical direction, increase the distance between the measurement probe and the standard substrate, thereby changing the thickness h of the simulated coating i , then return to step 2.2), repeat this m times, and obtain two datasets with m corresponding elements {[k0, k1, k2,......k m [h0, h1, h2,......h m}; 2.8), taking the data set {[k0, k1, k2,......k m , [h0, h1, h2,......h m} with the thickness size h i as the dependent variable h and the feature quantity size k i as the independent variable k, perform data fitting between the two by the least squares method to obtain the fitting result, i.e., the thickness curve h = f(k); (3) Construct a calibration relationship from the substrate to be measured to the standard substrate 3.1), Place the substrate to be measured without a coating layer below the top of the measurement probe, and closely fit it to the substrate to be measured, and simulate the coating layer thickness h when measuring for the 0th time r0 = 0, and then obtain the average characteristic quantity k in the manner of steps 2.2)-2.5) r0 ; 3.2), Move the measurement probe to infinity, i.e., an environment where the probe is not affected by any magnetic induction, and simulate the thickness h of the coating with an infinite thickness r∞ , and then obtain the corresponding average characteristic quantity k in the manner of steps 2.2)-2.5) r∞ ; 3.3), calculate the characteristic quantity k0 where the coating thickness h is 0 and the characteristic quantity k at infinity from the thickness curve h = f(k) ∞ ; 3.4), substitute (k0, k r0 ), (k ∞ , k r∞ ) into the linear model, where, k r0 and k r∞ as the characteristic quantity k to be calibrated x , k0 and k ∞ as the calibrated characteristic quantity, the calibration relationship k s = g(k x ) for use during measurement; (4) Measure the thickness of the coating layer of the substrate to be measured 4.1), Place the substrate to be measured with a coating layer below the top of the measurement probe and closely fit it to the coating layer of the substrate to be measured. Then, in the manner of steps 2.2)-2.5), obtain the average characteristic quantity and use this average characteristic quantity as the characteristic quantity k to be calibrated x Substitute it into the calibration relationship k s = g(k x ), and obtain the calibrated characteristic quantity k s ; 4.2), bring the obtained calibrated characteristic quantity k s as the characteristic quantity k into the thickness curve h = f(k), and obtain the thickness h of the coating on the substrate to be measured x of the size, that is, h x = f(k s ).

2. The method for detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field according to claim 1, wherein The frequency of the sine current I(t) is 10 Hz and the amplitude is 5 V.

3. The method for detecting the thickness of a coating based on the magnetic induction characteristics of a low-frequency magnetic field according to claim 1, wherein The magnetic signals of the n cycles are magnetic signals of 10 cycles.

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