Machine vision is to use camera instead of human eyes and computer to replace human brain. The working principle of machine vision for printing and packaging quality inspection is also very simple. The image of printed matter is captured by camera and compared with the standard image (template) of the printed matter in the computer. If the difference is found and exceeds the set tolerance range, it is determined as unqualified product. Of course, the actual machine vision system is much more complex. The common visual inspection system is composed of lighting source, optical lens, camera, image signal processing and executing mechanism. A good lighting source and lighting system is one of the key to the success or failure of the whole system. It plays a very important role. It is not simply lighting objects. First of all, the lighting source should be able to truly reproduce the color of the printed matter without causing color distortion, which requires that the luminous spectrum of the light source is close to the sunlight, and at the same time, it should ensure that there is sufficient brightness and uniform brightness in the entire lighting area; secondly, the design of the light source and lighting system should avoid some special areas with high reflectivity, such as gilding, laser anti-counterfeiting signs, etc Finally, in some applications, the combination of light source and lighting scheme should be considered, and the object feature quantity should be highlighted as much as possible to increase the contrast. In a word, when choosing the light source and designing the lighting system, it is necessary to determine the geometric shape, illumination brightness, uniformity and spectral characteristics of the light source according to the specific conditions of the printed matter. Of course, the luminous efficiency and service life of the light source should also be considered. An optical lens is equivalent to the lens of the human eye. The quality of optical lens will affect the ability of vision system to distinguish objects, the contrast of image and the range of clear imaging (depth of field). The advantages and disadvantages of these characteristics are limited by the aberration of the lens itself. The quality of the lens is determined by whether the aberration correction is good or not. The common aberrations are spherical aberration, coma aberration, astigmatism, field curvature, distortion and chromatic aberration. The lens passes through the optical information of the object to be photographed, while the electrical information of the object is processed in the computer. The function of the camera is to complete the conversion from optical information to electrical information. In the camera, the key device is a chip called vision sensor. On the chip, there are cells (pixels) that look like small pools of water, but they hold electric charges instead of water. The light carrying the optical information of the object to be photographed reaches the surface of the chip through the lens, and the light information becomes a charge signal (exposure). There are more electric charges in the places with light intensity and less charges in the places with weak light, corresponding to the light and shade degree of different areas on the printed matter. Different colors are decomposed into three basic colors, namely red, green and blue, and stored in different pixels. The charge generated by exposure is read out in the specified order (scanning mode), amplified and processed, and finally output from the camera. That's how the camera works. High quality image information is the original basis for correct judgment and decision-making of the system. From the application principle of the camera, we can simply summarize the requirements of its performance. First, there should be enough dynamic range to accurately reflect the bright and dark parts of the printed matter; second, enough resolution (enough number of pixels) should be able to clearly reflect the tiny details on the printed matter; third, it should be able to truly reproduce the color of the printed matter; fourth, the speed of the camera reading (line frequency or frame rate) should match the printing speed ,Ensure enough accuracy and 100% acquisition. Of course, the camera has other performance indicators, which will not be listed here. According to the actual situation, the cameras used in printing inspection can be divided into two categories: linear array camera and area array camera. The linear array camera can only get one line of information of the image at a time, and the object to be photographed must pass in front of the camera in the form of a straight line in order to obtain a complete image. Therefore, it is very suitable for the image detection of objects (printed matter) moving at a certain speed. And the area array camera can get the information of the whole image at a time, like the digital cameras we use every day. Image signal processing is the core of machine vision system, which is equivalent to human brain. How to process and calculate the image is the key and difficult point in the development of machine vision system. "Compared with the template, find out the difference", the working principle of machine vision is relatively simple, but the real implementation is very complex. The whole image signal processing usually includes: (1) preprocessing, (2) feature extraction and recognition, (3) measurement. When the high-speed moving print passes through the camera, compared with the template image, it often causes the position movement, rotation, and sometimes even floating, which makes the collected image deformed and distorted; the instability of the light source and the influence of the external environment make the collected image brightness uneven and color change; other external equipment, power supply and acquisition equipment itself will also introduce certain Interference and noise. Therefore, in the process of installing the vision system, on the one hand, we should eliminate these unfavorable factors, on the other hand, we should eliminate and weaken their influence through image preprocessing. To sum up, machine vision is a combination of light, machine, electricity and computer technology, which is very suitable for the measurement, inspection and identification of repeatability in the process of mass production. 2. The application of machine vision in the printing and packaging industry 2.1 online inspection and offline inspection visual inspection system can be divided into online detection system and offline detection system according to the carrier installed. The online detection system is installed on offset press, gravure press, flexo press, code printing machine and other printing equipment to detect the printing quality in real time. Print can be sheet or web. The visual inspection part of online inspection system and offline inspection system is basically the same in function, and the main difference lies in the processing method of defective products. When a defect is found, the online detection system usually records the location of the defect (length: web paper, number of sheets: single sheet), or mark the edge of the paper with inkjet printer, labeling machine and marking machine. The number detection records the error number or the number of adjacent sheets. The off-line detection system is usually installed on the inspection machine or rewinder. In addition to the function of automatic detection of good and bad products, the system can automatically detect the bad products according to different positions. 2.2 quality analysis, tracking and management using testing equipment for quality detection can provide real-time alarm and detailed and perfect analysis report in the whole process of detection. The field operator can adjust the problems in the work according to the real-time alarm and analysis report of the automatic detection equipment. Managers can track the production process according to the analysis report of test results, which is more conducive to the management of production technology. Because customers require, high-quality testing equipment, not only stay in the detection of good and bad printed matter, but also require the ability to analyze after the event. Some quality inspection equipment can not only improve the qualified rate of finished products, but also help manufacturers to improve the process flow and establish quality management system to achieve a long-term and stable quality standard.3. What can machine vision bring us? Obviously, although China is already a big printing country, China is not a printing power. We want to go to the world, in addition to using advanced printing equipment and testing equipment, but also to improve the management level. Management is productivity. We need efficiency from management and cost from management. Of course, all these should be guaranteed under the premise of the same quality and service. The installation of automatic visual inspection system requires a certain amount of investment in the early stage. However, the advantages of inspection system in quality inspection will bring long-term benefits to enterprises in terms of quality and labor cost. The application of visual quality inspection in printing and packaging industry can be in pre press, printing process and post press, but at present, most applications focus on post press quality inspection. There are tens of thousands of printing enterprises in China, and few of them actually use the detection system. In addition to the special industries such as banknote printing and ticket printing, they are mainly in the packaging industry. The packaging industry has higher requirements for printing, and has special requirements for quality and color. Packaging printing plant must have unique production capacity advantages, technical advantages and quality advantages in order to meet the requirements of customers for printing capacity, cycle and quality. The emergence of tobacco printing enterprises that provide printing services specifically for the tobacco industry is an example. In order to promote machine vision inspection, printing and packaging enterprises should know more about machine vision. Where to install it, how to install it, how to achieve the best effect, and what specific benefits can be brought, all of which need to be popularized and publicized constantly. The practitioners of printing and packaging industry should put forward their own specific needs, learn and master the concept and operation of machine vision inspection; the practitioners of machine vision industry should understand the printing process, production process, operation mode and quality standards. Only by the close combination of the two, can we have an automatic visual inspection system which is really in line with China's national conditions, and improve the quality and quality The purpose of efficiency and cost reduction.
Dongguan Beite packaging materials Co., Ltd. - located in Dongguan City, the world's manufacturing city, is a subsidiary of Beite packaging (China) Co., Ltd., with a plant area of 6000 square meters. It is a flexible packaging enterprise integrating R & D, production and sales. It has a complete set of domestic advanced production equipment, a scientific and humanized management system and a group of experienced technical personnel For backing, to provide customers with quality products and first-class service.
The company produces composite bags, roll film, fruit bags, aluminum foil bags, suction mouth bags, self-supporting bags, zipper bags, anti-static bags, special-shaped bags, etc., which are suitable for the internal and external packaging of food, daily chemicals, electronics, toys, medicine, cosmetics, industrial products and other industries.
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