A design method for a fully automatic batching system is introduced for the batching process in the protective material production industry. The system uses an industrial computer as the master station and PLC, frequency converter, and weighing instrument as slave stations. The Profibus bus is used for master-slave communication, and the Zijin Bridge configuration software is used to design the human-machine interface on the IPC. The operation results show that the system not only has high efficiency, but also is stable and reliable.

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In the protective material production industry, ingredients are usually evenly mixed together in a certain proportion to form a new material, so ingredients are an important component of production in this industry. In the production process, various raw materials must be strictly mixed uniformly in proportion, which must be completed by batching machinery.

At present, factories generally use two methods. One method involves manually weighing and then mixing various raw materials in proportion into a batching machine. Another method is automatic weighing and automatic mixing.

Due to the fact that many raw materials are in powder or granular form, the human body is prone to inhaling dust and other debris during manual batching, leading to the occurrence of occupational diseases, increased production risks and labor costs. At the same time, the variety and quantity of ingredients are numerous, making it difficult to manage manual batching on site and prone to mismatching. This not only makes quality difficult to guarantee, but also increases management costs. In order to ensure product quality and improve production efficiency, it is required to adopt an accurate and reliable automatic batching system.

A batching system based on PLC, industrial computer, and weighing instrument

In the existing batching system of Henan Xixia Protection Materials Group, workers first transport the materials to the weighing workshop. After weighing, the materials are manually sent to the batching machine for batching. The weighing workshop uses Hangzhou Sifang's weighing instruments for weighing, which are connected to the industrial control host through RS232 port. The industrial control host located in the central control room is responsible for recording the weighing results and displaying the weighing data. At the same time, the control personnel can manually control the start and stop of the batching process through the control circuit in the central control room.

This method is inefficient, and at the same time, the DOS program developed in C language is running on the host, which has poor scalability and difficult human-computer interaction, and cannot complete all the requirements for automatic batching. In order to improve productivity and reduce costs, it is necessary to adopt a fully automated batching system.

The new system adopts a master-slave structure. Using an industrial control computer as the upper host and Siemens PLC [2], frequency converter, and weighing instrument as the lower slaves. The host is in a dominant position, realizing communication management and control of each slave. The RS-232 asynchronous communication port of the industrial control computer is connected to the PLC through level conversion, forming a physical channel for communication between the upper and lower computers; Connect the other RS-232 port of the host to the communication port of the weighing instrument to form a second physical channel.

The upper computer adopts a polling method to communicate with the slave stations one by one. The upper computer transmits the results of task planning to the PLC. During the control process of the PLC, the upper computer uses the upper computer connection command to monitor the operating status and data area content of the lower computer, reads the internal status of the PLC and real-time data of the weighing instrument in real time, and displays them on the upper computer.

Overall, the system has the following functions:

① Fully automatic ingredient preparation. After setting the formula, the system automatically weighs the ingredients according to the formula without the need for operator intervention.

② It has report function. It can generate daily reports, real-time reports, monthly reports, annual reports, etc.

③ Dynamically add and modify reports. The system grants technical or operational personnel the authority to modify and add formulas by setting permissions, while recording the date and operator number of the modification.

④ Power outage recovery function. The system is capable of restoring measurement records prior to a sudden power outage.

⑤ LAN sharing function. The host can share data within the local area network, making it convenient for workshop supervisors to understand project progress and other situations.

Composition of System 1

The entire fully automatic batching and mixing system is composed of an industrial computer PLC、 It consists of industrial weighing instruments, frequency converters, vibration motors, mixers, sensors, conveyor belts, and other components.

The upper level industrial control computer provides a human-machine interaction interface to complete functions such as control information input, data management, data display, storage, statistics, and reporting. The upper level computer adopts IPC810 industrial control computer, and its main tasks are as follows:

The industrial control host first reads a formula with a certain number according to the instructions of the operator. Then, based on the proportion and order of ingredients in the formula, it sends a command to the PLC to start the ingredients, so that the PLC can start a specific frequency converter. During the ingredient preparation process, the industrial control host reads the status words of the PLC in real-time through polling to understand the operating status of the PLC and its subordinate equipment; On the other hand, real-time reading of weighing data from the weighing instrument installed on the batching machine, according to the batching strategy, when the weighing approaches the set value in the formula, the host sends a command to the PLC to stop the batching process. After all the ingredients on a recipe are prepared, the entire ingredient process pauses and waits for instructions from the operator.

During the operation of the system, the PLC communicates in real-time with the upper computer to ensure the consistency between the data displayed on the interface and the actual data on site. The operation commands and set parameters issued by the operator on the upper computer can be sent to the PLC in real time. The main work of the PLC is to receive commands sent by the upper computer and control the start, stop, and speed of the vibration motor through a frequency converter; 2) Write the operating status of the frequency converter in real-time into the memory data area for the industrial control computer to read. 3) Prepare its various statuses in the form of status words for the industrial control computer to read in real-time.

2 Control Strategy and Ingredient Process

By analyzing the characteristics of the ingredient process, it is found that the ingredient process has the following features:

① The controlled object is a unidirectional irreversible system. The raw materials cannot be returned to the conveyor belt from the batching machine.

② It has obvious time delay. When the ingredients reach the set value, the PLC controls the motor to stop conveying the raw materials. At this time, some of the raw materials on the conveyor belt cannot be recovered, so the system has obvious time delay.

③ The controlled characteristic is switchable. The start and stop control of the system are all on/off signals.

④ The ingredient system is linear within the normal working area.

Therefore, we consider adopting control strategies such as fast, slow, and early issuance of stop feeding instructions, while utilizing the interlocking technology of PLC to ensure the smooth progress of batching. After the system is started, the industrial control computer sends a start feeding signal to the PLC, which controls the frequency converter to drive the motor for rapid feeding. At the same time, the industrial control host continuously reads the weighing data of the weighing instrument through the serial port. When the weight value approaches the set value, the industrial control host sends a control command to the PLC to stop feeding.

At this time, the PLC controls the frequency converter to perform slow addition. By estimating the residual amount of raw materials on the conveyor mechanism in advance, the difference between the set value and the actual feeding is equivalent to the residual amount of raw materials on the conveyor mechanism. When the PLC truly issues a stop command, which is executed by the frequency converter to control the motor to stop. After stopping, there is no residual raw material on the conveyor mechanism, and the batching accuracy meets the requirements. The process is shown in Figure 1.

Software Design for Industrial Control Host

The main tasks completed by industrial computers are as follows:

① Provide animated displays of the ingredient process.

② Send control instructions to the PLC and read the operating status of the PLC.

③ Read the weighing signal on the weighing instrument, display the weighing value on the monitor, and send instructions to the PLC based on the weighing data.

④ Database and reports, saving ingredient data, printing reports.

⑤ Addition and modification of formulas.

⑥ Other functions such as auxiliary alarm for ingredient failure.

Automatic material weighing machine

Figure 1 Main Ingredient Process Flow Chart

Interface design of ingredient software

The upper industrial control computer uses Zijin Bridge configuration software to design the human-machine interface. Industrial control configuration software is actually a software development platform that can be developed by users according to their own needs. We can develop a user-friendly human-machine interface for the entire monitoring system on this platform according to the process requirements. Operators can interact with on-site equipment in real time through this interface. Zijin Bridge software is HMI/SCADA industrial automation configuration software, which provides a highly integrated and visual development environment. This software has the following characteristics:

Multiple communication functions. The Zijin Bridge configuration software supports the following communication functions:

1) Supports serial communication methods such as RS232, RS422, RS485, as well as wireless radio, telephone dialing, telephone polling dialing, and other methods;

2) Ethernet communication supports both wired Ethernet and wireless Ethernet;

3) All device drivers support mobile network standards such as GPRS, CDMA, GSM, etc.

Convenient development system. Rich components and controls form a powerful HMI development system; The enhanced transition color and progressive color functions fundamentally solve the problem of many similar software programs severely affecting screen refresh speed and system efficiency when using transition color and progressive color excessively; More flexible and diverse vector subgraphs make the production of engineering images faster; Provide an object-oriented programming approach with built-in indirect variables, intermediate variables, database variables, and support for custom functions and menus.

Openness. The openness of Zijin Bridge configuration software is manifested in the following aspects:

1) Support accessing databases through VBA using EXCEL;

2) The software adopts an open architecture and fully supports DDE, OPC, ODBC/SQL, ActiveX, and DNA standards. Provide external access interfaces in various forms such as OLE, COM/DCOM, and dynamic link libraries, making it easy for users to use commonly used development tools (such as VC++, VB, etc.) for deep secondary development;

3) The architecture of the I/O driver program for Zijin Bridge configuration software is an open structure, and the source code for its interface is fully open. Users can develop new drivers on their own.

Database functionality The Zijin Bridge configuration software has a built-in real-time database, and the real-time database also has multiple functional blocks to complete data processing and storage, which can achieve various functions such as accumulation, statistics, control, and linearization.

Supports multiple devices and buses. Supporting PLCs, regulators, intelligent instruments, intelligent terminals, and intelligent modules produced by most well-known domestic and foreign manufacturers; In addition, it also supports standard fieldbuses such as Profibus, Can, LonWorks, and Modbus.

I/O points of the 2 systems

The Zijin Bridge configuration software uses real-time database points to represent I/O points. After analysis, the system requires three I/O points and two digital control points to control the start and stop of the motor through PLC. Therefore, the data links for these two points are selected as the two digital input and output channels of the PLC. A simulation point is used to represent real-time data read from the weighing instrument, therefore, the data link of this point is the measurement value of the weighing instrument.

Communication program design

The communication program design mainly includes three parts, one is the communication between the host and the PLC; The second part is the communication between the host and the weighing instrument; The three parts are the communication between PLC and frequency converter.

1. Communication between the host and PLC

Configuration software usually comes with mainstream PLC driver programs built-in. First, create a virtual PLC device in the Zijin Bridge configuration software, and the model of the virtual device must be consistent with the model of the real PLC being used. If the required PLC model cannot be found in the configuration software, the software manufacturer can be commissioned to develop a new PLC driver of that model for free. Virtual devices are used to map real devices. Here, the PLC we use is simensS7-300, and the host is set to communicate with the PLC through serial port 1.

Communication between the host and the weighing instrument

For the weighing instrument, we use the weighing instrument from Hangzhou Sifang. In order to ensure good communication between the instrument and the configuration software, we have specially commissioned Zijinqiao Company to develop a driver program for the instrument. Firstly, we select a device type we need from the driver list of the configuration software, and create a virtual device for that type to map the real weighing instrument. Then, we set the communication port and protocol between the instrument and the computer.

Communication between PLC and frequency converter

Due to the variety of raw materials in the batching workshop, we have set up multiple conveyor belts for the convenience of batching. Therefore, one PLC of the batching system needs to be connected to multiple frequency converters. For this purpose, we used the Profibus bus for communication between the PLC and the frequency converter. We inserted a dedicated Profibus communication module into the frequency converter, set the slave address of the frequency converter, and then connected it to the PLC through the Profibus connector. The PLC programmed the frequency converter to send and receive messages, sent control words to the frequency converter, and read status words back from the frequency converter.

The CPU315-2DP serves as a Profibus master station, and each frequency converter that communicates with the master station can be regarded as a Profibus slave station. During communication, the master station selects the slave station to transmit data based on the address characters in the communication message. The slave station itself cannot actively send data, and there is no direct transmission of information between the slave stations. The frequency converter models selected for the system are all Siemens MicroMaster430 series [4]. The main communication between PLC and frequency converter involves two concepts. One is the data message, and the other is the control word and status word.

1) Communication message. Each message starts with the character STX, followed by the length specification LGE and address byte ADR, and then the data characters used. The message ends with the checksum BCC of the data block, and the main fields are explained as follows:

The STX area is an ASCII character (02hex) of one byte, representing the beginning of a message; The LGE area is a byte that indicates the number of bytes following this message.

The ADR area is a byte that represents the address of the slave node (i.e. frequency converter).

The BCC area is a one byte checksum used to check if the information is valid. It is the result of XOR operations on all bytes before BCC in this information. If the result of the checksum operation indicates that the information received by the frequency converter is invalid, it will discard this information and not send a response signal to the main station.

2) Control words and status words. PLC can read and write the parameter values of the frequency converter through the PKW area of the frequency converter, thereby changing or understanding the working status of the frequency converter. In this system, the PLC reads the data from this area and places it in a specific data area for the industrial computer to query. The query results are displayed on the industrial computer.

Conclusion

This system completes the required automatic batching tasks through the coordinated work of industrial computers, PLCs, and frequency converters. Since its launch in May 2008, the system has been batching more than 100 tons per day and completing about 10 formulas. It can not only display working conditions in real time, but also provide functions such as formula modification and addition; Actual operation has shown that the system runs stably and reliably, with a beautiful human-machine interface and easy operation. At the same time, the system is developed using configuration software, which can provide convenience for future upgrades.

(Excerpted from "Electrical Technology", the original title is "Design of Automatic Batching System Based on PLC and Weighing Instruments", written by Tian Fei, Cui Shilin, etc.). ）