RU2656115C1 - Method of metallic non-magnetic tubes wall thickness eddy current control - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to methods of non-magnetic metallic products non-destructive testing and can be used to control the thickness of a metal product and the thickness of the dielectric coating of its surface. Essence of the claimed invention lies in the fact that the method of eddy current monitoring of the wall thickness of metallic non-magnetic pipes comprises a stage, on which 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 gap value and the wall thickness for a fixed value of the specific electric conductivity of the pipe wall metal, corresponding to the used for finding the functional dependence of the pipe samples, and for determining the wall thickness, the measured value of the phase of the introduced voltage of the third frequency is corrected by the amount of the correction proportional to the difference between the measured value of the phase of the introduced voltage of the second frequency and its value for the samples used to determine the functional dependence of the phase of the introduced voltage of the third frequency on the gap value and the wall thickness.

EFFECT: technical result is an increase in the reliability of control over the thickness of the pipes and simplification of the determination of the function of converting the values of the introduced voltages of the eddy current converter into the value of the controlled parameter.

1 cl, 5 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 the metal product and the thickness of the dielectric coating of its surface.

A known method of eddy current control of the thickness of non-magnetic electrically conductive metal sheets, based on the excitation using an overhead transformer eddy current transducer in the object of control of eddy currents of the same frequency, measuring the complex value of the applied voltage of the eddy current transducer, which determines the value of the controlled parameter of the object of control (Nondestructive testing. Reference / under Edited by V.V. Klyuyev: in 8 volumes, T 2: in 2 books: Book 1: Leak test, Book 2: Eddy current contact role. - M.: Mechanical Engineering, 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 electrical conductivity of the material, the amplitude-phase detuning from the influence of changes in the electrical conductivity of the material or the gap on the sheet thickness measurements can be carried out.

The disadvantages of this method are the lack of detuning from the effects of changes simultaneously of two influencing factors and the small ranges of detuning from changes in each of them.

A known method of eddy current control of the thickness of metal objects, implemented in a device for controlling the thickness of metal products (SU 375468 A1, IPC6 G01B 7/06, publ. 03/23/1973, bull. No. 16), based on the excitation using an overhead eddy current transducer in the object of control of eddy currents of two frequencies and the measurement of the introduced voltages of two frequencies. The frequencies are chosen in such a way that the applied voltage of the eddy current high-frequency transducer depends only on the gap between the transducer and the test object and the electrical conductivity of the material, and the applied low-frequency voltage depends on the gap, electrical conductivity of the material and the thickness of the object. The method provides phase detuning from the effect on the high-frequency and low-frequency signals of the eddy current transducer of changes in the electrical conductivity of the material, as well as detuning from the influence of changes in the gap, carried out by adjusting the phase of the low-frequency signal depending on the amplitude and phase of the high-frequency. Due to the presence of two-frequency excitation of the eddy current transducer, an effective detuning from the influence on the measurement results of the thickness of the gap changes between the eddy current transducer and the test object in a significant range of its changes is provided.

The disadvantages of this method are the small range and the complexity of the detuning from the influence of changes in the electrical conductivity of the material.

A known method of eddy current control of metallic non-magnetic objects (SU 1176231 A1, IPC4 G01N 27/90, publ. 08/30/1985, bull. No. 32), selected as a prototype, based on the excitation using an overhead eddy current transducer in the object of control of eddy currents three frequencies, the first of which is selected from the condition of a negligible value of the depth of penetration of the magnetic field compared with the thickness of a flat object, the second frequency is selected from the condition of approximate equality of the depth of penetration of the magnetic field to half the thickness of the object, the third frequency is selected from the condition that the depth of penetration of the magnetic field of the thickness of the object is exceeded, the insertion stresses of three frequencies are measured. The insertion voltage of the eddy current transducer at the first frequency depends only on the gap h between the transducer and the test object, the insertion voltage on the second frequency depends on the gap h and the specific electrical conductivity of the material σ, and the introduced voltage at the third frequency depends on the gap h, the specific electrical conductivity of the material σ and the thickness of the object T.

The value of the amplitude of the introduced voltage of the first frequency determines the value of the gap h between the eddy current transducer and the control object. The value of the specific components of the applied voltage of the second frequency and the calculated value of the gap determine the value of the specific electrical conductivity of the material σ, and the values of the complex components of the applied voltage of the third frequency and the calculated values of the gap h and the specific electrical conductivity of the material a determine the thickness of the object T. To determine the parameters of the object control: the gap h, the electrical conductivity of the material σ and the thickness of the object T, the functions of the inverse azovaniya obtained by numerical analysis of functional dependencies introduced by the eddy current transducer voltages from said object parameters. Thus, it is possible to separately control the values of h, σ, and T.

The disadvantage of this method is the low reliability of the control of the wall thickness of metal non-magnetic pipes with significant ranges of changes in the parameters of the object. This is due to the high errors of approximation 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 planar object of control 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 of 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 control object differs from the flat one.

The proposed invention allows to increase the reliability of controlling the wall thickness of metal non-magnetic pipes and simplify the determination of the function of converting the values of the introduced voltages of the eddy current transducer into the value of the controlled parameter.

According to the method of eddy current control of the wall thickness of non-magnetic metal pipes in the test object, eddy currents of three frequencies are excited using an overhead eddy current transducer, the first of which is selected from the condition of a negligible depth of penetration of the magnetic field compared to the wall thickness, the second frequency is selected from the condition of an approximate equality of depth magnetic field penetration half the wall thickness, the third frequency is selected from the condition that the penetration depth is exceeded magnetically Field of the wall thickness measured voltage introduced three frequencies. The value of the amplitude of the applied voltage of the first frequency determines the gap between the eddy current transducer and the control object. The wall thickness is determined on the basis of the experimental functional dependence of the phase of the applied 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 the pipe samples used to find the functional dependence. At the same time, to determine the wall thickness, the measured phase value of the applied voltage of the third frequency is adjusted by the amount of correction proportional to the difference between the measured phase value of the applied voltage of the second frequency and its value for the samples used to determine the functional dependence of the phase of the applied voltage of the third frequency on the value of the gap and wall thickness.

The main difference that provides the technical result: the use of the experimental dependence of the phase of the applied voltage of the third frequency as a function of the transformation on the gap and wall thickness for a fixed value of the specific electrical conductivity of the metal of the pipe wall with the correction of the voltage measured during the phase control by a value proportional to the phase difference the measured applied voltage of the second frequency and its value corresponding to those used to find the function conversion to pipe samples.

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

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

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

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

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

The device (Fig. 1) that implements a method of eddy current control of the wall thickness of non-magnetic metal pipes contains the first 1 (G1), second 2 (G2) and third 3 (G3) harmonic signal generators, overhead eddy current transducer 4 (ETC), 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 with the first, second and third inputs of the overhead eddy current transducer 4 (ETC) and with the first, second and third inputs of the analog conversion unit 5 (BAP). The output of the eddy current transducer 4 (VTP) is connected to the fourth input of the analog conversion unit 5 (BAP). Six outputs of the block analog conversion 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 indicating unit 7 (BI).

One of the possible design options for the overhead eddy current transducer 4 (ETC), most often used in eddy current thickness gauges, 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 included in the opposite direction.

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

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

The output signals of the generators 1-3 (G1-G3) are fed to the field winding 8 of the overhead eddy current transducer 4 (ETC). The current of this winding has three harmonic components of the frequencies ƒ ₁ , ƒ ₂ and ƒ ₃ and creates a three-frequency magnetic field. The measuring 9 and compensation 10 windings of the eddy current transducer 4 (ETC) are turned on in the opposite direction; therefore, in the absence of an electrically conductive object near the eddy current transducer, the output signal of the eddy current transducer is zero. In the presence of an electrically conductive object near the eddy current transducer 4 (ECP), the three-frequency magnetic field induces eddy currents of three frequencies in the controlled product. The magnetic field of these eddy currents causes the appearance of the output signal (insertion voltage) of the eddy current transducer 4 (ETC). Block analog conversion 5 (BAP) carry out the selection of the complex signal components of the eddy current transducer, due to each of the three frequency components of the magnetic field of the 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 a device that implements 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 voltage frequencies ƒ ₁ , ƒ ₂ , ƒ ₃ :

,

,

,

,

,

.

Owing to the previously indicated choice of frequencies of harmonic signal generators, the extracted components of the eddy current transducer signal at the first frequency depend only on the gap h between the eddy current transducer 4 (ECP) and object 11, the signal components at the second frequency depend on the gap h and the specific conductivity of the material σ, and the components the signal at the third frequency - from the gap h, the conductivity of the material σ and the thickness T of the object.

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

;

;

;

;

.

.

Further computational conversion of the 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 clearance values h. The quantities of thickness and clearance samples used are determined by the required accuracy and measurement range, and for a wide range of control tasks are about ten.

In accordance with the proposed method, the computing unit 6 (WB) calculates the value of the gap h. To do this, use the function of the inverse transformation of the relative value of the amplitude A _{1 of the} introduced voltage of the first frequency to the value of the gap h (Fig. 3), determined by numerically analyzing the experimental dependence of the amplitude A ₁ on the gap h. This function is approximated by a dependence of the form with sufficient accuracy to solve a wide range of control problems

,

,

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;

And ₁₀ is the value of the amplitude at h = 0.

Next, the intermediate value of the wall thickness T is calculated 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} . A view of this functional relationship is shown in FIG. 4. To determine the value of T, first determine the discrete values h _i and h _{i + 1} closest 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 thickness T is calculated under the assumption that the dependence is linear in a small range of changes in the gap h:

.

.

Further computational transformations provide detuning from the influence on the result of monitoring changes in the specific electrical conductivity of the material σ. For this, the phase value ϕ _{20 of the} applied voltage of the second frequency is determined for the measured gap h and the value of the specific 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, frequency ƒ ₂ , electrical conductivity σ ₀ and the design parameters of the eddy current transducer.

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

Δϕ ₂ = 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 those used in determining the conversion functions of the samples. As the results of mathematical and physical modeling show, the phase difference Δϕ ₃ , due to the difference in the specific electric conductivity of the material of the controlled pipe σ from its value σ ₀ corresponding to the samples used to determine the conversion functions, is related to the phase difference Δϕ ₂ due to the same reason, a linear dependence kind of

Δϕ ₃ = s Δϕ ₂ ,

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

s = s (T).

The dependence s (T) with a high degree of approximation is described by the function s (T) = exp (k + mT + nT ² ),

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

When determining the value of the factor s necessary for calculating the value of Δϕ ₃ , the previously calculated thickness T is used. Then, the corrected phase value of the applied voltage of the third frequency is calculated, which corresponds to the samples used to determine the conversion functions:

ϕ ₃₀ = ϕ ₃ -Δϕ ₃ .

Then, using the new corrected phase value of the applied voltage of the third frequency ϕ ₃₀ , the specified value of the thickness T is recalculated using the dependence of FIG. 4. The found updated value of the thickness T is again used for sequential calculations of the values of s, Δϕ ₃ , ϕ ₃₀ and the new updated value of the thickness T. The described cycle of calculations is repeated (2 ... 5) times depending on the required accuracy and degree of discreteness of the gap values. The thickness value T calculated in the last cycle is taken as the measurement result of the controlled parameter T.

The display unit 7 (BI) displays the result of the control.

The effectiveness of using the proposed method of 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 electrical conductivity of the material was confirmed by laboratory and production tests of a prototype of the device when controlling the wall thickness of light-alloy drill pipes made of D16T duralumin with an outer diameter 147 mm and wall thickness in the range (5 ... 15) mm. An overhead transformer eddy current differential transducer was used, the design of which is schematically shown in FIG. 2. The frequencies of the components of the excitation current of 80 kHz, 2500 Hz, and 125 Hz were used. To determine the conversion functions, 11 pipe samples with wall thicknesses from the specified range and with a specific electric conductivity of the material of 16 MSm / m were used. To change the gap value, 12 gap samples (dielectric plates) with a thickness of (1 ... 15) mm were used. To determine the functional dependence of the measured signals on the changes in the electrical conductivity, a change in the temperature of the samples was used in the range (-12 ... + 85) ° С. To test the detuning efficiency from the influence of changes in electrical conductivity, samples with a specific electrical conductivity of 18 MSm / m and 20 MSm / m were used. The range of variation of the values of the factor s used to adjust the phase values of the applied voltage of the third frequency according to the results of measuring the phase of the second frequency was from 3.1 to 4.3.

The test results of the prototype of the device showed that when using the proposed control method in the specified range of changes of the influencing parameters, the absolute error of measuring the wall thickness does not exceed (0.2 ... 0.3) mm. Without the use of detuning from interfering factors (for example, from changes in electrical conductivity with a change in temperature in the indicated range), the measurement error increases by an order of magnitude.

Claims (1)

A method of eddy current control of the wall thickness of metallic non-magnetic pipes, based on the excitation of an eddy current transducer of three frequencies by means of an applied eddy current transducer in an object to control eddy currents, the first of which is selected from the condition of a negligible value of the magnetic field penetration depth compared to the wall thickness, the second frequency is selected from the condition equal to the depth of penetration of the magnetic field to half the wall thickness, the third frequency is chosen from the condition of exceeding the penetration depth of of the field thickness of the wall thickness, the insertion stresses of three frequencies are measured, the value of the gap between the eddy current transducer and the test object is determined by the amplitude of the applied voltage of the first frequency, characterized in that the wall thickness is determined on the basis of the experimental functional dependence of the phase of the applied voltage of the third frequency on the gap value and thickness walls for a fixed value of the specific electrical conductivity of the metal of the pipe wall corresponding to the functions used to find the total dependence of the pipe samples, and to determine the wall thickness, the measured phase value of the applied voltage of the third frequency is adjusted by a correction proportional to the difference between the measured phase value of the applied voltage of the second frequency and its value for samples used to determine the functional dependence of the phase of the applied voltage of the third frequency on the gap value and wall thickness.

Images