Metal conductivity detection method based on pulsed eddy current

Abstract

The invention discloses a metal conductivity detection method based on a pulsed eddy current method, which comprises the following steps: detecting N standard conductor test pieces with the same size and known conductivity by using a pulsed eddy current detector to correspondingly obtain induced electromotive force output signals of the N standard conductor test pieces; after the induced electromotive force output signal is converted into a curve based on an eddy current apparent time constant, the peak value time and the peak value size of the corresponding curve are determined; fitting to obtain a relation curve between the conductivity of the standard conductor test piece and the peak time or peak size; acquiring an induced electromotive force output signal of the to-be-tested conductor test piece, converting the output signal into an eddy-current-based apparent time constant curve of the to-be-tested conductor test piece, and determining peak time or peak size characteristic quantity of the to-be-tested conductor test piece based on the eddy-current apparent time constant curve; and substituting the peak time or peak size of the to-be-tested conductor test piece into the relation curve of the conductivity of the standard conductor test piece and the peak time or peak size for comparison to obtain the conductivity of the to-be-tested conductor test piece. The metal conductivity and the peak value time or peak value characteristic quantity of the eddy current apparent time constant curve are in a linear relation, so that the method has the advantages of easy acquisition of a standard relation curve, high detection precision and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of nondestructive testing, and in particular relates to a metal conductivity detection method based on pulse eddy current. Background Art

[0002] Pulse eddy current (PEC) detection technology is a non-contact detection method based on the principle of electromagnetic induction. Pulse eddy current (PEC) technology has been used for corrosion detection of pipelines and containers in the petrochemical, electric power and other industries as early as the 1980s, but there are fewer studies on metal conductivity detection, mainly because metal conductivity detection faces many challenges. Traditional detection methods often require complex equipment and operating procedures, and have strict requirements on the detection environment, and the detection accuracy is difficult to meet actual needs. At the same time, some methods may be limited by factors such as material properties, specimen shape and size, and cannot be widely used in the conductivity detection of various metal materials. Summary of the invention

[0003] The present invention proposes a metal conductivity detection method based on pulse eddy current, adopts non-contact detection technology, based on pulse eddy current coupling technology, and utilizes the characteristic that the time constant of eddy current is insensitive to lift-off changes to perform conductivity detection on conductor specimens.

[0004] The technical problems to be solved are:

[0005] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A method for detecting metal conductivity based on pulse eddy current, the key technology of which is to follow the following steps:

[0007] Step 1: Use a pulsed eddy current detector to test N standard conductor specimens of the same size and known conductivity, and obtain the induced electromotive force output signals ε(t) of the N standard conductor specimens, where N is a natural number greater than or equal to 2;

[0008] Step 2: Convert the induced electromotive force output signal ε(t) of the N standard conductor specimens obtained in step 1 into the eddy current apparent time constant curve τ ATC (t), and determine all the apparent time constant curves τ based on the eddy current ATC (t) Peak time T tr and the peak value τ(T tr );

[0009] Step 3: Based on the conductivity of N standard conductor specimens and the corresponding eddy current apparent time constant curve τ ATC (t), the conductivity of the standard conductor specimen and the peak time T are obtained by fitting rr Or the peak value τ(T tr) relationship curve;

[0010] Step 4: Use a pulsed eddy current detector to measure the induced electromotive force output signal of the conductor specimen to be tested, and convert the induced electromotive force output signal of the conductor specimen to be tested into the eddy current apparent time constant curve τ of the conductor specimen to be tested. ATC (t), and determine the apparent time constant curve τ of the conductor specimen to be tested based on the eddy current ATC Peak time T tr and the peak value τ(T tr );

[0011] Step 5: According to the peak time T obtained in step 4 tr (or peak size τ(T tr )) Substitute the conductivity and peak time T of the standard conductor specimen obtained in step 3 into tr (or peak size τ(T tr )) is compared with the relationship curve to obtain the conductivity of the conductor specimen to be tested.

[0012] Among them, the time domain characteristics of the pulsed eddy current depend on the impedance characteristics of the conductor specimen under test. When the excitation disappears, the eddy current in the conductor specimen under test diffuses to the depth in the shape of a smoke ring under the effect of electromagnetic induction, and decays over time under the Joule effect of the conductor specimen resistance. Before the eddy current penetrates to half the thickness of the plate, the distribution of the eddy current density is independent of the thickness d of the conductive plate, and the penetration speed vz of the eddy current in the normal depth of the medium surface can be approximated as:

[0013]

[0014] Derivation of the penetration depth D in a uniform half-space:

[0015]

[0016] μ represents the permeability of the medium, σ is the conductivity of the medium, C2 is a constant, and t represents time, which is zero time when the pulse excitation disappears.

[0017] According to the PEC circuit coupling model, the relatively concentrated eddy current density in the inspected conductor specimen can be regarded as a current loop, and the dynamic behavior of the induced eddy current loop in the conductor specimen follows the time constant τ=L / R of the first-order inductive circuit, where the inductance L is affected by the magnetic permeability, electrical conductivity and size of the current loop of the material, and the resistance R depends on the resistivity of the material and the size of the current loop.

[0018] Considering the rich frequency components of the pulse eddy current, the attenuation coefficient of the equivalent current loop is called the apparent time constant. Due to the diffusion of the eddy current, the apparent time constant of the equivalent current loop increases monotonically with time. This apparent time constant curve τ ATCThe growth rate of (t) decreases rapidly to a certain value after the eddy current diffuses to 1 / 2 of the conductivity of the conductor specimen, and the corresponding moment is recorded as the peak time T tr , the corresponding τ ATC (t) The curve amplitude is recorded as the steady-state value τ(T tr ).

[0019] The present invention is based on the PEC circuit coupling model theory, starting from the disturbance mechanism of the conductivity of the tested object on the eddy current diffusion, and by finding out τ AT The peak time T of the transient characteristic value of the c(t) curve tr , steady-state value τ(T tr ) and the conductivity σ of the conductor specimen to solve the technical problem of non-contact conductivity detection.

[0020] Through the above design, the apparent time constant of the eddy current is used to relate the diffusion characteristics of the induced eddy current to the conductivity parameters of the tested object.

[0021] Furthermore, each of the standard conductor specimens corresponds to a metal conductivity; the metal conductivity of the standard conductor specimens ranges from 7.6 to 58 MS / m.

[0022] Furthermore, the standard conductor specimen and the conductor specimen to be tested are made of non-magnetic metal.

[0023] Furthermore, the pulse eddy current detector includes a pulse current transmitter, a detection probe, and a signal acquisition machine. The detection probe includes a transmitting coil and a receiving coil. The signal output end of the pulse current transmitter is connected to the transmitting coil, the signal acquisition end of the signal acquisition machine is connected to the receiving coil, and the signal feedback end of the signal acquisition machine is used to connect to a computer.

[0024] Among them, the pulse current transmitter generates a pulse current waveform, which is then loaded onto the excitation coil. When the detection probe is close to the metal being tested, the induced eddy current in the metal generates an induced electromotive force on the receiving coil, which is then input into the computer through the signal acquisition machine.

[0025] Furthermore, in step 2, the induced electromotive force output signal ε(t) of the standard conductor specimen is converted into an eddy current apparent time constant curve τ ATC The conversion formula of (t) is:

[0026]

[0027] If the above N output signals are converted into time constant curves τ respectively through the calculus characteristics of the exponential function ATC (t): The above formula is obtained by further improvement, that is, it has better adaptability to system noise through integral operation.

[0028] Furthermore, the peak time T tr is the apparent time constant curve based on eddy current τ ATC The growth rate of (t) corresponds to the time when the eddy current generated by the pulse eddy current detector rapidly rises to the peak value after diffusing to 1 / 2 of the conductivity of the conductor specimen; the peak time T tr Corresponding to the apparent time constant curve τ based on eddy current ATC The amplitude of (t) is the steady-state value τ(T tr ).

[0029] The beneficial effect of the present invention is that the constraint characteristics of the conductivity of the conductor specimen on the dynamic distribution of eddy current can be observed through the apparent time constant. Compared with the traditional method of obtaining the PEC response change rate, the time constant τ obtained by the proposed integral algorithm (3) is ATC The noise of the PEC signal will not be amplified. The metal conductivity is linearly related to the peak time or peak size characteristic quantity of the eddy current apparent time constant curve, so it has the advantages of easy acquisition of the standard relationship curve and high detection accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is the principle diagram of the pulsed eddy current detector in the present invention;

[0031] Figure 2 Based on the eddy current apparent time constant curve τ for metal plate specimens with various conductivity ATc (t)

[0032] Figure 3 is the conductivity of the metal plate specimen and the peak time T tr The relationship curve of

[0033] Figure 4 is the conductivity of the metal plate specimen and the peak value τ(T tr ) relationship curve;

[0034] Figure 5 It is a detection flow chart of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings.

[0036] Example:

[0037] In this embodiment, chromium, brass, metal and copper are selected as standard conductor specimens. These metals are selected as standard conductor specimens because they have different conductivity values ​​and cover the conductivity range applicable to the present invention, and can better fit the standard relationship curve. The conductivity of the metal specimens is 7.6MS / m, 15MS / m, 38MS / m and 58MS / m respectively. Bronze is selected as the metal plate specimen to be tested. Bronze is selected as the metal plate specimen to be tested in order to verify the applicability of the method of the present invention to different metal materials. The conductivity is 10MS / m. The following is the test content of the metal plate specimen to be tested:

[0038] A metal conductivity detection method based on pulsed eddy current, combined with Figure 3 , Figure 4 It can be seen that the following steps are followed:

[0039] Step 1: Use a pulsed eddy current detector to test four standard plate specimens with known conductivity, and obtain the induced electromotive force output signals ε(t) of the four standard plate specimens. The conductivity of the four standard metal plate specimens is 7.6MS / m, 15MS / m, 38MS / m and 58MS / m respectively. The other parameters of the four standard metal plate specimens are consistent.

[0040] In this embodiment, the pulsed eddy current detector uses a weak magnetic coupling sensor to collect the induced electromotive force of the test piece, which can avoid the interference of the excitation magnetic field on the data and enhance the measurement sensitivity under large lift-off conditions.

[0041] In this embodiment, the power of the weak magnetic coupling sensor is: 36W to 120W (12V, 3A to 10A)

[0042] In this embodiment, see Figure 1 The pulse eddy current detector includes a pulse current transmitter, a detection probe, and a signal acquisition machine. The detection probe includes a transmitting coil and a receiving coil. The signal output end of the pulse current transmitter is connected to the transmitting coil, the signal acquisition end of the signal acquisition machine is connected to the receiving coil, and the signal feedback end of the signal acquisition machine is used to connect to a computer.

[0043] Step 2: Convert the induced electromotive force output signals ε(t) of the four standard plate specimens obtained in step 1 into the eddy current apparent time constant curve τ ATC (t), determine all the eddy current apparent time constant curves τ ATC Peak time T tr Specific curves see Figure 2 ,

[0044] The peak time T tr The peak time T corresponds to the moment when the maximum eddy current density penetrates to half the conductivity of the metal.tr Corresponding to the apparent time constant curve τ based on eddy current ATC The amplitude of (t) is the peak value τ(T tr ).

[0045] In step 2, the induced electromotive force output signal ε(t) of the standard conductor specimen is converted into an eddy current apparent time constant curve τ ATC The conversion formula of (t) is:

[0046]

[0047] Solve the apparent time constant curve τ based on the eddy current by integration operation ATC (t), reducing the impact of signal noise on solution accuracy.

[0048] Step 3: Based on the conductivity of the four standard plate specimens and the corresponding eddy current apparent time constant curve τ ATC (t), the conductivity of the standard conductor specimen and the peak time T are obtained by fitting tr For details, see Figure 3 , the conductivity of the standard conductor specimen and the peak value τ(T tr ), see Figure 4 ,

[0049] Step 4: Use a pulsed eddy current detector to measure the induced electromotive force output signal of the metal plate specimen to be tested, and convert the induced electromotive force output signal of the metal plate specimen to be tested into the eddy current apparent time constant curve τ of the metal plate specimen to be tested ATC (t), see Figure 2 And determine the metal plate specimen to be tested based on the eddy current apparent time constant curve τ ATC (t) Peak time T tr , the peak time T tr Corresponding to the apparent time constant curve τ based on eddy current ATC The amplitude of (t) is the peak value τ(T tr );

[0050] Step 5: According to the peak time T obtained in step 4 tr or peak value τ(T tr ) is substituted into the value obtained in step 3. Figure 3 or Figure 4 The conductivity of the conductor specimen to be tested is obtained by comparison.

[0051] The key point of the present invention is based on the eddy current apparent time constant curve τ ATC (t) Peak time T tr or peak value τ(T tr ) to make conductivity calibration curve, and T tr or τ(Ttr ) are linearly related to resistivity. It should be noted that the standard conductor specimen used should be the same material and structure as the actual workpiece to be tested. This solution is suitable for conductivity testing of non-ferromagnetic metals.

[0052] The above are only preferred implementations of the present invention. It should be pointed out that a number of modifications and improved technical solutions made by those skilled in the art without departing from the technical solution should also be deemed to fall within the scope of protection required by the claims.

Claims

1. A method for detecting metal conductivity based on pulse eddy current, characterized in that: Follow these steps: Step 1: Use a pulsed eddy current detector to test N standard conductor specimens of the same size and known conductivity, and obtain the induced electromotive force output signals ε(t) corresponding to the N standard conductor specimens, where N is a natural number greater than or equal to 2; Step 2: Convert the induced electromotive force output signal ε(t) of the N standard conductor specimens obtained in step 1 into the eddy current apparent time constant curve τ ATC (t), and determine all the apparent time constant curves τ based on the eddy current ATC (t) Peak time T tr and the peak value τ(T tr ); Step 3: Based on the conductivity of N standard conductor specimens and the corresponding eddy current apparent time constant curve τ ATC (t), the conductivity of the standard conductor specimen and the peak time T are obtained by fitting tr and the peak value τ(T tr ) relationship curve; Step 4: Use a pulsed eddy current detector to measure the induced electromotive force output signal of the conductor specimen to be tested, and convert the induced electromotive force output signal of the conductor specimen to be tested into the eddy current apparent time constant curve τ of the conductor specimen to be tested. ATC (t), and determine the apparent time constant curve τ of the conductor specimen to be tested based on the eddy current ATC (t) Peak time T tr and the peak value τ(T tr ); Step 5: Substitute the peak time T obtained in step 4 tr Or the peak value τ(T tr ) is substituted into the conductivity and peak time T of the standard conductor specimen obtained in step 3. tr (or peak size τ(T tr )) is compared with the relationship curve to obtain the conductivity of the conductor specimen to be tested.

2. The metal conductivity detection method based on pulse eddy current according to claim 1 is characterized in that: Each of the standard conductor specimens corresponds to a metal conductivity; the metal conductivity range of the standard conductor specimens is 7.6-58MS / m.

3. The metal conductivity detection method based on pulse eddy current according to claim 1 is characterized in that: The standard conductor specimen and the conductor specimen to be tested are made of non-magnetic metal.

4. The metal conductivity detection method based on pulse eddy current according to claim 1 is characterized in that: The pulse eddy current detector includes a pulse current transmitter, a detection probe, and a signal acquisition machine. The detection probe includes a transmitting coil and a receiving coil. The signal output end of the pulse current transmitter is connected to the transmitting coil, the signal acquisition end of the signal acquisition machine is connected to the receiving coil, and the signal feedback end of the signal acquisition machine is used to connect to a computer.

5. The metal conductivity detection method based on pulse eddy current according to claim 1, characterized in that: In step 2, the induced electromotive force output signal ε(t) of the standard conductor specimen is converted into an eddy current apparent time constant curve τ ATC The conversion formula of (t) is:

6. The metal conductivity detection method based on pulse eddy current according to claim 1 or 5, characterized in that: The peak time T tr and the peak value τ(T tr ) is based on the eddy current apparent time constant curve τ ATC The growth rate of (t) corresponds to the moment when the eddy current generated by the pulsed eddy current detector rapidly rises to the peak value after diffusing to 1 / 2 thickness of the conductor specimen.

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