Permanent magnet crack detection method and device

Abstract

The invention provides a permanent magnet crack detection method and a permanent magnet crack detection device, which are characterized in that a pair of current probes are utilized to introduce stable direct current or alternating current into a permanent magnet to be detected, and one or more potential probes are utilized to measure potential differences of contact points of the probes and the permanent magnet relative to a reference end. By comparing the measured values of the to-be-measured permanent magnet and the crack-free standard permanent magnet which are the same in shape, size and material, whether current density distribution abnormity caused by cracks exists in the to-be-measured permanent magnet or not is judged, and then the crack degree of the to-be-measured permanent magnet is evaluated and screened. Compared with the existing machine vision, eddy current, ultrasonic and magnetic powder detection technologies, the method has the beneficial effects that the requirement on the surface smoothness of the permanent magnet is low, the detection depth is large, the permanent magnet is not easily interfered by the residual magnetic flux density of the permanent magnet, and the detection efficiency is high.

Description

Technical Field

[0001] This invention belongs to the field of permanent magnet detection technology, and specifically relates to a method and apparatus for detecting cracks in permanent magnets. Background Technology

[0002] Density fluctuations, uneven shrinkage, and stress during the pressing, sintering, and machining processes of permanent magnets can cause internal cracks and defects, reducing their mechanical strength. In applications such as permanent magnet motors, cracked permanent magnets may disintegrate under high-speed centrifugal force or instantaneous torque, resulting in quality accidents. Crack detection in permanent magnets is a crucial quality control measure in the permanent magnet manufacturing industry.

[0003] Currently, the common methods for detecting cracks in permanent magnets are machine vision and eddy current testing. Machine vision is mostly used to inspect permanent magnets with smooth surfaces and metallic coatings, but it is less effective for uncoated permanent magnets with very low reflectivity. Furthermore, machine vision cannot detect internal cracks that do not extend to the surface, resulting in a certain probability of missed detections. Eddy current testing, based on eddy currents generated in metallic materials by an alternating magnetic field, is mostly used to inspect non-magnetic or low-coercivity magnetic metal components. Its detection depth is limited for high-coercivity permanent magnets, and the results are easily affected by the residual magnetic flux density of the permanent magnet. In addition, magnetic particle testing and ultrasonic testing are also relatively mature crack detection methods. Magnetic particle testing requires uniformly spreading dry magnetic powder or a magnetic powder suspension on the workpiece surface. The stray leakage magnetic field near the crack alters the powder's arrangement, revealing the crack's morphology. This method is less effective at detecting internal defects and has relatively low detection efficiency. Ultrasonic testing is suitable for thick, continuous media but is less commonly used for inspecting small, multi-shaped permanent magnets.

[0004] Therefore, given the shortcomings of existing technologies, it is necessary to propose a solution to address the technical problems existing in the current technologies. Summary of the Invention

[0005] This invention provides a method and apparatus for detecting cracks in permanent magnets. By applying a stable current to the permanent magnet under test, the potential at one or more fixed points on the surface of the permanent magnet is detected, thereby inferring whether cracks exist in the permanent magnet that can affect the current density distribution. Using this invention, the detection sensitivity can be improved, the false negative rate can be reduced, and it is applicable to conductive permanent magnets, including neodymium iron boron, samarium cobalt, and alnico permanent magnets.

[0006] To address the technical problems existing in the prior art, the technical solution of the present invention is as follows: A method for detecting cracks in permanent magnets includes the following steps: Step S1: Select a standard permanent magnet as the testing standard for the same batch; Step S2: Measure the potential difference at at least one fixed position of the standard permanent magnet using a detection device, and use this potential difference as the standard potential difference; Step S3: Measure the potential difference at the corresponding fixed position of the permanent magnet to be tested using the detection device in the same manner, and record the potential difference as the sample potential difference; Step S4: Compare the standard potential difference with the sample potential difference to determine whether there are cracks in the permanent magnet.

[0007] As a further improvement, step S3 includes the following steps: Fix the permanent magnet to be tested onto the base; A stable DC or AC current is supplied to the permanent magnet under test using at least one pair of current probes. Using at least one potential probe, at at least one fixed detection point on the surface of the permanent magnet under test, measure the magnitude of one or more DC or AC potential differences between the contact point and the reference end.

[0008] As a further improvement, a permanent magnet with the same shape, size, and material as the permanent magnet to be tested and without cracks is selected as a standard permanent magnet.

[0009] As a further improvement, in step S4, By comparing the differences in potential difference measurements of a batch of permanent magnets with the same shape, size, and material, we can analyze whether there are individual magnets with abnormal current distribution or abnormal resistance caused by cracks.

[0010] As a further improvement, in step S4, By comparing the sample potential difference of the permanent magnet sample to be tested with the standard potential difference, it is possible to determine in real time whether there is an abnormal current density distribution or resistance abnormality caused by cracks in the permanent magnet sample being tested.

[0011] As a further improvement, in steps S2 and S3, A current probe is used to pass DC currents of opposite polarities to a permanent magnet twice, and the average of the absolute values ​​of the DC potential difference measured twice is taken for comparison; or an AC current is passed to a permanent magnet using a current probe, and the effective value, average value, or peak value of the AC potential difference is measured for comparison.

[0012] This invention also discloses a permanent magnet crack detection device, comprising a base, a top plate, and a detection unit, wherein, The base is used to support and fix the permanent magnet to be tested; The top plate is adapted to the base, located above the base, and can move up and down relative to the base; The detection unit is used to pass a stable current to at least one fixed position of the permanent magnet under test and measure the potential difference at at least one fixed position of the permanent magnet, so as to determine whether there is a crack in the permanent magnet based on the measured sample potential difference and the standard potential difference. The detection unit includes at least one pair of current probes and at least one pair of potential probes, which are mounted on the top plate. After the top plate is lowered, the tips of the probes form good electrical contact with the surface of the permanent magnet to be tested.

[0013] As a further improvement, a pair of current probes and two pairs of potential probes are set up, with the probes connected to the power supply and measuring instruments by wires; The current probe is electrically connected to the power supply and is used to supply a stable current to the permanent magnet; The potential probe is electrically connected to the measuring instrument and is used to measure the potential difference at a fixed position on the surface of the magnet.

[0014] As a further improvement, both the base and the top plate are made of insulating material; the probe is made of conductive material.

[0015] As a further improvement, the base has a groove that can firmly fit the permanent magnet to be tested; The probe has a flexible or extendable tip to adapt to variations in the thickness of the permanent magnet or curved surfaces.

[0016] The beneficial effects of this invention are as follows: by applying a stable current to the permanent magnet under test, the potential at one or more fixed points on the surface of the permanent magnet is detected, thereby inferring whether there are cracks in the permanent magnet that can affect the current density distribution. This invention does not require specific optical properties of the permanent magnet surface, is less susceptible to interference from the residual magnetic flux density of the permanent magnet under test, and does not require magnetization of the permanent magnet. The permanent magnet crack detection device provided by this invention uses fixed detection points on the permanent magnet under test for both current and potential probes, which helps improve the repeatability of the detection results; multiple potential probes are set on the top plate, which helps improve detection sensitivity and reduce the probability of missed detections. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a component for detecting cracks in permanent magnets according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the probe contact point and crack on a permanent magnet according to an embodiment of the present invention; Figure 3 A schematic diagram of the current density distribution on a crack-free permanent magnet; Figure 4 A schematic diagram of the current density distribution on a cracked permanent magnet; Figure 5This is a comparison chart of potential probe measurements on permanent magnets with and without cracks. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] Conversely, this invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the invention as defined in the claims. Furthermore, to provide a better understanding of the invention, certain specific details are described in detail below. However, those skilled in the art will fully understand the invention even without these detailed descriptions.

[0020] This invention discloses a method for detecting cracks in permanent magnets, comprising the following steps: Step S1: Select a standard permanent magnet as the testing standard for the same batch; Step S2: Measure the potential difference at at least one fixed position of the standard permanent magnet using a detection device, and use this potential difference as the standard potential difference; Step S3: Measure the potential difference at the corresponding fixed position of the permanent magnet to be tested using the detection device in the same manner, and record the potential difference as the sample potential difference; Step S4: Compare the standard potential difference with the sample potential difference to determine whether there are cracks in the permanent magnet.

[0021] Furthermore, step S3 includes the following steps: Fix the permanent magnet to be tested onto the base; A stable DC or AC current is supplied to the permanent magnet under test using at least one pair of current probes. Using at least one potential probe, at at least one fixed detection point on the surface of the permanent magnet under test, measure the magnitude of one or more DC or AC potential differences between the contact point and the reference end.

[0022] In the above technical solution, a permanent magnet with the same shape, size and material as the permanent magnet to be tested and without cracks is selected as a standard permanent magnet and fixed on an insulating base. A pair of metal probes are placed at both ends of a standard permanent magnet, maintaining good electrical contact. These metal probes are hereinafter referred to as current probes. The output of a power supply is connected to the current probes, supplying a stable direct current or alternating current to the standard permanent magnet. A metal probe is placed at a fixed position on the upper surface of a standard permanent magnet, or several metal probes are simultaneously placed at several different fixed positions on the upper surface of the standard permanent magnet, maintaining good electrical contact. The one or more metal probes are hereinafter referred to as potential probes. The potential difference between the contact points of the one or more potential probes and a reference terminal is measured using a voltmeter, and the measured potential difference is recorded as the standard potential difference. The reference terminal refers to a fixed probe selected from current probes and voltage probes. Replace the standard permanent magnet with the permanent magnet sample to be tested. Using the current probe, apply a DC or AC current of the same value to the permanent magnet sample at the same location as described above. Using the potential probe, measure the potential difference between the probe contact point and the reference end at the same location as described above, and record one or more potential differences as the sample potential difference. Compare the standard potential difference with the sample potential difference. If the two batches of potential difference values ​​are the same, it indicates that the current density distribution in the permanent magnet sample and the standard permanent magnet is the same, and there are no cracks in the permanent magnet sample. If the two batches of potential difference values ​​are different, it indicates that the current density distribution in the permanent magnet sample and the standard magnet is different, and there are cracks in the permanent magnet sample. Furthermore, the degree of cracking inside the permanent magnet under test can be assessed based on the magnitude of the aforementioned differences, and a series of thresholds of different sizes can be set to screen the permanent magnet under test at different levels.

[0023] Furthermore, to eliminate the Schottky diode effect caused by the oxide layer that may exist at the contact point between the metal probe and the permanent magnet, direct currents with opposite polarities are passed through twice, and the average of the absolute values ​​of the potential difference measured twice is taken. Alternatively, an alternating current is passed through the permanent magnet using a current probe, and the effective value, average value, or peak value of the alternating potential difference is measured for comparison.

[0024] Furthermore, to address the special case where the crack direction is completely parallel to the current direction, the standard permanent magnet and the permanent magnet under test are rotated together by 90 degrees and then compared and measured again.

[0025] In the above technical solution, the permanent magnet crack detection device includes a base, a top plate, and a detection unit, wherein, The base is used to support and fix the permanent magnet to be tested; The top plate is adapted to the base, located above the base, and can move up and down relative to the base; The detection unit is used to pass a stable current to at least one fixed position of the permanent magnet under test and measure the potential difference at at least one fixed position of the permanent magnet, so as to determine whether there is a crack in the permanent magnet based on the measured sample potential difference and the standard potential difference. The detection unit includes at least one pair of current probes and at least one pair of potential probes, which are mounted on the top plate. After the top plate is lowered, the tips of the probes form good electrical contact with the surface of the permanent magnet to be tested.

[0026] In a preferred embodiment, a pair of current probes and two pairs of potential probes are provided, and the probes are connected to the power supply and the measuring instrument by wires. The current probe is electrically connected to the power supply and is used to supply a stable current to the permanent magnet; The potential probe is electrically connected to the measuring instrument and is used to measure the potential difference at a fixed position on the surface of the magnet.

[0027] Preferably, the base and the top plate are made of insulating materials, including plastic, resin, ceramic, and wood; The probe is made of conductive materials, including graphite, gold, silver, copper, aluminum, nickel, iron, and their alloys.

[0028] Preferably, the base has a groove for securely fitting the permanent magnet; Preferably, the probe has a flexible, extendable, or bendable tip to adapt to variations in the thickness of the permanent magnet or to a curved surface.

[0029] Example of a permanent magnet crack detection device Figure 1 This is an exploded view of the permanent magnet crack detection device in this embodiment, including a base 1, a permanent magnet 2, a top plate 3, a current probe 4, and a potential probe 5. The base 1, made of insulating material, supports and secures the permanent magnet 2. A groove on the base 1, its shape and internal dimensions closely matching the outer contour of the permanent magnet 2, allows the permanent magnet 2 to be firmly fitted into the groove. The top plate 3, located above the base 1 and the permanent magnet 2, can move vertically relative to them. The top plate 3 is also made of insulating material. A pair of current probes 4 are mounted on the top plate 3, positioned close to both ends of the permanent magnet 2. Five potential probes are also mounted on the top plate 3. To obtain a large potential difference signal, the potential probes 5 should be spaced as far apart as possible, but not exceeding the dimensions of the permanent magnet 2. Both the current probes 4 and the potential probes 5 are P100 type retractable spring probes.

[0030] The specific implementation method for detecting cracks in permanent magnets using this component is as follows: The top plate 3 moves downward, allowing the tips of the current probe 4 and the potential probe 5 to form good electrical contact with the upper surface of the permanent magnet 2. (See also...) Figure 2The current probe 4 is connected to the two output terminals of the DC power supply via a pair of wires, and a stable DC current is passed into the permanent magnet 2. Five potential probes are connected to five voltage measurement channels via wires, and the negative terminals of the voltage measurement channels are connected to the negative terminal of the DC power supply. The potential difference between the contact points of the five probes and the negative terminal of the power supply is measured. In this embodiment, the power supply is a DC power supply, and the reference terminal for voltage measurement is the probe connected to the negative terminal of the power supply. Figure 2 The dashed line represents a crack in the permanent magnet.

[0031] Finite element method (FEM) simulation can be used to compare the current density distribution in a permanent magnet with and without cracks. The simulation parameters are as follows: permanent magnet size 30*30*5 mm, material Nd-Fe-B, permanent magnet conductivity 7.14*10⁻⁶. 5 S / m; Current intensity 3 A; Crack is a straight line, 20 mm long, 0.2 mm wide, and filled with air; To simulate the uncertainty of crack location, the crack center point is offset by 5 mm towards the positive X-axis and tilted at 30°. See Figure 3 If there are no cracks in the permanent magnet, the current injected from the positive terminal of the current probe will diffuse throughout the entire permanent magnet and then converge at the negative terminal of the current probe. The current density distribution exhibits high symmetry. See also Figure 4 If a crack exists in the permanent magnet, the current will bypass the crack, which has extremely low conductivity. The current direction and current density distribution in the region near the crack are significantly different from those in the crack-free state.

[0032] Finite element method (FEM) simulations can be used to compare the measurements of five potential probes under conditions of presence and absence of cracks in a permanent magnet. (Participants...) Figure 5 If there are no cracks in the permanent magnet, the current can flow smoothly from the positive terminal to the negative terminal of the current probe. The permanent magnet is equivalent to a resistor with a small resistance value, and the readings of each potential probe are small. If there are cracks in the permanent magnet, the current needs to travel a long distance in the permanent magnet. The permanent magnet is equivalent to a resistor with a large resistance value, and the readings of each potential probe increase significantly.

[0033] In practice, the surface of the permanent magnet or the tip of the probe may have an oxide layer with low conductivity, resulting in a Schottky diode effect, meaning the conductivity of the permanent magnet 2 is related to the direction of the current. To eliminate this phenomenon, the current probe 4 can be alternated between positive and negative polarities and measured again, and the average of the absolute values ​​of the potential difference measured before and after the polarity alternation can be used for comparison. Alternatively, the DC power supply can be changed to an AC power supply, and an AC current can be passed through the permanent magnet using the current probe, and the effective value, average value, or peak value of the AC potential difference can be measured for comparison. Furthermore, if the crack is completely parallel to the current direction, the crack has no effect on the distribution of the current density and cannot be detected. To deal with this special case, the standard permanent magnet and the permanent magnet under test can be rotated together by 90 degrees and measured again for comparison.

[0034] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting cracks in permanent magnets, characterized in that, Includes the following steps: Step S1: Select a standard permanent magnet as the testing standard for the same batch; Step S2: Measure the potential difference at at least one fixed position of the standard permanent magnet using a detection device, and use this potential difference as the standard potential difference; Step S3: Measure the potential difference at the corresponding fixed position of the permanent magnet to be tested using the detection device in the same manner, and record the potential difference as the sample potential difference; Step S4: Compare the standard potential difference with the sample potential difference to determine whether there are cracks in the permanent magnet.

2. The method for detecting cracks in permanent magnets according to claim 1, characterized in that, Step S3 includes the following steps: Fix the permanent magnet to be tested onto the base; A stable DC or AC current is supplied to the permanent magnet under test using at least one pair of current probes. Using at least one potential probe, at at least one fixed detection point on the surface of the permanent magnet under test, measure the magnitude of one or more DC or AC potential differences between the contact point and the reference end.

3. The method for detecting cracks in permanent magnets according to claim 1, characterized in that, Select a permanent magnet that has the same shape, size, and material as the permanent magnet to be tested and is free of cracks, as the standard permanent magnet.

4. The method for detecting cracks in permanent magnets according to claim 2, characterized in that, In step S4, By comparing the differences in potential difference measurements of a batch of permanent magnets with the same shape, size, and material, we can analyze whether there are individual magnets with abnormal current distribution or abnormal resistance caused by cracks.

5. The method for detecting cracks in permanent magnets according to claim 2, characterized in that, In step S4, By comparing the sample potential difference of the permanent magnet sample to be tested with the standard potential difference, it is possible to determine in real time whether there is an abnormal current density distribution or resistance abnormality caused by cracks in the permanent magnet sample being tested.

6. The method for detecting cracks in permanent magnets according to claim 2, characterized in that, In steps S2 and S3, A current probe is used to pass DC currents of opposite polarities to a permanent magnet twice, and the average of the absolute values ​​of the DC potential difference measured twice is taken for comparison; or an AC current is passed to a permanent magnet using a current probe, and the effective value, average value, or peak value of the AC potential difference is measured for comparison.

7. A permanent magnet crack detection device, characterized in that, Includes a base, a top plate, and a detection unit, among which, The base is used to support and fix the permanent magnet to be tested; The top plate is adapted to the base, located above the base, and can move up and down relative to the base; The detection unit is used to pass a stable current to at least one fixed position of the permanent magnet under test and measure the potential difference at at least one fixed position of the permanent magnet, so as to determine whether there is a crack in the permanent magnet based on the measured sample potential difference and the standard potential difference. The detection unit includes at least one pair of current probes and at least one pair of potential probes, which are mounted on the top plate. After the top plate is lowered, the tips of the probes form good electrical contact with the surface of the permanent magnet to be tested.

8. The permanent magnet crack detection device according to claim 7, characterized in that, Set up a pair of current probes and two pairs of potential probes. The probes are connected to the power supply and the measuring instrument by wires. The current probe is electrically connected to the power supply and is used to supply a stable current to the permanent magnet; The potential probe is electrically connected to the measuring instrument and is used to measure the potential difference at a fixed position on the surface of the magnet.

9. The permanent magnet crack detection device according to claim 7, characterized in that, The base and top plate are both made of insulating material; the probe is made of conductive material.

10. The permanent magnet crack detection device according to claim 7, characterized in that, The base has a groove that can firmly fit the permanent magnet to be tested; The probe has a flexible or extendable tip to adapt to variations in the thickness of the permanent magnet or curved surfaces.

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