| With the great progress of the electronics manufacturing industry,printed circuit boards(PCBs)are the basic core components of electronic products.Traditional testing methods for PCB can no longer meet increasingly stringent quality requirements and the needs of automated and efficient production.The Automatic Optic Inspection(AOI)method based on machine vision technology has the advantages of non-contact,highspeed,and automatic.Among them,the 3D AOI technology using structured light and other three-dimensional imaging methods can quickly and accurately obtain the threedimensional information of the PCB surface,which has gradually become a new trend of quality inspection in the PCB manufacturing process.For the application scenarios of PCB quality inspection,This paper mainly aims at the problems of typical structured light 3D imaging system,such as small height measurement range and large calibration error.A telecentric tilted 3D structured light imaging system is built,which uses a telecentric lens and Scheimpflug tilt-shift projector to enlarge the common range of depth of field.The calibration algorithm of the corresponding imaging model is developed,and the system's global parameters is optimized based on Bundle Adjustment.Finally,The 3D reconstruction method of the telecentric structured light system is proposed,which can quickly obtain high-precision3 D imaging results.The main contents are listed as follows:(1)Aiming at the particularity of the telecentric lens imaging model,a telecentric model calibration method based on the invariance of external parameters is proposed,focusing on solving the problem of parameter ambiguity in the telecentric model.The calibration results are analyzed and compared through experiments,which proves the feasibility of the telecentric model calibration method proposed in this paper.(2)Aiming at the eccentricity error problem of the circular target used in the system,the iterative eccentricity error correction method of the control point is proposed,and the experiment proves that it can effectively reduce the reprojection error of the control point.In this paper,the Scheimpflug pinhole model of the twodimensional rotation angle parameter is used to describe the imaging process of the tilted projector in the system,and the corresponding calibration method is proposed,reducing the coupling influence between the parameters through parameter separation and optimization.(3)Aiming at the problem of poor correlation of system components' pose parameters,the system calibration and global parameter optimization are carried out by introducing the beam adjustment(Bundle Adjustment).The test shows that the system calibration and imaging errors can be effectively reduced.Then a threedimensional reconstruction method of the telecentric FPP system is proposed.The position of the optical center is determined by the virtual camera plane,and the problem of parameter degradation of the telecentric model is solved.(4)In response to the real-time requirements of PCB online inspection,this paper designs the system software imaging algorithm framework based on the CUDA programming model and GPU parallel computing.Tests show that the system can perform 3D imaging operations with 5 million points in only 0.2s,which basically meets real-time requirements for PCB online inspection.The telecentric tilted 3D structured light imaging system designed in this paper is mainly aimed at PCB quality inspection scenes with small field of view and high precision requirements.Tests and analysis show that the system in this paper can reach a height measurement range of 8mm within a field of view of 30mm*30mm,The proposed calibration algorithm and three-dimensional reconstruction method can ensure that the measurement error within this range is less than 10 um.The systems can achieve a relatively large measurement range and high measurement accuracy to meet the requirements of PCB online inspection. |
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