The basic equipment of the chemical water treatment system of a power plant has good controllability. It is monitored and managed in real time using PLC and industrial control computer. The basic composition of the system is shown in Figure 1.
Three Siemens SIMATICS 5-115U programmable logic controllers (PLCs) are used to control the primary equipment on site. The PLC system consists of three sets: #1 makeup water treatment unit, #2 makeup water treatment unit, and #3 condensate treatment unit.
The operator station uses two Advantech 586 industrial PCs, and the software development platform uses IntelliTion's FIX 5.5 configuration software. FIX 5.5 is a complete industrial automation software capable of data acquisition and control, alarm, and graphical data display. This version runs on Windows or Windows NT and uses a graphical user interface. Correspondingly, its internal graphics processing is based on third-generation graphics technology.
The data communication system uses the SINECL2 network, which connects SIMATIC series programmable logic controllers (PLCs) and industrial computers into a network. SINECL2 is a token bus network, and the network transmission medium is twisted pair cable or optical fiber. Each node is connected to the bus network via a bus connector. In this system, data communication is achieved between the three PLCs and between the PLCs and the two industrial computers.
Figure 1 System structure diagram
This configuration demonstrates the advantages of a distributed control system: decentralized control functions and centralized operation management. Decentralized control functions mean faster real-time system response and dispersed system hazards, while centralized operation management facilitates centralized management. The configuration also features redundancy.
2. PLC and its programming
2.1 SIMATIC S5-115U Hardware Composition and Programming Overview
The SIMATIC S5-115U programmable logic controller (PLC) adopts a standard modular structure. The power supply, CPU, and various I/O modules are all plugged into a motherboard, and expansion motherboards can be added according to different I/O points. The fine hierarchical structure of input, output modules, and memory makes this device highly adaptable to different configurations. Through the communication processor and local area network, communication between PLCs and with computers can be easily realized.
The programming language for SIMATICS 5-115U is STEP 5, which has three expression methods: Control System Flowchart (CSF), Ladder Diagram (LAD), and Statement List (STL). Among them, Statement List (STL) is closest to the internal control program of the machine and has much richer functions than the other two methods. Therefore, STL is used for all programming applications in this system.
The most significant feature of STEP5 is its use of structured programming methods and the provision of a large number of standard function blocks, such as the multiplication function block FB242 and the communication function block FB244, which greatly simplifies the programming work. Moreover, the programs written are clear and easy to read, modify and test. This advantage is especially evident when writing large and complex programs.
To accomplish complex tasks, the entire program can be divided into independent program blocks. STEP5 has five block types: Organization Block (OB), Program Block (PB), Sequence Block (SB), Function Block (FB), and Data Block (DB). Organization Blocks (OB) manage user programs, forming the interface between the operating system and the control program; all other block types are called and executed from here. Function Blocks (FB) are used to implement repeatedly called or particularly complex program functions. These function blocks can be provided by the system as standard function blocks or can be written by the user. For example, the standard function block FB242 can implement 16-bit binary multiplication, and FB244 can implement data transfer between the CPU and the communication processor. These function blocks can be directly called when needed.
2.2 Design of a large-scale SIMATICS 5-115U program
Taking the control program of the #1 water supply treatment unit of this system as an example, the following statements are mainly contained in the organization block OB1 to complete the unconditional call of each function block.
JUFB1 (defines the data transmitted from PLC1 to two industrial computers)
JUFB2 (defines the data transmitted from two industrial computers to PLC1)
JUFB231 (Complete the basic setup for communication between PLC1 and two industrial computers)
JUFB232 (Complete the basic settings for communication between PLC1 and PLC2/PLC3)
JUFB4 (enables automatic control and seamless switching)
JUFB3 (Pneumatic and Electric Door Control)
JUFB10 (implements analog signal processing function)
JUFB11 (Alarm Handling)
Within FB1 and FB2, the data requiring communication is written into the corresponding bits of a data block, such as DB10, to ensure consistency with the variables in the communication processor. FB231 calls two standard function blocks provided by STEP5 itself, FB244 (send data) and FB245 (receive data), and then fills in necessary parameters such as interface and job number according to the communication processor to achieve data communication. Within FB232, the data to be transmitted between the three PLCs is defined according to the data bits allocated by the communication processor. Within FB4, based on the production process requirements and operating specifications, the data transmitted by other function blocks and I/O modules is fully utilized to achieve automatic control and seamless switching of the system. Considering the similarity of multiple controlled objects, several representative function blocks FB20, FB30, and FB40 are compiled. For example, FB20 is called multiple times within FB4 to ensure consistency between the program step time in the PLC and the display time on the industrial control computer screen. Furthermore, FB20 calls the multiplication function block FB244. FB3 controls field equipment such as pneumatic doors, electric doors, and pumps based on the automatic program step instructions issued by FB4. FB10 is responsible for processing all analog quantities. Here, the square root function block FB5 is called. FB11 performs alarm processing on the analog quantities based on the data converted by FB10. It is important to pay attention to the correspondence between the analog quantities and the digital quantities inside the PLC to ensure the accuracy of the analog quantity display and alarm.
3. Design of Industrial Control Computer Monitoring and Management Software
The industrial control computer monitoring and management software was developed using the FIX5.5 software platform. FIX5.5 is a high-precision modular software system that includes more than a dozen software modules. This article mainly introduces the software modules used in the development of this application software.
(1) System Configuration Module (SCU): It mainly completes the configuration of network, I/O drivers, database name, system startup parameters and initial startup tasks. Intellution and third-party manufacturers have written more than 300 I/O drivers for PLCs and I/O cards, such as drivers for products of SIEMENS, OMRON, MODICON, ABB and other companies, and provide I/O driver development kits for users to develop their own I/O drivers.
(2) Data Scan, Alarm and Control Module (SAC): It is used to scan field data, condition signals, convert data formats and data types, determine alarm conditions and realize remote control output. The SAC sends the processed field data to the real-time database or sends the remote control output data to the I/O driver to realize the remote control output function.
(3) Real-time Database Management Module (DATABASEBUILDER): This module provides interactive functionality for creating and displaying/modifying real-time databases online. It is the primary data source for system operation. Users need to perform a significant amount of work here, primarily filling in variable label names to establish a one-to-one correspondence between field data and variable labels in the database, enabling the retrieval of this data in other modules. Filling in variable label names requires both standardization and skill. First, it is necessary to adhere to the syntax requirements of the FIX software; second, label names should be defined according to certain classification standards to improve efficiency in future data queries and applications.
(4) DRAW Module: FIX has an intuitive, object-based graphical user interface (GUI) that simplifies the graphics development process. To create a screen, you can use the toolbox provided by DRAW to generate objects such as valves, pumps, and log tables, and define their dynamic characteristics. That is, the objects change their state, size, color, rotation, and movement based on field data, such as the opening or closing of valves and the turning or stopping of pumps. These tasks can be completed in a dialog box, mainly by linking the variable labels in the real-time database with the corresponding objects, so that the object state changes with the field data. To reduce graphics development time, Intellution also provides a graphics library of commonly used equipment objects, which contains a variety of international standard graphics that can be pasted into the user screen at any time. At the same time, users can save their own commonly used graphics to the graphics library for future use.
(5) Display module (VIEW): Its main function is to dynamically display the screen created by DRAW. It can switch between multiple screens, change the screen shape, input data, and realize monitoring, which is the actual operation screen provided to the user.
4 System Functions
The system has both host computer monitoring and analog panel monitoring functions, which operate in parallel with each other as hot standby modes. The host computer screen offers three operating modes: host computer manual, host computer automatic, and host computer monitoring, and the switching between modes is seamless.
When the system requires monitoring via the analog panel, the host computer screen selects the host computer monitoring mode. The system status is then determined by the analog panel's M/A switch. When M/A is set to manual, one-to-one manual operation can be performed on the analog panel using the buttons. When M/A is set to automatic, the system can achieve automatic control of water treatment via an automatic program.
When the system requires monitoring by a host computer, the host computer screen can be used to select between manual and automatic functions. When manual mode is selected, one-to-one manual operation of the local device can be achieved on the host computer screen. When automatic mode is selected, automatic start/stop control can be performed on the host computer screen; the automatic control program is the same as the automatic control function of the analog panel.
5. Conclusion
In large and medium-sized production systems, the era of using programmable logic controllers (PLCs) on a standalone basis has passed. The organic integration of PLCs with industrial control computers and the application of computer networks have greatly improved the level of automation control and management on the production site, which is an inevitable trend.
