Mutual inductance type magnetism gathering eddy current sensor

Abstract

The invention discloses a mutual inductance type magnetism gathering eddy current sensor. The mutual inductance type magnetism gathering eddy current sensor is formed by combining a conical magnetic shielding shell, a mutual inductance type exciting coil and a mutual inductance type receiving coil. Due to the fact that structures such as the mutual inductance coil and the conical magnetic shielding shell are added, magnetic lines are concentrated in the sensor, and the magnetic leakage effect and the background magnetic field are reduced. And by introducing the structure of the conical mutual inductance coil, the effect of enhancing the magnetic field can be achieved, the magnetic field at the central position of the sensor can be enhanced along with the increase of the inclination angle, and the signal strength is improved. According to the sensor, the defect detection capability is improved from two aspects of signal excitation and signal receiving, the identification rate of micro crack defects is effectively improved, and the sensitivity of signal detection is enhanced.

Description

Technical Field

[0001] The invention relates to a mutual inductance eddy current sensor for detecting groove defects in aluminum plates, which belongs to the technical field of electromagnetic non-destructive testing. Since the mutual inductance coil can increase the magnetic field strength and the magnetic field area is concentrated toward the center, the recognition rate of tiny defects can be effectively improved and the sensitivity of the detection signal can be enhanced. Background Art

[0002] In many fields of modern industry, aluminum alloys are widely used in aerospace, automobile manufacturing, electronic equipment housings, architectural decoration and other industries due to their excellent physical and chemical properties, such as light weight, high strength, and corrosion resistance. However, during use, defects such as notches are prone to occur. Notch defects in aluminum alloys can weaken the strength of materials or structures, leading to stress concentration, thereby increasing the risk of structural failure and even causing equipment failure or collapse. Therefore, in order to ensure the safe use of structures, it is of great engineering value to conduct quantitative non-destructive testing technology research on notch defects. At present, the commonly used methods for detecting notch defects include: eddy current testing, ultrasonic testing, X-ray testing, infrared thermal imaging testing, etc. Ultrasonic testing technology uses the principle of wave reflection. This method measures a relatively large thickness range and accurately locates defects. However, ultrasonic testing requires the use of coupling agents during the test process, which is inconvenient to operate, and the detection signal will also be aliased, and the signal is not easy to separate. X-ray detection of material volume defects, but the detection equipment is complex and large, the inspection speed is slow, and it is difficult for X-ray to detect planar defects. Infrared thermal imaging detects surface and subsurface defects of materials, but it is difficult to achieve quantitative detection. Among the numerous non-destructive testing technologies, eddy current testing technology has multiple advantages such as easy operation, convenient portability, high sensitivity, non-destructive testing and low cost. In addition, it does not require coupling agent and is very suitable for aluminum alloy defect detection.

[0003] Eddy current sensors have the characteristics of large linear measurement range, simple structure, and are not affected by media such as oil and dirt. They also have wide frequency response and strong anti-interference ability, and are widely used in various fields. However, eddy current sensors used in aluminum alloy defect detection face many difficulties, such as insufficient detection sensitivity and inability to detect tiny defects. Therefore, it is necessary to develop a mutual inductance magnetic eddy current sensor to improve the ability to identify tiny defects in aluminum alloys. However, eddy current sensors are often interfered by magnetic noise during detection, which reduces the detection sensitivity. In order to reduce and eliminate the interference of the background magnetic field and to better obtain the signal of the disturbed magnetic field due to the defects in the test piece, structures such as magnetic shielding need to be introduced. In addition, the geometric shape and parameters of the sensor coil will also have an important influence on the distribution of the magnetic field. Therefore, it is very important to optimize the sensor parameters and design a reasonable sensor structure to improve the excitation magnetic field of the sensor.

[0004] The present invention selects to use a mutual inductance excitation coil to enhance the excitation magnetic field, and adopts a mutual inductance receiving coil as a signal receiving device at the position where the magnetic field is the strongest, further enhancing the magnetic field strength of the received signal, and is matched with a conical Permalloy magnetic shielding shell to increase the defect detection efficiency and reduce the influence of background noise. Summary of the invention

[0005] The purpose of the present invention is to propose a highly sensitive mutual inductance eddy current sensor for aluminum alloy defect detection. In order to solve the problems of low signal strength and detection sensitivity of ordinary coils, the signal amplitude is improved from two aspects: the excitation signal and the receiving signal. By adding structures such as a conical magnetic shielding shell, a mutual inductance excitation coil and a mutual inductance receiving coil, the excitation magnetic field and the receiving magnetic field strength are enhanced, and the magnetic lines of force are bound inside the sensor to shield the interference of the background magnetic field, so that the magnetic field is concentrated in the local area of ​​the test piece, and the central magnetic field of the test piece area to be tested is concentrated, thereby increasing the accuracy and efficiency of defect detection.

[0006] In order to achieve the above object, the present invention adopts the following design scheme:

[0007] A mutual inductance magnetic eddy current sensor, comprising a conical magnetic shielding shell 1, a mutual inductance excitation coil 2, and a mutual inductance receiving coil 3; the characteristics are as follows: the top of the conical magnetic shielding shell 1 adopts a fully enclosed structure, a funnel-shaped space is reserved inside for installing the mutual inductance excitation coil 2, and the outer bottom end is lower than the inner part, so that it is convenient to install the mutual inductance receiving coil 3 on the bottom; the mutual inductance excitation coil 2 is composed of two funnel-shaped excitation coils combined up and down to generate an excitation magnetic field. Since it is composed of two excitation coils, mutual inductance will be generated, thereby enhancing the excitation magnetic field; the mutual inductance receiving coil Coil 3 is composed of inner and outer planar circular spiral wires. The outer coil and the inner coil are wound in the same way and are placed at the center of the conical magnetic shielding shell, directly below the mutual induction excitation coil 2. The mutual induction receiving coil 3 is in the same plane as the bottom of the conical magnetic shielding shell 1. Since it is composed of two coils, mutual induction will be generated, thereby enhancing the received magnetic field and reducing external magnetic field interference; the interior of the conical magnetic shielding shell 1 maintains a consistent inclination angle with the mutual induction excitation coil 2, and the axes of the conical magnetic shielding shell 1, the mutual induction excitation coil 2 and the mutual induction receiving coil 3 coincide.

[0008] The mutual inductance magnetic eddy current sensor is characterized in that a mutual inductance excitation coil 2 is installed inside the conical magnetic shielding shell 1, which is made of high magnetic permeability Permalloy material to reduce the influence of background noise in the air and bind the magnetic lines of force inside the magnetic shielding cover to play a role in magnetic conduction and shielding.

[0009] The mutual inductance eddy current sensor is characterized in that the mutual inductance excitation coil 2 is composed of two upper and lower conical excitation coils, the diameter of which increases uniformly from the bottom to the top, and its cross-section is a parallelogram, which focuses the magnetic field on the part of the area to be measured, and the two coils enhance the magnetic field through mutual inductance.

[0010] The mutual inductance magnetic field collecting eddy current sensor is characterized in that: the mutual inductance receiving coil 3 is located directly below the conical magnetic shielding shell 1 and the mutual inductance excitation coil 2, and is used to measure the total magnetic field after the magnetic field excited by the mutual inductance excitation coil 1 in the longitudinal direction is superimposed on the magnetic field of the test piece. The magnetic field intensity at this position is the largest, the received signal magnetic field intensity is the strongest, and the two coils can enhance the received magnetic field through mutual inductance.

[0011] The mutual inductance eddy current sensor is characterized in that the sensor can improve the magnetic field strength from both the excitation and reception aspects, reduce the influence of leakage magnetic field and background magnetic field, and improve the recognition rate of tiny groove defects in aluminum plates within the effective skin depth range.

[0012] The present invention can achieve the following beneficial effects:

[0013] 1. The conical magnetic shielding shell can install the mutual inductance receiving coil inside it, which can play a role of magnetic conductivity, and the magnetic lines of force are bound inside the sensor, while shielding the noise of the background magnetic field and reducing the leakage magnetic field in the air.

[0014] 2. Compared with the single coil in the conventional eddy current sensor, the mutual inductance excitation coil generates a more concentrated and stronger magnetic field and a stronger eddy current distribution at the receiving signal location and near the measured object;

[0015] 3. The mutual inductance receiving coil is located directly below the conical magnetic shielding housing and the mutual inductance excitation coil, so that more magnetic lines of force pass through the receiving signal device, and a stronger magnetic field can be received through mutual inductance; BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 : Schematic diagram of the structure of mutual induction magnetic eddy current sensor;

[0017] Figure 2 : Schematic diagram of conical magnetic shielding housing;

[0018] Figure 3 : Schematic diagram of mutual inductance excitation coil;

[0019] Figure 4 : Schematic diagram of mutual inductance receiving coil;

[0020] Figure 5 : Schematic diagram of eddy current distribution of two sensors;

[0021] Figure 6 : Schematic diagram of magnetic field line distribution of two sensors;

[0022] Figure 7 : Schematic diagram of the change of magnetic field strength with distance under the excitation of two sensors;

[0023] Figure 8 : Schematic diagram of the comparison of the differential signals of the two sensors;

[0024] In the figure, 1. conical magnetic shielding shell, 2. mutual inductance excitation coil, 3. mutual inductance receiving coil. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0026] The schematic diagram of the mutual induction eddy current sensor is as follows: Figure 1 As shown. The sensor includes a conical magnetic shielding shell 1, a mutual inductance excitation coil 2 and a mutual inductance receiving coil 3. The detection object is an aluminum alloy plate with a specification of 110×110×5 (unit: mm). The metal screen shell is made of high magnetic permeability Permalloy and assembled on the excitation coil; through the through hole at the center of the excitation coil. The mutual inductance excitation coil is conical. The bottom inner diameter, outer diameter, height and wire diameter of the coil play a decisive role in the induced eddy current density and secondary magnetic field strength in the tested piece. The specific size is determined according to the simulation results. According to the sensor structure, the coil is wound uniformly in the same direction with an enameled wire with a diameter of 0.06mm; the mutual inductance receiving coil is wound with a 0.06mm enameled wire, and the inner and outer circles are wound in the same direction. It is placed at the center of the bottom of the mutual inductance excitation coil and the conical magnetic shielding shell, and the total magnetic field in the axial direction is measured by superposition of the excitation magnetic field of the mutual inductance excitation coil and the detection magnetic field of the tested piece, and the magnetic field strength can be further enhanced by mutual inductance.

[0027] A two-dimensional axisymmetric model was established in the finite element simulation, and the final sensor size was determined by the orthogonal experimental method based on the simulation results. Figure 2 As shown, the top is a fully enclosed structure with a thickness of a = 3.2 mm, a height of b = 1.8 mm from the top of the conical magnetic shielding shell to the top of the mutual inductance excitation coil, and a distance of L = 1.4 mm from the bottom of the conical magnetic shielding shell to the bottom of the mutual inductance excitation coil. Figure 3 As shown in (a), the excitation coil height H = 7mm, the tilt angle is θ = 45°, the mutual inductance excitation coil through-hole is inserted into the conical magnetic shielding shell, the mutual inductance receiving coil is placed at the bottom, and the signal line is led out from the bottom. The cross-sectional diagram of the mutual inductance excitation coil is shown in Figure 3 (b) shows the inner diameter R at the center of the coil.i =2.5mm, outer diameter R o =9.5mm, coil thickness D = 3.5mm.

[0028] A square wave with a frequency of f=5000Hz and an amplitude of 5V is passed into the coil, and the excitation effects of the mutual inductance eddy current sensor and the conical coil sensor are studied. Figure 5 The eddy current distribution is shown in Figure 1. The maximum value of the induced current density of the conical coil and the mutual inductance is 2.09╳10 4 A / m 2 and 1.23╳10 5 A / m 2 The induced current density of the mutual inductance type is larger, and the eddy current distribution of the conical coil is not as concentrated as that of the mutual inductance type. The conical coil is mostly concentrated in the upper part, while the eddy current of the mutual inductance type is more concentrated in the center of the bottom. Figure 6 The figure shows the distribution of magnetic field lines. The maximum values ​​of the magnetic field strength of the conical coil and the mutual inductance are 6.11mT and 27.54mT, respectively. The magnetic field strength of the mutual inductance is more than 4 times that of the conical coil. It can be seen that most of the magnetic field lines of the mutual inductance pass through the inside of the sensor, and the magnetic field in the conical coil is mostly concentrated on the upper part of the sensor. Extract the Z component of the magnetic field strength at the center position, and make a difference between the defect and non-defective signals to obtain the differential signal. △ The larger the value of T, the higher the sensitivity of the sensor. Figure 8 As shown in the figure, it can be seen that the mutual inductance eddy current sensor is more sensitive than the conical coil sensor. As the defect radius increases, the differential signal increases first and then remains unchanged. This is because the size of the identifiable defect is limited by the position and size of the sensor. The appropriate sensor size needs to be adjusted according to the size of the detection target. Therefore, the performance of the magnetic sensor is better than that of the ordinary coil sensor of the same size.

[0029] The present invention designs a mutual inductance type magnetic eddy current sensor, which adopts a conical magnetic shielding shell and a mutual inductance excitation coil structure, and is matched with a mutual inductance receiving coil as a receiving device, and is designed for the purpose of improving the signal strength of the excitation signal and the received signal. When the sensor is working, current is passed through the mutual inductance excitation coil, and the coil generates a dynamic magnetic field. At the same time, eddy currents and induced magnetic fields are generated within the skin depth of the test piece. The eddy currents and magnetic fields are disturbed by the defects and act on the detection element. At this time, the mutual inductance receiving coil detects the total magnetic field after the magnetic field excited by the coil and the magnetic field detected by the test piece are superimposed. Through simulation, it is found that the magnetic field strength differential signal of the mutual inductance type magnetic eddy current sensor varies more than that of the conical eddy current sensor at the same defect radius, indicating that it has higher sensitivity, which verifies the characteristics of the magnetic eddy current sensor in improving defect recognition capabilities and enhancing signal strength.

Claims

1. A mutual induction type magnetic eddy current sensor, comprising a conical magnetic shielding shell, a mutual induction type excitation coil and a mutual induction type receiving coil; characterized in that: The top of the conical magnetic shielding shell adopts a fully enclosed structure, and a conical space is reserved inside for installing the mutual inductance excitation coil, and the outer bottom end is lower than the inner part, and the mutual inductance receiving coil is installed on the bottom; the mutual inductance excitation coil is composed of two conical excitation coils combined up and down to generate an excitation magnetic field. Since it is composed of two excitation coils and generates mutual inductance, the excitation magnetic field is enhanced; the mutual inductance receiving coil is composed of inner and outer planar circular spiral wires, and the outer coil and the inner coil are wound in the same way and are placed at the center of the conical magnetic shielding shell, directly below the mutual inductance excitation coil. The mutual inductance receiving coil and the bottom of the conical magnetic shielding shell are in the same plane. Since it is composed of two coils and generates mutual inductance, the received magnetic field is enhanced and external magnetic field interference is reduced; the inside of the conical magnetic shielding shell maintains a consistent inclination angle with the mutual inductance excitation coil, and the axes of the conical magnetic shielding shell, the mutual inductance excitation coil and the mutual inductance receiving coil coincide.

2. The mutual induction type magnetic eddy current sensor according to claim 1, characterized in that: A mutual induction excitation coil is installed inside the conical magnetic shielding shell. The mutual induction excitation coil is made of high magnetic permeability material to reduce the influence of background noise in the air and bind the magnetic lines of force inside the magnetic shielding cover to play a role of magnetic conduction and shielding.

3. The mutual induction type magnetic eddy current sensor according to claim 1, characterized in that: The mutual induction excitation coil consists of two upper and lower conical excitation coils, the diameter of which increases evenly from the bottom to the top, and its cross-section is a parallelogram, which focuses the magnetic field on the area to be measured, and the two conical excitation coils enhance the magnetic field by mutual induction.

4. The mutual induction type magnetic eddy current sensor according to claim 1, characterized in that: The mutual inductance receiving coil is located directly below the conical magnetic shielding shell and the mutual inductance excitation coil, and is used to measure the total magnetic field after the magnetic field excited by the mutual inductance excitation coil in the longitudinal direction is superimposed on the magnetic field of the test piece. The magnetic field intensity is the largest at this position, and the received signal magnetic field intensity is the strongest. The two coils can enhance the received magnetic field through mutual inductance.

5. The mutual induction type magnetic eddy current sensor according to claim 1, characterized in that: The sensor can improve the magnetic field strength from both the excitation and reception aspects, reduce the influence of leakage magnetic field and background magnetic field, and at the same time improve the recognition rate of tiny groove defects in aluminum plates within the effective skin depth range.

Images