JP5011056B2 - Eddy current inspection probe and eddy current inspection device - Google Patents

Description

  The present invention relates to an eddy current inspection of a subject, and more particularly to an eddy current inspection probe and an eddy current inspection device suitable for detecting a defect or the like existing in an object.

  In the eddy current inspection method, which is one of the non-destructive inspection methods, an eddy current inspection probe having an excitation coil and a detection sensor is brought close to a conductive object, an eddy current is induced in the object by the excitation coil, and the object is induced. The detected eddy current disturbance (or magnetic flux density disturbance associated therewith) is detected by a detection sensor, and abnormal portions such as defects (cracks, etc.), plate thickness changes, and structural discontinuities existing in the subject are detected. Although this eddy current inspection method has good sensitivity on the surface side of the subject (in other words, on the side where the eddy current inspection probe is arranged), the eddy current decreases in the plate thickness direction due to the skin effect. Conventionally, it has been applied to, for example, inspection of a surface layer portion of a thick plate and inspection of a thin plate.

  Therefore, for example, for the purpose of application to the inspection of the back side (inside and back side) of the thick plate, the axial direction is substantially perpendicular to the inspection surface of the subject and is symmetrically separated from each other in the radial direction. An eddy current inspection probe including two arranged excitation coils and a detection sensor (detection coil) arranged between the two excitation coils has been proposed (for example, see Patent Document 1). In this eddy current inspection probe, currents flowing in opposite directions are passed through two exciting coils, and eddy currents induced in a region between these exciting coils are superimposed to generate a strong eddy current. Further, for example, a configuration is disclosed in which a distance adjustment mechanism for adjusting the interval between the two excitation coils is provided so that the interval between the two excitation coils becomes an optimum value according to the plate thickness of the subject (for example, a patent) Reference 2).

Japanese Patent No. 3796570 JP 2005-43154 A

However, there are the following problems in the above-described prior art.
That is, the detection sensor of the eddy current inspection probe is arranged on the symmetry axis between the two excitation coils, although not clearly described. For example, when inspecting a defect (such as a crack) extending in a direction parallel to the arrangement direction of the two exciting coils, depending on the depth position of the defect, the center other than the end of the defect can be detected. May not be detected. In such a case, it is unclear whether the detection result corresponds to the end of a continuous defect or a single defect, and even if it can be determined that the detection result corresponds to the end of a continuous defect, It is unclear how it is formed continuously in combination. Therefore, it has been difficult to accurately grasp the properties such as defects.

  An object of the present invention is to provide an eddy current inspection probe and an eddy current inspection apparatus capable of accurately grasping properties such as defects.

In order to achieve the above object, the present invention provides a pair of exciting coils that are symmetrically arranged in such a manner that their axial directions are substantially perpendicular to the test surface of a subject and are radially spaced from each other. An eddy current inspection probe that is disposed between the excitation coils and has only one detection sensor that detects a change in eddy current induced in the subject by the excitation coil, wherein the only detection sensor includes: The position is shifted from the axis of symmetry between the pair of exciting coils.
Preferably, the one detection sensor is arranged with a position shifted from a central axis connecting the radial center positions of the pair of excitation coils.
In order to achieve the above object, the present invention includes a pair of excitation coils that are symmetrically arranged so that the axial direction is substantially perpendicular to the examination surface of the subject and radially spaced from each other; In the eddy current inspection probe, which is disposed between the pair of excitation coils and includes first and second detection sensors for detecting a change in eddy current induced in the subject by the excitation coil, the first detection The sensor is arranged with a position shifted from a symmetric axis between the pair of exciting coils and a position shifted from a central axis connecting the radial center positions of the pair of exciting coils, and the second detection sensor is symmetric with the second detecting sensor. It arrange | positions in the position where an axis | shaft and the said central axis cross.

  According to the present invention, properties such as defects can be accurately grasped.

  Embodiments of the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic view showing the configuration of the eddy current inspection apparatus according to the first embodiment of the present invention, and FIG. 2 is a plan view showing the structure of the eddy current inspection probe. In FIG. 1, the eddy current inspection probe is shown in a side sectional view taken along a section AA in FIG.

  1 and 2, the eddy current inspection apparatus according to the present embodiment causes the eddy current inspection probe 3 having the excitation coils 1A and 1B and the detection sensor 2 to approach the subject 4 and causes the subject 4 to be in contact with the excitation coil 1A and 1B. An eddy current is induced, and a change in the eddy current induced in the subject 4 (or a change in magnetic flux density associated therewith) is detected by the detection sensor 2 to evaluate the property of the subject 4 (for example, the presence or absence of defects). is there. One of the eddy current inspection devices includes a scanning mechanism (not shown) for controlling the position of the eddy current inspection probe 3, a transmitter 5 for applying an excitation signal to the excitation coils 1A and 1B, and a detection signal from the detection sensor 2. Detection signal amplification unit 6 to be amplified, detection signal sampling unit 7 that samples the detection signal amplified by this detection signal amplification unit 6 at a preset time interval, and detection signal input from this detection signal sampling unit 7 In addition, detection data in which the position signal of the eddy current inspection probe 3 from the scanning mechanism (in other words, the position signal of the detection sensor 2) is correlated one-to-one is created, and an image based on the detection data is created. And a display processing unit 9 for displaying the detected data and images on the monitor 8.

  Another eddy current inspection device amplifies the detection signal from the means (not shown) for outputting the moving time of the eddy current probe 3, the transmitter 5 for applying the excitation signal to the excitation coils 1A and 1B, and the detection sensor 2. The detection signal amplification unit 6, the detection signal sampling unit 7 that samples the detection signal amplified by the detection signal amplification unit 6 at a preset time interval, and the detection signal input from the detection signal sampling unit 7 Detection data is created by associating the movement time of the eddy current inspection probe 3 from the start of scanning (in other words, the movement time of the detection sensor 2) in a one-to-one manner, and an image based on the detection data is created and the created detection is performed. And a display processing unit 9 for displaying data and images on the monitor 8.

  The exciting coils 1A and 1B of the eddy current inspection probe 3 are symmetrically arranged so that the axial direction is substantially perpendicular to the inspection surface of the subject 4 and spaced apart from each other in the radial direction. The exciting coils 1A and 1B are supplied with currents in opposite directions so that the eddy currents induced in the region between the exciting coils 1A and 1B (in other words, the region where the detection sensor 2 is arranged) are superimposed. It has become.

  Here, as a major feature of the present embodiment, the detection sensor 2 is intentionally shifted from the symmetry axis P between the excitation coils 1A and 1B on the central axis O connecting the radial center positions of the excitation coils 1A and 1B. It is arranged at a position (in other words, a position shifted from an error range of about ± 0.2 mm or about 1% of the interval between the exciting coils 1A and 1B becomes a central position). As the detection sensor 2, it is possible to use, for example, a hall element or a flack gate element using a detection coil or a magnetic measurement element, a GMR element using a giant magnetoresistance effect, an element using an MI effect, or the like.

  The operation of the eddy current inspection apparatus of this embodiment will be described with reference to FIG. First, the excitation signal from the transmitter 5 is applied to the excitation coils 1A and 1B of the eddy current inspection probe 3 (step 101), and the eddy current inspection probe 3 is disposed in the healthy part of the subject 4 and detected from the detection sensor 2. The balance is adjusted so that the signal (detected value) becomes zero (step 102). Then, the eddy current inspection probe 3 is placed at the inspection start position of the subject 4 (step 103), and then the eddy current inspection probe 3 is scanned on the inspection surface of the subject 4 and induced in the subject 4 by the detection sensor 2. A change in the eddy current is detected (step 104). Then, the display processing unit 9 creates detection data and an image based on the detection signal of the detection sensor 2 input via the detection signal amplification unit 6 and the detection signal collection unit 7 (step 105), and the detection data and image Is displayed on the monitor 8 (step 106). As a result, the inspector visually checks the detection data or image displayed on the monitor 8 to determine whether there is a defect or the like.

  Next, the effect of this embodiment is demonstrated using a comparative example.

  FIG. 4A is a plan view showing the structure of the eddy current inspection probe according to the comparative example, and FIG. 4B is a side sectional view taken along a section BB in FIG. 4A.

  In the eddy current inspection probe 10 of this comparative example, the detection sensor 2 is arranged at a position where the central axis O and the symmetry axis P intersect.

  Then, for example, the eddy current inspection probe 10 is scanned by a scanning mechanism or manually in a direction parallel to the arrangement direction of the excitation coils 1A and 1B, and the defect 11 (hereinafter referred to as the defect 11) extending in the direction parallel to the arrangement direction of the excitation coils 1A and 1B. (Referred to as “parallel defect”) (see FIG. 5A), detection data (waveform data) associated with the position or movement time of the detection sensor 2 is created (see FIG. 5B). ). This detection data has positive and negative peak values corresponding to the vicinity of the end portion of the parallel defect 11, and the detection value corresponding to the central portion other than the end portion is zero. First, when one exciting coil 1B approaches the parallel defect 11, the eddy current induced by the exciting coil 1B changes to obtain a positive peak value, and then both exciting coils 1A and 1B become parallel defects. When approaching 11, the change in the eddy currents induced by the exciting coils 1A and 1B cancels each other and the detected value becomes zero. Further, when the exciting coil 1B moves away from the parallel defect 11, it is induced by the exciting coil 1A. This is because the eddy current changes and a negative peak value is obtained. Then, when an image based on this detection data is created, only the image corresponding to the end of the parallel defect 11 is generated as shown by the hatched portion in FIG. 5C, and the image corresponding to the center is not created (note that The two-dot chain line in FIG. 5C is an imaginary line of the parallel defect 11). Therefore, the inspector can visually determine the presence or absence of a defect by visually checking the detection data or image displayed on the monitor 8, but cannot accurately grasp the nature of the defect.

  For example, the eddy current inspection probe 10 is scanned in a direction orthogonal to the arrangement direction of the excitation coils 1A and 1B, and the defect 12 extending in the direction orthogonal to the arrangement direction of the excitation coils 1A and 1B (hereinafter referred to as “orthogonal defect”). ) (See FIG. 6A), a detection signal cannot be obtained. Therefore, detection data and an image cannot be created (see FIG. 6B and FIG. 6C), and the presence or absence of a defect cannot be determined.

  In contrast, for example, when the parallel defect 11 is inspected by scanning the eddy current inspection probe 3 of the present embodiment in a direction parallel to the arrangement direction of the exciting coils 1A and 1B (see FIG. 7A), for example, FIG. Detection data as shown in (b) is created. This detection data has a peak value corresponding to the vicinity of the end portion of the parallel defect 11, and further, a detection value corresponding to the central portion other than the end portion (in FIG. 7B, lower than the above-described peak value). Detection value). This is because even when both exciting coils 1A and 1B approach the parallel defect 11, at the position of the detection sensor 2 where the distance from the exciting coils 1A and 1B is different, the eddy currents induced by the exciting coils 1A and 1B are reduced. This is because a difference occurs in the change (change due to the influence of the parallel defect 11). When an image based on this detection data is created, an image of the entire defect can be created as shown by the hatched portion in FIG. Therefore, the inspector can visually determine the presence or absence of the parallel defect 11 by visually observing the detection data or image displayed on the monitor 8, and can accurately grasp the property of the defect.

  The inventors of the present application conducted a test inspection in order to confirm the effects of the above-described embodiment. FIG. 8A is a side view showing the structure of the subject 4A used for the test examination, and FIG. 8B is a plan view of the subject 4A viewed from the direction of arrow C in FIG. 8A. is there.

The subject 4A has two lateral holes 11A and 11B extending in the horizontal direction (vertical direction in FIG. 8B) as artificial defects (however, the depth H of the lateral hole 11A from the upper surface, which is the inspection surface). ₁ <depth H _{2 of the} lateral hole 11B) is formed. Then, the eddy current inspection probe is arranged so that the arrangement direction of the exciting coils 1A, 1B is parallel to the extending direction of the horizontal holes 11A, 11B of the subject 4A, and the eddy current inspection probe is arranged in the extending direction of the horizontal holes 11A, 11B. The test inspection was performed by scanning (thus, it can be said that the lateral holes 11A and 11B of the subject 4A correspond to the parallel defects described above). At this time, images on the plan view of the subject 4A created by the display processing unit 9 and displayed on the monitor 8 (however, the range D shown in FIG. 8B) are shown in FIGS. 9A and 9B. ). When the eddy current inspection probe 10 of the comparative example is used, as shown in FIG. 9A, an image corresponding to the end portion of the horizontal holes 11A and 11B appears, but an image corresponding to the central portion other than the end portion appears. Absent. On the other hand, when the eddy current inspection probe 3 of the present embodiment is used, as shown in FIG. 9B, not only the end portions of the lateral holes 11A and 11B but the entire image appears.

  As described above, in the present embodiment, the detection sensor 2 of the eddy current inspection probe 3 is arranged at a position shifted from the symmetry axis P between the excitation coils 1A and 1B, thereby associating the position or movement time of the detection sensor 2 with each other. The detection data of the detected sensor 2 can have not only a peak value corresponding to the vicinity of the end portion of the parallel defect 11 but also a detection value corresponding to the central portion other than the end portion. An image of the entire defect can be created based on this detection data. Therefore, the inspector can accurately grasp the property of the parallel defect 11 by visually observing the detection data or image created by the display processing unit 9 and displayed on the monitor 8.

  For example, when the orthogonal defect 12 is inspected using the eddy current inspection probe 3 of the present embodiment (see FIG. 10A), for example, detection data as shown in FIG. 10B is obtained. This detection data has a detection value corresponding to almost the whole of the orthogonal defect 12 although it is small. However, it is difficult to create an image based on this detection data (see FIG. 10C).

  A second embodiment of the present invention will be described.

  Fig.11 (a) is a top view showing the structure of the eddy current test | inspection probe by this embodiment, FIG.11 (b) is a sectional side view by the cross section EE in Fig.11 (a). Note that. In the present embodiment, the same parts as those in the first embodiment are denoted by the same reference numerals, and description thereof will be omitted as appropriate.

  In the present embodiment, the eddy current inspection probe 13 is spaced apart from each other in the radial direction so that the axial direction is substantially perpendicular to the inspection surface of the subject 4 as in the eddy current inspection probe 3 of the first embodiment. An excitation coil 1A, 1B arranged symmetrically, and a detection sensor 2 arranged between the excitation coils 1A, 1B and detecting changes in eddy currents induced in the subject 4 by the excitation coils 1A, 1B. Have. The detection sensor 2 is arranged so as to be displaced from the central axis O and the symmetry axis P.

  For example, when the parallel defect 11 is inspected by scanning the eddy current inspection probe 13 in a direction parallel to the arrangement direction of the exciting coils 1A and 1B (see FIG. 12A), for example, as shown in FIG. Detection data is created. This detection data has a peak value corresponding to the vicinity of the end portion of the parallel defect 11, and further, a detection value corresponding to the central portion other than the end portion (in FIG. 12B, it is lower than the above-described peak value). Detection value). When an image based on the detection data is created, an image of the entire defect can be created as shown by the hatched portion in FIG. Therefore, the inspector can visually determine the presence or absence of the parallel defect 11 by visually observing the detection data or image displayed on the monitor 8, and can accurately grasp the property of the defect.

  Further, for example, when the eddy current inspection probe 13 is scanned in a direction orthogonal to the arrangement direction of the excitation coils 1A and 1B to inspect the orthogonal defect 12 (see FIG. 13A), for example, as shown in FIG. Detection data is created. This detection data has a detection value corresponding to almost the entire orthogonal defect 12, and this detection value is higher than when the eddy current inspection probe of the first embodiment is used. This is because the detection sensor 2 in the first embodiment detects a change in eddy current flowing parallel to the orthogonal defect 12, whereas the detection sensor 2 in the present embodiment detects eddy current flowing obliquely in the orthogonal defect 12. It is because the change of is detected. Based on this detection data, an image of the entire defect can be created as indicated by the hatched portion in FIG. Therefore, the inspector can visually check the detection data or image displayed on the monitor 8 to determine the presence or absence of the orthogonal defect 11 and accurately grasp the property of the defect.

  A third embodiment of the present invention will be described.

  FIG. 14 is a diagram illustrating the overall configuration of the eddy current inspection apparatus according to the present embodiment, and FIG. 15 is a plan view illustrating the structure of the eddy current inspection probe according to the present embodiment. In addition, in FIG. 14, the eddy current inspection probe is shown by the side sectional view by the cross section FF in FIG. Moreover, in this embodiment, the same code | symbol is attached | subjected to the part equivalent to the said embodiment, and description is abbreviate | omitted suitably.

  In the present embodiment, the eddy current inspection probe 14 includes excitation coils 1A and 1B that are symmetrically arranged with their axial directions being substantially perpendicular to the inspection surface of the subject 4 and spaced apart from each other in the radial direction. Detection sensors 2A and 2B are arranged between the excitation coils 1A and 1B and detect changes in eddy currents induced in the subject 4 by the excitation coils 1A and 1B. The detection sensor 2A is arranged so that the position is shifted from the center axis O and the symmetry axis P, and the detection sensor 2B is arranged at a position where the center axis O and the symmetry axis P intersect.

  The eddy current inspection apparatus includes detection signal amplification units 6A and 6B that amplify detection signals from the detection sensors 2A and 2B, respectively, and detection signals amplified by the detection signal amplification units 6A and 6B at predetermined time intervals, respectively. Detection signal sampling units 7A and 7B to be sampled, and detection data in which the position or movement time of the detection sensor 2A is associated with the detection signals input from the detection signal sampling unit 7A are created and input from the detection signal sampling unit 7B The detection data of the detection sensor 2B that associates the position or movement time of the detection sensor 2B with the detection signal is created, and an image based on the addition processing of the detection data of the detection sensor 2A and the detection data of the detection sensor 2B is created. A display processing unit 15 for displaying the created detection data and image on the monitor 8.

  Then, for example, when the parallel defect 11 is inspected by scanning the eddy current inspection probe 14 in a direction parallel to the arrangement direction of the exciting coils 1A and 1B (see FIG. 16A), for example, as shown in FIG. The detection data of the detection sensor 2A and the detection data of the detection sensor 2B as shown in FIG. The position of the detection sensor 2A is arranged such that the amount of deviation from the central axis O and the central axis P is larger than the position of the detection sensor 2 in the second embodiment. Therefore, the detection data of the detection sensor 2A has a small detection value peak corresponding to the end portion of the parallel defect 11, and conversely, a detection value corresponding to the center portion becomes large, and these values are almost the same (in other words, paraphrase). For example, the degree of detection of the central portion of the parallel defect 11 is increased). On the other hand, the position of the detection sensor 2B is the same as the position of the detection sensor 2 in the comparative example. Therefore, the detection data of the detection sensor 2B has a large peak value corresponding to the end of the parallel defect 11, and the detection value corresponding to the center is zero (in other words, the end of the parallel defect 11). Detection is improved). Therefore, an image of the entire defect can be created based on the addition of these detection data, as indicated by the hatched portion in FIG. 16D, and the accuracy of the image can be increased. Therefore, the inspector can visually check the detection data or image displayed on the monitor 8 to determine the presence or absence of the orthogonal defect 11 and accurately grasp the property of the defect.

  For example, when the eddy current inspection probe 14 is scanned in a direction orthogonal to the arrangement direction of the exciting coils 1A and 1B to inspect the orthogonal defect 12, the detection data of the detection sensor 2A is the detection sensor in the second embodiment. Similar to the detection data of No. 2 (see FIG. 13B described above), it has a detection value corresponding to almost the entire orthogonal defect 12. Based on this detection data, an image of the entire defect can be created. Therefore, the inspector can visually check the detection data or image displayed on the monitor 8 to determine the presence or absence of the orthogonal defect 11 and accurately grasp the property of the defect.

  In the above description, the case of detecting a defect of an object has been described as an example. However, the present invention is not limited to this. For example, the present invention may be applied to the case of detecting an abnormal part such as a plate thickness change or a structural discontinuity. Needless to say.

It is the schematic showing the structure of the eddy current test | inspection apparatus by the 1st Embodiment of this invention. It is a top view showing the structure of the eddy current inspection probe by the 1st Embodiment of this invention. It is a flowchart for demonstrating the operation | movement in the 1st Embodiment of this invention. It is the top view and side sectional view showing the structure of the eddy current inspection probe by a comparative example. It is a figure for demonstrating the detection data and image which are obtained when a parallel defect is test | inspected using the eddy current test | inspection probe of a comparative example. It is a figure for demonstrating the detection data and image which are obtained when an orthogonal defect is test | inspected using the eddy current test | inspection probe of a comparative example. It is a figure for demonstrating the detection data and image which are obtained when a parallel defect is test | inspected using the eddy current test | inspection probe of the 1st Embodiment of this invention. It is the side view and top view showing the structure of the subject used for the test | inspection. It is a figure showing the image obtained when using the eddy current test | inspection probe of a comparative example in a test test | inspection, and the image obtained when using the eddy current test probe of the 1st Embodiment of this invention. It is a figure for demonstrating the detection data and image which are obtained when an orthogonal defect is test | inspected using the eddy current test | inspection probe of the 1st Embodiment of this invention. It is the top view and side sectional view showing the structure of the eddy current inspection probe by the 2nd Embodiment of this invention. It is a figure for demonstrating the detection data and image which are obtained when a parallel defect is test | inspected using the eddy current test | inspection probe of the 2nd Embodiment of this invention. It is a figure for demonstrating the detection data and image which are obtained when an orthogonal defect is test | inspected using the eddy current test | inspection probe of the 2nd Embodiment of this invention. It is the schematic showing the structure of the eddy current test | inspection apparatus by the 3rd Embodiment of this invention. It is a top view showing the structure of the eddy current inspection probe by the 3rd Embodiment of this invention. It is a figure for demonstrating the detection data and image which are obtained when a parallel defect is test | inspected using the eddy current test | inspection probe of the 3rd Embodiment of this invention.

Explanation of symbols

1A, 1B Excitation coil 2 Detection sensor 2A Detection sensor 2B Detection sensor 3 Eddy current inspection probe 8 Monitor (display)
9 Display processing unit (display processing means)
13 Eddy Current Inspection Probe 14 Eddy Current Inspection Probe 15 Display Processing Unit (Display Processing Means)
O Center axis P Symmetry axis

Claims (6)

A pair of excitation coils disposed symmetrically with an axial direction substantially perpendicular to the test surface of the subject and spaced apart from each other in the radial direction; and the excitation coil disposed between the pair of excitation coils. An eddy current inspection probe having only one detection sensor for detecting a change in eddy current induced in the subject by
The eddy current inspection probe is characterized in that the only one detection sensor is arranged with a position shifted from an axis of symmetry between the pair of exciting coils. 2. The eddy current inspection probe according to claim 1, wherein the only one detection sensor is arranged with a position shifted from a central axis connecting the radial center positions of the pair of exciting coils. 3. A pair of excitation coils disposed symmetrically with an axial direction substantially perpendicular to the test surface of the subject and spaced apart from each other in the radial direction; and the excitation coil disposed between the pair of excitation coils. In an eddy current inspection probe having first and second detection sensors for detecting a change in eddy current induced in the subject by
The first detection sensor is arranged with a position shifted from the axis of symmetry between the pair of excitation coils and a position shifted from a center axis connecting the radial center positions of the pair of excitation coils,
The eddy current inspection probe, wherein the second detection sensor is arranged at a position where the symmetry axis and the central axis intersect. A pair of excitation coils arranged symmetrically with their axial directions being substantially perpendicular to the test surface of the subject and spaced apart from each other in the radial direction, and the excitation coils disposed between the pair of excitation coils The detection data or the detection data of the detection sensor associated with the eddy current inspection probe having only one detection sensor for detecting a change in the eddy current induced in the subject by the position and the movement time of the detection sensor In an eddy current inspection apparatus comprising a display processing means for creating an image based on the above and displaying the created detection data or image on a display,
The eddy current inspection apparatus is characterized in that the only one detection sensor is arranged with a position shifted from the axis of symmetry between the pair of exciting coils. 5. The eddy current inspection apparatus according to claim 4, wherein the one detection sensor is arranged with a position shifted from a central axis connecting the central positions of the pair of exciting coils. A pair of excitation coils arranged symmetrically with their axial directions being substantially perpendicular to the test surface of the subject and spaced apart from each other in the radial direction, and the excitation coils disposed between the pair of excitation coils The first detection in which the eddy current inspection probe having first and second detection sensors for detecting a change in eddy current induced in the subject is associated with the position or movement time of the first detection sensor. The detection data created by creating detection data of the second detection sensor associated with the detection data of the sensor and the position or movement time of the second detection sensor, or creating an image based on the detection data Or in an eddy current inspection apparatus comprising a display processing means for displaying the image on a display,
The first detection sensor is arranged with a position shifted from the axis of symmetry between the pair of excitation coils and a position shifted from a center axis connecting the radial center positions of the pair of excitation coils,
The second detection sensor is arranged at a position where the axis of symmetry and the central axis intersect.

Images