System and method for calibrating three-dimensional measurement results based on variable large-size polyhedron

Abstract

The present invention discloses a system and method for calibrating three-dimensional measurement results based on a variable large-size polyhedron, which can accurately calibrate the measurement result data of a large-scale three-dimensional measurement device on site. The system includes: at least four standard balls and corresponding standard ball fixing devices, at least four laser trackers, a time synchronization trigger, a controller, and at least one target ball. The at least four standard balls are arranged by the standard ball fixing devices so that the lines connecting the centers of the standard balls form the edges of a virtual space polyhedron. The target ball is movably arranged at a position tangent to the surface of the standard balls. At least four laser trackers are arranged on the periphery of the formed virtual space polyhedron, and the measurement range of each laser tracker covers all the standard balls. For each standard ball, the global coordinates corresponding to the center of the standard ball are calculated as the basis for calibrating the three-dimensional measurement results of the three-dimensional measurement device.

Description

Technical Field

[0001] The present invention relates to the field of three-dimensional measurement calibration technology, and in particular to a system and method for calibrating three-dimensional measurement results based on a variable large-size polyhedron. Background Art

[0002] In applications involving full-factor 3D measurement of large, complex parts, such as large and complex components, 3D measurement equipment such as laser trackers, industrial photogrammetry equipment, and 3D laser scanners often requires the coordinated use of multiple, or even multiple, different types of 3D measurement equipment to achieve high measurement accuracy and obtain full-body measurement data. However, various 3D measurement devices vary in their principles, measurement ranges, and accuracy. This multi-system collaborative measurement inevitably leads to issues with coordinate system integration and on-site calibration of the different measurement devices.

[0003] Chinese invention patent application publication number CN109341746A discloses a stereo standard for multi-system collaborative measurement and calibration. Using target mounts and a reference cubic mirror mounted at the four corners of a fixed frame, in conjunction with a spherical target, a high-precision coordinate measuring machine (CMM) provides stable three-dimensional coordinates of four spatial points. The coordinate systems of different measuring devices are aligned with the standard values ​​of the stereo standard, achieving a unified coordinate system. However, the fixed frame structure of the stereo standard in this solution is only suitable for "large" measuring devices within a range of one or two meters, and cannot be applied to the measurement of large objects measuring tens of meters, such as aircraft, ship locks, and even buildings.

[0004] A Chinese invention patent application with application publication number CN103591891A discloses a method for tracing the precision control field of an indoor space measurement and positioning system. The indoor space measurement and positioning system simultaneously measures global control points and measured points, and uses high-precision adjustment to replicate the precision of the global control points to the measured points to improve measurement accuracy. Although the measurement range of this solution can reach a large measurement range of 50 meters to a certain extent, the measurement of global control points and measured points relies on the direct measurement results of the same laser tracker. In essence, it increases the number of measurement points and makes mutual corrections, and the accuracy can only reach ±2×10 -5 L (L is the length of the object being measured), it is impossible to calibrate the measurement results of the global control points and the measured points with a higher level of accuracy. Summary of the Invention

[0005] At least one of the purposes of the present invention is to overcome the problems existing in the above-mentioned prior art and provide a system and method for calibrating three-dimensional measurement results based on a variable large-size polyhedron, which can accurately calibrate the measurement result data of a large-scale three-dimensional measurement device on site.

[0006] In order to achieve the above objectives, the technical solutions adopted by the present invention include the following aspects.

[0007] A system for calibrating three-dimensional measurement results based on a variable-size polyhedron comprises: at least four standard spheres and corresponding standard sphere fixing devices, at least four laser trackers, a time synchronization trigger, a controller, and at least one target sphere;

[0008] The at least four standard balls are arranged by a standard ball fixing device so that the lines connecting the centers of the standard balls form the edges of a virtual space polyhedron; the target ball is movably arranged at a position tangent to the surface of the standard balls;

[0009] The at least four laser trackers are arranged on the periphery of the formed virtual space polyhedron, and the measurement range of each laser tracker covers the entire standard sphere;

[0010] The controller and the time synchronization trigger are in communication connection with each laser tracker; the controller is configured to control all the laser trackers through the time synchronization trigger, synchronously measure the target ball on the surface of the standard ball at the same time, obtain at least four sets of three-dimensional coordinates of the center of the target ball, and calculate the global three-dimensional coordinates of the target ball after unifying the coordinate system of all laser trackers; for each standard ball, at least the global three-dimensional coordinates of the center of the target ball at four different tangent positions on its surface are obtained, and the global coordinates corresponding to the center of the standard ball are calculated as the basis for calibrating the three-dimensional measurement results of the three-dimensional measurement equipment.

[0011] Preferably, the standard sphere is fixed from below by support rods, and the position of each laser tracker is higher than the highest vertex of the tetrahedron, so that the standard sphere is within the downward measurement range of each tracker; alternatively, the standard sphere is fixed from above by a hoisting method, and the position of each laser tracker is lower than any vertex of the tetrahedron, so that the standard sphere is within the upward measurement range of each tracker; alternatively, a part of the standard sphere is fixed from below by support rods, and the other part is fixed from above by a hoisting method.

[0012] Preferably, the apparatus further comprises a plurality of target balls and corresponding target ball seats, wherein the plurality of target balls are arranged at a plurality of positions tangent to the surface of the standard ball through the target ball seats.

[0013] Preferably, the centers of at least three of the at least four standard balls are set on the same plane, and the distance between any two of the at least three standard balls is greater than 10 meters and less than 56 meters; the distance between any other standard ball other than the at least three standard balls and the plane where the at least three standard balls are located is set to be greater than 3 meters and less than 20 meters.

[0014] A method for calibrating three-dimensional measurement results based on a variable-size polyhedron employs the system and comprises the following steps:

[0015] Step 1: Obtain a first 3D measurement result from a 3D measurement device;

[0016] Step 2: Obtain the three-dimensional coordinates of the center of the target ball from the laser tracker;

[0017] Step 3: Calculate the global three-dimensional coordinates of the center of the target ball;

[0018] Step 4: Calculate the global three-dimensional coordinates of the center of the standard ball according to the global three-dimensional coordinates of the centers of the multiple target balls as the second three-dimensional measurement result;

[0019] Step 5: Obtain the 3D coordinate offset of the measurement result of the 3D measuring device according to the global coordinates of the centers of the standard balls in the second 3D measurement result and the first 3D measurement result and the distances between the centers of the two balls.

[0020] Preferably, obtaining the three-dimensional coordinates of the center of the target ball from the laser tracker includes: the controller controls all laser trackers through a time synchronization trigger, and for each standard ball, synchronously measures the target ball at a position tangent to the surface of the standard ball at the same time to obtain at least four sets of three-dimensional coordinates of the center of the target ball.

[0021] Preferably, before calculating the global three-dimensional coordinates of the center of the target ball, the method further includes: controlling each laser tracker to synchronously measure a distance to each of the centers of the four target balls, taking the centers of the target balls as orientation points, establishing a global three-dimensional coordinate system based on the principle of spatial six-degree-of-freedom adjustment and a weighted rank-deficient free network adjustment model, performing system orientation calculation based on a barycenter reference, and obtaining the global three-dimensional coordinates of the center of each laser tracker;

[0022] According to the global three-dimensional coordinates of the center of each laser tracker and the three-dimensional coordinates of the center of the target ball obtained by each laser tracker, a global coordinate conversion is performed to calculate the global three-dimensional coordinates of the center of each target ball.

[0023] Preferably, it further includes: sequentially restricting multiple standard balls to five three-dimensional spaces with length, width and height of 10m×10m×3m, 20m×20m×6m, 30m×30m×9m, 40m×40m×12m, and 50m×50m×15m, and repeating steps 2 to 4 in sections to obtain the global three-dimensional coordinates of the centers of the five groups of standard balls.

[0024] Preferably, before step 4, the method further includes: calibrating the diameter and roundness of each standard ball and / or target ball by a three-coordinate measuring machine.

[0025] Preferably, each step of the method is set to be performed at a temperature within the range of (20±5)°C, and the temperature change per hour is ≤1.0°C.

[0026] In summary, due to the adoption of the above technical solution, the present invention has at least the following beneficial effects:

[0027] At least four standard balls are arranged through standard ball fixing devices so that the lines connecting the centers of the standard balls form the edges of a virtual space polyhedron; and a target ball is movably arranged at a position tangent to the surface of the standard balls, so that at least four laser trackers are arranged on the periphery of the formed virtual space polyhedron, and the measurement range of each laser tracker covers all the standard balls. The global coordinates corresponding to the centers of the standard balls can be calculated, and the measurement result data of a large-scale three-dimensional measuring device exceeding 50 meters can be accurately calibrated on-site, and the global three-dimensional coordinate accuracy of the collaborative measurement of the three-dimensional measuring device can be improved to an accuracy range of ≤±(10μm+1μm / m), and the calibration result can be traced back to the 633nm national standard. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 3D is a schematic diagram of a three-dimensional structural arrangement of a system for calibrating three-dimensional measurement results based on a variable large-size polyhedron according to an exemplary embodiment of the present invention.

[0029] Figure 2 yes Figure 1 The system is shown in a perspective view looking down at the calibration structure arrangement.

[0030] Figure 3 yes Figure 1 、 Figure 2 Top view of the system shown.

[0031] Figure 4 is a perspective view of a system bottom-up calibration structure arrangement for calibrating three-dimensional measurement results based on a variable large-size polyhedron according to an exemplary embodiment of the present invention.

[0032] Figure 5 2 is a schematic diagram of the arrangement structure of the standard ball and the target ball according to an exemplary embodiment of the present invention.

[0033] Figure 6 FIG. 4 is a schematic diagram of a controller structure according to an exemplary embodiment of the present invention.

[0034] Figure 7 4 is a flow chart of a method for calibrating three-dimensional measurement results based on a variable-size polyhedron according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments to make the purpose, technical solutions and advantages of the present invention more clearly understood. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0036] Figures 1 to 4 A schematic diagram illustrates the structure of a system for calibrating three-dimensional measurement results based on a variable-size polyhedron according to an exemplary embodiment of the present invention. The system in this embodiment primarily comprises: at least four standard spheres, with lines connecting their centers forming the edges of a virtual tetrahedron (dashed lines in the figure); support rods and support bases, and / or suspension rods and suspension bases, for securing the standard spheres; at least four laser trackers positioned around the virtual tetrahedron; a time synchronization trigger and controller communicatively connected to each laser tracker; and at least one movable target sphere positioned tangentially to the surface of the standard spheres.

[0037] The controller is configured to control all laser trackers through a time synchronization trigger, synchronously measure the target sphere at the tangent position to the surface of the standard sphere at the same time (the time difference between any two laser trackers is less than 5μs), obtain at least four sets of three-dimensional coordinates of the target sphere center, and calculate the global three-dimensional coordinates of the target sphere center after unifying the coordinate system of all laser trackers; for each standard sphere, the global three-dimensional coordinates of the target sphere center at at least four different tangent positions on its surface are obtained, and based on this, the global coordinates corresponding to the center of the standard sphere are calculated as the basis for calibrating the measurement results of three-dimensional measurement equipment such as industrial photogrammetry equipment and three-dimensional laser scanners.

[0038] Each standard ball is set within the measurement range of any one of the first laser tracker, the second laser tracker, the third laser tracker, and the fourth laser tracker; for example, Figure 2 As shown, the standard ball is fixed from below (relative to the up and down direction in the figure) by a support rod, and (by upgrading the platform and measuring gantry) each laser tracker is positioned higher than the highest vertex of the tetrahedron, so that the standard ball is within the top-view measurement range of each tracker. In other embodiments, as Figure 4 As shown, the standard sphere can be fixed from above by hoisting, and each laser tracker can be positioned lower than any vertex of the tetrahedron so that the standard sphere is within the upward measurement range of each tracker.

[0039] At the measurement site, 3D measurement equipment, such as industrial photogrammetry equipment and 3D laser scanners, first scans the contours of the four standard spheres that form the virtual space tetrahedron. This generates spherical point cloud data for each standard sphere. The 3D coordinates of the standard sphere's center are calculated from this point cloud data and used as the 3D measurement result. Next, an operator moves a target sphere to multiple tangent positions on the surface of the standard sphere, or multiple target spheres are positioned at multiple tangent positions on the surface of the standard sphere using target sphere holders. All laser trackers are synchronized and measured at a preset sampling frequency to obtain the global 3D coordinates of the target spheres at these multiple tangent positions. The global 3D coordinates of the standard sphere's center are then accurately calculated based on the diameters of the standard and target spheres. This is then used to correct the measurements made by the 3D measurement equipment, achieving precise calibration of the 3D measurement results and improving the global 3D coordinate accuracy of the collaborative measurements of the 3D measurement equipment to within ±(10μm+1μm / m).

[0040] The centers of the first, second, and third standard balls are located on the same plane, and the distance between any two of them is greater than 10 meters and less than 56 meters. The center of the fourth standard ball is greater than 3 meters and less than 20 meters from the plane formed by the first, second, and third standard balls. When the four standard balls are fixedly arranged, their centers form the four vertices of a virtual space tetrahedron. In other embodiments, five or more standard balls can be used, and their centers can form a virtual space polyhedron with more faces. The centers of the standard balls can all be located at the vertices of the polyhedron, or some can be located at the edges of the polyhedron.

[0041] like Figure 5 As shown, each standard sphere has a diameter of 10 to 30 cm, with a diameter measurement uncertainty of U = 1.5 μm (coverage factor k = 2); the roundness measurement uncertainty is U = 0.2 μm (k = 2). Depending on the 3D measurement device being calibrated, a polished metal standard sphere, a matte metal standard sphere, a polished ceramic standard sphere, or a matte ceramic standard sphere can be used. The diameter and roundness of the standard spheres are calibrated using a coordinate measuring machine before calibration, and the calibrated diameter and roundness serve as the basis for global coordinate calculation. The target sphere can be a 0.5- or 1.5-inch diameter metal, ceramic, or integrated target sphere, with an optical center error of ≤ ±12.5 μm. The target sphere can be fixed to the surface of the standard sphere using a target sphere holder by the operator, ensuring that the target sphere is tangent to the spherical surface of the standard sphere. Furthermore, the same target sphere can be moved to multiple tangent locations on the surface of the standard sphere. At each location, multiple laser trackers are controlled to perform synchronous measurements to obtain the global 3D coordinates of the multiple target spheres. In a preferred embodiment, multiple target balls can be set at multiple tangential positions on the surface of the standard ball by using a cascade magnetic target ball seat that is easy to detach and install, for example, Figure 5 The target ball seat shown is triangular in shape, with a magnetic arc component provided on each side, which can flexibly connect multiple target ball seats together and make the target ball tangent to the surface of the standard ball.

[0042] The measurement radius of each laser tracker is 80m, and it has been tested by the indoor 50m long length standard device of the China Institute of Testing Technology (measuring range: 0-56m; measurement uncertainty: U = 0.14μm + 1×10 -7 L (k = 2, L is the measurement length), and the 50m long length standard device is calibrated by the 633nm national secondary standard device (measurement uncertainty: U = 2.5×10 -11 nm (k=1)) calibration, so the measurement results in the system of the present invention can be traced back to the 633 nm national standard.

[0043] Figure 6 A controller according to an exemplary embodiment of the present invention is shown, namely an electronic device 310 (for example, a computer server with a program execution function), which includes at least one processor 311, a power supply 314, and a memory 312 and an input / output interface 313 that are communicatively connected to the at least one processor 311; the memory 312 stores instructions that can be executed by the at least one processor 311, and the instructions are executed by the at least one processor 311 to enable the at least one processor 311 to execute the method disclosed in any of the aforementioned embodiments; the input / output interface 313 may include a display, a keyboard, a mouse, and a USB interface for inputting and outputting data; the power supply 314 is used to provide power to the electronic device 310.

[0044] Figure 7 A flow chart of a method for calibrating three-dimensional measurement results based on a variable large-size polyhedron according to an exemplary embodiment of the present invention is shown. The method adopts the systems described in the above embodiments and mainly includes the following steps.

[0045] Step 701: Obtain a first 3D measurement result from a 3D measurement device. The 3D measurement device performs contour scanning on the four standard spheres that form the virtual space tetrahedron, obtaining spherical point cloud data for each standard sphere. Based on the point cloud data, the diameters and roundness of the standard spheres, the 3D coordinates of the center of each standard sphere and the distances between the centers of the standard spheres are calculated and used as the 3D measurement result. The 3D measurement device can include multiple industrial photogrammetry devices, 3D laser scanners, and other devices. After the measurement data from all 3D measurement devices are unified, the global coordinates of the centers of all standard spheres and the distances between each center are used as the first 3D measurement result, which is the calibration target of the present invention.

[0046] Step 702: Obtain the 3D coordinates of the target sphere's center from the laser tracker. The controller uses a time-synchronized trigger to control all laser trackers. For each reference sphere, the controller simultaneously measures the target sphere at a position tangent to the reference sphere's surface, obtaining at least four sets of 3D coordinates of the target sphere's center. In a preferred embodiment, more target spheres and corresponding target sphere mounts can be used, enabling simultaneous acquisition of the 3D coordinates of at least four target spheres corresponding to at least four reference spheres.

[0047] Step 703, calculating the global three-dimensional coordinates of the target sphere's center: Before the calculation, the system may also be initialized to establish global three-dimensional coordinates. Specifically, each laser tracker is controlled to synchronously measure its own distance to the centers of multiple (≥4) target spheres. The target sphere centers are used as orientation points. Based on the principle of spatial six-degree-of-freedom adjustment and a weighted rank-deficient free network adjustment model, a global three-dimensional coordinate system is established. The system orientation is calculated based on the center of gravity reference to obtain the global three-dimensional coordinates of the center of each laser tracker. Based on the global three-dimensional coordinates of the center of each laser tracker and the three-dimensional coordinates of the target sphere center obtained by each laser tracker, a global coordinate conversion is performed to calculate the global three-dimensional coordinates of the center of each target sphere.

[0048] Step 704 calculates the global 3D coordinates of the center of the standard sphere based on the global 3D coordinates of the centers of the multiple target spheres, as the second 3D measurement result. The distance from the optical center of each target sphere to its surface (i.e., the target sphere radius) is known, and the diameter and roundness of each standard sphere are also known values ​​calibrated by a coordinate measuring machine. The coordinates of the target sphere centers corresponding to each standard sphere can be fitted into the surface point cloud coordinates of a 3D sphere centered at the center of the standard sphere and with the sum of the target sphere radius and the standard sphere radius as its radius. Therefore, based on the global 3D coordinates of the target sphere center, combined with the target sphere diameter, the standard sphere diameter, and the roundness, the global 3D coordinates of the center of each standard sphere can be calculated.

[0049] Step 705 : Obtain the 3D coordinate offset of the measurement result of the 3D measuring device according to the global coordinates of the centers of the standard balls in the second 3D measurement result and the first 3D measurement result and the distances between the centers of the standard balls.

[0050] After calibration in the above steps, when the 3D measuring equipment performs 3D scanning measurement on the actual measured parts on site, after obtaining the first 3D measurement result, the global coordinates of the center of the standard ball are corrected according to the 3D coordinate offset, so that a higher-precision third 3D measurement result can be obtained.

[0051] In a preferred embodiment, each of the above steps is performed within a temperature range of (20±5)°C, with a temperature change of ≤1.0°C per hour. Furthermore, shock-absorbing pads can be installed on the bottom of the laser tracker, on the standard ball mounting seat, and other locations to isolate external vibrations. A windshield can also be installed around the laser tracker to prevent interference from air convection and heat radiation that could affect measurement accuracy.

[0052] In various embodiments, multiple standard spheres can be sequentially confined within five three-dimensional spaces with length, width, and height dimensions of (10m×10m×3m), (20m×20m×6m), (30m×30m×9m), (40m×40m×12m), and (50m×50m×15m). The above steps are repeated to obtain the global three-dimensional coordinates of the centers of the five standard spheres, and calibration is performed five times, further improving calibration accuracy. During the calibration process, each laser interferometer can sample the three-dimensional coordinate data of the target sphere's center at a frequency of 1000 points / second and fit the most accurate value to calculate the global three-dimensional coordinates of the centers of the standard spheres. During calibration, the placement and number of standard spheres can be increased or decreased based on the important dimensions of the workpiece being measured or the key inspection areas, but the lines connecting the centers of the standard spheres form the edges of a spatial polyhedron.

[0053] All or part of the steps in the above-described method embodiments may be accomplished by hardware associated with program instructions. The aforementioned program may be embodied in the form of a software product, which is stored in a storage medium and includes instructions for causing a computer device (such as a personal computer, server, or network device) to execute all or part of the method described in the embodiments of the present invention. The aforementioned storage medium includes various media capable of storing program code, such as a mobile storage device, read-only memory, a magnetic disk, or an optical disk.

[0054] The above description is only a detailed description of the specific embodiments of the present invention, and does not limit the present invention. Various substitutions, modifications and improvements made by those skilled in the relevant art without departing from the principles and scope of the present invention should be included in the scope of protection of the present invention.

Claims

1. A system for calibrating three-dimensional measurement results based on a variable-size polyhedron, characterized in that: include: At least four standard balls and corresponding standard ball fixing devices, at least four laser trackers, a time synchronization trigger, a controller, and at least one target ball; The at least four standard balls are arranged by a standard ball fixing device so that the lines connecting the centers of the standard balls form the edges of a virtual space polyhedron; the target ball is movably arranged at a position tangent to the surface of the standard balls; The at least four laser trackers are arranged on the periphery of the formed virtual space polyhedron, and the measurement range of each laser tracker covers the entire standard sphere; The controller and the time synchronization trigger are in communication with each laser tracker. The controller is configured to control all laser trackers via the time synchronization trigger to simultaneously measure the target sphere on the surface of the standard sphere, obtain at least four sets of three-dimensional coordinates of the target sphere's center, and calculate the global three-dimensional coordinates of the target sphere after unifying the coordinate systems of all laser trackers. For each standard sphere, the global three-dimensional coordinates of the target sphere's center at least four different tangent positions on its surface are obtained, and the global coordinates corresponding to the center of the standard sphere are calculated as a basis for calibrating the three-dimensional measurement results of the three-dimensional measurement device. The standard sphere is fixed from below by support rods, and the position of each laser tracker is higher than the highest vertex of the tetrahedron, so that the standard sphere is within the downward measurement range of each tracker; alternatively, the standard sphere is fixed from above by a hoisting method, and the position of each laser tracker is lower than any vertex of the tetrahedron, so that the standard sphere is within the upward measurement range of each tracker; alternatively, part of the standard sphere is fixed from below by support rods, and the other part is fixed from above by a hoisting method; The centers of at least three of the at least four standard balls are arranged on the same plane, and the distance between any two of the at least three standard balls is greater than 10 meters and less than 56 meters; the distance between any other standard ball other than the at least three standard balls and the plane on which the at least three standard balls are located is set to be greater than 3 meters and less than 20 meters; The diameter of the standard sphere is 10-30 cm, the uncertainty of diameter measurement is: U=1.5 μm, the uncertainty of roundness measurement is: U=0.2 μm; the time difference between any two laser trackers is less than 5 μs.

2. The system according to claim 1, wherein: The device further comprises a plurality of target balls and corresponding target ball seats, wherein the plurality of target balls are arranged at a plurality of positions tangent to the surface of the standard ball through the target ball seats.

3. A method for calibrating three-dimensional measurement results based on a variable-size polyhedron, characterized in that: The system according to claim 1 or 2 comprises the following steps: Step 1: Obtain a first 3D measurement result from a 3D measurement device; Step 2: Obtain the three-dimensional coordinates of the center of the target ball from the laser tracker; Step 3: Calculate the global three-dimensional coordinates of the center of the target ball; Step 4: Calculate the global three-dimensional coordinates of the center of the standard ball according to the global three-dimensional coordinates of the centers of the multiple target balls as the second three-dimensional measurement result; Step 5: Obtain the 3D coordinate offset of the measurement result of the 3D measuring device according to the global coordinates of the centers of the standard balls in the second 3D measurement result and the first 3D measurement result and the distances between the centers of the two balls.

4. The method according to claim 3, characterized in that The method of obtaining the three-dimensional coordinates of the center of the target ball from the laser tracker includes: a controller controls all laser trackers through a time synchronization trigger, and for each standard ball, synchronously measures the target ball at a position tangent to the surface of the standard ball at the same time to obtain at least four sets of three-dimensional coordinates of the center of the target ball.

5. The method according to claim 3, characterized in that Before solving the global three-dimensional coordinates of the center of the target ball, the method also includes: controlling each laser tracker to synchronously measure the distance to the centers of the four target balls, taking the centers of the target balls as orientation points, establishing a global three-dimensional coordinate system based on the principle of spatial six-degree-of-freedom adjustment and a weighted rank-deficient free network adjustment model, and solving the system orientation based on the center of gravity benchmark to obtain the global three-dimensional coordinates of the center of each laser tracker; According to the global three-dimensional coordinates of the center of each laser tracker and the three-dimensional coordinates of the center of the target ball obtained by each laser tracker, a global coordinate conversion is performed to calculate the global three-dimensional coordinates of the center of each target ball.

6. The method according to claim 3, characterized in that Further including: Sequentially confine multiple standard balls to five three-dimensional spaces with length, width and height of 10m×10m×3m, 20m×20m×6m, 30m×30m×9m, 40m×40m×12m, and 50m×50m×15m, and repeat steps 2 to 4 in sections to obtain the global three-dimensional coordinates of the centers of the five sets of standard balls.

7. The method according to claim 3, characterized in that Before step 4, the method further includes: calibrating the diameter and roundness of each standard ball and / or target ball by using a three-coordinate measuring machine.

8. The method according to any one of claims 3 to 7, characterized in that Each step of the method is set to be performed at a temperature within the range of (20±5)°C, and the temperature change per hour is ≤1.0°C.

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