Fiber Bragg Grating Strain Sensor Sensitivity Calibration Device and Method for Pile Driving

Abstract

A fiber Bragg grating (FBG) strain sensor sensitivity calibration device and method for pile driving, the device comprising a loading platform (1), a reaction steel plate (2), an oblique support (4), a straight support (5), a reaction support (3), a pull rod (6), a loading rod (8), a loading plate (7), an oil cylinder (9), a T-shaped bracket A (10), a T-shaped bracket B (11), a PLC controller (15), an oil pump (14), an oil delivery pipe (13), and an oil return pipe (12). The method controls the oil pump to output pressure to the oil cylinder through the PLC controller, and the output pressure can simulate the reciprocating loading and unloading during the pile driving process. The strain generated by the steel pipe pile is obtained based on the pressure, and a graph is drawn using the data of the strain and the fiber Bragg grating wavelength. A linear fit is then performed, and the slope of the obtained straight line is the sensitivity coefficient of the fiber Bragg grating. The present invention can accurately measure the sensitivity coefficient of the fiber Bragg grating during the pile driving process and accurately obtain parameters such as the strain, axial force, and lateral friction resistance of the steel pipe pile.

Description

Technical Field

[0001] The invention relates to a sensitivity calibration device and method of an optical fiber Bragg grating strain sensor for pile driving, belonging to the technical field of civil engineering test and experiment. Background Art

[0002] In recent years, fiber optic sensing technology has developed rapidly. Fiber optic sensing technology is often used as the main means of solid deformation monitoring and is widely used in fields such as civil engineering. At present, fiber optic Bragg grating (FBG) strain sensing technology is gradually being used to monitor strain during pile driving tests. In order to ensure the authenticity of the monitoring data and the accuracy of the measurement results, the strain sensitivity of the fiber optic Bragg grating (FBG) strain sensor must be calibrated according to different usage requirements and environmental conditions before use. The pile driving process is a repeated loading and unloading process, and the pile size is large. So far, there is no reliable and accurate method for calibrating the sensitivity of fiber optic Bragg grating (FBG) strain sensors for large pile driving tests. Accurate sensitivity is an important parameter to ensure the accuracy and repeatability of measurement results. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to propose a device and method for calibrating the sensitivity of a fiber Bragg grating strain sensor for pile driving in order to solve the problem of sensitivity calibration of the fiber Bragg grating strain sensor for large-scale pile driving test.

[0004] The technical solution implemented by the present invention is as follows: a fiber Bragg grating strain sensor sensitivity calibration device for pile driving includes a loading platform, a reaction steel plate, a diagonal brace, a straight brace, a reaction brace, a pull rod, a loading rod, a loading plate, an oil cylinder, a T-bracket A, a T-bracket B, an oil pump, an oil supply pipe, an oil return pipe and a PLC controller.

[0005] The loading platform is an I-shaped steel and is placed on the horizontal ground; two reaction steel plates are vertically welded at a certain distance from both sides of the I-shaped steel; between the two reaction steel plates, one reaction steel plate is close to the oil cylinder, and the other reaction steel plate is close to one end of a horizontally placed steel pipe pile to be calibrated, and a loading plate and a loading rod are installed between the other end of the steel pipe pile and the oil cylinder, and the oil cylinder applies force to the steel pipe pile through the loading rod and the loading plate; the reaction support is welded on both sides of the I-shaped steel of the loading platform; the straight support is welded between the reaction support and the reaction steel plate; the oblique support is welded between the reaction steel plate and the top of the reaction support; a groove is opened in the middle of the top of the reaction steel plate; two T-shaped bracket B, the loading plate is welded to the loading rod and the lower end of the loading plate is kept from contacting the surface of the I-beam; the pull rod is placed in the groove above the two reaction steel plates, and both ends of the pull rod are threaded, and the pull rod can fix the two ends of the reaction steel plates through nuts; the return oil pipe and the oil supply pipe are connected between the oil pump and the oil cylinder, and the oil pump can control the required loading force through the PLC controller; the space between the reaction steel plate and the loading plate is the steel pipe pile that needs to be calibrated, and two T-shaped brackets A are installed under the steel pipe pile that needs to be calibrated, and the upper end of the T-shaped bracket A is arc-shaped, and the size of the arc can be changed according to the diameter of the calibrated steel pipe pile.

[0006] The center line of the oil cylinder output shaft, the center line of the loading rod, the center of the loading plate and the center line of the horizontal steel pipe pile are on the same horizontal line.

[0007] A sensitivity calibration method for a fiber Bragg grating strain sensor used for pile driving, comprising the following steps:

[0008] (1) Provide steel pipe piles required for calibration of model tests.

[0009] (2) The fiber Bragg grating strain sensors are symmetrically arranged on the outer wall of the steel pipe pile and protected.

[0010] (3) Place the steel pipe pile on the calibration platform of the device by a crane, and place a level meter on the steel pipe pile to check whether the steel pipe pile is placed horizontally.

[0011] (4) Place the pull rod in the groove above the two reaction steel plates and fix both ends with screws to prevent excessive displacement.

[0012] (5) Connect two symmetrical strings of fiber Bragg grating strain sensors to the demodulator, turn on the PLC controller, and set the parameters.

[0013] (6) Turn on the PLC controller that controls the operation of the oil cylinder and apply load to the steel sheet piles in stages;

[0014] 1) Apply load in stages, which can be divided into 5-7 levels.

[0015] 2) The load includes a step-by-step loading and unloading process to simulate the pile driving process. After each level of loading, the load is maintained for 2 minutes, and then unloaded to 0. Then, the second level of loading is applied and unloaded to 0. This loading and unloading process is repeated.

[0016] (7) The change of the fiber Bragg grating wavelength in the demodulator is recorded simultaneously during the load application process.

[0017] (8) Calculate the change in the strain of the steel pipe pile using the following formula:

[0018] ;

[0019] Among them, ε is the strain generated by the steel pipe pile; F is the force output by the cylinder; E is the elastic modulus of the steel pipe pile; R is the outer diameter of the steel pipe pile; r is the inner diameter of the steel pipe pile.

[0020] (9) The strain generated by the steel pipe pile at each level of load is calculated by the formula. The change in the fiber Bragg grating wavelength corresponding to each level of loading can be known through the demodulator. Then, the strain and fiber Bragg grating wavelength change curve is drawn and linear fitting is performed. The slope of the fitted straight line is the sensitivity coefficient.

[0021] The present invention has the following beneficial effects: the device can accurately calibrate the sensitivity coefficient of a fiber Bragg grating strain sensor during pile driving; the method realistically simulates the entire pile driving process and accurately monitors the strain, axial force, and lateral friction of steel pipe piles during the sinking process. This invention is of great significance for improving the measurement accuracy of fiber Bragg grating strain sensors. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a side view of the device of the present invention;

[0023] Figure 2 It is a side view of the T-bracket;

[0024] Figure 3 It is the side view of the reaction steel plate;

[0025] Figure 4 is the loading and unloading curve of the cylinder output;

[0026] Figure 5 is the calibration curve fitted by strain and fiber Bragg grating wavelength;

[0027] In the figure, 1 is the loading platform, 2 is the reaction steel plate, 3 is the reaction support, 4 is the diagonal support, 5 is the straight support, 6 is the pull rod, 7 is the loading plate, 8 is the loading rod, 9 is the oil cylinder, 10 is the T-bracket A, 11 is the T-bracket B, 12 is the return oil pipe, 13 is the oil supply pipe, 14 is the oil pump, 15 is the PLC controller, 16 is the nut, 17 is the steel pipe pile, and 18 is the fiber Bragg grating strain sensor. DETAILED DESCRIPTION

[0028] The present embodiment provides a sensitivity calibration device for a fiber Bragg grating strain sensor for a large-scale pile driving test, comprising a loading platform 1, a reaction steel plate 2, a reaction support 3, a diagonal support 4, a straight support 5, a pull rod 6, a loading plate 7, a loading rod 8, an oil cylinder 9, a T-bracket A 10, a T-bracket B 11, an oil return pipe 12, an oil supply pipe 13, an oil pump 14, a PLC controller 15, a pull rod nut 16, and the like.

[0029] The structure of the calibration device in this embodiment is as follows Figure 1 As shown,

[0030] The loading platform 1 is made of I-shaped steel, and its overall dimensions are 5m long × 0.5m wide × 0.5m high.

[0031] The outer dimensions of the reaction steel plate 2 are 0.5m x 0.5m x 10mm in length x width x thickness. The reaction steel plate 2 is vertically welded at a distance of 0.5m from both ends of the loading platform 1. There is a groove in the middle of the upper part of the reaction steel plate 2. Figure 3 shown.

[0032] The reaction support 3 is an I-shaped steel and is welded to both ends of the loading platform 1; the straight support 5 is made of I-shaped steel and is welded between the reaction steel plate 2 and the reaction support 3; the diagonal support 4 is made of I-shaped steel, one end of which is welded to the top of the reaction support 3 and the other end is welded 10mm below the top of the reaction steel plate 2; two T-shaped brackets B are welded to the loading platform 1; the oil cylinder is cylindrical with a diameter of 30mm and a height of 35mm, and is placed on the T-shaped bracket B. The bottom of the oil cylinder 9 is in close contact with the reaction steel plate 2. The loading rod has a diameter of 40mm and a length of 4.8m. The loading plate is made of steel, and its overall dimensions are length × width × thickness of 0.5m × 0.5m × 10mm. The loading plate 7 is welded to the loading rod 8 and moves with the loading rod 8. The loading plate 7 does not contact the loading platform 1;

[0033] The oil return pipe 12 and the oil delivery pipe 13 are connected to the oil pump 14 , and the PLC controller 15 controls the oil pump 14 to load and unload the oil cylinder 9 .

[0034] The steel pipe pile 17 is 4 m long, 140 mm in outer diameter, and 4 mm in wall thickness. The fiber Bragg grating strain sensor is attached to the symmetrical outer wall of the steel pipe pile, and the steel pipe pile is placed on a T-bracket A. The T-bracket is as follows: Figure 2 shown.

[0035] The operation process of the calibration device of a fiber Bragg grating strain sensor for pile driving test in this embodiment is as follows:

[0036] (1) Provide the steel pipe piles 17 required for calibration of the model test. The parameters of the steel pipe piles are as follows:

[0037] ;

[0038] (2) Fiber Bragg grating strain sensors 18 are symmetrically arranged on the outer wall of the steel pipe pile 17 and protected;

[0039] (3) Place the steel pipe pile 17 on the T-shaped bracket A on the loading platform 1 by a crane, and place a level on the steel pipe pile 17 to check whether the steel pipe pile 17 is placed horizontally:

[0040] (4) Place the tie rod 6 in the groove above the two reaction steel plates 2 and fix both ends with screws 16 to prevent the top of the reaction steel plates 2 from deforming during loading;

[0041] (5) Connect two symmetrical strings of fiber Bragg grating strain sensors 17 to the demodulator, turn on the PLC controller 15, and set the parameters:

[0042] (6) Turn on the PLC controller 15 that controls the operation of the oil cylinder 19, so that the oil pump 14 can deliver oil to the oil cylinder 9 through the oil delivery pipe 13, causing the oil cylinder 9 to generate thrust and transmit it to the loading rod 8, thereby realizing the loading and unloading of the steel pipe pile:

[0043] 1) The loading is applied in 6 levels;

[0044] 2) The load includes two stages: step-by-step loading and step-by-step unloading to simulate the pile driving process. The loading and unloading time curve is as follows: Figure 4 As shown, each level is maintained for 2 minutes.

[0045] (7) During the load application process, the change of the fiber Bragg grating wavelength in the demodulator is recorded simultaneously:

[0046] (8) Use formula (1) to calculate the change in steel pipe pile strain corresponding to each level of loading:

[0047] (a);

[0048] Where: ε is the strain generated by the steel pipe pile, F is the force output by the cylinder, E is the elastic modulus of the steel pipe pile, R is the outer diameter of the steel pipe pile, and r is the inner diameter of the steel pipe pile.

[0049] (9) According to the principle of fiber Bragg grating strain sensor:

[0050] (b);

[0051] in, is the offset of the fiber Bragg grating central wavelength; is the strain sensitivity coefficient; is the grating strain.

[0052] (10) According to formula (a) and formula (b), the sensitivity coefficient is:

[0053] (C);

[0054] (11) The strain generated by the steel pipe pile at each level of load is calculated by formula (a). The change in the fiber Bragg grating wavelength corresponding to each level of loading can be obtained by the demodulator, as shown in the following table:

[0055] ;

[0056] (12) Draw the strain and fiber Bragg grating wavelength variation curve and perform linear fitting, such as Figure 5 As shown, the slope of the fitted straight line is the sensitivity coefficient, which is 1.187.

Claims

1. A sensitivity calibration device for fiber Bragg grating strain sensors used in pile driving, characterized in that: The device includes a loading platform, a reaction steel plate, a diagonal brace, a straight brace, a reaction brace, a pull rod, a loading rod, a loading plate, an oil cylinder, a T-bracket A, a T-bracket B, an oil pump, an oil supply pipe, an oil return pipe and a PLC controller; The loading platform is an I-shaped steel and is placed on the horizontal ground; two reaction steel plates are vertically welded on both sides of the I-shaped steel; between the two reaction steel plates, one reaction steel plate is close to the oil cylinder, and the other reaction steel plate is close to one end of the horizontally placed steel pipe pile to be calibrated, and a loading plate and a loading rod are installed between the other end of the steel pipe pile and the oil cylinder, and the oil cylinder applies force to the steel pipe pile through the loading rod and the loading plate; the reaction support is welded on both sides of the I-shaped steel of the loading platform; the straight support is welded between the reaction support and the reaction steel plate; the oblique support is welded between the reaction steel plate and the top of the reaction support; a groove is opened in the middle of the top of the reaction steel plate; there are two T-shaped brackets under the oil cylinder, and the loading The plate is welded to the loading rod and keeps the lower end of the loading plate from contacting the surface of the I-beam. The T-bracket B is welded under the oil cylinder. The pull rod is placed in the groove above the two reaction steel plates. Both ends of the pull rod are threaded, and the pull rod fixes the two ends of the reaction steel plates through nuts. The return oil pipe and the oil supply pipe are connected between the oil pump and the oil cylinder. The oil pump controls the required loading force through a PLC controller. The space between the reaction steel plate and the loading plate is a steel pipe pile that needs to be calibrated. There are two T-brackets under the steel pipe pile that needs to be calibrated. The upper end of the T-bracket is an arc pile. The size of the arc is changed according to the diameter of the calibrated steel pipe pile.

2. The sensitivity calibration device for fiber Bragg grating strain sensors used in pile driving according to claim 1, characterized in that: The center line of the oil cylinder output shaft, the center line of the loading rod, the center of the loading plate and the center line of the horizontal steel pipe pile are on the same horizontal line.

3. A method for calibrating the sensitivity of a fiber Bragg grating strain sensor using the device for calibrating the sensitivity of a fiber Bragg grating strain sensor for pile driving according to any one of claims 1 and 2, characterized in that: The method comprises the following steps: (1) Provide steel pipe piles required for model testing; (2) The fiber Bragg grating strain sensors are symmetrically arranged on the outer wall of the steel pipe pile and protected; (3) Place the steel pipe pile on the calibration platform of the device by a crane, and place a level meter on the steel pipe pile to check whether the steel pipe pile is placed horizontally; (4) Place the tie rod in the groove above the two reaction steel plates and fix both ends with nuts to prevent excessive displacement; (5) Connect two symmetrical strings of fiber Bragg grating strain sensors to the demodulator, turn on the PLC controller, and set the parameters; (6) The PLC controller controls the oil cylinder to apply loads to the steel pipe piles in stages; (7) Record the change of fiber Bragg grating wavelength in the demodulator during the load application process; (8) Calculate the change in the strain of the steel pipe pile using the following formula: ; Where, ε is the strain generated by the steel pipe pile; F is the force output by the oil cylinder; E is the elastic modulus of the steel pipe pile; R is the outer diameter of the steel pipe pile; r is the inner diameter of the steel pipe pile; (9) The strain generated by the steel pipe pile at each level of load is calculated by the formula, and the change in the fiber Bragg grating wavelength corresponding to each level of loading is known through the demodulator. Then, the strain and fiber Bragg grating wavelength change curve is drawn and linear fitting is performed. The slope of the fitted straight line is the sensitivity coefficient.

4. The sensitivity calibration method of a fiber Bragg grating strain sensor for pile driving according to claim 3, characterized in that: The load is applied in stages, ranging from 5 to 7 levels.

5. The sensitivity calibration method of a fiber Bragg grating strain sensor for pile driving according to claim 3, characterized in that: The load includes step-by-step loading and unloading to simulate the piling process; after each level of loading, the load is maintained for 2 minutes and then unloaded to 0; then the load is loaded to the second level and unloaded to 0, and this process is repeated to the highest level and then unloaded to 0.

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