Talking about the precise control of PLC in printing machine
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1 Introduction
In recent years, with the improvement of automation technology in China, factory automation has also reached a new level. As an emerging industrial controller, PLC has unique advantages in terms of small size, complete functions, low price and high reliability, and has been widely used in various fields.
In order to make the product performance stable and easy to maintain, we have adopted a control scheme based on PLC as the main controller. Because the two-color printing machine requires easy operation and high precision, it has more input and output points, so it uses two-machine communication. The upper computer adopts 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 the air valve. The lower position 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 acquisition of the adjustment motor. At the same time, a Mitsubishi 5.7-inch touch screen was selected, which is mainly responsible for the speed display of the water roller motor, the adjustment display, and the fault display of the whole machine. The system is reliable in operation, convenient in maintenance, simple and intuitive in operation, greatly improving the grade of the offset printing machine and being well received by users.
2 System structure
Among them, the upper computer and the lower computer use RS485 communication, which is convenient and reliable. For multicolor machines, security is important. In the design, each unit must take into account the safety control, including the emergency stop of the unit, safety button; also consider convenient operation, including each unit should have a punctual, anti-point button. Therefore, on the one hand, the input points increase a lot; on the other hand, the routing is also very inconvenient. The use of two-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 saves the wiring and facilitates the control.
Since the printing press is a high-precision machine, the quality of the printed matter lies in the precision of machining and installation. On the other hand, it depends on the water path, the balance of the ink path and the accuracy of the pressing. Each color group of the two-color machine has a water path and an ink path device. In order to facilitate the adjustment of the water roller speed, each water roller is controlled by a frequency converter, and the main motor speed 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 0~10V DC voltage output according to the corresponding algorithm, which is well realized. Multiple speed adjustment requirements.
Tuning is a cumbersome process in the printing process. Especially for multi-color machines, the accuracy of the alignment of each group will have a great impact on the printed products. If the overprint is not accurate, the printed matter will appear to be literally overlapping or the image is unclear. 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 tuning, it will waste a lot of time and the accuracy is not high. In order to achieve automatic pattern making, we installed a potentiometer on the plate roller, and transmitted the analog quantity to the 4A/D through the potentiometer. After the PLC process, the rotation precision of the plate roller can be well controlled within the plate making range.
As a new type of human-machine interface, the touch screen has received attention since its appearance. Its ease of use, powerful functions and excellent stability make it ideal for industrial environments. Users can freely combine text, buttons, graphics, numbers, etc. to process or monitor 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, and the efficiency cannot be improved. However, using the human-machine interface, the operator can clearly indicate and inform the operator of the current condition of the machine, making the operation simple and vivid. The use of 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 the production cost, and relatively increase the added value of the whole set of equipment. Mitsubishi touch screen and Mitsubishi PLC have good versatility, can monitor and modify the program online, and do not have to troublesomely plug and unplug the interface.
3 Software design
3.1 Paper design
The overall electrical design of the printing press is still relatively complicated, and the time requirements are also very strict. Proximity switches are installed in many places on the machine to detect different points in time. In the printing process, the quality of the paper is an important part of the quality of the machine. The so-called paper is good or bad. It refers to the phenomenon of no arrogance, double sheeting, etc. If there is arbitrage and double-fold phenomenon, at high speed, the damaged paper will be caught in the machine, thus destroying the rubber. It brings great losses to users.
In the experiment, we found that the procedure compiled according to the above process has no problem at low speed, but after the speed increases to 7000r/h, there will be suffocation. The reason is mainly due to the reaction time of the optical head and the time lag of the magnet action. During the execution of the program, the cyclic scan mode is adopted. In order to advance the output of the electromagnet, the input and output refresh commands of the interrupt and Mitsubishi programming commands are used in the design to make the electromagnet output execute immediately, and the electromagnet action time is advanced, even if At a speed of 12000r/h, the faulty paper can also be locked well, solving a big problem in paper feeding.
3.2 clutch pressure design
Depressurization and pressure play a very important role in printing. The accuracy of the clutch pressure has a direct impact on the quality of the printed product. If the pressure is too early, the embossing roller will be soiled, which will bring a lot of inconvenience to the operation; if the pressure is too early, the last paper will not be printed with a complete pattern, resulting in waste of paper.
During printing, the plate roller and the rubber roller are pressed together, and the rubber roller and the embossing roller are pressed together. In our machines, all the pressures are pneumatic, and each cylinder has an action time. Since the printing speed is multi-speed, between 3000~12000r/h, different speeds can be selected according to user needs. However, the cylinder operating time is constant, and the gear turning angle is constant. Therefore, when the machine speed is different, the pressing time is also different. In order to solve this problem, we calculate the delay time of the machine for different machine speeds based on theoretical calculations. When the comparison instruction is used, 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 pressure and pressure.
3.3 Human Machine Interface Design
In the man-machine interface, 7 screens are designed, including overall graphics, fault display, machine speed and count display, water roller speed display, and adjustment monitoring. The fault display uses the indicator, and the bit component can be used to achieve the flashing effect, 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 water volume. Just press the histogram to increase the water volume and also facilitate monitoring.
4. Conclusion
A set of electrical design of the printing press belongs to the system design, including hardware and software design, covering a wide range.