A pulsed eddy current probe and detection method for eliminating lift-off effect

Abstract

This invention discloses a pulsed eddy current probe that eliminates the lift-off effect, comprising a housing, within which are an excitation coil, a first receiving coil, and a second receiving coil. The coil spacing between the excitation coil and the first receiving coil is fixed; however, the coil spacing between the excitation coil and the second receiving coil is adjustable. Different spacings between the first and second receiving coils and the excitation coil allow for the acquisition of transient magnetic fields at different spatial locations. The excitation coil is connected to an excitation circuit, and the first and second receiving coils are connected to a signal conditioning circuit. This invention proposes a pulsed eddy current probe and detection method that eliminates the lift-off effect. It can acquire the crossover point of a characteristic quantity-differential signal independent of lift-off, and this characteristic quantity can be acquired in a single detection, overcoming the influence of the lift-off effect on quantitative defect analysis and improving the detection accuracy of pulsed eddy currents.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic nondestructive testing technology, specifically to a pulsed eddy current probe and testing method that eliminates the lift-off effect. Background Technology

[0002] Metal components are widely used in industries such as petrochemicals and power. During use, due to the interaction of environment, load, and materials, defects such as corrosion or cracks may occur, affecting their safe operation. Therefore, regular inspection of metal components is necessary. Pulsed eddy current testing technology has been widely used in the inspection of metal components in recent years due to its advantages such as high speed, high safety, and low cost.

[0003] Pulsed eddy current testing technology uses pulsed current as the excitation signal. When the excitation current jumps from a high (low) level to zero, a magnetic field is induced in space. This magnetic field includes a primary magnetic field generated by the jump in excitation current and a secondary magnetic field generated by the induced eddy currents in the component. The latter is related to defects in the component, and its measurement and analysis using a magnetically sensitive element (such as a coil) can obtain information about the component's defects. According to the principle of pulsed eddy current testing, any factor affecting the induced eddy currents will affect the test results. Since the mutual inductance between the coil and the measured object decreases rapidly with increasing lift of the probe coil from the surface of the measured object, and the eddy current density in the measured object also changes significantly with small changes in lift (this effect is called the lift-off effect), the pulsed eddy current testing signal is easily affected by the lift-off effect. Furthermore, the influence of the lift-off effect on the signal often overlaps with the defect signal and can easily overwhelm the defect signal, thereby reducing the detection accuracy and hindering on-site testing. Therefore, there is an urgent need to study methods and probes to eliminate the lift-off effect.

[0004] The main methods for designing and optimizing pulsed eddy current probes and identifying signal characteristic quantities are as follows. Currently, differential probes are commonly used in conjunction with characteristic quantity-lift-off crossover points to reduce the lift-off effect. A typical structure of a differential probe is shown below. Figure 5 As shown, a typical pulsed eddy current differential probe usually consists of an excitation coil 1, a first receiving coil 2, and a second receiving coil 3, with the distance between the excitation coil 1 and the first receiving coil 2 being equal to the distance between the excitation coil 1 and the second receiving coil 3. The output of the probe is the differential signal between the first receiving coil 2 and the second receiving coil 3. Therefore, when the first receiving coil 2 is located in the defect-free area of ​​the specimen and the second receiving coil 3 is located at the defect location 4 of the specimen, the output result is the defect signal. Figure 6The differential signals from the first receiving coil 2 and the second receiving coil 3 are shown in the figure when the probe lift-off is 2 mm, 4 mm, and 6 mm. As can be seen from the figure, the differential probe cannot directly eliminate the lift-off effect, but the differential signals under different lift-off conditions intersect at a single point, namely the lift-off crossover point, which is independent of the probe lift-off. Based on this, using a differential probe in conjunction with the lift-off crossover point can reduce the influence of the lift-off effect. However, to obtain the lift-off crossover point, it is necessary to obtain the differential signals of the probe under two or more lift-off conditions, i.e., it is necessary to repeatedly measure the same position under different lift-off conditions, which reduces detection efficiency. Therefore, the above-mentioned probe and feature quantity are inconvenient for field use. Summary of the Invention

[0005] 1. The technical problem to be solved:

[0006] To address the aforementioned technical problems, this invention provides a pulsed eddy current probe and detection method for eliminating the lift-off effect.

[0007] 2. Technical Solution:

[0008] A pulsed eddy current probe for eliminating lift-off effect includes a housing, within which are an excitation coil, a first receiving coil, and a second receiving coil. The coil spacing between the excitation coil and the first receiving coil is fixed; however, the coil spacing between the excitation coil and the second receiving coil is adjustable. Different spacings between the first and second receiving coils and the excitation coil allow for the acquisition of transient magnetic fields at different spatial locations. The excitation coil is connected to an excitation circuit, and the first and second receiving coils are connected to a signal conditioning circuit. The excitation circuit generates a square wave current with a certain duty cycle, which acts on the excitation coil. When the square wave current is at its rising or falling edge, the change in current causes the excitation coil to generate a changing magnetic field. This changing magnetic field acts on the conductor specimen being tested, inducing eddy currents. The diffusion of these eddy currents in the conductor specimen further generates a transient magnetic field, i.e., a secondary magnetic field, in space. The first and second receiving coils acquire the secondary magnetic fields at different spatial locations, convert them into voltage signals, and after amplification and filtering by the signal conditioning circuit, convert them into digital signals for input to a computer.

[0009] Furthermore, the housing consists of a base and an end cap; the base has a first slot, a second slot, and a first sliding groove; the excitation coil, the first receiving coil, and the second receiving coil are wound on the first coil frame, the second coil frame, and the third coil frame, respectively; the first coil frame is engaged in the first slot; the second coil frame is engaged in the second slot; the third coil frame is engaged in the probe holder; the probe holder is located in the first sliding groove and is fixed to two bosses by bolts; the two bosses are located on both sides of the first sliding groove; each of the two bosses has a limiting block; a guide groove is formed between the two limiting blocks; the probe holder is slidably connected in the guide groove; the bosses have threaded holes, and the probe holder has oblong holes, and the bolts pass through the oblong holes of the probe holder and are threaded into the threaded holes.

[0010] The end cap has two sockets, in which a first aviation plug and a second aviation plug are installed respectively. The excitation coil is connected to the excitation circuit through the first aviation plug; the first receiving coil and the second receiving coil are connected to the signal conditioning circuit through the second aviation plug.

[0011] Furthermore, the first coil frame, the second coil frame, and the third coil frame are all composed of a winding rod, an upper plate, and a lower plate; the upper plate and the lower plate of the first coil frame and the second coil frame are provided with buckles.

[0012] Furthermore, the end cap is provided with a third slot, a fourth slot, and a second sliding groove; the first coil frame is engaged in the third slot; the second coil frame is engaged in the fourth slot; and the upper end of the third coil frame is located in the second sliding groove.

[0013] A pulsed eddy current detection method for eliminating lift-off effect includes the following steps:

[0014] Step 1: Place the first receiving coil and the second receiving coil in the defect-free area of ​​the conductor specimen to be tested, and obtain the induced voltage signal of the defect-free area, i.e., the reference signal;

[0015] Step 2: Place the first receiving coil and the second receiving coil in the area to be tested of the conductor specimen, and obtain the induced voltage signal of the area to be tested, i.e. the detection signal;

[0016] Step 3: Differentiate the detection signal with the reference signal to obtain the differential signal; differentiate the differential signal to obtain the differential detection signals of the first receiving coil and the second receiving coil; plot the signal curve waveforms of the differential detection signals of the first receiving coil and the second receiving coil as a function of time; the signal curves of the differential detection signals of the first receiving coil and the second receiving coil as a function of time intersect at one point, which is the differential signal intersection point;

[0017] Step 4: The horizontal axis corresponding to the differential signal crossover point is the differential signal crossover point time. The differential signal crossover point time is directly proportional to the thickness of the conductor specimen. The thinner the specimen, the more severe the defect. Furthermore, the differential signal crossover point is unrelated to lift-off and is not affected by the lift-off effect. The defect situation can be directly reflected by the magnitude of the differential signal crossover point time.

[0018] 3. Beneficial effects:

[0019] This invention proposes a pulsed eddy current probe and detection method to eliminate the lift-off effect. It can obtain the cross-point of the feature quantity-differential signal that is independent of the lift-off effect, and this feature quantity can be obtained in only one detection. This overcomes the influence of the lift-off effect on the quantitative analysis of defects and improves the detection accuracy of pulsed eddy current. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the front structure of the probe in Embodiment 1 of the present invention;

[0021] Figure 2 This is a schematic diagram of the connection structure between the base and the probe holder in Embodiment 1 of the present invention;

[0022] Figure 3 This is a schematic diagram of the back structure of the probe in Embodiment 1 of the present invention;

[0023] Figure 4 This is a schematic diagram of the structure of the first coil frame, the second coil frame, and the third coil frame in Embodiment 1 of the present invention;

[0024] Figure 5 This is a schematic diagram of a typical pulsed eddy current differential probe structure in the background technology;

[0025] Figure 6 The background section shows the waveforms of pulsed eddy current differential signals under different lift-off conditions.

[0026] Figure 7 This is a structural diagram of the stepped plate specimen of Embodiment 2 of the present invention;

[0027] Figure 8 This is a waveform diagram of the signal curve when the probe of Embodiment 2 of the present invention detects an aluminum stepped plate specimen;

[0028] Figure 9 This is a waveform diagram of the signal curve when the probe of Embodiment 2 of the present invention detects a carbon steel stepped plate specimen;

[0029] Figure 10 This is a graph showing the relationship between the differential signal crossover time and the lift-off in Embodiment 2 of the present invention;

[0030] Figure 11 This is a graph showing the relationship between the time of the differential signal crossover point and the thickness in Embodiment 2 of the present invention. Detailed Implementation

[0031] The present invention will now be described in detail with reference to the accompanying drawings.

[0032] Example 1

[0033] As attached Figure 1-4 As shown, a pulsed eddy current probe for eliminating lift-off effect includes a housing 1, within which are an excitation coil, a first receiving coil, and a second receiving coil. The coil spacing between the excitation coil and the first receiving coil is fixed. The coil spacing between the excitation coil and the second receiving coil is adjustable. Different spacings between the first and second receiving coils and the excitation coil allow for the acquisition of transient magnetic fields at different spatial locations. The excitation coil is connected to an excitation circuit, and the first and second receiving coils are connected to a signal conditioning circuit. The excitation circuit generates a square wave current with a certain duty cycle, which acts on the excitation coil. When the square wave current is at its rising or falling edge, the change in current causes the excitation coil to generate a changing magnetic field. This changing magnetic field acts on the conductor specimen being tested, inducing eddy currents. The diffusion of eddy currents in the conductor specimen further generates a transient magnetic field, i.e., a secondary magnetic field, in space. The first and second receiving coils acquire the secondary magnetic fields at different spatial locations, convert them into voltage signals, and after amplification and filtering by the signal conditioning circuit, convert them into digital signals for input to a computer.

[0034] The housing 1 consists of a base and an end cap; the base has a first slot 2, a second slot 3 and a first sliding groove 4; the excitation coil, the first receiving coil and the second receiving coil are respectively wound on the first coil frame 5, the second coil frame 6 and the third coil frame 7; the first coil frame 5 is engaged in the first slot 2; the second coil frame 6 is engaged in the second slot 3; the third coil frame 7 is engaged in the probe holder 8; the probe holder 8 is located in the first sliding groove 4 and is fixed to two bosses 9 by bolts; the two bosses 9 are located on both sides of the first sliding groove 4; each of the two bosses 9 is provided with a limiting block 10; a guide groove is formed between the two limiting blocks 10; the probe holder 8 is slidably connected in the guide groove; the bosses 9 are provided with threaded holes 11, the probe holder 8 is provided with oblong holes 12, and the bolt passes through the oblong holes 12 of the probe holder 8 and is threaded into the threaded holes 11.

[0035] The end cap has two sockets, in which a first aviation plug 13 and a second aviation plug 14 are installed respectively. The excitation coil is connected to the excitation circuit through the first aviation plug 13; the first receiving coil and the second receiving coil are connected to the signal conditioning circuit through the second aviation plug 14.

[0036] The first coil frame 5, the second coil frame 6, and the third coil frame 7 are all composed of a winding rod 15, an upper plate 16, and a lower plate 17; the upper plate 16 and the lower plate 17 of the first coil frame 5 and the second coil frame 6 are provided with buckles 18.

[0037] The end cap is provided with a third slot 19, a fourth slot 20 and a second sliding groove 21; the first coil frame 5 is engaged in the third slot 19; the second coil frame 6 is engaged in the fourth slot 20; the upper end of the third coil frame 7 is located in the second sliding groove 21.

[0038] In this embodiment, the pulsed eddy current probe is used by selecting a certain area of ​​the conductor specimen to be tested as the defect-free area and another area of ​​the conductor specimen to be tested as the area to be tested. The entire probe (including the first receiving coil and the second receiving coil) is placed in the defect-free area to acquire the induced voltage signal of the defect-free specimen, i.e., the reference signal. Then, the probe is moved and placed in the area to be tested of the conductor specimen to acquire the induced voltage signal of the area to be tested, i.e., the detection signal. The detection signal is differentially divided with the reference signal to obtain the differential detection signal. The differential signal is differentiated to obtain the differential detection signal of the first receiving coil and the second receiving coil. The waveform of the differential detection signal of the first receiving coil and the second receiving coil as a function of time is plotted. The two differential detection signal curves intersect at a point, i.e., the differential signal crossover point. The horizontal axis corresponding to the differential signal crossover point is the differential signal crossover point time. The differential signal crossover point time is proportional to the thickness of the conductor specimen; the thinner the specimen, the more severe the defect. The differential signal crossover point is independent of lift-off and is not affected by the lift-off effect. The magnitude of the differential signal crossover point time can directly reflect the defect situation.

[0039] Example 2

[0040] Verification experiment: Using the probe from Example 1 to... Figure 7 The stepped plate specimen shown was tested. The material of the stepped plate specimen can be a non-ferromagnetic material (such as aluminum) or a ferromagnetic material (such as carbon steel). The stepped plate specimen is divided into 6 steps, each 120 mm long and wide, with thicknesses of 2.0 mm, 4.0 mm, 6.0 mm, 8.0 mm, 10.0 mm, and 12.0 mm, respectively. Different degrees of corrosion are simulated by the thickness of the steps. Assuming that the 12.0 mm thick step is a defect-free area, then the steps of other thicknesses are defective areas compared to the 12.0 mm thick step. The thinner the step, the more severe the corrosion in the defective area.

[0041] The probe from Example 1 was used to detect the defect-free area of ​​the stepped plate specimen to obtain a reference signal; then, other defective areas were detected to obtain detection signals; differential processing was performed to obtain a differential signal, which was then differentiated to obtain a differential detection signal. The waveforms of the differential detection signals of the first and second receiving coils over time were plotted, with the waveform of the signal obtained after detecting the aluminum stepped plate specimen shown below. Figure 8 As shown, the signal curve waveform obtained after testing the carbon steel stepped plate specimen is as follows. Figure 9 As shown. Among them, by Figure 8 and Figure 9 It can be seen that when the object being detected is a non-ferromagnetic or ferromagnetic material, the signal curves of the first receiving coil and the second receiving coil intersect at a single point, namely the differential signal crossover point; the horizontal axis corresponding to the differential signal crossover point is the time of the differential signal crossover point.

[0042] Signal waveforms under different lift-off conditions were acquired using the probe from Example 1. The relationship between the differential signal crossover time and the lift-off was analyzed, and the results are as follows: Figure 10 As shown. By Figure 10 It can be seen that the crossover time of the differential signal is almost independent of the lift-off.

[0043] Similarly, the probe of Example 1 was used to acquire signal curve waveforms in detection areas of different thicknesses, and the relationship between the differential signal crossover time and the specimen thickness was analyzed. The results are as follows: Figure 11 As shown. Figure 11 This indicates that the time of the differential signal crossover point is directly proportional to the specimen thickness.

[0044] Combination Figure 10 and Figure 11 It can be seen that the pulse eddy current detection method and probe of this embodiment 1 can eliminate the lift-off effect and improve the detection accuracy of pulse eddy current.

[0045] Although the present invention has been disclosed above with reference to preferred embodiments, these are not intended to limit the invention. Any person skilled in the art can make various changes or modifications without departing from the spirit and scope of the invention. Therefore, the scope of protection of the present invention should be defined by the scope of the claims of this application.

Claims

1. A method of eddy current testing for eliminating lift-off effect, characterized in that: The application relates to a pulsed eddy current probe for eliminating the lift-off effect; the pulsed eddy current probe for eliminating the lift-off effect comprises a shell, an excitation coil, a first receiving coil and a second receiving coil are arranged in the shell; the coil spacing of the excitation coil and the first receiving coil is a fixed value; the coil spacing of the excitation coil and the second receiving coil can be adjusted; the spacings of the first receiving coil and the second receiving coil from the excitation coil are different, and transient magnetic fields at different spatial positions can be obtained; the excitation coil is connected with an excitation circuit, and the first receiving coil and the second receiving coil are connected with a signal conditioning circuit; the excitation circuit generates a square wave current with a certain duty ratio to act on the excitation coil; when the square wave current is at a rising edge or a falling edge, the change of the current causes the excitation coil to generate a changing magnetic field; the changing magnetic field acts on a conductor specimen to be detected to cause the conductor specimen to induce eddy current; the diffusion of the eddy current in the conductor specimen further generates a transient magnetic field, i.e. a secondary magnetic field, in space; the first receiving coil and the second receiving coil are used to respectively obtain the secondary magnetic fields at different spatial positions, the secondary magnetic fields are converted into voltage signals, and the voltage signals are converted into digital signals after being amplified and filtered by the signal conditioning circuit and then input into a computer. The application also relates to a pulsed eddy current detection method for eliminating the lift-off effect. Step 1: the first receiving coil and the second receiving coil are placed in a defect-free area of a conductor specimen to be detected to obtain an induced voltage signal of the defect-free area, i.e. a reference signal; Step 2: the first receiving coil and the second receiving coil are placed in a detection area of the conductor specimen to be detected to obtain an induced voltage signal of the detection area, i.e. a detection signal; Step 3: the detection signal and the reference signal are differentiated to obtain a differential signal; the differential signal is differentiated to obtain differential detection signals of the first receiving coil and the second receiving coil; a signal curve waveform diagram of the differential detection signals of the first receiving coil and the second receiving coil changing with time is drawn; the signal curves of the differential detection signals of the first receiving coil and the second receiving coil intersect at a point, i.e. a differential signal intersection point; Step 4: the abscissa corresponding to the differential signal intersection point is a differential signal intersection point time; the differential signal intersection point time is proportional to the thickness of the conductor specimen; the thinner the thickness, the more serious the defect; the differential signal intersection point is irrelevant to the lift-off and is not affected by the lift-off effect; the size of the differential signal intersection point time can directly reflect the defect condition.

2. The method of claim 1, wherein, The shell is composed of a base and an end cover; the base is internally provided with a first clamping groove, a second clamping groove and a first sliding groove; the excitation coil, the first receiving coil and the second receiving coil are respectively wound on a first coil skeleton, a second coil skeleton and a third coil skeleton; the first coil skeleton is clamped in the first clamping groove; the second coil skeleton is clamped in the second clamping groove; the third coil skeleton is clamped in a probe holder; the probe holder is located in the first sliding groove and is fixed on two bosses through bolts; the two bosses are located on the two sides of the first sliding groove; limiting blocks are arranged on the two bosses; a guide groove is formed between the two limiting blocks; the probe holder is slidingly connected in the guide groove; threaded holes are arranged on the bosses, and a waist-shaped hole is arranged on the probe holder; the bolts are threadedly connected in the threaded holes after passing through the waist-shaped hole of the probe holder.

3. A method of eddy current testing for eliminating lift-off effect according to claim 2, characterized in that, Two insertion holes are arranged on the end cover, and a first jack and a second jack are respectively arranged in the two insertion holes, and the excitation coil is connected with the excitation circuit through the first jack; the first receiving coil and the second receiving coil are connected with the signal conditioning circuit through the second jack.

4. A method of eddy current testing for eliminating lift-off effect according to claim 2 or 3, characterized in that, The first coil former, the second coil former and the third coil former are all composed of a winding rod, an upper disc body and a lower disc body; buckles are arranged on the upper disc body and the lower disc body of the first coil former and the second coil former.

5. A method of eddy current testing for eliminating lift-off effect according to claim 4, characterized in that, Third and fourth buckling grooves and a second sliding groove are arranged on the end cover; the first coil former is buckled in the third buckling groove; the second coil former is buckled in the fourth buckling groove; and the upper end of the third coil former is located in the second sliding groove.

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