Analysis and Research on Screening Registration Accuracy Based on Machine Vision
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First, the principle of overprint error detection
Overprinting is a printing technique that uses multiple colors of ink to print a stacked pattern based on the same reference point. The printing needs to be re-applied by multiple inks during the printing process, which requires accurate alignment of each coloring. If several colors are not aligned, the printed print will be completely unrecognizable, that is, the overprint error will occur.
In the printing process, the detection of the overprinting error is realized by detecting the mark on the specific position of the printed product. This mark is commonly referred to as the Mark line (printing mark lines of each color), and is also referred to as color in the case of not being confused. Standard. A commonly used color mark mark form is shown in Figure 1. In the case of accurate overprinting, such as (a) picture, the color code is coincident, the picture is clear; (b) the picture shows the case of large overprint error. The two color-coded lines do not overlap and are staggered from each other, indicating that there is a large overprinting error. Due to the inaccurate overprinting, the image is very blurred overall. Therefore, lateral and vertical overprinting errors can be obtained by observing the degree of coincidence of the respective color marking lines.
a Overprinting accurate b Imprinting is not accurate
The traditional visual inspection is inefficient and easy to introduce human error. With the improvement of social production efficiency, the drawbacks of visual inspection are becoming more and more obvious, and the intelligent detection technology based on machine vision has been gradually put into practical use. The use of machine vision for online inspection not only has the advantages of high precision and high efficiency, but also overcomes many shortcomings of traditional visual inspection.
Second, an overview of machine vision
Machine vision is to use the machine instead of the human eye to make measurements and judgments, that is, to simulate the human visual function with a computer, extract information from the image of the objective object, process it and understand it, and finally use it for actual detection, measurement and control. The biggest feature of machine vision technology is its speed, large amount of information and many functions. Machine vision system refers to the conversion of the ingested target into an image signal by machine vision products (ie, image capture device, divided into CMOS and CCD), and transmitted to a dedicated image processing system, which is transformed according to pixel distribution, brightness, color, and the like. Digital signals; the image system performs various operations on these signals to extract the characteristics of the target, and then controls the action of the device on the basis according to the result of the discrimination.
The machine vision system is mainly composed of industrial camera, lens, light source, image analysis and processing software. These four parts are all in a professional field, and there are quite a lot of performance and specifications. There are many things to consider when matching with the application. . However, each part cannot be individually determined whether it can be applied to an application. Simply put, the composition of machine vision is different because of the different professions involved in each part, and the application problem is inseparable, plus the application itself has another A kind of expertise that makes the difficulty of making machine vision systems increase.
The advantages of machine vision systems are:
2.1 Non-contact measurement, no damage to the observer and the observer, thus improving the reliability of the system.
2.2 has a wide spectral response range, for example using infrared measurements that are invisible to the human eye, extending the visual range of the human eye.
2.3 Long-term stable work, it is difficult for humans to observe the same object for a long time, while machine vision can perform measurement, analysis and recognition tasks for a long time.
With the continuous application of machine vision products and technologies in industrial technology, the concept of machine vision is no longer as unfamiliar as in previous years. People have gradually begun to accept machine vision inspection technology and have been widely used.
Third, factors affecting the accuracy of screen printing overprinting
According to the actual printing situation, there are many factors affecting the printing quality, such as: pre-press graphic information design processing, the influence of printing plate production, the influence of printing registration, the influence of plate quality and the influence of ink color matching. This article focuses on the impact of print registration accuracy on print quality.
Since it is necessary to distinguish the color of the ink when it is used for color printing, it is generally necessary to print a light color and then print a dark color. First, the secondary printing is mainly printed, and the color sequence should be arranged when making the printing plate. Considering the factors affecting the quality of screen printing from the aspects of printing overprinting, the following four points are mainly summarized: 1 the influence of the operator; 2 the influence of the substrate and ink; 3 the influence of the squeegee; 4 the influence of the screen printing machine.
According to the structure and movement of the screen printing machine, the accuracy of the machine affecting the overprinting is also very significant. Especially the composition and movement of the machine. The screen printing machine is based on machine vision technology.
Fourth, the working principle and the main working process
According to the working characteristics of the screen printing machine, a series of actions such as image taking, signal conversion, and table motion are performed by using machine vision technology to achieve accurate overprinting.
1. Working principle
The movement of one position of the image to another is performed according to the motion matrix. When the image is deviated, it must first complete its rotational motion and then achieve its translation. details as follows:
It is assumed that the image is moved from the start position of p1q1 to the end position of pq, and it is known that the rigid body has rotated by the angle θ from the start position to the end position. Set from the starting position around the point P1 to the angle θ, reach the position of p1q', and then shift from the position of p1q' to the end position pq, as shown in the figure:
2. The main working process based on machine vision technology
A basic machine vision simple system is shown:
The main working process of multi-color screen printing overprint detection and control based on machine vision technology is as follows:
2.1 Finishing the printing of the first color, at this step, the detection and control of the registration accuracy is not required.
2.2 The image acquisition part sends a start pulse to the camera and the illumination system according to the preset program and delay.
2.3 Complete the scanning of the image under the control of the computer signal.
2.4 The image information collected is digitized by A/D or the digital video data digitized by the camera is directly received, and the image capturing portion stores the digital image in the memory of the processor or the computer.
2.5 processor processes, analyzes, and identifies the image to obtain measurement results or logic control values.
2.6 processing results control the movement of the workbench, achieve image registration and correct motion errors.
Compared with the traditional method, it not only has the advantages of high precision, high speed, high degree of automation, but also overcomes the shortcomings of traditional visual inspection, which can easily introduce human error, and can improve the flexibility and automation of production.
Based on the above analysis, if there is an overprint error, the four-color components of C M Y K are not exactly coincident in the printing process, and the RGB three components of the pattern are not exactly coincident on the 24-bit true color image acquired by the CCD. Therefore, our machine vision-based intelligent detection is to divide the image of the product to be tested and the standard template image into three sub-graphs of R, G and B respectively. Apply the template matching method to detect R, G and B on the image to be detected. The coordinates of the template subgraph, if the coordinate error between the three template subgraphs of R, G, and B is less than a certain threshold, it can be considered that there is no overprinting error, otherwise the overprinting error is considered. Once the processing unit determines that there is an overprint error, the corresponding control command is issued to correct it by the executing device, thereby achieving accurate overprinting.
All in all, machine vision is a more complex system. However, due to the superiority of machine vision technology, people are increasingly applying this technology. Not only can it save manpower, but it can also improve accuracy to meet people's needs. Therefore, the application fields of machine vision systems are more and more extensive, not only widely used in industry, agriculture, national defense, transportation, medical care, finance, sports, entertainment, etc., but also in screen printing technology. Application, and better control of the printing process, thus improving the quality of the print.