Method for synchronously measuring thickness and conductivity of non-magnetic material based on eddy current phase characteristics and application thereof

Abstract

The invention belongs to the technical field of non-magnetic material nondestructive testing, and discloses a non-ferromagnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics, and the method comprises the specific steps: S1, estimating the conductivity and thickness range of a to-be-tested piece based on data such as a product specification or an industrial standard; s2, determining excitation frequency according to a frequency selection criterion (high frequency enables the skin effect depth to be smaller than 1 / 3 of the thickness, and low frequency enables the skin effect depth to be larger than two times of the thickness of the sample); s3, an eddy current method is adopted to obtain the impedance variable quantity phase of the to-be-measured non-ferromagnetic material; and S4, substituting the phases into corresponding simplified model formulas respectively, and obtaining the conductivity and the thickness at the same time. Aiming at the problems of multiple calibration, blindness in excitation frequency selection, low single-parameter measurement efficiency and the like in conductivity or thickness measurement by an existing eddy current method, double-parameter output of single measurement is realized, multiple calibration and inversion calculation are not needed, the calculation is simple, rapid and accurate, the hardware cost is low, and the applicability is wide.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of non-destructive testing of non-magnetic materials, and particularly relates to a non-magnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics and application thereof. BACKGROUND

[0002] With the development of manufacturing industry towards high-end and precision, the requirements for metal plates are becoming more and more strict. Thickness and conductivity are two key indicators for evaluating the quality and performance of metal materials, and the determination of the conductivity and thickness of metal materials can timely check the wear, aging and corrosion degree of the metal materials, and reflect the changes of physical and chemical properties through the change trend of the material quality. Therefore, the accurate determination of the conductivity and thickness of metal materials is very important and has certain value and significance.

[0003] At present, the thickness measurement or conductivity measurement of conductive materials commonly used are ultrasonic method, four-probe method and eddy current method. The eddy current method is based on electromagnetic induction phenomenon, is a non-destructive testing method, has the advantages of low cost, high precision, non-contact and no coupling agent, and is widely used in the fields of health monitoring, electromagnetic parameters and thickness measurement of metal plates.

[0004] However, the existing eddy current method has the following limitations in measuring conductivity or thickness: the conductivity and thickness variables are seriously coupled during the measurement process, single frequency point measurement cannot realize the decoupling of multiple variables, multiple calibrations are required, and the calibration workload increases exponentially; the inversion model based on integral equation is a nonlinear and severely ill-posed inversion model, and has the problems of convergence iteration algorithm and large high-performance computer iteration, and low real-time performance; the existing measurement method selects the frequency according to experience, is not suitable for different conductivity / thickness materials, and the measurement method cannot select the optimal multi-frequency combination for different thickness or conductivity, which causes signal interference and low efficiency; therefore, the complexity and high precision degree of the eddy current method cannot be improved in quality. SUMMARY

[0005] The application provides a non-ferromagnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics, which simplifies and analyzes the classic Dodd-Deeds analytical model to obtain the relationship between the impedance phase characteristics of the eddy current sensor and the conductivity and thickness, and determines the selection basis of the frequency, so as to solve the technical problems of complex calibration, single parameter measurement, blind selection of excitation frequency and low calculation efficiency in the prior art, and realize fast and high-precision double-parameter synchronous output.

[0006] In order to achieve the above purpose, the application provides the following technical scheme:

[0007] In a first aspect, a non-magnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics comprises the following steps:

[0008] S1. Estimate the conductivity range of the sample to be tested. and thickness range ;

[0009] S2. Select high and low excitation frequencies based on frequency selection criteria;

[0010] S3. The phase of the impedance change of the non-ferromagnetic material under test is obtained by eddy current method.

[0011] Substituting S4 and the phase into the corresponding simplified model formulas, we can obtain the conductivity and thickness simultaneously.

[0012] Preferably, the frequency selection criterion described in step 2 is:

[0013] High-frequency excitation frequency is : Select conductivity and minimum thickness To ensure that the skin effect depth is at the minimum thickness 1 / 3, that is, satisfying condition;

[0014] low frequency excitation frequency : Select conductivity and maximum thickness Ensure the skin penetration depth is at maximum thickness. twice that, which satisfies condition;

[0015] in The excitation frequency is selected based on the thickness and conductivity of the test piece, using the vacuum permeability as an example, which improves the measurement accuracy.

[0016] Preferably, the method and steps for obtaining the phase of the impedance change in step 3 are as follows:

[0017] 1) High frequency is applied sequentially through an eddy current sensor and low frequency motivational signal;

[0018] 2) The measuring sensors acquire data in the air. and the impedance value when placed on the sample to be tested ;

[0019] 3) Calculate the impedance change ;

[0020] 4) Extracting high-frequency phase and low frequency phase .

[0021] Preferably, in step 4, the simplified model formula is:

[0022] ,

[0023] wherein, is the spatial frequency eigenvalue, whose value is the inverse of the coil radius; by substituting the phase value into the simplified model, the conductivity and the thickness can be directly solved.

[0024] In the second aspect, a storage medium is burned with the application implemented by the first aspect for health monitoring, electromagnetic parameter, and thickness measurement of non-magnetic metal materials.

[0025] Compared with the prior art, the beneficial effects of the present application are:

[0026] (1) High precision: the relative errors of the conductivity and thickness measurement are less than 2.57% and 2.8%, respectively;

[0027] (2) Calibration-free: directly substitute the impedance phase value into the simplified model formula for calculation, without sample calibration or inversion iteration;

[0028] (3) High efficiency and low cost: the dual-frequency excitation hardware requirement is lower than that of the multi-frequency / sweep frequency system, and supports single measurement dual-parameter output;

[0029] (4) Wide applicability: suitable for industrial online detection and portable device development of all non-magnetic metal materials such as titanium alloy, aluminum alloy, and copper. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is the detection principle diagram of the non-ferromagnetic conductivity and thickness of the present application;

[0031] Figure 2 is the flowchart of the non-magnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics of the present application;

[0032] Figure 3 is the comparison diagram of the actual value and the measured value of the conductivity of the present application;

[0033] Figure 4 is the comparison diagram of the actual value and the measured value of the conductivity of the present application. DETAILED DESCRIPTION

[0034] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. The described embodiments are only a part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0035] The embodiment describes in detail the detection principle of non-ferromagnetic conductivity and thickness, as shown in Figure 1 .

[0036] When the coil passing through the alternating current is close to or placed on the sample to be measured, the sample to be measured will generate eddy current due to electromagnetic induction, and the strength and distribution of the eddy current are closely related to the electromagnetic parameters and thickness of the material. Therefore, by measuring the impedance or voltage signal of the coil, the electromagnetic parameters and thickness of the measured sample can be inferred.

[0037] According to the approximate solution of the classical Dodd-Deeds analytical model, the analytical expression of the impedance of the eddy current sensor is as follows:

[0038]

[0039] wherein,

[0040]

[0041]

[0042]

[0043] wherein, and are the inner and outer diameters of the coil, and are the coil distance and height, c is the thickness of the sample to be measured, is the conductivity, is the vacuum permeability, is the excitation current angular frequency, N is the number of turns of the coil, is a characteristic frequency, and its value is the reciprocal of the coil radius.

[0044] When measuring the conductivity using the eddy current method, the skin depth is usually smaller than the thickness of the sample to be measured, so a higher excitation frequency is required. When the excitation frequency is large, , then can be approximately simplified as:

[0045]

[0046] Further, the relationship between the impedance phase and the conductivity can be obtained:

[0047]

[0048] Under this condition, the phase characteristics are mainly affected by the conductivity and are independent of the thickness.

[0049] Further derivation gives a simplified model for representing the thickness using the phase characteristics:

[0050]

[0051] When the skin depth is greater than the thickness of the measured sample, a lower excitation frequency is needed when measuring the thickness of the measured sample by using the eddy current method. The above equation can be simplified as:

[0052]

[0053] Further, the relationship between the impedance phase and the thickness is obtained as follows:

[0054]

[0055] At this time, the tangent value of the impedance phase and the thickness are in a linear relationship. However, when the frequency is too low, the sensitivity is low, the signal is mixed with a signal, and the measurement result is inaccurate. Therefore, the relationship between the impedance phase and the thickness is further modified as follows:

[0056]

[0057] A simplified model for representing the thickness by using the phase characteristics is further derived as follows:

[0058]

[0059] According to the simplified model of the conductivity and the thickness, it can be seen that the eddy current sensor can obtain the phase by loading a suitable excitation frequency, and the conductivity and the thickness can be directly calculated by using the above formula. The calculation formula is simple, and it is not necessary to perform multiple calibrations or complex inversion processes, so that the detection efficiency is greatly improved.

[0060] In addition, compared with the sweep frequency and multi-frequency eddy current method, the detection speed of the method is faster, the requirement for the hardware circuit is lower, and the measurement precision and the applicability are significantly improved

[0061] Embodiments of the present application provide a non-ferromagnetic material thickness and conductivity synchronous measurement method based on eddy current phase characteristics, as shown in Figure 2 The steps are as follows:

[0062] S1, according to the product specification or industry standard of the measured sample, the conductivity range and the thickness range are estimated

[0063] S2, based on the frequency selection criterion, high and low excitation frequencies are selected;

[0064] S3, the impedance change phase of the measured non-ferromagnetic material is obtained by using the eddy current method;

[0065] S4, the phase is substituted into the corresponding simplified model formula, and the conductivity and the thickness are obtained at the same time. ​​​

[0066] Preferably, the frequency selection criterion in step 2 is:

[0067] High-frequency excitation frequency : select the conductivity and thickness minimum value , to ensure that the skin depth is 1 / 3 of the minimum thickness, that is, to meet the condition ; Low-frequency excitation frequency

[0068] : select the conductivity and thickness maximum value , to ensure that the skin depth is 2 times of the maximum thickness, that is, to meet the condition . Where is the vacuum permeability. According to the actual thickness and conductivity of the sample to be tested, the excitation frequency can be selected to effectively avoid the blindness of excitation frequency selection and improve the measurement accuracy.

[0069] Preferably, the impedance change amount phase acquisition method in step 3 is as follows:

[0070] 1) Load high-frequency ( ) and low-frequency (

[0071] ) excitation signals in turn through the eddy current sensor; 2) Measure the impedance values of the sensor in air ( ) and when placed on the sample to be tested (

[0072] ) respectively; 3) Calculate the impedance change amount ;

[0073] 4) Extract high-frequency phase and low-frequency phase

[0074] . Preferably, in step 4, the simplified model formula of the conductivity and thickness is:

[0075] ;

[0076] .

[0077] Substitute the high-frequency and low-frequency phases obtained in step 3 into the corresponding simplified model formula respectively, and the conductivity and thickness can be obtained simultaneously according to the formula calculation. The simplified model is modified on the basis of the Dodd-Deeds analytical model to ensure the accuracy and efficiency of the model.

[0078]

[0079] ​​In one specific embodiment, the conductivity and thickness of three non-ferromagnetic materials, TC4 titanium, 1060 aluminum and TU1 copper, were measured. The eddy current sensor parameters were: inner radius 5 mm, outer radius 5 mm, height 5 mm, distance 0 mm, number of turns 50, and the thickness of the test piece was 2.5 ~ 4 mm.

[0080] According to the international standard, the conductivity range was predicted to be 33 ~ 36 MS / m and 55 ~ 60 MS / m, respectively.

[0081] Specifically, according to the frequency selection criterion, the high frequency excitation frequencies were 730, 12 and 6.6 kHz, respectively, and the low frequency excitation frequencies were 4, 0.075 and 0.07 kHz, respectively.

[0082] The conductivity and thickness values were obtained by measurement, as shown in Figure 3 and 4 The measured values were compared with the actual values. The measured values were highly consistent with the actual values, and in addition, the corrected thickness simplified model was closer to the actual thickness.

[0083] In combination with Table 1 and Table 2, the measurement results of the non-magnetic material thickness and conductivity synchronous measurement method based on the eddy current phase characteristics of the present application are described in detail.

[0084] Table 1

[0085]

[0086] Table 2

[0087]

[0088] The relative error of the corrected thickness simplified model was reduced from 18.75% before correction to within 2.85%. The measurement method of the present application has high accuracy, and effectively determines the conductivity and thickness of non-ferromagnetic materials. At the same time, the test piece represents the low, medium and high conductivity range of non-magnetic materials, and the measurement method proposed by the present application can be applied to non-ferromagnetic materials in a wide range of 0.5-58.5 MS / m.

[0089] The above-described embodiments are only used to illustrate the technical solutions of the present application, but not limit the present application; although the present application is described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

Claims

1. A method for simultaneously measuring the thickness and conductivity of nonferromagnetic materials based on eddy current phase characteristics, characterized in that, Includes the following steps: S1. Estimate the conductivity range of the sample to be tested. and thickness range ; S2. Select high and low excitation frequencies based on frequency selection criteria; S3. The phase of the impedance change of the non-ferromagnetic material under test is obtained by eddy current method. Substituting S4 and the phase into the corresponding simplified model formulas, we can obtain the conductivity and thickness simultaneously.

2. The method for synchronously measuring the thickness and conductivity of nonferromagnetic materials based on eddy current phase characteristics according to claim 1, characterized in that, The frequency selection criterion is as follows: High-frequency excitation frequency is Choose conductivity and minimum thickness This makes the skin effect depth the minimum thickness. 1 / 3; Low-frequency excitation frequency is Choose conductivity And the maximum thickness, so that the skin effect depth is the maximum thickness. 2 times; in is the vacuum permeability.

3. The method for synchronously measuring the thickness and conductivity of nonferromagnetic materials based on eddy current phase characteristics according to claim 1, characterized in that, The steps of the method for obtaining the phase of impedance change are as follows: (1) Eddy current sensor is sequentially loaded with high frequency and low frequency motivational signal; (2) The measuring sensors respectively acquire data in the air and the impedance value when placed on the test piece ; (3) Calculate the impedance change ; (4) Extracting high-frequency phase and low frequency phase .

4. The method for synchronously measuring the thickness and conductivity of nonferromagnetic materials based on eddy current phase characteristics according to claim 1, characterized in that, The simplified model formula is: ， ； in, The spatial frequency characteristic value is the reciprocal of the coil radius; substituting the phase value into this simplified model yields the conductivity. and thickness .

5. A storage medium programmed with the method described in claim 1 for applications in health monitoring, electromagnetic parameter measurement, and thickness measurement of non-magnetic metallic materials.

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