A New Online Inspection System for Printing Defects of Tobacco Marks

1 Overview

In recent years, the competition for domestic cigarette pack printing has become increasingly fierce, fine packs of cigarette products have emerged, cigarette pack design and printing processes have become more and more complex, and the materials used have become more and more sophisticated. Gravure, offset, flexo, screen printing, UV printing, UV glazing, holographic hot stamping, laser aluminum foil and other technologies have come into battle. A variety of printing technology combinations of cigarette packs can be seen everywhere. With the complication and diversification of the printing process, the requirements for inspection of finished products have become higher and higher. After defects occur in various processes (such as flying ink, knives, misprints, etc.), they eventually flow into the final inspection process. If all of them are manually completed, the workload is extremely high, and it is difficult to maintain long-term stability by relying on human vision testing. It is prone to fatigue and missed detection, and it will flow into the tobacco factory or the user's hands, causing quality accidents.

According to the repetitive principle of printing, the online defect detection system of printing defects continuously shoots the printing pattern through a high-speed camera, and compares it with an intact reference. When the difference between the two exceeds the set range, the inspection system determines the printing defect. Produce, save defect patterns and sound and light alarms, and control the labeling machine to label defective paper.

The earliest used for print quality inspection was the technology of grayscale comparison between the standard image and the detected image. Nowadays, the commonly used technology is based on RGB three primary colors for comparison. From the actual use, the factors that affect the detection ability are as follows.

1) Problems with printed materials

In addition to common white cardboard and coated paper, printed materials also have a large proportion of transfer paper (including gold and silver cardboard and laser paper). In addition to printing, there are also printed materials such as plain bronzing and holographic positioning bronzing. The strong reflection characteristics make it difficult to detect under ordinary lighting conditions, and the convex hull pattern also causes difficulty in detection due to the low color difference characteristics.

2) Paper snake running problem caused by equipment fluctuation

In the printing process, as the tension and speed fluctuate, the paper will have a serpentine running phenomenon during the advancement process, manifesting different degrees of stretching in the direction of motion, and different degrees of deviation in the width direction to the image. Collection and comparison make it difficult.

3) The problem of detection accuracy

The detection system based on the camera is based on the color information of the image. If the size of the defect or the color difference exceeds the observation range of the camera, this defect can not be detected theoretically, or it can not be trusted, how to make the detection accuracy and the quality standard of the enterprise. Consensus is the main problem faced by testing equipment vendors.

4) Post-detection processing issues

Testing is just a means of quality management. The purpose of testing is to guide production and eliminate the occurrence of continuous waste products in a timely manner. At the same time, it is also necessary to provide judgment basis for finished products. At present, most online and domestic inspection systems at home and abroad have not proposed a better solution.

5) On-line testing equipment installation station problem

When the conditions permit, on-line inspection equipment can be installed on all production equipment such as printing presses, hot stamping machines, slitting machines, etc. However, for most companies, choosing the right equipment at the most suitable location can reduce costs, Can improve equipment utilization.

6) Combination of test data and enterprise production quality management system

How to set up test data information through the network to establish a database within the enterprise to achieve data sharing, and then provide the correct basis for production management, quality control, is the main content of the test system data management.

These key issues are extremely important for printing companies in the selection of testing equipment. This paper studies a machine vision inspection system for on-line defect inspection of high-speed printing products. Through the unique optical system, the obvious defects are detected and marked, prompting the next process and user; and prompting crew members to promptly handle the continuous defects to avoid greater losses; through the C/S networked parallel structure, the image data Distributed processing and centralized statistics management.

2 system description

1) System composition

Print quality online detection system uses multiple color line array cameras to simultaneously capture large-format prints. Image data is processed through FPGA/DSP acquisition cards. Image processing, defect extraction, and classification are performed by the corresponding processing unit. Defect data passes through high-speed Ethernet. The network transmits to the server for statistics and management, outputs alarm signals and defect position information, and synchronizes with the production line through the photoelectric encoder.

2) Imaging Design

The hardware core component of the detection system is a CCD camera, which will affect the system's detection methods, detection capabilities, and subsequent image processing operations and data processing methods. Commonly used CCD cameras are divided into two types of area arrays and line arrays according to the imaging method.

The area camera takes a single exposure image of the subject and sends the entire image to the image processor for processing. The disadvantage of the area camera is that as the circumference of the plate becomes longer, the number of cameras required for shooting increases, and the use and maintenance are inconvenient.

Each exposure of the line camera performs data acquisition on only one line of the subject. Through the continuous motion of the object, the camera continuously exposes the image. Finally, multiple lines of data are combined to form an entire image. Linear array CCD cameras are widely used in surface inspection due to their small footprint and simple light source design.

The line scan operation of the linear CCD camera is very similar to a conventional scanner. The sensor in the camera scans a row of images each time a moving object passes through it, and then merges all the collected lines into a complete two through an image capture card. Dimensional image.

3) Lighting design

The requirements for the lighting system of the printed matter camera are: 1 Brightness enough; 2 Preventing glare from entering the camera; 3 No flicker; 4 Uniform wavelength distribution of the light source;

According to the above requirements, there are two kinds of light sources available, white LED light source and trichromatic fluorescent light source. Compared with incandescent tungsten light bulbs and fluorescent lamps, white LED light sources have the advantages of small size, low heat generation, low power consumption, and long service life. However, there is a problem of high maintenance costs; a rare earth tricolor straight tube fluorescent lamp is highly efficient. Energy-saving new light source, good color rendering, is a true daylight-type light source, has been widely used in television camera lighting, although life is less than the LED light source, but the low cost, easy maintenance, this system uses this type of light source.

The light source structure design uses four fluorescent lamps to be incident on the surface of the roller at high and low angles respectively. The low-angle light highlights the surface profile of the printed product, and the high-angle light compensates the overall brightness. In order to prevent the specular reflection light from entering the lens, diffuse transmission filtering is used for high-angle light.

4) Processor structure

In print production, the observation size of printed products is larger than 650mm, and the printing accuracy is very high (0.1mm/pixel). The single camera and single processor cannot complete the processing of huge data volume (above 100MBbyte/s), so the multi-camera is used. In the structure, synchronous and parallel processing is performed on different areas, and the processing result is transmitted to the server through the high-speed Ethernet for data management and statistics. The key problem that the system has to solve is the synchronization problem.

There are two types of synchronization problems. One is the synchronization of acquisition and processing, and the other is the synchronization of transmission of defective data. The synchronization of acquisition and processing is achieved by a pulse encoder, and each processor is triggered by a pulse coded signal at the same time. Synchronization of the print defect data upload on the same layout is guaranteed by the fixed timing generated by the pulse encoder.

3 System Features

1) Hot stamping and holographic trademark detection technology

Through unique laser holographic lighting technology, accurate extraction of bronzing and holographic trademark features can be achieved, and printing defects can be detected at the same time, as shown in FIG. 3 .

2) Image Precise Positioning Technology

Independently developed a signal positioning technology called ONE, which can automatically track the serpentine motion of printed paper. This technology is the basis for the stable operation of the detection system.

3) Sub-area detection technology

Different areas can be set with different detection accuracy, and the detection of a certain area can also be shielded.

4) Automatic adjustment technology for detection accuracy

According to the fluctuating state of the printing press, the detection accuracy can be automatically adjusted to reach the optimal detection effect.

5) Forecasting Technology of Defect Trend

Arbitrarily set the defect alarm threshold and the labeler's action threshold, forecast the occurrence of defects, and promptly identify major defects.

6) High-speed image processing technology

Using self-developed multi-threaded processing algorithm, it can work stably for a long time at a printing speed of 300m/min.

7) Management of historical data

Can print, count, display, query historical defect data; save time can reach any number of years as needed.

8) Multi-screen display technology

In addition to the control cabinet, an optional monitor is installed at the key work station to display the operation interface, defect data, and real-time images, respectively, so that the operator can easily check the quality status and system operation status at any time.

9) Friendly Human-Machine Interface

After the operator sets the parameters, it can automatically detect the start, stop, change roll and equipment fluctuation status, and no manual intervention is required from the beginning to the end of printing. Different defects can be set to alert the operator with different alarm sounds.

4 Use effect

In this study, two horizontal linear array CCD cameras with a horizontal resolution of 4K and a vertical resolution of 15K were used to detect the 820-mm-wide print layout, parallel processing and distributed control via Gigabit Ethernet, and client/server method. Conduct centralized data query and management.

1) Algorithm efficiency

In this system, the image processing part is all written using MMX/SSE optimization instruction set, which realizes the parallel processing of single instruction and multiple data. When the speed of the production line is the largest (300m/min), the detection system captures a complete version of the image with a period of 211ms. The processing time (128ms) is sufficient to meet the requirements of real-time performance, and the maximum printing speed of 500m/min can be met.

2) Detection accuracy

The detection accuracy of this system depends on the detection resolution and detection level.

CCD is a discrete sampling device. According to the Nyquist Sampling Theorem, the minimum defect size can be detected at more than 2 times the detection resolution. For example, when the above camera is used to observe a 410mm web, the lateral pixel resolution is achieved at a printing speed of 200m/min. The rate is 0.1mm, the longitudinal resolution is 0.22mm, and the smallest defect that can detect stable detection is 0.2mm x 0.44mm;

The detection level is an important function of this system. Since the strictness of each position detection requirement of printed products is different, for example, the barcode area is the most stringent, and the glue area or the cutting area is the most loose, it is unrealistic to use the same level for all areas, which will cause many unnecessary alarms. Therefore, the setting of the region level actually uses different thresholds for different regions. These thresholds are set in advance according to the requirements of quality management before system detection starts.

3) Check out the defect type

Figure 4 shows four typical printing defect and defect detection legends. The actual images are color pictures. Of the printing defects, flying ink accounted for more than 80%, and the most continual defect was the knife-like defects. Such defects were small due to small size, and sometimes they were not easily detected by the naked eye. In this example, they passed the neighborhood. Analysis of the pixels detected a 0.25 mm wide knife wire.

5 Summary

The online quality inspection technology and system for printing products has always been monopolized by Europe, America and Japan. The system uses the industry's most advanced hardware equipment (including cameras, acquisition cards and processors), software with independent intellectual property rights (registration number: 2008SR12259) and invention patents (200410061396.7 and 200410061395.2), and can effectively print on high-speed printing production lines. The occurrence of printing defects such as knife wire, flying ink, ink string, leak, dirty version, change roll, ink dot, overprint deviation, etc. is detected and alarmed. It is installed and used at Dali Maideng Printing Co., Ltd., and the operation is stable and reliable.

In addition, the online detection technology for printing defects is not a panacea, but also has a technical bottleneck. For example, the resolution of the longitudinal pixels is far greater than the detection accuracy of the overprint system. Therefore, this system is insensitive to overprint runs below 0.2 mm, and can be achieved in combination with still image devices. Better detection results. In addition, it is not sensitive to low contrast chromatic aberration. It is necessary to perform color sampling with a color difference meter.