Direct-current electric field sensing system with rotating structure

Abstract

The invention relates to a direct-current electric field sensing system with a rotating structure, which realizes accurate measurement of a direct-current electric field. The system comprises a laser, a rotating structure and a detector, the rotating structure comprises a single-mode fiber slip ring, a collimating mirror, a polarizing film, a 1 / 4 wave plate, a crystal and a multimode fiber slip ring. Experimental results show that the scheme has the advantages of high response speed and strong anti-electromagnetic interference capability, and the aims of inhibiting light beam offset, realizing light beam collimation and achieving high-precision measurement can be fulfilled by utilizing the rotating structure and the optical fiber slip ring.

Description

Technical Field

[0001] The invention belongs to the field of electric field, and relates to a direct current electric field sensing system, and in particular to a direct current electric field sensing system containing a rotating structure. Background Art

[0002] Accurate measurement of electric field has always been the research direction of power science. Electric sensors mostly use electrical electric field measurement technology and use the charge induction principle to measure the electric field in space. With the continuous development of materials science and optical sensing technology, optical electric field measurement technology provides a better solution for electric field measurement. Optical electric field sensors are superior to traditional electrical electric field sensors in terms of performance and have been widely studied.

[0003] In recent years, the rapid development of materials and optical technology has provided many new solutions for electric field measurement. Optical electric field sensors based on optical sensing principles have significant advantages over traditional electrical sensors in terms of response speed, frequency band, volume, and insulation performance. In general, optical electric field sensors use the changes in medium properties under the action of an external electric field to convert electric field signals into corresponding light signals for measurement.

[0004] However, both the integrated optical waveguide electric field sensor and the body effect interferometric electric field sensor are used to measure AC electric fields, and are not suitable for measuring extremely low frequency electric fields and DC electric fields. This is because under the action of DC or extremely low frequency electric fields, the internal charge of the sensor crystal drifts and accumulates on the crystal surface. The direction of the electric field generated by the accumulated charge is opposite to the direction of the external electric field to be measured, so the electric field strength inside the crystal is gradually offset, the sensor output signal changes, and the DC electric field cannot be accurately measured.

[0005] Therefore, it is necessary to establish a DC electric field sensing system to achieve accurate measurement of the DC electric field. Summary of the invention

[0006] A schematic diagram of a DC electric field sensing system with a rotating structure according to the present invention is shown in FIG. Figure 1 As shown, the system includes a laser, a rotating structure, and a detector; the rotating structure includes a single-mode optical fiber slip ring, a collimator, a polarizer, a quarter wave plate, a crystal, and a multi-mode optical fiber slip ring.

[0007] The technical solution of the present invention utilizes electro-optical sensing, mainly based on the primary electro-optical effect, to convert the electric field signal into an optical signal, separate and extract the electric field component through subsequent signal processing, and inversely calculate the electric field intensity to be measured. The present invention utilizes the primary electro-optical effect of the crystal and the principle of interferometric phase detection to convert the electric field signal to be measured into a light intensity signal. The light intensity signal is converted into an electrical signal using a detector, which facilitates subsequent signal collection and processing. Within the linear working range of the electro-optical sensing unit, there is the following linear relationship between the output voltage signal and the electric field to be measured:

[0008] U out =k1+k2E z

[0009] Where U out is the output voltage signal, k1 is the DC component of the photodetector output, k2 is the proportionality coefficient between the electric field strength and the output voltage, E z is the modulated electric field strength to be measured.

[0010] The optical fiber slip ring is used to realize the transmission of optical signals from a rotating platform to data transmission of a stationary platform. In the present invention, single-mode optical fiber slip rings and multi-mode optical fiber slip rings are used.

[0011] Fiber optic slip rings use optical fiber as the transmission medium. They are devices that require 360-degree rotation of light guide and transmission of optical fiber signals. They are the best technical solution for data transmission between rotating connected system components. They are particularly suitable for applications that require unlimited, continuous or intermittent rotation, and at the same time require data and signal transmission from a fixed position to a rotating position. They can improve mechanical performance, simplify system operation, and avoid damage to optical fibers due to the rotation of active joints.

[0012] The crystal is the core element of the technical solution of the present invention, which is responsible for converting the external electric field signal into an optical signal. Its performance will directly affect the sensitivity, measurement range and environmental temperature adaptability of the electro-optical sensing unit. Therefore, the following factors should be considered when selecting an electro-optical crystal: 1. It has good light transmission performance, small light intensity loss in the optical path caused by light reflection and absorption, and uniform crystal texture. 2. It has a large electro-optical coefficient, and the intensity of the electro-optical effect is greater under the same optical path, which is conducive to improving the sensitivity of the sensor and realizing the miniaturization of the sensor. 3. It has stable physical and chemical properties. The electro-optical crystal should have a high hardness and melting point, not easy to be decomposed and deliquesced by light, and have a high light damage threshold, which is convenient for the packaging of the sensor and improves the working stability. The crystal of the technical solution of the present invention is barium metaborate or potassium dihydrogen phosphate or lithium niobate.

[0013] The technical solution of the present invention sets a polarizer after the crystal, and the intensity of the output light changes with the phase difference. A 1 / 4 wave plate with the same optical axis direction as the crystal optical axis is set in front of the crystal. Its function is to introduce a 90° inherent phase difference between the o light and the e light, which is equivalent to introducing a DC bias of light intensity. When the external electric field intensity is much smaller than the half-wave voltage, the electro-optical sensing unit is in the linear working range.

[0014] The rotating structure involved in the present invention is the core and key of the technical solution. DC electric field sensing requires a rotating structure to complete the measurement, but the spatial gap of the rotating structure is large. Especially in the measurement of strong DC electric field, metal cannot be used in the structure. Generally, plastic devices are used, and the gap is large. During the rotation process, the light beam will be offset, resulting in the inability to align the light beam. After the optical fiber slip ring is used, the interface is turned to achieve gapless operation, which can suppress the light beam offset, realize light beam collimation, and achieve high-precision measurement.

[0015] The collimator mirror involved in the present invention has an input end placed on the front side of a single-mode optical fiber slip ring, and an output end placed on the rear side of a multi-mode optical fiber slip ring. The present invention studies three types of collimators: metal shell aspheric optical fiber collimator mirror, metal sleeve aspheric optical fiber collimator mirror, and glass sleeve optical fiber collimator mirror. Based on the experimental results, the characteristics are summarized as follows: 1. Metal shell aspheric optical fiber collimator mirror, large beam diameter (2mm), long working distance, high coupling efficiency; large volume, and metal. 2. Metal sleeve aspheric optical fiber collimator mirror, medium beam diameter, maximum working distance of 100mm, maximum beam diameter of about 0.5mm, medium coupling efficiency; small volume diameter, and metal sleeve. 3. Glass sleeve optical fiber collimator mirror, compact volume, no metal; short working distance, low coupling efficiency, and small beam diameter.

[0016] The beneficial effects of the present invention are as follows: the solution has the advantages of fast response speed, strong anti-electromagnetic interference capability, etc., and the use of a rotating structure and an optical fiber slip ring can suppress beam deviation, realize beam collimation, and achieve the purpose of high-precision measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the DC electric field sensing system. DETAILED DESCRIPTION

[0018] A schematic diagram of a DC electric field sensing system with a rotating structure according to the present invention is shown in FIG. Figure 1 As shown, the system includes a laser, a rotating structure, and a detector; the rotating structure includes a single-mode optical fiber slip ring, a collimator, a polarizer, a quarter wave plate, a crystal, and a multi-mode optical fiber slip ring.

[0019] The technical solution of the present invention utilizes electro-optical sensing, mainly based on the primary electro-optical effect, to convert the electric field signal into an optical signal, separate and extract the electric field component through subsequent signal processing, and inversely calculate the electric field intensity to be measured. The present invention utilizes the primary electro-optical effect of the crystal and the principle of interferometric phase detection to convert the electric field signal to be measured into a light intensity signal. The light intensity signal is converted into an electrical signal using a detector, which is convenient for subsequent signal collection and processing. Within the linear working range of the electro-optical sensing unit, there is the following linear relationship between the output voltage signal and the electric field to be measured: U out =k1+k2E z , where Uout is the output voltage signal, k1 is the DC component of the photodetector output, k2 is the proportionality coefficient between the electric field strength and the output voltage, E z is the modulated electric field strength to be measured.

[0020] Fiber optic slip rings are used to realize the transmission of optical signals from a rotating platform to a stationary platform. Single-mode fiber optic slip rings and multi-mode fiber optic slip rings are used in the present invention. Fiber optic slip rings use optical fiber as the transmission medium. They are devices that can rotate 360 ​​degrees to guide light and transmit optical fiber signals. They are the best technical solution for data transmission between rotating connected system components. They are particularly suitable for applications that require unlimited, continuous or intermittent rotation, and at the same time require the transmission of data and signals from a fixed position to a rotating position. They can improve mechanical performance, simplify system operation, and avoid damage to optical fibers due to the rotation of active joints. They can be used in conjunction with traditional electronic merge rings to form optoelectronic hybrid merge rings that transmit power and high-speed data. Fiber optic connectors can be selected from FC, SC, ST, SMA, and LC.

[0021] The crystal is the core element of the technical solution of the present invention, responsible for converting the external electric field signal into an optical signal. Its performance will directly affect the sensitivity, measurement range and environmental temperature adaptability of the electro-optical sensing unit. Therefore, the following factors should be considered when selecting an electro-optical crystal: 1. It has good light transmission performance, small light intensity loss in the optical path caused by light reflection and absorption, and uniform crystal texture. 2. It has a large electro-optical coefficient, and the intensity of the electro-optical effect is greater under the same optical path, which is conducive to improving the sensitivity of the sensor and realizing the miniaturization of the sensor. 3. It has stable physical and chemical properties. The electro-optical crystal should have a high hardness and melting point, not easy to be decomposed and deliquesced by light, and have a high light damage threshold, which is convenient for sensor packaging and improves working stability.

[0022] According to the technical solution of the present invention, the following crystals were studied: barium borate (BBO), potassium dihydrogen phosphate (KDP), and lithium niobate (LN). KDP crystal is characterized by a large electro-optic coefficient and good light transmission ability, but it has natural birefringence and is easy to deliquesce, and has poor environmental stability. BBO crystal is characterized by a negative uniaxial crystal with natural birefringence, extremely low absorption coefficient, weak piezoelectric ringing effect, good temperature stability, stable physical and chemical properties, not easy to deliquesce, good light transmission performance, and high laser damage threshold; the disadvantage is that the ideal application band is 3μm-5μm, the electro-optic coefficient is relatively small, which is not conducive to the production of high-sensitivity sensors. LN crystal is characterized by good physical and chemical stability, and has a transmittance of up to 98% in the 400-5000nm band; although it also has natural birefringence, it has a large electro-optic coefficient and is the preferred material for making high-sensitivity electro-optical sensing units. The basic parameters are shown in Table 1.

[0023] Table 1. Basic parameters of crystal

[0024]

[0025] For the electric field strength to be measured of 100kV / m=100V / mm, the half-wave voltage is set to 10000V, and the linear region voltage is set to 1000V. At this time, the nonlinear error of the measurement can theoretically reach about 1.6%. The crystal length is calculated to be 19mm by the sensor half-wave voltage formula of the transverse electro-optical effect. Therefore, the electrode of the crystal is used as the sensing plate, with a length of 19mm, a thickness of 100nm, a plate spacing of d=10mm, and an electric field strength to be measured of E. Assuming the induced voltage between the plates is U, then U=E*d.

[0026] The electrodes of the crystal sensor can be regarded as a voltage source controlled by the electric field strength to be measured. Because of its extremely high internal resistance, the key to controlling the induced voltage signal generated by it is that the high-insulation switch and related circuits used should have a fairly high input impedance and strictly prevent charge leakage. It can be estimated that the discharge time constant is about seconds. When the switch is disconnected, the capacitor is charged, and the insulation impedance of the crystal and the switch is greater than 10 12 Ω, its charging time constant is quite large, so the electric field sampling time after the switch is turned off mainly depends on the drift time of the charge in the crystal. The high insulation switch uses MRE05-2A79-HI from Germany's Menge Company, with a control voltage of 5V and an overall insulation resistance of up to 100TΩ.

[0027] The technical solution of the present invention sets a polarizer after the crystal, and the intensity of the output light changes with the phase difference. A 1 / 4 wave plate with the same optical axis direction as the crystal optical axis direction is set in front of the crystal. Its function is to introduce a 90° inherent phase difference between the o light and the e light, which is equivalent to introducing a DC bias of light intensity. When the external electric field intensity is much smaller than the half-wave voltage, the electro-optical sensing unit is in the linear working range. It is calculated that when the external electric field is in the range of [-Eπ / 2, Eπ / 2], the nonlinear error is 18%; when the external electric field is in the range of [-Eπ / 10, Eπ / 10], the nonlinear error is 1.6%; when the external electric field is in the range of [-Eπ / 12, Eπ / 12], the nonlinear error is 0.1%. According to the electric field characteristics to be measured, by reasonably designing parameters such as laser wavelength and crystal length, the half-wave voltage of the electro-optical sensing unit can be effectively adjusted to control the nonlinear error of input and output within the allowable range.

[0028] The output optical signal reaches the detector after transmission. The detector converts the laser signal modulated by the electro-optical sensing unit into an electrical signal to facilitate the subsequent signal collection and processing. The core component of the detector is the photodiode. The light intensity change caused by the electro-optical effect in the actual measurement process is generally at the μW level, which is easily interfered by the dark current noise of the photodiode, resulting in a decrease in the signal-to-noise ratio. Therefore, a high-voltage reverse bias topology circuit is used in the detection circuit design. On the one hand, it improves the detector response speed, and on the other hand, it improves the detection sensitivity. After reasonable design, the equivalent noise power can meet the requirements. Under good light conditions, low amplification gain is used as much as possible in the future to reduce the noise introduced by the amplifier circuit in the photodetector.

[0029] The key to the implementation of the present invention is packaging. In order to improve the overall mechanical strength of the electro-optical sensing unit, ensure the stability of each optical device during measurement and improve the convenience of use, optical devices such as collimators, polarizers, 1 / 4 wave plates and crystals are packaged in a cavity shell made of organic glass material. The packaging shell is processed by a CNC machine tool, and the precise grooving matches the geometric shape of the optical device to ensure that the optical axis angle relationship and the optical path collimation between the optical devices meet the design requirements. The packaged electro-optical sensing laser is input through a single-mode optical fiber, modulated by the electro-optical sensing unit under an external electric field, and the light intensity changes with the external electric field, and is output through a multi-mode optical fiber. Fiber optic transmission allows the laser source and the photodetector to be arranged in an area away from a strong electric field, reducing electromagnetic interference during the measurement process and achieving accurate measurement of the electric field signal.

[0030] The rotating structure involved in the technical solution of the present invention is the core and key of the present invention. DC electric field sensing requires a rotating structure to complete the measurement, but the spatial gap of the rotating structure is large. Especially in the measurement of strong DC electric field, metal cannot be used in the structure. Generally, plastic devices are used, and the gap is large. During the rotation process, the light beam will be offset, resulting in the inability to collimate the light beam. After adopting the optical fiber slip ring, the interface is turned to achieve gapless operation, which can suppress the light beam offset and achieve light beam collimation.

[0031] The laser light source is used as the reference light source of the sensor, and its stability is extremely important. This embodiment adopts a central wavelength of 1550nm, an output optical power adjustable within 20mW, an output power error less than 0.03dB, an output spectrum line width less than 1nm, and an FC / PC optical fiber output interface; the lithium niobate crystal is the core component of the electro-optical sensing unit, in order to maximize the sensitivity of the electro-optical sensing unit; a pair of polarizers with mutually perpendicular polarization directions are provided in the electro-optical sensing unit to realize interferometric phase detection, and a polarization beam splitter prism is used as a polarizer and analyzer in the optical path, with an operating wavelength of 1550nm, a laser anti-reflection film coated on the end face, and a light transmittance greater than 95%; the 1 / 4 wave plate introduces an inherent phase difference of π / 2 between the o light and the e light in the optical path of the electro-optical sensing unit, thereby adjusting The working point of the electro-optical sensing unit is in the optimal linear region; the optical fiber collimator is used to collimate the light beam from the optical fiber to the free space, and re-couple the collimated light beam into the optical fiber for transmission at the end; the optical fiber is selected as the medium waveguide for optical signal transmission because it has the advantages of low loss, light weight, and strong anti-electromagnetic interference ability. It is widely used in the fields of communication and sensing, and can achieve long-distance signal transmission; the slip ring connector is used to achieve flexible connection of the optical fiber, which is convenient for adjusting the posture of the sensor and realizing the rotation of the sensor; the remote monitoring unit mainly realizes the control and measurement of the output signal. The operator controls the remote switch to close or the sensor to rotate through the command transmitter connected to the industrial computer to eliminate charge drift, and controls the switch to open while measuring and recording the output signal, thereby realizing the measurement of the DC electric field.

[0032] The collimator mirror involved in the present invention has an input end placed on the front side of a single-mode optical fiber slip ring, and an output end placed on the rear side of a multi-mode optical fiber slip ring. The present invention studies three types of collimators: metal shell aspheric optical fiber collimator mirror, metal sleeve aspheric optical fiber collimator mirror, and glass sleeve optical fiber collimator mirror. The characteristics are as follows: 1. Metal shell aspheric optical fiber collimator mirror, large beam diameter (2mm), long working distance, high coupling efficiency; large volume, and metal. 2. Metal sleeve aspheric optical fiber collimator mirror, medium beam diameter, maximum working distance of 100mm, maximum beam diameter of about 0.5mm, medium coupling efficiency; small volume diameter, and metal sleeve. 3. Glass sleeve optical fiber collimator mirror, compact volume, no metal; short working distance, low coupling efficiency, and small beam diameter.

[0033] This embodiment uses a 1550nm single-mode fiber-coupled single-frequency high-precision laser source, with a line width of up to 100kHZ and a fiber-coupled output power of up to 40mw. The laser has the characteristics of compact structure, narrow line width, and high beam quality. At the same time, an advanced cavity stabilization design is adopted to ensure high and stable output of the laser. In addition, the laser output adopts single-mode fiber-coupled output, uses an FC / PC fiber interface, and can be debugged through the RS232 serial port, which is flexible and convenient to operate.

[0034] The detector is the main device for detecting laser intensity. When the laser irradiates the photosensitive area of ​​the photodetector, the photodiode in it can convert the light intensity signal into a voltage signal for subsequent signal collection and processing. This embodiment uses the PDA10CS-EC photodetector produced by THORLABS: its photosensitive area is 1mm 2 The detectable laser wavelength is 900-1700nm, the maximum noise is 500μV, the frequency band range is 17MHz, the amplification gain is adjustable within the range of 70dB, the output voltage is proportional to the laser intensity, and it can be connected to an oscilloscope and other equipment through a BNC interface to facilitate signal acquisition and processing.

[0035] Experimental results show that the scheme has the advantages of fast response speed and strong anti-electromagnetic interference ability. By using the rotating structure and fiber slip ring, the beam deviation can be suppressed and the beam can be collimated to achieve the purpose of high-precision measurement.

Claims

1. A DC electric field sensing system with a rotating structure, characterized in that: The system comprises a laser, a rotating structure and a detector; the rotating structure comprises a single-mode optical fiber slip ring, a collimator, a polarizer, a quarter wave plate, a crystal and a multi-mode optical fiber slip ring.

2. A DC electric field sensing system with a rotating structure as claimed in claim 1, characterized in that: A polarizing plate is placed behind the crystal, and a quarter wave plate with the optical axis direction being the same as that of the crystal is placed in front of the crystal.

3. A DC electric field sensing system with a rotating structure as claimed in claim 1, characterized in that: The crystal is barium metaborate, potassium dihydrogen phosphate or lithium niobate.

4. A DC electric field sensing system with a rotating structure as claimed in claim 1, characterized in that: The input end of the collimator is arranged at the front side of the single-mode optical fiber slip ring, and the output end of the collimator is arranged at the rear side of the multi-mode optical fiber slip ring.

5. A DC electric field sensing system with a rotating structure as claimed in claim 1, characterized in that: The collimator is a metal shell aspheric optical fiber collimator, a metal sleeve aspheric optical fiber collimator, or a glass sleeve optical fiber collimator.

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