Application of machine vision in printing quality inspection
1. Working principle of machine vision inspection system
1. The meaning of machine vision
Machine Vision (Machine Vision), as the name suggests, combines the ability to automatically control a machine with visual sensing. Visually speaking, people perceive the changes in the external world through the eyes, then analyze and make judgments through the brain, and finally complete the actions by the hands and feet. Machine vision is to complete this process through automatic control equipment with computing capabilities. Specifically, machine vision uses a camera to replace the human eye and a computer to replace the human brain for measurement and judgment. Machine Vision System (Machine Vision System) refers to the machine vision products (that is, image pickup device, divided into two kinds of CMOS and CCD) to convert the captured target into an image signal, and send it to a dedicated image processing system. According to the pixel distribution and brightness, Color and other information are converted into digital signals; the image system performs various operations on these signals to extract the characteristics of the target, such as: area, length, number, position, etc .; and then outputs the results according to the preset tolerance and other conditions, such as: Size, angle, offset, number, pass / fail, yes / no, etc .; finally, control the equipment actions on site according to the result of the discrimination.
Machine vision technology is an interdisciplinary field involving artificial intelligence, neurobiology, psychophysics, computer science, image processing, pattern recognition and many other fields. It is characterized by fast speed, high precision, high degree of automation and intelligence, non-contact, strong objectivity, high reliability, large amount of information, many functions, and high production efficiency. It is precisely because of the characteristics of machine vision, so in the modern automated production process, people use machine vision systems widely in the field of condition monitoring, finished product inspection and quality control, and are widely used in food and beverage, cosmetics, pharmaceuticals, building materials And in chemical industry, metal processing, electronics manufacturing, packaging and printing, automobile manufacturing and other industries.
Machine vision is used for quality inspection in the printing and packaging industries. Its basic working principle is to capture (collect) the image on the printed matter with a camera, and compare it with the standard image (template) of the printed matter in the computer. If a difference is found And beyond the set tolerance range, it is judged as unqualified product. The actual machine vision system is much more complicated.
2. Structure and working principle of visual inspection system
The visual inspection system is mainly composed of four parts: illumination light source, optical lens, camera, image information processing and execution mechanism.
â‘ Light source and lighting system
Illumination light source and lighting system is one of the key components of the entire visual inspection system, and 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, and at the same time, ensure that there is sufficient brightness and the brightness in the entire illumination area Uniform; secondly, the design of the light source and the lighting system should avoid some special areas with high reflectivity on the printed matter, 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; In some applications, it should also be considered that the cooperation of the light source and the lighting scheme should highlight the object feature as much as possible, and the obvious difference between the part that needs to be detected and the unimportant parts should be generated as much as possible to increase the contrast. At the same time, it should also ensure sufficient overall brightness, and the change of the object position should not affect the quality of imaging. Transmitted light and reflected light are generally used in machine vision applications. For the reflected light, the relative position of the light source and the optical lens, the texture of the object surface, the geometric shape of the object, the background and other factors should be fully considered. In short, when selecting a light source and designing a lighting system, the geometry, brightness, uniformity, spectral characteristics, luminous efficiency, and service life of the required light source should be determined according to the specific conditions of the printed product.
â‘¡Optical lens
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 inspection system to distinguish objects, affect the contrast of the image and the range of clear imaging. The imaging quality of a lens, that is, whether it is good or not for aberration correction, can be measured by the aberration size. Common aberrations include spherical aberration, coma, astigmatism, field curvature, distortion, and chromatic aberration. .
â‘¢Camera
We know that the optical lens passes the optical information of the object being photographed, and the computer processes the electrical information of the object being photographed. Then the camera is the device that completes the conversion from optical information to electrical information.
A. Working principle
In cameras, the key device is a chip called a vision sensor. On the chip, cells like small pools are arranged neatly, 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, that is, exposure. Where there is more light, there is more charge, and where there is less light, there is 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, and stored in different units. The charge signals generated by the exposure are sequentially processed through amplification and analog-to-digital conversion in the scanning order, and finally output from the camera.
B. Classification
The cameras used for printing quality detection can be divided into two categories according to the actual situation, linear array cameras and area array cameras. The linear array camera can only obtain one line of information of the 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, so it is very suitable for image detection of objects moving at a certain speed and uniform speed. The area array camera can obtain the information of the entire image at a time, so the camera speed is faster. For example, the digital cameras we use every day.
The photoelectric conversion device is the core device constituting the camera. At present, typical photoelectric conversion devices mainly include CCD and CMOS image sensors. CCD is currently the most commonly used image sensor for machine vision. It integrates photoelectric conversion, charge storage, charge transfer, and signal reading, and is a typical solid-state imaging device. The outstanding feature of CCD is that charge is used as a signal, but unlike its device, which uses current or voltage as a signal. This type of imaging device forms a charge packet through photoelectric conversion, and then transfers and amplifies the output image signal under the action of a driving pulse. A typical CCD camera is composed of an optical lens, a timing and synchronization signal generator, a vertical driver, and an analog / digital signal processing circuit. The development of CMOS (Complementary Metal Oxide Semiconductor) image sensors first appeared in the early 1970s. In the early 1990s, with the development of VLSI manufacturing technology, CMOS image sensors developed rapidly. The CMOS image sensor integrates the photosensitive element array, the image signal amplifier, the signal reading circuit, the analog-to-digital conversion circuit, the image signal processor, and the controller on a single chip, and also has the advantage of random access for local pixel programming. At present, CMOS image sensors are widely used for their good integration, low power consumption, wide dynamic range, and almost no smearing of output images. [next]
C. Requirements for camera inspection of printed matter
It can be seen from the working principle of the above camera that if the system can correctly make judgments and decisions, it must require: a. First, the quality of the original image information should be high; b. The camera must have sufficient dynamic range, that is, on the printed product The bright and dark parts can be accurately reflected; c, the camera must have sufficient resolution, that is, the minute details on the printed product can be clearly reproduced; d, the camera should be able to truly reproduce the color of the printed product; e. The reading speed of the camera should be consistent with the advancing speed of the printed product to ensure sufficient accuracy and 100% collection.
â‘£Image information processing and execution mechanism
The processing and execution mechanism of image information, namely the computer, is the core of the machine vision system, which is equivalent to the human brain. The entire image information processing process usually includes:
A. Pretreatment. When the high-speed moving print passes the camera, compared with the template image, it often produces positional movement, rotation, and sometimes even flutter, which deforms the collected image. The instability of the light source and the influence of the external environment make the The brightness of the collected image is uneven, and the color changes; other external equipment, power supply, and the collection equipment itself will also introduce certain interference and noise. To this end, 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. Common image preprocessing operations mainly include image enhancement, data encoding and transmission, smoothing, edge sharpening, segmentation and so on. After these processes, the quality of the image has been improved to a considerable extent, which not only improves the visual effect of the image, but also facilitates the analysis, processing and recognition of the image by the computer.
B. Feature extraction and recognition;
C. Measurement. In a simple sentence, it is "find the difference compared to the template".
In summary, machine vision is a technology that integrates light, machine, electricity, and computer, and is very suitable for repeatable measurement, inspection, and identification in the mass production process.
Second, the application of machine vision in the printing and packaging industry
1. Online detection and offline detection
The visual inspection system can be divided into two types: an online inspection system and an offline inspection system according to the carrier it is installed on.
The online inspection system is installed on printing equipment such as offset presses, gravure presses, flexo presses, and code printers, and is used 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 difference is mainly in the way of handling unqualified products. When a defect is found, the online inspection system usually records the location of the defect (roll paper: length, single sheet: number of sheets), or uses a printer, labeling machine, marking machine to mark on the edge of the paper , Number detection records 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 function of the online inspection system, the inspection machine usually also has a warehouse function, that is, the good and bad products are automatically assigned to different warehouses according to the inspection results.
2. Quality analysis, tracking and management
In addition to providing real-time alarms and detailed and comprehensive analysis reports for the entire inspection process, on-site operators can also perform real-time alarms and analysis reports for fully automated inspection systems to respond to problems that arise during work. Adjustment. And managers can also track the production process based on the analysis report of the test results, which is more conducive to the management of production technology. In other words, 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 achieve a long-term stable quality standard.
Third, the promotion and popularization of machine vision inspection in the printing and packaging industry
At present, although China is a big printing country, it is not yet a printing power. If we want to go to the world, we must use advanced printing equipment and testing equipment. Although installing an automated visual inspection system requires a certain amount of investment in the early stage, its advantages 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 pre-press, printing process and post-press processing, but most of the current applications are mainly focused on post-press quality inspection. There are tens of thousands of printing companies in China, but few of them actually use detection systems. Apart from special industries such as printing banknotes and tickets, they are mainly in the packaging industry. The packaging industry has high requirements for printing and special requirements for quality and color. This requires that packaging and printing plants must have unique capacity advantages, technical advantages and quality advantages in order to meet the comprehensive requirements of customers on printing volume, cycle and quality.
To promote machine vision inspection, we must first enable printing and packaging companies to learn more about machine vision. Where to install, how to install, how to achieve the best effect, what specific benefits can bring, all these need to be constantly popularized and publicized. 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 methods and quality standards, only the two are closely combined In order to produce an automated visual quality inspection system that truly meets China's national conditions, we can achieve the fundamental purpose of improving quality, improving efficiency, and reducing costs.
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