The basic equipment of a chemical water treatment system in a power plant is highly controllable. It is monitored and managed in real-time using PLCs (Programmable Logic Controllers) and industrial control computers. The fundamental structure of the system is illustrated in Figure 1.
Three Siemens SIMATIC S5-115U programmable controllers are used to manage the on-site primary equipment. The control system is divided into three independent units: #1 make-up water treatment, #2 make-up water treatment, and #3 condensate treatment. This division allows for more efficient and localized control over different parts of the system.
The operator interface consists of two Advantech 586 industrial computers, which run FIX5.5 configuration software from Intellution. This software is a comprehensive platform for industrial automation, supporting data acquisition, control, alarm handling, and graphical display. It operates under Windows or Windows NT environments and uses a user-friendly graphical interface. The graphics processing is based on third-generation technology, enhancing visual clarity and usability.
Data communication within the system is facilitated through the SINEC L2 network. This token bus network connects the SIMATIC PLCs and industrial computers, enabling seamless data exchange. The network can use either twisted pair cables or fiber optics, with each node connected via a bus connector. In this setup, communication occurs between the three PLCs and the two industrial computers, ensuring real-time coordination across the system.
Figure 1: System Structure Diagram
The system design reflects the benefits of a distributed control system. It features decentralized control functions, allowing for fast real-time responses and reduced risk concentration, while centralizing operations for easier management. The configuration also includes redundancy, improving system reliability and fault tolerance.
2. PLC and Its Programming
2.1 Hardware Components and Programming Overview of SIMATIC S5-115U
The SIMATIC S5-115U is built on a modular architecture, with power supply, CPU, and I/O modules all mounted on a single motherboard. Additional expansion boards can be added as needed. This design offers flexibility in configuration, making it adaptable to various I/O requirements. Communication with the computer is simplified through a communication processor and local network integration.
The programming language for the S5-115U is STEP5, which supports three methods: Control System Flow Chart (CSF), Ladder Diagram (LAD), and Statement List (STL). Among these, STL is most similar to the internal machine code and provides more advanced functionality. Therefore, it is widely used in practical applications.
One of the key features of STEP5 is its structured programming approach, which includes numerous standard function blocks like FB242 (multiplication) and FB244 (communication). These blocks significantly reduce programming complexity and improve code readability, especially for large and complex systems.
STEP5 allows programs to be divided into separate blocks, such as Organization Blocks (OB), Program Blocks (PB), Sequence Blocks (SB), Function Blocks (FB), and Data Blocks (DB). OBs serve as the main entry point for user programs, managing interactions between the operating system and control logic. FBs are particularly useful for repeated or complex tasks, and they can be either standard or custom-built.
2.2 Design of Large-Scale SIMATIC S5-115U Programs
As an example, the control program for the #1 make-up water treatment unit utilizes several function blocks called from the organization block OB1. These include:
- JU FB1: Defines data transmitted from PLC1 to the two industrial PCs.
- JU FB2: Defines data sent from the PCs back to PLC1.
- JU FB231: Sets up communication between PLC1 and the two PCs.
- JU FB232: Sets up communication between PLC1 and other PLCs.
- JU FB4: Handles automatic control and bumpless switching.
- JU FB3: Manages pneumatic and electric valves.
- JU FB10: Processes analog signals.
- JU FB11: Handles alarms.
In FB1 and FB2, data is written to specific bits in a data block, ensuring consistency with the communication processor variables. FB231 uses standard function blocks like FB244 (send) and FB245 (receive), with parameters set according to the communication processor. Similarly, FB232 defines data transmission between PLCs based on allocated bits.
FB4 integrates data from other blocks and I/O modules to achieve smooth automatic control. Repeated or similar tasks are handled by function blocks like FB20, FB30, and FB40, which help synchronize PLC and screen times. Analog processing is managed by FB10, which calls the square function block for alarm processing. Proper mapping between analog and digital values ensures accurate display and alarms.
3. Industrial Computer Monitoring and Management Software
The monitoring and management software runs on the FIX5.5 platform, a high-precision modular system with multiple software modules. Key modules include:
- **System Configuration Module (SCU):** Configures network settings, I/O drivers, and database names. Over 300 I/O drivers are available for various PLCs and devices.
- **Data Scanning, Alarm, and Control Module (SAC):** Handles field data scanning, signal conditioning, and remote control. It sends data to the real-time database or outputs it through I/O drivers.
- **Real-Time Database Builder:** Allows interactive creation and modification of the real-time database. Accurate tag naming is crucial for data association and future queries.
- **Drawing Module (DRAW):** Provides a GUI for creating dynamic graphics. Users can generate objects like valves and pumps, linking them to real-time data for visual updates.
- **Display Module (VIEW):** Dynamically displays created graphics, allowing users to switch views, input data, and monitor the system.
4. System Functions
The system supports both host computer and analog disk monitoring, operating in hot standby mode. Three working modes are available: manual, automatic, and monitoring. Switching between modes is smooth and uninterrupted.
When monitored via the analog disk, the system's state depends on the M/A switch. Manual mode allows direct operation, while automatic mode enables full automation. When using the host computer, users can select manual or automatic control, performing one-to-one device operations or automated start/stop commands.
5. Conclusion
In large and medium-scale production systems, the era of single-machine PLCs has passed. Integration with industrial computers and network technologies has significantly enhanced automation and management capabilities. This trend is essential for modern industrial control systems.
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