Embedded dot detection system: new method of printing quality control
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In the printing process, the ink amount control is mainly done manually, which is disadvantageous for 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 quantity control is difficult, combined with the idea of embedded system, we propose the research and development of embedded dot detection system based on color print quality control.
Research status
So far, the quality control tests for common color prints are mainly as follows.
1. Manual direct observation. The worker uses a simple hand-held magnifying glass to observe the printed product, and then according to experience, after adjusting the sample, the water supply and the amount of ink are appropriately adjusted. This method is completely manual, and the accuracy of ink adjustment depends entirely on the worker's experience.
2. Density measurement method. The principle of printing coloring is the color subtractive method, that is, the ink printed on the paper, selectively absorbs light of a certain wavelength from the irradiated light, and reflects other light to form a color. Therefore, in 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 quality control process of the printed matter, the density is measured by the density meter, and the detector indirectly reflects the thickness of the ink layer by the density value.
3. Chroma measurement method. The instrument used for chromaticity measurement is a colorimeter, which obtains the instrument response value proportional to the tristimulus value of the color by direct measurement, and converts the tristimulus value of the color. The method of obtaining the tristimulus value of 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 detecting 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 offline detection, that is, workers take samples, use manual observation or instrument detection, and then adjust the amount of ink properly, can not directly detect the print screen, can only detect special Designed control strips. Moreover, the direct observation method relies to a large extent on the experience and quality of the workers, while in the latter two methods, the cost of the measuring instruments is high.
In terms of online testing, many manufacturers have developed their own printed image color monitoring and control systems. Heidelberg's CPC image control system, which uses spectrophotometer measurement technology to scan images, is an attempt to control color from simple plane density control to three-dimensional spectrum. It uses dual detection of chromaticity and density to complete a split within 30 seconds. The printing sheet is processed, and the information of 160,000 measuring points is processed, and after comparing 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 scans a special color strip on the printed matter. The system can automatically adjust the main color settings in both the horizontal and vertical directions and display it on the smart print monitor. Out of color control. Manroland insists on monitoring and controlling print quality and the printing process through print ink density.
Although the color control systems of these famous printers have their own characteristics, in general, there are still some limitations. For example, most of them can only detect special test strips; only the whole machine can be introduced, and the cost increase is large for domestic Most printers are unacceptable and have higher requirements for the quality of workers.
Based on the above situation, we apply the embedded system to print inspection and propose the development of embedded dot detection system based on color print quality control.
Embedded dot detection system
An embedded system is a collection of hardware and software that can work independently, without the aid of other systems, and can be connected to other systems to work together. The detection system can work independently for the ordinary printing machine, and intuitively obtain the detection result, and can be used as the online detection and real-time feedback of the ink when the printing system is further automated in the future.
Considering that the ink adjustment of the printing machine is mainly to adjust the ink volume of 32 ink segments, we now use one area image as the detection object, and 32 ink segments can be adjusted one by one in a scanning manner. We make such a device, as long as the image of the detected area is placed at the designated position of the device for detection, the calculation result, that is, the amount of ink of each color ink required is displayed. Figure 1 is a block diagram showing the system configuration of the detecting device.
The principle of the detection device:
1. First, the image of the detected area of the printed product is illuminated, and the light passes through the optical imaging system to reach the photoelectric image sensor;
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 process it by digital image method;
4. Finally, identify the ink coverage and related parameters of each ink color in the detected area, and send the result to the display module, or directly feedback to the printing control platform, and then compare with the relevant parameters of the sample, and finally guide the ink supply system. The adjustment of the amount of ink supplied to the segment.
In summary, the basic functions to be completed by the detecting device are: (1) collecting and storing images of a given area; (2) processing the collected image information; and (3) outputting the operation result (ie, displaying or Feedback operation result).
Application prospect
In the general printing machine, it is basically manual or semi-automatic ink adjustment, which has high requirements on the operator's 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. The expected printing effect reduces the operator's experience requirements, reduces the scrap rate, and reduces printing costs.
In the automatic printing process, the detecting device can be used as a feedback link of the closed-loop control of the ink path, detecting and feeding back the ink information of each ink segment from the printed product, and providing the printing control platform to compare with the relevant parameters of the sample, thereby correcting The amount of ink supplied to each ink segment is automatically controlled in the true sense of the printing process.