RU2784787C1 - Method for eddy current control of wall thickness of metal non-magnetic pipes - Google Patents

Abstract

FIELD: current control.

SUBSTANCE: invention is intended for eddy current control of the wall thickness of metal non-magnetic pipes. The substance of the invention lies in the fact that excitation is carried out using an attached eddy current transducer in the object of control of eddy currents of three frequencies, the first of which is selected from the condition of a negligibly small value of the penetration depth of the magnetic field compared to the wall thickness, the second frequency is selected from the condition of approximate equality of the penetration depth of the magnetic field to half the wall thickness, the third frequency is selected from the condition of exceeding the penetration depth of the magnetic field of the wall thickness, the introduced voltages of three frequencies are measured, the value of the gap between the eddy current transducer and the test object is determined from the value of the amplitude of the introduced voltage of the first frequency, the wall thickness is determined on the basis of the experimental functional dependence phase of the introduced voltage of the third frequency on the value of the gap and wall thickness for a fixed value of the specific electrical conductivity of the metal of the pipe wall, corresponding to those used when and functional dependence of pipe samples, and for detuning from changes in the specific electrical conductivity of the metal of the pipe wall, the measured value of the phase of the introduced voltage of the third frequency is corrected by a correction value equal to the product of the difference between the measured value of the phase of the introduced voltage of the second frequency and its value for those used in finding the conversion function of pipe samples and the correction factor, while the value of the correction factor is determined by the obtained values of the pipe wall thickness and gap using the linear dependence of the correction factor on these values.

EFFECT: increasing the reliability of monitoring the wall thickness of metal non-magnetic pipes.

1 cl, 6 dwg

Description

The invention relates to methods for non-destructive testing of non-magnetic metal products and can be used to control the thickness of a metal product and the thickness of the dielectric coating on its surface.

A known method of eddy current control of the thickness of non-magnetic electrically conductive metal sheets, based on excitation using an overhead transformer eddy current transducer in the object of control of eddy currents of one frequency, measuring the complex value of the introduced voltage of the eddy current transducer, which determines the value of the controlled parameter of the control object (Non-destructive testing. Handbook / under V. V. Klyuev: in 8 volumes T 2: in 2 books: Book 1: Leak tightness control Book 2: Eddy current control - M.: Mashinostroenie, 2003. - 688 p.: p. 418-422). Due to the different influence on the value of the introduced voltage of the sheet thickness, the gap between the eddy current transducer and the sheet surface, as well as the specific electrical conductivity of the material, when controlling the sheet thickness, amplitude-phase detuning can be carried out from the influence of changes in the electrical conductivity of the material or gap on the results of measuring the sheet thickness.

The disadvantage of this method is the lack of the possibility of detuning from the influence of changes at the same time two influencing factors and small ranges of detuning from changes in each of them.

A known method of eddy current control of metal non-magnetic objects (SU 1176231 A1, MPK4 G01N 27/90, publ. 08/30/1985), based on excitation using an attached eddy current transducer in the control object of eddy currents of three frequencies, the first of which is selected from the condition of a negligible value the depth of penetration of the magnetic field compared to the thickness of the object, the second frequency is selected from the condition that the penetration depth of the magnetic field is approximately equal to half the thickness of the object, the third frequency is selected from the condition that the penetration depth of the magnetic field exceeds the thickness of the object, the introduced voltages of three frequencies are measured. The input voltage of the eddy current transducer at the first frequency depends only on the gap h between the transducer and the test object, the input voltage at the second frequency depends on the gap h and the electrical conductivity of the material σ, and the input voltage at the third frequency depends on the gap h, the electrical conductivity of the material a and the thickness of the object T.

By the value of the amplitude of the introduced voltage of the first frequency, the value of the gap h between the eddy current transducer and the test object is determined. The values of the complex components of the introduced voltage of the second frequency and the calculated value of the gap determine the value of the electrical conductivity of the material σ, and the values of the complex components of the introduced voltage of the third frequency and the calculated values of the gap h and the electrical conductivity of the material σ determine the value of the thickness of the object T. To determine the parameters of the object control: the gap h, the specific electrical conductivity of the material σ and the thickness of the object T, the functions of the inverse transformation are used, obtained as a result of a numerical analysis of the functional dependences of the introduced voltages of the eddy current transducer on the indicated parameters of the object. Thus, it is possible to separately control the values of h, σ and Т.

The disadvantage of this method is the low reliability of the control of the wall thickness of metal non-magnetic pipes with a significant range of changes in the parameters of the object. This is due to high approximation errors of the real functional dependences of the introduced voltages of the eddy current transducer on the parameters of the object by the proposed analytical expressions even in the case of a flat test object and a significant increase in these errors in the case of a curvilinear shape of the object, which occurs when testing pipes. Another disadvantage of the known method is the difficulty in determining the function of converting the values of the introduced voltages of the eddy current transducer into the value of the controlled parameter, especially when the shape of the test object differs from flat.

A known method of eddy current control of metal non-magnetic objects (RU 2656115 C1, IPC G01N 27/90 (2006.01), publ. 05/31/2018), selected as a prototype, based on excitation using an attached eddy current transducer in an eddy current control object of three frequencies, the first of which are selected from the condition of a negligibly small value of the penetration depth of the magnetic field compared to the wall thickness, the second frequency is selected from the condition that the penetration depth of the magnetic field is approximately equal to half the wall thickness, the third frequency is selected from the condition that the penetration depth of the magnetic field of the wall thickness is exceeded, the introduced voltages of three frequencies. The value of the amplitude of the introduced voltage of the first frequency determines the value of the gap between the eddy current transducer and the test object. The wall thickness is determined on the basis of the experimental functional dependence of the phase of the introduced voltage of the third frequency on the value of the gap and wall thickness for a fixed value of the electrical conductivity of the pipe wall metal, corresponding to the pipe samples used in finding the functional dependence. To tune out the change in the specific electrical conductivity of the metal of the pipe wall, the measured value of the phase of the introduced voltage of the third frequency is corrected by a correction value equal to the product of the difference between the measured value of the phase of the introduced voltage of the second frequency and its value for the pipe samples used in finding the conversion function and the correction factor, the value of which is determined value of the pipe wall thickness and is related to it by an exponential dependence.

The disadvantage of this method is the low reliability of the control of the wall thickness of metal non-magnetic pipes while changing the influencing parameters of the object in significant ranges. This is due to the actual dependence of the correction factor not only on the pipe wall thickness, but also largely on the gap between the eddy current transducer and the test object, which is not taken into account in the prototype.

The proposed invention improves the reliability of the control of the wall thickness of metal non-magnetic pipes.

According to the method of eddy current control of the wall thickness of metal non-magnetic pipes in the test object, using an attached eddy current transducer, eddy currents of three frequencies are excited, the first of which is selected from the condition of a negligibly small value of the penetration depth of the magnetic field compared to the wall thickness, the second frequency is selected from the condition of approximate equality of the depth penetration of the magnetic field to half the wall thickness, the third frequency is selected from the condition of exceeding the penetration depth of the magnetic field of the wall thickness, the introduced voltages of three frequencies are measured. The value of the amplitude of the introduced voltage of the first frequency determines the value of the gap between the eddy current transducer and the test object. The values of the phase of the introduced voltage of the third frequency and the gap determine the thickness of the pipe wall. At the same time, in order to tune out the influence of changes in the specific electrical conductivity of the pipe wall metal, the value of the phase of the introduced voltage of the third frequency is corrected by a correction value equal to the product of the correction factor and the difference between the measured value of the phase of the introduced voltage of the second frequency and its value for those used when finding the conversion function of pipe samples. The value of the correction factor is determined by the obtained values of the pipe wall thickness and gap using the linear dependence of the correction factor on these values.

The main difference that provides the technical result: taking into account the dependence of the correction factor used to correct the value of the phase of the introduced voltage of the third frequency, not only on the thickness of the pipe wall, but also on the gap between the eddy current transducer and the test object, which provides better detuning from the influence of changes in the specific electrical conductivity of the metal of the pipe wall.

In FIG. 1 shows a block diagram of a device that implements the proposed method.

In FIG. 2 shows a cross-section of the eddy current transducer and part of the pipe.

In FIG. 3 shows a view of the function of inverse conversion of the relative value of the amplitude of the introduced voltage of the first frequency A ₁ into the value of the gap h.

In FIG. 4 shows the function of the inverse transformation of the phase ϕ ₃ of the introduced voltage of the third frequency into the value of the pipe wall thickness T for different values of the gap h at a fixed value of the electrical conductivity of the material σ ₀ .

In FIG. 5 shows the functional dependence of the phase ϕ ₂₀ of the introduced voltage of the second frequency on the gap h at a fixed value of the electrical conductivity of the material σ ₀ .

In FIG. 6 shows the functional dependence of the correction factor s on the pipe wall thickness T for different values of the gap h.

The device (Fig. 1) that implements the method of eddy current control of the wall thickness of metal non-magnetic pipes contains the first 1 (G1), the second 2 (G2) and the third 3 (G3) harmonic signal generators, an attached eddy current transducer 4 (VTP), an analog conversion unit 5 (BAP), computing unit 6 (WB), display unit 7 (BI).

The outputs of the first, second and third harmonic signal generators 1-3 (G1-G3) are connected respectively to the first, second and third inputs of the attached eddy current transducer 4 (ECT) and to the first, second and third inputs of the analog conversion unit 5 (BAP). The output of the eddy current transducer 4 (EC) is connected to the fourth input of the analog conversion unit 5 (BAP). The six outputs of the analog conversion unit 5 (BAP) are each connected to a separate input of the computing unit 6 (WB). The output of the computing unit 6 (WB) is connected to the input of the display unit 7 (BI).

One of the possible design options for an attached eddy current transducer 4 (ECC) contains an excitation winding 8 (Fig. 2), a measuring winding 9 and a compensation winding 10. The measuring winding 9 and the compensation winding 10 are connected in opposite directions.

When monitoring, the eddy current transducer 4 (ECP) is located near the object of control 11. Harmonic signal generators 1-3 (G1-G3) generate harmonic signals with frequencies ƒ ₁ , ƒ ₂ and ƒ ₃ .

The frequency values ƒ ₁ , ƒ ₂ and ƒ ₃ must satisfy the conditions under which, the penetration depth of the magnetic field of the first frequency is negligible compared to the thickness of the pipe wall, the penetration depth of the second frequency magnetic field is approximately equal to half the wall thickness, the penetration depth of the magnetic field of the third frequency exceeds the pipe wall thickness.

The output signals of the generators 1-3 (G1-G3) are fed to the excitation winding 8 of the attached eddy current converter 4 (ECT). The current of this winding has three harmonic frequency components ƒ ₁ , ƒ ₂ and ƒ ₃ and creates a three-frequency magnetic field. Measuring 9 and compensation 10 windings of the eddy current transducer 4 (ECT) are connected in opposite directions, therefore, in the absence of an electrically conductive object near it, the output signal of the eddy current transducer is zero. In the presence of an electrically conductive object near the eddy current transducer 4 (ECT), a three-frequency magnetic excitation field induces eddy currents of three frequencies in the controlled product. The magnetic field of these eddy currents causes the output signal (introduced voltage) of the eddy current converter 4 (ECT). Block analog conversion 5 (BAP) carry out the selection of the complex components of the signal of the eddy current transducer 4 (ECT), due to each of the three frequency components of the magnetic field of eddy currents. To perform this function, the analog conversion unit 5 (BAP) includes frequency-selective blocks and amplitude-phase detection blocks used, for example, in devices that implement the analog method and the prototype method.

The output signals of the analog conversion unit 5 (BAP) are proportional to the amplitudes of the real and imaginary complex components of the introduced voltages of the frequencies ƒ ₁ , ƒ ₂ , ƒ ₃ :

Due to the previously mentioned choice of harmonic signal generator frequencies, the selected components of the eddy current transducer signal at the first frequency depend only on the gap h between the eddy current transducer 4 (ECT) and the object 11, the signal components at the second frequency depend on the gap h and the specific electrical conductivity of the material σ, and the components signal at the third frequency - from the gap h, the electrical conductivity of the material σ and the thickness T of the object.

Computing unit 6 (WB) performs computational conversion of the output signals of the analog conversion unit 5 (BAP) into the measured value of the controlled parameter. For this, the amplitude of the introduced voltage of the first frequency A ₁ and the phases ϕ ₂ and ϕ ₃ of the introduced voltages of the second and third frequencies are calculated:

Further computational conversion of measurement information signals is carried out using conversion functions determined experimentally using pipe samples of various thicknesses T with a fixed specific electrical conductivity of the material σ ₀ at various values of the gap h. The number of thickness and gap samples used is determined by the required accuracy and measurement range, and for a wide range of inspection tasks is about ten.

In accordance with the proposed method, the computing unit 6 (WB) calculates the value of the gap h. For this, the function of inverse conversion of the relative value of the amplitude A ₁ of the introduced voltage of the first frequency into the value of the gap h (Fig. 3) is used, which is determined by numerical analysis of the experimental dependence of the amplitude A ₁ on the gap h. This function with sufficient accuracy for solving a wide range of control problems is approximated by a dependence of the form

where a is a coefficient depending on the outer diameter of the pipe, the design parameters of the eddy current transducer and the range of changes in the gap h;

A ₁₀ - amplitude value at h=0.

Next, the intermediate value of the wall thickness T is determined under the assumption that the specific electrical conductivity of the pipe material σ is equal to the specific electrical conductivity of the samples σ ₀ . For this, the functional dependence of the pipe wall thickness T(h, ϕ ₃ ) on the gap h and phase ϕ ₃ is used. This functional relationship is shown in Fig. 4. To determine the value of T, first determine its discrete values h _i and h _i+1 nearest to the measured value h, corresponding to the thicknesses of the samples used to determine the dependence shown in FIG. 4. Next, the corresponding values of T _i (h _i , ϕ ₃ ) and T _i+1 (h _i+1 , ϕ ₃ ) are calculated. The value of the thickness T is calculated assuming the linearity of the dependence in a small range of changes in the gap h:

Further computational transformations provide detuning from the influence of changes in the electrical conductivity of the material σ on the control result. For this, the value of the phase ϕ ₂₀ of the introduced voltage of the second frequency is determined for the measured gap h and the value of the electrical conductivity σ ₀ corresponding to the samples used to determine the conversion functions. The experimental dependence of the phase ϕ ₂₀ on the gap h is shown in Fig. 5. With a high degree of approximation, this dependence is approximated by the function

ϕ ₂₀ =-exp(b+ch+dh ² ),

where b, c and d are experimentally determined coefficients depending on the outer diameter of the pipe, the frequency ƒ ₂ , the value of the electrical conductivity σ ₀ and the design parameters of the eddy current transducer.

Next, the phase difference Δϕ ₂ is calculated between the measured value of the phase ϕ ₂ of the introduced voltage of the second frequency and its value ϕ ₂₀ for the pipe samples used in determining the transformation functions:

Δϕ ₂ \u003d ϕ ₂ -ϕ ₂₀ .

The next computational operation is to determine the phase difference Δϕ ₃ between the measured value of the phase ϕ ₃ of the introduced voltage of the third frequency and its value ϕ ₃₀ for the samples used in determining the conversion functions. As the results of mathematical and physical modeling show, the phase difference Δϕ ₃ due to the difference in the specific electrical conductivity of the material of the controlled pipe σ from its value σ ₀ corresponding to the samples used to determine the transformation functions is associated with the phase difference Δϕ ₂ due to the same reason, a linear dependence kind

Δϕ ₃ \u003d sΔϕ ₂ ,

where the factor s is a function of the pipe wall thickness T and the gap h:

s=s(T, h).

This functional relationship is shown in Fig. 6. With an acceptable degree of approximation, the dependence s(T, h) is described by the function

s(T, h)=e ₀ +e ₁ T+e ₂ Th+e ₃ h,

where e ₀ , e ₁ , e ₂ and e ₃ are experimentally determined coefficients depending on the outer diameter of the pipe, the value of electrical conductivity σ ₀ , the values of the second ƒ ₂ and third ƒ ₃ frequencies and the design parameters of the eddy current transducer.

When determining the value of the multiplier s, necessary to calculate the value of Δϕ ₃ , the values of the gap h and thickness T calculated earlier are used. Then the corrected value of the phase of the introduced voltage of the third frequency is calculated, corresponding to the samples used to determine the conversion functions:

ϕ ₃₀ \u003d ϕ ₃ -Δϕ ₃ .

Further, using the new corrected value of the phase of the introduced voltage of the third frequency ϕ ₃₀ , the refined value of the thickness T is recalculated using the dependence of FIG. 4. The found refined value of the thickness T is again used for sequential calculations of the values of the values s, Δϕ ₃ , ϕ ₃₀ and the new refined value of the thickness Т. The thickness value T calculated in the last cycle is taken as the result of measuring the controlled parameter T.

Indication block 7 (BI) indicates the result of the control.

The efficiency of using the proposed method for eddy current control of the wall thickness of metal non-magnetic pipes under conditions of significant changes in the gap between the eddy current transducer and the surface of the object and the specific electrical conductivity of the material was confirmed by the results of laboratory tests of a prototype device when controlling the wall thickness of light-alloy drill pipes made of D16T duralumin with an outer diameter of 147 mm and wall thickness in the range (5…15) mm. An overhead transformer differential eddy current transducer was used, the design of which is schematically shown in Fig. 2. The frequencies of the excitation current components were 100 kHz, 2500 Hz, and 125 Hz. To determine the conversion functions, 11 pipe samples with wall thicknesses from the specified range and with a specific electrical conductivity of the material of 16 MSm/m were used. To change the value of the gap, 12 gap samples (dielectric plates) with a thickness of (1…15) mm were used. To determine the functional dependence of the measured signals on changes in electrical conductivity and to check the effectiveness of detuning from the effect of changes in electrical conductivity, a change in the temperature of the samples in the range (-10 ... + 80) ° С was used. The range of change in the values of the multiplier s used to correct the phase values of the introduced voltage of the third frequency based on the results of measuring the phase of the second frequency was from 3.2 to 5.3.

The test results of a prototype device showed that when using the proposed control method while changing all the influencing parameters in the indicated ranges, the absolute error in measuring the wall thickness does not exceed 0.25 mm, which is approximately 15% less than when implementing the prototype method.

Claims (1)

A method for eddy current control of the wall thickness of metal non-magnetic pipes, including excitation using an attached eddy current transducer in the object of control of eddy currents of three frequencies, the first of which is selected from the condition of a negligibly small value of the penetration depth of the magnetic field compared to the wall thickness, the second frequency is selected from the condition of approximate equality the depth of penetration of the magnetic field to half the wall thickness, the third frequency is selected from the condition of exceeding the penetration depth of the magnetic field of the wall thickness, the introduced voltages of three frequencies are measured, the value of the gap between the eddy current transducer and the test object is determined from the value of the amplitude of the introduced voltage of the first frequency, the wall thickness is determined on the basis of the experimental functional dependence of the phase of the introduced voltage of the third frequency on the value of the gap and wall thickness for a fixed value of the specific electrical conductivity of the metal of the pipe wall, respectively which is used in finding the functional dependence of the pipe samples, and in order to tune out the change in the specific electrical conductivity of the metal of the pipe wall, the measured value of the phase of the introduced voltage of the third frequency is corrected by a correction value equal to the product of the difference between the measured value of the phase of the introduced voltage of the second frequency and its value for those used in finding the function conversion of pipe samples and the correction factor, characterized in that the value of the correction factor is determined from the obtained values of the pipe wall thickness and the gap using a linear dependence of the correction factor on these values.

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