Pulse eddy current detection method suitable for metal blind hole depth under lift-off disturbance condition

Abstract

The invention relates to the technical field of nondestructive testing, in particular to a pulsed eddy current testing method suitable for the depth of a metal blind hole under the lift-off disturbance condition. The method comprises the following steps: S1, detecting a conductor test piece without a blind hole by adopting a pulsed eddy current detector to obtain an induced electromotive force output signal # imgabs0 # as a blank control group; the method can be used for pulsed eddy current detection of the depth of the metal blind hole under the lift-off disturbance condition, breaks through the limitation of a traditional method on the lift-off distance, is high in lift-off disturbance adaptability, can accurately detect in a large lift-off range, improves the detection precision through characteristic curve conversion and standard curve comparison, can obtain comprehensive defect information, and is suitable for popularization and application. In practical application, the method plays an important role in industrial production quality control, equipment maintenance safety detection and the like, the depth of the blind hole can be rapidly and accurately detected, the product quality and the equipment safety are guaranteed, the production efficiency is improved, and the cost is reduced.

Description

Technical Field

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

[0002] Based on the principle of electromagnetic induction, the pulsed eddy current (PEC) detection method passes a pulse current into an excitation coil to excite a pulse magnetic field, generating pulse eddy currents (transient eddy currents) in the conductor specimen in the magnetic field. The magnetic field generated by the pulse eddy currents induces a voltage signal on the detection coil that changes with time, thereby achieving the detection purpose.

[0003] Usually, the pulsed eddy current method determines the parameters and defects of the inspected material based on the amplitude and zero-crossing time of the response signal. However, this criterion is very sensitive to the intensity of the excitation signal and the lift-off disturbance, and is usually only applicable to metal defect detection within a lift-off of 5 mm and when the lift-off is fixed.

[0004] Based on this, a conductor blind hole depth detection method based on pulse eddy current is proposed to provide a technical solution to solve the above problems.

[0005] Lift-off disturbance refers to the change in the distance between the test probe and the surface of the conductor specimen being tested during pulsed eddy current testing. This distance change can have a significant impact on the test results because it changes the distribution and attenuation characteristics of the eddy current. When the lift-off distance increases, the intensity of the eddy current will weaken, and the curve shape of its decay time constant will also change, which in turn affects the reliability of the feature quantities extracted based on these curves for defect detection such as blind hole depth. Summary of the invention

[0006] The purpose of the present invention is to provide a conductor blind hole depth detection method based on pulse eddy current to solve the problems raised in the above background technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a conductor blind hole depth detection method based on pulsed eddy current, comprising the following steps:

[0008] Step 1: Use a pulsed eddy current detector to test the conductor specimen without blind holes, and obtain the induced electromotive force output signal ε0(t) as a blank control group;

[0009] Step 2: For N conductor specimens of the same specification with known blind hole depths, use a pulsed eddy current detector to test the top and back of the blind hole, and obtain the corresponding output signals ε of the induced electromotive force of the N surface blind holes. i1 (t) and the induced electromotive force output signal ε of N subsurface blind holes i2 (t), (i=1,2,3…N);

[0010] Step 3: Set ε0(t), ε i1 (t) and ε i2 (t) is converted into the characteristic curve τ0(t), τ based on the apparent time constant of eddy current decay i1 (t) and τ i2 (t), (i=1,2,3…N);

[0011] Step 4: τ i1 (t)-τ0(t) to obtain the differential time constant curve of N surface blind holes and extract its minimum value A i1 , (i=1,2,3…N), and τ i2 (t)-τ0(t) to obtain the differential time constant curve of N sub-surface blind holes and extract its minimum value A i2 ,(i=1,2,3…N);

[0012] Step 5: A i1 -A i2 Get the difference feature of N minimum values, and convert A i1 -A i2 Plotted as a standard curve against blind hole depth.

[0013] Step 6: Repeat S2 to S4 for the specimen with unknown blind hole depth within 50 mm of lift-off disturbance, obtain the difference characteristic quantity A1-A2 between the minimum value of the surface blind hole and the sub-surface blind hole, and finally determine the depth of the blind hole to be tested by comparing it with the standard curve.

[0014] Furthermore, the pulse eddy current detector includes a pulse current transmitter, a detection probe, and a signal acquisition machine. The pulse current transmitter and the signal acquisition machine are both connected to the detection probe, and the signal feedback end of the signal acquisition machine is used to connect to a computer.

[0015] Furthermore, 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, and the signal acquisition end of the signal acquisition machine is connected to the receiving coil.

[0016] Furthermore, in the third step, the induced electromotive force output signal ε(t) is converted into a characteristic curve τ based on the apparent time constant of eddy current decay. ATC The conversion formula of (t) is:

[0017]

[0018] In the formula, d represents the differential sign, and t is the reciprocal of the sampling frequency of the signal acquisition machine, that is, the sampling time step.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention can be used for pulse eddy current detection of the depth of metal blind holes under lift-off disturbance conditions, breaking through the limitation of lift-off distance by traditional methods, having strong adaptability to lift-off disturbances, and being able to accurately detect within a larger lift-off range. The detection accuracy is improved through characteristic curve conversion and standard curve comparison, and comprehensive defect information can be obtained. In practical applications, it plays an important role in industrial production quality control and equipment maintenance safety detection, and can quickly and accurately detect the depth of blind holes, ensure product quality and equipment safety, improve production efficiency and reduce costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the pulsed eddy current detection principle of the present invention;

[0022] Figure 2 The PEC characteristic curve of the present invention based on the apparent time constant of the attenuation of the induced eddy current;

[0023] Figure 3 This is a model diagram of the pulsed eddy current test piece of the present invention;

[0024] Figure 4 Schematic diagram of differential apparent time constant curve of surface defects and sub-surface defects of the present invention;

[0025] Figure 5 The characteristic value A of the present invention i1 -A i2 Schematic diagram of the change law with the depth of the blind hole;

[0026] Figure 6 It is a connection diagram of the detection probe of the present invention. DETAILED DESCRIPTION

[0027] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The "apparent time constant of eddy current decay" is an important parameter that describes the decay characteristics of eddy current in a conductor. During the pulse eddy current testing process, when a pulse current is passed through the excitation coil to generate a pulse magnetic field, the pulse eddy current generated in the conductor specimen will gradually decay over time. The apparent time constant of eddy current decay reflects the speed of this decay, which is related to factors such as the material, geometry, and internal defects of the conductor. By converting the induced electromotive force output signal to obtain a characteristic curve based on the apparent time constant of eddy current decay, the situation inside the conductor can be analyzed more intuitively, providing an important basis for detecting defects such as the depth of blind holes.

[0029] Please also read Figure 1-Figure 6 , a conductor blind hole depth detection method based on pulse eddy current, comprising the following steps:

[0030] Step 1: Use a pulsed eddy current detector to test the conductor specimen without blind holes, and obtain the induced electromotive force output signal ε0(t) as a blank control group;

[0031] Step 2: For N conductor specimens of the same specification with known blind hole depths, use a pulsed eddy current detector to test the top and back of the blind hole, and obtain the corresponding output signals ε of the induced electromotive force of the N surface blind holes. i1 (t) and the induced electromotive force output signal ε of N subsurface blind holes i2 (t), (i=1,2,3…N);

[0032] Step 3: Set ε0(t), ε i1 (t) and ε i2 (t) is converted into the characteristic curve τ0(t), τ based on the apparent time constant of eddy current decay i1 (t) and τ i2 (t), (i=1,2,3…N), such as Figure 2 As shown;

[0033] The conversion formula is as follows:

[0034]

[0035] In the formula, d represents the differential sign, and t is the reciprocal of the sampling frequency of the signal acquisition machine, that is, the sampling time step.

[0036] Step 4: τ i1 (t)-τ0(t) to obtain the differential time constant curve of N surface blind holes and extract its minimum value A i1 , (i=1,2,3…N), and τ i2 (t)-τ0(t) to obtain the differential time constant curve of N sub-surface blind holes and extract its minimum value A i2 ,(i=1,2,3…N);

[0037] Step 5: A i1 -A i2 Get the difference feature of N minimum values, and convert A i1 -A i2 Plotted as a standard curve against blind hole depth.

[0038] Step 6: Repeat S2 to S4 for the unknown blind hole depth of the test piece with a lift-off disturbance of less than 50 mm, obtain the minimum difference characteristic value A1-A2 between the surface blind hole and the sub-surface blind hole, and finally determine the depth of the blind hole to be tested by comparing it with the standard curve. Step 3: Convert the induced electromotive force output signal ε(t) into a characteristic curve τ based on the eddy current decay time constant ATC (t);

[0039] See also Figure 6 , in this embodiment, the pulse eddy current detector includes a pulse current transmitter, a detection probe, and a signal acquisition machine;

[0040] Here, 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.

[0041] In this embodiment, an aluminum plate is selected as a conductor specimen. Figure 1 As shown, the PEC sensor located on the z = 100mm plane consists of a transmitting coil and a receiving coil, whose radii are 50mm and 20mm respectively. A 400mm×400mm×10mm metal is placed on the XOY plane. With (x,y) = 0,60)mm as the center, a 60×10×dmm blind hole parallel to the y axis is set on the upper surface of the metal plate, where the blind hole depth d is 2mm, 3mm, 4mm, 5mm, 6mm, 7mm and 8mm respectively;

[0042] Set up 8 standard aluminum plate specimens with known blind hole depths, of which one aluminum plate as the control group has no blind hole defects, and the blind hole depths of the other 7 standard aluminum plate specimens are 2mm, 3mm, 4mm, 5mm, 6mm, 7mm and 8mm respectively. The other parameters of the 8 standard aluminum plate specimens are the same. The pulse eddy current detector is used to test the top and back of the blind holes of the 7 standard aluminum plate specimens and the position of the blind holes of the aluminum plate of the blank control group, and the induced electromotive force output signal ε(t) of the corresponding 15 standard aluminum plate specimens is obtained;

[0043] The induced electromotive force output signal ε(t) of 15 standard aluminum plate specimens is converted into the apparent time constant curve τ based on eddy current decay. ATC (t), the 14 τ ATC (t) Compared with the control group without blind hole defects ATC (t) Subtract to obtain 14 differential characteristic curves, and determine the minimum value A of the differential characteristic curves corresponding to the 7 types of surface blind holes i1 The minimum value of the differential characteristic curve corresponding to the 7 sub-surface blind holes A i2; see the specific curve Figure 4 ;

[0044] Convert the induced electromotive force output signal ε(t) of the standard conductor specimen into a curve τ based on the apparent time constant of eddy current decay ATC The conversion formula of (t) is:

[0045]

[0046] Solve the eddy current decay time constant curve τ by integral operation ATC (t), reduce the impact of signal noise on solution accuracy;

[0047] According to the blind hole depth and corresponding characteristic value A of 7 standard aluminum plate specimens i1 -A i2 , the blind hole depth and characteristic value A of the standard conductor specimen are obtained by fitting i1 -A i2 For details, see Figure 5 ;

[0048] The pulsed eddy current detector is used to measure the induced electromotive force output signal of the aluminum plate specimen to be tested (in this case, 3mm, 5mm and 8mm deep blind holes are used as examples) on the plane of z = 50mm, 60mm and 80mm, and the induced electromotive force output signal of the aluminum plate specimen to be tested is converted into a differential characteristic curve of the aluminum plate specimen to be tested based on the eddy current decay time constant;

[0049] Substitute the difference A1-A2 between the minimum value A1 of the differential characteristic curve corresponding to the surface blind hole obtained in step 4 and the minimum value A2 of the differential characteristic curve corresponding to the subsurface blind hole into the blind hole depth and A of the standard conductor specimen obtained in step 5. i1 -A i2 The relationship curve is compared, such as Figure 5 As shown, the blind hole depth of the conductor specimen to be tested is obtained.

[0050] The lift-off disturbance will significantly change the curve of the eddy current decay time constant τ ATC (t), thereby destroying the minimum value A of the differential characteristic curve i1 The reliability of blind hole depth detection based on characteristic quantities.

[0051] Depend on Figure 4 It can be seen that the difference A between the minimum values ​​of the surface blind hole and the subsurface blind hole under the same lift-off parameters is i1 -A i2 It increases monotonically with the increase of blind hole depth, Figure 5 It can be seen that the difference A of the minimum characteristic quantity under the same blind hole depth condition i1 -A i2The data under the lift-off condition of 80mm is plotted as red "○", the data under the lift-off condition of 70mm is displayed as black "+", the data under the lift-off condition of 60mm is designated as green "☆", and the data under the lift-off condition of 50mm is marked as pink "□". Figure 5 It can be seen that the difference in the minimum value of the differential D-ATC curve of the same blind hole depth is distributed more concentratedly under the five lift-off conditions. This characteristic creates the possibility for quantifying the blind hole depth under large lift-off disturbance.

[0052] Taking a 3mm deep blind hole as an example, the maximum offset of the difference in the minimum value under lift-off disturbance is +5.26μs, while the minimum difference feature of a 4mm deep blind hole under 100mm lift-off is +8.96μs away from the feature of a 3mm deep blind hole. Therefore, the offset of the feature caused by lift-off disturbance is less than 60% of the difference in the feature corresponding to the 1mm blind hole depth change. Therefore, the difference in the minimum time of the differential D-ATC curve can be used as a feature for the depth detection of aluminum plate blind holes under large lift-off disturbance.

[0053] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0054] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A conductor blind hole depth detection method based on pulsed eddy current, characterized in that: The following steps are involved: S1: The conductor specimen without blind holes is tested by pulse eddy current detector, and the induced electromotive force output signal ε0(t) is obtained as the blank control group; S2: For N conductor specimens of the same specification with known blind hole depths, a pulsed eddy current detector is used to test the top and back of the blind holes, and the corresponding output signals ε of the induced electromotive force of the N surface blind holes are obtained. i1 (t) and the induced electromotive force output signal ε of N subsurface blind holes i2 (t), (i=1,2,3…N); S3: ε0(t), ε i1 (t) and ε i2 (t) is converted into the characteristic curve τ0(t), τ based on the apparent time constant of eddy current decay i1 (t) and τ i2 (t), (i=1,2,3…N); S4: τ i1 (t)-τ0(t) to obtain the differential time constant curve of N surface blind holes and extract its minimum value A i1 , (i=1,2,3…N), and τ i2 (t)-τ0(t) to obtain the differential time constant curve of N sub-surface blind holes and extract its minimum value A i2 ,(i=1,2,3…N); S5: A i1 -A i2 Get the difference feature of N minimum values, and convert A i1 -A i2 Plotted as a standard curve against blind hole depth. S6: Repeat S2 to S4 for the specimen with unknown blind hole depth within 50 mm of lift-off disturbance, obtain the difference characteristic quantity A1-A2 between the minimum value of the surface blind hole and the sub-surface blind hole, and finally determine the depth of the blind hole to be tested by comparing it with the standard curve.

2. A method for detecting the depth of a conductor blind hole based on pulsed eddy current according to claim 1, characterized in that: The pulse eddy current detector comprises a pulse current transmitter, a detection probe and a signal acquisition machine. The pulse current transmitter and the signal acquisition machine are both connected to the detection probe, and the signal feedback end of the signal acquisition machine is used to connect to a computer.

3. The method for detecting the depth of a conductor blind hole based on pulsed eddy current according to claim 2, characterized in that: The detection probe comprises a transmitting coil and a receiving coil. The signal output end of the pulse current transmitter is connected to the transmitting coil, and the signal acquisition end of the signal acquisition machine is connected to the receiving coil.

4. The method for detecting the depth of a conductor blind hole based on pulsed eddy current according to claim 1, characterized in that: In S3, the induced electromotive force output signal ε(t) is converted into a characteristic curve τ based on the apparent time constant of eddy current decay ATC The conversion formula of (t) is: In the formula, d represents the differential sign, and t is the reciprocal of the sampling frequency of the signal acquisition machine, that is, the sampling time step.

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