In the printing process, the ink volume control is mainly done manually, which is very detrimental to the stability of the printing quality, the further improvement of the printing speed and the further automation of the printing process in the future. Based on the current situation that ink volume control is difficult, combined with the idea of ​​embedded systems, we propose the research and development of an embedded dot detection system based on color print quality control.
Research status
So far, the quality control tests of common color printed products mainly include the following.
1. Manual direct observation method. Workers use simple hand-held magnifying glass to observe the printed products, and then adjust the water supply and ink supply according to experience and after comparing with the sample. This method is completely done manually, and the accuracy of ink adjustment depends entirely on the worker's experience.
2. Density measurement method. The principle of printing color is the subtractive method of pigment, that is, the ink printed on paper, selectively absorbs light of a certain wavelength from the irradiated light, and reflects other light, thereby forming a color. Therefore, within a certain range of the ink layer, the amount of light absorbed is proportional to the thickness and concentration of the ink layer, so the density value reflects the thickness of the ink layer. In the process of printing quality control, it is measured with a density meter, and the inspector indirectly reflects the thickness of the ink layer through the density value.
3. Colorimetry. The instrument used for colorimetric measurement is a colorimeter. Through direct measurement, the instrument response value proportional to the tristimulus value of the color is obtained, and the tristimulus value of the color is converted. The method of obtaining the tristimulus value by the colorimeter is completed by the internal optical analog integration of the instrument, that is, the color filter is used to correct the spectral characteristics of the instrument light source and the detection element, so that the output electrical signal size is proportional to the tristimulus value of the color.
The above three methods currently have great limitations: basically all are offline detection, that is, workers take samples, use manual observation or instrument detection, and then adjust the amount of ink appropriately, none of them can directly detect the print screen, only special Designed control strip. Moreover, the direct observation method relies heavily on the experience and quality of the workers. Among the latter two methods, the cost of measuring instruments is relatively high.
In terms of online inspection, many manufacturers have developed their own color monitoring and control system for printed images. Heidelberg's CPC image control system, which uses spectrophotometer measurement technology to scan images, is an attempt from simple planar density control to three-dimensional spectral color control. It uses dual detection of chromaticity and density, and can complete a split in 30 seconds. For the detection of printed sheets, the information of 160,000 measurement points is processed, and then compared with the standard data, the ink area information is displayed on the color touch screen. The color control system developed by Mitsubishi also uses a spectrophotometer, but it scans a special color bar on the printed matter. The system can automatically adjust the setting of the main color in both horizontal and vertical directions and display it on the intelligent printing monitor The color control situation. Manroland insists on monitoring and controlling the printing quality and printing process through the ink density of the printed matter.
Although the color control systems of these famous printing manufacturers have their own characteristics, in general, there are still some limitations. For example, most of them can only detect special test strips; they can only be introduced by the whole machine. Most printing plants are unacceptable, and the quality of workers is more demanding.
Based on the above status quo, we apply the embedded system to printing inspection and propose the development of an embedded dot inspection system based on color printing quality control.
Embedded network detection system
An embedded system is a collection of hardware and software. Generally, it can work independently without the help of other systems. It can also be connected to other systems and work together. This detection system can work alone for ordinary printing presses to obtain the detection results intuitively. When the printing system is further automated in the future, it can be used as online detection and real-time feedback of ink.
Considering that the ink adjustment of the printing press is mainly to adjust the ink amount of 32 ink segments, so we now use one area image as the detection object, and the 32 ink segments can be adjusted one by one by scanning. We make such a device, as long as the image of the detected area is placed in a designated position of the device for detection, the calculation result, that is, the amount of ink of each color required can be displayed. Figure 1 is a block diagram of the system composition of the detection device.
The principle of the detection device:
1. First, illuminate the image of the detected area of ​​the printed product, and the light reaches the photoelectric image sensor through the optical imaging system;
2. After the photoelectric sensor, the image information is converted into an electrical signal and output;
3. Collect the output signal of the photoelectric sensor, and use the method of digital image processing;
4. Finally, identify the ink coverage and related parameters of each ink color in the detected area, and send the results to the display module, or directly feedback to the printing control platform, and then compare with the related parameters of the sample, and finally guide each ink The adjustment of the ink volume in the segment.
In summary, the basic functions to be completed by the detection device are: (1) data collection and storage of images in a given area; (2) processing of the collected image information; (3) output of calculation results (that is, display or Feedback operation results).
Application prospect
In general printing presses, the ink is adjusted manually or semi-automatically, which requires high operator experience. After using the detection device, the ink supply status of each ink segment can be directly obtained online, and the ink supply can be adjusted based on this to achieve The expected printing effect, thereby reducing the experience requirements of the operator, reducing the scrap rate, can also reduce printing costs.
In the automatic printing process, the detection device can be used as the feedback link of the closed loop control of the ink path, detect and feed back the ink information of each ink segment from the printed product, and provide it to the printing control platform to compare with the relevant parameters of the sample sheet to correct The ink supply of each ink segment can achieve automatic control in the true sense of the printing process.
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