JP2005345350A - Optical fiber current sensor, and calibration device therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber current sensor of a wide dynamic range excellent in linearity, and capable of reducing an unstable factor, and a calibration device of small size capable of calibrating accurately the optical fiber current sensor. <P>SOLUTION: This optical fiber current sensor using a Faraday effect is provided with a phase modulation means (phase modulator 101) for phase-modulating both propagation lights with a prescribed angular frequency, a ratio measuring means (CPU 201) for measuring a ratio between phase-modulated components of different degrees included in an interference light of the both propagation lights, and a current value processing means (CPU 201) for finding a value of a current of an object, based on the ratio between the phase-modulated components. The dynamic range is widened, the linearity is excellent, and the unstable factor is reduced, because of using the ratio between the phase-modulated components. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical fiber current sensor using the Faraday effect, and more particularly to an optical fiber current sensor having a wide dynamic range, good linearity and less instability, and a small-sized calibration apparatus that can accurately calibrate an optical fiber current sensor.

  The fiber optic current sensor uses a fiber optic loop to form a Sagnac interference system, and circularly polarized light propagating through the fiber optic loop in the forward / reverse circulatory direction causes the Faraday effect to act on each of the waves due to the current. The phase difference generated in the light is detected using the interference of light, and the current is obtained from this phase difference.

  Since the principle and details of this optical fiber current sensor are described in detail in Non-Patent Documents 1 and 2, these are replaced with the disclosure of the background art of the present invention. The optical fiber current sensor includes a loop type and a reflection type. The configuration of the loop type is as described in FIG. 2 (FIG. 2) of Non-Patent Document 1, and the configuration of the reflection type is non-patent. This is as described in FIG.

  In an optical fiber current sensor, in order to calibrate the relationship between the current and the phase difference between light (also referred to as calibration), a current having a known value is supplied to one power cable, and the value of this current and the optical fiber current sensor In general, a method of checking whether or not the current value measured in (1) matches. Using a multi-turn electric coil wound from the outside to the inside of the optical fiber loop instead of one power cable, the current capacity is measured by measuring the current flowing through the electric coil by the number of turns using the optical fiber loop. A large calibration current can be obtained with a small calibration device with a small amount of current.

  That is, the calibration device shown in FIG. 6 includes an electric coil 601 wound from the outside to the inside of the optical fiber loop 401, a current source 602 that applies a desired current to the electric coil, and a signal processing unit of the optical fiber current sensor. And a personal computer 603 for informing a correct current value applied by the current source 602 to 402.

“Plasma current measurement by Sagnac interferometric photocurrent transformer (CT)”, Ichinose et al., Journal of Plasma and Fusion Research, Vol. 76, No. 6, 593-600, June 2000 “Development of optical fiber current sensor for DC electric railway”, Hayashiya et al., Proceedings of 32nd Meeting on Lightwave Sensing Technology, pp. 141-146, December, 2003

  In the technique disclosed in Non-Patent Document 1, the amplitude of the fundamental wave S1 having the phase modulation frequency ωm is used as a signal representing the output of the optical fiber current sensor, that is, the current value. However, since the fundamental wave S1 is a sine wave function of 2θ (θ is a phase difference caused by current), the relationship between the target current and the output current value does not become a straight line when the target current increases. If the measurement is performed only in a region where the sine wave function exhibits linearity, the magnitude of the current that can be measured is restricted, and the dynamic range of the optical fiber current sensor is narrowed. The fundamental wave S1 having the phase modulation frequency ωm has a constant A proportional to the intensity of the light source light and a phase modulation amplitude φ as factors. When the constant A and the amplitude φ change due to the state change of the light source and the phase modulator, the fundamental wave S1 also changes, so the output of this optical fiber current sensor becomes unstable.

  Further, in the calibration device of FIG. 6 in which a large calibration current is obtained with a small calibration device by passing a current through the electrical coil 601, the magnetic field h0 originally intended to be given by the electrical coil portion passing inside the optical fiber loop 401 is obtained. In addition, there is a magnetic field h5 caused by an electric coil portion passing outside the optical fiber loop 401, and the magnetic field h5 affects the propagation light of the optical fiber loop 401. Therefore, one power cable is connected to the inside of the optical fiber loop. The environment is different from the case of passing through and accurate calibration cannot be performed.

  Accordingly, an object of the present invention is to provide an optical fiber current sensor that solves the above-described problems and has a wide dynamic range, good linearity and less instability, and a calibration device that can accurately calibrate the optical fiber current sensor in a small size. is there.

  In order to achieve the above object, the optical fiber current sensor according to the present invention propagates circularly polarized light in the forward and reverse direction of the optical fiber loop that is circulated around the current to be measured, and the position generated between the two propagation lights. In the optical fiber current sensor for measuring the current of interest from the phase difference, between the phase modulation means for applying phase modulation to the two propagation lights at a predetermined angular frequency and the phase modulation components of different orders included in the interference light of the two propagation lights Ratio measuring means for measuring the ratio and current value processing means for obtaining the value of the target current based on the ratio between the phase modulation components are provided.

  The ratio measuring unit may measure a ratio between a phase modulation component of a fundamental frequency and a phase modulation component of a double frequency.

  The current value processing means applies the ratio between the phase modulation components obtained from the current of interest to the relationship between the ratio between the phase modulation components obtained according to the current having a known value in advance and the known current value. You may obtain | require the value of an electric current.

  The current value processing means stores the relationship between the known current value and the phase modulation component ratio in a table as discrete values, and searches this table with the phase modulation component ratio obtained from the target current. Then, the value of the target current may be obtained.

  The current value processing means, when the phase modulation component ratio obtained from the target current deviates from the discrete value of the phase modulation component ratio stored in the table, a plurality of phase modulation component ratios The current value of the target may be obtained by performing an interpolation operation using the discrete values of a plurality of known currents corresponding to the discrete values.

  A calibration device according to the present invention is a calibration device that applies a known current to the optical fiber current sensor according to any one of claims 3 to 5, and a current source that changes a current applied to the two electric coils, A storage means for storing the ratio between phase modulation components obtained from the ratio measurement means for each value of applied current, and a transfer means for transferring the ratio between phase modulation components to the current value processing means. is there.

  An electric coil that wraps around from the outside of the optical fiber loop to an inside of the optical fiber loop and an electrical coil that wraps around from the outside of the optical fiber loop to the inside are wound around the optical fiber loop. You may make it apply the electric current used as a direction.

  The present invention exhibits the following excellent effects.

  (1) An optical fiber current sensor having a wide dynamic range, good linearity, and less instability can be obtained.

  (2) A calibration apparatus that can accurately calibrate the optical fiber current sensor in a small size can be obtained.

  Hereinafter, an embodiment of an optical fiber current sensor of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 1, the optical fiber current sensor according to the present invention is roughly composed of an optical system 100 and a signal processing circuit 200. The optical system 100 shows only the phase modulator 101, the light receiver 102, and the light source 103 that are directly related to the signal processing circuit 200, and the optical fiber loop and the like will be described in detail later. In the present embodiment, the signal processing circuit 200 performs digital processing, and thus includes a CPU 201, a memory 202, and a peripheral analog element group.

  The optical fiber current sensor measures a ratio between phase modulation means for phase-modulating the propagation light of the optical system 100 at a predetermined angular frequency and phase modulation components of different orders included in the interference light obtained from the optical system 100. And a current value processing means for obtaining the value of the target current based on the ratio between the phase modulation components. Here, the ratio measuring means and the current value processing means are realized by the CPU 201. The details of the ratio measuring means and the current value processing means will be clarified in the description of the operation described later.

  An output terminal of a D / A converter 204 is connected to a phase modulator 101 as a phase modulation means via a driving amplifier 203. The D / A converter 204 D / A converts the phase modulation amplitude value commanded by the CPU 201.

  An output terminal of the light receiver 102 that receives the interference light is connected to a lock-in amplifier 206 via an amplifier 205 for amplification. The lock-in amplifier 206 extracts the phase modulation components S1, S2, S4... For the fundamental wave frequency of phase modulation and a plurality of higher-order frequencies such as double and quadruple frequencies. A plurality of output terminals of the lock-in amplifier 206 are connected to the A / D converter 208 via the multiplexer 207. The multiplexer 207 selects one of the phase modulation components appearing at each terminal of the lock-in amplifier 206 and supplies it to the A / D converter 208. The CPU 201 issues the selection command.

  The output end of the amplifier 205 is connected to the light source 103 via the supply current control circuit 209. The supply current control circuit 209 functions to keep the intensity of the light source light constant.

  A D / A converter 210 is connected to the CPU 201, and an amplifier 211 for measurement value output is connected to the output end of the D / A converter 210, and the current value measured outside the optical fiber current sensor is output as an analog signal. It can be done.

  As shown in FIG. 2, the optical system 100 includes a light source 103 that generates light source light, a light receiver 102 that receives interference light, and guides the light source light to a subsequent coupler 104 and transmits interference light from the coupler 104. A first-stage coupler 105 that leads to the light receiver 102, a polarizer 106 that changes the light source light guided from the first-stage coupler 105 to the subsequent-stage coupler 104 into linearly polarized light, and a light source light that has passed through the polarizer 106 is split into two to be emitted. At the same time, the two return lights are combined (interfered) and returned to the first-stage coupler 105, the subsequent-stage coupler 104, the transmission optical fibers 107 and 108 connected to both emission ends of the coupler 104, and one of the transmission optical fibers A phase modulator 101 provided in the optical fiber 107, a delay dummy fiber 109 formed in the middle of the transmission optical fiber 107, and each transmission optical fiber Λ / 4 elements 111 and 112 provided at the terminals of 07 and 108, and optical fiber loops (sensing) installed at both ends of the λ / 4 elements 111 and 112 and connected around the current I to be measured 110). This optical system 100 propagates circularly polarized light having the same rotation direction in directions opposite to each other so that a phase difference is generated between both propagation lights by the Faraday effect.

  The optical system 100 of FIG. 2 of this embodiment is equivalent to the optical system of FIG. On the other hand, the signal processing circuit of FIG. 2 of Non-Patent Document 1 is a circuit that performs analog processing, whereas the signal processing circuit 200 of FIG. 1 of this embodiment performs digital processing except for light source control.

  Next, the operation of the optical fiber current sensor according to the present invention will be described with reference to FIGS.

  The light source light emitted from the light source 103 exits the coupler 105, is converted into linearly polarized light by the polarizer 106, is branched by the coupler 104, and is transmitted to both ends of the optical fiber loop 110 via the transmission optical fibers 107 and 108. Incident. At that time, both linearly polarized light is converted into circularly polarized light in the same rotational direction by the λ / 4 elements 111 and 112. That is, if one is clockwise circularly polarized light, the other is also converted to clockwise circularly polarized light, and if one is counterclockwise circularly polarized light, the other is also converted to counterclockwise circularly polarized light. One of the light incident on the optical fiber loop 110 circulates around the current to be measured in the clockwise direction (CW), and the other circulates in the counterclockwise direction (CCW) and returns to the transmission optical fibers 107 and 108. Further, when each light is transmitted through the transmission optical fiber 107, the phase is modulated by the phase modulator 101 and delayed by the dummy fiber 109.

  Due to the magnetic field accompanying the current I to be measured passing through the inside of the optical fiber loop 110, the phase (velocity) of both CW and CCW light changes due to the Faraday effect. Both the CW and CCW lights returning to the coupler 104 interfere with each other when combined by the coupler 104 and become interference light. The interference light is branched by the coupler 105 and received by the light receiver 102. This interference light includes information on the phase difference 2θ caused by the current I to be measured.

  The CPU 201 samples the phase modulation component S1 of the fundamental frequency, the phase modulation component S2 of the double frequency, the phase modulation component S2 of the quadruple frequency, and the phase modulation component S4 of the quadruple frequency while switching the multiplexer 207 at appropriate time intervals. Based on the sampled data, the ratio measuring means in the CPU 201 measures the ratio S1 / S2 between the phase modulation component S1 at the fundamental frequency and the phase modulation component S2 at the double frequency by calculation. S1, S2, S4, S1 / S2 are as follows according to Non-Patent Document 1.

  Further, the relationship between the current I to be measured and the phase change θ of the propagating light is shown according to Non-Patent Document 1 as follows.

θ = nVeI (5)
However, n: number of turns of the optical fiber loop Ve; Verde constant Since the number of turns n and the Verde constant Ve are fixed values, there is a one-to-one relationship between the phase change θ and the current I, and both the ratio S1 / S2 and the phase change θ Since there is a one-to-one relationship, a one-to-one relationship is also established between the ratio S1 / S2 and the current I. Therefore, in the present embodiment, the optical fiber current sensor is a table in the memory 202 that shows the relationship between the known current value I given by the calibration performed in advance and the phase modulation component ratio S1 / S2 measured at that time. (See FIG. 4B), the current value processing means in the CPU 201 searches this table with the phase modulation component ratio S1 / S2 obtained from the current to be measured, and the table is obtained by this search. The current value I assigned from is used as the current value to be measured.

Since the capacity of the memory 202 is finite, the known current value and the ratio between phase modulation components stored in the table are discrete values (values larger than the resolution provided by the A / D converter 208 and the CPU 201). It has become. Therefore, the phase modulation component ratio S1 / S2 obtained from the current to be measured may deviate from the discrete value of the phase modulation component ratio S1 / S2 stored in the table. At that time, the current value to be measured may be obtained by performing an interpolation operation using the discrete values of a plurality of known currents corresponding to the discrete values of the ratios between the plurality of phase modulation components. For the interpolation calculation, linear interpolation, tan ⁻¹ function, or the like can be used.

  The CPU 201 sends the current value thus measured to the D / A converter 210 to convert it into an analog voltage. As a result, the measured value is amplified by the amplifier 211 and output to the outside in analog form.

  In the present invention, since the value of the target current is obtained based on the ratio between the phase modulation components, the influence of the non-linearity of the sine wave function is eliminated, and the target current and the target current are not affected by the magnitude of the target current. The relationship with the output current value is a straight line. Therefore, it is possible to measure with a wide dynamic range and good linearity.

  Further, in the present invention, since the constant A and the amplitude φ, which are unstable factors, are removed by taking the ratio between the phase modulation components, even if the constant A and the amplitude φ change, the measurement is not affected. (However, the amplitude φ is constant by controlling the amplitude of the phase modulator 101 so that S4 / S2 is constant).

  In the present embodiment, a table is used to search for the phase modulation component ratio S1 / S2, and the current value I is assigned. However, the current value I may be obtained by calculating an equation. That is, the ratio S4 / S2 = J4 (2φ · sin ωmα) / J2 (2φ · sin ωmα) of the quadrature frequency phase modulation component S4 and the double frequency phase modulation component S2 is constant. If the amplitude is controlled, J1 (2φ · sinωmα) / J2 (2φ · sinωmα) in the equation (4) of the ratio S1 / S2 becomes a constant K, and the phase change θ is

It becomes. By substituting the phase change θ obtained by the equation (6) into the equation (5), the current I to be measured can be obtained. Since the lock-in amplifier outputs the amplitude of each component as a DC voltage, the terms cos ωt, cos 2ωt, and cos 4ωt are eliminated.

  Hereinafter, an embodiment of a calibration apparatus of the present invention will be described in detail with reference to the accompanying drawings.

  As shown in FIG. 3, the calibration device according to the present invention includes an electric coil 301 and an optical fiber loop 110 that wrap around the optical fiber loop 110 of the optical fiber current sensor described above from the outer side to the inner side. The electric coils 302 that wrap around from the opposite outer side to the inner side are hung so that currents in the same direction are applied to the respective electric coils 301 and 302 inside the optical fiber loop 110. Current source 303 for changing the current applied to 302, storage means 304 for storing the ratio between phase modulation components obtained from the ratio measuring means of the optical fiber current sensor for each value of the applied current, and between the phase modulation components Transfer means for transferring the ratio to the current value processing means (included in a personal computer as the storage means 304) . The two electric coils 301 and 302 are arranged in a figure 8 shape such that one side passing through the inside of the optical fiber loop 110 is in contact with each other as shown in the figure. The two electric coils 301 and 302 have the same number of turns, winding pitch, wire diameter, wire length, winding diameter and the like.

  Here, the optical fiber current sensor is divided into an optical fiber loop 110 and a signal processing unit 250, and the signal processing unit 250 accommodates members shown in FIGS. 1 and 2 other than the optical fiber loop 110.

  In this calibration device, currents i of the same magnitude are supplied in reverse directions to the electric coils 301 and 302 so that the directions of the currents i are the same on the sides where the two electric coils 301 and 302 are in contact with each other. . As a result, the magnetic fields h1 and h2 generated by the current flowing in one side of the electric coils 301 and 302 are in the same direction. Since the optical fiber loop 110 is circulated around the one side, the magnetic fields h1 and h2 similarly act on the light propagating through the optical fiber loop 110. Therefore, the Faraday effect due to the current i × the number of turns × 2 is generated in the light propagating through the optical fiber loop 110.

  On the other hand, when attention is paid to both sides passing through the outside of the optical fiber loop 110 for each of the electric coils 301 and 302, the optical fiber loop 110 has the same magnetic fields h3 and h4 generated by the current i flowing through the electric coils 301 and 302. In the existing space, they are opposite to each other. Therefore, for example, at the illustrated point P, the magnetic field h3 is directed in the same direction as the CW light propagating through the optical fiber loop 110, the magnetic field h4 is directed in the opposite direction to the CW light, and at the Q point facing the point P. The magnetic field h3 is directed in the opposite direction to the CW light, and the magnetic field h4 is directed in the same direction as the CW light. Accordingly, the influence of these magnetic fields h3 and h4 on the CW light is canceled out. The same applies to CCW light.

  In this way, in the calibration device of the present invention, two electric coils that wrap around from the outer side to the inner side of the optical fiber loop are used, so the influence of the magnetic field caused by the electric coil portion that passes through the outer side of the optical fiber loop is canceled out. It is possible to perform an accurate calibration equivalent to a case where one power cable passes through the inside of an optical fiber loop while taking advantage of a calibration current equivalent to a large current with a small current by using an electric coil. become.

  At the time of calibration, the current source 303 discretely changes the current applied to the two electric coils 301 and 302 of the current source 303 at appropriate intervals. In synchronization with this, the storage unit 304 stores the phase modulation component ratio S1 / S2 obtained from the ratio measuring unit of the optical fiber current sensor for each value I of the current used for calibration (applied current i × number of turns). To do. As a result, a table as shown in FIG. 4A is created in the storage means 304 using the current I as a search word (or address) and the phase modulation component ratio S1 / S2 as stored data. When the table for the necessary dynamic range range is created, the table in the storage unit 304 is transferred to the memory 202 of the optical fiber current sensor in FIG. At that time, a table as shown in FIG. 4B is created in the memory 202 with the phase modulation component ratio S1 / S2 as a search word (or address) and the current I as read data.

  In this embodiment, the calibration device provides a table for the optical fiber current sensor. However, even if the calibration device of the present invention is used for calibration of an optical fiber current sensor that does not use a table, Similarly, it is possible to obtain an effect that accurate calibration can be performed. The calibration device of the present invention can also be applied to calibration of a current sensor using a Rogowski coil or Hall CT. The calibration device of the present invention can also be applied to calibration of an optical fiber current sensor that uses lead glass as a sensing fiber to detect current from rotation of the polarization direction.

  Next, the optical fiber current sensor of the present invention is compared with the optical fiber current sensor of the background art. FIG. 5 shows current values and errors when the phase difference is changed when the calibration is performed at the phase difference of 60 deg for the present invention and the background art. The phase difference of 60 deg corresponds to the most linear region in the sine function. Each current value is shown as a relative output with reference = 1 when the phase difference is 60 deg.

  As shown in the figure, in the graph of the present invention, the current value changes linearly with respect to the change in phase difference, whereas in the background art graph, the current value changes in a curve with respect to the change in phase difference. Yes. That is, an error occurs in the background art, and the error varies depending on the phase difference. In contrast, the present invention does not cause an error regardless of the phase difference.

It is a block diagram of the optical fiber electric current sensor which shows one Embodiment of this invention. It is a block diagram of the optical system of the optical fiber current sensor of FIG. It is a block diagram of the calibration apparatus which shows one Embodiment of this invention. (A) is a figure of the table which the calibration apparatus of FIG. 3 produces, (b) is a figure of the table memorize | stored in the optical fiber current sensor of FIG. FIG. 6 is a relationship diagram between a phase difference and an output current value. It is a block diagram of the calibration apparatus of background art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Optical system 101 Phase modulator 102 Light receiver 103 Light source 200 Signal processing circuit 201 CPU
202 Memory 206 Lock-in amplifier 208 A / D converter 301, 302 Electric coil

Claims (7)

  In an optical fiber current sensor that propagates circularly polarized light in the forward and reverse circulation directions of an optical fiber loop that circulates around the current to be measured, and measures the current from the phase difference generated between the two propagation lights. Phase modulation means for applying phase modulation to the propagating light at a predetermined angular frequency, ratio measuring means for measuring the ratio between the phase modulation components of different orders included in the interference light of both propagation lights, and the ratio between the phase modulation components An optical fiber current sensor comprising: current value processing means for obtaining a value of a target current based on the current value processing means.   2. The optical fiber current sensor according to claim 1, wherein the ratio measuring unit measures a ratio between a phase modulation component of a fundamental frequency and a phase modulation component of a double frequency.   The current value processing means applies the ratio between the phase modulation components obtained from the current of interest to the relationship between the ratio between the phase modulation components obtained according to the current having a known value in advance and the known current value. 3. The optical fiber current sensor according to claim 1, wherein a current value is obtained.   The current value processing means stores the relationship between the known current value and the phase modulation component ratio in a table as discrete values, and searches this table with the phase modulation component ratio obtained from the target current. 4. The optical fiber current sensor according to claim 3, wherein a current value of the target is obtained.   The current value processing means, when the phase modulation component ratio obtained from the target current deviates from the discrete value of the phase modulation component ratio stored in the table, a plurality of phase modulation component ratios 5. The optical fiber current sensor according to claim 4, wherein the current value of the target is obtained by performing an interpolation operation using discrete values of a plurality of known currents corresponding to the discrete values.   A calibration device for providing a known current to the optical fiber current sensor according to any one of claims 3 to 5, wherein a current source that changes a current applied to the two electric coils and the ratio for each value of the applied current Calibration of an optical fiber current sensor, comprising storage means for storing the phase modulation component ratio obtained from the measurement means, and transfer means for transferring the phase modulation component ratio to the current value processing means. apparatus. An electrical coil that wraps around the optical fiber loop from the outside to the inside of the optical fiber loop and an electrical coil that wraps around from the outside to the inside opposite to the optical fiber loop are wound around, and the respective electrical coils are the same inside the optical fiber loop. 7. The optical fiber current sensor calibration apparatus according to claim 6, wherein a directional current is applied.

Images