JPH05196707A - Optical magnetic-field sensor - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a magnetic field sensor with a wide dynamic range. [Structure] A light emitting section 1 and a magneto-optical element 5 having a Faraday effect in which a plane of polarization rotates in proportion to the strength of a magnetic field to be measured.
And are connected by an optical fiber 2. A polarizer 4 is arranged on the side of the light emitting unit 1 of the magneto-optical element 5, and an analyzer 7 whose principal axis is determined so as to form a specific angle with the principal axis of the polarizer 4 is arranged on the output side. A light receiving element 12 that receives the light transmitted through the analyzer 7 and converts it into an electric signal is provided. A signal processing circuit 19 for detecting the rotation angle θ of the polarization plane from the electric signal output from the light receiving element 12 to detect the strength of the magnetic field is provided. Signal processing circuit 19
A high-frequency blocking filter 20, an analog / digital converter 23, a storage element 24 that stores an inverse trigonometric function value corresponding to a digital input value and can be read at high speed, and a digital / analog conversion that converts the output of the storage element into an analog output. A container 25 is provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field sensor for detecting magnetic field strength by applying the Faraday effect.

[0002]

2. Description of the Related Art As a magnetic field sensor of this type, Japanese Patent Laid-Open No.
The one described in Japanese Patent Publication No. 9-159076 is conventionally known. FIGS. 3 and 4 show the conventional technique disclosed in this publication. The one shown in FIG. 3 is a magnetic field sensor for measuring the strength H of the magnetic field itself, and the one shown in FIG. 4 is a current I flowing through a conductor. It is applied to a current transformer which measures the intensity H of the magnetic field formed by the current and measures the current I by this.

In FIG. 3, light emitted from a light emitting section 1 composed of a light emitting diode or the like is guided to a condenser lens 3 through an optical fiber 3 and is converted into light close to parallel light by the condenser lens 3 to be polarized. The polarized light that is incident on the child 4 and is extracted by the polarizer 4 is incident on the magneto-optical element 5. The magneto-optical element 5 is formed of a so-called Faraday effect, for example, lead glass, a magnetic film, or the like, depending on the strength H (in the direction of the arrow in the drawing) of the applied magnetic field. Therefore, the polarization plane of the polarized light incident on the one end surface of the magneto-optical element 5 is rotated by θ in proportion to the magnetic field strength H and is emitted from the other end surface. The emitted light is composed of a polarizing prism or a polarizing beam splitter, and is incident on an analyzer 7 whose principal axis is set at a position of 45 ° with the polarizer 4. Here, as shown in FIG. , Y component). From FIG. 5, each vector component is represented by the following equation. Ex = E · sin (45 ° + θ) (1) Ey = E · cos (45 ° + θ) (2)

These vector lights become light quantity signals Px and Py collected by the condenser lenses 8 and 9, respectively,
It is guided to the light receiving elements 12 and 13 via the optical fibers 10 and 11. Here, since the light quantity is the square of the magnitude of the vector, the light quantity signals Px and Py are as follows. Px = Ex ² = E ² ・ Sin ² (45 ° + θ) = (E ² / 2) ・ (1 + sin2θ) (3) Py = Ey ² = E ² ・ Cos ² (45 ° + θ) = (E ² / 2) ・ (1-sin2θ) (4)

The light receiving elements 12 and 13 are formed of, for example, photodiodes, and receive light amount signals Px and Px.
It is a photoelectric converter that outputs a current signal proportional to y. Taking the case where the magnetic field is an alternating magnetic field as an example, this current signal is expressed by the following equations (5) and (6) by the current-voltage conversion amplifiers 14 and 15.
Are converted into voltage signals Vx and Vy. Note that K _{1 in} the equation is a constant. Vx = K ₁ (1 + sin2θ) (5) Vy = K ₁ (1-sin2θ) (6)

The voltage signal Vx is supplied to the analog adders 16 and 17
Is input to the + input terminal of the analog adder 16 and the voltage signal Vy is input to the + input terminal of the analog adder 17, respectively. The output of the analog adder 17 is input to the denominator input terminal of the divider 18. As a result, the signal V ₀ output from the divider 18 is represented by the following equation (7). V ₀ = (Vx−Vy) / (Vx + Vy) = sin2θ (7) In the formula (7), when θ is sufficiently small, the following formula (8)
Is established. V ₀ = 2θ = K ₂ · H (8) (where, K ₂ is a proportional constant and V ₀ = 2θ is an approximate value.) That is, as is apparent from the equation (8), the output is given to the divider 18. The magnetic field strength H is detected by the signal V ₀ .

FIG. 4 shows an example in which the magnetic field sensor is applied to the current transformer as described above, and detects the alternating current I flowing through the conductor 6. In this example, a hole through which the conductor 6 penetrates is formed in the magneto-optical element 5, and the light emitted from the polarizer 4 passes through the magneto-optical element 5 through the optical path indicated by the dotted line in the figure, and then the analyzer 7 Be led to. Others are the same as the magnetic field sensor shown in FIG.

Therefore, the polarized light transmitted through the magneto-optical element 5 has its plane of polarization rotated in proportion to the intensity H of the magnetic field generated in proportion to the current I, and the output V _{0 of the} divider 18 is given by It becomes the same as in (7), and since the following equation (9) is obtained in the range where θ is sufficiently small, the current I flowing through the conductor 6 is detected. V ₀ = 2θ = K ₃ · H (9) (where K ₃ is a proportional constant and V ₀ = 2θ is an approximate value)

[0009]

However, the conventional optical magnetic field sensor shown in FIGS. 3 and 4 has a drawback that the linearity can be obtained only in the range where θ is sufficiently small as described above. .. For example, the range of θ that satisfies the accuracy of 1% is slightly within ± 7.0 °. Further, as is known from the equations (3), (4) or (5), (6), the ratio (modulation degree) of the measured signal component to the bias component included in the light amount signal is only 24% when θ = 7 °. Absent. This means that the signal / noise ratio is small when attempting to measure a low magnetic field region, and sufficient measurement accuracy cannot be obtained. As described above, the conventional optical magnetic field sensor has a drawback that it can measure only a magnetic field in a very narrow range. The present invention is
The present invention has been proposed to solve the above-mentioned problems of the conventional technology, and an object thereof is to provide an optical magnetic field sensor having a wide measurement range.

[0010]

As means for achieving the above object, the present invention provides a magneto-optical element having a Faraday effect in which a plane of polarization rotates in proportion to the strength of a magnetic field to be measured, and a magnetic optical element having the Faraday effect. A polarizer arranged on the light source side of the optical element,
An analyzer having a principal axis determined so as to form a specific angle with the principal axis of the polarizer that receives the light transmitted through the magneto-optical element, a light receiving element that receives the light transmitted through the analyzer and converts an electrical signal, In an optical magnetic field sensor equipped with a signal processing circuit for detecting the rotation angle of the polarization plane from an electric signal output from a light receiving element to detect the strength of a magnetic field, the signal processing circuit includes a high frequency blocking filter, an analog / Digital converter,
The present invention is characterized in that a storage element that stores an inverse trigonometric function value corresponding to a digital input value and that can be read at high speed, and a digital / analog converter that converts the output of the storage element into an analog output and outputs the analog output are characterized.

[0011]

According to the present invention having the above-mentioned structure, the electric signal obtained by the light receiving element is output as the trigonometric function f (sin2θ) of the rotation angle of the polarization plane. After passing this output through a high frequency blocking filter, A / D conversion,
Input to the memory element. In the storage element, the value of θ corresponding to the digital input value is read at high speed. The read value of 2θ is D / A converted and used as the output of the magnetic field sensor.

[0012]

EXAMPLES (1) First Example Hereinafter, a first example of the present invention will be specifically described with reference to FIG.

Reference numeral 1 is a light emitting portion including a light emitting element such as an LED and a driving circuit, 2 is an optical fiber for guiding the light of the light emitting portion, 3
Is a condenser lens for converting the light emitted from the optical fiber 2 into a substantially parallel light beam, 4 is a polarizer for linearly polarizing the light from the condenser lens 3, and 5 is a magnetic field to be measured acting on the polarizer. A magneto-optical element whose plane of polarization rotates in proportion to the intensity of H, and 7 is another analyzer which is arranged such that the direction of the principal axis is at a position of 45 ° with respect to the principal axis of the polarizer.

Reference numeral 8 is a condenser lens, 10 is an optical fiber, and the parallel rays emitted from the analyzer 7 are focused by the condenser lens 8 and are incident on the optical fiber 10. The other end of the optical fiber 10 is coupled to the light receiving element 12, the light quantity signal that has passed through the optical fiber 10 is converted into a current, and then the output of the current-voltage light tube amplifier 14 is input to the signal processing circuit 19 to output the output signal. V
_{Get 0} .

The structure of the signal processing circuit 19 is as follows. That is, 20 is a high-frequency pass filter, and 21 is a low-frequency pass filter that divides an input signal into an AC signal component to be measured and a DC bias signal component. 18 is a divider connected so as to divide the output of the high frequency pass filter 20 by the output of the low frequency pass filter 21. 22 will be done later A
A high-frequency blocking filter as a pre-stage for D / D conversion, and 23 is A
It is a / D converter. 24 is so-called Read on
A memory element called a memory (ROM). This storage element 24 has a one-to-one correspondence with the input digital signal value (A).
The value of sin ⁻¹ (A) corresponding to is stored in advance,
This is read out and output at high speed according to the input value. Reference numeral 25 is a D / A converter for converting a digital output signal from the ROM into an analog signal. The operation of the magnetic field sensor thus configured will be described below.

The light from the light emitting section 1 passes through the optical fiber 2 and reaches the collective lens 3, where it becomes substantially parallel rays. This light is linearly polarized by the polarizer 4 and passes through the magneto-optical element 5. During this time, when the magnetic field H acts in the direction of the arrow shown in FIG. 1, the polarization plane rotates by θ in proportion to the strong field intensity. The emitted light of the magneto-optical element 5 passes through the analyzer 7. Since the main axis of the analyzer 7 is arranged at an angle of 45 ° with the main axis of the polarizer 4, as can be seen from the optical vector diagram shown in FIG. 5, the emitted light of the analyzer 7 is expressed by the following equation. It Ex = E · sin (45 ° + θ) (10)

The beam diameter of this light is narrowed by the condenser lens 8, becomes a light amount signal Px, and is guided to the light receiving element 12 via the optical fiber 10. The amount of light is the square of the magnitude of the vector, and is as follows. Px = Ex ² = E ² ・ Sin ² (45 ° + θ) = (E ² / 2) ・ (1 + sin2θ) (11)

The light receiving element 12 is formed of a photodiode, for example, and is a photoelectric converter that outputs a current signal proportional to the received light amount signal Px. Taking the case where the magnetic field is an alternating magnetic field as an example, this current signal is converted into a voltage signal Vx by the current-voltage conversion amplifier 14. Vx = K ₁ (1 + sin2θ) (12) (where K ₁ is a constant).

The voltage signal Vx is supplied to the high frequency pass filter 20.
And the low frequency pass filter 21. Therefore, the output Vx _A of the high frequency pass filter 20 is (1
The DC bias component of the equation (2) is removed, and the following equation is obtained. Vx _A = K ₁ sin2θ (13) The output Vx _D of the low frequency pass filter 21 has the following expression after the AC component is removed. Vx _D = K ₁ (14)

Vx _A is applied to the numerator input terminal of the divider 18 by V
Since x _D is input to the denominator input terminal of the divider 18, the output signal Vx _{0 of the} divider is represented by the following equation (15). Vx ₀ = sin2θ (15)

The signal represented by the equation (15) is converted into a digital signal by the A / D converter after the high frequency is removed by the high frequency blocking filter. With respect to this signal, in the storage element 24, a value of θ corresponding to Vx ₀ in a one-to-one correspondence with Vx ₀ is previously stored in the storage element 24, and Vx ₀ is input by reading this value at high speed. Will be output in real time. At this stage, the output is sufficient, but in the embodiment shown in FIG. 1, the analog output V ₀ is obtained via the D / A converter 25. Therefore, in the present embodiment, the detectable range of θ is −90 ° ≦ 2θ ≦ 90 °, and therefore −45 ° ≦ θ ≦ 45 °, which is approximately 6 times the range of the conventional technique.

Further, as is clear from the equation (15), the characteristic variation of the light emitting section 1, the optical fibers 2 and 10, the light receiving element 1
2. It is not affected by the characteristic variation of the current-voltage conversion amplifier 14. Further, since there are only two optical fibers, 2 and 10, the optical system is simpler than the conventional three.

(2) Second Embodiment FIG. 2 shows a second embodiment of the present invention. The configuration of the second embodiment is such that between the light emitting section 1 and the current-voltage conversion amplifier 14,
This is the same as the first embodiment. In the second embodiment, the output of the current-voltage conversion amplifier 14 is input to the low frequency pass filter 21, while the output of the low frequency pass filter 21 is fed back to the light emitting unit as a signal for controlling the light emission amount. .. The output of the current-voltage conversion amplifier 14 is simultaneously input to the A / D converter via the high frequency blocking filter 22. 24 is 1: 1 to the input digital signal value (A)
Is a storage element (ROM) in which a value of sin ^-1 (A / k-1) corresponding to is stored and stabbed in advance and is read out and output at high speed according to an input value. Reference numeral 25 is a D / A converter that converts a digital output signal from the ROM into an analog signal.

In the second embodiment, the operation between the light emitting section 1 and the current-voltage conversion amplifier 14 is the same as that of the first embodiment shown in FIG. However, the signal from which the AC component has been removed by the low-frequency pass filter 21 is fed back to the light emitting unit as a signal for controlling the amount of light emission.
The constant K ₁ in the equation 2) is controlled so that K ₁ = K (constant), and the voltage signal Vx is not affected by the characteristic variation from the light source unit 1 to the current-voltage conversion amplifier 14. Vx = K (1 + sin2θ) (16)

The signal represented by the equation (16) is converted to a digital signal by the A / D converter after the high frequency is removed by the high frequency blocking filter. This signal group is input to the storage element 24 and then left. In the storage element 24, from the relationship of the equation (16), Vx
The value of θ corresponding to 1 to 1 is stored in advance. By reading this value at high speed, Vx is input and θ is output in real time. At this stage, the output is sufficient, but in the other embodiment shown in FIG.
The analog output V ₀ is obtained via the converter 25.

Also in the second embodiment, the range of detectable θ is −45 ° ≦ θ ≦ 45, as in the first embodiment.
The optical system is simple, without being affected by the characteristic variation between the light emitting unit 1 and the current-voltage conversion amplifier 14.
In addition, there is an effect that the signal processing circuit is simplified as much as the high frequency pass filter 20 and the divider 18 in the first embodiment are unnecessary.

[0027]

As described above, according to the present invention, in the signal processing circuit, the value stored in advance corresponding to the input is read out directly from the storage element at a high speed, so that the magnetic field in a wide range is high. An effect that an optical magnetic field sensor that can detect with accuracy can be provided is obtained.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a first embodiment of an optical magnetic field sensor of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the optical magnetic field sensor of the present invention.

FIG. 3 is a configuration diagram showing an example of a conventional optical magnetic field sensor.

FIG. 4 is a configuration diagram showing a state in which the optical magnetic field sensor of FIG. 3 is applied to a current transformer.

FIG. 5 is a vector diagram for explaining the actions of a polarizer and an analyzer.

[Explanation of reference numerals] 4 ... Polarizer 5 ... Magneto-optical element 7 ... Analyzer 12 ... Light receiving element 14 ... Current / voltage conversion amplifier 18 ... Divider 19 ... Signal processing circuit 20 ... High frequency pass filter 21 ... Low frequency pass filter 22 ... high frequency blocking filter 23 ... A / D converter 24 ... storage element

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takashi Nakajima 2-1, Ukishima-cho, Kawasaki-ku, Kawasaki-shi, Kanagawa Kanagawa Prefecture Hamakawasaki factory

Claims (1)

[Claims] 1. A magneto-optical element having a Faraday effect in which a plane of polarization rotates in proportion to the strength of a magnetic field to be measured, a polarizer arranged on the light source side of the magneto-optical element, and a magneto-optical element transmitted through the magneto-optical element. An analyzer having a principal axis that receives light and forms a specific angle with the principal axis of the polarizer, a light receiving element that receives the light that has passed through the analyzer and converts an electrical signal, and an electrical output from the light receiving element. In an optical magnetic field sensor including a signal processing circuit for detecting the rotation angle of the polarization plane from a signal to detect the strength of a magnetic field, the signal processing circuit is provided with a value corresponding to a value input to the processing circuit in advance. An optical magnetic field sensor, comprising: a storage element for storing the data; and an output means for reading out a value corresponding to an input value from the storage element when the signal is input to the signal processing circuit and outputting the read value.