Principles and technical points of printing quality inspection

Machine vision replaces the human eye with a video camera and replaces the human brain with a computer. The working principle of machine vision for printing and packaging quality inspection is also very simple. A camera captures (collects) the image of the product and compares it with the standard image (template) of the product in the computer. If a difference is detected and the set tolerance is exceeded The range, that is, it is determined to be a non-conforming product. Of course, the actual machine vision system is much more complicated.

Daheng's image quality inspection products have been widely adopted in China. Now we will discuss the detection principle and technical points of Daheng's image quality inspection products.

A typical visual inspection system consists of an illumination source, an optical lens, a camera, and an 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 plays a very important role. It is not simply to illuminate objects. First of all, the lighting source should be able to truly reproduce the color of the printed product, and it cannot cause color distortion. This requires that the light source's emission spectrum be close to daylight, and at the same time, it should ensure sufficient brightness and uniform brightness throughout the entire lighting area; followed by the light source and The design of the lighting system should avoid some special areas of high reflectivity on the printed products, such as: gold foil, laser anti-counterfeit signs, etc. overflow the collected images, resulting in reduced detection accuracy or simply can not be detected; Finally, in some applications, It is necessary to consider that the cooperation of the light source and the lighting scheme should highlight the feature quantity of the object as much as possible and increase the contrast. In short, when choosing the light source and designing the lighting system, it is necessary to determine the geometry, lighting brightness, uniformity, and spectral characteristics of the light source according to the specific conditions of the printed product. 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 affects the ability of the vision system to resolve objects, affecting the contrast of the image and the range of clear imaging (depth of field). However, the pros and cons of these characteristics are limited by the aberration of the lens itself. The quality of aberration corrected by the lens determines the imaging quality of the lens. Common aberrations include spherical aberration, coma aberration, astigmatism, field curvature, distortion and color difference.

The lens passes the optical information of the object being photographed, and the computer processes the electrical information of the object being photographed. The role of the camera is to convert the optical information into electrical information. In the camera, the key device is a chip called a vision sensor. Neatly arranged on the chip are units (pixels) like small pools, but they are not filled with water, but electric charges. The light that carries the optical information of the subject reaches the surface of the chip through the lens. The light information becomes the charge signal (exposure). The light has strong local charge, and the weak light has less charge, which corresponds to the lightness and darkness of different areas on the print. Different colors are decomposed into three primary colors of red, green and blue, which are stored in different pixels. The charge generated by the exposure is sequentially read out in a prescribed order (scanning mode), amplified and processed, and finally output from the camera. This is how the camera works.

High quality image information is the original basis for the correct judgment and decision of the system. From the application principle of the camera, we can simply summarize the requirements for its performance. First, there must be sufficient dynamic range. The bright and dark parts of the prints can be accurately reflected. Second, there must be sufficient resolution (enough pixels). The tiny details on the prints are all Can be clearly reflected; Third, to be able to truly reproduce the color of the print; Fourth, the speed of the camera readout (line frequency or frame rate) to match the speed of the print product, to ensure adequate accuracy and 100% collection. Of course, there are other performance indicators for the camera, which are not listed here.

The cameras used for the detection of printed products are divided into two categories according to the actual situation: line array cameras and area array cameras. The line camera can only obtain one line of information of an image at a time, and 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) that move at a constant speed and at a uniform speed. The area array camera can obtain the information of the whole image once, like the digital camera we use everyday.

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 key and difficult point in the development of machine vision systems. "Compared to the template, to find out the difference", the working principle of machine vision is relatively simple to say, but it is very complicated to implement. The processing of the entire image signal usually includes: (1) preprocessing, (2) feature extraction and identification, and (3) measurement. When a high-speed moving print passes through the camera, compared with the template image, it often causes the position to move, rotate, and sometimes even flutter, so that the captured image is distorted; the light source is unstable and the external environment influences the collected image. The brightness of the image is uneven and the color changes; other external devices, power supplies, and acquisition devices themselves introduce some interference and noise. For this reason, in the process of installing the vision system, on the one hand, these unfavorable factors should be eliminated, and on the other hand, the influence of the image preprocessing should be eliminated and diluted.

In summary, machine vision is a combination of optical, mechanical, electrical, and computer technologies that is ideally suited for repeatable measurement, inspection, and identification in mass production.

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 on-line inspection system and an off-line inspection system according to the carriers it installs.
The on-line inspection system is installed on printing equipment such as offset printing presses, gravure printing presses, flexographic printing presses, and code printing presses to detect print quality in real time. Prints can be either sheet or web. The visual inspection part of the on-line inspection system and off-line 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 inspection system usually records the location of the defect (length: web, number of sheets: single sheet), or uses a printer, labeler, marking machine to mark the edge of the paper, number Check the record error number or the number of adjacent sheets.

Off-line inspection systems are usually installed on inspection machines or rewinders. In addition to the position detection and marking functions of the online inspection system, the inspection machine usually has a binning function and automatically assigns good and bad products to different bins based on the detection results.

2.2 Quality Analysis, Tracking and Management

The use of inspection equipment for quality inspection provides real-time alarms and detailed and complete analysis reports for the entire 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 follow the analysis report of the test results to track the production process, which is more conducive to the management of production technology. Because of the high-quality testing equipment required by customers, not only does it stop at the quality of the printed matter, but it also requires the ability to analyze afterwards. Some quality inspection equipment can not only improve the qualification rate of finished products, but also help manufacturers to improve the process and establish a quality management system to achieve a long-term stable quality standards.

3 What can machine vision bring us

Obviously, although China is already a big printing country, China is not yet a printing power. We must go to the world, in addition to the use of advanced printing equipment and testing equipment, but also to improve the management level. Management is productivity, management needs efficiency, and management requires costs. Of course, all these must be guaranteed under the same quality and service. The installation of an automated visual inspection system requires a certain amount of investment in the early stages. However, the advantages of the inspection system in quality inspection will give companies long-term benefits in quality and labor costs.

The application of visual quality inspection in the printing and packaging industry can be prepress, during printing and after printing, but most of the current applications focus on post-print quality inspection. There are tens of thousands of printing companies in China, and few really use inspection systems. In addition to special industries such as banknote printing and ticketing, they are mainly in the packaging industry. The packaging industry has high requirements for printing, and the requirements for quality and color are rather special. Packaging and printing plants must have unique production advantages, technological advantages, and quality advantages to meet customer requirements for print volume, cycle, and quality. The emergence of cigarette printing companies that provide printing services specifically for the tobacco industry is an example.

To promote machine vision inspection, we must first make printing and packaging companies more aware of machine vision. Where it is installed, how it is installed, how best to achieve results, and what specific benefits it can bring, all of this needs to be continuously popularized and promoted. The practitioners of the printing and packaging industry have to put forward their own specific needs, learn and master the concept and operation of machine vision inspection; practitioners in the machine vision industry must understand the printing process, production process, operation methods and quality standards, only two closely In order to achieve the goal of improving quality, improving efficiency, and reducing costs, it is possible to have an automated visual inspection system that truly meets China's national conditions.

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