Three-dimensional laser point cloud-based cable force measurement method for stay cable

Abstract

Disclosed in the present invention is a three-dimensional laser point cloud-based cable force measurement method for a stay cable. Stay cable actual measurement line shape fitting is performed by utilizing point cloud data, obtained by means of three-dimensional laser scanning, of the stay cable, and on the basis of a finite element calculation method, a line shape that is calculated by a stay cable finite element model and is the same as the actually measured line shape of the stay cable is found, thereby determining an actual cable force of the stay cable, and achieving the purpose of accurately measuring the cable force of the stay cable. According to the present invention, a three-dimensional laser scanning technology is introduced and applied to cable force measurement of a stay cable, and for the defects of a common cable force measurement method, the characteristics of non-contact measurement, high measurement precision, high measurement efficiency and the like of the three-dimensional laser scanning technology are utilized so as to achieve rapid measurement of the line shape and the section size of the stay cable. On the basis of finite element calculation and analysis, the three-dimensional laser point cloud-based cable force measurement method for the stay cable is provided, thereby achieving rapid and efficient measurement of the cable force of the stay cable.

Description

A method for measuring the force of stay cables based on three-dimensional laser point clouds Technical Field

[0001] The present invention belongs to the field of point cloud reverse modeling, and in particular relates to a method for measuring the force of a stay cable based on three-dimensional laser point clouds. Background Art

[0002] Cable-stayed bridges are a mature cable-stayed bridge structure with a greater span capacity than traditional beam bridges. They are widely used in river and sea crossing projects. Cable-stayed bridges have several symmetrically distributed cables, making them highly statically indeterminate structures with relatively complex load conditions. The cables connect the towers and the main beam, transferring loads between them and are key load-bearing components of cable-stayed bridges. Therefore, accurately measuring cable tension is crucial for evaluating the operational status of cable-stayed bridges.

[0003] In engineering practice, commonly used methods for measuring cable tension include pressure sensor, electromagnetic field, and vibration frequency methods. The pressure sensor method is generally only applicable to cable tension measurement during the tensioning process of cable construction. The electromagnetic field method requires pre-measurement of cable material properties and fabrication of induction coils, which is very costly. The vibration frequency method is an indirect method for identifying cable tension. First, the fundamental frequency or several frequencies of the cable must be accurately measured, and then the cable tension is calculated based on relevant theories. This places high demands on the accuracy of the vibration pickup used to identify the cable frequency and the vibration test signal.

[0004] In recent years, the application of 3D laser scanning technology in bridge measurement has shown a growing trend. 3D laser scanning uses high-speed laser scanning to rapidly acquire 3D coordinate data of the surface of the measured object over a large area and at high resolution. This technology can rapidly and extensively collect spatial point information, generating point cloud data. Therefore, how to use 3D laser scanning to measure stay cable tension has become a hot topic in this field.

[0005] Summary of the Invention

[0006] Purpose of the invention: In view of the shortcomings of the existing methods for measuring the tension of inclined cables, the purpose of the present invention is to provide a method for measuring the tension of inclined cables based on three-dimensional laser point clouds, which can quickly and efficiently complete the measurement of the tension of inclined cables.

[0007] Technical solution: A method for measuring the force of a stay cable based on a three-dimensional laser point cloud of the present invention comprises the following steps:

[0008] Step 1: Use a 3D laser scanner to scan the cable surface of the cable-stayed bridge to obtain the point cloud data {x, y, z} of a single cable;

[0009] Step 2: Fit the cross-sectional dimensions of a single cable to the point cloud data {x, y, z} to determine the cross-sectional dimensions r of the cable. r and the measured linear coordinates of the cable {x ci ,y ci ,z ci};

[0010] Step 3: Establish the finite element model of the cable;

[0011] Step 4: Calculate and analyze the finite element model of the inclined cable, and extract the linear coordinates {x Fi ,z Fi} and cable forces;

[0012] Step 5: The linear coordinates {x Fi ,z Fi}After fitting, the measured linear coordinates of the cable {x ci ,y ci ,z ci} for comparison. If the two linear shapes are the same, the cable force calculated by the finite element model of the cable is the actual cable force. If the two linear shapes are different, the cable tension is added to the finite element model of the cable, and steps 3 to 5 are repeated until the two linear shapes are the same to obtain the actual cable force.

[0013] Furthermore, step 1 specifically includes the following steps:

[0014] Step 1.1: Use a 3D laser scanner to scan the cable surface of the cable-stayed bridge to collect point cloud data of the cable surface;

[0015] Step 1.2: Rotate the coordinate system of the cable surface point cloud data to a coordinate system in which the cable length is horizontal. In the coordinate system, the cable length is the x-axis, the horizontal axis perpendicular to the cable length is the y-axis, and the cable height is the z-axis. Then extract the point cloud data {x, y, z} of a single cable from it.

[0016] Furthermore, step 2 specifically includes the following steps:

[0017] Step 2.1: Segmentation of the cable point cloud data: In the cable point cloud data {x, y, z}, a point cloud segment {x i ,y i ,z i},as follows:

[0018] Where m is the number of point cloud segments, rounded up, and ΔL is in the range of 0.1 m to 1 m, ensuring that there are enough points when fitting the circular curve of the cross section of the point cloud segment to reduce the random error of fitting; x max Segment the point cloud {x i ,y i ,z i} i The maximum value of x min Segment the point cloud {x i ,y i ,z i} i The minimum value of .

[0019] Step 2.2: Determine the cross-sectional projection angle of the point cloud segment: According to the point cloud segment {x i ,y i ,z i} Two points determine the projection angle α, point A (x A ,y A ,z A ) is {x i ,y i ,z i}The point corresponding to the minimum value of the z-axis direction, point B (x B ,y B ,z B ) is the point corresponding to the minimum value in the z-axis direction on the side corresponding to point A in the x-axis direction, and is calculated as follows:

[0020] Among them, x i,zA and y i,zA For {x i ,y i ,z i} i Minimum value z i,min The x and y coordinate values ​​of the corresponding point, x i,zB and y i,zB For {x i ,y i ,z i The z value on the side corresponding to point A in the x-axis direction i Minimum value z i,min，xB The x and y coordinate values ​​of the corresponding point;

[0021] Project point A and point B onto the xz axis plane. The coordinates of point A are (x A ,z A ), the coordinates of point B are (x B ,z B ), calculate the point cloud segment projection angle α based on point A and point B as follows:

[0022] Step 2.3: Cross-sectional projection of point cloud segment: Point cloud segment {x i ,y i ,z i} Take point A as the reference point and rotate it in the xz-axis plane with a rotation angle of α to obtain the rotated point cloud segment {x w ,y w ,z w}, calculated as follows: x w =(x i -x A )cosα-(z i -z A )sinα+x A y w =y i z w =(x i -x A )sinα+(z i -z A )cosα+z A

[0023] Where, α is the projection angle of the point cloud segment;

[0024] Cut the rotated point cloud into segments {x w ,y w ,z w} projected onto the yz axis plane, the point cloud segment is projected onto the cross section perpendicular to the cable tangent line, and the cross-sectional point cloud {y w ,z w};

[0025] Step 2.4: Circular curve fitting of the cross section of the point cloud segment: Use the least squares method to fit the circular curve of the cross section of the point cloud segment. Assume that the circular curve is: (yy r ) 2 +(zz r ) 2 =r 2

[0026] The cross-sectional point cloud {y w ,z w}The coordinates of the center of the fitted circle (y r ,z r ) and radius r, as follows:

[0027] in b=n∑y w z w -∑y w ∑zw

[0028] Where n is the cross-sectional point cloud {y w ,z w}Number of midpoints;

[0029] Step 2.5: Fitting the cross-section circle curve of the point cloud with low arc coverage: When the cross-section point cloud {y w ,z w}When the coverage rate on the arc is low, the cross-sectional point cloud {y w ,z w The error in estimating the radius is large, so it is necessary to estimate the coordinates of the center of the circle under the condition of a given radius of the circle. Suppose the circle is: (yy r ) 2 +(zz r ) 2 =r0 2

[0030] Where r0 is the radius of the circular curve obtained by cutting the cross section of the point cloud with a higher arc coverage rate. Then, the cross-sectional point cloud {y w ,z w}The coordinates of the center of the fitted circle (y r ,z r );

[0031] Step 2.6: Calculate the linear coordinates of the point cloud segment: According to the radius r of the fitted circle and the coordinates of the circle center (y r ,z r )Calculate the point cloud segment {x w ,y w ,z w The centroid coordinates (x0, y0, z0) within the . y0=y r z0=z r

[0032] Where x A is the x-axis coordinate value of point A, ΔL is the horizontal length of the point cloud segment along the x-axis;

[0033] Take point A (x A ,z A ) as the reference point, rotate the centroid coordinates (x0, y0, z0) in the xz axis plane to the initial point cloud segment {x i ,y i ,z i}, take point A as the reference point, rotate in the xz axis plane, the rotation angle is -α, and get the linear coordinates of the point cloud segment (xc ,y c ,z c ), as follows: x c =(x0-x A )cos(-α)-(z0-z A )sin(-α)+x A y c =y0 z c =(x0-x A )sin(-α)+(z0-z A )cos(-α)+z A

[0034] Step 2.7: Calculation of measured linear coordinates and cross-sectional dimensions of the cable: Repeat steps 2.2 to 2.6 to calculate the linear coordinates (x c ,y c ,z c ) and the radius r of the cross-section fitting circle, the measured linear coordinate points {x ci ,y ci ,z ci} is the linear coordinate point (x c ,y c ,z c ), the measured cross-sectional size of the cable is the average value r of the radius r of the circular curve fitted by all point cloud segments r , calculated as follows:

[0035] Where m is the number of point cloud segments.

[0036] Furthermore, step 3 specifically includes the following steps:

[0037] Step 3.1: Establish the straight line component of the inclined cable: Set the measured linear coordinate point {x ci ,y ci ,z ci} Project to the xz axis plane and extract the coordinates of the connection point P1 between the cable and the main beam (x P1 ,z P1 ) and the coordinates of the connection point P2 between the cable and the tower (x P2 ,z P2 ), establish the two-dimensional straight line component of the inclined cable with points P1 and P2;

[0038] Step 3.2: Section property setting: Assign section properties to the cable component. The section type is solid circle and the section radius is the measured section size r of the cable. r ;

[0039] Step 3.3: Material property setting: For the cable material properties, obtain the elastic modulus E, density ρ and linear expansion coefficient α from the cable engineering drawings. c Three parameters;

[0040] Step 3.4: Boundary condition setting: Set both ends of the cable component to be hinged, that is, do not constrain the rotation angle, but constrain the displacement in the x, y, and z directions;

[0041] Step 3.5: Select the element type: assign the cable component to the beam element, and select the B21 two-node plane linear beam element;

[0042] Step 3.6: Apply gravity load: Set the vertical downward gravity acceleration g in the z-axis direction in the cable finite element model to simulate the self-weight of the cable. g is 9.8m / s 2 ;

[0043] Step 3.7: Calculation of initial temperature of temperature field: By “cooling” the cable by a temperature difference Δt, the cable is shortened. Due to the constraint of boundary conditions, a tension ΔT is generated in the cable. This simulates the application of tension ΔT in the cable. The calculation is as follows:

[0044] Where, E is the elastic modulus of the cable, A is the cross-sectional area of ​​the cable, and α c is the linear expansion coefficient of the cable;

[0045] Step 3.8: Temperature field setting: Set the temperature field for the finite element model of the inclined cable in the temperature field analysis step, and reduce the temperature from Δt to 0℃ to simulate the tensioning process of the inclined cable.

[0046] Furthermore, step 4 specifically includes the following steps:

[0047] Step 4.1: Analysis and calculation of the finite element model of the cable;

[0048] Step 4.2: Coordinates of the cable unit nodes {x Fi ,z Fi} and support reaction R F Extraction, after the calculation of the finite element model of the inclined cable is completed, the coordinate points of all unit nodes {x Fi ,z Fi} and the support reaction R at the cable-beam connection P1 F .

[0049] Furthermore, step 5 specifically includes the following steps:

[0050] Step 5.1: Determine the linear shape of the finite element model of the cable: Set the node coordinates {x Fi ,z Fi} two adjacent points are fitted into a straight line, using a multi-segment straight line f i (x) to approximate the shape of the cable linear curve, and set the multi-segment straight line function f i (x) is: f i (x) = a i x+b i

[0051] Among them, a i is the slope of the straight line between each two adjacent points, b i is the intercept of the straight line between each two adjacent points, as follows: b i =z F(i) -a i x F(i)

[0052] Where x F(i) and x F(i+1) is the x-axis coordinate value of the coordinates of two adjacent points in the node coordinates of the cable unit, z F(i) and z F(i+1) is the z-axis coordinate value of the coordinates of two adjacent points in the node coordinates of the inclined cable unit;

[0053] Step 5.2: Calculate the corresponding points of the measured linear coordinates of the inclined cable: ci ,y ci ,z ci} Projected onto the xz-axis plane, calculate the measured linear coordinates of the inclined cable in the multi-segment straight line function f i The corresponding points with the same x-axis coordinate value in (x) are converted into x in the measured linear coordinates of the inclined cable. ci Substitute f i (x), calculate the z-axis coordinate of the corresponding point di , then the corresponding point is {x ci ,z di}, calculated as follows: z di =f i (x ci )

[0054] Step 5.3: Calculation of the distance between the measured linear coordinate point of the inclined cable and the corresponding point: Since the measured linear coordinate {x ci ,z ci} and the corresponding point {x ci ,z di} have the same x-axis coordinate value, then the average distance Z between them is:

[0055] Where n is the number of measured linear coordinates of the inclined cable;

[0056] Step 5.4: Comparison of the cable line shape: When Z is smaller, it means that the measured line shape of the cable is closer to the line shape calculated by the finite element model of the cable. Determine whether Z meets the allowable error ε as follows: |Z|≤ε

[0057] If the condition is satisfied, it means that the measured linear shape of the inclined cable is approximately the same as the linear shape calculated by the finite element model of the inclined cable. Under this condition, the support reaction force R F is the actual cable force of the inclined cable. If it is not satisfied, it is necessary to increase the initial tension force ΔT in step 3.7 and repeat steps 3 to 5 to make the linear shape calculated by the finite element model of the inclined cable close to the measured linear shape of the inclined cable until the condition is satisfied. The actual cable force R of the inclined cable is obtained. F .

[0058] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0059] This invention applies 3D laser scanning technology to the measurement of stay-cable tension. Addressing the shortcomings of conventional cable-tension measurement methods, a 3D laser point cloud-based method is proposed. This method leverages the non-contact, high-accuracy, and high-efficiency characteristics of 3D laser scanning technology to rapidly measure the linear shape and cross-sectional dimensions of the stay-cable. Combined with finite element analysis, the method finds a finite element model of the stay-cable that matches the measured linear shape, thereby determining the actual cable tension and enabling rapid and efficient measurement of the cable's tension. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG1 is a flowchart of the overall steps of a method for measuring the force of a stay cable based on a three-dimensional laser point cloud according to the present invention;

[0061] FIG2 is a further schematic diagram of step 1 of the present invention;

[0062] FIG3 is a flow chart of further detailed steps of step 2 of the present invention;

[0063] FIG4 is a schematic diagram of the calculation of the projection angle α of the cross-section of the point cloud segment in step 2.2 of the present invention;

[0064] FIG5 is a flowchart showing further details of steps 3 to 5 of the present invention. DETAILED DESCRIPTION

[0065] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0066] This paper uses two cables, A6 and A26, from a domestic cable-stayed bridge as its research subjects. These cables have actual cable tension data, which can be used to compare and verify the technical solution proposed in this paper. As shown in Figures 1 to 5, a cable tension measurement method based on a 3D laser point cloud includes the following steps:

[0067] Step 1: Use a 3D laser scanner to scan the cable surface of the cable-stayed bridge to obtain the point cloud data {x, y, z} of a single cable of the cable-stayed bridge to be measured.

[0068] Step 2: Fit the cross-sectional dimensions of the cable to determine the cross-sectional dimensions r of the cable r and the measured linear coordinates of the cable {x ci ,y ci ,z ci}, the specific steps are as follows:

[0069] Step 2.1: Segmentation of the cable point cloud data. In the cable point cloud data, cut out point cloud segments {x i ,y i ,z i}, the calculation is as follows:

[0070] Where m is the number of point cloud segments, rounded up.

[0071] Step 2.2: Determine the cross-sectional projection angle of the point cloud segment. i ,y i ,z i}Inside point A (x A ,y A ,z A ) and point B (x B ,y B ,z B ) Calculate the projection angle α as follows:

[0072] Among them, x i,zA and y i,zA For {x i ,y i ,z i} i Minimum value z i,min The x and y coordinates of the corresponding point. i,zB and y i,zB For {x i ,y i ,z i The z value on the side corresponding to point A in the x-axis direction i Minimum value z i,min，xB The x and y coordinate values ​​of the corresponding point.

[0073] As shown in Figure 4, point A and point B are projected onto the xz axis plane. At this time, the coordinates of point A are (x A ,z A ), the coordinates of point B are (x B,z B ). Calculate the point cloud segment projection angle α based on point A and point B as follows:

[0074] Step 2.3: Projection of cross section of point cloud segment. i ,y i ,z i} Take point A as the reference point and rotate it in the xz-axis plane with a rotation angle of α to obtain the rotated point cloud segment {x w ,y w ,z w}, calculated as follows: x w =(x i -x A )cosα-(z i -z A )sinα+x A y w =y i z w =(x i -x A )sinα+(z i -z A )cosα+z A

[0075] Then cut the rotated point cloud into segments {x w ,y w ,z w} projected onto the yz-axis plane to obtain the cross-sectional point cloud {y w ,z w}.

[0076] Step 2.4: Fitting the cross-section circle curve of the point cloud segment. Calculate the cross-section point cloud {y w ,z w}The coordinates of the center of the fitted circle (y r ,z r ) and radius r, calculated as follows:

[0077] Let the circular curve be: (yy r ) 2 +(zz r ) 2 =r 2

[0078] The cross-sectional point cloud {y w ,z w}The coordinates of the center of the fitted circle (y r ,z r ) and radius r, as follows:

[0079] in b=n∑y w z w -∑y w ∑z w

[0080] Where n is the cross-sectional point cloud {y w ,z w}The number of midpoints.

[0081] Step 2.5: Circular curve fitting of the cross section of the point cloud segment with low arc coverage. When the arc coverage of the cross section of the point cloud segment is low, estimate the coordinates of the circle center under the condition of a given circle radius. The given circle radius is the radius of the circular curve obtained by the cross section of the point cloud segment with high arc coverage. Then, calculate the cross section point cloud {y w ,z w}The coordinates of the center of the fitted circle (y r ,z r ).

[0082] Step 2.6: Calculate the linear coordinates of the point cloud segment. According to the radius r of the fitted circle and the coordinates of the circle center (y r ,z r )Calculate the point cloud segment {x i ,y i ,z i}Linear coordinates (x c ,y c ,z c ), calculated as follows: y0=y r z0=z r

[0083] Where x A is the x-axis coordinate value of point A, and ΔL is the horizontal length of the point cloud segment along the x-axis.

[0084] Take point A (x A ,z A ) as the reference point, rotate the centroid coordinates (x0, y0, z0) in the xz axis plane to the initial point cloud segment {x i ,y i ,z i}, take point A as the reference point, rotate in the xz axis plane, the rotation angle is -α, and get the linear coordinates of the point cloud segment (x c ,y c ,z c), as follows: x c =(x0-x A )cos(-α)-(z0-z A )sin(-α)+x A y c =y0 z c =(x0-x A )sin(-α)+(z0-z A )cos(-α)+z A

[0085] Step 2.7: Calculation of measured linear coordinates and cross-sectional dimensions of the inclined cable. ci ,y ci ,z ci} is the linear coordinate of all point cloud segments (x c ,y c ,z c ), the measured cross-sectional size of the cable is the average value r of the radius r of the circular curve fitted by all point cloud segments r , calculated as follows:

[0086] Where m is the number of point cloud segments.

[0087] The comparison of measured cross-sectional dimensions of the inclined cables is shown in Table 1.

[0088] Table 1 Measured cross-sectional dimensions of the inclined cables

[0089] Step 3: The finite element model of the cable is established and analyzed using ABAQUS finite element analysis software. The specific steps for establishing the finite element model of the cable are as follows:

[0090] Step 3.1: Create the straight line component of the inclined cable. ci ,y ci ,z ci} Project to the xz axis plane and extract the coordinates of the cable-beam connection point P1 (x P1 ,z P1 ) and the coordinates of the tower connection point P2 (x P2 ,z P2 ). Create a 2D straight line component of the cable using points P1 and P2.

[0091] Step 3.2: Set the cross-section properties. Assign the cross-section properties to the cable components, the cross-section type is solid circle, and the cross-section radius is r. r .

[0092] Step 3.3: Material property settings. Elastic modulus E, density ρ and linear expansion coefficient αc All are obtained from the engineering drawings of the inclined cables.

[0093] Step 3.4: Set boundary conditions. Set both ends of the cable component to hinged, that is, do not constrain the rotation angle, but constrain the displacement in the x, y, and z directions.

[0094] Step 3.5: Select the element type. Assign the cable component a beam element and select the B21 two-node plane linear beam element.

[0095] Step 3.6: Apply gravity load. Set the vertical downward gravity acceleration g in the z-axis direction in the cable finite element model to simulate the self-weight of the cable. g is 9.8m / s 2 .

[0096] Step 3.7: Calculate the initial temperature of the temperature field. By "cooling" the cable by a temperature difference of Δt, the cable is shortened. Due to the constraints of the boundary conditions, a tension ΔT is generated in the cable. This simulates the application of a tension ΔT in the cable. The calculation is as follows:

[0097] Where, E is the elastic modulus of the cable, A is the cross-sectional area of ​​the cable, and α c is the linear expansion coefficient of the cable.

[0098] Step 3.8: Temperature field setting: In the temperature field analysis step, set the temperature field for the cable finite element model, and reduce the temperature from Δt to 0°C to simulate the cable tensioning process.

[0099] Step 4: Perform calculation and analysis on the finite element model of the inclined cable and extract the linear coordinates {x Fi ,z Fi} and the cable force R F , the specific steps are as follows:

[0100] Step 4.1: Analysis and calculation of the finite element model of the cable.

[0101] Step 4.2: Coordinates of the cable unit nodes {x Fi ,z Fi} and support reaction R F Extract. After the calculation of the finite element model of the inclined cable is completed, all the unit node coordinate points {x Fi ,z Fi} and the support reaction R at the cable-beam connection P1 F .

[0102] Step 5.1: Determine the linear shape of the finite element model of the cable. Fi ,z Fi} two adjacent points are fitted into a straight line, using a multi-segment straight line fi (x) to approximate the shape of the cable linear curve. Let the multi-segment straight line function f i (x) is: f i (x) = a i x+b i

[0103] Among them, a i is the slope of the straight line between each two adjacent points, b i is the intercept of the straight line between each two adjacent points, as follows: b i =z F(i) -a i x F(i)

[0104] Where x F(i) and x F(i+1) is the x-axis coordinate value of the coordinates of two adjacent points in the node coordinates of the cable unit, z F(i) and z F(i+1) It is the z-axis coordinate value of the coordinates of two adjacent points in the node coordinates of the inclined cable unit.

[0105] Step 5.2: Calculate the corresponding points of the measured linear coordinates of the inclined cable. ci ,y ci ,z ci} projected onto the xz axis plane, and the x ci Substitute f i (x), calculate the z-axis coordinate of the corresponding point di , then the corresponding point is {x ci ,z di}, calculated as follows: z di =f i (x ci )

[0106] Step 5.3: Calculate the distance between the measured linear coordinate point of the inclined cable and the corresponding point. ci ,z ci} and the corresponding point {x ci ,z di} have the same x-axis coordinate value, then the average distance Z between them is:

[0107] Where n is the number of measured linear coordinates of the cable.

[0108] Step 5.4: Comparison of the cable shape. The smaller Z is, the closer the measured cable shape is to the shape calculated by the finite element model. Determine whether Z meets the allowable error ε as follows: |Z|≤ε

[0109] If the condition is satisfied, it means that the measured linear shape of the inclined cable is approximately the same as the linear shape calculated by the finite element model of the inclined cable. Under this condition, the support reaction force R F is the actual cable tension of the cable. If it is not satisfied, it is necessary to increase the initial tension ΔT in step 3.7 and repeat steps 3 to 5 to make the linear shape calculated by the finite element model of the cable close to the measured linear shape of the cable until the condition is satisfied and the actual cable tension R is obtained. F .

Claims

1. A method for determining the cable force of a stay cable based on three-dimensional laser point cloud, characterized in that, It includes the following steps: Step 1: Use a three-dimensional laser scanner to scan the stay cable surface of the stay cable bridge to be measured, and obtain the point cloud data {x, y, z} of a single stay cable; Step 2: Fit the point cloud data {x, y, z} of a single stay cable to determine the cross-sectional dimension r of the stay cable r and the measured linear coordinates {x ci , y ci , z ci} of the stay cable; Step 3: Establish a finite element model of the stay cable; Step 4: Perform computational analysis on the finite element model of the stay cable, and extract the cable profile coordinates {x Fi , z Fi} and the cable forces calculated from the finite element model of the stay cable; Step 5: Fit the cable profile coordinates {x Fi , z Fi} obtained from the finite element model of the stay cable with the measured cable profile coordinates {x ci , y ci , z ci}. If the cable profiles are the same, the cable force calculated from the finite element model of the stay cable is the actual cable force of the stay cable at this time; if the cable profiles are different, increase the cable tension in the finite element model of the stay cable and repeat Steps 3 to 5 until the cable profiles are the same to obtain the actual cable force of the stay cable.

2. The method for measuring the cable force of a stay cable based on three-dimensional laser point cloud according to claim 1, characterized in that, Step 1 specifically includes the following steps: Step 1.1: After using a three-dimensional laser scanner to scan the stay cable surface of the stay cable bridge to be measured, collect the point cloud data of the stay cable to be measured; Step 1.2: Rotate the coordinate system of the stay cable point cloud data to a coordinate system where the cable length direction is horizontal. In the coordinate system, the cable length direction is the x-axis, the horizontal direction perpendicular to the cable length is the y-axis, and the cable height direction is the z-axis. Then extract the point cloud data {x, y, z} of a single stay cable from it.

3. A method for measuring the cable force of a stay cable based on three-dimensional laser point cloud according to claim 1, characterized in that, Step 2 specifically includes the following steps: Step 2.1: Segmentation of the point cloud data of stay cables: In the point cloud data {x, y, z} of the stay cables, point cloud segments {x i , y i , z i} are intercepted at every ΔL along the x-axis direction according to the horizontal length, as follows: where m is the number of point cloud segments, rounded up, and the value of ΔL ranges from 0.1 m to 1 m to ensure that there are enough points for fitting the circular curve of the cross-section of the point cloud segment to reduce the fitting random error; x max is the point cloud segment {x i , y i , z i The maximum value of x in i ; x min is the point cloud segment {x i , y i , z i}, and the minimum value of x in i ; Step 2.2: Determination of the projection angle of the cross-section of the point cloud segment: Based on two points within the point cloud segment {x i ,y i ,z i}, the projection angle α is determined. Point A (x A ,y A ,z A ) is the point corresponding to the minimum value in the z-axis direction within {x i ,y i ,z i}, and point B (x B ,y B ,z B ) is the point corresponding to the minimum value in the z-axis direction on the side corresponding to point A in the x-axis direction. The calculation is as follows: where x i,zA and y i,zA are the x i , y i , z i coordinates of the point with the minimum z i value within {x i,min , y i,zB , z i,zB}, and x i and y i are the x i and y coordinates of the point corresponding to the minimum z i value on the side corresponding to point A in the x-axis direction within {x i,min，xB , y , z}; Project point A and point B onto the xz-axis plane. At this time, the coordinates of point A are (x A , z A ), and the coordinates of point B are (x B , z B ). Calculate the projection angle α of the point cloud segment according to point A and point B as follows: Step 2.3: Projection of the cross-section of the point cloud segment: The point cloud segment {x i , y i , z i} takes point A as the reference point and rotates within the xz-axis plane by an angle of α to obtain the rotated point cloud segment {x w , y w , z w}, and the calculation is as follows: x w = (x i - x A ) cos α - (z i - z A ) sin α + x A y w = y i z w = (x i - x A ) sin α + (z i - z A ) cos α + z A In the formula, α is the projection angle of the point cloud segment; Segment the rotated point cloud {x w , y w , z w} and project it onto the yz-axis plane to complete the projection of the segmented point cloud onto the cross-section perpendicular to the cable tangent, obtaining the cross-sectional point cloud {y w , z w}; Step 2.4: Fitting of the cross-sectional circular curve of the point cloud segment: Use the least squares method to fit the cross-sectional circular curve of the point cloud segment. Assume the circular curve is: (y - y r ) 2 +(z - z r ) 2 = r 2 The center coordinates (y w , z w ) and radius r of the circular curve fitted to the cross-sectional point cloud {y r , z r} are calculated by the least squares method as follows: Among them b = n∑y w z w -∑y w ∑z w where n is the number of points in the cross-sectional point cloud {y w , z w}; Step 2.5: Fitting of the cross-sectional circular curve of the point cloud segment with low arc coverage rate: When the coverage rate of the cross-sectional point cloud {y w , z w} on the arc is low, the error in estimating the radius of the cross-sectional point cloud {y w , z w} is large. It is necessary to estimate the center coordinates under the condition of a given circular curve radius. Let the circular curve be: (y - y r ) 2 +(z - z r ) 2 = r0 2 where \(r_0\) is the radius of the circular curve obtained from the cross-section of the point cloud segment with a high arc coverage rate, and then the center coordinates \((y w , z w ) of the circular curve fitted to the cross-section point cloud \(\{y r , z r \}\) are calculated according to the least squares method in step 2.4; Step 2.6: Calculate the linear coordinates of the point cloud segment: Calculate the centroid coordinates (x0, y0, z0) within the point cloud segment {x r , z r } according to the radius r of the fitted circular curve and the center coordinates (y w , y w , z w} as follows: y0 = y r z0 = z r where x A is the x-axis coordinate value of point A, and ΔL is the horizontal length of the point cloud segment along the x-axis; With point A (x A , z A ) as the reference point, rotate the centroid coordinates (x0, y0, z0) to the position of the initial point cloud segment {x i , y i , z i} in the xz-axis plane. With point A as the reference point, perform a rotation in the xz-axis plane with a rotation angle of -α to obtain the linear coordinates (x c , y c , z c ) of the point cloud segment, as follows: x c =(x0 - x A )cos(-α) - (z0 - z A )sin(-α) + x A y c = y0 z c = (x0 - x A )sin(-α) + (z0 - z A )cos(-α) + z A Step 2.7: Calculation of the measured alignment coordinates and cross-sectional dimensions of the stay cable: Repeat Steps 2.2 to 2.6 to calculate the alignment coordinates (x c , y c , z c ) and the radius r of the fitted circular curve of the cross-section of all point cloud segments of the stay cable. The measured alignment coordinate points {x ci , y ci , z ci} are the alignment coordinate points (x c , y c , z c ) of all point cloud segments. The measured cross-sectional dimension of the stay cable is the average value r r of the radius r of the fitted circular curve of all point cloud segments, and the calculation is as follows: In the formula, m is the number of point cloud segments.

4. A method for measuring the cable force of a stay cable based on three-dimensional laser point cloud according to claim 1, characterized in that, Step 3 specifically includes the following steps: Step 3.1: Establishment of the straight cable component: Project the measured linear coordinate points {x ci , y ci , z ci} of the stay cable onto the xz-axis plane, and extract the coordinates (x P1 , z P1 ) of the connection point P1 between the stay cable and the main girder and the coordinates (x P2 , z P2 ) of the connection point P2 between the stay cable and the cable tower. Establish a two-dimensional straight cable component with points P1 and P2; Step 3.2: Section property setting: Assign the section property to the stay cable component. The section type is a solid circle, and the section radius is the measured section size r of the stay cable r ; Step 3.3: Material property setting: For the material properties of stay cables, obtain the elastic modulus E, density ρ, and coefficient of linear expansion α from the engineering drawings of stay cables. c These three parameters; Step 3.4: Setting of boundary conditions: Set both ends of the stay cable component as hinged, that is, do not restrict the rotation angle, and restrict the displacements in the x, y, and z directions; Step 3.5: Selection of element type: Assign the stay cable component as a beam element, and select the B21 two-node planar linear beam element; Step 3.6: Apply gravity load: Set the gravitational acceleration g acting vertically downward in the z-axis direction in the finite element model of the stay cable to simulate the self-weight effect of the stay cable, where g is 9.8 m / s 2 ; Step 3.7: Calculation of the initial temperature of the temperature field: Shorten the stay cable by the "temperature drop" temperature difference value Δt of the stay cable. Due to the constraints of the boundary conditions, a tensile force ΔT is generated in the stay cable, simulating the application of the tensile force ΔT in the stay cable; The calculation is as follows: where E is the elastic modulus of the stay cable, A is the cross-sectional area of the stay cable, and α c is the coefficient of linear expansion of the stay cable; Step 3.8: Setting of the temperature field: Set the temperature field for the finite element model of the stay cable in the temperature field analysis step. The temperature drops from Δt to 0 °C to simulate the tensioning process of the stay cable.

5. A method for measuring the cable force of a stay cable based on three-dimensional laser point cloud according to claim 1, characterized in that Step 4 specifically includes the following steps: Step 4.1: Analyze and calculate the finite element model of the stay cable; Step 4.2: The node coordinates {x Fi , z Fi} of the stay cable element and the support reaction R F are extracted. After the calculation of the finite element model of the stay cable is completed, all the node coordinate points {x Fi , z Fi} and the support reaction R F at the cable-beam connection P1 are extracted.

6. The method for measuring the cable force of a stay cable based on 3D laser point cloud according to claim 1, wherein Step 5 specifically includes the following steps: Step 5.1: Determination of the alignment of the finite element model of the stay cable: Fit the adjacent two points within the node coordinates {x Fi , z Fi} of the stay cable element into a straight line, and use multiple straight lines f i (x) to approximate the shape of the alignment curve of the stay cable. Let the multiple straight line function f i (x) be: f i (x) = a i x + b i where a i is the slope of the straight line between every two adjacent points, and b i is the intercept of the straight line between every two adjacent points, as follows: b i = z F(i) - a i x F(i) where x F(i) and x F(i+1) are the x-axis coordinate values of two adjacent points in the cable element node coordinates, and z F(i) and z F(i+1) are the z-axis coordinate values of two adjacent points in the cable element node coordinates; Step 5.2: Calculation of corresponding points of the measured alignment coordinates of the stay cable: Project the measured alignment coordinates {x ci , y ci , z ci} of the stay cable onto the xz-axis plane, and calculate the corresponding points with the same x-axis coordinate values in the multi-segment straight-line function f i (x). Substitute x ci in the measured alignment coordinates of the stay cable into f i (x) to calculate the z-axis coordinate z di of the corresponding point. Then the corresponding point is {x ci , z di}, and the calculation is as follows: z di = f i (x ci ) Step 5.3: Calculation of the distance between the measured alignment coordinate points of the stay cables and the corresponding points: Since the x-axis coordinate values of the measured alignment coordinates {x ci , z ci} and the corresponding points {x ci , z di} are the same, the average distance Z between them is: In the formula, n is the number of measured linear coordinates of the stay cable; Step 5.4: Comparison of the stay cable alignment: The smaller Z is, the closer the measured alignment of the stay cable is to the alignment calculated by the finite element model of the stay cable. Judge whether Z meets the allowable error ε as follows: |Z| ≤ ε If satisfied, it indicates that the measured alignment of the stay cable is approximately the same as the alignment calculated by the finite element model of the stay cable. The reaction force R of the bearing obtained under this condition F is the actual cable force of the stay cable. If not satisfied, the initial tension ΔT in step 3.7 needs to be increased, and steps 3 to 5 are repeated to make the alignment calculated by the finite element model of the stay cable approach the measured alignment of the stay cable until the condition is satisfied, and the actual cable force R of the stay cable is obtained F .

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