Machine vision is to replace human eyes with cameras and human brains with computers. The working principle of machine vision for printing and packaging quality detection is also very simple. Use the camera to capture (collect) the image of the printed product and compare it with the standard image (template) of the printed product in the computer. Scope, that is determined to be unqualified products. Of course, the actual machine vision system is much more complicated.
Daheng Image () printing quality inspection products are widely used in China. Now we will discuss the inspection principles and technical points in conjunction with Daheng Image (Printing Quality Inspection) products.
The usual visual inspection system consists of the illumination light source, optical lens, camera, image signal processing and execution mechanism. A good lighting source and lighting system is one of the keys to the success or failure of the entire system, and it plays a very important role. It is not simply to illuminate objects. First of all, the illumination light source must be able to reproduce the color of the printed product without causing color distortion. This requires the light emission spectrum of the light source to be close to daylight. At the same time, it should ensure sufficient brightness and uniform brightness in the entire illumination area; The design of the lighting system should avoid some special areas with high reflectivity on the printed products, such as: hot stamping, laser anti-counterfeiting signs, etc., which will overflow the collected images, resulting in reduced detection accuracy or no detection at all; It is necessary to consider the cooperation of the light source and the lighting scheme as much as possible to highlight the characteristics of the object and increase the contrast. In short, when selecting a light source and designing a lighting system, the geometry, brightness, uniformity, and spectral characteristics of the light source should be determined according to the specific conditions of the printed matter. Of course, the luminous efficiency and use of the light source must also be considered life.
The optical lens is equivalent to the lens of the human eye. The quality of the optical lens will affect the ability of the visual system to distinguish objects, affect the contrast of the image and the range of clear imaging (depth of field). The quality of these characteristics is limited by the aberration of the lens itself. Whether the lens is correct for aberration determines the imaging quality of the lens. Common aberrations include spherical aberration, coma, astigmatism, field curvature, distortion, and chromatic aberration.
The lens passes the optical information of the photographed object, and the computer processes the electrical information of the photographed object. The role of the camera is to complete the conversion of optical information to electrical information. In cameras, the key device is a chip called a vision sensor. The cells (pixels) like small pools are neatly arranged on the chip, but they do not hold water, but charge. The light carrying the optical information of the photographed object reaches the surface of the chip through the lens, and the optical information becomes a charge signal (exposure), where there is more charge in the light intensity and less charge in the weak area, which corresponds to the brightness of different areas on the printed product; Different colors are decomposed into three primary colors of red, green and blue and stored in different pixels. The charges generated by the exposure are read out in sequence in the prescribed order (scanning mode), after amplification and other processing, and finally output from the camera. This is how the camera works.
High-quality image information is the original basis for the system to make correct judgments and decisions. From the application principle of the camera, we can simply summarize the performance requirements. First, there must be sufficient dynamic range, and the bright and dark parts of the printed product can be accurately reflected; second, there must be sufficient resolution (enough number of pixels), and the minute details on the printed product Can be clearly reflected; third, to be able to truly reproduce the color of the printed product; fourth, the speed of the camera's readout (line frequency or frame rate) should match the speed of the printed product to ensure sufficient accuracy and 100% collection. Of course, the camera has other performance indicators, which are not listed here.
The cameras used for printed matter detection are divided into two categories according to the actual situation, linear array cameras and area array cameras. Linear array cameras can only obtain one line of image information at a time. The object to be photographed must pass in front of the camera in a straight line to obtain a complete image. Therefore, it is very suitable for image detection of objects (prints) moving at a constant speed at a certain speed. The area array camera can obtain the information of the entire image at once, like the digital camera we use every day.
The processing of image signals is the core of the machine vision system, which is equivalent to the human brain. How to process and calculate images is the focus and difficulty of machine vision system development. "Compared with the template, find the difference", the working principle of machine vision is relatively simple, but it is actually very complicated to implement. The processing of the entire image signal usually includes: (1) preprocessing, (2) feature extraction and recognition, and (3) measurement. When the high-speed moving print passes through the camera, compared with the template image, it often produces positional movement, rotation, and sometimes even flutter, which deforms and distorts the collected image; the instability of the light source and the influence of the external environment The brightness of the image is uneven, and the color changes; other external devices, power supplies, and acquisition devices themselves can also introduce some interference and noise. For this reason, in the process of installing the vision system, on the one hand, we must eliminate these unfavorable factors, on the other hand, we must eliminate and dilute their influence through image preprocessing.
In summary, machine vision is a combination of light, machine, electricity, and computer technology, which is very suitable for repeatable measurement, inspection, and identification in the mass production process.
2 Application of machine vision in the printing and packaging industry
2.1 Online detection and offline detection
The visual inspection system can be divided into an online inspection system and an offline inspection system according to the carrier on which it is installed.
The online detection system is installed on printing equipment such as offset presses, gravure presses, flexo presses, code printers, etc., to detect print quality in real time. The printed product can be a single sheet of paper or roll paper. The visual inspection part of the online inspection system and the offline inspection system are basically the same in function, and the main difference is the processing method of the defective product. When a defect is found, the online inspection system usually records the location of the defect (length: roll paper, number of sheets: single sheet), or uses a printer, labeling machine, marking machine to make marks on the edge of the paper, number Detect and record the wrong number or the number of adjacent sheets.
The offline inspection system is usually installed on the inspection machine or rewinding machine. In addition to the position recording and marking functions of the online inspection system, the inspection machine usually has a warehouse function, which automatically distributes good and bad products into different warehouses according to the inspection results.
2.2 Quality analysis, tracking and management
The use of testing equipment for quality testing can provide real-time alarms and detailed and complete analysis reports throughout the testing process. The on-site operator can adjust the problems in the work according to the real-time alarm and analysis report of the automatic detection equipment. The manager can track the production process based on the analysis report of the test results, which is more conducive to the management of production technology. Because what customers demand, high-quality testing equipment is not only to detect the quality of the printed matter, but also requires the ability to analyze afterwards. Some quality testing equipment can not only improve the pass rate of finished products, but also assist manufacturers to improve the process flow, establish a quality management system, and reach a long-term stable quality standard.
3 What machine vision can bring us
Obviously, although China is already a printing power, China is not yet a printing power. We want to go to the world, in addition to using advanced printing equipment and testing equipment, we must also improve management. Management is productivity. It requires efficiency from management and costs from management. Of course, all this must be guaranteed on the premise of the same quality and service. The installation of an automated visual inspection system requires a certain investment in the early stage. However, the advantages of the inspection system in quality inspection will bring long-term benefits to the company in terms of quality and labor costs.
The application of visual quality inspection in the printing and packaging industry can be in pre-press, during and after printing, but most applications are currently focused on post-press quality inspection. There are tens of thousands of printing companies in China, and few of them actually use detection systems. Apart from special industries such as banknote printing and ticketing, they are mainly in the packaging industry. The packaging industry has high requirements for printing and special requirements for quality and color. Packaging and printing plants must have unique productivity advantages, technical advantages and quality advantages in order to meet customer requirements for print volume, cycle and quality. One example is the emergence of cigarette printing companies that specifically provide printing services for the tobacco industry.
To promote machine vision inspection, we must first enable printing and packaging companies to understand more about machine vision. Where to install it, how to install it, how to achieve the best results, and what specific benefits it can bring, all this needs constant popularization and publicity. Practitioners in the printing and packaging industry should put forward their specific needs, learn and master the concept and operation of machine vision inspection; practitioners in the machine vision industry should understand the printing process, production process, operation method and quality standards, only the two are closely related Only by combining can we have an automated visual inspection system that truly meets China's national conditions, and can we achieve the goals of improving quality, improving efficiency, and reducing costs.
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