Study Of Key Technologies On Line-Scan Automatic Optical Inspection

With the development of industrialization and informatization, the demand of the modern manufacturing industry for AOI(Automatic Optical Inspection) technology has been rising rapidly. The Line-scan AOI is very important for the inspection of products which are large size and high precision. In FPD, precision electronics, semiconductor, glass, textile, papermaking and other high-tech or basic manufacturing industries, the Line-scan AOI has become the key equipment. As the fast development of Chinese manufacturing industry in recent years, the localization of Line-scan AOI is very important for "Made in China". However, the localization is still of a low level, since we started late and the foundation is poor. To shorten the distance with technological leaders, the thorough and systematic research work on the key techniques of the Line-scan AOI are necessary.Combining with practical projects background, some key issues of the Line-scan AOI are discussed in this dissertation, and some new approaches are proposed.1. The system structure and key technologiesThe Line-scan AOI system structure is introduced, and the key technologies in every sub-system are analyzed.2. Alignment, adjustment and calibration in optical-mechanical sub-systemThe optical-mechanical structures and parameter model of the Line-scan AOI system with single camera are analyzed. The methods of alignment and adjustment for each degree of freedom are proposed, and the flow of adjustment and calibration is analyzed and designed. In accordance with the methods, the "shaft encoder disk AOI" is developed, which embeds 2.7μm optical resolution. Based on the research of single-camera system, the methods of alignment, adjustment and calibration in multiple-cameras system are proposed. In accordance with the methods, the "furniture plate hole AOI" is developed, which embeds 80μm optical resolution and gains pixel-level inspection accuracy.3. The optimization technology of motion controlThe problems of color deviation and zigzag shape on boundary in images are analyzed, which are discovered in the line-scan system for flexible products. The problems are resolved after the improvement of motion control method, and the quality of images has been improved greatly. The methods of closed loop control mode, velocity curve design and error analysis are proposed for the AOI systems with rigid line-scan mechanism. In accordance with the methods, the "LCD/TP ITO AOI" system is developed, which embeds 2-3μm optical resolution and gains sub-pixel-level inspection accuracy.4. The quality improvement of image acquisitionBased on the research of tri-line CCD camera hardware design, a new correction method through caching three-channel data separately is proposed, which satisfies the high requirements of image correction in large format line-scan system. The method which combines white balance and flat-field correction is proposed, and is used in image correction for "PCB Automatic Visual Inspector", a color line-scan machine vision system.5. The parallel processing methods to accelerate large scale image processingThe image acquisition structure and the buffer method are designed, to satisfied line-scan AOI systems which need high resolution and high processing speed. The parallel processing method by multi-core CPU is researched, and the acceleration experiment of the Ostu algorithm by this method has been done. As a result, the linear feature of this method has been certificated. The parallel processing method by GPU is researched, and the acceleration experiment of the Ostu algorithm by this method has also been done. The mode of sharing graphics memory has been proved to be effective. The experimental result of B/W PCB line image processing by the opening algorithm on FPGA platform shows that the processing speed could be accelerated remarkably.From the five aspects mentioned above, some important problems of the Line-scan AOI engineer application are resolved. And the transformation of AOI equipments from theory to application could be strongly accelerated by our research results.