JP3542315B2 - Shield for coil - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a coil shield for reducing electromagnetic coupling between a coil and an external object.
[0002]
[Prior art]
In a position sensor, a metal sensor, an automatic train stop system, and the like, a resonance circuit is incorporated, and a change in the resonance frequency of the resonance circuit is used. Further, in a system for transmitting and receiving information by a loop antenna, a resonance circuit including the loop antenna is formed, and it is required that the resonance frequency of the resonance circuit does not change unlike a position sensor or the like. The resonance frequency and Q value of the resonance circuit having the inductance L, the capacitance C, and the resistance R are expressed by the following equations (1) and (2), respectively.
Resonance frequency = 1 / {2π (LC) ^1/2 } (1)
Q value = (L / C) ^1/2 / R (2)
[0003]
For example, in a position sensor or a metal sensor, an alternating current is applied to a coil in a resonance circuit to generate an alternating magnetic field, and when a metal body approaches the coil, an eddy current is induced in the metal body to cause a secondary alternating current. A magnetic field is generated, and the inductance L of the coil changes due to the secondary AC magnetic field, and the change in the inductance L changes the resonance frequency of the resonance circuit. To detect.
[0004]
In the automatic train stop system, a grounding element including a resonance circuit whose resonance frequency is changed by a signal from a traffic light is installed on the ground, and a vehicle element including an oscillation circuit oscillating at a certain frequency is installed on the train. I have. In order to stop the train, the grounding element is controlled so that the resonance frequency of the grounding element is different from the oscillation frequency of the vehicle element, and the vehicle element is electromagnetically coupled to the vehicle element by passing the train. The oscillation frequency is converted to the resonance frequency of the grounding element, and this frequency conversion is detected on the train side to stop the train.
[0005]
Therefore, in the above-described resonance circuit incorporated in a position sensor, a metal sensor, or the like, it is a necessary minimum condition that a resonance frequency does not change due to an object other than an object to be detected. The minimum condition is that the resonance frequency does not change in the built-in resonance circuit, and it is also desired that the Q value is high in any of the resonance circuits.
[0006]
By the way, a position sensor, a metal sensor, or the like may be installed on a desk, a housing of a machine, a floor, or the like. In some cases, the desk or the housing of the machine is made of iron, and the floor or the like also contains iron materials such as reinforcing bars and steel frames. Also, even in the automatic train stop system, the ground rails may be installed on concrete sleepers or slab roadbeds, and iron materials such as reinforcing bars are also present in these. However, when a conductive object such as an iron material exists near the coil in the resonance circuit, the inductance L of the coil decreases, and the resonance frequency of the resonance circuit increases as compared with the standard state.
[0007]
The reason is understood from the case where a current is virtually passed through the coil. That is, the AC magnetic field generated by the current flowing through the coil induces an eddy current in the conductive object, and the eddy current generates a secondary AC magnetic field. This secondary AC magnetic field is generated so as to cancel the AC magnetic field generated by the coil. In other words, even if the same amount of current flows through the coil, if a conductive object is present in the vicinity, the substantial AC magnetic field generated by the coil decreases, and the inductance L of the coil decreases. As a result, as is apparent from equation (1), the resonance frequency of the resonance circuit including the coil increases as compared with the standard state.
[0008]
In addition, when an eddy current is induced in an external object, the eddy current loss represented by the following equations (3) and (4) is reduced when the skin thickness is sufficiently large and sufficiently small, respectively. Occurs in objects. Then, the eddy current loss becomes a resistance component of the resonance circuit, and the resistance R of the resonance circuit increases. In the equation (3) (4) below, H _m is the amplitude of the magnetic field, the magnetic permeability of μ outside of the object, [rho is the resistivity of the external world of the object, f is the frequency of the alternating magnetic field, t is outside of the object Is the thickness.
Eddy current loss = (πtμH _m f) ² / 6ρ (3)
Eddy current loss = {H _m ² (μρf) ^1/2 } / 4π (4)
[0009]
Then, as described above, when eddy current loss occurs in an external object and the resistance R of the resonance circuit increases, the Q value of the resonance circuit decreases as is apparent from the equation (2). Since iron has a high magnetic permeability μ, the eddy current loss generated by the iron material is large as is apparent from the equations (3) and (4). For this reason, when a coil is installed near the iron material, the resistance R of the resonance circuit including the coil increases considerably, and as a result, the Q value decreases considerably.
[0010]
In other words, simply by placing the coil in the resonance circuit incorporated in the position sensor or the like near the iron material, the resonance frequency of the resonance circuit changes and the Q value also decreases. Does not work. To avoid such inconvenience, a coil in the resonance circuit may be shielded so that a magnetic field generated by the coil does not reach an external object, for example, an iron material in a floor or the like.
[0011]
FIGS. 7C and 7D show a conventional example of a coil shield applied to an automatic train stop system (for example, JP-A-10-53135 and JP-A-8-91217). The coil shielding body 11 is made of only ferrite, and is provided between the winding part 13 and the core part 14 of the coil 12 and the iron material 15 which is an object capable of causing electromagnetic coupling with the coil 12. The electromagnetic coupling between the coil 12 and the iron material 15 is reduced.
[0012]
[Problems to be solved by the invention]
As shown in FIG. 7A, a coil in a resonance circuit having a resonance frequency and a Q value of 129.78 kHz and 222 in a standard state where the coil shield 11 is not provided and the iron material 15 is not present, respectively. 7B, the resonance frequency and the Q value of the resonance circuit become 130.27 kHz and 125, respectively, and the resonance frequency changes and the Q value as described above. Decreases.
[0013]
On the other hand, when the coil 12 of this resonance circuit is provided with the above-described conventional coil shield 11 as shown in FIG. 7C, the resonance frequency and the Q value become 120.05 kHz and 231 respectively. When the coil 12 provided with the coil shield 11 is brought close to the iron material 15 as shown in FIG. 7D, the resonance frequency and the Q value of the resonance circuit become 120.24 kHz and 175, respectively. That is, in the coil 12 provided with the coil shield 11, the Q value of the resonance circuit approaches the standard state as compared with the case where the coil 12 not provided with the coil shield 11 is brought close to the iron material 15. Thus, the change in the resonance frequency due to the presence or absence of the iron material 15 is also suppressed to 0.19 kHz.
[0014]
However, since the resonance frequency changes from 129.78 kHz to 120.05 kHz only by providing the coil shield 11 in the coil 12 in the resonance circuit, the resonance circuit does not operate normally with this. Therefore, the invention of the present application provides a coil shielding body in which the resonance frequency does not easily change even when the coil is provided in the resonance circuit, and the resonance frequency does not easily change and the Q value does not decrease due to electromagnetic coupling with an external object. It is intended to be.
[0015]
[Means for Solving the Problems]
The coil shield according to claim 1 includes a soft magnetic material that directly faces the winding portion of the coil in the axial direction of the coil, and a magnetic field generated from the coil is strongest in the vicinity of the facing portion. The soft magnetic material is effectively magnetized. Then, on the object side of the outside world opposite to the coil of the soft magnetic material, the direction of the magnetic field by the coil and the direction of the magnetic field by the magnetized soft magnetic material are opposite to each other, and the leakage magnetic flux from the coil is weakened.
[0016]
For this reason, the electromagnetic coupling between the coil and the external object is reduced, and the resonance frequency of the resonance circuit including the coil is hardly changed by the external object. In the core of the coil, the direction of the magnetic field generated by the coil is the same as the direction of the magnetic field generated by the magnetized soft magnetic material. Therefore, the magnetic flux in the core of the coil is strengthened, and the inductance of the coil is increased. The soft magnetic material preferably has a relative permeability of 10 or more so that a sufficient magnetic field is generated from the magnetized soft magnetic material.
[0017]
On the other hand, a conductive material that directly faces the core of the coil in the axial direction of the coil is also provided. During conductive material eddy currents are induced by the magnetic field of the coil, moreover, the area A of the conductive material is 0.5S ₁ or more in the eddy currents in the conductive material because is effectively induced. Since the secondary magnetic field due to the eddy current is generated so as to cancel the magnetic field generated by the coil, the leakage magnetic flux from the coil is effectively reduced . The resistivity of the conductive material is preferably 1 × 10 ⁻² Ω · cm or less so that an eddy current is effectively induced. When the projected area in the axial direction of the entire coil including the winding part and the core part is S ₂ , the area A of the conductive material is preferably 2S ₂ or less. Outside the coil. Since the magnetic field outside the coil is opposite to the magnetic field inside the core of the coil, if the area A is more than 2S ₂ , the effect of weakening the magnetic flux leaking from the coil is reduced.
[0018]
Therefore, the reduced electromagnetic coupling between the coil and the outside world of the object, the resonance frequency of the resonance circuit including the coil rather difficulty varies with the outside world of the object, an eddy current loss occurring in the outside world of the object also have little. In addition, since the direction of the magnetic field generated by the coil and the direction of the magnetic field generated by the conductive material are reversed in the core of the coil, the magnetic flux in the core of the coil is weakened, and the inductance of the coil is reduced.
[0019]
In other words, the change in coil inductance can be suppressed by canceling the increase in the inductance of the coil due to the soft magnetic material and the decrease in the inductance due to the conductive material. Even if the coil in the resonance circuit is provided with the coil shield, the resonance frequency can be reduced. Is unlikely to change from the standard state. Also, both the soft magnetic material and the conductive material weaken the magnetic flux leakage from the coil on the object side of the external world, so the electromagnetic coupling between the coil and the external world object is reduced, and the resonance frequency of the resonance circuit including the coil is affected by the external object. Hard to change.
[0020]
In the coil shield according to claim 2, since the resistivity of the soft magnetic material is 1 Ω · cm or more, the soft magnetic material is a non-conductive soft magnetic material having a high resistivity. Therefore, an eddy current is hardly induced in the soft magnetic material, an eddy current loss generated in the soft magnetic material is small, and an eddy current loss generated in the coil shield is small. Further, since both the soft magnetic material and the conductive material weaken the magnetic flux leaking from the coil on the object side in the outside, eddy current loss generated in the object in the outside is small. Therefore, a decrease in the Q value of the resonance circuit including the coil is suppressed. Further, the change in the inductance of the coil is suppressed by canceling the increase due to the soft magnetic material and the decrease due to the conductive material. Therefore, even if the coil is provided with the coil shield, the resonance frequency of the resonance circuit including the coil does not easily change.
[0021]
In the coil shield according to the third aspect, since the soft magnetic material has a resistivity of 1 Ω · cm or more, the soft magnetic material is a non-conductive soft magnetic material having a high resistivity, and a vortex generated in the soft magnetic material. Low current loss. Furthermore, since the relative magnetic permeability of the conductive material is 1.0 ± 0.1, this conductive material is a non-magnetic conductive material, and as is apparent from the equations (3) and (4), the vortex generated in the conductive material. Low current loss. Further, since both the soft magnetic material and the conductive material weaken the magnetic flux leaking from the coil on the object side in the outside, eddy current loss generated in the object in the outside is small. Therefore, a decrease in the Q value of the resonance circuit including the coil is suppressed. Further, the change in the inductance of the coil is suppressed by canceling the increase due to the soft magnetic material and the decrease due to the conductive material. Therefore, even if the coil is provided with the coil shield, the resonance frequency of the resonance circuit including the coil does not easily change.
[0022]
The coil shield according to claim 4, since the width w ₁ of the soft magnetic material is at least the width W of the winding of the coil, the magnetic field of the coil is small leaking from between the soft magnetic material and the conductive material. Therefore, at ambient to the object side is leakage flux from the coil is weakened effectively, electromagnetic coupling between the object of the coil and the outer boundary is reduced. As a result, the resonance frequency of the resonance circuit including the coil is hardly changed by an external object, and the eddy current loss generated in the external object is small. However, the inductance of the coil increases. As shown in FIG. 8, the strength of the magnetic field generated by the coil rapidly decreases as the distance from the winding of the coil increases. Therefore, when the winding portion is cylindrical, the average diameter is D, and when the winding portion is other than cylindrical, (diameter of the inscribed circle of the winding portion + diameter of the circumscribed circle of the winding portion) / 2 is D, the distance h from the coil to the soft magnetic material is preferably 0.2D or less so that the soft magnetic material is effectively magnetized .
[0023]
Their to, by offsetting the decrease due to the increase and the conductive material by a soft magnetic material of the inductance of the coil, the change in inductance of the coil can be suppressed. As a result, even if the coil is provided with the coil shield, the resonance frequency of the resonance circuit including the coil does not easily change.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an embodiment and an example of the present invention will be described with reference to FIGS. FIG. 1 shows a coil shield of the present embodiment. The coil shield 21 includes a soft magnetic material 22 facing the winding portion 13 of the coil 12 in the axial direction of the coil 12 and a conductive material 23 facing the core portion 14 of the coil 12 in the axial direction of the coil 12. I have it.
[0025]
The resistivity of the soft magnetic material 22 is 1 Ω · cm or more, and the relative magnetic permeability of the conductive material 23 is 1.0 ± 0.1. The width w ₁ of the soft magnetic material 22 is not less than W, where W is the width of the winding portion 13 in a plane perpendicular to the axis of the coil 12, and S ₁ is the projected area of the core portion 14 in the axial direction. The area A of the conductive material 23 is 0.5 S ₁ or more. Since the width w ₁ of the soft magnetic material 22 is equal to or larger than the width W of the winding portion 13, the magnetic field 24 leaking from between the soft magnetic material 22 and the conductive material 23 is small.
[0026]
As shown in FIG. 2, the soft magnetic material 22 is magnetized by the magnetic field 24 of the coil 12 to generate a magnetic field 25. On the iron material 15 side, the direction of the magnetic field 24 and the direction of the magnetic field 25 are opposite to each other, and the leakage magnetic flux from the coil 12 is weakened. Therefore, the electromagnetic coupling between the coil 12 and the iron material 15 is reduced, and the resonance frequency of the resonance circuit including the coil 12 is hardly affected by the presence or absence of the iron material 15. Even if the iron material 15 is present, the electromagnetic coupling between the iron material 15 and the coil 12 is weak, so that the eddy current loss generated in the iron material 15 is small.
[0027]
Further, since the resistivity of the soft magnetic material 22 is as high as 1 Ω · cm or more, eddy current is hardly induced in the soft magnetic material 22 and eddy current loss generated in the coil shield 21 is small. However, since the direction of the magnetic field 24 and the direction of the magnetic field 25 in the core portion 14 are the same, the magnetic flux in the core portion 14 is strengthened, and the inductance L of the coil 12 increases. On the other hand, since the area A of the conductive material 23 is 0.5 S ₁ or more, an eddy current is effectively induced in the conductive material 23 by the magnetic field 24.
[0028]
Since the secondary magnetic field 26 due to the eddy current is generated so as to cancel the magnetic field 24, the leakage magnetic flux from the coil 12 is weakened, and the electromagnetic coupling between the coil 12 and the iron material 15 is reduced. Therefore, the resonance frequency of the resonance circuit including the coil 12 is hardly affected by the presence or absence of the iron material 15. Even if the iron material 15 exists, the electromagnetic coupling between the iron material 15 and the coil 12 is weak, so that the eddy current loss generated in the iron material 15 is small.
[0029]
Further, since the relative magnetic permeability of the conductive material 23 is 1.0 ± 0.1, as apparent from the equations (3) and (4), even if an eddy current is induced in the conductive material 23, The loss is small. Further, since the magnetic field 26 is generated so as to cancel the magnetic field 24 as described above, the magnetic flux in the core portion 14 is weakened, and the inductance L of the coil 12 decreases.
[0030]
As described above, since the electromagnetic coupling between the coil 12 and the iron material 15 is reduced by the soft magnetic material 22 and the conductive material 23, the resonance frequency of the resonance circuit including the coil 12 is hardly affected by the presence or absence of the iron material 15. Furthermore, the eddy current loss generated in the iron material 15 is small, and the eddy current loss generated in the soft magnetic material 22 and the conductive material 23 is also small. Less loss. As a result, an increase in the resistance R of the resonance circuit including the coil 12 is suppressed, and a decrease in the Q value of the resonance circuit is suppressed as is apparent from the above-described equation (2).
[0031]
On the other hand, the magnetic field 25 generated from the soft magnetic material 22 increases the inductance L of the coil 12, so that the resonance frequency of the resonance circuit including the coil 12 decreases as is apparent from the above-described equation (1). Further, since the secondary magnetic field 26 due to the eddy current in the conductive material 23 reduces the inductance L of the coil 12, the resonance frequency of the resonance circuit including the coil 12 is increased as is apparent from the above-described equation (1). increase.
[0032]
FIG. 4 shows the relationship between the area A of the conductive material 23 and the resonance frequency of the resonance circuit, and includes the coil 12, but does not include the coil shield 21 and does not include the iron material 15. The resonance frequency of the resonance circuit in the state is also shown. The decrease in the resonance frequency with respect to the resonance frequency in the standard state at the area A = 0 corresponds to the increase in the inductance L of the coil 12 due to the soft magnetic material 22.
[0033]
As is clear from FIG. 4, as the area A increases from 0, the increase in the inductance L of the coil 12 due to the soft magnetic material 22 is offset by the decrease in the inductance L of the coil 12 due to the conductive material 23. As a result, the resonance frequency increases. For this reason, if the area A where the resonance frequency is equal to the resonance frequency in the standard state is adopted within the above-mentioned range of 0.5 S ₁ or more, the change in the resonance frequency of the resonance circuit is suppressed.
[0034]
【Example】
In the present embodiment, a resonance circuit including the coil 12 having a winding portion 13 having a width W = 30 mm and an average diameter D = 250 mm was prepared. As shown in FIG. 7A, the resonance frequency and the Q value of this resonance circuit in the standard state where the coil shielding body 21 is not provided and the iron material 15 is not present are 129.78 kHz and 222, respectively. Was. However, when a section steel as the iron material 15 is arranged 90 mm below the coil 12 as shown in FIG. 7B, the resonance frequency and the Q value of this resonance circuit become 130.27 kHz and 125, respectively. Changed by 0.49 kHz and the Q value decreased by 97.
[0035]
Further, in this embodiment, the soft magnetic material 22 made of Ni—Zn ferrite having a resistivity of 1 × 10 ⁴ Ω · cm and a thickness of 3 mm and a resistivity of 2.83 × 10 ⁻⁶ Ω · cm A coil shield 21 including an Al conductive material 23 having a length of 1 mm was prepared, and the coil shield 21 was provided on the coil 12. Since the distance h between the winding portion 13 and the soft magnetic material 22 is preferably 0.2 D or less, that is, 50 mm or less in this case, h is set to 10 mm.
[0036]
As shown in FIG. 5, in a resonance circuit in which the coil shield 21 is provided in the coil 12, in a case where a shaped steel as the iron material 15 is disposed 90 mm below the coil 12, the conductive material 23 is used. The relationship between the area A, the resonance frequency, and the Q value was tested, and the results in FIG. 6 were obtained. Since 0.5S _{1 to} 2S ₂ is 190 to 1231 cm ² , FIG. 6 shows that 314 cm ² (diameter 200 mm) is set as the area A of the conductive material 23 in which the resonance frequency is substantially equal to the resonance frequency in the standard state within this range. Selected.
[0037]
When the coil 12 is provided with the coil shield 21 and the iron material 15 is not present, the resonance frequency of the resonance circuit is 129.84 kHz. Accordingly, the change in the resonance frequency from the standard state (129.78 kHz) due to the provision of the coil shield 21 in the coil 12 is 0.06 kHz, and the resonance frequency hardly changes even if the coil shield 21 is provided.
[0038]
Further, when the coil 12 is provided with the coil shield 21 and the iron material 15 is present, the resonance frequency and the Q value of the resonance circuit are 129.94 kHz and 167, respectively. The change in resonance frequency due to the presence or absence of the iron material 15 in the state where the coil shielding body 21 is provided in the coil 12 is 0.10 kHz, and the change in the resonance frequency of the iron material 15 in the state where the coil shielding body 21 is not provided in the coil 12. The change in the resonance frequency is smaller than the change in the resonance frequency due to the presence or absence of 0.49 kHz.
[0039]
In addition, when the coil 12 was not provided with the coil shield 21, the Q value was reduced by 97 due to the presence of the iron material 15. However, when the coil 12 was provided with the coil shield 21, The decrease in the Q value due to the presence of 15 is suppressed to 55. 7C and 7D, even when the coil 12 is provided with the coil shielding body 21, the resonance frequency is hardly changed from the standard state, and it depends on the presence or absence of the iron material 15. The change in the resonance frequency is 0.19 kHz in the conventional example, but as small as 0.10 kHz in the present embodiment.
[0040]
Further, in the coil shield 21 of the present embodiment, the soft magnetic material 22 made of Ni—Zn ferrite has an annular shape, and the amount of ferrite by the central opening is smaller than that of the conventional coil shield 11. Few. Since Al is considerably less expensive than Ni-Zn ferrite, the coil shield 21 of the present embodiment is less expensive than the conventional coil shield 11.
[0041]
Although the soft magnetic material 22 made of Ni-Zn ferrite is used for the coil shielding body 21 of the above embodiment, the soft magnetic material other than Ni-Zn ferrite and the soft magnetic material other than oxide soft magnetic material are used. May be used. Further, only for the purpose of not changing the resonance frequency depending on the presence or absence of the iron material 15, the soft magnetic material 22 does not need to be non-conductive, and the conductive material 23 does not need to be non-magnetic.
[0042]
【The invention's effect】
The coil shield according to the invention of the present application is composed of a soft magnetic material and a conductive material. For this reason, the change in the inductance of the coil can be suppressed by canceling the increase in the inductance of the coil due to the soft magnetic material and the decrease in the inductance due to the conductive material, and the resonance can be suppressed even if the coil in the resonance circuit is provided with the coil shield. The frequency is hard to change from the standard state.
[0043]
In addition, since both the soft magnetic material and the conductive material weaken the leakage flux from the coil to the external object side, the electromagnetic coupling between the coil and the external object is reduced, and the eddy current loss generated in the external object is small. Therefore, if the coil in the resonance circuit is provided with a coil shield, the resonance frequency is hardly changed by an external object, and the Q value is hardly reduced. Furthermore, if the soft magnetic material is made non-conductive and the conductive material is made non-magnetic, the eddy current loss generated in the coil shield is small, and even if the coil in the resonance circuit is provided with the coil shield, the Q value is reduced. Is difficult to decrease.
[Brief description of the drawings]
FIG. 1 is a side sectional view showing a state in which an embodiment of the present invention is provided in a coil.
FIG. 2 is a side sectional view showing a relationship between a magnetic field of a soft magnetic material and a magnetic field of a coil according to an embodiment of the present invention.
FIG. 3 is a side sectional view showing a relationship between a magnetic field generated by a conductive material and a magnetic field generated by a coil according to the embodiment of the present invention.
FIG. 4 is a graph showing a relationship between an area A of a conductive material and a resonance frequency of a resonance circuit in one embodiment of the present invention.
FIG. 5 is a side sectional view showing a state where an embodiment of the present invention is provided in a coil and an iron material is present near the coil.
FIG. 6 is a graph showing the relationship between the area A of the conductive material and the resonance frequency and Q value of the resonance circuit in the embodiment of the present invention shown in FIG. 5;
7A is a side sectional view of a coil, FIG. 7B is a side sectional view of a state in which an iron material is present near the coil, and FIG. 7C is a coil provided with a conventional coil shield. (D) is a side sectional view of a state in which a conventional coil shield is provided between a coil and an iron material.
FIG. 8 is a graph showing a relationship between a distance of a coil from a winding portion and a magnetic field intensity.
[Explanation of symbols]
12 coil, 13 winding part, 14 core part, 21 shield for coil, 22 soft magnetic material, 23 conductive material, 24 to 26 magnetic field

Claims (4)

A soft magnetic material that directly faces the winding part of the coil in the axial direction of the coil;
A conductive material that directly faces the core of the coil in the axial direction ,
Wherein the projected area of the core portion as S _1, the shaft and the conductive material of the area A is 0.5S ₁ or Der Ru coil shield in a plane perpendicular to the axial direction. The coil shield according to claim 1, wherein the soft magnetic material has a resistivity of 1 Ω · cm or more. 3. The coil shield according to claim 2, wherein the relative permeability of the conductive material is 1.0 ± 0.1. The width of the winding portion before Symbol plane and is W, coil shield according to any one of claims 1-3 width w ₁ of the soft magnetic material in the plane is not less than W .

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