Composite standard part and calibration method suitable for calibration of structured light measurement system

Abstract

The application discloses a composite standard part suitable for a structured light measurement system calibration, which is used in a linear structured light or circular structured light measurement calibration system and comprises a bottom plate, wherein a sawtooth annular gauge is hollowed out in the middle of the bottom plate, the annular gauge is shaped as follows: the diameter of a center hollow annular ring is the smallest, and the diameters of hollow annular rings from the center hollow annular ring to both sides are sequentially and equally increased in steps, and the annular gauge is used for circular structured light calibration. The device structure is simple, easy to realize, convenient to operate and relatively strong in practicality, and the standard part can simultaneously realize linear structured light and circular structured light calibration. Meanwhile, a calibration method of a linear / circular structured light measurement system based on the composite standard part is provided.

Description

Technical Field

[0001] This invention belongs to the field of precision measurement, and in particular relates to a composite standard and calibration method suitable for the calibration of structured light measurement systems. Background Technology

[0002] Non-contact measurement is an important research direction in the field of machine vision. It uses sensors based on optical and wave principles to acquire the three-dimensional information of the object being measured. Its advantages include minimal impact on the object, ease of automation, low requirements for the working environment, and short measurement cycle. Non-contact measurement mainly includes infrared measurement, ultrasonic measurement, multi-camera measurement, and structured light measurement. Structured light measurement, with its advantages of high measurement density, high speed, high accuracy, and ability to measure large objects, has wide applications in defense, automotive, and aerospace industries. It is mainly used for workpiece positioning measurement, 3D model reconstruction, pipe internal wall damage detection, aperture measurement, and appearance measurement. One key technical issue is the calibration of structured light.

[0003] Currently, the following schemes are mainly used for structured light calibration: Morinaka et al. proposed a three-dimensional reconstruction method for distortion caused by any refractive medium, but this method only establishes a nonlinear mapping table that associates image points with two-dimensional points on the calibration plate. Therefore, the refractive distortion of the cylindrical surface still needs careful analysis, and refractive distortion correction is the key to omnidirectional structured light visual measurement. Zhu et al. proposed a binocular structured light calibration method. The camera in the system forms a binocular system with another auxiliary camera to obtain the coordinates of the light stripes formed by the blank plane plate intersecting with the structured light, so as to fit the light plane parameters. This method has low instrument requirements and is easy to operate. However, during the calibration process, the binocular observation range of the structured light plane is limited by the system structure, resulting in relatively low measurement repeatability. Albertazzi et al. calibrated the system by measuring the inner stepped cylindrical gauges of 28 regions with different diameters to obtain the polynomial relationship between the pixel radius of the ring light stripe and the true radius. The inner surface is calculated based on this relationship. This method relies on a high-precision calibration target with a complex structure, and its operation is very complicated, requiring a precision moving device. Zhuang et al. improved the system resolution to 100 μm through theoretical analysis of the error sources. In her research, the system was calibrated by measuring a standard ring gauge. This calibration method requires precise alignment of the detector with the ring gauge, making it difficult to operate. Furthermore, this calibration only considers magnification. Zhang et al. improved the system by stretching the laser and the conical reflector into the tube from opposite directions. This improvement helps reduce system errors because the radial beam is refracted once by the glass tube, instead of twice as much as in other systems. He proposed a very complex calibration technique: using two high-precision electronic theodolites to measure the three-dimensional coordinates of multiple light points generated by the intersection of the laser plane and a thin tire tread. Clearly, this method is very complex and inefficient due to frequent manual operations. Summary of the Invention

[0004] The purpose of this invention is to provide a composite standard component suitable for calibrating line / circular structured light measurement systems. The component has a simple structure, is easy to implement, convenient to operate, and highly practical. Furthermore, this standard component can simultaneously calibrate line / circular structured light systems. A calibration method for line / circular structured light measurement systems based on the composite standard component proposed in this invention is also provided.

[0005] To achieve the above-mentioned objectives, the technical solution of this invention is as follows:

[0006] A composite standard component suitable for calibration of structured light measurement systems, used in measurement and calibration systems for line structured light or circular structured light, the composite standard component includes a base plate, the center of which has a serrated annular gauge. The annular gauge has the following shape: the diameter of the central hollowed-out ring is the smallest, and the diameters of the hollowed-out rings on both sides increase sequentially in equal increments. The annular gauge is used for calibration of circular structured light.

[0007] The base plate is symmetrically provided with calibration steps on both sides of the ring gauge. The calibration steps are arranged with the highest step in the middle and the steps decreasing in size from the highest step to both sides. The calibration steps are used for line structure cursor calibration.

[0008] A further preferred technical solution is that the diameter of the first hollow ring of the ring gauge is 48mm, and the diameters of the hollow rings on both sides are 52mm, 56mm, 60mm, 64mm, 68mm, 72mm, 76mm, 80mm, 84mm and 88mm respectively, for a total of 11 hollow rings.

[0009] Furthermore, the height of each step in the calibration step, from lowest to highest, is 2mm, 2.5mm, 3mm, and 4mm.

[0010] This invention also proposes a structural orifice determination method based on the above-described composite standard parts, comprising the following steps:

[0011] Step 1: Based on Zhang Zhengyou's camera calibration algorithm, perform camera calibration on the checkerboard region of the target image to obtain the camera's internal parameters, as well as the camera's external parameters and distortion parameters corresponding to the target image.

[0012] Step 2: Place the composite standard part intersecting with the structured light to be calibrated, so that light stripes appear on the composite standard part where circular and line structured light intersect with the composite standard part respectively;

[0013] Step 3: While maintaining the intersection between the composite standard part and the structured light to be calibrated, use a camera to take pictures of the corresponding structured light stripes;

[0014] Step 4: Perform distortion correction on the structured light stripe image obtained in Step 3 using the distortion coefficients in the camera external parameters obtained in Step 1;

[0015] Step 5: Preprocess the structured light image after correction in Step 4 and extract the center of the light stripe;

[0016] Step 6: Based on the structured light image obtained in Step 5, the pixel coordinates of the center of the light stripe in the image can be obtained. Using the pixel coordinates of the center of the light stripe, the number of pixels corresponding to the height of the step in the structured light image and the number of pixels corresponding to the ring can be calculated respectively.

[0017] Step 7: Based on the number of pixels corresponding to the step height and the radius of the ring in the structured light image obtained in Step 6, perform least squares fitting with the actual size of the composite standard part to achieve optical plane calibration of line structured light and circular structured light.

[0018] in,

[0019] In step 1, the camera's internal parameters are intrinsic parameter matrices, the camera's external parameters are rotation and translation matrices, and the camera's distortion parameters are radial distortion and tangential distortion.

[0020] In step 5, the Steger algorithm is used to extract the center of the light stripe.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] As can be seen from the technical solution provided by the present invention, while maintaining the intersection of the composite standard and the structured light to be calibrated, the calibration tool can move back and forth at will, which is simple to operate and does not require a precision displacement device. Moreover, the devices and structures involved are not complicated and are easy to implement. In addition, since no additional devices such as precision moving devices, electronic theodolites, and auxiliary cameras are required, the experimental cost required to realize the whole solution is also low. Attached Figure Description

[0023] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0024] Figure 1 This is a schematic diagram of the overall structure of the line / circle structured light measurement and calibration system of the present invention;

[0025] Figure 2 for Figure 1 Side view;

[0026] Figure 3 for Figure 1 A three-dimensional schematic diagram of the composite standard component of the centerline / circle structured light measurement calibration system;

[0027] Figure 4 for Figure 3 Top view;

[0028] Figure 5 A flowchart of a structural calibration method based on composite standard parts provided in an embodiment of the present invention. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will now be clearly and completely described in conjunction with the accompanying drawings.

[0030] Combined with appendix Figure 1 , Figure 2 As shown, this embodiment of the invention provides a composite standard part suitable for the calibration of a line / circle structured light measurement system. The line / circle structured light measurement system includes a camera 4 mounted on a frame, a line structured light generator 2 mounted on the frame, and a circular structured light generator 3. The line / circle structured light measurement system is a measurement system using existing technology. Other structures are not described in detail in this embodiment. The following focuses on the composite standard part 1 for the calibration of the line / circle structured light measurement system, which is the core contribution of this invention.

[0031] Combined with appendix Figure 3 Appendix Figure 4 As shown, the composite standard component 1 for calibration of structured light measurement systems provided by the present invention includes a base plate, wherein a serrated annular gauge 1.2 is hollowed out in the center of the base plate. The annular gauge is used for calibration of circular structured light. The annular gauge 1.2 has the following shape: the diameter of the central hollowed-out ring is the smallest, and the diameters of the hollowed-out rings on both sides increase sequentially in equal steps. The diameter of the first hollowed-out ring of the annular gauge 1.2 is 48mm, and the diameters of the hollowed-out rings on both sides that increase sequentially in equal steps are 52mm, 56mm, 60mm, 64mm, 68mm, 72mm, 76mm, 80mm, 84mm and 88mm, respectively, for a total of 11 hollowed-out rings.

[0032] The base plate is symmetrically provided with calibration steps 1.1 on both sides of the ring gauge 1.2. The calibration steps 1.1 have a central highest step, and the steps decrease in height sequentially from the highest step to both sides. The height of each calibration step is 2mm, 2.5mm, 3mm, and 4mm from low to high. The calibration steps are used for line structure calibration.

[0033] This invention also proposes a structural orifice determination method based on the above-described composite standard parts, combined with the attached... Figure 5 The following instructions are provided, including the following steps:

[0034] Step 1: Based on Zhang Zhengyou's camera calibration algorithm, perform camera calibration on the checkerboard region of the target image to obtain the camera's intrinsic parameters (intrinsic parameter matrix), and the camera's extrinsic parameters (rotation matrix and translation matrix) and distortion parameters (radial distortion and tangential distortion) corresponding to the target image.

[0035] Step 2: Place the composite standard part intersecting with the structured light to be calibrated, so that light stripes appear on the composite standard part where circular and line structured light intersect with the composite standard part respectively;

[0036] Step 3: While maintaining the intersection between the composite standard part and the structured light to be calibrated, use a camera to take pictures of the corresponding structured light stripes;

[0037] In actual operation, the structured light plane intersects with the composite standard part. The structured light plane illuminates the composite standard part, producing a bright circular structured light stripe and two stepped line structured light stripes. This avoids interference between the structured light curve and the light on the surface of the composite standard part, which is beneficial for extraction.

[0038] Step 4: Perform distortion correction on the structured light stripe image obtained in Step 3 using the distortion coefficients (radial distortion and tangential distortion) in the camera external parameters obtained in Step 1;

[0039] Step 5: Perform preprocessing (binarization, ROI, etc.) and light stripe center extraction on the structured light image after correction in Step 4 (such as Steger algorithm).

[0040] Step 6: Based on the structured light image obtained in Step 5, the pixel coordinates of the center of the light stripe in the image can be obtained. Using the pixel coordinates of the center of the light stripe, the number of pixels corresponding to the height of the step in the structured light image and the number of pixels corresponding to the ring can be calculated respectively.

[0041] Step 7: Based on the number of pixels corresponding to the step height and the radius of the ring in the structured light image obtained in Step 6, perform least squares fitting with the actual size of the composite standard part to achieve optical plane calibration of line structured light and circular structured light.

[0042] Normally, calibration can be completed with just one image. In this embodiment, only one image is needed to complete the calibration, thus improving calibration efficiency. Traditional line structured light calibration requires multiple photos, and circular structured light calibration also requires multiple acquired images. However, the composite standard studied in this paper only requires one image to achieve optical plane calibration of both line structured light and circular structured light.

[0043] The above-described scheme of the present invention allows the calibration tool to move freely back and forth while maintaining the intersection between the composite standard and the structured light to be calibrated. The operation is simple, does not require a precision displacement device, and the involved devices and structures are not complex and easy to implement. In addition, since no additional devices such as precision moving devices, electronic theodolites, and auxiliary cameras are required, the experimental cost required to implement the entire scheme is also low.

[0044] Through the above description of the embodiments, those skilled in the art can clearly understand that the above embodiments can be implemented through a software plus hardware platform. Based on this understanding, the technical solutions of the above embodiments can be embodied in the form of a software product, which can be stored on an NVIDIA development board, making the entire system constitute a product and more conveniently executing the methods described in the various embodiments of the present invention.

[0045] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A structural orifice determination method based on composite standard parts, characterized in that, Includes the following steps, Step 1: Based on Zhang Zhengyou's camera calibration algorithm, perform camera calibration on the checkerboard region of the target image to obtain the camera's internal parameters, as well as the camera's external parameters and distortion parameters corresponding to the target image. Step 2: Place the composite standard part intersecting with the structured light to be calibrated, so that light stripes appear on the composite standard part where circular and line structured light intersect with the composite standard part respectively; The composite standard component includes a base plate with a serrated annular gauge hollowed out in the center. The annular gauge has the following shape: the diameter of the central hollowed-out ring is the smallest, and the diameters of the hollowed-out rings on both sides increase sequentially in equal increments. The annular gauge is used for the calibration of circular structured light. The base plate is symmetrically provided with calibration steps on both sides of the ring gauge. The calibration steps are arranged with the highest step in the middle and the steps decreasing in equal order from the highest step to both sides. The calibration steps are used for line structure cursor calibration. Step 3: While maintaining the intersection between the composite standard part and the structured light to be calibrated, use a camera to take pictures of the corresponding structured light stripes; Step 4: Perform distortion correction on the structured light stripe image obtained in Step 3 using the distortion coefficients in the camera external parameters obtained in Step 1; Step 5: Preprocess the structured light image after correction in Step 4 and extract the center of the light stripe; Step 6: Based on the structured light image obtained in Step 5, the pixel coordinates of the center of the light stripe in the image can be obtained. Using the pixel coordinates of the center of the light stripe, the number of pixels corresponding to the height of the step in the structured light image and the number of pixels corresponding to the ring can be calculated respectively. Step 7: Based on the number of pixels corresponding to the step height and the radius of the ring in the structured light image obtained in Step 6, perform least squares fitting with the actual size of the composite standard part to achieve optical plane calibration of line structured light and circular structured light.

2. The structural calibration method based on composite standard parts according to claim 1, characterized in that, In step 1, the camera's internal parameters are intrinsic parameter matrices, the camera's external parameters are rotation and translation matrices, and the camera's distortion parameters are radial distortion and tangential distortion.

3. The structural calibration method based on composite standard parts according to claim 1, characterized in that, In step 5, the Steger algorithm is used to extract the center of the light stripe.

4. The structural calibration method based on composite standard parts according to claim 1, characterized in that, The diameter of the first hollow ring of the ring gauge is 48mm, and the diameters of the hollow rings on both sides are 52mm, 56mm, 60mm, 64mm, 68mm, 72mm, 76mm, 80mm, 84mm and 88mm respectively, for a total of 11 hollow rings.

5. The structural calibration method based on composite standard parts according to claim 1, characterized in that, The height of each step in the calibration steps, from lowest to highest, is 2mm, 2.5mm, 3mm, and 4mm.

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