The basic equipment of a power plant's chemical water treatment system is highly controllable and is monitored and managed in real time using PLCs (Programmable Logic Controllers) and industrial control computers. The system’s fundamental structure is illustrated in Figure 1, showcasing the integration of various components for efficient operation.
Three Siemens SIMATIC S5-115U programmable controllers are used to manage the primary on-site equipment. These controllers are organized into three distinct sets: #1 make-up water treatment unit, #2 make-up water treatment unit, and #3 condensate treatment unit. This setup ensures precise control over different parts of the system.
For operator interaction, two Advantech 586 industrial computers are utilized, running on the FIX5.5 configuration software from Intellution. This software provides a comprehensive platform for data acquisition, control, alarm management, and graphical display. It operates under Windows or Windows NT environments and uses a user-friendly graphical interface, leveraging third-generation graphics technology for enhanced visualization.
The communication network employed in this system is the SINEC L2 network, which connects the SIMATIC PLCs and industrial computers. As a token bus network, it supports twisted pair or fiber optic cables for reliable data transmission. Each node is connected via a bus connector, enabling seamless communication between the three PLCs and the two industrial computers.
Figure 1 presents the system structure diagram, illustrating the interconnected components and their roles in the overall process. The system design reflects the benefits of a distributed control system, where control functions are decentralized for faster response times and safety, while operations are centralized for easier management. Additionally, the system incorporates redundancy features to enhance reliability.
In terms of PLC programming, the SIMATIC S5-115U uses STEP5 as its programming language, offering three methods: Control System Flowchart (CSF), Ladder Diagram (LAD), and Statement List (STL). Among these, STL is most commonly used due to its closer alignment with the internal machine code and richer functionality.
STEP5 also supports structured programming, providing standard function blocks like FB242 (multiplication) and FB244 (communication), which simplify complex programming tasks. Programs can be divided into blocks such as Organization Blocks (OB), Program Blocks (PB), Sequence Blocks (SB), Function Blocks (FB), and Data Blocks (DB), allowing for modular and maintainable code.
For example, in the control program for the #1 make-up water treatment unit, multiple function blocks are called within the OB1 block. These include data exchange between the PLC and industrial PCs, communication settings, automatic control, valve and pump control, analog processing, and alarm handling. Each block is designed to handle specific tasks, ensuring smooth and efficient operation.
The industrial computer monitoring and management software is built on the FIX5.5 platform. Key modules include the System Configuration Module (SCU), Data Scanning and Alarm Module (SAC), Real-Time Database Builder, Drawing Module (DRAW), and Display Module (VIEW). These modules enable network configuration, data acquisition, real-time database management, graphical representation, and dynamic display of system status.
The system offers both host computer monitoring and analog disk monitoring, operating in a hot standby mode. Users can switch between manual, automatic, and monitoring modes without disturbance. When using the analog disk, the M/A switch determines whether the system is in manual or automatic control. Similarly, the host computer allows users to manually or automatically control the system, ensuring flexibility and responsiveness.
In conclusion, modern production systems have moved beyond single-machine PLC applications. The integration of PLCs with industrial computers and the use of networked systems have significantly improved automation and management efficiency. This trend highlights the importance of advanced control strategies in achieving reliable and efficient industrial operations.
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