Principles and technical points of the printing quality inspection system
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
Machine vision is to replace the human eye with a video camera and replace the human brain with a computer. The working principle of machine vision for printing and packaging quality inspection is also very simple. The image of the printed product is captured (acquired) by the camera, compared with the standard image (template) of the printed product in the computer, if the difference is found and exceeds the set tolerance The range is judged as a non-conforming product. Of course, the actual machine vision system is much more complicated.
A typical visual inspection system consists of an illumination source, an optical lens, a camera, an image signal processing and an actuator. A good lighting source and lighting system is one of the keys to the success of the whole system. It plays a very important role. It is not simply to illuminate objects. Firstly, the illumination source should be able to truly reproduce the color of the print, and it can not cause the color distortion. This requires the illumination spectrum of the light source to be close to daylight, and at the same time ensure sufficient brightness and uniform brightness throughout the illumination area; secondly, the light source and The design of the lighting system should avoid some special areas with high reflectivity on the printed products, such as bronzing, laser anti-counterfeiting marks, etc., which cause the collected images to overflow, resulting in reduced detection accuracy or no detection at all; finally, in some applications, It is necessary to consider the cooperation of the light source and the illumination scheme as much as possible to highlight the feature quantity of the object and increase the contrast. In short, when selecting the light source and designing the lighting system, it is necessary to determine the geometric shape, illumination brightness, uniformity, spectral characteristics of the light source, etc. according to the specific conditions of the printed product, and of course, also consider the luminous efficiency and use of the light source. 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 distinguish objects, affecting the contrast of the image and the extent of the sharp image (depth of field). The merits of these characteristics are limited by the aberration of the lens itself, and the lens's excellent or bad aberration correction 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 object being photographed, while the computer processes the electrical information of the object being photographed. The role of the camera is to complete the conversion of the optical information to the electrical information. In cameras, the key device is a chip called a vision sensor. The cells (pixels) like small pools are arranged neatly on the chip, but they are not water, but electric charges. The light carrying the optical information of the object reaches the surface of the chip through the lens, and the light information becomes a charge signal (exposure). The place of the light intensity is rich, and the charge of the weak place is less, corresponding to the brightness 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 predetermined order (scanning mode), amplified, and otherwise 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-making of the system. From the application principle of the camera, we can simply summarize the performance requirements. First, there must be enough dynamic range, the bright and dark parts of the print can be accurately reflected; second, there must be enough resolution (sufficient number of pixels), the tiny details on the prints are Can be clearly reflected; third, to be able to truly reproduce the color of the print; Fourth, the speed of the camera read (line or frame rate) should match the speed of the print, to ensure sufficient accuracy and 100% collection. Of course, there are other performance indicators for the camera, which are not listed here.
The cameras used for printing inspection are divided into two categories according to the actual situation, line array cameras and area array cameras. A line camera can only obtain one line of information 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, so it is very suitable for image detection of an object (printed product) moving at a constant speed at a constant speed. The area camera can get the entire image information 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 key and difficult point in the development of machine vision systems. “Changing the difference compared to the template”, the working principle of machine vision is simpler, but the real implementation is very complicated. The processing of the entire image signal usually includes:
(1) Pretreatment
(2) Feature extraction and recognition
(3) Measurement. When a high-speed moving print passes through the camera, compared with the template image, it tends to move, rotate, and sometimes even flutter, causing the captured image to be distorted; the instability of the light source and the influence of the external environment make the collected The brightness of the image is uneven and the color changes; other external devices, power supply and acquisition devices themselves introduce certain interference and noise. To this end, in the process of installing the vision system, on the one hand to eliminate these unfavorable factors, on the other hand, through image pre-processing to eliminate and dilute their effects.
In summary, machine vision is a combination of light, machine, electricity, and computer technology, which is extremely suitable for repeatability measurement, inspection and identification in mass production.
2, the application of machine vision in the printing industry
2.1 Online detection and offline detection
The visual inspection system can be divided into an online detection system and an offline detection system according to the carrier on which it is installed.
The on-line inspection system is installed on printing equipment such as offset printing machines, gravure printing machines, flexo printing machines, and printing machines to detect printing quality in real time. The print can be a single sheet of paper or a roll of paper. The visual detection part of the online detection system and the offline detection system are basically the same in function, and the most important difference lies in the processing method of the defective product. When a defect is found, the on-line inspection system usually records the location of the defect (length: web, number of sheets: sheet), or uses a printer, labeler, marking machine to mark the paper, number Detect the error number or the number of the adjacent sheet.
Off-line inspection systems are usually installed on inspection machines or winders. In addition to the position recording and marking functions of the online inspection system, the inspection machine usually has a binning function, which automatically assigns good and bad products to different bins according to the detection results.
2.2 Quality Analysis, Tracking and Management
The use of testing equipment for quality testing provides real-time alarms and detailed and comprehensive analysis reports throughout the 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 of the high-quality testing equipment required by customers, not only do they stay in the good or bad of the printed matter, but also require post-analysis capabilities. Some quality testing equipment can not only improve the qualified rate of finished products, but also help manufacturers improve the process, establish a quality management system, and achieve a long-term stable quality standard.
3. What can machine vision bring to us?
Obviously, although China is already a printing power, China is not a printing power. We have to go to the world, in addition to the use of advanced printing equipment and testing equipment, but also to improve management. Management is productivity, efficiency is to management, and cost is to management. Of course, all this 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 stage. However, the superiority of the inspection system in quality inspection will bring long-term benefits to the company in terms of quality and labor costs.
Visual quality inspection can be applied in the printing and packaging industry before, during and after printing, but most applications today focus on post-print quality inspections. There are tens of thousands of printing companies in China, and there are very few real-use detection systems. In addition to special industries such as printing money and tickets, it is mainly in the packaging industry. The packaging industry has higher requirements for printing, and the requirements for quality and color are special. Packaging and printing plants must have unique capacity advantages, technical advantages and quality advantages in order to meet customer requirements for printing, cycle and quality. The emergence of a tobacco-printing company that provides printing services specifically for the tobacco industry is an example.
To promote machine vision inspection, we must first make printing and packaging companies understand machine vision. Where to install, how to install, how to achieve the best results, and what specific benefits can be brought about, all of which require constant popularization and publicity. Practitioners in the printing and packaging industry should propose their own specific needs, learn and master the concepts and operations of machine vision inspection; practitioners in the machine vision industry should understand the printing process, production process, operation methods and quality standards, only the two are close In combination, an automated visual inspection system that truly conforms to China's national conditions can be achieved in order to improve quality, improve efficiency, and reduce costs.