Abstract: Through specific applications in major engineering structures such as bridges and buildings, this paper analyzes and studies many important issues related to the long-term strain monitoring system of fiber optic grating engineering structures, including design methods, basic structure, sensor network optimization, system temperature compensation, on-site sensor deployment, and protection technology, and provides solutions. This system was applied to monitor the reinforcement of large, heavy-duty working steel crane beams. Comparison with theoretical results demonstrates that using a fiber optic grating strain sensing network to detect strain in large components is feasible. Keywords: Fiber Bragg Grating; strain; engineering structures; monitoring [b][align=center]Design of Monitoring System Based on Fiber Bragg Grating Strain Sensor Cao Hai-yan, Tian Yue-xin[/align][/b] Abstract: In the process of the application of the sensors in bridges, architectures and other critical engineering structures, systematic analysis is made on the key problems, such as the devise of FBG long-term strain monitoring system, basic structure, optimization of sensing network, temperature compensation of the system, layout of sensor and protecting crafts. The system designed is applied to monitor the strengthening of the large crane beam. It is feasible to monitor the large structure with the scheme of FBG strain sensor by contrast with the theory. Key words: Fiber Bragg Grating; strain; engineering structures; monitoring 1 Introduction Fiber optic sensing technology is a new stage in the development of sensing technology after electrical measurement technology. Compared with electromechanical sensors, fiber optic sensors have many advantages, such as: small size, light weight, anti-electromagnetic interference, corrosion resistance, high temperature resistance, flexibility and convenience, etc. Fiber Bragg grating (FBG) sensors, in addition to possessing the advantages of ordinary fiber optic sensors, also have some unique advantages, the most important being the wavelength modulation and strong multiplexing capability of the sensing signal. Their benefits include: the measurement signal is unaffected by factors such as fiber bending loss, connection loss, light source fluctuations, and detector aging; it avoids the problems of unclear phase measurements found in interferometric fiber optic sensors; by cascading multiple FBGs onto a single fiber and embedding (or bonding) the fiber to the structure under test, information from several measurement targets can be obtained simultaneously, enabling quasi-distributed measurement. They are widely used for real-time online monitoring of parameters such as stress, strain, and temperature in engineering structures, as well as structural parameters such as deformation, cracks, and overall integrity [1,2]. Strain is an important physical characteristic parameter of materials and structures. The premise of structural monitoring is to extract parameters that reflect structural characteristics from the structure. The strain signal best reflects local structural characteristics and facilitates structural safety evaluation and damage localization; therefore, strain is one of the most important parameters for health monitoring of important engineering structures. Through extensive research and practice by domestic and international peers, strain measurement has been focused on fiber optic grating (BGF) sensing technology [3,4]. The main work of this paper is to present the basic structure and implementation scheme of a BGF strain sensing monitoring system, discuss key issues in engineering applications, propose solutions, and successfully apply it to the long-term monitoring of major engineering structures. 2 Design of the Fiber Optic Grating (FGF) Strain Sensing Monitoring System [align=center] Figure 1 Basic Structure of the Fiber Optic Grating Distributed Sensing Monitoring System[/align] Figure 1 shows the basic structure of the fiber optic grating strain sensing monitoring system. In the system, the broadband light source output light is input to circulator port 1, and output from port 2 to a 1×8 optical switch. Each optical switch route consists of several gratings with different center wavelengths connected in series to form a sensing array. Through the reflected light wavelengths of different fiber optic gratings… corresponding to each measurement point on the measured object, the system senses the stress and strain at each distributed measurement point, causing a change in the wavelength of the reflected light. The changed reflected light is transmitted from the measurement site via a transmission fiber, input to circulator port 2, and then output from port 3. The magnitude of wavelength change is detected by a fiber Bragg grating demodulation system, converted into an electrical signal by a photodetector, and then output. Finally, a computer system analyzes and evaluates the state of the measured object. Based on the basic structure of the fiber Bragg grating distributed strain monitoring system shown in Figure 1, the implementation plan of the entire system is divided into the following steps: 1) Determine the strain distribution of the structure: Based on the specific structure and engineering application, determine the location of the measurement points and the measurement distribution method, roughly estimate the strain range of each measurement point, and calculate the general strain distribution of the entire structure. 2) Determine the center wavelength of the fiber Bragg grating at each measurement point: Based on the estimated strain distribution state of each measurement point, especially the maximum strain value of each measurement point, the position of each measurement point is correlated with the wavelength of the corresponding fiber Bragg grating. When using distributed sensing, ensure that the wavelength distribution of each measurement point has a certain interval. The size of the interval depends on the maximum strain value and strain property (tensile strain or compressive strain) of each measurement point to avoid wavelength overlap of gratings connected in series during operation. 3) Determine the sensor structure and installation method: Based on the monitoring requirements and actual engineering conditions, select the sensor structure (patent type, embedded type, etc.) and installation method (adhesive or welding, etc.), and determine the embedding and protection process. 4) Determine the fiber Bragg grating demodulation system: Based on the fiber Bragg grating wavelength change value corresponding to the maximum strain change value at the corresponding measuring point, and the wavelength distribution interval at each point, calculate the sum of the wavelength change values ​​and interval values ​​at all measuring points. Then multiply by the corresponding wavelength balance coefficient of 1.2 to 1.8 to determine the required wavelength demodulation range of the fiber Bragg grating demodulator. Combined with the required measurement accuracy, select the appropriate fiber Bragg grating demodulator and matching demodulation and data analysis software. 5) Determine the fiber Bragg grating sensor sensitivity coefficient K: Based on the selected fiber Bragg grating sensor structure and installation method, select the sensitivity coefficient K value and set it in the demodulation software. The measurement results directly display the strain value. 6) Analysis and evaluation of the overall structural state: Based on the measured strain values ​​at each measuring point on the structure, perform specific program calculations to determine the overall strain distribution state of the structure and issue an alarm for the limiting state. 3 Key issues in engineering applications 3.1 Embedding and protection of fiber optic grating strain sensors The embedding process of the sensor must achieve two objectives: first, to ensure that the strain of the structure is completely transmitted to the sensor and characterized by the sensor; second, to ensure that the service life of the sensor meets the purpose of long-term monitoring of major engineering structures [5]. Compared with other fields (such as machinery), the construction and service environment of large engineering structures (such as civil engineering) is harsh, so the embedding process of the sensor is the key to the engineering of sensing technology. (1) Adhesive grating strain sensor The installation process of this type of sensor is divided into the following steps: selection of adhesive - grinding - cleaning - pasting - sensor protection - laying of optical cable. Among them, the selection of adhesive is the first thing to consider in the installation of this type of sensor. The performance of the adhesive must meet the following points: it can form a thin, gapless adhesive layer with high shear strength; it has a wide working range from low temperature to high temperature; it has good linearity, small transmission and hysteresis; and it has high elongation. To ensure the steel plate is firmly adhered to the reinforcing steel, a smooth, even surface of suitable size must be ground on the steel using a grinder. After cleaning with alcohol, apply a layer of epoxy resin evenly, then attach the sensor, ensuring the grating axis aligns with the steel reinforcement axis. Gently press two magnets onto the upper surface of the sensor, allowing the epoxy resin to cure naturally. After curing, apply another layer of resin around the sensor's edges. Finally, wrap several layers of gauze around the sensor for protection. At this point, the sensor installation and protection are essentially complete. Next comes the laying and protection of the optical cable, requiring attention to three points: a. When binding the optical cable along the reinforcing steel, it should be slightly loose and placed under the steel reinforcement to prevent direct impact from concrete pouring, vibration, and molding processes; b. Avoid folding or bending the optical cable to prevent excessive light intensity loss; c. At the section of the optical cable emerging from the component, use a strong plastic tube to encase the cable to prevent breakage during pouring. (2) Embedded grating strain sensors: These sensors are relatively easy to install. They are generally tied to the reinforcing bars in the structure with wire to keep them aligned with the direction of force on the concrete. The sensing part is completely wrapped with gauze to ensure that the sensor is only subjected to axial stress and to avoid shearing. The protection of the optical cable is the same as above. 3.2 Temperature compensation: Temperature compensation is required in the fiber optic grating strain monitoring system. There are various compensation methods, such as using a special polarization-type sensing fiber and adjusting the stress around the elliptical fiber core to eliminate stress-induced birefringence, thereby reducing the insensitivity to temperature. However, this method is suitable for use under low strain conditions. Alternatively, the thermal expansion coefficients of the fiber optic grating support material can be appropriately designed to reduce or eliminate temperature sensitivity. In practical engineering applications, linear compensation is often used. The basis of this method is that within the measurement range of 0℃～100℃ and 0～1% strain, the influence of strain-temperature cross-sensitivity is ignored, and the influence on the measurement results is very small. The effects of strain and temperature on the grating wavelength are treated as independent linear superpositions. Based on the above principle, two near-position gratings are arranged in areas with significant temperature changes, such as the anchorage end, the root, middle, and end of the anchor cable, the shaded and sun-facing sides of the bridge, and the bridge deck or bottom. One is a measuring grating, affected by both temperature and strain; the other is a temperature compensation grating, affected only by temperature. Note that the supporting materials of the two gratings must be identical, and they should be in the same temperature field. Therefore, the wavelength changes of the two gratings caused by temperature changes should be the same. Subtracting the wavelength drift caused by temperature changes from the wavelength drift of the measuring grating yields the wavelength drift caused by strain alone, thus achieving temperature compensation. 3.3 Distributed Sensing and Networked Fiber Bragg Grating Sensing Technology One of the biggest advantages of this technology is its ability to perform distributed measurements. Using wavelength division multiplexing (WDM), multiple fiber Bragg grating sensors with different center wavelengths can be connected in series on a single fiber. By correlating the wavelength values ​​with the measurement point locations, distributed measurements can be achieved. Currently, a maximum of about 20 sensors can be connected in series in the laboratory. However, in engineering applications, the demodulation range of the demodulator, the wavelength resolution bandwidth, and the range of variation of the measured parameter must be considered. Therefore, before designing the network, it is essential to understand the strain range of the object under test, rationally select the wavelength of the fiber Bragg grating, design the wavelength spacing of the cascaded gratings, and leave a certain margin; one should not blindly pursue the number of cascaded gratings. 4. Test Analysis The test object is a large, heavy-duty working steel crane beam. 4.1 Measurement Point Layout The test object is the arc end of a 28-meter span crane beam reinforced by welding. Two measurement points are symmetrically arranged on both sides of the reinforcing plates at the arc end of the crane beam, and one measurement point is arranged on one side of the R-plate at the arc end of the crane beam. Each measurement point is composed of a strain rosette formed by fiber Bragg gratings, i.e., each point has three fiber Bragg gratings. In addition, one fiber Bragg grating is set as a temperature compensation point, for a total of ten points. Nine fiber Bragg gratings are used to form a strain rosette to test the plane stress at three measurement points, and one fiber Bragg grating is used as a temperature compensation measurement point. Since the three stress measurement points are relatively close, the temperature changes should be similar to those at the temperature measurement point. 4.2 Test Scheme The test used a 440/80t crane for loading. The crane trolley lifted a known weight Q and brought it as close as possible to the test end. The reading was taken after the crane stopped at the designated position L. The test conditions are shown in Figure 2. [align=center] Figure 2: Position of the test crane[/align] In the actual test, the lifting weight Q was 141.57t. There were four working conditions, and the position L of the crane used for the test varied, as shown in Table 1. Combining the test results of the crane wheel pressure from the previous project, the crane wheel pressure P can be calculated based on the lifting weight Q. Then, based on the crane position L, the support reaction force R of the crane beam at the test location under each working condition can be calculated to determine the theoretical stress value at the measuring point at the arc end of the reinforced crane beam under the test working conditions. Table 1 Test Data 4.3 Test Results and Analysis To compare with the measured results, the arc end of the welded reinforcement design scheme was simulated using three-dimensional block finite element units. ALGOREAS finite element method was used for analysis and calculation. The elastic modulus E, Poisson's ratio, and dimensions of the calculation model were the same as the reinforcement design dimensions. The finite element calculation results of the maximum principal stress at the arc end are compared with the measured results in Table 2. Table 2 Comparison of Stress Calculation and Measured Results As can be seen from Table 2, the results of the measuring points on the inner and outer sides of the upper span of the reinforcement plate are different, indicating that the reinforcement plate is subjected to external bending. However, the average value of the measured results on both sides is in good agreement with the finite element calculation results, reflecting the actual stress distribution and changes of the arc end reinforcement plate. The test data shows that there is a deviation between the crane rail and the centerline of the crane beam, causing additional stress at the crane end. The differences in this test mainly come from the deviation of the measuring point position (weld type at both ends, distance of the measuring point from the weld) and the difference in the calculation model (difference between the actual working state of the crane beam and the ideal model). Similarly, the measured values ​​at the measuring points on the R-plate deviated significantly from the finite element calculation results. This was mainly due to testing only one side, which was affected by out-of-plane forces and measuring point deviations. The above comparative analysis demonstrates that fiber optic sensing testing technology is feasible for analyzing monitoring data at the arc end. 5. Conclusion The crane beam's working environment is subject to significant electromagnetic interference, heavy loads, and frequent crane operation. Due to the high working stress level of the crane beam, oblique fatigue cracks occurred at the arc end. Therefore, a bidirectional welded arc steel plate reinforcement method was used. To test the reinforcement effect, strain gauge tests were previously conducted, but the test signals were subject to significant interference and exhibited poor stability. Fiber Bragg grating (FBG) sensing technology was then used for strain testing. Compared to strain gauge measurements, FBG strain measurement offers higher resolution and sensitivity, making it particularly suitable for detecting very small strains in structures, and is an advanced detection technology. FBG strain measurement uses wavelength modulation and digital transmission, exhibiting strong resistance to environmental interference and good long-term stability. It has great application prospects in long-term structural health monitoring and safety assessment, and has significant social and economic benefits. The innovative points of this paper are: 1. An innovative construction scheme and implementation scheme for the long-term strain monitoring system of fiber Bragg grating engineering structure are given. 2. The key issues in engineering applications, such as the optimization of the sensor network, the temperature compensation of the system, the deployment of field sensors, and the protection process, are comprehensively analyzed and studied, and solutions are given. References: [1] Liu Jianping et al. Real-time monitoring and strain analysis of fiber Bragg grating sensor network [J]. Microcomputer Information, 2006, 4-1: 186-188. [2] Zhan Yage et al. Application of fiber Bragg grating sensor [J]. Physics, 2004, 33 (1): 58-61. [3] Huang Shanglian et al. Fiber strain sensor and its application in structural health monitoring [J]. Measurement and Control Technology, 2004, 23 (5): 1-5. [4] Yang Yaoen et al. Study on temperature and strain sensing characteristics of FP fiber sensor [J]. Sensor Technology, 2005, 24 (4): 24-26. [5] Zheng Wei et al. Application of fiber optic grating sensor in safety monitoring of Shuibuya panel dam [J]. Microcomputer Information, 2006, 4-1: 206-207.