A zero magnetic field leakage detection system based on a magnetic shielding structure

Abstract

The application discloses a zero magnetic field leakage detection system based on a magnetic shielding structure. When the zero magnetic field leakage detection system scans a defect of a tested piece, the mobile carrier is placed on the tested piece first, and then the mobile carrier is controlled to move uniformly on the tested piece. In the moving process, the magnetizer magnetizes the tested piece to saturation. At this time, the magnetic flux of the permanent magnet is divided into two routes. One route spreads to the inside of the tested piece, and the other route spreads in the air. In the spreading process of the magnetic flux in the tested piece, when the defect position is encountered, the magnetic conductivity of the defect position is distorted, the magnetic flux produces magnetic refraction leakage to the air, and simultaneously, the magnetic diffusion and the magnetic compression effect brought by the background magnetic field are accompanied, and finally, the leakage magnetic field is formed above the defect. The magnetic sensor captures the leakage magnetic signal and sends the leakage magnetic signal to the PC end, so that the shape and size of the defect are judged.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnetic flux leakage nondestructive testing, and more particularly relates to a zero magnetic field magnetic flux leakage detection system based on a magnetic shielding structure. BACKGROUND

[0002] Nondestructive testing refers to a detection means for evaluating the integrity of a measured object by using a physical method without damaging the measured object, and is an important guarantee for system safety and reliability. It is widely used in industries, aerospace, nuclear industry, such as defect detection of materials for rail, oil and gas pipelines, nuclear industry, aerospace, etc. Magnetic flux leakage detection is one of common nondestructive testing methods. It is concerned by domestic and foreign scholars due to its high reliability, adaptability to complex environment, and ability to simultaneously detect defects of inner and outer walls of a pipeline with a wall thickness of 30 mm or less.

[0003] Magnetic flux leakage detection is performed under the condition that a test piece is externally magnetized to saturation, and a leakage magnetic field is formed at a defect position through magnetic refraction, magnetic diffusion, and magnetic compression. The size of the leakage magnetic field is affected by the magnetic saturation degree of the test piece, the defect shape and size, and the background magnetic field size. When a magnetizing device operates on the test piece, if the surface of the test piece is smooth and defect-free, the magnetic force lines pass through the inside of the test piece. When passing through the defect position, the magnetic induction lines are blocked at the defect due to the distortion of the magnetic permeability, and are refracted at the defect side, and are diffused into the air on the defect surface. However, due to the existence of a strong background magnetic field, the interaction between the magnetic fields causes the magnetic force lines to be squeezed under the action of another magnetic field, resulting in magnetic diffusion and reverse magnetic compression. In summary, the leakage magnetic field is formed by magnetic refraction, magnetic diffusion, and magnetic compression. By using a sensor to detect the leakage magnetic field, the amplitude and gradient change of the leakage magnetic field can be extracted, and the defect can be qualitatively and quantitatively analyzed. The sensitivity of the magnetic flux leakage sensor and the lift-off height have always been a hot spot in the field of magnetic flux leakage detection. Whether the magnetic sensor based on measuring the absolute magnetic field size or the gradient magnetic sensor, they are all advancing to a more sensitive and higher lift-off detection field. The advantage of the magnetic flux leakage detection based on magnetic shielding is that the background magnetic field approaches 0, the diffusion range of the leakage magnetic field is larger, and the signal-to-noise ratio is better when the magnetic sensor is used to measure the leakage magnetic field.

[0004] The distance between the probe and the test piece is called the lift-off, and the change of the lift-off will greatly interfere with the defect. The detection signal of the defect under a large lift-off becomes weak, greatly increasing the detection difficulty. The existing magnetic flux leakage detection sensor is difficult to overcome the signal detection under a large lift-off. Only by using the magnetic shielding method, reducing the background magnetic field at the leakage magnetic field, and making the diffusion range of the leakage magnetic field larger, can the detection signal value be better under the same lift-off. SUMMARY

[0005] The purpose of the present application is to overcome the deficiencies of the prior art, and provide a zero magnetic field leakage detection system based on a magnetic shielding structure, which improves the magnetic shielding cover structure to make the size of the two vector direction magnetic fields in the shielding range tend to 0, so that the sensor can detect under a larger lift, and the signal-to-noise ratio of the detection signal is better.

[0006] To achieve the above-mentioned purpose of the application, the zero magnetic field leakage detection system based on a magnetic shielding structure comprises a mobile carrier, a magnetizer, a magnetic shielding cover and a magnetic sensor.

[0007] The mobile carrier is used for carrying the magnetizer and the magnetic sensor; a hollow rectangular frame is arranged at each of the left and right ends of the mobile carrier, and a permanent magnet is arranged in each of the rectangular frames; two parallel rectangular strips are mounted on the top of the rectangular frames, so as to connect the two rectangular frames into a whole, and a support frame with a sliding wheel is mounted on the outer side surface of each of the two rectangular frames, so that the mobile carrier is similar to a moving trolley; two rectangular holes are arranged on the inner side surface of each of the two rectangular frames, and a magnetic yoke is arranged in each of the rectangular holes, and the two ends of the magnetic yoke are located directly above the permanent magnets; a group of screw holes are arranged in the middle of each of the rectangular strips, and a fixing member of the magnetic sensor is suspended on the screw hole position through a long screw rod.

[0008] The magnetizer comprises two permanent magnets and a magnetic yoke; the permanent magnets and the magnetic yoke are fixed in the mobile carrier and have a U-shaped structure, and the polarities of the two permanent magnets are opposite, and are used for forming a uniform horizontal magnetization field in the tested piece;

[0009] The magnetic sensor is composed of two single-axis Hall sensors which are perpendicular to each other, one of which is used for measuring the magnetic field in the x-axis direction, and the other of which is used for measuring the magnetic field in the z-axis direction; the magnetic sensor is surrounded by the magnetic shielding cover;

[0010] The magnetic shielding cover has a U-shaped structure, and a rectangular opening is arranged above the magnetic shielding cover; the magnetic shielding cover is arranged above the magnetic sensor, and then the magnetic shielding cover and the magnetic sensor are fixed in the fixing member of the magnetic sensor.

[0011] When the double-axis zero magnetic field leakage detection system scans the defects of the tested piece, the mobile carrier is first placed on the tested piece, and then the mobile carrier is controlled to move at a constant speed on the tested piece; in the moving process, the permanent magnets generate magnetic flux which goes out from the N pole and enters the S pole, and in the process from the N pole to the S pole, part of the magnetic flux enters the tested piece to propagate, and the other part of the magnetic flux propagates in the air.

[0012] In the propagation process of the magnetic flux in the tested piece, when the magnetic flux encounters a defect position, the magnetic permeability of the defect position is distorted, and the magnetic flux leaks into the air through magnetic refraction;

[0013] Most of the magnetic flux propagating in the air enters the magnetic shielding cover, and a small part of the magnetic flux still propagates in the air and generates a magnetic field component B1 in the z-axis direction below the magnetic sensor.

[0014] The magnetic flux entering the magnetic shielding cover is divided into three paths at the rectangular opening, wherein two paths of the magnetic flux propagate along two sides of the magnetic shielding cover, and the other path of the magnetic flux leaks to the air through the rectangular opening and generates a z-axis downward magnetic field component B2 below the magnetic sensor; B1 and B1 are superposed to obtain a background magnetic field;

[0015] Finally, the magnetic field leaked due to the magnetic refraction at the defect position in the air is accompanied by the magnetic compression effect of the background magnetic field, and finally forms a magnetic leakage field above the defect;

[0016] The magnetic sensor captures the magnetic leakage signals in the x and z axis directions, and then sends them to the pc end, and the pc end processes and analyzes the magnetic leakage signals to determine the shape and size of the defect.

[0017] The purpose of the application is achieved as follows:

[0018] The zero magnetic field magnetic leakage detection system based on the magnetic shielding structure is used to scan the defects of the tested piece, the mobile carrier is placed on the tested piece, and then the mobile carrier is uniformly moved on the tested piece by controlling the mobile carrier, the magnetizer magnetizes the tested piece to saturation during the movement, at this time, the magnetic flux of the permanent magnet is divided into two paths, one path propagates to the inside of the tested piece, and the other path propagates in the air, during the propagation of the magnetic flux in the tested piece, when the defect position is encountered, the magnetic conductivity of the defect position is distorted, the magnetic flux is leaked to the air due to the magnetic refraction, and is accompanied by the magnetic compression effect of the background magnetic field, and finally forms a magnetic leakage field above the defect; the magnetic sensor captures the magnetic leakage signal and sends it to the pc end, so that the shape and size of the defect are determined.

[0019] Meanwhile, the zero magnetic field magnetic leakage detection system based on the novel magnetic shielding structure has the following beneficial effects:

[0020] (1) The magnetic shielding cover with a u-shaped opening is adopted, the poor shielding effect of the z-axis magnetic field of the traditional u-shaped magnetic shielding cover is compensated, the sensor works in a zero magnetic field background, and the sensor can still detect the defect signal under a large lift-off;

[0021] (2) Compared with the traditional magnetic leakage detection system, the sensitivity to small defects is significantly improved, and the signal background noise is effectively reduced;

[0022] (3), by changing the structure of the magnetic shield cover, the air background magnetic field at the defect approaches zero, the magnetic field in the air enters the magnetized structure, and at the same time the partial opening above the magnetizer divides the magnetic shield magnetic circuit into three ways, two of which propagate along the two side communication parts, and one enters the air below to generate a downward magnetic field component, which offsets the upward magnetic field generated by the high magnetic permeability shield, so that the background magnetic field approaches 0 in the X and Z directions. Such improvements not only improve the magnetic field trend, but also achieve weak magnetization, low-power detection mode. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a zero magnetic field leakage magnetic detection system architecture based on a magnetic shielding structure of the present application;

[0024] Figure 2 is a structure diagram of the tested piece;

[0025] Figure 3 is a traditional leakage magnetic detection principle diagram;

[0026] Figure 4 is a traditional u-shaped magnetic shielding structure magnetic field distribution diagram;

[0027] Figure 5 is a new type of magnetic shield cover structure diagram and its magnetic field distribution diagram;

[0028] Figure 6 is a defect detection signal diagram of the tested piece. DETAILED DESCRIPTION

[0029] The specific embodiments of the present application will be described below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present application. It should be particularly noted that in the following description, when the detailed description of the known functions and designs may obscure the main content of the present application, these descriptions will be omitted here.

[0030] EMBODIMENT

[0031] Figure 1 is a zero magnetic field leakage magnetic detection system architecture based on a magnetic shielding structure of the present application.

[0032] In this embodiment, as shown in Figure 1 , a zero magnetic field leakage magnetic detection system based on a magnetic shielding structure of the present application, comprising: a mobile carrier, a magnetizer, a magnetic shield cover, a magnetic sensor;

[0033] The mobile carrier is used for carrying the magnetizer and the magnetic sensor; each of left and right ends of the mobile carrier is provided with a hollow rectangular frame, and each of interiors of the rectangular frames is placed with a permanent magnet; the top of the rectangular frame is installed with two parallel rectangular strips, so as to connect the two rectangular frames into a whole, and each of outer sides of the two rectangular frames is further installed with a set of support frames with sliding wheels, so as to make the mobile carrier similar to a moving trolley; two rectangular holes are arranged on the inner side of the two rectangular frames, for placing a magnetic yoke, and two ends of the magnetic yoke are just located above the permanent magnets; a set of screw holes are arranged in the middle of each of the rectangular strips, and a fixing part of the magnetic sensor is hung through a long screw rod at the screw hole position;

[0034] In the embodiment, the support frame is provided with a plurality of screw holes, and the height of the mobile carrier is adjusted by adjusting the installation position of the screw holes of the support frame;

[0035] In the embodiment, as shown in Figure 2 , the tested piece is a flat plate test piece with three artificial cracks, the material is 45# steel, the moving speed of the mobile carrier is 0.04 m / s, and the direction is marked in the figure, wherein the size of the tested piece is 500 mm*20 mm*10 mm, the depth of the three artificial cracks is 4 mm, the width is 2 mm, and the length of the cracks 1-3 is 12 mm, 14 mm and 16 mm respectively, and the relative position of the defects is marked in the figure. The mobile carrier is placed on the flat plate tested piece, and the vertical distance (lift-off) between the magnetic sensor in the fixing part and the surface of the tested piece is adjusted to 6 mm by adjusting the suspension height of the long screw rod;

[0036] The magnetizer comprises two permanent magnets and a magnetic yoke; the permanent magnets and the magnetic yoke are fixed in the mobile carrier, and have a U-shaped structure, and the polarities of the two permanent magnets are opposite, for forming a uniform horizontal magnetization field in the tested piece;

[0037] In the embodiment, the permanent magnet is 50 mm long, 40 mm wide and 20 mm high, and the magnetic yoke is 180 mm long, 40 mm wide and 20 mm high;

[0038] The magnetic sensor is two single-axis Hall sensors perpendicular to each other, one for measuring the x-axis direction magnetic field and the other for measuring the z-axis direction magnetic field; the periphery of the magnetic sensor is placed with a magnetic shielding cover;

[0039] The magnetic shielding cover has a U-shaped structure, and a rectangular opening is arranged above the magnetic shielding cover; after the magnetic shielding cover is covered above the magnetic sensor, the magnetic shielding cover and the magnetic sensor are fixed in the fixing part of the magnetic sensor together;

[0040] In the embodiment, the conventional magnetic flux leakage detection principle is as shown in Figure 3As shown, the U-shaped structure formed by the yoke and the two permanent magnets is used to form a uniform horizontal magnetization field inside the test piece, and the test piece is magnetized to a saturated (or near-saturated) state. When there is a defect on the surface of the test piece, due to the magnetic force line extrusion at the defect, the magnetic permeability is small, and part of the magnetic flux leaks into the air to form a magnetic leakage field, and the magnetic sensor is used to pick up the magnetic leakage field signal.

[0041] Figure 4 It is the structure of the traditional u-shaped magnetic shield, at this time the magnetic circuit of the permanent magnet is divided into two ways, one way into the test piece, and the other way propagates in the air. The magnetic flux in the air is again divided into two ways at the u-shaped magnetic shield. Magnetic flux 1 propagates in the magnetic shield, and magnetic flux 2 still propagates in the air. Because the magnetic shield has high magnetic permeability, the z-axis component magnetic field is much larger than the x-axis at this time. Therefore, the traditional u-shaped magnetic shield can only effectively shield the x-axis magnetic field component, and the shielding effect on the z-axis is not good. In order to improve the shielding effect, the present application uses high magnetic permeability material to make the magnetic shield, and sets a rectangular opening with a size of 1 cm long, 1 cm wide and 0.5 cm high at the middle position above the magnetic shield, as shown in (a) of Figure 5 Under the new magnetic shield, the magnetic flux distribution is shown in (b) of Figure 5 The magnetic flux propagating in the air from the permanent magnet is decomposed into four magnetic fluxes at the magnetic shield. Magnetic flux 1, magnetic flux 2 and magnetic flux 3 enter the magnetic shield, and magnetic flux 4 still propagates in the air. In the magnetic shield, magnetic flux 1 and magnetic flux 2 propagate on both sides of the magnetic shield, respectively. Magnetic flux 3 leaks into the air at the opening and generates a downward component, which cancels out with the magnetic flux 4 in the air, so that the background magnetic field below the magnetic shield approaches 0.

[0042] When the dual-axis zero magnetic field leakage detection system scans the test piece defect, first place the mobile carrier on the test piece, and then move the mobile carrier at a constant speed on the test piece by controlling the mobile carrier. In the moving process, the permanent magnet generates magnetic flux from the N pole and the S pole. In the process from the N pole to the S pole, part of the magnetic flux enters the test piece to propagate, and the other part of the magnetic flux propagates in the air;

[0043] In the propagation process of the magnetic flux in the test piece, when the defect position is encountered, the magnetic permeability at the defect position is distorted, and the magnetic flux leaks into the air by magnetic refraction;

[0044] Most of the magnetic flux propagating in the air enters the magnetic shield, and a small part still propagates in the air and generates a z-axis upward magnetic field component B1 below the magnetic sensor;

[0045] The magnetic flux entering the magnetic shield is divided into three ways at the rectangular opening. Two of the magnetic fluxes propagate along the two sides of the magnetic shield, and the other magnetic flux leaks into the air through the rectangular opening and generates a z-axis downward magnetic field component B2 below the magnetic sensor. B1 and B1 are superimposed to obtain the background magnetic field;

[0046] Finally, the magnetic field leaked due to magnetic refraction at the defect location is accompanied by the magnetic compression effect brought by the background magnetic field in the air with magnetic diffusion, and finally forms a magnetic leakage field above the defect;

[0047] The magnetic sensor captures the magnetic leakage signals in the x and z axis directions, and then sends them to the PC end. The PC end processes and analyzes the magnetic leakage signals to determine the shape and size of the defect.

[0048] In this embodiment, the detection result of the flat plate type test piece is as shown in Figure 6 The crack area is marked in Figure 2 It can be seen that at the crack, the sensor output voltage changes, as shown in Figure 6 (a), the X-axis outputs a peak, as shown in Figure 6 (b), the Z-axis sensor outputs an up-down mutation signal, and the defect information and the signal are one-to-one correspondence. Based on this, the purpose of nondestructive testing is achieved.

[0049] Although the above describes the specific embodiments of the present application for the purpose of facilitating the understanding of the present application by those skilled in the art, it should be clear that the present application is not limited to the scope of the specific embodiments, and for those skilled in the art, it is obvious that various changes are within the spirit and scope of the present application defined and determined by the appended claims, and all the inventions utilizing the concept of the present application are within the scope of protection.

Claims

1. A zero magnetic field leakage magnetic flux detection system based on a magnetic shielding structure, characterized in that, include: Mobile carrier, magnetizer, magnetic shield, magnetic sensor; The mobile carrier is used to carry the magnetizer and the magnetic sensor. A hollowed-out rectangular frame is set at each of the left and right ends of the mobile carrier, and a permanent magnet is placed inside each rectangular frame. Two parallel rectangular strips are installed on the top of the rectangular frames, connecting them into a whole. A set of support frames with sliding wheels is also installed on the outer sides of each of the two rectangular frames, making the mobile carrier resemble a mobile cart. Two rectangular holes are set on the inner sides of the two rectangular frames for placing the magnetic yoke, with the two ends of the yoke positioned directly above the permanent magnet. A set of screw holes is set in the middle of each rectangular strip, and a fixing component for suspending the magnetic sensor is suspended at the screw hole positions via a long screw. The magnetizer includes two permanent magnets and a yoke; the permanent magnets and the yoke are fixed inside the moving carrier in a U-shaped structure, and the two permanent magnets are placed with opposite polarities to form a uniform horizontal magnetization field inside the test piece. The magnetic sensor consists of two mutually perpendicular single-axis Hall sensors, one measuring the magnetic field in the x-axis direction and the other measuring the magnetic field in the z-axis direction; a magnetic shield is placed around the magnetic sensor. The magnetic shielding cover has a U-shaped structure and a rectangular opening at the top. After the magnetic shielding cover is placed over the magnetic sensor, it is then fixed together with the magnetic sensor inside the fixing component of the magnetic sensor. When the zero magnetic field leakage detection system scans for defects in the test piece, the moving carrier is first placed on the test piece. Then, the moving carrier is controlled to move at a constant speed on the test piece. During the movement, the permanent magnet generates magnetic flux that exits from the N pole and enters from the S pole. During the process from the N pole to the S pole, part of the magnetic flux enters the test piece and propagates, while the other part of the magnetic flux propagates in the air. During the propagation of magnetic flux within the test piece, when it encounters a defect location, the permeability at the defect location is distorted, and the magnetic flux is refracted and leaks into the air. Most of the magnetic flux propagating in the air enters the magnetic shield, while a small portion still propagates in the air and generates an upward magnetic field component B1 along the z-axis directly below the magnetic sensor. The magnetic flux entering the magnetic shield is divided into three paths at the rectangular opening. Two of the magnetic fluxes propagate along the sides of the magnetic shield, while the other magnetic flux leaks into the air through the rectangular opening and generates a downward magnetic field component B2 along the z-axis directly below the magnetic sensor. The background magnetic field is obtained by superimposing B1 and B2. Ultimately, the magnetic field leaking due to magnetic refraction at the defect location, along with the magnetic diffusion and magnetic compression effect brought about by the background magnetic field, eventually forms a leakage magnetic field above the defect. The magnetic sensor captures the leakage magnetic field signals in the x and z axes and then sends them to the PC. The PC processes and analyzes the leakage magnetic field signals to determine the shape and size of the defect.

2. The zero magnetic field leakage detection system based on a magnetic shielding structure according to claim 1, characterized in that, The support frame is provided with multiple screw holes, and the height of the moving carrier can be adjusted by adjusting the installation position of the screw holes of the support frame.

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