Application of machine vision technology in the qu

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Application of machine vision technology in packaging and printing quality inspection

machine vision is to use machines instead of human eyes to measure and judge. Machine vision system refers to converting the captured object into image signals through machine vision products (i.e. image pickup devices, divided into CMOS and CCD), which are transmitted to a special image processing system, and then converted into digital signals according to pixel distribution, brightness, color and other information; The image system performs various operations on these signals to extract the characteristics of the target, and then controls the on-site equipment action according to the discrimination results

the characteristic of machine vision system is to improve the flexibility and automation of production. In some dangerous working environments that are not suitable for manual operation or where artificial vision is difficult to meet the requirements, machine vision is often used to replace artificial vision; At the same time, in the process of mass industrial production, the efficiency of using artificial vision to check the product quality is low and the accuracy is not high. Using machine vision detection method can greatly improve the production efficiency and the degree of automation of production. And machine vision is easy to realize information integration, which is the basic technology to realize computer integrated manufacturing

a typical industrial machine vision application system includes the following parts: light source, lens, CCD camera, image processing unit (or image capture card), image processing software, monitor, communication/I/O unit, etc. First, the camera is used to obtain the image signal of the measured target, and then it is converted into a digital signal through a/D conversion and transmitted to the special image processing system. According to the pixel distribution, brightness, color and other information, various operations are carried out to extract the characteristics of the target, and then the judgment results are output according to the preset judgment criteria to control the driving actuator for corresponding processing. Machine vision is a comprehensive technology, including digital image processing technology, mechanical engineering technology, control technology, light source lighting technology, optical imaging technology, sensor technology, analog and digital video technology, computer software and hardware technology, man-machine interface technology, etc Machine vision emphasizes practicality and requires to be able to adapt to the harsh environment of industrial sites. It should have reasonable cost performance, universal industrial interfaces, high fault tolerance and security, and strong versatility and portability It emphasizes real-time and requires high speed and precision

the output of the vision system is not the image video signal, but the detection result after operation and processing, such as size data. After the upper computer such as PC and PLC obtains the detection results in real time, it commands the motion system or i/o system to perform corresponding control actions, such as positioning and sorting. From the operating environment of the visual system, it can be divided into (2) raw materials, which can be divided into PC based system and PLC Ba, and some adopt ordinary belt sed system. The PC based system makes use of its openness, high programming flexibility and good windows interface. At the same time, the overall cost of the system is low. Taking data transmission company of the United States as an example, the system contains high-performance image capture card, which can generally connect multiple lenses. In terms of supporting software, there are several levels from low to high, such as DLL for c/c++ programming under windows95/98/nt environment, visual control ActiveX provides graphical programming environment under VB and vc++, and even object-oriented machine vision configuration software under windows. Users can use it to quickly develop complex and advanced applications. In the system based on PLC, the function of vision is more like an intelligent sensor. The image processing unit is independent of the system and exchanges data with PLC through serial bus and i/o. The system hardware generally uses high-speed ASIC or embedded computer for image processing, and the system software is solidified in the image processor. The menu displayed in the monitor is configured through a simple device similar to the game keyboard, or the software is developed on PC and then downloaded. The system based on PLC embodies the characteristics of high reliability, integration, miniaturization, high speed and low cost. The representative manufacturers are Panasonic of Japan and Siemens of Germany

Siemens of Germany has accumulated more than 20 years of experience in industrial image processing. SIMATIC video is the first high-performance monochrome and color image processing system, and has become a very important product in SIMATIC automation system. SIMATIC vs710, launched in 1999, is the first intelligent, integrated, distributed gray-scale industrial vision system with PROFIBUS interface in the industry. It integrates image processor, CCD and i/o into a small chassis, and provides PROFIBUS connection mode (communication rate up to 12mbps) or integrated i/o and RS232 interfaces. More importantly, through the configuration of Pro vision parametric software under PC Windows, vs 710 combines the flexibility of PC, the reliability of PLC, distributed network technology and integrated design for the first time, making Siemens find a perfect balance between PC and PLC systems. The application of machine vision system in printing the packaging of more than 800 pieces of works exhibited in this lantern exhibition

1, automatic print quality detection

the detection system used by automatic print quality detection equipment mostly uses high-definition, high-speed camera lens to shoot standard images, and sets certain standards on this basis; Then take the detected image, and then compare the two. The CCD linear sensor converts the light change of each pixel into an electronic signal. After comparison, as long as the detected image is different from the standard image, the system considers the detected image as unqualified. All kinds of errors in the printing process are only the difference between the standard image and the detected image for the computer, such as stains, ink dot color difference and other defects

the first technology used to detect the quality of printed matter is the gray comparison between the standard image and the detected image. Now the more advanced technology is the comparison based on RGB three primary colors. What is the difference between automatic machine detection and human eye detection? Take human vision as an example. When we focus on a printed matter, if the contrast color of the printed matter is relatively strong, the smallest defect that can be found by human eyes is the defect with obvious contrast color and no less than 0.3mm; However, it is difficult to maintain a sustained and stable visual effect by relying on human ability. However, in another case, if you are looking for defects in prints of the same color system, especially quality defects in a light color system, the defects that human eyes can find need to have at least 20 gray level differences. The automatic machine can easily find the defect with the size of 0.10mm, even if the defect is only one gray level different from the standard image

but in practical use, even the same panchromatic contrast system has different ability to distinguish color differences. Some systems can find defects with large changes in contour and color difference, while others can identify extremely small defects. For white cardboard and some simple style prints, such as the Kent cigarette label in Japan and the Marlboro cigarette label in the United States, it may be enough to simply turn off the oil return valve for detection, while most domestic prints, especially various labels, have many characteristics, with too many flash elements, such as gold and silver cardboard, hot stamping, embossing or polishing prints, This requires that the quality inspection equipment must have enough ability to find a very small gray level difference, perhaps five gray level differences, or a more stringent one. This is crucial to the domestic label market

the accuracy of the comparison between the standard image and the image of the inspected printed matter is the key problem of the detection equipment. Usually, the detection equipment collects the image through the lens. In the middle part of the lens range, the image is very clear, but the image of the edge part may produce phantom, and the detection result of the phantom part will directly affect the accuracy of the whole detection. From this point of view, if only the comparison of the whole area is not suitable for some fine prints. If the obtained image can be subdivided again, for example, the image can be divided into 1024dpi x 4096dpi or 2048dpi x 4096dpi, the detection accuracy will be greatly improved, and at the same time, because the phantom of the edge part is avoided, the detection result will be more stable

using testing equipment for quality testing can provide real-time reports and detailed and perfect analysis reports of the whole process of testing. The on-site operator can rely on the timely alarm of the full-automatic detection equipment and adjust the problems in the work in time according to the real-time analysis report. Perhaps the reduction will not only be one percentage point of the scrap rate. The manager can track the production process according to the analysis report of the detection results, which is more conducive to the management of production technology. Because the high-quality testing equipment required by customers is not only to detect the good and bad of printed matter, but also to have the ability of post analysis. What some quality testing equipment can do can not only improve the qualified rate of finished products, but also help manufacturers improve their process flow, establish a quality management system, and achieve a long-term stable quality standard

(to be continued)

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