Tank top three-coordinate measuring method for gas jacking of large LNG storage tank

Abstract

The invention provides a tank top three-coordinate measuring method for gas jacking of a large LNG (Liquefied Natural Gas) storage tank, which is characterized in that a measuring system is used for measuring, the measuring system comprises more than three measuring units and cameras, each camera corresponds to a measuring point, and the measuring points are preset specific three-coordinate points; and each measuring point at least needs to be corresponding to three measuring units at the same time. The measuring unit comprises a two-axis turntable and a laser range finder, wherein the laser emission source is used as the original point of the measuring unit. The method comprises the following operation steps: firstly, calibrating an original point of a measurement unit under a global coordinate system; then, the two-axis rotary table is adjusted through a preliminary aiming algorithm and a precise aiming algorithm, a laser spot is made to coincide with the measuring point, and the laser spot is a focus formed by converging laser through a focus lens; and finally, calculating the three coordinates of the measuring point in the global coordinate system in the process that the measuring point is jacked along with the tank top gas. Therefore, the spatial position of the tank top of the large LNG storage tank in the gas jacking process can be accurately monitored.

Description

Technical Field

[0001] The present invention relates to the technical field of engineering surveying, and particularly to a three - coordinate measurement method for the tank top of large - scale LNG storage tanks during gas - lifting. Background Art

[0002] Liquefied natural gas (LNG) storage tanks are widely used in multiple fields, including the storage and supply of liquefied natural gas, the separation and liquefaction of natural gas, energy supply, industrial heating and fuel, power generation, ship fuel, and liquefied petroleum gas. With the development of new energy technologies and the increasing demand for natural gas, the construction demand for LNG storage tanks is constantly rising. Gas - lifting is a key step in the construction of LNG storage tanks, with high technical requirements and strict risk control. During this process, the commonly used balancing device relies on a cable device to limit the degrees of freedom during the tank - top lifting process. However, during gas - lifting, the tank top may be continuously affected by external factors, and these changes may cause the tank top to become unbalanced.

[0003] To solve this technical problem, as disclosed in the patent document with Chinese patent application number 202410260348.8 and publication date of June 14, 2024, it discloses a tank - top balancing device and method for large - scale LNG cryogenic storage tanks during gas - lifting, including n cable assemblies, n traction machines connected to the n cable assemblies one - to - one, a balance monitoring module, a communication module, and a control module. The n cable assemblies are distributed at equal circumferential angles around the center of gravity of the tank top and are respectively connected to the tank top; the n traction machines are respectively installed on the top of the tank wall. The balance monitoring module is used to monitor the position and pose of the tank top; the control module determines whether the tank top is tilted based on the pose of the tank top. If the determination is yes, it calculates the required movement distance of the cable assembly near the lowest point of the tank top based on the position and pose of the tank top, outputs a control signal, and sends it to the traction machine connected to this cable assembly through the communication module. The traction machine pulls the cable assembly connected to it based on the control signal to make the tank top return to a horizontal pose.

[0004] The above - mentioned literature determines whether the tank top is tilted based on the pose of the tank top by the control module. However, since the length of the rope changes when it is stressed, the measurement accuracy is affected by various factors. In addition, real - time measurement of the tank top is not mentioned in the literature, so small changes in the tank top cannot be monitored in a timely manner. During the lifting process, the tank top may rotate and skew, which will cause changes in the positions of the measurement points. If the measurement unit device is not adjusted, it is impossible to ensure that the measurement points coincide with the laser spots, resulting in inaccurate measurement data. Therefore, it is very necessary to propose a method for measuring the three - coordinates of the measurement points on the top of large - scale LNG storage tanks. Summary of the Invention

[0005] The present invention aims to provide a three - coordinate measurement method for the tank top during the gas - lifting of a large - scale LNG storage tank, calculating the three - coordinates of the measurement points in the global coordinate system during the process of the tank top gas - lifting. First, ensure that the laser spot and the measurement point are on the same plane of the tank top. Then, globally calibrate the origin of the measurement unit. Next, use the preliminary aiming and precise aiming algorithms to adjust the two - axis turntable so that the laser spot formed by the measurement unit coincides with the measurement point. Finally, during the process of the measurement point following the gas - lifting of the tank top, calculate its three - coordinates in the global coordinate system, and the measurement data has good accuracy.

[0006] To achieve the above - mentioned purpose, the present invention provides a three - coordinate measurement method for the tank top during the gas - lifting of a large - scale LNG storage tank. The measurement is carried out through a measurement system, which includes more than three measurement units and cameras. Each camera corresponds to a measurement point, and each measurement point needs to be simultaneously aimed at by at least three measurement units. The measurement unit includes a two - axis turntable and a laser rangefinder, and the laser emission source is the origin of the measurement unit. The measurement units are arranged on the tank wall of the storage tank, and the cameras are fixedly installed on the tank top and move with the lifting of the tank top. The operation steps of this method are as follows: First, ensure that the laser spot and the measurement point are on the same plane of the tank top. Then, globally calibrate the origin of the measurement unit. Next, use the preliminary aiming and precise aiming algorithms to adjust the two - axis turntable so that the laser spot coincides with the measurement point. Finally, during the process of the measurement point following the gas - lifting of the tank top, calculate its three - coordinates in the global coordinate system. It also includes the following steps: S1 Globally calibrate the origin position of the measurement unit. The global coordinate system is established with a preset origin O t and marked as O t X t Y t Z t , and obtain the three - coordinates of the origin of each measurement unit in the global coordinate system; S2 Record the laser spots at three different positions: The first laser spot is the spot formed at the initial position, called the initial spot; the second laser spot is formed after the first turntable rotates clockwise by a minimum step angle; the third laser spot is formed after the second turntable rotates counterclockwise by a minimum step angle. The three - coordinates in the measurement coordinate system are respectively A ( x c1 , y c1 , z c1 ), B ( x c2 , yc2 , z c2 ), C ( x c3 , y c3 , z c3 ), and fit these three - coordinate points into a plane, which is called the plane where the measurement points are located. And ensure that the three laser spots A ( x c1 , y c1 , z c1 ), B ( x c2 , y c2 , z c2 ), C ( x c3 , y c3 , z c3 ) and the measurement point M ( x c4 , y c4 , z c4 ) are in the same plane and also within the shooting field of view of the camera; S3 Establish the conversion relationship between the measurement coordinate system and the coordinate system of the plane where the measurement points are located. Taking the position of the laser spot A as the origin O p , establish the coordinate system of the plane where the measurement points are located, marked as O p X p Y p Z p . Determine the Z p - axis vector of the plane where the measurement points are located, and this vector is the normal vector of the plane where the measurement points are located. Define the X p - axis vector as the vector , that is, the vector pointing from point A to point B . Then, through the X p - axis vector and Z pThe cross product operation of the axis vectors gives Y p axis vectors. Thus, the coordinate system of the plane where the measurement point is located is obtained. Using the direction cosine matrix, the rotation matrix R can be derived, while the translation vector T is determined by A the three - coordinate vectors of the point in the measurement coordinate system; S4 Three laser spots at different positions are captured by the camera. Using the camera imaging plane and the plane where the measurement point is located, a homography matrix is constructed, and the corresponding relationship between the measurement point in the camera imaging plane and the plane where the measurement point is located can be known. Then, according to the known R and T , the three coordinates of the measurement point in the measurement coordinate system can be obtained. Finally, substituting into the coordinate formula and solving inversely, the angles that the two - axis turntable of the laser spot to the measurement point needs to rotate can be obtained; A S5 Since there are errors in the rotation matrix, translation vector, and homography matrix during the calculation processes of steps S3 and S4, an accurate aiming algorithm is also required to make the laser spot coincide with the measurement point. In this algorithm, a two - dimensional pixel coordinate system is adopted, which is a commonly used system in image processing and computer vision for determining the specific position of each pixel in the image. This coordinate system is mainly used to measure and adjust the relative position between the laser spot and the measurement point. u and v axes refer to two reference axes in this coordinate system, which are used to describe the horizontal and vertical positions of the laser spot in the image. The first turntable rotates clockwise by a minimum step angle, and the pixel distance between the laser spot and the measurement point is calculated. If the pixel distance becomes larger, the rotation direction is changed and rotation continues; if the pixel distance becomes smaller, rotation continues in the current direction until the pixel distance from the laser spot to the measurement point becomes larger, and the adjustment of the first turntable is completed. Next, the second turntable is rotated and made to rotate counterclockwise by a minimum step angle. The pixel distance between the laser spot and the measurement point is calculated. If the pixel distance becomes larger, the rotation direction is changed and rotation continues; if the pixel distance becomes smaller, rotation continues in the current direction until the pixel distance from the laser spot to the measurement point becomes larger, and the adjustment of the second turntable is completed. If the pixel distance from the laser spot to the measurement point is not within the threshold range, the entire adjustment process is restarted until the pixel distance between the laser spot and the measurement point reaches within the threshold range, and the two - axis turntable stops rotating; S6 Control the two - axis turntables in other measurement units in the same way so that the laser spots formed by the measurement units coincide with the measurement point. During the process of the measurement point rising with the tank top gas, calculate its three coordinates in the global coordinate system. Description of the Drawings

[0007] Figure 1 ​Schematic diagram of the measurement system for the gas jacking of the LNG storage tank of the present invention.

[0008] Figure 2 Exploded view of the measurement unit of the present invention.

[0009] Figure 3 Schematic diagram of the position of the laser emission source of the present invention.

[0010] Figure 4 Schematic diagram of the origin calibration of the measurement unit of the present invention.

[0011] Figure 5 Flowchart of the three - coordinate measurement method for the tank top measurement points of the present invention.

[0012] Figure 6 Flowchart of the preliminary aiming of the present invention.

[0013] Figure 7 Schematic diagram of the three - dimensional coordinate system and the camera imaging model of the present invention.

[0014] Figure 8 Flowchart of the precise aiming of the present invention. Detailed implementation manners

[0015] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific implementation manners.

[0016] As Figure 1 shown, a three - coordinate measurement method for the tank top of a large - scale LNG storage tank gas jacking is introduced. This method involves setting n measurement units 11, and using the origin of the i - th measurement unit 11 O ci (i = 1,..., n) to establish a measurement coordinate system, marked as O ci X ci Y ci Z ci where X ci axis is the rotation axis of the first turntable in the two - axis turntable, Y ci axis is the rotation axis of the second turntable in the two - axis turntable. The global coordinate system is established with a preset origin O t and marked as O t X t Y t Z tThis measuring method includes more than three measuring units 11. One camera 16 corresponds to one measuring point 14, and at least three measuring units 11 should correspond to the measuring point 17 simultaneously. In this embodiment, the camera 16 is fixedly installed on the tank top 13 and moves with the lifting of the tank top 13. The measuring units 11 are arranged at the top of the tank wall 12. The laser spot formed by the origin of the measuring unit 11 coincides with the measuring point by using the preliminary aiming and precise aiming algorithms. Calculate the three coordinates of the measuring point in the global coordinate system during the gas lifting process of the tank top as follows: (1), where O t1 ( X 1 , Y 1 , Z 1 ), O t2 ( X 2 , Y 2 , Z 2 ), O t3 ( X 3 , Y 3 , Z 3 ) are the three coordinates of the origins of the three measuring units in the global coordinate system. (( X 7 , Y 7 , Z 7 ) are the three coordinates of the measuring point in the global coordinate system. l 10 、 l 11 、 l 12 are the distances from the origins of the three measuring units to the measuring point respectively. These distance data are all measured by the laser rangefinder, so as to calculate the three coordinates of the measuring point in the global coordinate system during the lifting of the tank top, and accurately monitor the spatial position of the tank top of the large LNG storage tank during the gas lifting process through the three coordinates of the measuring point.

[0017] Such as Figure 2 and Figure 3As shown in the figure, a numerically controlled measuring device for measuring points of a large LNG storage tank is introduced. The measuring unit includes a two-axis turntable and a laser rangefinder 1. The two-axis turntable includes a first turntable 2, a cantilever bracket 3, a second turntable 4 and a fixing plate 5. The first turntable 2 is arranged at the top of the tank wall. The cantilever bracket 3 is installed on the first turntable 2 through a bracket mounting plate 6. A second turntable 4 is arranged on the cantilever bracket 3. The laser rangefinder 1 is installed on the rotating disk of the second turntable 4 through the fixing plate 5. The laser emission source 7 of the laser rangefinder 1 is located at the intersection of the rotation axis 8 of the first turntable 2 and the rotation axis 9 of the second turntable 4. The first turntable 2 drives the cantilever bracket 3 to rotate in the horizontal direction, and the second turntable 4 drives the laser rangefinder 1 to rotate in the vertical direction. In this embodiment, a counterweight 10 is further included. The counterweight 10 is arranged at one end of the bracket mounting plate 6 away from the cantilever bracket 3 to prevent the center of the measuring unit from shifting due to the excessive weight of the cantilever bracket 3. The shown measuring coordinate system is established based on the origin of the measuring unit O c is established and marked as O c X c Y c Z c , where X c axis is the rotation axis 8 of the first turntable 2 in the two-axis turntable, that is, axis ω 1 , Y c axis is the rotation axis 9 of the second turntable 4 in the two-axis turntable, that is, axis ω 2 . The laser rangefinder 1 can rotate in the directions of ω 1 and ω 2 .

[0018] As Figure 4 shown, a global calibration method for the origin position of the measuring unit is introduced. The global coordinate system is established with a preset origin O t and marked as O t X t Y t Z t . The measuring unit 11 is arranged on the tank wall 12 of the LNG storage tank. There is a tank top 13 inside the tank wall 12. The three-coordinate measuring device 15 presets three reference points 14. In this embodiment, the three coordinates of the origin 7 of the measuring unit 11 in the global coordinate system are set as O t1 ( X 1 , Y1 , Z 1 )、 O t2 ( X 2 , Y 2 , Z 2 )、 O t3 ( X 3 , Y 3 , Z 3 ), three reference points are arbitrarily set on the tank wall 12 by using the three-coordinate measuring device 15 O t4 ( X 4 , Y 4 , Z 4 )、 O t5 ( X 5 , Y 5 , Z 5 )、 O t6 ( X 6 , Y 6 , Z 6 ), and then the laser spot formed by the laser rangefinder in the measuring unit 11 is made to coincide with the three reference points 14 preset by the three-coordinate measuring device 15 in sequence. The three coordinates of the origin 7 of the three measuring units 11 in the global coordinate system are calculated by using the distance formula between two points through the three reference points 14 preset by the three-coordinate measuring device 15

[0019] , , , wherein l 1 、 l 2 、 l 3 are the distances from the origin of the first measuring unit to the three reference points preset by the three-coordinate measuring device l 4 、 l 5 、l 6 The distances from the origin of the second measuring unit to three preset reference points of the three-coordinate measuring device l 7 、 l 8 、 l 9 The distances from the origin of the third measuring unit to three preset reference points of the three-coordinate measuring device. These distance data are all obtained by measuring with a laser rangefinder. In this embodiment, the solution of the above equation is prior art and will not be elaborated here.

[0020] As Figure 5 shown, this flow chart describes the three-coordinate measuring method for the entire tank top measuring points, including aiming at three preset reference points to calibrate the origin of the measuring unit, and then making the laser spot formed by the measuring unit coincide with the measuring point and calculating its three coordinates to complete the precise space measurement task.

[0021] As Figure 6 shown, a control method for the preliminary aiming of the gas lift measuring points of a large LNG storage tank is introduced. The three coordinates of three laser spots A 、 B 、 C and the measuring point M in the measurement coordinate system are calculated by the coordinate formula: ; In equation (5), ( x cj , y cj , z cj ), where j = 1, 2, 3 represents the three coordinates of the three laser spots A 、 B 、 C in the measurement coordinate system; j = 4 represents the three coordinates of the measuring point M in the measurement coordinate system. α is the angle of rotation of the two-axis turntable on the ω 2 axis, θ is the angle of rotation of the two-axis turntable on the ω 1 axis.

[0022] Establish the conversion relationship between the measurement coordinate system and the plane coordinate system where the measuring point is located, and determine the Z p axis vector of the plane where the measuring point is located. This vector is the normal vector of the plane where the measuring point is located. Define X p axis vector as vector , that is, from point A pointing to pointB vector. Through X p axis vector and Z p axis vector through cross product operation to obtain Y p axis vector. Thus, the coordinate system of the plane where the measurement point is located is obtained. Using the direction cosine matrix, the rotation matrix R can be deduced, and the translation vector T is determined by A the three-coordinate vectors of the point. The coordinate transformation parameters t x , t y , t z , α , β, γ T represent the pose transformation of the coordinate system of the plane where the measurement point is located relative to the measurement coordinate system, where t x , t y , t z T is represented by T , indicating the position of the origin of the coordinate system of the plane where the measurement point is located in the measurement coordinate system, α , β, γ T is represented by R , indicating the rotation angle of the coordinate system of the plane where the measurement point is located relative to the measurement coordinate system. Among them: the cosines of the pairwise angles between the measurement coordinate system and the axes of the plane where the measurement point is located form a 3×3 matrix R . Taking A point as the origin of the coordinate system of the plane where the measurement point is located, where: T = x 1 , y 1 , z 1 T .

[0023] ; In equation (6): represents the three basis vectors of the measurement coordinate system. , represent the three basis vectors of the coordinate system of the plane where the measurement point is located. Transforming the coordinate system from the measurement coordinate system to the coordinate system of the plane where the measurement point is located, and finding the three coordinates of the laser spot in the coordinate system of the plane where the measurement point is located, which can be achieved through formula (7). Formula (7) is as follows: ; ​​​​In formula (7), ( x pi , y pi , z pi ), where i=1,2,3, represents three laser spots A , B , C The three coordinates of the measuring point in the plane coordinate system; i=4, indicating the measuring point M The three coordinates of the measuring point in the plane coordinate system, R The cosine of the angles between the measurement coordinate system and the plane coordinate system where the measurement point is located forms a 3×3 matrix. T represents the position of the origin of the plane coordinate system where the measurement point is located in the measurement coordinate system. From (7), we can find ( x pi , y pi , z pi ) The three coordinates of the three laser spots in the plane coordinate system where the measurement point is located. Then use the coordinates of the three laser spots in the pixel coordinate system A ( u 1 , v 1 ) 、 B ( u 2 , v 2 ) 、 C ( u 3 , v 3 ) and the coordinates of the measuring point in the plane coordinate system ( x p1 , y p1 ) 、( x p2 , y p2 ) 、( x p3 , y p3 ), establish the homography matrix from the camera imaging plane to the plane where the measurement point is located ; ; Combining formulas (8), (9), and (10), we can get the coefficients of the homography matrix. is the homography matrix. Then, through the transformation relationship of the homography matrix, the pixel coordinates of the measurement points are converted into the coordinates in the plane coordinate system where the measurement points are located. According to the formula: , In equation (11), ([[]] u 4 , v 4 ) are the coordinates of the measurement point in the pixel coordinate system, ([[]] x p4 , y p4 ) are the coordinates of the measurement point in the plane coordinate system where the measurement point is located. Then, according to formula (7), the three coordinates of the measurement point M in the measurement coordinate system can be obtained ([[]] x c4 , y c4 , z c4 ). According to formula (5), the angles of α and θ that the two-axis turntable needs to rotate can be solved.

[0024] As Figure 7 shown, it shows the geometric relationship in three-dimensional space, which includes a plane where the measurement point is located, the global coordinate system, and the measurement coordinate system. On the plane where the measurement point is located, taking the position of the laser spot A as the origin O p , a plane coordinate system where the measurement point is located is established and marked as O p X p Y p Z p . Points A , B , C and M are connected to the corresponding points in the camera imaging plane by dotted lines, where A point is the spot formed by the first laser spot at the initial position and is called the initial spot; B point is the spot formed by the second laser spot after the first turntable rotates clockwise by a minimum step angle; C point is the spot formed by the third laser spot after the second turntable rotates counterclockwise by a minimum step angle; M point is the measurement point. The measurement coordinate system is established with the origin O ci (i = 1,..., n) of the measurement unit to establish the measurement coordinate system, marked as O ci Xci Y ci Z ci , the global coordinate system is established with a preset origin O t and marked as O t X t Y t Z t , and the three axes represent the reference frame of the entire system. A camera imaging plane is also shown in the figure, and the coordinate axes u and v define the coordinate system of the camera imaging plane, on which four pixel coordinate points are marked A '( u 1 , v 1 ) B ' ( u 2 , v 2 ) C '( u 3 , v 3 ) and M ' ( u 4 , v 4 ).

[0025] As Figure 8 shown, this flowchart introduces a control method for accurately aiming at the gas lift measurement points of large LNG storage tanks. In this algorithm, a two-dimensional pixel coordinate system is adopted, which is a commonly used system in image processing and computer vision and is used to determine the specific position of each pixel in the image. This coordinate system is mainly used to measure and adjust the relative position between the laser spot and the measurement point. u and vThe axis refers to two reference axes in this coordinate system, which are used to describe the horizontal and vertical positions of the laser spot in the image; the first turntable rotates clockwise by a minimum step angle, and the pixel distance between the laser spot and the measurement point is calculated. If the pixel distance becomes larger, the rotation direction is changed and the rotation continues; if the pixel distance becomes smaller, the rotation continues in the current direction until the pixel distance from the laser spot to the measurement point becomes larger, and the adjustment of the first turntable is completed. Next, the second turntable is rotated and it rotates counterclockwise by a minimum step angle. The pixel distance between the laser spot and the measurement point is calculated. If the pixel distance becomes larger, the rotation direction is changed and the rotation continues; if the pixel distance becomes smaller, the rotation continues in the current direction until the pixel distance from the laser spot to the measurement point becomes larger, and the adjustment of the second turntable is completed. If the pixel distance from the laser spot to the measurement point is not within the threshold range, the entire adjustment process is restarted until the pixel distance between the laser spot and the measurement point reaches within the threshold range, and the two-axis turntable stops rotating.

[0026] The working principle of the present invention: First, the global coordinate calibration of the origin of the measurement unit is performed. Then, three laser spots at different positions are recorded: the first laser spot is the spot formed at the initial position and is called the initial spot; the second laser spot is formed after the first turntable rotates clockwise by a minimum step angle; the third laser spot is formed after the second turntable rotates counterclockwise by a minimum step angle. The three coordinates of the three laser spots in the measurement coordinate system are calculated in turn using the coordinate formula of the measurement unit, and these three coordinates are fitted into a plane, which is called the plane where the measurement point is located, and it is ensured that the three laser spots and the measurement point are in the same plane and also within the shooting field of view of the camera. Secondly, through the images of the three laser spots taken by the camera, the mapping relationship between the camera imaging plane and the plane where the measurement point is located is determined using the homography matrix, and the measurement point is transferred from the pixel coordinate system to the measurement coordinate system. Then, the coordinate formula is used to determine the angle required for the two-axis turntable to rotate from the initial point of the laser spot A to the measurement point. Through precise aiming adjustment, the precise measurement of the three coordinates of the measurement point at the top of the large LNG storage tank can be achieved. Then, the same method is used to make the laser spots formed by other measurement units coincide with the measurement point. Finally, the three coordinates of the measurement point in the global coordinate system during the lifting of the tank top are calculated, and the spatial position of the tank top of the large LNG storage tank during the gas lifting process is accurately monitored through the three coordinates of the measurement point. This measurement method is simple and can improve the accuracy of the three coordinates of the measurement point.

Claims

1. A three-coordinate measurement method for the tank top of a large LNG storage tank for gas lifting, which is measured by a measurement system. The measurement system includes more than three measurement units and cameras, each camera corresponds to a measurement point, and each measurement point corresponds to at least three measurement units. The measurement unit includes a two-axis turntable and a laser rangefinder, wherein the laser emission source is the origin of the measurement unit. The measurement unit is set on the tank wall of the storage tank, and the camera is fixedly installed on the tank top and moves with the lifting of the tank top. The measurement point is arranged on a preset plane of the tank top and is ensured to be within the camera's shooting field of view, characterized in that: The following steps are also included: S1 performs global calibration on the origin position of the measurement unit. The global coordinate system is based on the preset origin. O t Create, mark O t X t Y t Z t , find the three coordinates of the origin of each measurement unit in the global coordinate system; The S2 measurement system has a total of n measurement units, with the origin of the i-th measurement unit O ci (i=1,...,n) establish a measurement coordinate system, marked as O ci X ci Y ci Z ci ,in X ci The axis is the rotation axis of the first turntable in the two-axis turntable. Y ci The axis is the rotation axis of the second turntable in the two-axis turntable. The three coordinates of the three laser spots in the measurement coordinate system are calculated in turn using the coordinate formula. The three coordinates are fitted into a plane, which is called the plane where the measurement point is located. Through the three laser spot images taken by the camera, the camera imaging plane and the plane where the measurement point is located are used to construct a homography matrix to obtain the three coordinates of the measurement point in the measurement coordinate system, and then the laser spots formed by at least three measurement units are controlled to coincide with the measurement point, and the three coordinates of the measurement point in the tank jacking process in the global coordinate system are calculated, and these three coordinates are used to accurately monitor the spatial position of the tank top of the large LNG storage tank during the gas jacking process; S3 uses a two-dimensional pixel coordinate system to determine the specific position of each pixel in the image. The first turntable is rotated clockwise by a minimum step angle to calculate the pixel distance between the laser spot and the measuring point. If the pixel distance increases, the rotation direction is changed and the rotation is continued; if the pixel distance decreases, the rotation is continued in the current direction until the pixel distance from the laser spot to the measuring point increases, and the adjustment of the first turntable is completed; then the second turntable is rotated and rotated counterclockwise by a minimum step angle to calculate the pixel distance between the laser spot and the measuring point. If the pixel distance increases, the rotation direction is changed and the rotation is continued; if the pixel distance decreases, the rotation is continued in the current direction until the pixel distance from the laser spot to the measuring point increases, and the adjustment of the second turntable is completed; if the pixel distance from the laser spot to the measuring point is not within the threshold range, the entire adjustment process is restarted until the pixel distance from the laser spot to the measuring point reaches the threshold range, and the two-axis turntable stops rotating; S4 calculates the three coordinates of the measuring point in the global coordinate system as it rises with the tank top gas.

2. A three-coordinate measurement method for the tank top of a large LNG storage tank for gas lifting according to claim 1, characterized in that: The step S1 also includes: Assume that the three coordinates of the origins of the three measurement units in the global coordinate system are O t1 ( X 1, Y 1, Z 1) O t2 ( X 2, Y 2, Z 2) O t3 ( X 3, Y 3, Z 3) Using any three known reference points in the global coordinate system O t4 ( X 4, Y 4, Z 4) O t5 ( X 5, Y 5, Z 5) O t6 ( X 6, Y 6, Z 6). Let the laser spot formed by the laser rangefinder coincide with the three known reference points in the global coordinate system in turn, and calculate the three coordinates of the origin of the three measurement units in the global coordinate system through the distance formula between the two points. ； ； in l 1. l 2. l 3 is the distance from the origin of the first measurement unit to the three known reference points, l 4. l 5. l 6 is the distance from the origin of the second measurement unit to the three known reference points, l 7. l 8. l 9 is the distance from the origin of the third measurement unit to three known reference points, and these distance data are all measured by a laser rangefinder.

3. The method for measuring the three-coordinates of the tank top for gas lifting of a large LNG storage tank according to claim 1 is characterized in that: Step S2 also includes: S2.1 records the laser spots at three different positions: the first laser spot is formed at the initial position, which is called the initial spot; the second laser spot is formed after the first turntable rotates clockwise by a minimum step angle; the third laser spot is formed after the second turntable rotates counterclockwise by a minimum step angle; the three coordinates of the three laser spots in the measurement coordinate system are calculated in sequence using the coordinate formula of the measurement unit, and the three coordinates are fitted into a plane, which is called the plane where the measurement point is located, and it is ensured that the three laser spots and the measurement point are in the same plane and are also in the camera's shooting field of view; S2.2 constructs a homography matrix through the camera imaging plane and the plane where the measuring point is located to determine the angle that the two-axis turntable needs to rotate from the initial point of the laser spot to the measuring point; at the same time, through image processing technology, the coordinates in the pixel coordinate system are extracted from the measuring point and laser spot image taken by the camera; S2.3 uses the camera imaging plane and the plane where the measurement point is located to construct a homography matrix. By calculating the three coordinates of the measurement point in the measurement coordinate system and substituting them into the coordinate formula for inverse solution, the angle that the two-axis turntable needs to rotate is determined.

4. The method for measuring the three-coordinates of the tank top for gas lifting of a large LNG storage tank according to claim 1 is characterized in that: The step S4 also includes: using the three coordinates of the origins of the three measurement units in the global coordinate system, calculating the three coordinates of the measurement point in the global coordinate system, as follows: （4）； in O t1 ( X 1, Y 1, Z 1) O t2 ( X 2, Y 2, Z 2) O t3 ( X 3, Y 3, Z 3) are the three coordinates of the origin of the three measurement units in the global coordinate system. O t7 ( X 7, Y 7, Z7) are the three coordinates of the measurement point in the global coordinate system, l 10 , l 11 , l 12 They are the distances from the origins of the three measuring units to the measuring points. These distance data are measured by a laser rangefinder, so as to calculate the three coordinates of the measuring points in the global coordinate system as the tank top is lifted. The three coordinates of the measuring points can be used to accurately monitor the spatial position of the tank top during the gas lifting process of the large LNG storage tank.

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