DCS is short for Data Communication Subsystem. A DCS is a distributed control system , while a PLC (Programmable Logic Controller) is simply a control "device." The difference lies in the distinction between a "system" and a "device." A system can perform the functions and coordination of any device, while a PLC only performs the functions specific to its own unit.
Since its introduction in 1975, DCS has undergone approximately three major transformations. In the 1970s, the hardware, operating system, and monitoring software of the operator stations were all proprietary, developed by individual DCS manufacturers, and lacked dynamic flowcharts; communication networks were primarily polling-based. The 1980s saw a shift, with token-based communication networks becoming more common. In the 1990s, general-purpose systems emerged for operator stations, and by the late 1990s, some communication networks adhered to TCP/IP protocols, while others began using Ethernet. Overall, the changes primarily occurred in I/O boards, operator stations, and communication networks. Controllers, relatively speaking, saw fewer changes. Operator stations evolved from dedicated machines to general-purpose machines, such as PCs and minicomputers. Currently, their operating systems generally use UNIX, with some smaller systems using NT. UNIX is generally considered more stable, while NT is prone to crashes. I/O boards saw the introduction of fieldbuses into DCS systems.
A DCS system comprises three main parts: controllers with I/O components, a communication network, and a human-machine interface (HMI). The HMI includes operator stations, engineer stations, and historical data stations. The controller I/O components connect to the production process, the operator stations connect to humans, and the communication network links these two parts into a system. Therefore, the operator station is a crucial component of the DCS; the engineer station configures the controller and operator stations; and the historical data station records historical production data. Recent developments have also included dynamic data servers for HMIs .
A DCS system controller and I/O components can typically operate for 16-20 years, while the operator station, because it has moving parts, is more prone to failure, such as hard drives, keyboards, CRTs, floppy drives, etc. After 6-8 years of operation, the probability of failure is relatively high. Therefore, operator stations are frequently replaced during the operation of a DCS.
The controller of a DCS (Distributed Control System) has undergone relatively minor changes. These changes primarily manifest in the arrangement and number of control algorithms, the number of I/O points accessed, and the size of the memory. The operating system is generally dedicated. The operator station, however, has seen significant changes. Before the 1980s, operator stations typically lacked hard drives and dynamic flowcharts, and could display a limited number of tags, such as 500 tags (tags refer to logical relationships like AI, DI, loops, and switch quantities). In the 1980s, operator stations capable of displaying 5000 tags appeared, and in the 1990s, those capable of displaying 30,000 tags emerged. Simultaneously, general-purpose display software running on Microsoft's NT platform also appeared. Initially, this general-purpose software was only used on PLC operator stations, but it was gradually applied to DCS as well. Its tag capacity can reach 10,000 or even more.
Since the advent of fieldbus technology in the late 1990s, it has ushered in a new era of automation control technology—the digital age. Fieldbus is a digital, bidirectional, intelligent control system that integrates fieldbus-driven local devices, control systems, and predictive maintenance software to improve the informatization and intelligence level of power plants, ultimately achieving a qualitative improvement in the power plant's control level, economic benefits, and competitiveness.
Most fieldbus systems have already undergone IEC 61508 certification, and their communication protocols have passed IEC 1158-2 (1993) and ISAS 50.02-1992 standard fieldbus 'Ff-81631.25kbits/s certification. They have been preliminarily verified in practical applications in power plants using a "proven in use" approach. Similar to the initial application of DCS systems in power plants, the functional safety evaluation of fieldbus systems will gradually be recognized and verified through practical use.
Fieldbus technology has revolutionized the field of automation. This technology represents the future direction of automation development. Digital communication is an unstoppable trend, an inevitable consequence of modern technological advancements. The bidirectional digital communication fieldbus signal technology itself will undoubtedly bring tangible benefits to the safe and economical operation of thermal power plants and improve their management.
Fieldbus, utilizing bus-driven I/O cards, high-speed field communication networks, and intelligent field devices, greatly simplifies the installation and commissioning of control systems and forms the foundation for the digitalization, intelligentization, and informatization of power plants. The basic data acquired by fieldbus from field devices is essential for improving power plant management.
Research on the application of fieldbus in Chinese power plants has developed rapidly along with the configuration of control systems for newly built 600MW thermal power units, the selection level of automation devices, and the increasing requirements for the overall layout and installation of power plants.
Composition of process control station
A DCS process control station is a complete computer system, mainly composed of a power supply, CPU (Central Processing Unit), network interface, and I/O. I/O: The control system needs to establish input and output channels for signals; this is I/O. I/O in a DCS is generally modular, with one or more I/O channels on each module used to connect sensors and actuators (control valves). I/O Units: Typically, a process control station consists of several racks, each capable of holding a certain number of modules. The rack containing the CPU is called the CPU unit; only one CPU unit can exist in the same process station. The other racks, used only to house I/O modules, are called I/O units.
In terms of networks
The DCS network is the central nervous system of the entire system. It is a safe, reliable, dual-redundant, high-speed communication network with better system scalability and openness. PLCs, on the other hand, are primarily designed for individual operation. When communicating with other PLCs or host computers, they typically use a single-network structure, and their network protocols often do not conform to international standards. PLCs lack adequate network security protection measures; therefore, we employ dual redundancy for power supply, CPU, and network.
In terms of hardware
All I/O modules in a DCS system have CPUs, enabling them to assess the quality of acquired and output signals and perform scalar conversions. They can be hot-swapped in case of faults and replaced randomly. In contrast, PLC modules are simply electrical conversion units without intelligent chips; a fault in one of these units will cause the entire system to malfunction.
Domestic DCS system
In the mid-1980s, many organizations and individual technicians wanted to get involved in the development and production of DCS (Distributed Control Systems). However, the large capital investment, the massive amount of software engineering required, the need for coordination among software developers, hardware engineers, and other engineers, and the sheer complexity of DCS development (involving computer, communication, control, and display technologies) meant that a few technicians couldn't master all the necessary technologies, lacked the necessary capital, and couldn't produce results quickly enough. Many technicians simply couldn't sustain the effort. Many could only admire DCS from afar. Many organizations, such as the automation departments of several large steel companies, also withdrew from DCS development due to insufficient funds. For example, the Acheng Relay Factory began DCS development in 1984 and achieved considerable success. However, due to insufficient funds and mismanagement, it too was forced to essentially withdraw from the DCS competition. STD bus products, intermediate products in control systems, saw numerous manufacturers in the early 1990s, with annual sales reaching hundreds of millions in China (comparable to current domestic DCS sales). However, due to the heavy workload for end users, they are also withdrawing from the market.
In the mid-1990s, most DCS components, except for function blocks, were readily available on the market. For example, controllers could use PC motherboards, and DCS networks could use Ethernet. Human-machine interfaces (HMIs) could use general-purpose monitoring software such as KingSCADA. The market situation had changed significantly compared to the 1980s. Another wave of DCS development emerged, with various brands of DCS systems appearing on the market, resulting in approximately a hundred different systems. Upon closer analysis, however, these systems were all repetitive at a lower level, offering limited functionality and failing to achieve economies of scale.
With the involvement of relevant government departments, new entities began to enter the DCS development field. These departments provided strong support in terms of both funding and projects, leading to rapid development. Companies like Beijing Hollysys, Shanghai Xinhua, and Zhejiang University Control System Co., Ltd. have achieved sales of several hundred units each, essentially reaching large-scale production. While they haven't yet reached the level of imported systems, they can still meet basic control requirements. In terms of system structure and I/O board production, they have reached a considerably high level, not far from international standards. For example, the I/O boards produced in Wuxi are aesthetically pleasing. Human-machine interface software has also been developed domestically, such as KingSCADA, which sells 2,000 units annually and has shown good performance in use.
The most challenging aspect of DCS development lies in controller development. Firstly, the hardware uses PC cards and generic operating systems like Microsoft NT or the Canadian-developed QNX, resulting in high prices and reduced profits. Secondly, the development of function blocks is the most difficult. The number of function blocks is virtually unlimited. The workload is immense, requiring collaboration between software engineers and engineers to create functional blocks that function well. Due to my country's previous planned economy, technical talent was not highly valued, and individual units often held private ownership. It was difficult for a single unit to concentrate so many skilled technical personnel. Furthermore, my country's software developers are not only few in number but also accustomed to independent programming, lacking effective communication with field technicians. Consequently, the developed function blocks often exhibit unsatisfactory characteristics. Domestically produced DCS systems not only have a limited number of function blocks, but according to user feedback, for example, the truth tables of sequential controller function blocks are not as user-friendly as those of foreign systems. Similarly, self-adjusting function blocks are generally not used in the field, and even when used, they are often unreliable.
In short, due to relatively small sales volume and insufficient tracking of user feedback, my country still lacks a set of thoroughly tested and proprietary functional codes. This software is not commercially available; it must be developed in-house. Furthermore, the programs written will differ depending on the operating system. This is unrelated to whether or not a controller hardware is needed. Using a fieldbus system also requires algorithms, as does using a PC for both control and display.
No problems have been found with the configuration software used for control strategies so far.
Another issue is the controller's power supply system. In my country, integrated power supplies are generally used. Because DCS systems in my country are very inexpensive and have low profit margins, DCS manufacturers lack the funds to invest in developing power supply systems. The power redundancy of foreign DCS systems is shown in the diagram below. When installing two power supplies into the cabinet, they need to be adjusted separately with a load. During adjustment, the load on both power supplies must be the same, and they must be adjusted to have the same output before being installed in the cabinet. The power system includes a power detection board that displays the output status of both power supplies. Both power supplies can supply power to the load simultaneously. Ideally, they should operate in a balanced voltage state. Twenty years of operation have proven the power supply system to be effective.
The domestically produced DCS uses two redundant power supplies, one in operation and one on standby. If the AC input power of one power supply experiences a step drop, it can switch to the backup power supply. However, if the input voltage ramps down, it cannot switch to the backup power supply. This is one of the drawbacks of the power system.
The N+1 power supply scheme has not been adopted in my country's DCS systems. Imported DCS systems using N+1 power supplies also have many problems. N+1 power supply means that once the power of each inserted module is known, the total power required by all modules is summed. Since the power of each module's power supply is fixed, dividing the total required power by the power of each module's power supply gives the number of power supplies. Any decimal places are rounded up to the nearest integer, N, and then one is added to obtain N+1.
The third problem is the low sales volume. There is a lack of third-party assistance in improving the system, such as expert systems, real-time databases, and interfaces with digital instruments and PLCs.
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