JPH08233866A - Photomagnetic field sensor - Google Patents

Abstract

PURPOSE: To measure a magnetic field in the state that the relationship between the magnitude of the field and the output is linear and a phase angle is small by so providing that the relative main axis angle between polarizers and the Faraday rotary angle of a magneto-optic element satisfy a specific formula. CONSTITUTION: Specific formulae I and II are θ <= arccot (βhotanϕ) and θ>= arccot (αhotanϕ), where ho = rated magnetic field/(saturated magnetic field of magneto-optic element), β is an amount decided from according to an error and applied magnetic field/rated magnetic field. The a is represented by α= (2-C<2> +(4-7C<2> /2)<1/2> ]/2C<2> }<1/2> (C is a modulation factor). It is so set as to satisfy the formulae I and II. Then, the Faraday rotary angle θ is regulated by regulating the thickness of a magneto-optic element 6, and the relative main axis angles θ of two polarizers 14, 15 are regulated by regulating the quarrying angle of polarizing glass for forming the polarizers 14, 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical magnetic field sensor for measuring magnetic field strength by utilizing the Faraday effect of a magneto-optical element, and more particularly to a power transmission line and a distribution line for supplying electric power and a power receiving and transforming facility (hereinafter referred to as "cubicle"). ) And GIS (GAS
INSULATED SWITCH GEAR) and the like, the present invention relates to an optical magnetic field sensor that detects the magnitude of a current by measuring the strength of a magnetic field generated around an electric wire and a general optical magnetic field sensor that measures an alternating magnetic field.

[0002]

2. Description of the Related Art Current sensors used in cubicles and GIS to detect anomalies by measuring the magnitude of current flowing through power transmission lines or distribution lines, which are electric power transmission routes from power plants to consumption areas. The sensor of the transformer type has been used so far. However, this transformer-type current sensor has problems such as large size, heavy weight, and poor insulation.

The principle of the optical magnetic field sensor is that a magnetic field generated around a conductor (for example, a transmission line) by an electric current is measured by using the Faraday effect of a magneto-optical material, and a current value is obtained from the measured magnetic field. It is in. The advantages of the optical magnetic field sensor are high withstand voltage, high insulation, non-contact, small size and light weight, and no need for a power source or electric circuit on the high voltage side.

FIG. 1 shows the basic structure of a current measuring optical magnetic field sensor that has been used so far. Light emitted from a light emitting diode or other light source 1 passes through an optical fiber 2, a lens 3 and a first polarization beam splitter (hereinafter referred to as “PBS”) 4, and becomes linearly polarized light. Furthermore, half-wave plate 5
And enters the magneto-optical element 6. When the light passes through the magneto-optical element 6, it receives optical rotation according to the strength of the magnetic field to be measured (hereinafter simply referred to as “magnetic field”). Further, the light passes through the second PBS 7 and has an intensity according to the strength of the magnetic field, and is condensed on the optical fiber 9 by the lens 8.

In the conventional optical magnetic field sensor, the half-wave plate 5 is used.
Is used because the plane of polarization is rotated by 45 degrees and the first PBS4
This is because the relative principal axis angle between the second PBS 7 and the second PBS 7 is 45 degrees. Further, the magnetic field and the path of the light passing through the magneto-optical element 6 are parallel to each other. The characteristics of the half-wave plate 5 and the magneto-optical element 6 are hardly changed even if the arrangements thereof are exchanged.

The light focused on the optical fiber 9 is guided to the photodetector 10 by the optical fiber 9 and photoelectrically converted. After that, the signal processing circuit 11 separates the AC component and the DC component, and the divider outputs the AC voltage component / DC voltage component. The measured magnetic field is represented by the effective value calculated in this way. The AC voltage component / DC voltage component is obtained in order to detect the desired magnetic field by eliminating the fluctuation of the light intensity emitted from the light source and the fluctuation of the light amount due to the fluctuation of the optical fiber.

Magneto-optical materials used for such an optical magnetic field sensor include lead glass, ZnSe, BGO and BS.
There are paramagnetic materials such as O, and diamagnetic materials. But,
Recently, it has been promoted to use the optical magnetic field sensor for current measurement of power transmission lines and distribution lines, GIS, current transformers for measuring instruments in cubicles. Since such an optical magnetic field sensor is required to have high sensitivity, small size, and low price, a magnetic garnet having high mass productivity and high magnetic sensitivity, and further, a Bi-substituted magnetic garnet for a magneto-optical element are required. The development of optical magnetic field sensors using the is becoming popular.

As a magneto-optical element, RIG (Bi: Rare E
When a ferromagnetic material exhibiting a multi-domain structure with perpendicular magnetization such as arth iron garnet) is used, the light transmitted through this magneto-optical element causes a diffraction phenomenon, and 0th-order light, 1st-order light, ---, n-order light And spreads according to the order (however, when the magnetic field is 0, only odd-order light). This is because the magnetic domain serves as a phase grating. If all these diffracted light is captured, the linearity between the magnetic field and the output from the sensor is good (eg
JP-A-5-126924). However, since all diffracted light can be captured only after proper selection of lenses and precise placement of lenses and optical components, mass productivity of such optical magnetic field sensors must be low. I don't get.

On the other hand, if not all the diffracted light is taken in, the alignment at the time of manufacturing the optical magnetic field sensor becomes easy, and an inexpensive optical magnetic field sensor can be realized, but the output from the magnetic field and the signal processing circuit can be realized. The relationship between and becomes out of the linear relationship, and it becomes impossible to satisfy the 1PS class ratio error required for the optical magnetic field sensor for measuring a large alternating current. Here, the 1PS class specific error is JEC
The linearity of the sensor output with respect to an arbitrary magnetic field defined by (Electrical Standards Committee Standard) is defined by the following equation (4). R = (K _n −K) / K (4) In the above formula (4), R = ratio error, K _n = (sensor output when rated magnetic field H ₀ is applied) /
(Rated magnetic field H ₀ ), K = (sensor output when an arbitrary magnetic field is applied) / (arbitrary magnetic field), The rated magnetic field H ₀ varies depending on the purpose of use of the optical magnetic field sensor. When the ratio error R = 0%, a perfect linear relationship is established between the magnitude of the magnetic field and the sensor output.

Here, the 1PS class defined by JEC means that the ratio error R is (1) ± 1% or less at the rated magnetic field H ₀ ,
(2) ± 1.5% or less in a magnetic field of 0.2H ₀ ,
(3) A standard within a range of ± 3.0% or less in a magnetic field of 0.05 H ₀ . As described above, when a ferromagnetic material exhibiting a multi-domain structure with perpendicular magnetization is used for the magneto-optical element of an optical magnetic field sensor, it has the characteristic of high sensitivity, while it has a straight line between the magnetic field and the output from the signal processing circuit. There is a dilemma that a good relationship has a low mass productivity, and a good relationship has a bad linear relationship.

[0011]

As described above, the optical magnetic field sensor using a ferromagnetic material exhibiting a multi-domain structure with perpendicular magnetization in the magneto-optical element has high sensitivity, but is a main characteristic of the optical magnetic field sensor. If an attempt is made to improve the linear relationship between the magnetic field and the output from the signal processing circuit, alignment between the optical components becomes difficult and mass productivity deteriorates. On the other hand, in the configuration in which the alignment is easy, various problems occur such that the linear relationship between the magnetic field and the output is deteriorated and the phase angle is increased. It is also possible to improve the linear relationship between the magnetic field and the output from the signal processing circuit by introducing a square root calculator or a square root calculation circuit into the signal processing circuit. In order to obtain a good linear relationship between and, it is necessary to use a highly accurate square root arithmetic unit or square root arithmetic circuit, which has the disadvantage that the optical magnetic field sensor becomes expensive.

The present invention has been made in view of the above problems, and is a highly sensitive and highly accurate measurement, that is, the linear relationship between the magnitude of the magnetic field and the output is good and the phase angle is small. It is an object of the present invention to provide an optical magnetic field sensor that enables low-cost measurement in addition to excellent mass productivity.

[0013]

In order to achieve this object, an optical magnetic field sensor according to the present invention comprises a first polarizer, a second polarizer, a first polarizer and a second polarizer. In a magneto-optical sensor having a magneto-optical element located between them, a relative principal axis angle φ (φ> 0) between the first and second polarizers.
And Farade rotation angle θ (θ> 0) of the magneto-optical element are in a range that satisfies the two inequalities (1) and (2). θ ≦ arccot (βh ₀ tanφ) (1) θ ≧ arccot (αh ₀ tanφ) (2) In the above equations (1) and (2), h ₀ = H ₀ / H _S (H
₀ is the rated magnetic field, H _S is the saturation magnetic field of the magneto-optical element)

In the above equation (1), β represents the square root of the positive solution of the solutions of the cubic equation of u shown in equation (3). That is, β = u ^1/2 . 64 (η ² -1) u ³ + [64 (η ² −r ² ) +4 (η ² r ² −1)] u ² + [16 (η ² −r ⁴ ) + 4r ⁴ (η ² −1)] ^{u + r 2 (η 2 -r} 2) = 0 (3) above formula (3), η = 1 + R (R represents the ratio _{error) r = h 1 / h 0} h 1 = H 1 / H S (H 1 in representing the applied magnetic field) where (2), alpha = ^{{[2-C 2 + (4-7C 2} /2) 1/2 ] / 2
C ² } ^1/2 (C represents the degree of modulation)

[0015]

The relationship between the output from the [action] field and the signal processing circuit, an input to the magneto-optical element I _i, and the output from the photodetector and I _o, from I _o is the following formula (5) Desired. However, I _in indicates the absolute value of the current value obtained from the photodetector when the amount of light input to the magneto-optical material is directly input to the photodetector. I _o = I _in [cos θ cos φ + (H ₁ / H _s ) sin θ sin φ sin (ωt)] ² (5) where θ is the Faraday rotation angle of the magneto-optical material and φ is the relative principal axis between the two polarizers. Angle, H _S is the saturation magnetic field, H ₁ is the externally applied magnetic field, ω is the frequency of the AC magnetic field (frequency of current),
t is time.

The output I _O is separated into a DC component I _DC and an AC component I _AC in the signal processing circuit, which are expressed by the following equations (6) and (7), respectively. ^{_{I DC = I in [cos 2}} θcos 2 φ + (H 1 2 / 2H s 2) sin 2 θ sin 2 φ] (6) I AC = I in [H 0 sin2θsin2φ sinωt - (H 1 2 / 2H s 2) ^{sin 2 θsin 2 φcos (2ωt)} ] and (7), the divider in the signal processing _{circuit, I 1 = I AC / I} DC (8) is obtained. The effective value C of I ₁ is expressed by the following equation (9). C = _VAC / _VDC (9)

Here, V _AC is the effective value of I _AC , and V _DC is I _DC
It is expressed by the following equations (10) and (11). V _DC = I _in ^{[cos 2 θcos 2 φ + (H} 1 2 / 2H S 2) sin 2 θsin 2 φ] (10) V AC = I in ^{_{[(H 1 2 / 8H S 2}} ) sin 2 2θsin 2 2φ + C of ^{_{(H 1 4 / 8H S 4}} ) sin 2 θsin 2 φ) ] ^1/2 (11) (9) is called the modulation degree corresponding to the sensitivity of the sensor. In this case, as is clear from equations (9), (10), and (11), C does not have a linear relationship with the externally applied magnetic field, and the phase where the measured value (H sin ωt) becomes 0 And I ₁
It can be seen that the phase is out of phase with = 0 (hereinafter, this phase shift is referred to as "phase angle").

In equations (10) and (11), x = sin θ sin φ, y = cos θ cos φ (12) h ₀ = H ₀ / H _S , h ₁ = H ₁ / H _S, η = 1 + R (13) When placed, the following equation (14) is derived from the equations (4), (9), (10) and (11). _{^{η = (y 2 + h 1}} 2 x 2/2) (2x 2 y 2 + h 0 2 x 4/8) 1/2 / (y 2 + h 0 2 x 2/2) (2x 2 y 2 + h 1 ^{2 x 4/8) 1/2 (} 14) _{where, t = y / h 0 x} , putting a _{r = h 1 / h 0 (} 15), equation (14) can be transformed to the following equation (16) . η = (2t ² + r ² ) (16t ² +1) ^1/2 / (2t ² +1) (16t ² + r ² ) ^1/2 (16) If u = t ² , then equation (16) becomes It is expressed by equation (3). 64 (η ² -1) u ³ + [64 (η ² -r ² ) + 4 (η ² r ² -1) u ² + 16 (η ² -r ⁴ ) + 4r ² (η ² -1) u + r ² (Η ² −r ² ) = 0 (3) Since u = t ² , u is positive, and β is u = ^{1/2 when} the solution of t is β. It is expressed as. β = y / h ₀ x

Therefore, θ can be expressed by the following equation (17) using β, h ₀ and φ. θ = arccot (βh ₀ tanφ) (17) As described above, the saturation magnetic field H _S of the magneto-optical element, the rated magnetic field H ₀ using the optical magnetic field sensor, the applied magnetic field H ₁ and the required ratio error R are determined. , (17), it is possible to obtain a group of curves showing the relationship between θ and φ.

Range 20 of 1PS class ratio error defined by JEC
Is shown in FIG. Further, φ = 20 ° when a material having a Faraday rotation angle θ = 45 ° is used for the magneto-optical element,
FIG. 3 shows the calculation result of the applied magnetic field dependency of the ratio error when the angle is 30 ° and 40 °. As is clear from FIG.
In order to satisfy the specific error of 1 PS class, it can be seen that if the relative error at 20% of the rated magnetic field H ₀ is within -1.5%, the 1 PS class is sufficiently satisfied with other magnetic fields.

Therefore, in the case of r = h ₁ / h ₀ = 0.2, in order to satisfy (absolute value of the ratio error R) ≦ 0.015, the equation (1
It can be seen from 7) that the following inequality (1) should be satisfied. θ ≦ arccot (βh ₀ tanφ), 0 <θ, 0 <φ (1) Next, assuming that the modulation degree C is W = (y / h ₁ x) ² , equations (6), (7), ( It can be expressed by the following equation (18) from 9). ^{C 2 W 2 + (C 2} -2) W 2 + (C 2 / 4- 1/8) = 0 (18) Therefore, the roots satisfying W> 0 is expressed by the following equation (19). _{W = (y / h 1 x} ) = ^{[2C 2 + (4-7C 2/2} ) 1/2 ] / 2C ² (19) where a new parameter alpha = {[2C ² + ( 4-7C ²
Introducing / 2) ^1/2 ] / 2C ² } ^1/2 , αh ₁ xy = 0, and θ is represented by the following equation (20). θ = arccot (αh ₀ tanφ) (20)

The larger the degree of modulation C, the better the S / N ratio of the sensor output, and the more accurate sensor can be obtained. Therefore, the modulation degree C in the rated magnetic field H ₀ is desired to be 1% or more. In order to make the modulation degree C in the rated magnetic field H ₀ 1% or more, the following inequality (2) may be satisfied. θ ≧ arccot (αh ₀ tanφ) (2) Here, C ≧ 0.01.

As can be seen from the equations (1) and (2), these relationships are independent of the value of the saturation magnetic field H _S , and therefore, if the magneto-optical element is a ferromagnetic material having perpendicular magnetization, the material is Equations (1) and (2) generally hold. Further, in the range of θ and φ satisfying the expressions (1) and (2), the ratio error R at 20% of the rated magnetic field H ₀ is (absolute value of R) ≦ 1.5%, and the modulation factor at the rated magnetic field H ₀ is Since C is C ≧ 1%, it is possible to realize an accuracy (ratio error) of 1 PS class or higher and a sensitivity (modulation degree) of 1% or higher by adjusting θ and φ.

[0024]

【Example】

(Example 1) Magnetic garnet ((YbTbB
i) θ that satisfies that the absolute value of the ratio error R at 20% of the rated magnetic field when ₃ Fe ₅ O ₁₂ ) is used is 1.5% or less and the modulation C in the rated magnetic field is 1% or more. The result of calculation of the relationship of φ is shown by the diagonal lines in FIG. Here, the saturation magnetic field of the magneto-optical material = 1450 [Oe], the rated magnetic field =
It was set to 700 [Oe]. Solid lines 24 and 25 in FIG. 4 are boundary lines having a relative error of 1.5% or less and a modulation degree of 1% or more.
FIG. 5 shows a schematic configuration of the optical magnetic field sensor according to the embodiment of the present invention. The light from the light source 1 reaches the lens 3 via the optical fiber 2 and is condensed. The condensed light is refracted at a right angle by the total reflection prism 12. Next, the light passes through the first polarizer 14, the magneto-optical element 6, and the second polarizer 15 in that order, and is refracted at right angles by the total reflection prism 13 again. afterwards,
The light is condensed on the optical fiber 10 by the lens 8, guided to the photodetector 10, and photoelectrically converted into an electric signal. This electric signal is processed in the signal processing circuit 11.

The light source 1 is a light-emitting diode having a wavelength of 0.85 μm, the optical fibers 2 and 9 are multimode fibers, the lenses 3 and 8 are self-occurring lenses, and the polarizer 1
Polarizing glass is used for 4 and 15, and (YbT
bBi) ₃ Fe ₅ O ₁₂ was used, and the photodetector 10 was a Si photodiode. The signal processing circuit 11 is configured as a circuit that separates the output from the photodetector 10 into a direct current component and an alternating current component, and outputs the alternating current component / direct current component by a divider, and measures the effective value of this output. The Faraday rotation angle θ adjusts the thickness of the magneto-optical element 6, and the relative principal axis angle φ of the two polarizers 14 and 15 adjusts the cut-out angle of the polarizing glass constituting the polarizers 14 and 15. Therefore, each was adjusted.

The Faraday rotation angle θ of this optical magnetic field sensor
The relationship between the relative principal axis angle φ of the two polarizers is shown by the diagonal line in Fig. 4.
Within the section, rated magnetic field H₀Error in the magnetic field of 20% of
R was set to -1.5% and -0.5%, and five units were manufactured.
In this case, the rated magnetic field H_o(H _o= 700 [O_e])
The calculated modulation factors are 25% and 15%, respectively. Figure 4
Of the relative error R = -0.5% and the degree of modulation = 15% in the shaded area
The curve is shown by the dotted line 21. Of the 10 optical magnetic field sensors
Table 1 shows the relationship between θ and φ.

Within the range satisfying the relationship between θ and φ shown in Table 1, an AC effective magnetic field of 50 Hz is applied to the optical magnetic field sensor shown in FIG. 5 in the range of 0-700 [Oe], and the magnitude of the magnetic field is increased. The effective value of the output voltage from the signal processing circuit 11 due to the change of the height was measured, and the ratio error and the modulation factor were obtained from the relationship between the effective value of the output voltage and the AC magnetic field. Table 1 shows the ratio error and the modulation factor of the optical magnetic field sensors having different θ and φ. 1
And No. Since very good agreement is found in each group of No. 2, as a representative example, No. From 1 θ = 2
0 °, φ = 45 °, No. From 2, θ = 20 °, φ = 3
Experimental values of the optical magnetic field sensor at 0 ° are shown by black circles and white circles in FIGS. 6 and 7, respectively. Further, the calculated value of the ratio error obtained from the above-described calculation formula is shown by a solid line 22 and a dotted line 23 in FIG. The solid line indicates the ratio error at a magnetic field of 20% of the rated magnetic field H ₀ = -1.
In the case of 5%, the dotted line is the case where the ratio error is −0.5%.

The solid line and the dotted line showing the calculation result and the experimental value are in very good agreement, which indicates that the calculation formula is correct. Also, as is clear from FIG.
The degree of modulation in the rated magnetic field was 25%, 15% or more as designed. Next, FIG. 8 shows the results of examining changes in the phase angle due to changes in the magnetic field. 0-70 in each optical magnetic field sensor
It was found that the phase angle was almost 0 ° in the range of the magnetic field of 0 [Oe].

Therefore, the optical magnetic field sensor according to the present invention is more accurate in the ratio error and phase angle than the 1PS class, and sufficiently satisfies the requirement that the degree of modulation in the rated magnetic field ≧ 1%. As described above, in the optical magnetic field sensor according to the present embodiment, the relationship between the magnitude of the magnetic field and the output is linear,
The phase angle is also small, and the magnetic field can be measured with high sensitivity. Further, since the alignment between the optical components is easy, the mass productivity is excellent.

[0030]

The optical magnetic field sensor according to the present invention can maintain a good linear relationship between the magnitude of the magnetic field and the output from the signal processing circuit in a wide magnetic field range. Since the phase angle is small, it is possible to measure the magnetic field with high sensitivity and high accuracy. Further, it is excellent in mass productivity and can be reduced in price. Further, the optical magnetic field sensor according to the present invention can be manufactured regardless of the material of the magneto-optical element.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a basic configuration of a conventional optical magnetic field sensor.

FIG. 2 is a graph showing a range of a ratio error of 1PS class defined by JEC.

FIG. 3 is a graph showing a relationship between a magnetic field and a ratio error at various relative principal axis angles φ.

[Fig. 4] Rated magnetic field = 700 [Oe], for a magneto-optical element (Y
6 is a graph showing a range of a Faraday rotation angle θ and a relative principal axis angle φ when bTbBi) ₃ Fe ₅ O ₁₂ is used.

FIG. 5 is a schematic diagram showing a basic configuration of an embodiment of an optical magnetic field sensor according to the present invention.

FIG. 6 is a graph showing the relationship between the magnetic field and the ratio error.

FIG. 7 is a graph showing the relationship between magnetic field and modulation factor.

FIG. 8 is a graph showing a relationship between a magnetic field and a phase angle.

[Explanation of symbols]

 1 Light source 2,9 Optical fiber 3,8 Lens 4,7 Polarizer (PBS) 5 Half-wave plate 6 Magneto-optical element 10 Photodetector 11 Signal processing circuit 12,13 Total reflection prism 14,15 Polarizer

Claims (1)

[Claims] 1. A magneto-optical sensor having a first polarizer, a second polarizer, and a magneto-optical element located between the first polarizer and the second polarizer, wherein There are two inequalities (1) between the relative principal axis angle φ (φ> 0) between the two polarizers and the Faraday rotation angle θ (θ> 0) of the magneto-optical element.
And an optical magnetic field sensor having a range satisfying (2). θ ≦ arccot (βh ₀ tanφ) (1) θ ≧ arccot (αh ₀ tanφ) (2) In the above equations (1) and (2), h ₀ = H ₀ / H _S (H
₍₀ is the rated magnetic field, H _S is the saturation magnetic field of the magneto-optical element) In the above equation (1), β is the square root of the solution of the cubic equation of u shown in equation (3), which is positive. Represent That is, β = u ^1/2 . 64 (η ² -1) u ³ + [64 (η ² -r ² ) + 4 (η ² r ² -1)] u ² + [16 (η ² -r ⁴ ) + 4r ² (η ² -1)] ^{u + r 2 (η 2 -r} 2) = 0 (3) above formula (3), η = 1 + R (R represents the ratio _{error) r = h 1 / h 0} h 1 = H 1 / H S (H 1 in representing the applied magnetic field) where (2), alpha = ^{{[2-C 2 + (4-7C 2} /2) 1/2 ] / 2
C ² } ^1/2 (C represents the degree of modulation)