Abstract : This paper presents a case study of the design of a control system for a vulcanizing workshop in a tire factory. It introduces a developed monitoring system where a PLC controls the production sequence, and an Ethernet module facilitates data communication between the PLC and the host industrial computer. Keywords: Vulcanizing machine; Computer control; Industrial Ethernet; MC protocol 1 Introduction With the development of 4C technologies (Computer, Control, CRT), a control mode consisting of monitoring software and a PLC (Programmable Logic Controller) has become a popular trend in computer control systems. In this type of control mode, the system is generally divided into upper and lower levels. The industrial control computer acts as the host computer, mainly responsible for data storage, analysis, processing, complex calculations, status display, and printing. The PLC acts as the lower computer, mainly responsible for data acquisition, status judgment, and input/output control. This mode fully utilizes the characteristics of both microcomputers and PLCs, achieving complementary advantages. This paper is based on a case study of the expansion and renovation project of a vulcanizing workshop in a tire factory. Considering the good reliability and real-time performance of industrial Ethernet, it is adopted as the monitoring network to achieve real-time detection, recording, storage, and alarm functions for key parameters in the tire vulcanizing process. Tire vulcanization is a complex chemical reaction process and the final critical stage in tire manufacturing. To ensure the final product meets specified quality standards, all chemical and physical reactions occurring during this process must strictly adhere to a predetermined pattern. Therefore, a PLC is necessary to strictly control key parameters of the vulcanizing machine, such as temperature, pressure, and time, ensuring they fluctuate along a predetermined trajectory. Furthermore, on-site personnel must have a clear understanding of the vulcanization status of each tire; therefore, real-time display, recording, and alarm functions for these key parameters in various ways (digital or graphical) are required. The monitoring system described in this paper effectively accomplishes these tasks. 2. System Composition This workshop primarily produces passenger car tires and light and medium-duty truck tires. The vulcanizing chamber diameter is generally below 65°, therefore, a widely used hydraulic bladder dual-mold vulcanizing machine is employed. One vulcanizing unit can vulcanize two tires simultaneously. The control of the vulcanizing machine directly affects the quality of the final product. The monitoring system described in this paper directly controls the vulcanizing machine through a PLC in the control station; therefore, the structure of the control station directly impacts the performance of the entire system. The vulcanization process sequence control utilizes a Mitsubishi Q (Q02) series PLC, along with its matching power supply module, basic rack, analog input (AI) module, analog output (A0) module, digital input (DI) module, digital output (DO) module, and various temperature and pressure sensors to detect, control, and alarm key parameters. A Mitsubishi touchscreen or tablet PC is used to display key parameters and set control parameters for a single vulcanizing machine. The tablet PC communicates with the PLC, directly reading values ​​from the PLC's internal data units. Therefore, a Mitsubishi serial communication module (C24) is installed on the PLC rack to communicate with the tablet PC, adhering to the serial communication protocol specified by Mitsubishi. The tablet PC runs a VB program that collects key parameter data via the serial port and transmits the data to the host computer via Ethernet. The vulcanizing machine, where the touchscreen is located, communicates with the host industrial computer via a Mitsubishi E71 Ethernet module. The monitoring network serves as the carrier of data transmission. Considering the reliability and real-time nature of system data acquisition and information transmission, the monitoring computer and PLC adopt a high-speed and reliable standard industrial Ethernet communication method with a communication rate of 10Mbps. This method allows for easy networking with other information management systems without adding any hardware, forming an integrated management and control network system to improve the company's overall management level. The monitoring station mainly performs functions such as real-time data recording and display, monitoring parameter changes in the vulcanization process, statistical analysis and reporting of various data, and historical data query. It is mainly operated by two high-performance industrial control computers with a hard drive capacity sufficient to store at least 10 years of data based on the required data acquisition rate. Its basic hardware configuration includes: two Advantech high-performance PLCs: CPU PIII 1.0G, 128M memory, 30G hard drive, 17-inch color monitor, network card, and hub, etc. The basic software configuration includes: System software: Windows 2000 Professional; Application software: Microsoft Office, monitoring software written in VC++. Figure 1 shows the system structure diagram. The system is divided into three layers: the first layer is the host computer monitoring station composed of two industrial control computers; the second layer is the industrial Ethernet composed of hubs, twisted pairs, and transceivers; the third layer is the control station, which is divided into two types according to specific implementation details: tablet vulcanizing machine group and touch screen vulcanizing machine group. The figure only shows the hardware connections involved in communication. Figure 1 System Structure Diagram 3 Communication Steps and Communication Protocol Data transmission between the monitoring station and the control station is based on Ethernet. Considering the requirements for real-time generation and reliability in the transmission process, TCP/IP is selected as the communication protocol, and a C/S mode (client/server mode) is adopted to enable connection-oriented communication between the control station and the monitoring station. At the same time, since the lower-level control station is a Mitsubishi PLC, it communicates with the industrial control computer through the Ethernet module E71. Therefore, the data packets transmitted in the communication process must also follow Mitsubishi's MC protocol. During communication, the host computer acts as the server in the communication mode. It listens for connection requests from each customer terminal and checks the validity of each connection at regular intervals (usually one sampling period). If valid, it sends a data packet to the lower-level device according to the MC protocol format to read the corresponding parameter values. Upon receiving a data packet, it first checks its validity. If valid, it unpacks and analyzes the corresponding parameter values; otherwise, it discards the packet. The customer terminal is actually an E71 module, which has a built-in fixed communication program (initially, the parameters used by this program need to be set, such as IP address, port number, protocol, and communication method). This program is responsible for analyzing the correctness of data packets sent by each external device. If correct, it executes the relevant operations (such as reading or writing the value of a data unit in the PLC) according to the command information contained in the data packet and encapsulates the resulting data information into a packet according to the MC protocol format and sends it to the host computer. If incorrect, it discards the packet and turns on the error indicator light of the E71 module. If no data packet is received within a set time period, the connection is forcibly disconnected. Since the firmware does not include connection establishment functionality, and the E71 module, as the client in the communication mode, must actively attempt to establish a connection with the host computer (send a connection establishment request) when no connection is established, this task must be accomplished by writing corresponding instructions in the firmware. Network parameter settings are done through GXDeveloper. After running the software and creating a new program, configure the settings in the NetworkParameter section under the Parameter item. Key settings include: Network type (select Ethernet here); Start 1/ONo., Network No., Group No., Station No., which identify the network location of a PLC; Operation settings specify the IP address of the Ethernet module, the data type in communication (binary or ASCII), and the connection opening method (PLC actively opening the connection or external device actively opening the connection); Initial settings specify how long the Ethernet module should wait after not receiving a data packet to check the existence of an external device, the number of checks, the interval between checks, and DNS settings; Openset settings specify the communication protocol (TCP or UDP), the connection opening method (active or passive), whether to check the existence of external devices, the IP address of the external device, and the port number of the external device's related programs. The client connection establishment process involves the PLC, as the client, actively establishing a communication connection with the host computer through network parameter settings. The PLC must also ensure the robustness of the connection between itself and the host computer. If a connection error occurs at any time, the connection must be re-established. Therefore, in the PLC's sequential control program, it is necessary to detect the connection's open status bit. If it is OFF, a timer must be used to repeatedly establish the connection until the status changes to ON. In this method, the open status bit changes from ON to OFF in two ways: * When a shutdown command is received from an external device or the PLC actively requests shutdown, a shutdown instruction is executed. * When the Ethernet module does not receive a data packet within a set time and, after confirming the existence of the external device, assumes the external device does not exist (due to abnormal shutdown of the external device, abnormal exit of the monitoring software, or disconnection of the network cable, etc.), the connection is forcibly closed by the module's internal firmware. The ladder diagram logic steps of the PLC's sequential control program are shown in Figure 2. After successfully opening the connection, the connection open status bit will automatically be set to ON; after successfully closing the connection, the connection open status bit will automatically be set to OFF. [img=370,347]http://www.chuandong.com/uploadpic/THESIS/2009/4/2009040313045363703J.jpg[/img] Figure 2 PLC Communication Sequential Control Program Logic Steps [align=left] Server-side Connection Establishment Steps The server-side monitoring software is written in VC++ and uses a derived class of CSOCKET to complete the communication. The CSOCKET class is a high-level encapsulation of WINSOCKET. The client and server implement synchronous communication and hide the implementation details of the TCP/IP protocol. Only related WINDOWS events and service function calls are needed to complete the data sending and receiving. The communication part defines a server socket object and several client socket objects (the number is determined by the number of lower-level machines), and a client socket chain manages all client sockets. The server socket is mainly responsible for listening for connection requests and updating client sockets. Each time a new connection request is detected, a new client socket is generated based on the IP address of the requesting lower-level machine, and inserted into the program's client socket chain. The client socket stores established connection information. When a connection request is sent and successfully established, a client socket object is generated, which serves as the basis for data transmission and reception between the upper-level and lower-level machines. The process of establishing a server-side connection is shown in Figure 3. [Image: http://www.chuandong.com/uploadpic/THESIS/2009/4/2009040313102725995A.jpg] Figure 3: Server-side communication process flowchart. The protocol, or protocol protocol, consists of two parts: serial port and Ethernet. This article discusses its application in Ethernet, so the serial port part is not described. It specifies the data format (the specific meaning of each data bit in the frame) in the data packets sent and received when external devices transmit data with the Ethernet module. The same frame contains both ASCII and binary versions; this article only discusses its ASCII form. The system uses the QnAcompatible3E frame, which is suitable for communication with the E71 module. Its fields are shown in Figure 4. Figure 4: MC Protocol Frame Structure [align=left] The header is a TCPhP header; since derived class communication is used in VC, this item is transparent. The message type identifies whether the data packet is sent by an external device (command packet, "5000") or by the Ethernet module (response packet, "D000"). The Q protocol header specifies the location of the lower-level device in the network where the data packet originated and to which it was sent, as well as how many bytes remain after this to mark the end of the frame. The command code indicates the type of command, uniquely identified by a main command and a subcommand; this is not included in response packets. The data field contains either command data or response data. 4. Monitoring Software Design The host computer monitoring software mainly performs functions such as real-time data display, storage, analysis, reporting, and alarm generation for the vulcanization process. It is written in VC++. The functional structure diagram of the software is shown in Figure 5. Figure 5. Functional Structure Diagram of Monitoring Software. The network communication module mainly performs network data transmission and reception according to the steps described in the previous section; real-time data display and alarms are also performed. Whenever a set of correct data is received, real-time data display and real-time curve plotting are performed based on its value. The real-time curves are plotted in the form of a circular coordinate system, showing the real-time curves of all key parameters of the vulcanizing machines within a day. Alarms are also provided for real-time data exceeding the specified range, as well as network communication status alarms. The data analysis and historical data query section mainly operates on historical data. By querying the historical database, the vulcanizing process that has occurred on each vulcanizing machine is analyzed from multiple aspects, including data and curves, to find potential errors in the vulcanizing process. The database management function mainly includes data storage, retrieval, and maintenance. Data received from the network needs to be stored in the database immediately for later analysis. Similarly, the historical database needs to be queried during data analysis. This part is completed using [unclear]. To facilitate data management, the program automatically generates a file every day. Each file maintains a table for each vulcanizing machine to record parameters. The reporting function mainly includes statistical reports of historical data, historical curve data and real-time curves, and data printing output. 5. Conclusion The tire industry is a mixed industry, involving highly real-time information, large volumes of data, and diverse types of data. The design and implementation of its workshop automation system is therefore quite complex. The system, after on-site debugging and trial operation, is performing well. Practice has proven that the system described in this paper effectively achieves automatic monitoring of the production process, reduces the labor intensity of workers, improves work efficiency, and can form a computer-integrated manufacturing system (CIMS) for the enterprise, together with computer manufacturing systems in other workshops.