A thread gauge digital model error analysis method based on three-dimensional point cloud

Abstract

The application discloses a kind of based on three-dimensional point cloud's thread gauge digital model error analysis method, comprising the following steps: obtaining three-dimensional point cloud digital thread plug gauge model;Precision quantitative error analysis is carried out to thread gauge digital model, and the source of model error is analyzed;Non-quantitative error analysis is carried out to thread gauge digital model, and the overall evaluation of the quality of digital model reconstruction is carried out, and whether the model reconstructed can be used as standard model is judged in combination with the quantitative error obtained.The application provides a new direction for the development of physical measurement to digital measurement, solves the problems of difficult processing, high cost and complex verification process of measurement standard in the process of thread gauge measurement comparison, and the thread gauge digital model constructed can realize production assembly calibration at the same time, effectively improving work efficiency.

Description

Technical Field

[0001] This invention relates to the field of metrological verification, specifically to an error analysis method for a digital model of a thread gauge based on three-dimensional point clouds. Background Technology

[0002] Thread gauges are measuring instruments with standard ordinary thread profiles that can reflect the boundary conditions of the internal and external threads being inspected. They are classified according to their performance into working thread gauges and calibration thread gauges.

[0003] my country is a major producer and user of threaded steel, with various types of screws and nuts already used in all aspects of production and daily life. Therefore, it is essential to inspect the produced threads to ensure their qualification and replaceability. Physical measurement is currently the most widely used method, and with the advent of the digital age, digital measurement is gradually emerging, making digitalization an inevitable trend in the metrology industry. Based on the current thread gauge transfer process and the characteristics of the gauges themselves, the contradictions of the traditional metrological assurance model are becoming increasingly prominent. The main problems are: thread gauges are prone to wear due to long-term coordination and comparison, making it difficult to maintain accuracy; there are numerous verification instruments, but their practicality is limited; there are many types of standards with complex structures, resulting in long transfer routes and large accumulated errors when transferring them to working instruments, causing significant accuracy loss, and tracing the source of errors is also difficult; existing standards are diverse and costly to process. While corresponding measurement transfer and traceability systems have been established for general-purpose standards, an effective measurement transfer system is lacking for a large number of specialized standards. Establishing a comprehensive measurement transfer system would be a huge undertaking and time-consuming.

[0004] The traditional measurement value transfer methods of existing thread gauges are not suitable for the rapidly changing industrial production development. Therefore, there is an urgent need to study the geometric accuracy analysis methods and comparison methods of digital models suitable for gauges to meet the needs of thread gauge metrology work and to be suitable for metrological assurance in various fields. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides an error analysis method for a digital model of thread gauges based on three-dimensional point clouds. This invention offers a new direction for the development of physical metrology towards digital metrology, and solves the problems of difficult and costly processing of metrological standards and cumbersome verification procedures in the process of thread gauge metrological comparison. The constructed digital model of thread gauges can realize simultaneous production, assembly, and calibration operations, effectively improving work efficiency.

[0006] An error analysis method for a digital model of a thread gauge based on 3D point cloud includes the following specific steps:

[0007] Step 1: Propose a digital comparison and transfer process for thread gauges and obtain a 3D point cloud digital model of the thread gauges;

[0008] Step 2: Perform accuracy quantification error analysis on the 3D point cloud thread gauge digital model, analyze the sources of model error, including standard model design error and / or instrument detection and acquisition error of actual measurement model and / or point cloud processing error;

[0009] Step 3: Perform non-quantitative error analysis on the 3D point cloud thread gauge digital model, conduct an overall evaluation of the reconstruction quality of the digital model, and combine the quantitative error obtained in Step 2 to determine whether the reconstructed thread gauge digital model can be used as a standard model.

[0010] Preferably, the step one of obtaining the 3D point cloud digital model of the thread gauge includes the following steps:

[0011] S11: A three-dimensional thread measurement machine composed of a high-precision Z-axis, X-axis, high-precision grating system, contact scanning probe, and high-precision rotary axis is used to collect point cloud data of thread gauge surface.

[0012] S12: The adaptive BPA reconstruction algorithm is used to reconstruct the collected point cloud model to obtain a model with good mesh topology connection and good overall quality.

[0013] Preferably, the calculation of each error value in step S2 includes the following steps:

[0014] S21: Analyze and calculate the design and machining errors of the digital model of the M16*2-6g thread gauge, mainly including the STL format output error and the tolerance range of each parameter during the machining process;

[0015] S22: Calculate the instrument detection and acquisition error that exists in the process of acquiring the surface point cloud data of M16*2-6g thread gauge using a three-dimensional thread comprehensive measuring machine;

[0016] S23: Based on the point cloud data generated in step S22, obtain the thread gauge parameters between each point cloud according to the following formula;

[0017] Center coordinates ;

[0018] Da Jia ;

[0019] trails ;

[0020] Median diameter ;

[0021] pitch ;

[0022] Among them, Q i This indicates the intersection of the pitch diameter line and the thread.

[0023] Tooth angle ;

[0024] Where K Qi For point Q i The slope.

[0025] Preferably, the thread quantity model is compared with the reconstructed quantity model in step one, including the following steps:

[0026] S31: Adopt the best fit alignment to make the two coincide, and use the Iterative Correspondin Points (ICP) algorithm to establish the alignment relationship between the two models;

[0027] S32: Calculate the deviation values ​​of each point in the reconstructed volumetric model from the corresponding points in the standard volumetric model;

[0028] S33: Use the "3D Comparison" command to obtain a three-dimensional error comparison diagram of the model, as well as the overall distribution of 3D comparison errors;

[0029] S34: Analyze the quantization error and non-quantization error obtained in steps two and three, and determine whether the quality of the reconstructed model obtained in step two is good and whether it can be used for digital metrology.

[0030] Preferably, the thread gauge is an M16*2-6g thread gauge.

[0031] The beneficial effects of this invention are as follows: Compared with existing technologies, this invention creatively proposes a method for extracting parameters and analyzing errors in a three-dimensional thread gauge digital model, enabling the construction of a digital model of the thread gauge and the analysis of errors in the measured model. This provides a direction for the development from physical metrology to digital metrology, solving the problems of difficult and costly processing of metrological standards and cumbersome verification procedures in the process of comparing physical thread gauges. The constructed digital model of the thread gauge allows for simultaneous production, assembly, and calibration operations, effectively improving work efficiency. It provides a research foundation for the digitalization of thread metrology and offers valuable reference for the digital development of the entire metrology industry. Attached Figure Description

[0032] Figure 1 This is a flowchart of the method of the present invention.

[0033] Figure 2 This is the result of reconstructing a point cloud model using the design model of this invention.

[0034] Figure 3 This is a graph showing the output error of the STL design model of this invention.

[0035] Figure 4 This is a diagram showing the intersection of the thread mean diameter line and the thread in this invention.

[0036] Figure 5 This is a result diagram of the thread gauge parameter detection system of the present invention.

[0037] Figure 6 This is a comparison diagram of the design model and the actual measured model of the thread gauge of this invention. Detailed Implementation

[0038] The invention will be further described below with reference to the accompanying drawings and embodiments.

[0039] like Figure 1-6 As shown, this embodiment of the invention provides an error analysis method for a digital model of a thread gauge based on three-dimensional point clouds, which specifically includes the following implementation steps:

[0040] Step 1: Propose a digital comparison and transfer process for thread gauges, and obtain a 3D point cloud digital model of the M16*2-6g thread gauge, including the following steps:

[0041] S1: Using SmartTouch intelligent scanning technology, a three-dimensional thread comprehensive measuring machine consisting of a high-precision Z-axis, X-axis, high-precision grating system, contact scanning probe, and high-precision rotating axis is used to collect point cloud data of thread gauge surface through a real-time force analysis device and an intelligent force sensing device to maintain the same contact force at any position and on any inclined plane.

[0042] S2: Point cloud denoising is performed using statistical filtering combined with bilateral filtering algorithm, and the point cloud is divided into grids according to the spatial grid method to achieve simplified sampling. The normal vector of the sampling point and the neighboring point is calculated. If the product of the two is less than zero, the point cloud in that region is retained, resulting in point cloud data with fewer outliers and a smooth surface.

[0043] S3: The adaptive BPA reconstruction algorithm is used to reconstruct the point cloud after the above processing.

[0044] Step 2: Perform precision quantification error analysis on the digital model of the thread gauge, and analyze the sources of model error, including standard model design error and / or instrument detection and acquisition error of the actual measurement model and / or point cloud processing error;

[0045] Specifically, the following steps are included:

[0046] S21: Analyze and calculate the design error of the digital model of the M16*2-6g thread gauge, mainly including the error of each parameter during the STL format output process.

[0047] In this embodiment, the analysis of thread gauge design error includes the following steps:

[0048] Step 1: When designing a standard model, the manufacturing process outputs an STL format model. This format approximates the original model using triangular facets, which introduces errors. These errors can be represented by the chord height, i.e., the radial distance ΔH between the triangle contour edge and the surface contour. The more triangular facets there are, the higher the overall accuracy. Therefore, the exported STL file is further subdivided to obtain the model design error as the radial distance Δh.

[0049] like Figure 3 As shown, assuming the radius of the arc is R, the corresponding central angle is... After point cloud subdivision, the absolute error Δh of the chord distance height derived from geometric theory is:

[0050] ;

[0051] Step 2: When exporting the STL model, select the export angle as... The major diameter d = 16 mm, the minor diameter d1 = 13.835 mm, and the mean diameter d2 = 14.701 mm. Substituting these values ​​into R, the error can be calculated using the formula. , , Due to the symmetry of the thread gauge and the continuity of the thread profile, the pitch P and the thread angle α can be ignored.

[0052] S22: The acquisition error using the three-dimensional thread measuring instrument can be obtained from the specifications in the instrument manual. The measurement errors for the major diameter, pitch diameter, and minor diameter are ±(4.0+L / 200)μm; the error for the pitch P is ±(0.9+L / 200)μm; and the error for the thread angle α is ±0.03°. Therefore, the instrument errors for each parameter are:

[0053] ;

[0054] S23: Point cloud error mainly occurs during the filtering, simplification, and reconstruction of the point cloud. This error is obtained by calculating the specific parameters of the thread gauge. In this embodiment, obtaining the point cloud error includes the following steps:

[0055] Step A: In the program, import the filtered and simplified point cloud data of the M16*2-6g thread gauge, and shift the point cloud to the origin of the coordinate system, ensuring that the thread centerline is parallel to the Z-axis. The result is as follows. Figure 5 As shown.

[0056] Step B: Obtain the three-dimensional coordinates of each point in the point cloud P, and rotate the coordinates to obtain multiple sets of maximum and minimum values ​​of the X and Y axes in order to calculate the coordinates of the central axis and the center O(x0, y0) of the imaginary cylinder.

[0057] ;

[0058] When rotating the coordinate system, the rotation angles of the X, Y, and Z axes are respectively ɵ x , ɵ y , ɵ z The rotation matrices for each axis are then:

[0059] ;

[0060] ;

[0061] ;

[0062] Each rotation of ɵ degrees corresponds to one revolution of the coordinate axis. The difference between the maximum and minimum x-coordinate values ​​of the point cloud data is obtained, and their average value is calculated as the major diameter d of the thread.

[0063] ;

[0064] Where ɵ alternates between 1° and 20°.

[0065] By rotating the coordinate axes, calculate the average minimum distance from each point in the set of angle points P to the center O in the XY plane, and use this average as the minor diameter d1:

[0066] trails ;

[0067] The pitch diameter d2 is located where the widths of the grooves and protrusions on the thread profile are equal in the axial section of the thread. Taking one side of the YZ section as an example, the intersection of the pitch diameter line and the point cloud is marked as points Q1~Q6.

[0068] ;

[0069] pitch ;

[0070] Among them, Q i This indicates the intersection of the pitch diameter line and the thread.

[0071] Tooth angle ;

[0072] Where K Qi For point Q i The slope.

[0073] like Figure 4 As shown in Table 1, the parameters of the digital model of the thread gauge reconstructed by the adaptive BPA algorithm are extracted.

[0074] Table 1. Measured data of various parameters of the thread gauge

[0075] parameter Measured value Theoretical value <![CDATA[Absolute error Δ3]]> relative error Large diameter d (mm) 15.9972 16.000 -0.0028 0.175‰ Minor diameter d1 (mm) 13.8168 13.835 -0.0182 1.3155‰ Mean diameter d2 (mm) 14.6979 14.701 0.0031 0.2126‰ Pitch P (mm) 2.0005 2.000 0.0005 0.250‰ Tooth angle α (°) 60.1489 60.000 0.1489 2.4817%

[0076] Combining the above errors, calculate the overall error of each parameter:

[0077] ;

[0078] Given the major, medium, and minor diameter tolerances of the thread quantity specification. If the value is 160μm, then its small error range is... pitch tolerance The value is 4μm, and the small error range is... Tooth angle tolerance The value is 0.5°, and the small error range is... As can be seen from the above formula, the errors of each parameter are all within a small range and their influence can be disregarded. Furthermore, the relative errors are all less than 5%. Therefore, it is preliminarily determined that this digital model can be used as a standard model.

[0079] Step 3: Based on the designed digital standard gauge model and the point cloud reconstruction model obtained in Step 2, perform corresponding model processing, align and compare the two models, perform point cloud deviation analysis, and output a test result report to achieve digital measurement of thread gauges. In this implementation, the corresponding processing is carried out in the Cloud Compare environment, including the following steps:

[0080] Step a: Open the design drawing SolidWorks model in the "Graphics Area" of the status bar as a reference model, and then import the digital standard model generated by point cloud processing and reconstruction as a test model.

[0081] Step b: Under the initial menu, manually adjust the model position using "Cross Section" to achieve approximate alignment; then call the "Fine Registration (ICP)" alignment algorithm to ensure the two models overlap. Next, call the "3D Comparison" command to obtain the deviation value of the test model compared to the standard model, thus obtaining a three-dimensional error comparison chart of the models, as shown below. Figure 5 As shown.

[0082] Step c: The software results show that the maximum deviation between the two models is 0.2870mm, and the minimum deviation is 0mm. The distance contour map indicates that the larger deviation occurs in the minor diameter portion. In actual production, the minor diameter does not have a specified deviation and can be determined through negotiation among the processing parties. Therefore, the superiority of the adaptive BPA reconstruction algorithm in establishing the digital model of the thread gauge is evident. The reconstructed thread gauge model and quality closely resemble the real model. Therefore, the improved BPA algorithm can be used to reconstruct a three-dimensional model, thereby completing the digital transmission of thread gauge measurements.

[0083] In this embodiment, the three-dimensional thread measuring machine includes a worktable, a high-precision Z-axis, an X-axis, a high-precision grating system, a contact scanning probe, and a high-precision rotary axis. The high-precision rotary axis is located at the bottom of the worktable, the Z-axis is in the vertical direction, and the X-axis is perpendicular to the Z-axis and located above the worktable. The contact scanning probe is mounted on the X-axis, and its electric displacement is connected to the high-precision grating system. The three-dimensional thread measuring machine is commercially available.

[0084] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for error analysis of a digital model of a thread gauge based on three-dimensional point cloud, characterized in that, The specific steps include the following: Step 1: Obtain a 3D point cloud digital model of the thread gauge; Step 2: Perform precision quantification error analysis on the acquired 3D point cloud digital model of the thread gauge, and analyze the sources of error in the model, including standard model design error and / or instrument detection and acquisition error of the actual model and / or point cloud processing error; Specifically, the following steps are included: S21: Analyze and calculate the design and machining errors of the digital model of the thread gauge, including the STL format output error and the tolerance range of the parameters during the machining process; S22: Calculate the instrument detection and acquisition error that exists in the process of acquiring point cloud data of thread gauge surface using a three-dimensional thread comprehensive measuring machine; S23: Based on the point cloud data generated in step S22, obtain the thread gauge parameters between each point cloud according to the following formula; Center coordinates ; Large diameter ; Small path ; Median diameter ; pitch ; Among them, Q i Indicates the intersection of the pitch diameter line and the thread; Tooth angle ; Where K Qi For point Q i The slope; Step 3: Perform non-quantitative error analysis on the 3D point cloud thread gauge digital model, conduct an overall evaluation of the model reconstruction quality, and combine the quantized error obtained in Step 2 to determine whether the reconstructed thread gauge digital model can be used as a standard model.

2. The error analysis method for the digital model of thread gauges based on three-dimensional point clouds according to claim 1, characterized in that, The step one, obtaining the 3D point cloud digital model of the thread gauge, includes the following steps: S11: Use a three-dimensional thread measurement machine to collect point cloud data of the thread gauge surface; S12: The adaptive BPA reconstruction algorithm is used to reconstruct the collected point cloud data model to obtain the grid topology connection model.

3. The error analysis method for the digital model of thread gauges based on three-dimensional point clouds according to claim 1, characterized in that, The 3D point cloud digital model of the thread gauge obtained in step one is compared with the reconstructed digital model of the thread gauge in step three, including the following steps: S31: Use the best fit alignment to make the two coincide, and use the corresponding point iterative algorithm to establish the alignment relationship between the two models; S32: Calculate the deviation values ​​of each point in the reconstructed volumetric model from the corresponding points in the standard volumetric model; S33: Use the 3D comparison command to obtain a three-dimensional error comparison diagram of the model, as well as the overall distribution of 3D comparison errors; S34: Analyze the quantization error and non-quantization error obtained in steps two and three, and determine whether the quality of the reconstructed model obtained in step two is good and whether it can be used for digital metrology.

4. The error analysis method for the digital model of thread gauges based on three-dimensional point clouds according to claim 1, characterized in that, The thread gauge is an M16*2-6g thread gauge.

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