On the precise control of PLC in printing machine
1 Introduction
In recent years, with the improvement of China's automation technology, factory automation has also reached a new level. As an emerging industrial controller, PLC has unique advantages in its small size, complete functions, low price and high reliability, and has been widely used in various fields.
As the largest printing machine manufacturer in China-Beiren Group Company, in order to make the product performance stable and easy to maintain, we have adopted a PLC-based control scheme. Because the two-color printing machine requires easy operation and high precision, it has many input and output points, so it adopts two-machine communication. The host computer uses Mitsubishi FX2N-80MR + 32EX + 4D / A, which is mainly responsible for the control of the main drive, the control of the clutch pressure of each unit, and the control of the air pump and air valve. The lower machine adopts Mitsubishi FX2N-64MR + 4A / D, which is mainly responsible for the control of the water roller motor, the speed control output of the main drive, and the data collection of the version-adjusting motor. At the same time, a Mitsubishi 5.7-inch touch screen is selected, which is mainly responsible for the display of the speed of the water roller motor, the adjustment of the display, and the failure display of the whole machine. The system is reliable in operation, convenient in maintenance, and simple and intuitive in operation, which greatly improves the grade of the offset press and is well received by users.
2 System structure
The system structure diagram is as follows:
Among them, the upper computer and the lower computer adopt RS485 communication, which is convenient and reliable. For multi-color machines, safety factors are important. In the design, each unit must take into account the safety control, including emergency stop and safety button of the unit; also consider the convenience of operation, including that each unit should have a punctual and anti-point button. Therefore, on the one hand, the input points increase a lot; on the other hand, routing is also very inconvenient. The dual-machine communication can solve this problem well. The wiring of each unit can enter the control cabinet closer to it according to the principle of proximity, which not only saves wiring, but also facilitates control.
Since the printing press is a high-precision machine, the quality of the printed matter is on the one hand the accuracy of mechanical processing and installation, on the other hand, it also depends on the balance of the water path, ink path and the accuracy of the pressure. Each color group of the two-color machine has waterway and ink path devices. In order to facilitate the adjustment of the speed of the water roller, each water roller is controlled by a frequency converter, and at the same time, the speed of the main motor also needs to be adjusted by the frequency converter. Therefore, in order to achieve multi-channel speed adjustment, we use Mitsubishi 4D / A digital-to-analog converter, which converts the digital quantity given by the PLC side into a 0 ~ 10V DC voltage output according to the corresponding algorithm, which is well achieved Multi-channel speed adjustment requirements.
In the printing process, version adjustment is a relatively tedious process. Especially for multi-color machines, the accuracy of each set of plate alignment will have a great impact on printed products. If overprinting is inaccurate, there will be literal overlap or unclear images in the printed matter. Generally speaking, the axial adjustment range of the printing plate is -2mm ~ + 2mm, and the circumferential adjustment range is -1mm ~ + 1mm. If you use manual adjustment, it will waste a lot of time, and the accuracy is not high. In order to realize automatic pattern making, we installed a potentiometer on the plate roller, and transferred the analog quantity to 4A / D through the potentiometer. After PLC processing, the rotation accuracy of the plate roller can be well controlled within the range of plate making.
As a new type of man-machine interface, the touch screen has attracted attention since its appearance. Its simple and easy-to-use, powerful functions and excellent stability make it very suitable for industrial environments. Users can freely combine text, buttons, graphics, numbers, etc. to process or monitor and manage information that may change at any time. With the rapid development of mechanical equipment, the previous operation interface needs to be operated by a skilled operator, which cannot improve efficiency. But using the man-machine interface can clearly indicate and inform the operator of the current status of the machine equipment, making the operation simple and vivid. Using the touch screen can also standardize and simplify the wiring of the machine, and also reduce the number of I / O points required for PLC control, reduce production costs, and relatively increase the added value of the entire equipment. Mitsubishi touch screen and Mitsubishi PLC have good versatility, can monitor and modify the program online, and do not need to repeat the plug and pull interface very cumbersome.
3 Software design 3.1 Paper design
The overall electrical design of the printing press is still relatively complicated, and the requirements on time are also very strict. Proximity switches are installed in many places of the machine to detect different time points. In the printing process, the quality of the paper is an important link that affects the quality of the machine. The so-called paper is good or bad. It refers to the phenomenon of no skewed sheets and double sheets. If there is skewed sheets and double sheets, under high-speed conditions, the damaged paper will be wrapped into the machine, thereby destroying the rubber. Bring great losses to users. The process flow is as follows:
In the experiment, we found that the program compiled according to the above process has no problem at low speed, but after the speed is increased to 7000r / h, there will be a phenomenon of skewed locking. The reason is mainly due to the lag of the reaction time of the optical head and the action time of the magnet. During the execution of the program, the cyclic scanning method is used. In order to advance the output of the electromagnet, in the design, I used the input and output refresh commands of the interrupt and Mitsubishi programming instructions to make the output of the electromagnet execute immediately, and advance the action time of the electromagnet. Even at the speed of 12000r / h, it can lock the faulty paper very well, which solves a big problem of paper feeding.
3.2 Design of clutch pressure
Separating pressure and closing pressure play a very important role in printing. The accuracy of the clutch pressure has a direct impact on the quality of the printing quality. If the pressure is too early, it will stain the platen roller and cause a lot of inconvenience to the operation; if the pressure is too early, the last piece of paper will not be printed with a complete pattern, causing paper waste.
In the design, the program flow of pressure separation and pressure combination is shown in the figure:
When printing, the plate roller and the rubber roller are pressed together first, and the rubber roller and the printing roller are pressed together. In our machine, the pneumatic pressure is all used for the combined pressure, and each cylinder has an action time. As the printing speed is multi-stage speed, between 3000 ~ 12000r / h, different speeds can be selected according to user needs. However, the cylinder operation time is fixed, and the gear rotation angle is fixed. Therefore, when the machine speed is different, the closing time is also different. In order to solve this problem, we find out the delay time of the machine for different machine speeds according to the theoretical calculation value. Using the comparison instruction, when the machine section speed is equal to the theoretical value, the corresponding time is delayed, so that the impression roller and the rubber roller are accurately pressed. After many tests, there is no problem with the pressure and pressure.
3.3 Human-machine interface design
In the man-machine interface, 7 pictures are designed, including the overall graphics, fault display, machine speed and count display, water roller speed display, version monitoring and so on. The fault display uses an indicator, and the flashing effect can be achieved by giving the bit element, so that the operator can easily know the fault location, and the overall feeling is very good. In the water roller speed display, a histogram is designed to show the increase in the amount of water. Just press the histogram to increase the amount of water, and it is also convenient for monitoring. as the picture shows:
4. Conclusion
The electrical design of a printing machine belongs to the system design, including hardware and software design, which covers a wide range. Here, I only briefly introduce some of the more important parts, there are many other details, and I will not list them one by one here. Using a set of Mitsubishi control systems feels reliable and convenient. During the mass production of the machine, no major problems were found. Its PLC has complete functions, reliable and durable, and simple instructions. Compared with other products, it feels that Mitsubishi's overall software system interface is relatively friendly, which brings great convenience to users in programming and maintenance. The touch screen and PLC have good versatility, and the program can be monitored and modified through the touch screen, which is unmatched by other products. In short, Mitsubishi's industrial control components have brought a lot of convenience to designers and users.
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