JP2001228230A - Magnetic sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic sensor capable of eliminating errors due to the individual differences of photodetectors, etc., and accurately measuring magnetic-field intensity. SOLUTION: By providing linearly polarized light outputted from a light source 10 for a polarization switching part 20, two pieces of polarized light with their planes of polarization which intersect each other at right angles are made alternately emergent in time sequence and transmit through a Faraday element 41. The two pieces of polarized light transmitted through the Faraday element 41 are made to transmit through a linear polarizer 42 and are converted into light intensity according to the angle of rotation of each plane of polarization. The two pieces of converted light are detected by a single photodiode 52. By using the single photodiode 52, it is possible to eliminate detection errors due to individual differences.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic sensor utilizing the phenomenon that the plane of polarization of light traveling in a Faraday element rotates in accordance with the intensity of a magnetic field, and more particularly to systematic fluctuations in light source intensity and light detection. TECHNICAL FIELD The present invention relates to a technique for avoiding a measurement error due to an influence of an individual difference (e.g., variation in sensitivity) of an instrument.

[0002]

2. Description of the Related Art The configuration of a conventional magnetic sensor is shown in FIG.
This will be described below. This magnetic sensor is roughly divided into a light source unit 10 that emits linearly polarized light, a sensor head unit 40 that captures linearly polarized light from the light source unit 10, and a light source unit 10 that collects two polarized lights output from the sensor head unit 40. , Lenses 51a and 51b, photodetectors 52a and 52b for converting each condensed polarized light into an electric signal, and an arithmetic processing unit 60 for processing the converted electric signals. The light source unit 10 and the sensor head 40 are connected to each other, and the sensor head 40 and the light receiving units 51a and 51b are connected to each other by optical transmission paths 30a, 30b, and 30c, respectively.

The light source unit 10 includes a light emitting element driving circuit 11 for driving the light emitting element, and a laser diode or LE that emits light by receiving an electric signal from the light emitting element driving circuit 11.
A light emitting element 12 such as D, a lens 13 for condensing the light of the light emitting element 12, and a linear polarizer 14 for converting the condensed light into linearly polarized light and outputting the linearly polarized light. The linearly polarized light output from the light source unit 10 is transmitted to the optical transmission path 30a.
Is transmitted to the sensor head 40 via the.

In the sensor head 40, the transmitted linearly polarized light enters and passes through the Faraday element 41. The plane of polarization of the linearly polarized light transmitted through the Faraday element 41 is rotated according to the intensity of the magnetic field to be detected. 7A shows the inclination of the plane of polarization of the linearly polarized light immediately before the light enters the Faraday element 41, and FIG.
Respectively show the inclination of the plane of polarization of the linearly polarized light emitted from. The linearly polarized light emitted from the Faraday element 41 enters the polarization beam splitter 42. This polarization beam splitter 42 is, as shown by a broken line in FIG.
With reference to the inclination of the plane of polarization of the linearly polarized light immediately before the light enters the Faraday element 41, that is, the inclination of the linear polarizer 14,
It is installed at an angle of 45 °. The linearly polarized light that has entered the polarization beam splitter 42 is separated into two-component polarized light that is orthogonal to each other and output. The two polarized lights separated and output are individually transmitted to the condensing lenses 51a and 51b via the optical transmission paths 30b and 30c.

The condensed polarized lights are converted into electric signals by the photodiodes 52a and 52b and output to the arithmetic processing unit 60. The arithmetic processing unit 60 calculates the magnetic field strength around the Faraday element 41 based on the ratio of the magnitudes of the two electric signals taken in.

[0006]

However, in the case of the conventional example having such a configuration, the polarization beam splitter 42 separates the light into two-component polarized light, and thereafter, the arithmetic processing unit 6
Each means up to the stage preceding 0 is individually provided corresponding to two-component polarized light. As a result, there is a problem that a detection error or the like due to an individual difference of each means as described below is inevitably generated.

Since the characteristics of the photodiodes 52a and 52b, such as the dark level and the sensitivity / temperature-dependent coefficient, do not match, the calculated magnetic field intensity includes an error.

Also, two different optical transmission paths 30b,
30c, the light between the reflection surfaces of the optical transmission path has a different phase and a different phase for each detector (for photodiode) when light having good coherence like a laser is used for detection. Occurs at the interference distance. as a result,
When the detection results are compared, a beat is detected due to interference such as when the wavelength of light slightly fluctuates. This beat becomes noise for the signal.

The present invention has been made in view of such circumstances, and has a high sensitivity magnetic sensor capable of removing an error due to an individual difference of a photodetector or the like and obtaining a detection result of a minute magnetic field. The purpose is to provide.

[0010]

The present invention has the following configuration to achieve the above object. That is, according to the first aspect of the present invention, there is provided a magnetic sensor for detecting a magnetic field intensity around a Faraday element, wherein (a) a light beam emitted from a single light source is switched to two polarized lights whose polarization planes are orthogonal to each other. (B) a Faraday element that transmits the two polarized lights and converts the intensity of the magnetic field to be detected into a rotation angle of the polarization plane of the two polarized lights; A) light converting means for converting the two polarized lights emitted from the Faraday element into light intensities corresponding to the rotation angles of the respective polarization planes; and (d) each light converted into light intensity by the light converting means. (E) a single light detecting means for receiving the light and converting it into an electric signal; and (e) detecting the magnetic field intensity based on the electric signals corresponding to the two polarized lights, which are alternately output in time series from the light detecting means. The required calculation means To have.

According to a second aspect of the present invention, in the magnetic sensor according to the first aspect, at least any one of between the light output means and the Faraday element and between the light conversion means and the light detection means. An optical fiber is used for one of the optical transmission paths.

[0012]

The operation of the present invention is as follows. That is, according to the first aspect of the present invention, two polarized lights, which are alternately output in a time series and whose polarization planes are orthogonal to each other, are rotated by the respective polarization planes in the process of transmitting through the Faraday element and output. Is done. The two polarized lights emitted from the Faraday element are converted by the light converting means into light intensities corresponding to the rotation angles of the respective polarization planes, and then sent to a single detecting means to be converted into electric signals. That is, since the two polarized lights converted into the light intensity are converted into electric signals by the same detecting means, errors due to individual differences of the light detecting means in the detection result are removed.

According to the second aspect of the present invention, since two polarized lights pass through the same optical transmission line made of an optical fiber, the two polarized lights are similarly affected by the characteristics of the optical fiber. Therefore, in the process of obtaining the magnetic field strength based on the electric signals corresponding to the two polarizations, the influence of the transmission characteristics of the optical fiber is canceled.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the magnetic sensor according to the present invention. The magnetic sensor according to the present embodiment is roughly divided into a light source unit 10 that generates light, a polarization switching unit 20 that receives light from the light source unit 10 and switches the rotation angle of a polarization plane, and a polarization switch that switches the rotation angle. , A sensor head 40 that transmits polarized light from the optical transmission path 30a and converts the intensity of a magnetic field into a rotation angle of a polarization plane, and transmits each polarized light transmitted through the sensor head 40. It comprises an optical transmission path 30b, a light receiving unit 50 that receives each transmitted polarized light, converts it into an electric signal, and outputs the electric signal, and an arithmetic processing unit 60 that calculates the magnetic intensity from each electric signal.

Hereinafter, the configuration and function of each section will be described in detail. The light source unit 10 includes a light emitting element driving circuit 11
A light emitting element 12 for generating light controlled by the light emitting element driving circuit 11, a lens 13 for receiving and condensing light emitted from the light emitting element 12, and a light transmitting element for transmitting the condensed light. And a linear polarizer 14 for converting into linearly polarized light and outputting the linearly polarized light. Note that a laser diode or an LED is used for the light emitting element 12.

The polarization switching section 20 includes a solenoid driving circuit 2
1, a solenoid coil 22, and a Faraday element 23 provided along the axis of the solenoid coil 22. The solenoid drive circuit 21 supplies a rectangular current P to the solenoid coil 22. The solenoid coil 22 to which the rectangular current P is supplied has a Faraday element 23.
Are generated alternately in the traveling direction of the linearly polarized light in the Faraday element 23 and in the opposite direction. As a result, the polarization plane of the linearly polarized light transmitted through the Faraday element 23 is 45 ° and − with respect to the polarization plane when no magnetic field is applied, that is, the polarization plane of the linear polarizer 14.
A 45 ° rotation is provided alternately. As a result, two polarizations whose polarization planes are orthogonal to each other are alternately output in time series from the polarization switching unit 20. The switching frequency of the rotation angle of the polarization plane is appropriately set within a range of, for example, 10 Hz to several tens of kHz. Light source unit 10 and polarization switching unit 20 described above
Corresponds to the light output means in the present invention.

In this embodiment, the polarization switching unit 20 sets the rotation angle of the polarization plane to ± 45 ° for the following reason. In a sensor head 40 to be described later, two time-series polarized lights transmitted through the Faraday element 41 are converted into linear polarizers 42.
Is used to convert the light into light intensity corresponding to the rotation angle of each polarized light. Since the plane of polarization of the linear polarizer 42 is set at the same angle as the plane of polarization of the linear polarizer 14 of the light source unit 10, the rotation angle of the plane of polarization of the polarized light incident on the linear polarizer 42 is ± 45 °. , The rotation angle can be taken out as the largest fluctuation of the light intensity.
In other words, the rotation angle of the polarization plane of the two polarized lights is set to ± 45 ° in order to detect with high sensitivity the measurement magnetic field that is a factor that changes the rotation angle of the polarization planes of the two polarizations transmitted through the Faraday element 41. I set it.

The two polarized lights alternately emitted in time series from the polarization switching section 20 are transmitted to the sensor head 40 via the optical transmission path 30a. In this embodiment, the optical transmission path 3
0a is a spatial transmission path, but an optical fiber may be used.

The sensor head 40 transmits the two time-series polarized lights transmitted through the optical transmission path 30a and converts the intensity of the magnetic field to be detected into the rotation angle of the plane of polarization of the two polarized lights. A Faraday element 41 and a linear polarizer 42 that converts two polarized lights emitted from the Faraday element 41 into light intensities corresponding to the rotation angles of the respective polarization planes.
And mirrors 43a and 43b for changing the optical path of the light passing through the linear polarizer. The Faraday element 41 corresponds to the Faraday element in the present invention, and the linear polarizer 42 corresponds to the light converting means in the present invention.

The Faraday element 41 is disposed so that the optical paths of two polarized lights passing through the Faraday element 41 are substantially parallel to the direction of the magnetic field to be measured. The linear polarizer 42 is arranged such that its polarization plane is at substantially the same angle as the polarization plane of the linear polarizer 14 of the light source unit 10.

The time-series light that has passed through the linear polarizer 42 is
The optical path is changed by the mirrors 43a and 43b, and is sent to the same optical transmission path 30b.

The time-series light transmitted through the optical transmission path 30b is received by the light receiving section 50. This light receiving unit 50
It includes a condensing lens 51 and a single photodiode 52 that converts received light into an electric signal. The photodiode 52 corresponds to a single light detecting unit in the present invention. In addition, this photodiode 52
It uses a back-illuminated InGaAs (Indium Gallium Arsenic) photodiode. When light is incident from the front of the element, the structure has a gold electrode on the front of the element.
Light is somewhat reflected by the electrodes and is lost. In this regard, by performing rear incidence, this problem is solved.

The electric signal detected by the photodiode 52 is given to the arithmetic processing unit 60. The arithmetic processing unit 60
2 which is alternately output in time series from the photodiode 52
The magnetic field strength around the Faraday element 41 is obtained based on the electric signals corresponding to the two polarizations. This arithmetic processing unit 60
Corresponds to the calculating means in the present invention.

Next, the operation of the embodiment device having the above-described configuration will be described. The light emitted from the light emitting element 12 becomes linearly polarized light by passing through the linear polarizer 14, and the linearly polarized light is transmitted to the polarization switching unit 20. Polarization switching unit 2
In the process of passing through the Faraday element 23, the linearly polarized light transmitted to 0 is converted so that the rotation angle of the plane of polarization is alternately ± 45 °. The two polarizations in the time series in which the rotation angles of the polarization planes are alternately switched are provided to the sensor head 40 via the optical transmission path 30a. The initial polarization 1 and the initial polarization 2 shown in FIG. 2 indicate the polarization 1 and the polarization 2 which are alternately applied to the sensor head 40 in a time series.
The polarization plane of the initial polarized light 1 is rotated by (+) 45 ° clockwise, and the polarized light of the initial polarized light 2 is rotated by (−) 45 ° counterclockwise.

The two light beams emitted from the polarization switching unit 20 in time series
The two polarized lights are spatially transferred and pass through the Faraday element 41 of the sensor head 40. At this time, since the two polarized lights have a relationship that their polarization planes are at right angles to each other, the polarization planes of the respective polarized lights have θ due to the magnetic field in the same direction to be measured.
As shown in FIG. 2, the polarization plane of the polarized light 1 becomes + θ with respect to the rotation angle of 45 ° of the initial polarized light 1 as shown in FIG.
Is rotated by -θ with respect to the rotation angle 45 ° of the initial polarized light 1. As described above, the polarization plane is rotated.
As the two polarized lights pass through the linear polarizer 42, the linear polarizer 42 outputs the light intensities corresponding to the rotation angles of the polarization planes of the respective polarized lights as represented by the following equations (1) and (2). Polarized light 1,
2 are extracted (polarized light 1 and polarized light 2 shown in FIG. 2).
See “Faraday rotation”). Polarized light 1 I ₀ · sin (45 + θ) (1) Polarized light 2 I ₀ · cos (45 + θ) ... (2) where I ₀ is the light intensity of the initial polarized light, and the linear polarizer 4
The polarization plane 2 is along the Y axis in FIG.

The two polarized lights converted into light intensities by the linear polarizer 42 are reflected by mirrors 43a and 43b, an optical transmission path 30b,
And the photodiode 51 via the lens 51 in that order.
2 and is converted into an electric signal. Each electric signal corresponding to the two polarized lights is supplied to the arithmetic processing unit 60, and the magnetic field intensity is calculated by the arithmetic processing described below.

Next, an operation method in the operation processing unit 60 will be described.
Will be described. Assuming that the two light passing through the linear polarizer 42
Polarize each individual detector (e.g.
C) and the amplifier that leads to this,
The measured light intensities of the two polarized lights are Jones Matri
(Jones Matrix), the following equation (3) and
It can be expressed by (4). Polarized light 1 [(-cos (α)^Two+ sin (α)^Two) sin (VLH)^Two-2cos (α) cos (VLH) sin (α) sin (VLH) + cos (α)^Two] I₀ ・ Gain_a + Dark_a ... (3) Polarized light 2 [(-sin (α)^Two+ cos (α)^Two) sin (VLH)^Two+ 2cos (α) cos (VLH) sin (α) sin (VLH) + sin (α)^Two] I₀ ・ Gain_b + Dark_b (4) In the above formulas (3) and (4), α is a Faraday element
The polarization plane of the initial polarization incident on the linear polarizer 42 and the polarization of the linear polarizer 42
Angle with the light surface (α = 45 ° in the embodiment), I₀Is the first
V is the Verde constant, L is the Faraday element
The length, H, of the measurement magnetic field acting on the Faraday element 41
World, Gain_a, Gain_b, Dark _a, Dark_b
Is the gain and dark bell of the two detectors and amplifier.
Both are functions of environmental conditions such as ambient temperature. Ma
VHL in the expressions (3) and (4) is the Faraday element 4
1 is polarized by the measuring magnetic field H
It is equal to the rotation angle θ of the surface.

For example, here, α in the above equation is 45 °,
Assuming that Dark _a and Dark _b are sufficiently small with respect to the input signal level, take the ratio of the detection values of the above equations (3) and (4), and assume the range of the minute measurement magnetic field where sin (VLH) ≒ VLH is satisfied. Then, the above ratio is expanded to a second-order series. The result can be expressed by the following equation (5). Gain _a (1 −4 VLH) / Gain _b (5) That is, in this series expansion, since the light source intensities I ₀ cancel each other, it can be expressed by a simple equation as in the above equation (5).

In the case of the present invention, since the light intensity of two polarized lights is converted into an electric signal by using a single detector (photodiode 52), the following equation (6) holds. Gain _a = Gain _b (6) From this relation, the above equation (5) can be expressed by a simple equation excluding Gain _a and Gain _b as in the following equation (7), and two systems can be detected. It is possible to obtain a detection result in which an error generated when measurement is performed using a device or an amplifier is removed. 1-4VLH ... (7)

The above equation (7) represents the ratio A / B of the electric signals A and B alternately given in time series from the photodiode 52.
be equivalent to. 1-4VLH = A / B (8)
The arithmetic processing unit 60 calculates the measurement magnetic field H, which is an unknown number, using the equation (8).

The method of calculating the measured magnetic field H from the electric signals A and B is not limited to the one using the equation (8), but can be calculated as follows. The relationship between the electric signals A and B and the rotation angle (Faraday rotation angle) θ of the polarization plane of the polarized light transmitted through the Faraday element 41 can also be expressed by the following equation (9). (A−B) / (A + B) = sin 2θ (9) Here, assuming that the measured magnetic field is small, 2θ≪1, so that sin 2θ can be approximated by 2θ. Then, equation (9) is expressed as follows. (A−B) / (A + B) = 2θ (10) From the relation of θ = VLH of the Faraday effect, the expression (10) is expressed as follows. (AB) / (A + B) = 2VLH (11) The measurement magnetic field H can also be calculated by using the equation (11).

As described above, according to the magnetic sensor according to the above-described embodiment, two linearly polarized lights whose polarization planes are orthogonal to each other are transmitted through the Faraday element 41 alternately in time series, and The two polarized lights are converted into light intensities corresponding to the rotation angles of the respective polarization planes, and the time-series polarized lights converted into these light intensities are converted into a single photodiode 5.
2, the detected magnetic field is converted into electric signals, and the measured magnetic field is calculated based on these electric signals. Therefore, according to this embodiment, the two photodetectors are compared with the conventional device shown in FIG. 6 in which electric signals are individually converted into electric signals using two systems of photodetectors. The measurement of the magnetic field strength can be accurately performed without causing a measurement error due to a difference in characteristics or the like.

The present invention is not limited to the above embodiment, but can be modified as follows.

(1) In the above embodiment, the optical transmission path 30b provided between the sensor head 40 and the light receiving section 50 is a spatial transmission path as shown in FIG. In this embodiment, the optical fiber 30b is used.

By using the optical fiber 30b, there is no need to guide the linearly polarized light from the linear polarizer 42 by changing the optical path using a mirror or the like. That is, by arranging the optical fiber 30b immediately after the linear polarizer 42, the linearly polarized light from the linear polarizer 42 can be easily guided to the light receiving unit 50. Further, since two polarized lights are transmitted alternately in time series on the same optical fiber 30b, measurement errors caused by a difference in transmission path characteristics, such as when transmitting using two optical fibers, are avoided. can do.

Further, by using the optical fiber 30b, loss of linearly polarized light transmitted when an obstacle such as an obstacle occurs between the detecting section 40 and the light receiving section 50 can be avoided.

Although an optical fiber is used between the sensor head 40 and the light receiving section 50 in this embodiment, an optical fiber may be provided between the polarization switching device 20 and the sensor head 40.

(2) Instead of the polarization switching section 20 shown in FIG. 1, a polarization switching means as shown in FIGS. 4 and 5 may be used. The polarization switching means of the present embodiment includes a drive motor 71
And a flat disk 73 attached to a shaft 72 of the drive motor 71, and a plurality of polarizers 73a and 73b alternately arranged on the disk 73 at equal intervals in a circular shape.

The polarizer 73a is arranged so that the angle of the plane of polarization thereof is 45 ° with respect to the angle of the plane of polarization of the linear polarizer 14 at the light incident position R _IN shown in FIG. On the other hand, the polarizer 73b has a light incident position R shown in FIG.
_{At IN} , the polarization plane is arranged so that the angle of the polarization plane is −45 ° with respect to the polarization plane of the linear polarizer 14.

The operation of this embodiment will be described below. When the drive motor 71 is driven, the flat disk 73 connected to the shaft 72 of the drive motor 71 also rotates in conjunction with the drive.
At this time, when the linearly polarized light R is emitted from the light source unit 10, the linearly polarized light R is applied to the polarizers 73a and 73 provided on the disk 73.
3b are transmitted alternately and sequentially. Since the two transmitted linearly polarized lights show an angle of ± 45 ° with respect to the polarization plane of the linear polarizer 14, two polarized lights whose polarization planes are orthogonal to each other are emitted alternately in time series. That is, it has the same function as the polarization switching unit 20 of the embodiment shown in FIG.

[0041]

As is apparent from the above description, according to the first aspect of the present invention, two polarized lights whose polarization planes are orthogonal to each other are output alternately in time series and transmitted through the Faraday element. The plane of polarization of the polarized light is rotated according to the measurement magnetic field. Then, after converting each polarized light transmitted through the Faraday element into a light intensity corresponding to a rotation angle of each polarization plane, the light is converted into an electric signal by a single light detecting means, and based on these electric signals, a measurement magnetic field is measured. Seeking strength. Therefore, there is no measurement error due to a difference in characteristics between individual photodetectors and the like, unlike a conventional device that converts an electric signal using two photodetectors.

According to the second aspect of the present invention, two polarized lights alternately emitted in a time series pass through the optical fiber which is the same optical transmission path, so that compared with the case where two systems of optical transmission paths are used. Thus, it is possible to avoid an error caused by a difference in characteristics of the optical transmission path. In addition, by using an optical fiber as the optical transmission path, the influence of obstacles on the transmission path can be avoided, so that the Faraday element is placed away from the light output means and light detection means, and the magnetic field at a remote location is measured. Can also be performed.

[Brief description of the drawings]

FIG. 1 is a block diagram of a magnetic sensor according to an embodiment of the present invention.

FIG. 2 is a diagram showing a rotation angle of a polarization plane of two initial polarizations emitted in time series and a rotation angle of a polarization plane of two detected polarizations in the example.

FIG. 3 is a block diagram of a magnetic sensor of a modified example using an optical fiber for an optical transmission path.

FIG. 4 is a front view of a polarization switching device according to a modification of the present invention.

FIG. 5 is a side sectional view of a polarization switching device according to a modification of the present invention.

FIG. 6 is a block diagram of a conventional magnetic sensor.

FIG. 7 shows the rotation of the plane of polarization of the initial polarization in the prior art;
FIG. 5 is a diagram illustrating a rotation angle of a polarization plane of the detected polarized light.

[Explanation of symbols]

 Reference Signs List 10 light source section 20 polarization switching section 30a optical transmission path 30b optical transmission path 40 sensor head 50 light receiving section 60 arithmetic processing section

Claims (2)

[Claims] 1. A magnetic sensor for detecting a magnetic field intensity around a Faraday element, wherein (a) a light beam emitted from a single light source is switched to two polarized lights whose polarization planes are orthogonal to each other and output alternately in time series. Light output means; (b) 2
A Faraday element that transmits two polarized lights and converts the intensity of the magnetic field to be detected into a rotation angle of the polarization plane of the two polarized lights; and (c) the two polarized lights emitted from the Faraday element are (D) a single light detecting means for receiving each light converted into the light intensity by the light converting means and converting the light into an electric signal; (E) arithmetic means for calculating the magnetic field strength based on the electric signals corresponding to the two polarized lights, which are alternately output in a time series from the light detecting means. 2. The magnetic sensor according to claim 1, wherein
A magnetic sensor, wherein an optical fiber is used in at least one of a light transmission path between the light output means and the Faraday element and between the light conversion means and the light detection means.