Control System of Powder Packing Machine Based on PLC

1 Preface

As we all know, the automation level of our country is improving constantly, and the application of automatic control technology in many fields has become more and more extensive. Packaging industry has a long history. After the manual weighing, relay control and weighing instrument control, PLC control technology has been widely used.

The packaging industry in China is composed of three parts. The first is the packaging basic industry mainly composed of packaging raw and auxiliary materials and packaging machinery and equipment. This one belongs to capital intensive and technology intensive industries, which has relatively high industrial added value, and has high entry threshold and industrial competition barriers. The second is the packaging products manufacturing industry composed of paper, plastic, metal and glass, which is basically labor-intensive enterprise, and is a general competitive processing industry, with the advantages of geographical cost, low entry barrier and relatively low production added value. But the market competition is also very fierce. The third is packaging services and packaging management industries such as packaging agency, research and management organization. This part belongs to knowledge intensive industry, with high added value and low entry threshold. Such enterprises are developing rapidly, but the overall development level is not very high, and the whole industry is not standardized. These three big development constitute the overall development of the packaging industry in China.

The biggest feature of packaging machinery is complex action, frequent, and there are many executive elements. In this case, other control equipment such as relay control logic must need a large number of intermediate relays. In this case, these intermediate relays can be replaced by programming the auxiliary relays in the case of PLC control.

Then, it is explained from the physical point of view: the former needs the specific electrical components to be combined, while the internal register of PLC can realize complex control logic without extra cost within the scope of PLC programming capacity. There are many kinds of packaging machines, which are liquid, block and bulk packaging machines according to the product status; According to the packaging industry, there are food, daily chemical industry, textile and other packaging machines; According to the packaging station, there are single station, multi station packaging machine;

According to the degree of automation, there are semi-automatic, automatic packaging machines, etc. Liquid packaging machine is the packaging equipment for liquid products, such as beverage filling machine, dairy filling machine, viscous liquid food packaging machine, liquid cleaning products and personal care products packaging machine, etc. all belong to the category of liquid packaging machine. Block packaging machine is the packaging equipment for packing block products. Nowadays, soap, cake, bread and so on are all used in the market. Generally, it is formed once, fast and convenient. The bulk packaging machine can be divided into particle packaging machine and powder packaging machine. Because the material is of particle type, the granular packing machine has good fluidity and is easy to control, and is usually used

The gravity feeding method has high precision and good effect. Powder packaging machine is also called powder packing scale, which is developed for powder packaging in fertilizer, chemical industry, food, pharmaceutical industry, etc. it can be fully applied to the semi-automatic metering and filling packaging of 5-50kg powder materials; It is equipped with spiral elevator, belt conveyor and package sealing device, manual bag, automatic bag clamping, metering, filling, bag dropping and conveying, manual auxiliary entering sealing station, hot sealing inner bag or sewing outer bag, and completing semi-automatic packaging process. Because of the characteristics of powder, the requirements for powder packaging machine are greatly improved. Because of the poor fluidity of powder, it is easy to block and block the pipeline. Therefore, the performance and precision of powder packaging machine are also required. According to the characteristics of powder packaging, the control system of powder packaging machine based on PLC is designed to improve the precision of powder packaging.

2 system structure

The whole packaging system consists of PLC (Siemens S7-200), packaging machine, digital weighing sensor (Mitsubishi f701), frequency converter and servo motor. PLC receives the weight standard signal of the digital weighing sensor f701, so as to control the frequency converter. The frequency converter controls the speed of the motor by changing the output frequency, thus changing the rotating speed of the screw rod on the packaging machine to control the speed of the feed. From the above process, it can be seen that the whole system is a closed-loop control system.

 According to the control operation requirements of weighing automatic quantitative powder packaging machine (for the packing of powder materials), Siemens S7-200 series PLC is selected as the control system. The PLC of this series has the characteristics of compact structure, modularization, strong expansibility and rich instruction set. The CPU model selected is cpu226ac/dc/rel, which provides 24 digital input and 16 digital output. The I / O interface circuit is adopted.

The optical coupling circuit is used, which has strong adaptability to the external interface. And 2 RS485 communication / programming ports, with PPI communication protocol, MPI communication protocol and free communication ability. It can communicate with the outside world easily. And the CPU can be used to process and extend i/o quickly, which is easy to install. PLC automatically scans the current state of each input and output point, refreshes the status of the output point according to the logic relation determined by the program, and controls the action of the corresponding motor through the frequency converter, thus completing the automatic control of the process flow. The Mitsubishi digital weighing sensor f701 is selected because of its high operability and good data calibration function, and can accurately measure the weight of powder. In addition, the hopper scale / packing weighing scale can be controlled by its quantitative feeding / discharging control function. Its timing control function and rich external interface (si/f, BCD output, RS-232C, RS-485, d/a converter, etc.) can be easily connected with external devices such as PC or program generator. The self checking function can check the internal circuit, alarm abnormally, monitor CPU action, greatly increase the reliability, besides, the digital analog filter function is integrated in the internal part. The packing machine is a powder packing machine. As its name implies, it includes the components of discharging hopper, inserting plate, feeding door, discharge door, discharge hopper, belt mechanism, material conveying screw rod, etc., and the material is transported by the screw rod driven by external motor.

The selected Mitsubishi inverter has the highest driving performance. The motor can get high precision and high response speed control, even v/f control and advanced flux vector control. The long life components and advanced life diagnosis methods make the inverter life up to 10 years; Besides, it also has rich network functions, supports CC link communication, RS-485 communication, sscnt3, and supports various main networks; The noise is low and the operation is convenient. For the selection of feed motor, we consider the power of the motor. Packaging products are mainly for small bags. We choose less motor power, which is controlled at about 1.1KW, and then indoor temperature is considered. With the features of large dust, one-way operation and 380V three-phase AC power supply, we choose Mitsubishi Motor.

3 control system

3.1 design requirements

The main control of the system is switch control. Except for a feeding motor, other actions are completed by the cylinder, but the whole system is controlled by PLC. After the system is started, the packing machine clamps the packing bag with clamp, and the digital weighing sensor starts to remove the tare weight of the package. Then the internal cylinder opens the feeding door (in the case of sufficient materials), and the external servo motor drives the feeding screw rod to feed in a fast speed under the control of PLC. When the load cell fails to reach the set fast weight, the system will continue to feed quickly; When the material weight reaches the fast adding setting value of the load cell, the sensor gives PLC a fast feeding end signal. The PLC controls the output frequency of the inverter to change the speed of the servo motor to feed in a slow speed. When the material weight reaches the load cell to add weight slowly, the sensor then gives PLC a slow feeding end signal, and the plug-in board is closed and the motor stops, After weighing instrument gives the weighing end signal, the strap is released and the whole process is completed.

3.2 software design

PLC terminal software is written in ladder language, and the software is microwinv3.2. The biggest advantage of S7-200 series is the same communication mode of free port. Free port communication is the operation mode of s7-200cpu communication port controlled by user program. Using free mode, user-defined communication protocol can be realized and various intelligent devices can be connected. Considering the actual situation, the lower limit switch is set in the packing machine storage tank to ensure the sufficient materials for this packing. In case of emergency stop, after handling the accident, the process can be completed smoothly by manual fast or slow acceleration. The above is a relatively perfect additional processing function designed in actual situation to ensure the smooth implementation of the packaging process.

The control system of the packaging machine has been running in the packaging factory for a long time. The practice proves that the production efficiency of the enterprise is greatly improved and the cost is saved for the enterprise. During the operation, the system performance is stable, the packaging accuracy is excellent, the sales volume of the products is greatly improved, and the user satisfaction also increases greatly.

4.Conclusion

The development of control technology greatly improves the production efficiency and reduces the labor intensity and production cost of workers. In modern industrial production, more and more attention is paid to the improvement of automation level. The system has fully played the advantages of PLC in the control field. The whole material transfer process is completed in a complete control system, and the coordination and design of each sub process is strict, so as to ensure the production is carried out under the safe and efficient conditions.

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