Giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero field blind spot

Abstract

The invention provides a wide-linearity, high-sensitivity and zero-field-blind-spot-free giant magneto-impedance sensor probe. The giant magneto-impedance sensor probe comprises a magneto-sensitive material and a solenoid. The magnetic sensitive material is arranged in the solenoid, and the magnetic sensitive material is not in contact with the solenoid; the magnetic sensitive material is a nanocrystalline magnetic conductive sheet subjected to copper-clad magnetic field annealing heat treatment, and the magnetic sensitive material is prepared according to the following method: arranging the nanocrystalline magnetic conductive sheet on a lower copper sheet, and then arranging an upper copper sheet on the magnetic conductive sheet to obtain an assembly; and annealing the obtained assembly under the action of a magnetic field, and removing the copper sheet to obtain the magnetic sensitive material. The probe has the advantages of wide linearity, high sensitivity and no zero field blind spot.

Description

Technical Field

[0001] The invention relates to a giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot, belonging to the technical field of sensors. Background Art

[0002] With the development of science and technology, human life and production have become increasingly inseparable from the application of various sensors. Among them, magnetic sensors are widely used in important fields such as attitude control, non-destructive testing, and magnetic sensitive switches due to their detection of magnetic signals. For example, magnetic sensors can be used in transportation for navigation, real-time speed measurement, attitude control, etc. In industrial production, magnetic switches designed using magnetic sensors can be used for some non-contact operating conditions and are more reliable. In 1992, Professor K. Mohri and others in Japan found that when an external magnetic field of 10Oe was applied to the axial direction of the FeCoSiB amorphous wire, the impedance of the wire changed by as much as 75%. This effect is called the giant magneto-impedance (GMI) effect. Magnetic sensors designed based on the giant magneto-impedance effect are widely used because of their high magnetic sensitivity, small size, low power consumption, high integration, and excellent stability. Therefore, it is still very necessary to improve the GMI effect and develop GMI sensors. At present, the sensitivity of giant magnetoimpedance sensors in near-zero magnetic field is still not high. The sensitivity of the conventional symmetrical giant magnetoimpedance effect in near-zero magnetic field is very low, which is called the zero-field blind spot. Although the asymmetric giant magnetoimpedance effect does not have a zero-field blind spot near the zero magnetic field, it also has the problem of overall sensitivity not being as high as the symmetrical giant magnetoimpedance effect, so its application value is not high.

[0003] As a key part of giant magneto-impedance sensors, magnetic sensitive materials need to have both the advantages of wide linear range and high sensitivity. However, existing magnetic sensitive materials often cannot take both into account, and it is difficult to ensure a linear response to the magnetic field environment within a sufficiently wide range. At present, amorphous and nanocrystalline magnetic sensitive materials are still the current research hotspots. People use various processing techniques to nanocrystallize amorphous materials to improve the magnetic sensitivity of materials, so that they have the advantages of wide linear range and high sensitivity. For example, Chinese patent document CN102061425A discloses a preparation process for magnetic sensitive materials with high sensitivity and wide linear range. Although this technology can achieve the goal of having both the advantages of high sensitivity and wide linear range at the same time, its process requirements are high and it is difficult to achieve. It is difficult to ensure the consistency of product quality of the same batch of products and different batches of products, and the output is difficult to meet the actual production requirements. The difficulty of industrial transformation is relatively large. Chinese patent documents CN105695682A and CN105679486A disclose methods for preparing magnetic sensitive materials with high sensitivity and wide linear range by using different technologies. Although the purpose of combining the two advantages is achieved, the effective linear range width of the magnetic sensitive material obtained by the technology is only 790A / m, and the sensitivity is 61% / Oe, which is still difficult to meet the requirements of magnetic sensors for high sensitivity and wide linearity. Chinese patent document CN102360683A discloses a composite material for magnetic core of magnetic sensitive probe, which is composited by magnetic core material 1 containing hard magnetic phase and soft magnetic core material 2, wherein the magnetic core material 1 containing hard magnetic phase is a magnetic material containing hard magnetic phase with asymmetric magneto-impedance effect characteristics, and the soft magnetic core material 2 is a soft magnetic material with symmetric giant magneto-impedance effect characteristics. However, the linear range width of the composite magnetic core obtained by the composite magnetic core is only 680A / m, and the magnetic core material 1 and material 2 obtained by different annealing processes cannot guarantee the process stability, and the operation of regulating the linear range width is not flexible enough.

[0004] Therefore, it is still very important to adopt new technologies to effectively improve the sensitivity of magnetic sensitive materials while increasing the linear range and enabling the prepared giant magneto-impedance sensor to cross the zero-field blind spot. For this reason, the present invention is proposed. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention provides a giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot. The probe of the present invention has the advantages of wide linearity, high sensitivity and no zero-field blind spot.

[0006] The present invention achieves the above object through the following technical solutions:

[0007] A giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot, comprising a magneto-sensitive material and a solenoid; the magneto-sensitive material is placed inside the solenoid, and the magneto-sensitive material is not in contact with the solenoid; the magneto-sensitive material is a nanocrystalline magnetic conductive sheet that has been subjected to copper-clad magnetic field annealing heat treatment.

[0008] Preferably according to the present invention, the magnetically sensitive material is placed at the inner axis center position of the solenoid; and the length of the solenoid is greater than the length of the magnetically sensitive material.

[0009] According to the present invention, the solenoid is used to generate a bias magnetic field.

[0010] Preferably, according to the present invention, the magnetic sensitive material is prepared according to the following method:

[0011] The nanocrystalline magnetic conductive sheet is arranged on the lower copper sheet, and then the upper copper sheet is arranged on the magnetic conductive sheet to obtain an assembly; the obtained assembly is then annealed under the action of a magnetic field, and then the copper sheet is removed to obtain a magnetic sensitive material;

[0012] Further preferably, the nanocrystalline magnetic sheet is made of 8-12 layers of nanocrystalline alloy strips that are stacked and bonded, and then go through a heat treatment process, a pressing and crushing process, and a die-cutting process, all of which are existing technologies in the field; the nanocrystalline alloy strip used is 1K107B nanocrystalline strip; and the magnetic sheet is a common commercially available product.

[0013] Further preferably, the thickness ratio of the lower copper sheet to the nanocrystalline magnetic conductive sheet is 1-5:2, the length ratio is 2:1, and the width ratio is 10:1.

[0014] Further preferably, the magnetic conductive sheet is located at the center of the lower copper sheet; and the length direction of the magnetic conductive sheet is parallel to the length direction of the lower copper sheet.

[0015] Further preferably, the upper copper sheet and the lower copper sheet have the same size, and the upper copper sheet and the lower copper sheet are aligned.

[0016] Further preferably, the angle between the direction of the magnetic field and the axial direction of the assembly is 0 to 45°, and more preferably, the direction of the magnetic field is parallel to the axial direction of the assembly; the axial direction of the assembly is parallel to the length direction of the upper surface of the lower copper sheet, the direction of the magnetic field is parallel to the upper surface of the lower copper sheet, and the upper surface of the lower copper sheet is the side of the lower copper sheet that contacts the magnetic conductive sheet.

[0017] Further preferably, the intensity of the magnetic field is 200-450 Oe.

[0018] Further preferably, the annealing treatment step is: placing the assembly in a magnetic field annealing furnace, heating it to 150-200°C in an air atmosphere, keeping it warm for 2-4 hours, then cooling it to room temperature and taking it out; the magnetic field is continuously and constantly applied during the entire annealing process, that is, the magnetic field is continuously and constantly applied during the process from heating to cooling to room temperature.

[0019] Preferably according to the present invention, the solenoid is made of two strands of enameled wire tightly wound side by side; further preferably, the enameled wire is made of copper, and the solenoid is made of two strands of enameled wire with a wire diameter of 0.3 mm tightly wound side by side.

[0020] The technical features and beneficial effects of the present invention are as follows:

[0021] 1. Compared with the existing methods for preparing magnetic sensitive materials, the method for preparing wide linearity and high-sensitivity giant magnetoimpedance sensor materials of the present invention can prepare materials that have a wide linear range while meeting sufficiently high sensitivity, while the magnetic sensitive materials prepared by the existing general magnetic sensitive material preparation technology can only prepare one of the two advantages of wide linear range and high sensitivity or cannot achieve a large value for both; the magnetic sensitive material of the present invention can achieve zero-field blind spot crossing in a simple and convenient manner by applying a DC bias magnetic field, and the bias magnetic field size is adjustable, which can be suitable for different application scenarios.

[0022] 2. The present invention uses a magnetic conductive sheet that has been fragmented. The cracks in the fragmented magnetic conductive sheet play a role of local pinning. After magnetic field annealing, the domain wall rotation and domain wall displacement process of the strip under longitudinal drive are affected by the change of the external magnetic field, thereby obtaining a magnetic sensitive material with high sensitivity and wider linearity.

[0023] 3. The method of preparing a wide linearity and high sensitivity giant magnetoimpedance sensor material of the present invention has the advantage of being easy to control the material properties. Compared with other inventions, the present invention can obtain magnetic sensitive materials with different sensitivities and linear ranges by changing process parameters such as magnetic field annealing time while keeping the annealing temperature and magnetic field size unchanged. This is an advantage that the existing general technology for preparing magnetic sensitive materials with both high sensitivity and wide linear range does not have.

[0024] 4. The magnetic sensitive sensor probe prepared by the present invention has the characteristics of miniaturization, crossing the zero field blind spot and easy adjustment of the response range. The width of the response interval can be flexibly controlled by adjusting the DC current. While achieving high sensitivity, it has the advantages of crossing the zero field blind spot and providing magnetic field direction identification. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a schematic diagram of the structure of the assembly of the present invention.

[0026] Figure 2This is a schematic diagram of the direction of applied magnetic field in Example 1 of the present invention.

[0027] Figure 3 This is a schematic diagram of the structure of the giant magneto-impedance sensor probe of the present invention, in which: 1 is a magneto-sensitive material, and 2 is a solenoid.

[0028] Figure 4 This is a curve showing the impedance change rate of Example 1 as a function of magnetic field.

[0029] Figure 5 This is a curve showing the impedance change rate of Example 2 as a function of the magnetic field.

[0030] Figure 6 This is a curve showing the impedance change rate of Example 3 as a function of magnetic field.

[0031] Figure 7 This is a curve showing the impedance change rate of comparative example 1 as a function of magnetic field.

[0032] Figure 8 This is a curve showing the impedance change rate of comparative example 2 as a function of magnetic field.

[0033] Fig. 9 This is a curve showing the impedance change rate of comparative example 3 as a function of magnetic field.

[0034] Fig.10 This is a curve showing the impedance change rate of comparative example 4 as a function of magnetic field. DETAILED DESCRIPTION

[0035] The present invention will be further described below with reference to specific embodiments and drawings, but is not limited thereto.

[0036] The materials used in the examples are all conventional materials and are available on the market, and the methods described are all prior art unless otherwise specified.

[0037] The nanocrystalline magnetic sheet used in the embodiment is 8 layers of nanocrystalline ribbons stacked and bonded together, and is made through a heat treatment process, a pressing and crushing process, and a die-cutting process, with a thickness of 0.2 mm; the nanocrystalline ribbon used is a 1K107B nanocrystalline ribbon, and other materials are commonly used materials in this field. The nanocrystalline magnetic sheet used is available from Ningbo Zhongke Biplus New Material Technology Co., Ltd.

[0038] The metal push clip used in the embodiment is a common office push clip, which can be a Deli 8591 metal push clip.

[0039] Example 1

[0040] A giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot, the giant magneto-impedance sensor probe comprises a magneto-sensitive material 1 and a solenoid 2; the magneto-sensitive material 1 is placed at the inner axis position of the solenoid 2; the magneto-sensitive material 1 is not in contact with the solenoid 2, and its structural schematic diagram is shown as follows Figure 3 shown.

[0041] The wide linearity, high sensitivity and zero-field blind spot giant magneto-impedance sensor probe is prepared according to the following method:

[0042] (1) Preparation of magnetic sensitive material 1:

[0043] A nanocrystalline magnetic conductive sheet with a length of 100 mm, a width of 5 mm, and a thickness of 0.2 mm is cut and placed horizontally on the center of a lower copper sheet with a length of 200 mm, a width of 50 mm, and a thickness of 0.5 mm. The length direction of the magnetic conductive sheet is parallel to the length direction of the lower copper sheet, and the width direction of the magnetic conductive sheet is parallel to the width direction of the lower copper sheet. Then, an upper copper sheet with the same size as the lower copper sheet is placed on the magnetic conductive sheet. The upper and lower copper sheets are aligned and then clamped with a metal push clamp to obtain an assembly. The structural schematic diagram is shown as follows. Figure 1 As shown; the obtained assembly is placed horizontally in a magnetic field annealing furnace, and the direction of the magnetic field is parallel to the axial direction of the assembly (the axial direction of the assembly is parallel to the length direction of the upper surface of the lower copper sheet, and the direction of the magnetic field is parallel to the upper surface of the lower copper sheet, and the upper surface of the lower copper sheet is the side of the lower copper sheet in contact with the magnetic conductive sheet). The schematic diagram of the magnetic field application direction is shown in Figure 2 As shown; the temperature is raised to 150°C in an air atmosphere, kept warm for 2 hours, then cooled to room temperature and taken out, the copper sheet is removed, and the magnetic sensitive material 1 is obtained; during the entire annealing process, an external DC magnetic field of 200Oe is continuously and constantly applied, that is, the magnetic field is continuously and constantly applied from the beginning of heating to the process of cooling to room temperature.

[0044] (2) Preparation of the solenoid coil 2: A solenoid coil 2 with a diameter of 8 mm is prepared by winding 208 turns of an enameled wire with a wire diameter of 0.3 mm in parallel in a double-strand manner. The enameled wire is made of copper.

[0045] (3) Insert the magnetic sensitive material 1 obtained in step (1) into the internal axis position of the solenoid 2 obtained in step (2), with the magnetic sensitive material 1 being parallel to the axis of the solenoid 2 and the axis of the magnetic sensitive material 1 being the length direction, to form a sensor probe.

[0046] The two ends of one wire in the solenoid 2 in the sensor probe are connected in parallel with an AC signal source and a signal receiver. The signal source provides an AC signal and the signal receiver detects the voltage signal at both ends of the solenoid. The two ends of the other wire in the solenoid 2 are connected to a DC power supply to form a loop to provide a DC bias magnetic field for testing. This is a commonly used test structure and method in this field.

[0047] The curve of the impedance change rate of the wide linear high sensitivity zero-field blind spot giant magneto-impedance sensor probe prepared in this embodiment versus the magnetic field is as follows: Figure 4 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 597A / m.

[0048] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 1042%, the linear range with a linear correlation coefficient of 0.995 is -546 to 546 A / m, the linear range width is 1092 A / m, the slope of the curve in the linear range is 0.586, and the sensitivity is 55% / Oe.

[0049] Example 2

[0050] A giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot is as described in Example 1, except that the holding time in the preparation of the magneto-sensitive material is 3 hours.

[0051] The curve of the impedance change rate of the wide linear high sensitivity zero-field blind spot giant magneto-impedance sensor probe prepared in this embodiment versus the magnetic field is as follows: Figure 5 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 539A / m.

[0052] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 1093%, the linear range with a linear correlation coefficient of 0.995 is -558 to 539 A / m, the linear range width is 1097 A / m, the slope of the curve in the linear range is 0.659, and the sensitivity is 62% / Oe.

[0053] Example 3

[0054] A giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot is as described in Example 1, except that the holding time in the preparation of the magneto-sensitive material is 4 hours.

[0055] The curve of the impedance change rate of the wide linear high sensitivity zero-field blind spot giant magneto-impedance sensor probe prepared in this embodiment versus the magnetic field is as follows: Figure 6 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 562A / m.

[0056] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 1038%, the linear range with a linear correlation coefficient of 0.995 is -562 to 562 A / m, the linear range width is 1124 A / m, the slope of the curve in the linear range is 0.606, and the sensitivity is 57% / Oe.

[0057] Comparative Example 1

[0058] A giant magneto-impedance sensor probe is as described in Example 1, except that the heat preservation time in the preparation of the magneto-sensitive material is 1 hour.

[0059] The curve of the impedance change rate of the wide linear high sensitivity giant magneto-impedance sensor probe without zero-field blind spot prepared in this comparative example as a function of the magnetic field is as follows: Figure 7 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 546A / m.

[0060] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 986%, the linear range with a linear correlation coefficient of 0.995 is -546 to 547 A / m, the linear range width is 1093 A / m, the slope of the curve in the linear range is 0.587, and the sensitivity is 55% / Oe.

[0061] Comparative Example 2

[0062] A giant magneto-impedance sensor probe is as described in Example 1, except that the heat preservation time in the preparation of the magneto-sensitive material is 5 hours.

[0063] The curve of the impedance change rate of the wide linear high sensitivity giant magneto-impedance sensor probe without zero-field blind spot prepared in this comparative example as a function of the magnetic field is as follows: Figure 8 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 551A / m.

[0064] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 981%, the linear range with a linear correlation coefficient of 0.995 is -598 to 504 A / m, the linear range width is 1102 A / m, the slope of the curve in the linear range is 0.587, and the sensitivity is 55% / Oe.

[0065] Comparative Example 3

[0066] A giant magneto-impedance sensor probe is as described in Example 1, except that the heat preservation time in the preparation of the magneto-sensitive material is 6 hours.

[0067] The curve of the impedance change rate of the wide linear high sensitivity giant magneto-impedance sensor probe without zero-field blind spot prepared in this comparative example as a function of the magnetic field is as follows: Fig. 9 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 535A / m.

[0068] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 990%, the linear range with a linear correlation coefficient of 0.995 is -535 to 536 A / m, the linear range width is 1071 A / m, the slope of the curve in the linear range is 0.575, and the sensitivity is 54% / Oe.

[0069] Comparative Example 4

[0070] A giant magneto-impedance sensor probe is as described in Example 1, except that the magneto-sensitive material is prepared according to the following method:

[0071] (1) Cut an amorphous strip with a grade of 1K107, a length of 100 mm, a width of 5 mm, and a thickness of 23 μm.

[0072] (2) placing the strip obtained in step (1) in a vacuum tube annealing furnace and annealing at 415° C. for 30 min to obtain a nanocrystalline strip. The entire annealing process is carried out in a nitrogen atmosphere.

[0073] The curve of the impedance change rate of the wide linear high sensitivity giant magneto-impedance sensor probe without zero-field blind spot prepared in this comparative example as a function of the magnetic field is as follows: Fig.10 During the measurement, the amplitude of the driving current is 10mA, the frequency is 55KHz, and the DC bias magnetic field is 18A / m.

[0074] The test results are as follows: the maximum giant magneto-impedance ratio of the sample is 1010%, the linear range with a linear correlation coefficient of 0.995 is -18 to 18 A / m, the linear range width is 36 A / m, the slope of the curve in the linear range is 13.152, and the sensitivity is 1266% / Oe.

[0075] To sum up the above analysis, by subjecting the magnetic conductive sheet to copper-clad magnetic field annealing heat treatment, the internal structure of the material can be fully improved, so that the material exhibits excellent giant magneto-impedance effect under longitudinal drive, and then by applying an external DC bias magnetic field, a giant magneto-impedance sensor probe with high sensitivity, wide linear range and no zero-field blind spot is obtained.

[0076] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and changes. Any modifications and improvements made within the scope of the principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot, characterized in that: The giant magneto-impedance sensor probe comprises a magneto-sensitive material and a solenoid; the magneto-sensitive material is placed inside the solenoid, and the magneto-sensitive material is not in contact with the solenoid; the magneto-sensitive material is a nanocrystalline magnetic conductive sheet that has been subjected to a copper-clad magnetic field annealing heat treatment.

2. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The magnetically sensitive material is placed at the inner axis center of the solenoid; the length of the solenoid is greater than the length of the magnetically sensitive material.

3. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The magnetic sensitive material is prepared according to the following method: The nanocrystalline magnetic conductive sheet is arranged on the lower copper sheet, and then the upper copper sheet is arranged on the magnetic conductive sheet to obtain an assembly; the obtained assembly is then annealed under the action of a magnetic field, and then the copper sheet is removed to obtain a magnetic sensitive material.

4. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 3, characterized in that: The nanocrystalline magnetic conductive sheet is made of 8-12 layers of nanocrystalline alloy strips which are laminated and bonded, and then go through a heat treatment process, a pressing and crushing process, and a die-cutting process; the nanocrystalline alloy strip used is 1K107B nanocrystalline strip.

5. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The thickness ratio of the lower copper sheet to the nanocrystalline magnetic conductive sheet is 1-5:2, the length ratio is 2:1, and the width ratio is 10:

1.

6. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The magnetic conductive sheet is located at the center of the lower copper sheet; the length direction of the magnetic conductive sheet is parallel to the length direction of the lower copper sheet; the upper copper sheet and the lower copper sheet have the same size, and the upper copper sheet and the lower copper sheet are aligned.

7. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The angle between the direction of the magnetic field and the axial direction of the assembly is 0 to 45°. Preferably, the direction of the magnetic field is parallel to the axial direction of the assembly; the axial direction of the assembly is parallel to the length direction of the upper surface of the lower copper sheet, and the direction of the magnetic field is parallel to the upper surface of the lower copper sheet, and the upper surface of the lower copper sheet is the side in contact with the magnetic conductive sheet.

8. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The intensity of the magnetic field is 200-450 Oe.

9. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The annealing treatment steps are: placing the assembly in a magnetic field annealing furnace, heating it to 150-200° C. in an air atmosphere, keeping it warm for 2-4 hours, cooling it to room temperature, and then taking it out; the magnetic field is continuously and constantly applied during the entire annealing process, that is, the magnetic field is continuously and constantly applied from the beginning of heating to the process of cooling it to room temperature.

10. The giant magneto-impedance sensor probe with wide linearity, high sensitivity and no zero-field blind spot according to claim 1, characterized in that: The solenoid is made of two strands of enameled wire tightly wound side by side; preferably, the enameled wire is made of copper, and the solenoid is made of two strands of enameled wire with a wire diameter of 0.3 mm tightly wound side by side.

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