CN102156213A - Fiber bragg grating current measurement method based on birefrigent effect - Google Patents

Abstract

The invention discloses a fiber bragg grating current measurement method based on birefrigent effect, belonging to the technical field of sensors and measurement. Devices used for the method comprise a broadband light source (11), a fiber coupler (12), a sensing fiber bragg grating (13), a support optical fiber (20), a permanent magnet (14), silicon slices (15 and 16), a magnet coil (17), a support (18), a drive current control unit (19), a spectrometer (19), a spectrometer (21) a spectrum analyzing unit (22) and an output and display unit (23). The method has the characteristics that the current measurement is realized on the basis of the principle that the sensing fiber bragg grating generates the birefrigent effect under the action of radial electromagnetic force so as to present two reflectance spectrums with different polarization states and the difference of the central wavelengths of the two reflectance spectrums is in direct proportion to the radial electromagnetic force; and as the differential measurement method is adopted, the temperature crossing sensitive problem of the fiber bragg grating sensing signals is solved.

Description

A Method for Measuring Fiber Bragg Grating Current Based on Birefringence Effect

technical field

The invention relates to a fiber grating current measurement method, which belongs to the technical field of sensors and measurements.

Background technique

Current measurement technology has broad application prospects and important significance in the field of power system and electronic technology. At present, the research on current measurement technology has become one of the hot spots, and the current sensing technology based on optical fiber provides a better solution for current measurement.

There are many kinds of physical effects on which the fiber optic current sensor is based, mainly electromagnetic induction, Faraday magneto-optic effect, thermal effect (temperature type), inverse piezoelectric effect, magnetic force type, magnetostrictive effect, etc. Optical fiber current sensors based on electromagnetic induction use traditional current transformers or air-core transformers (Rogowski coils) to sample transmission line currents, use active device modulation technology, use optical fibers as signal channels, and transmit optical signals converted from the high-voltage side to the ground for signal processing Processing, so as to obtain the device under test. Since the conventional current sensor is still used as the sensing head in the system, it can only be used as a transitional scheme from the electromagnetic current sensor to the all-optical fiber optic current sensor.

(Min-Cheol Oh, Jun-Kyu Seo, Kyung-Jo Kim, Hoon Kim, Jun-Whee Kim, and Woo-Sung Chu, Optical current sensors consisting of polymeric waveguide components, JOU RNAL OF LIGHTWAVE TECHNOLOGY, 28(12), 2010) proposed a current sensor based on Faraday’s magneto-optic effect. When linearly polarized light passes through a uniform isotropic medium in an external magnetic field, and the propagation direction of the light is consistent with the direction of the external magnetic field, the polarization plane of the linearly polarized light will occur The effect of rotation.

Fiber optic current sensors based on thermal effect (temperature type), inverse piezoelectric effect, magnetic force type, and magnetostrictive effect are fundamentally due to the action of current to force or generate displacement (length changes) on the fiber or fiber grating. Therefore, the detection of current is realized by using the principle of optical fiber interferometer or the principle of detecting strain or temperature by optical fiber grating.

The thermal effect (temperature type) fiber optic current sensor is based on the heat generated by the current to thermally expand the fiber and increase the length; the inverse piezoelectric effect fiber optic current sensor is that when a certain voltage is applied to the piezoelectric ceramic, it will deform, thus the length of the fiber Increase. Both of the above two principles can respectively constitute an interference type fiber optic current sensor or a fiber grating current sensor.

Optical fiber sensors that use magnetism to detect current can be divided into two categories, one is (Heredero RL, De Caleya RF, Guerrero H, et al. Micro-machined optical fiber current sensor. APPLIED OPTICS, 38 (25), 1999) The proposed magnetic force produces displacement, which changes the length of the interference cavity of the fiber optic FP interference sensor, thereby realizing current detection. The other type is (Yong Zhao, Qing-yao Meng, Kun Chen. Novel current measurement method based on fiber Bragg grating sensor technology .SENSORS AND ACTUATORS A: PHYSICAL, 126, 2006) based on the magnetic field generated by the measured current, the ferromagnetic material in the magnetic field is moved or deformed by the magnetic force, which causes the strain of the fiber grating and affects the reflection spectrum of the fiber grating. current is detected. The direction of the electromagnetic force applied to the fiber grating current sensor is along the axis of the fiber grating, so that the reflection wavelength of the fiber grating is shifted.

The magnetostrictive effect optical fiber current sensor generally fixes the optical fiber on the magnetostrictive material, and the magnetostrictive material is placed in the magnetic field. The stretching of the magnetostrictive material causes the length of the optical fiber to change, thereby changing the optical path in the optical fiber, causing a change in the optical phase. The measured magnetic field can be measured by using the interferometric method to detect the phase change, and then the measured current value can be obtained. Or (Deborah Reilly, Andrew J. Willshire, Grzegorz Fusiek, Pawel Niewczas, and James R. McDonald, A fiber-Bragg-grating-based sensor for simultaneous AC current and temperature measurement, IEEE SENSORS JOURNAL, 6(6), 2006) It is proposed that the fiber grating fixed on the magnetostrictive material is stretched, and the magnitude of the measured current can be obtained by detecting the shift of the reflection wavelength of the fiber grating. The disadvantage of this type of fiber optic current sensor is that due to the direct pasting method, there is a load effect at the pasting point, and as the current increases, there will be a hysteresis effect, which is difficult to overcome.

Contents of the invention

The purpose of the present invention is not only to overcome the shortcomings of the prior art, but also to have the advantages of low cost and simple signal demodulation method, and at the same time, it also solves the problem of temperature cross-sensitivity of the measurement signal of the fiber grating sensor.

Technical scheme of the present invention is as follows:

A current measurement method mainly includes a fiber grating radial load sensitive unit, an electromagnetic force generating device, a light source, an optical fiber coupler, a spectrometer, a spectrum analysis unit, an output and a display unit, and is characterized in that: the current measurement method mainly A sensing fiber grating is used to realize radial load measurement, and the fiber grating radial load sensing unit is composed of a sensing fiber grating, a section of supporting optical fiber and two upper and lower silicon wafers with smooth surfaces. The sensing fiber grating and the supporting fiber are placed in parallel between two silicon chips; the electromagnetic force generating device is composed of a permanent magnet, a bracket wound with an electromagnetic coil and a drive current controller, and the permanent magnet is fixed on On the top of the upper silicon wafer among the upper and lower silicon wafers, a bracket wound with an electromagnetic coil is fixed under the lower silicon wafer, and the drive current controller is connected to the electromagnetic coil through a wire; the light source passes through the The fiber coupler is connected to one end of the sensing fiber grating, and the measurement signal of the sensing fiber grating is reflected by itself and then connected to the spectrometer through the output end of the fiber coupler. Display unit processing.

In the present invention, the use of a sensing fiber grating to realize radial load measurement is to use the sensing fiber grating to generate birefringence under the action of radial electromagnetic force to produce two reflection spectra with different polarization states. It is realized by the principle that the difference of the central wavelength of the reflection spectrum is proportional to the radial electromagnetic force.

The technical feature of the present invention is that: the material of the frame wound with the electromagnetic coil is plastic, the inner diameter is 15mm, and the height is 100mm. The material and diameter of the supporting optical fiber section are exactly the same as those of the sensing optical fiber grating, and their surface coating layer is removed.

The present invention has the following characteristics: ①A differential signal processing method is adopted, which avoids the temperature cross-sensitive problem of the fiber grating measurement signal; ②The signal transmission loss is low, without distortion, and is suitable for long-distance measurement; ③The measurement probe part of the system is electrically insulated, Good security.

Description of drawings

Fig. 1 is a schematic diagram of the current measurement principle of the fiber Bragg grating based on the birefringence effect provided by the present invention.

Fig. 2 is a graph showing the relationship between the measured current and the FBG output wavelength difference when the number of coil turns in the electromagnetic force generating device of the present invention is different.

Fig. 3 is a graph showing the relationship between the measured current and the FBG output wavelength difference when the air gaps in the electromagnetic force generating device of the present invention are different.

Fig. 4 is a graph showing the relationship between the measured current and the FBG output wavelength difference when the area of the electromagnetic coil is different in the electromagnetic force generating device of the present invention.

Fig. 5 is a curve diagram of the relationship between the measured current and the wavelength difference of the sensing fiber grating realized by using the present invention after the system parameters are selected.

Fig. 6 is a change diagram of the reflection spectrum of the sensing fiber grating when the measured current changes by using the present invention.

(a) Measured current I=0

(b) Measured current I=0.3A

(c) Measured current I=0.5A

Detailed ways

The fiber grating current measuring method proposed by the present invention is described as follows in conjunction with the accompanying drawings:

The FBG current measurement system is shown in Figure 1. The sensing FBG and a section of supporting fiber are placed between two silicon wafers at a certain distance (the advantage of this is that the force on the sensing FBG is uniform). have the same material and diameter. A permanent magnet is placed on the top of the upper silicon chip, and a support wound with an electromagnetic coil is placed under the lower silicon chip. The light output by the broadband light source is transmitted to the sensing fiber grating through the coupler, and the optical signal with a specific reflection wavelength is received by the spectrum analyzer through the coupler. When the measured current is not applied, the optical spectrum analyzer can only monitor the optical signal with one reflection peak. When the measured current is applied to the electromagnetic coil, the coil current will generate a magnetic field, and the permanent magnet will be subjected to electromagnetic force in the magnetic field, which will generate a radial load on the optical fiber directly below it, causing the sensing fiber grating to generate radial strain, that is double refraction. As a result of the linear birefringence effect in two mutually orthogonal directions, the sensing fiber grating produces two reflection peaks, and with the increase of the external measured current, the electromagnetic force increases, and the compressed sensing fiber grating The resulting birefringence effect is more pronounced, causing the reflection spectra of the two orthogonal polarization states to become increasingly separated from each other. By detecting the amount of wavelength separation, that is, by detecting the difference between the reflected light peaks of two orthogonal polarization states, the magnitude of the measured electromagnetic force can be known. Since the electromagnetic force has a definite corresponding relationship with the measured current, it is realized. current measurement.

According to the fiber mode theory, under the condition of constant temperature, when only the radial force acts on the fiber, the axial stress of the fiber is assumed to be zero (actually, since the wavelength difference detection method is finally adopted, even if there is an axial stress Directional strain will also be eliminated in the measurement results), at this time, it will cause additional birefringence of the fiber, and it is this birefringence that causes two additional orthogonal polarization modes. Therefore, if a radial load is applied to a fiber grating, it will cause the separation of the reflection spectra of the two polarization states. The difference in the wavelength separation of these two orthogonal polarization modes can be expressed as:

${\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}}_{\mathrm{<span\; class="notranslate">\; B</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; x</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; F\&Lambda;</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; RE</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="no"\; filenames="HSA00000457862600012.png"\; state="\{\{state\}\}">no</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 11</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="0"\; label="no"\; filenames="HSA00000457862600012.png"\; state="\{\{state\}\}">no</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 11</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, Λ is the grating period, F is the external force acting along the fiber with length _L2 , R is the radius of the fiber cladding, n _{0, x} and n _{0, y} are the initial effective Refractive index, p ₁₁ and p ₁₂ are the elastic-optic coefficients of the optical fiber; E ₁ and v ₁ are the Young's modulus of elasticity and Poisson's ratio of the optical fiber, respectively.

It can be seen that the difference between the shifting amount of the wavelength in the two polarization states of the fiber grating is proportional to the radial load.

Since the current can generate a magnetic field, there is a corresponding relationship between the magnitude of the current and the magnetic field, and the permanent magnet can generate electromagnetic force under the action of the magnetic field. If the change of the current to be measured can be converted into electromagnetic force through a certain device, and this force If it is applied to the fiber grating, the above principle can be used to realize the measurement of the current. The specific scheme and structure proposed in this paper are shown in Figure 1. Among them, the helical tube structure is composed of electromagnetic coils and permanent magnets. After electrification, magnetic flux is generated in the magnetic circuit, and electromagnetic attraction is generated to the permanent magnets. Under the action of electromagnetic attraction, the permanent magnets move a small distance along the direction of the magnetic force lines. , so that the working air gap is reduced, resulting in a mechanical movement trend. At this time, the electromagnetic attraction F can be obtained from the energy balance, as shown in the formula (2)

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; wxya</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{\mathrm{<span\; class="notranslate">\; d\&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

In the formula, dδ is the moving distance of the permanent magnet under the action of electromagnetic attraction F, and dW _m is the change of magnetic energy stored in the magnetic system.

Ignoring the fringing phenomena of the magnetic flux across the air gap, and noting that the displacement of the permanent magnet only changes the magnetic energy stored in the air gap, write:

${\mathrm{<span\; class="notranslate">\; W</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; h</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; s\&delta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{<span\; class="notranslate">\; (</span>\frac{\mathrm{<span\; class="notranslate">\; \&Psi;</span>}}{\mathrm{<span\; class="notranslate">\; the\; s</span>}{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; s\&delta;</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; \&Psi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}{{\mathrm{<span\; class="notranslate">\; s\&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

Ψ为气隙磁通量，s为气隙的横截面面积，可简化认为是电磁线圈横截面积，μ₀为真空中的磁导率，W_m为气隙的磁能，δ为气隙减小的距离。In the formula, H is the magnetic field strength of the air gap magnetic field,

Ψ is the magnetic flux of the air gap, s is the cross-sectional area of the air gap, which can be simplified as the cross-sectional area of the electromagnetic coil, μ ₀ is the magnetic permeability in vacuum, W _m is the magnetic energy of the air gap, and δ is the reduction of the air gap distance.

Then according to formula (3), formula (2) can be written as:

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; wxya</span>}}_{\mathrm{<span\; class="notranslate">\; m</span>}}}{\mathrm{<span\; class="notranslate">\; d\&delta;</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{\mathrm{<span\; class="notranslate">\; \&Psi;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; s\&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 4</span>}<span\; class="notranslate">\; )</span>$

According to the magnetic circuit related theorem, the relationship between the air gap magnetic flux Ψ and the current can be obtained as

$\mathrm{<span\; class="notranslate">\; \&Psi;</span>}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; NIs</span>}}{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

In the formula, N is the number of turns of the coil.

Then combining formula (4) and formula (5), the electromagnetic attraction can be obtained as:

$\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}\frac{{{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; NI</span>}<span\; class="notranslate">\; )</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; \&delta;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

After obtaining the expression of electromagnetic attraction, the relationship between electromagnetic attraction and the current to be measured is established. In this way, by substituting equation (6) into equation (1), the relationship between the wavelength difference of the two polarization states of the fiber grating and the current can be obtained, as follows:

$\mathrm{<span\; class="notranslate">\; \&Delta;\&lambda;</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; \&mu;</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}{\mathrm{<span\; class="notranslate">\; n</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; I</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&delta;</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; L</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}\frac{\mathrm{<span\; class="notranslate">\; \&Lambda;</span>}}{\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; RE</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; \{</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; x</span>}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 11</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 3</span>}\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; no</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}<span\; class="notranslate">\; ,</span>\mathrm{<span\; class="notranslate">\; the\; y</span>}}^{\mathrm{<span\; class="notranslate">\; 3</span>}}<span\; class="notranslate">\; [</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 11</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; p</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; 5</span>}\mathrm{<span\; class="notranslate">\; v</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span><span\; class="notranslate">\; \}</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 7</span>}<span\; class="notranslate">\; )</span>$

Wherein, record the wavelength difference Δλ=Δλ _B,y −Δλ _B,x . For a specific device, s, N, δ, and L ₂ are all constants. If the fiber characteristic parameters are known, then Δλ∝I ² can be known. When a current passes, the electromagnetic force generated by the electromagnetic coil causes the sensing fiber grating to be subjected to radial stress, and the wavelength separation of the two orthogonally polarized reflections increases. The greater the current intensity, the more the separation, so the electromagnetic force is used to change the wavelength separation difference of the sensing fiber grating to sense the magnitude of the current intensity.

It is known that the vacuum magnetic permeability μ ₀ =4π×10 ^-7 H/m, the sensing fiber grating period Λ=0.5355μm, the fiber cladding radius R=62.5μm, and the Young’s modulus E ₁ of the fiber =7× 10 ¹⁰ Pa, the initial effective refractive index n _eff of the fiber core = 1.43, the elastic-optical coefficients p ₁₁ = 0.121, p ₁₂ = 0.270, and the Poisson's ratio v ₁ = 0.17.

The following is a simulation analysis of the influence of some structural parameters in the measurement system on the measurement characteristics.

Let s=10 ^-4 m ² , δ=2×10 ^-3 m, L ₂ =1.5×10 ^-2 m.

It can be seen from Figure 2 that when the number of turns of the electromagnetic coil is relatively small (N=900), the electromagnetic force caused by the change of the current makes the separation of the reflected wavelengths of the two polarization states of the sensing FBG not obvious. As the number of turns of the electromagnetic coil increases, the effect of the electromagnetic force increases, and the difference between the reflected wavelengths of the two polarization states increases gradually. However, when the number of turns increases, the volume of the electromagnetic coil will increase accordingly, the manufacturing process will be correspondingly complicated, and the current energy consumption will increase. Therefore, considering a compromise, the number of turns N=1500 is selected.

In the case that N=1500 has been determined, select parameters s=10 ^-4 m ² , L ₂ =1.5×10 ^-2 m. It can be seen from Figure 3 that when the number of turns is constant (N=1500), the electromagnetic force can be increased by adjusting the height of the air gap, the effect of wavelength separation is more obvious, and the current measurement sensitivity can be continuously increased at the same time. However, reducing the height of the air gap will also increase the difficulty of the manufacturing process. Combined with the actual situation, when the air gap width is below δ=10 ^-3 m, it will not be considered. Choose δ=1.5×10 ^-3 m which is easier to realize.

In the case where N=1500 has been determined, δ=1.5×10 ^-3 m, L ₂ =1.5×10 ^-2 m. It can be seen from Figure 4 that when the number of turns is constant (N=1500) and the air gap is constant (δ=1.5×10 ^-3 m), the electromagnetic force can also be increased by adjusting the area of the electromagnetic coil, thereby increasing the wavelength separation. This method is easier to realize, combined with the actual situation, so s=3.14×10 ^-4 m ² is selected.

Since the length L ₂ of the optical fiber in the magnetic field does not vary widely and has no obvious influence on the magnitude of the electromagnetic force, it is not considered as an adjustable parameter.

To sum up, select parameters: N=1500, s=3.14×10 ^-4 m ² , δ=1.5×10 ^-3 m, L ₂ =1.5×10 ^-2 m. The relationship curve between the final measured current and the difference between the reflected wavelengths of the two polarization modes of the sensing fiber grating is given, as shown in FIG. 7 .

It can be seen from Figure 5 that, with the increase of the measured current, the difference between the reflected wavelengths of the two polarization modes of the sensing fiber grating increases, and the measurement sensitivity increases. When the measured current is greater than 0.7A, the measurement characteristics have good linearity. At this time, the measurement sensitivity can reach 2nm/A. If the resolution of the wavelength detection device can reach 1pm, the current variation of 0.5mA can be distinguished.

According to Figure 1, a preliminary system experimental device was built, and the spectral change was measured by the spectrum analyzer during the increase of the measured current. It can be seen from Figure 6 that when the measured current is 0, there is no birefringence effect at this time There is only one reflection peak in the reflection spectrum of the sensing fiber Bragg grating; when the measured current increases, due to the birefringence effect of the sensing fiber Bragg grating, two polarization modes appear, and these two polarization modes increase with the increase of the electromagnetic force. The reflected wavelengths are separated from each other, and the larger the current, the more obvious the separation, which is consistent with the theoretical analysis results.

Claims (4)

1. A current measurement method, mainly comprising a fiber grating radial load sensitive unit, an electromagnetic force generating device, a light source, an optical fiber coupler, a spectrometer, a spectral analysis unit, an output and a display unit, characterized in that: the current measurement The method mainly uses a sensing fiber grating (13) to realize radial load measurement, and the fiber grating radial load sensitive unit consists of a sensing fiber grating (13), a section of supporting optical fiber (20) and two upper and lower pieces Silicon chips (15, 16) with smooth surfaces are composed, and the sensing fiber grating and supporting optical fiber are placed in parallel between the two silicon chips; the electromagnetic force generating device consists of a permanent magnet (14), a winding Composed of a support (18) with an electromagnetic coil (17) and a drive current controller (19), the permanent magnet is fixed on the top of the upper silicon wafer among the upper and lower silicon wafers, and the electromagnetic coil is wound The bracket is fixed below the lower silicon chip, and the drive current controller is connected to the electromagnetic coil through a wire; the light source (11) is connected to one end of the sensor fiber grating through the fiber coupler (12), and the sensor The measurement signal of the fiber grating is reflected by itself and connected to the spectrometer (21) through the output end of the fiber coupler, and the output signal of the spectrometer is transmitted to the spectrum analysis unit (22) through the signal line, and then processed by the output and display unit (23). 2. according to the described current measurement method of claim 1, it is characterized in that: the described utilization of a sensing fiber grating (13) to realize radial load measurement is to utilize sensing fiber grating to produce under radial electromagnetic force. Due to the birefringence effect, two reflection spectra with different polarization states appear, and the difference between the central wavelengths of the two reflection spectra is proportional to the radial electromagnetic force. 3. The current measuring method according to claim 1, characterized in that: the material of the support (18) wound with the electromagnetic coil (17) is plastic, the inner diameter is 15mm, and the height is 100mm. 4. The current measuring method according to claim 1, characterized in that: the material and diameter of the supporting fiber (20) are exactly the same as those of the sensing fiber grating (13), and their surface coatings are removed.

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