Abstract: This paper proposes a fiber optic current sensing system based on the LabVIEW virtual instrument design platform. This system utilizes the Faraday effect to sense the magnetic field strength generated by the current, exhibiting no hysteresis or saturation limitations. Its performance is sufficient for current sensing systems and is suitable for measuring a wide range of current variations. The waveform of the measured current can be directly observed and read using virtual technology, facilitating waveform display, current analysis, storage, and processing. Keywords: Virtual instrument; Fiber optic current sensing; Faraday effect Measurement System of Fiber-current Sensor Based on VI Abstract: A VI measurement system of fiber-current sensor was introduced using the VI-designed LabVIEW platform. This system measures the magnetic field of current in accordance with the Faraday Effect. It possesses advantages of non-hysteresis, non-saturation, and large-scale current measurement. The wave form of current can be directly observed using virtual instrument technology. Measure data can be displayed, analyzed, stored, and disposed of easily. Key words: VI; fiber-current sensor; Faraday Effect 1. Introduction A virtual instrument is an instrument that fully utilizes computer technology and can be designed and defined by the user. It typically consists of three parts: a computer, an instrument module, and software. The data acquisition card, GPIB card, VXI module, etc., in the instrument module are used for signal input and output. Virtual instruments have strong analytical processing capabilities. With the development of computer technology and virtual instrument technology, the traditional concept that users can only use the functions of instruments provided by manufacturers is changing, and the scope of user-designed and defined instruments is further expanded. The same virtual instrument can be used in more situations. LabVIEW is a virtual instrument development platform software developed by NI in the United States. LabVIEW has rich library functions and functional modules, and can easily communicate with general programming languages ​​such as Matlab and C to meet various needs [1][2]. Fiber optic current sensors were developed to meet the high reliability requirements of enterprises and factories using high voltage current, such as the power industry, for equipment that needs to operate continuously. Fiber optic current sensors initially used analyzers to measure the phase change of linearly polarized light to the magnetic field; later, it was proposed to use optical fibers as components for sensing the magnetic field of current. However, since the coefficient of phase change of optical fiber to the magnetic field (Feld constant) is very small, direct measurement is not accurate enough. Therefore, interferometry was used to convert the phase change into light energy change, and the phase change and current magnitude were calculated by observing the change of light energy. The phase signal is converted into light energy change by using interference, and the phase change is also changed from active demodulation to passive demodulation. This is because active modulation is more easily affected and consumes energy, thereby reducing the noise generated from the active demodulation part [3][4][5]. The fiber optic current based on virtual instrument technology designed in this paper is a new type of current measurement system. It applies virtual instrument technology to fiber optic current transformers, which can be used to measure bus current, display the parameters and waveforms of the measurement signal in real time, and analyze and store the measurement data. 2. Hardware composition of fiber optic current sensing system [align=center] Figure 1 Fiber optic current sensing system based on virtual instrument[/align] Figure 1 shows the block diagram of the entire hardware system. The fiber optic interference signal output by the fiber optic current sensing system is converted into an electrical signal by the photoelectric conversion circuit, and then the signal data is collected by the data acquisition card and transmitted to the software system of the virtual instrument. 2.1 Basic principle of fiber optic current sensing In the fiber optic current sensing system in this paper, the Faraday effect is used to sense the magnetic field strength generated by the current. The Faraday effect describes how, when an electromagnetic wave passes through a magnetic field parallel to the direction of light propagation, the polarization plane of the outgoing linearly polarized light rotates relative to the polarization plane of the incident light due to the magnetic field's influence. The amount of this rotation is proportional to the magnetic field strength and the distance the electromagnetic wave travels within the field. This effect of a magnetic field on electromagnetic waves is called the Faraday effect, an inherent characteristic of electromagnetic fields, discovered and named by the physicist Faraday. Therefore, in this system, we wind an optical fiber around the current being measured, ensuring the fiber is parallel to the magnetic field direction to maximize the effective Faraday effect. Since light is also an electromagnetic wave, it undergoes phase rotation due to the Faraday effect within the magnetic field. The magnitude of this rotation can be used to calculate the strength of the magnetic field being measured. In this system, the magnetic field is generated by current, and the total deflection angle of the polarization direction of the propagating linearly polarized light is: (1) Here, V is the Feld constant of the optical fiber, l is the length of the optical fiber affected by the Faraday effect, and H[sub]l[/sub] is the magnetic field component parallel to the direction of propagation of the optical fiber. According to Ampere's law, the current to be measured is wrapped in a loop with the optical fiber, and formula (1) is obtained by loop integration as (2) N is the number of loops of the optical fiber, i is the intensity of the current to be measured, so θ[sub]F[/sub] is a function of the number of loops of the optical fiber and the current to be measured. From the above analysis, it can be seen that under the condition of a closed optical path, the change in the polarization angle of the linearly polarized light passing through the optical fiber and surrounding the current-cutting conductor is proportional to the current enclosed by the optical fiber. 2.2 Fiber Optic Current Sensing Optical Path System [align=center] Figure 2 Fiber Optic Current Sensing Optical Path System[/align] The fiber optic current sensing optical path, as shown in Figure 2, consists of a laser connected to an optical isolator, an optical depolarizer, an optical circulator, an optical polarizer, a quarter-wave plate, a Faraday rotator, a sensing head, and a Faraday mirror. At the laser output end, an optical isolator is usually connected. An optical isolator is a component that only allows light waves to propagate in one direction. It is commonly used after the light source to prevent reflected light waves from returning to the laser's resonant cavity, thus affecting the normal operation of the laser or even burning it out. The laser output light is neither purely polarized nor unpolarized, but rather polarized light with residual polarization. Because the polarization direction of such polarized light is not fixed, the energy of linearly polarized light generated directly through the optical polarizer also varies with the angle between the polarized light and the optical polarizer. To avoid variations in optical power due to changes in the angle between the polarized light and the polarizer, an extinction polarizer is used in the system to eliminate the polarization state of the laser, thus generating a polarization-free light source with the same energy in all directions. After passing through the optical path, the polarizer produces stable polarized light, thereby reducing the intensity noise of the system response signal. The optical circulator utilizes Faraday's principle to prevent light from coupling along the fiber path, exhibiting unidirectional transmission characteristics and functioning as an isolator. The polarizer in the system generates linearly polarized light and splits it at a 45-degree angle, allowing the two beams to travel in the X and Y directions, and ensuring that the X and Y directions have the same optical energy in the polarization-maintaining fiber. The quarter-wave plate converts the linearly polarized light into left-handed circularly polarized light and right-handed circularly polarized light. Current sensing is performed in the fiber optic sensing head. In this system, the sensing head involves winding the fiber around the current at equal intervals. This is intended to ensure that every point on the fiber has the same phase change with the magnetic field, and that the bending loss at each point is also the same. Finally, a Faraday mirror is used to reflect the light. The Faraday mirror can rotate the light by 90 degrees, thus switching the path of light traveling on different paths and compensating for the slow changes in the refractive index of the fiber optic cable caused by temperature or vibration. The light passes through an optical isolator, optical depolarizer, optical circulator, optical polarizer, quarter-wave plate, sensing head, and Faraday mirror, then reverses through the fiber optic current sensing head, quarter-wave plate, and optical polarizer to complete a loop. Interference occurs at the optical polarizer, and the photoelectric conversion circuit converts the interference light into an electrical signal for signal analysis. 3. Fiber Optic Current Sensing Virtual Software System The virtual software for this fiber optic current sensing system uses NI's LabVIEW development platform. Using a graphical language, relevant drivers are written to enable communication with the computer. Then, relevant graphics or icons are connected, and appropriate methods and parameters are selected to construct a new virtual instrument. The software system has a user-friendly interface and is simple and convenient to operate. LabVIEW is a development environment based on a graphical programming language (G language). It shares many similarities with traditional programming languages ​​such as C, Pascal, and Basic, including similar data types, data flow control structures, program debugging tools, and hierarchical and modular programming features. However, the biggest difference lies in the language: traditional programming languages ​​use text-based programming, while LabVIEW uses a graphical language (i.e., various icons, graphic symbols, and connections) to write programs in the form of block diagrams. Programming with LabVIEW requires little programming experience because it uses terminology and icons familiar to test engineers, such as various knobs, switches, and waveforms, making the interface very intuitive. Therefore, LabVIEW is undoubtedly an excellent choice for test engineers without extensive programming experience. 3.1 Virtual Software System Functions This virtual software integrates relevant testing functions onto a single panel. Here, we focus on introducing LabVIEW's block diagram programming. The block diagram program is the core of the virtual instrument; the virtual oscilloscope instrument mainly uses it to complete data acquisition, processing, and display. The block diagram program of this system mainly includes five functional modules: data acquisition, waveform display, parameter measurement, spectrum analysis, and waveform storage and playback, as shown in Figure 3. [align=center]Figure 3 Fiber Optic Current Virtual Software Measurement System[/align] 3.1.1 Data Acquisition Module LabVIEW fully integrates built-in communication libraries for hardware such as GPIB, VXI, RS-232, RS-485, and pluggable data acquisition cards, providing numerous connection mechanisms. It achieves connection with external program code or software systems through DLLs, shared libraries, OLE, etc. This system directly calls the LabVIEW port operation icons InPort.vi and OutPort.vi for programming. These two functions are stored in the Memor template, the next level template under the Advanced sub-template of the functional module, respectively completing the functions of directly reading and outputting data from the device's physical address. As long as the physical address of each channel of the data acquisition card is known, it is easy to implement LabVIEW-driven control of the data acquisition card for data acquisition by setting the port parameters of InPort.vi and OutPort.vi. Here, we use the PCI21200 data acquisition card. The sampling frequency of this module can be adjusted between 1Hz and 250MHz, and the time base, i.e., the time represented by each horizontal division, can be varied between 4ns (4ns/Division) and 20000s (20000s/Division). 3.1.2 Waveform Display Module The software provides waveform display mode by using the display channel selection buttons "A" and "B" to display the waveform of one or two channels of input signal together. 3.1.3 Parameter Measurement Module The parameter measurement module includes the measurement of current, voltage parameters and time parameters such as frequency and period, and displays the measurement results. 3.1.4 Spectrum Analysis Module The spectrum analysis module uses the fast FFT algorithm to complete the frequency domain signal analysis. Spectrum analysis functions: (1) Provides 9 types of windowed analysis windows; (2) Completes the amplitude spectrum analysis and power spectrum analysis of the measured signal. 3.1.5 Data Storage Module The "Store" and "Read" buttons on the main panel can store relevant data to a floppy disk or hard disk; or the data read from the floppy disk or hard disk will be automatically displayed as waveforms and retained in the display window. 3.2 Current Difference Test Results In the system, we designed the rated current for the fiber optic current test to be 500A and the rated voltage to be 50kV. We used this virtual instrument system to measure the induced current in the fiber optic cable. A transformer was used to simulate the large current, and the data was processed through the virtual instrument. The resulting difference data is shown in Table 1: Table 1 Difference Test Data From the difference test results, the test accuracy is high and meets the actual requirements. 4. Conclusion The innovations of this work are: 1. Functional Expansion. This fiber optic current virtual software testing system has significantly expanded its functionality compared to traditional calibrators. This not only makes it more convenient to use but also improves the overall performance. The waveform display, with its storage oscilloscope function, provides an intuitive and real-time display of the measured signal parameters and waveforms. It also allows for analysis, storage, and other processing of the measurement data, which is not possible with traditional calibrators. 2. High Reliability and Stable Operation. High current sensing requires long-term outdoor operation in harsh environments. The electronic circuitry of the signal processing section is susceptible to electromagnetic interference and temperature fluctuations, leading to unstable operation. This fiber optic current sensing system features a simple insulation structure, small size, light weight, no ferromagnetic saturation, high feasibility, high sensitivity, large measurement range, and wide bandwidth. With the introduction of virtual instrument technology, the processing of the current signal generated by the fiber optic cable is completed by LabVIEW software, ensuring high reliability and stable operation. 3. Flexible openness. The VI program can be modified at any time according to user requirements, displaying measured values ​​at different points, changing the front panel style, and adjusting program functions. This VI program can be called as a sub-VI by other VIs, forming a more complex VI program with other parts to achieve more complete measurement functions. LabVIEW software can also interface with MATLAB and C language programs, enhancing numerical analysis and processing capabilities. Fiber optic current sensing technology is a hot topic in modern current measurement research and has broad development prospects. In today's increasingly information-driven world, applying virtual instruments to current sensing is a trend in sensing technology development. References: [1] Liao Kaijun, Liu Zhifei. Overview of virtual instrument technology. Foreign Electronic Measurement Technology, 2006, 02: 6-8 [2] Lei Zhenshan, Zhang Fengmei, Liu Zhaoni. Research on bridge monitoring technology based on fiber optic grating and virtual instrument. Microcomputer Information, 2005, 15: 128-130 [3] Liu Xiazhong, Liu Huijin, Zheng Xiaomin, Yang Liu. Design of hybrid current transformer based on virtual instrument technology. Relay, 2004, 23: 44-48 [4] Zhao Jingjing, Feng Lishuang. A new type of fiber optic current transformer. Microcomputer Information, 2005, 09: 135-136 [5] Xie Yubing, You Dahai, Huang Shangyou. Photoelectric current transformer test system based on LabVIEW. 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