Field calibration method and system for SMT 3D AOI equipment

Abstract

The invention discloses a field calibration method and system for SMT 3D AOI equipment, and relates to the surface mounting technology, and the method comprises the steps: feeding a calibration board into the SMT 3D AOI equipment according to the same board feeding mode as a common PCB, and carrying out the initialization; multiple groups of horizontal and vertical grating images are acquired; performing phase calculation on the grating image acquired by the camera by using a phase shift method to obtain a plurality of groups of horizontal and vertical phase diagrams; performing image processing on the calibration plate image acquired by the camera, calculating circle center coordinates of all the circular patterns, and calculating corresponding point coordinates of the circle center coordinates on the imaging plane of each projector; calculating coordinate values of all circle centers on the calibration plate in a world coordinate system according to the adopted Z-axis height and the distance between the circular patterns of the calibration plate; repeatedly calculating world coordinates; and determining optimized system parameters according to the projection process of conversion from the coordinate values of the world coordinate system to the camera pixel coordinate system. According to the invention, the internal parameters and the external parameters of the equipment can be quickly optimized, and the precision of the imaging system is recovered to the initial state.

Description

Technical Field

[0001] The present application relates to surface mount technology and data processing technology, and in particular to an on-site calibration method and system for SMT 3D AOI equipment. Background Art

[0002] In the SMT (surface mount technology) scenario, it is necessary to inspect the components of the chip components on the PCB (printed circuit board). 3D AOI (automatic optical inspection) equipment is an automated inspection equipment for quality control in the PCB manufacturing process. The 3D imaging system in the 3D AOI equipment usually consists of four projectors and cameras. The camera is placed downward at the center, and the four projectors are placed at similar horizontal heights in four directions around the camera, projecting downward and inward, so that the five devices of the projector and camera form a common field of view at the center. The camera is usually equipped with a telecentric lens, and the projector uses an ordinary industrial lens and adopts the Sham projection method, so that the focal planes of the four projectors coincide with the focal plane of the camera.

[0003] Cameras and projectors typically use structured light 3D reconstruction technology to reconstruct 3D point clouds. This requires system calibration of the camera and four projectors to determine the device's intrinsic and extrinsic parameters. Due to the high accuracy requirements for calibration in SMT 3D AOI equipment, the system calibration process is relatively complex. If the camera or any of the projectors experiences mechanical changes due to external factors such as vibration or temperature fluctuations, the quality and accuracy of the 3D reconstructed point cloud will be affected, thereby impacting the accuracy of AOI inspection. Recalibration can address this issue, but the process is complex and time-consuming. Summary of the Invention

[0004] Embodiments of the present application provide an on-site calibration method and system for SMT 3D AOI equipment. When the accuracy of the SMT 3D AOI equipment imaging system changes and no longer meets the inspection requirements, the internal and external parameters of the equipment are quickly optimized to restore the accuracy of the imaging system to its initial state.

[0005] The present application provides an on-site calibration method for SMT 3D AOI equipment. The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors. The camera is placed at a central position, the lens is mounted on the front end of the camera, and the projectors are arranged in multiple specified directions around the camera, symmetrically about the camera optical axis. The projector optical axis and the camera optical axis are arranged at an angle. The on-site calibration method includes:

[0006] Feed the calibration board into the SMT 3D AOI equipment in the same way as ordinary PCBs and initialize it. The calibration board has a regular shape and circular patterns arranged regularly on its surface.

[0007] Acquire the calibration plate image, and control the projector to sequentially project a set of horizontal and vertical phase-shifted structured light stripe grating images onto the calibration plate surface, and the camera sequentially acquires the images to obtain multiple sets of horizontal and vertical grating images;

[0008] The phase shift method is used to calculate the phase of the grating image captured by the camera to obtain multiple sets of horizontal and vertical phase images;

[0009] Perform image processing on the calibration plate image captured by the camera to calculate the center coordinates of all circular patterns, as well as the coordinates of the corresponding points on the imaging plane of each projector;

[0010] Calculate the coordinates of all the circle centers on the calibration plate in the world coordinate system based on the Z-axis height used and the spacing between the circular patterns on the calibration plate.

[0011] Move the imaging module along the Z axis to K different heights z k , repeatedly calculate to obtain K groups of camera pixel coordinates of the circle center, K groups of corresponding point pixel coordinates of the projector and world coordinates;

[0012] The optimized system parameters are determined according to the projection process of transforming the coordinate values ​​of the world coordinate system into the camera pixel coordinate system.

[0013] An embodiment of the present application also provides an on-site calibration system for SMT 3D AOI equipment. The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors, wherein the camera is placed at a central position, the lens is installed at the front end of the camera, and the projectors are arranged in multiple specified directions around the camera, symmetrically about the camera optical axis, and the projector optical axis and the camera optical axis are arranged at an angle. The on-site calibration system includes a processor and a memory, and the memory stores a computer program. When the computer program is executed by the processor, it implements the steps of the on-site calibration method for SMT 3D AOI equipment as described above.

[0014] The method of the embodiment of the present application can quickly optimize the internal and external parameters of the equipment and restore the accuracy of the imaging system to its initial state after the accuracy of the SMT 3D AOI equipment imaging system changes and no longer meets the detection requirements.

[0015] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference symbols are used throughout the drawings to represent the same components. In the drawings:

[0017] Figure 1 Schematic diagram of the 3D AOI equipment imaging system according to an embodiment of the present application;

[0018] Figure 2 The following is a schematic diagram of the basic process of the on-site calibration method of the SMT 3D AOI equipment in an embodiment of the present application.

[0019] Figure 3 SMT 3D AOI equipment for this application embodiment DETAILED DESCRIPTION

[0020] Exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0021] An embodiment of the present application provides an on-site calibration method for SMT 3D AOI equipment. The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors, wherein the camera is placed in a central position, the lens is installed at the front end of the camera, and the projectors are arranged in multiple specified directions around the camera, symmetrically about the camera optical axis, and the projector optical axis is set at an angle to the camera optical axis.

[0022] In the following embodiments of this application, a 3D AOI equipment imaging system including four projectors is used as an example for illustration. Figure 1 As shown, the camera is placed in the center, and the lens is a telecentric lens, which is installed in the front end of the camera. The projectors are placed in four directions around the camera, and are centrally symmetrical about the camera optical axis. The projector optical axis and the camera optical axis are set at an angle. The field of view of the camera and the four projectors coincide in front of the camera lens, and the field of view of the overlapping part is the measurement area. The four projectors all use the Sham projection method, so that the focal planes of the four projectors coincide with the focal plane of the camera. The projectors on the upper and lower sides of the camera are recorded as projector 1 and projector 2; the projectors on the left and right sides of the camera are recorded as projector 3 and projector 4. As shown Figure 2 As shown, the on-site calibration method described in the embodiment of the present application includes the following steps:

[0023] In step S201, the calibration board is fed into the SMT 3D AOI equipment in the same way as ordinary PCBs and initialized. The calibration board is of regular shape and has circular patterns arranged regularly on its surface. Figure 3 As shown in the specific example, the calibration plate is rectangular with a dark background, and has a light-colored circular pattern distributed in an array at equal intervals on the surface. The radius of the central circle is larger than that of the other circles to locate the center of the calibration plate, and the calibration plate is surrounded by a border. In more specific examples, the calibration plate is square with a dark background, and has white circular patterns distributed in an array at equal intervals on the surface. Alternatively, the calibration plate can be square with a light background, and has dark (e.g., black) circular patterns distributed in an array at equal intervals on the surface.

[0024] like Figure 3 As shown, the surface is patterned with 25 x 25 evenly spaced white circles. The center circle has a slightly larger radius than the others and is used to locate the center of the calibration plate. A white border surrounds the calibration plate for positioning and identification. The calibration plate covers the entire measurement area. Its thickness is consistent with that of a standard PCB.

[0025] In step S202, a calibration plate image is collected, and a projector is controlled to sequentially project a set of horizontal and vertical phase-shifted structured light stripe grating images onto the calibration plate surface, and the camera sequentially collects the images to obtain multiple sets of horizontal and vertical grating images.

[0026] In step S203, the phase of the grating image captured by the camera is calculated using the phase shift method to obtain multiple sets of horizontal and vertical phase maps. In the embodiment of the present application, four projectors are used as an example, and the camera captures an image of a calibration plate. Projectors 1-4 sequentially project a set of horizontal and vertical phase-shifted structured light stripe grating images onto the surface of the calibration plate, and the cameras sequentially capture them to obtain four sets of horizontal and vertical grating images. The phase shift method is used to calculate the phase of the grating image captured by the camera, and four sets of horizontal and vertical phase maps can be obtained. The phase shift method is a method of obtaining phase information by projecting multiple structured light images with phase differences.

[0027] In step S204, the calibration plate image captured by the camera is processed to calculate the center coordinates of all circular patterns, and the corresponding point coordinates of the center coordinates on the imaging plane of each projector are calculated. Specifically, the calibration plate image captured by the camera is processed and analyzed to identify and calculate the center coordinates of all circles. N is the total number of circles. Using the horizontal and vertical phase images obtained in step S103, the corresponding point coordinates of the circle center coordinates on the imaging plane of projectors 1-4 can be calculated respectively. (This example uses 25x25 circles, N=625).

[0028] In step S205 , the coordinate values ​​of all the circle centers on the calibration plate in the world coordinate system are calculated according to the adopted Z-axis height and the spacing between the circular patterns on the calibration plate.

[0029] In step S206, the imaging module is moved along the Z axis to K different heights z k , repeatedly calculate to obtain K groups of camera pixel coordinates of the circle center, K groups of corresponding point pixel coordinates of the projector and world coordinates. In a specific example, the imaging module is moved along the Z axis to K different heights z k , k=1,2,…,K. Repeat steps S202-S204 to obtain K groups of camera pixel coordinates of the circle center, pixel coordinates of the corresponding points of projectors 1-4, and world coordinates. The camera pixel coordinates of the circle center are recorded as The pixel coordinates of the corresponding point of the circle center on the imaging plane of projector 1-4 are The coordinate value of the circle center in the world coordinate system In a specific example, 3 different heights may be used, K=3.

[0030] In step S207 , the optimized system parameters are determined according to the projection process of converting the coordinate values ​​of the world coordinate system into the camera pixel coordinate system.

[0031] The method of the embodiment of the present application can quickly optimize the internal and external parameters of the equipment and restore the accuracy of the imaging system to its initial state after the accuracy of the SMT 3D AOI equipment imaging system changes and no longer meets the detection requirements.

[0032] In some embodiments, initialization includes: adjusting the Z-axis height of the imaging device to a height within any measurement range, adjusting the XY-axis position to align with the center of the calibration plate, and the field of view covers the entire calibration plate, and the calibration plate plane is perpendicular to the camera optical axis.

[0033] In some embodiments, calculating the coordinate values ​​of all circle centers on the calibration plate in the world coordinate system based on the Z-axis height and the spacing of the circular patterns on the calibration plate includes:

[0034] A world coordinate system is established based on the center position of the circle specified by the calibration plate and the specified height of the imaging device as the origin; the coordinate values ​​of all the center points of the circle on the calibration plate in the world coordinate system are calculated based on the currently used Z-axis height and the circle spacing of the calibration plate. For example, a world coordinate system is established with the XY position of the center point of the circle in the upper left corner of the calibration plate and the height of the imaging device Z=0 as the origin, and the coordinate values ​​of all the center points of the circle on the calibration plate in the world coordinate system are calculated based on the Z-axis height used in step S201 and the circle spacing of the calibration plate.

[0035] In some embodiments, determining optimized system parameters according to a projection process of transforming coordinate values ​​of a world coordinate system into a camera pixel coordinate system includes:

[0036] The projection process of the coordinate value of the world coordinate system to the camera pixel coordinate system is defined as The calculation process can be expressed as in are the camera telecentric lens model parameters, that is, the camera system parameters.

[0037] The projection process of the coordinate value of the world coordinate system to the projector pixel coordinate system is defined as where K p ,θ,K d , are the system parameters of the projector, namely the rotation matrix and translation vector.

[0038] In some embodiments, determining the optimized system parameters according to the projection process of converting the coordinate values ​​of the world coordinate system to the camera pixel coordinate system specifically includes:

[0039] The world coordinate system (x w ,y w ,z w ) is converted to the projector coordinate system (x p ,y p ,z p ), in and are the external parameters of the projector;

[0040] The projector coordinate system (x p ,y p ,z p ) is converted to the projector imaging plane coordinate (x n ,y n ), x n =x p / z p ,y n =y p / z p ;

[0041] Based on the projector imaging plane, add the projector lens distortion parameter K d =(k1,k2,p1,p2):

[0042]

[0043] Further add the Sham projection angle parameter θ:

[0044]

[0045] The distorted projector imaging plane coordinates (x s ,y s ) Using the projector internal parameters K p = Convert to pixel coordinates (u p ,v p ):

[0046]

[0047] Jointly optimize the system parameters of the camera and individual projectors.

[0048] In some embodiments, jointly optimizing system parameters of the camera and each projector includes:

[0049] The Levenberg-Marquardt nonlinear optimization algorithm is used to optimize the system parameters of the camera and each projector:

[0050]

[0051] The optimization initial values ​​use the existing camera and projector system parameters. If the calculated joint back-projection error is less than a certain threshold T, the optimization is completed. After the optimization is completed, the optimized system parameters replace the existing parameters.

[0052] Compared to existing solutions, the method of this application can simplify the calibration process of SMT 3D AOI equipment, making it possible to automatically complete the high-precision calibration process like PCB inspection. It does not require manual operation by the user and does not require the user to have relevant knowledge of machine vision.

[0053] The method of the present application can restore the accuracy of SMT 3D AOI equipment to the factory state within a few minutes, shorten the equipment offline maintenance time, and avoid the decline in AOI detection accuracy due to the decline in equipment accuracy.

[0054] An embodiment of the present application further provides an on-site calibration system for SMT 3D AOI equipment. The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors, wherein the camera is placed at a central position, the lens is mounted at the front end of the camera, and the projectors are arranged in multiple specified directions around the camera, symmetrically about the camera optical axis, and the projector optical axes are arranged at an angle to the camera optical axis. The on-site calibration system includes a processor and a memory, wherein the memory stores a computer program, and when the computer program is executed by the processor, the steps of the on-site calibration method for SMT 3D AOI equipment as described above are implemented.

[0055] It should be noted that, in the various embodiments of the present application, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the element.

[0056] The serial numbers of the above-mentioned embodiments of the present application are for description only and do not represent the advantages or disadvantages of the embodiments.

[0057] Through the description of the above implementation methods, those skilled in the art can clearly understand that the above-mentioned embodiment methods can be implemented by means of software plus the necessary general hardware platform, and of course can also be implemented by hardware, but in many cases the former is a better implementation method. Based on this understanding, the technical solution of the present application, or the part that contributes to the prior art, can be embodied in the form of a software product, which is stored in a storage medium (such as ROM / RAM, magnetic disk, optical disk), and includes a number of instructions for enabling a terminal (which can be a mobile phone, computer, server, air conditioner, or network device, etc.) to execute the methods described in each embodiment of the present application.

[0058] The embodiments of the present application are described above in conjunction with the accompanying drawings, but the present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of this application, ordinary technicians in this field can also make many forms without departing from the purpose of this application and the scope of protection of the claims, all of which are protected by this application.

Claims

1. A field calibration method for SMT 3D AOI equipment, characterized in that: The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors. The camera is placed in the center, the lens is installed in front of the camera, and the projectors are set in multiple specified directions around the camera, symmetrically about the camera optical axis. The projector optical axis and the camera optical axis are set at an angle. The on-site calibration method includes: Feed the calibration board into the SMT 3D AOI equipment in the same way as ordinary PCBs and initialize it. The calibration board has a regular shape and circular patterns arranged regularly on its surface. Acquire the calibration plate image, and control the projector to sequentially project a set of horizontal and vertical phase-shifted structured light stripe grating images onto the calibration plate surface, and the camera sequentially acquires the images to obtain multiple sets of horizontal and vertical grating images; The phase shift method is used to calculate the phase of the grating image captured by the camera to obtain multiple sets of horizontal and vertical phase images; Perform image processing on the calibration plate image captured by the camera to calculate the center coordinates of all circular patterns, as well as the coordinates of the corresponding points on the imaging plane of each projector; Calculate the coordinates of all the circle centers on the calibration plate in the world coordinate system based on the Z-axis height used and the spacing between the circular patterns on the calibration plate. Move the imaging module along the Z axis to K different heights z k , repeatedly calculate to obtain K groups of camera pixel coordinates of the circle center, K groups of corresponding point pixel coordinates of the projector and world coordinates; The optimized system parameters are determined according to the projection process of transforming the coordinate values ​​of the world coordinate system into the camera pixel coordinate system.

2. The on-site calibration method of the SMT 3D AOI equipment according to claim 1, characterized in that: The calibration plate is rectangular with a dark background. A light-colored circular pattern is distributed in an array at equal intervals on the surface. The radius of the central circle is larger than that of the other circles, which is used to locate the center of the calibration plate. There is a frame mark around the calibration plate.

3. The on-site calibration method of the SMT 3D AOI equipment according to claim 2, characterized in that: Initialization includes: Adjust the Z-axis height of the imaging device to any height within the measurement range, adjust the XY-axis position to align with the center of the calibration plate, and the field of view covers the entire calibration plate, and the calibration plate plane is perpendicular to the camera optical axis.

4. The on-site calibration method of the SMT 3D AOI equipment according to claim 1, wherein: Based on the Z-axis height used and the spacing between the circular patterns on the calibration plate, the coordinate values ​​of all the circle centers on the calibration plate in the world coordinate system are calculated as follows: Establishing a world coordinate system based on the center position specified by the calibration plate and the specified height of the imaging device as the origin; Based on the currently used Z-axis height and the circle spacing of the calibration plate, calculate the coordinate values ​​of all circle centers on the calibration plate in the world coordinate system.

5. The on-site calibration method of the SMT 3D AOI equipment according to claim 1, wherein: According to the projection process of the coordinate value of the world coordinate system to the camera pixel coordinate system, the optimized system parameters are determined as follows: The projection process of the coordinate value of the world coordinate system to the camera pixel coordinate system is defined as The calculation process can be expressed as in are the camera telecentric lens model parameters; The projection process of the coordinate value of the world coordinate system to the projector pixel coordinate system is defined as where K p ,θ,K d , are the system parameters of the projector.

6. The on-site calibration method of the SMT 3D AOI equipment according to claim 5, characterized in that: According to the projection process of the coordinate value of the world coordinate system to the camera pixel coordinate system, the optimized system parameters are determined to include: The world coordinate system (x w ,y w ,z w ) is converted to the projector coordinate system (x p ,y p ,z p ), in and are the external parameters of the projector; The projector coordinate system (x p ,y p ,z p ) is converted to the projector imaging plane coordinate (x n ,y n ), x n =x p / z p ,y n =y p / z p ; Add the projector lens distortion parameter K d =(k1,k2,p1,p2): Add the Sham projection angle parameter θ: The distorted projector imaging plane coordinates (x s ,y s )Use projector internal parameters Convert to pixel coordinates (u p ,v p ): Jointly optimize the system parameters of the camera and individual projectors.

7. The on-site calibration method of the SMT 3D AOI equipment according to claim 6, characterized in that: Jointly optimize the system parameters of the camera and each projector including: The Levenberg-Marquardt nonlinear optimization algorithm is used to optimize the system parameters of the camera and each projector: The optimization initial values ​​use the system parameters of the existing camera and projector. If the calculated joint back-projection error is less than a certain threshold T, the optimization is completed.

8. An on-site calibration system for SMT 3D AOI equipment, characterized in that: The 3D AOI equipment imaging system includes a camera, a lens, and multiple projectors, wherein the camera is placed in a central position, the lens is installed at the front end of the camera, and the projectors are arranged in multiple specified directions around the camera, symmetrically about the camera optical axis, and the projector optical axis and the camera optical axis are arranged at an angle. The on-site calibration system includes a processor and a memory, and the memory stores a computer program. When the computer program is executed by the processor, it implements the steps of the on-site calibration method of the SMT 3D AOI equipment according to any one of claims 1 to 7.

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