I. Key Differences
1. DCS
Communication is key to a DCS system. You could say the data highway is the spine of a DCS. Since its task is to provide a communication network between all components of the system, the design of the data highway itself determines the overall flexibility and security. The media for the data highway can be: a twisted pair of wires, coaxial cable, or fiber optic cable.
By analyzing the design parameters of a data highway, one can essentially understand the relative advantages and disadvantages of a specific DCS system.
(1) How much I/O information can the system process?
(2) How much information about control loops related to control can the system process?
(3) How many users and devices (CRT, control station, etc.) can it accommodate?
(4) How is the integrity of the transmitted data thoroughly checked?
(5) What is the maximum allowable length of the data highway?
(6) How many branch roads can the data highway support?
(7) Whether the data highway can support hardware (programmable controllers, computers, data recording devices, etc.) manufactured by other manufacturers.
To ensure the integrity of communication, most DCS manufacturers can provide redundant data highways.
To ensure system security, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules used to ensure that the transmitted data is received and understood in the same way as the sent data.
Currently, DCS systems generally use two types of communication methods: synchronous and asynchronous. Synchronous communication relies on a clock to regulate data transmission and reception, while asynchronous networks use a reporting system without a clock.
2. FCS
The key points of FCS are as follows:
(1) The core of the FCS system is the bus protocol, i.e. the bus standard.
Once the bus protocol of a particular type of bus is determined, the related key technologies and equipment are also determined. In terms of the basic principles of their bus protocols, all types of buses are the same, based on solving bidirectional serial digital communication transmission. However, due to various reasons, there are significant differences in the bus protocols of different types of buses.
To ensure the interoperability of fieldbus devices and make them truly open systems, the original IEC international standard explicitly stipulated that the user layer of the fieldbus communication protocol should include device description functionality. For interoperability, each fieldbus device is described using a Device Description (DD). A DD can be considered a driver for the device, containing all necessary parameter descriptions and the operating steps required by the master station. Because the DD includes all the information needed to describe device communication and is independent of the master station, true interoperability between field devices is achieved.
The answer to whether the actual situation is consistent with the above is no. Currently, the adopted international fieldbus standards include eight types, while the original IEC international standard was only one of those eight types, and its status was equal to the other seven types. Regardless of their market share, each of the other seven types of buses has a set of software and hardware support for its protocol. They can form systems and products. However, the original IEC fieldbus international standard was an empty framework without either software or hardware support. Achieving mutual compatibility and interoperability among these buses is, in its current state, virtually impossible.
Based on the above, can we conclude that the interoperability of open fieldbus control systems means that, for a specific type of fieldbus, as long as the bus protocol of that type of fieldbus is followed, the products are open and interoperable? In other words, regardless of the manufacturer or even the fieldbus company, as long as the products follow the bus protocol, are open and interoperable, they can form a bus network.
(2) The foundation of the FCS system is digital intelligent field devices.
Digital intelligent field devices are the hardware support and foundation of the FCS system. The reason is simple: the FCS system executes a two-way digital communication fieldbus signaling system between automatic control devices and field devices. If the field devices do not follow a unified bus protocol, i.e., the relevant communication rules, and do not possess digital communication capabilities, then the so-called two-way digital communication is just empty talk, and it cannot be called a fieldbus control system. Furthermore, a key characteristic of fieldbus is the addition of field-level control functions. If the field devices are not multifunctional and intelligent products, then the characteristics of a fieldbus control system do not exist, and the so-called advantages of simplified systems, convenient design, and easy maintenance are also meaningless.
(3) The essence of the FCS system is the on-site processing of information.
For a control system, whether using a DCS or a fieldbus, the amount of information the system needs to process is at least the same. In fact, using a fieldbus allows for the acquisition of more information from the field. The amount of information in a fieldbus system does not decrease, and may even increase, while the number of cables transmitting information is greatly reduced. This necessitates, on the one hand, significantly improving the information transmission capacity of cables, and on the other hand, enabling a large amount of information to be processed locally on-site, reducing the back-and-forth communication between the field and the control room. It can be said that the essence of fieldbus is the localization of information processing.
Reducing information round trips is an important principle in network design and system configuration. Reducing round trips often improves system response time. Therefore, in network design, nodes with high information exchange volumes should be placed on the same branch.
Reducing data transfer and minimizing system cabling can sometimes be contradictory. In such cases, the principle of saving investment should still be prioritized. If the response time of the chosen system allows, the cabling-saving solution should be selected. If the response time of the chosen system is more critical, and slightly reducing data transfer is sufficient, then the solution that reduces data transfer should be chosen.
Some field instruments with fieldbuses now come equipped with many function blocks. Although the performance of the same function block may vary slightly between different products, it is a common occurrence that many function blocks with similar functions exist on a single network branch. Which function block from the field instrument to select is a problem that system configuration must address.
The principle for considering this issue is to minimize information round trips on the bus. Generally, the function block on the instrument that outputs the most information related to this function can be selected.
II. Comparison of Typical Systems
By using fieldbus, users can significantly reduce field wiring, achieve multi-variable communication with a single field instrument, achieve full interoperability between devices from different manufacturers, increase field-level control functions, greatly simplify system integration, and make maintenance very convenient. In traditional process control systems, each field device requires a dedicated twisted-pair cable to transmit 4-20mA signals to the control room. In a fieldbus system, the twisted-pair cable from each field device to the junction box can still be used, but digital communication from the field junction box to the central control room is accomplished with only a single twisted-pair cable.
The editor has not yet calculated exactly how much cable can be saved by adopting a fieldbus control system. However, we can look at the total length of cables used in power plants with DCS systems related to automatic control systems to see the share of cable in infrastructure investment.
1. A power plant with 2×300MW coal-fired units
The thermal system is unit-based. Each unit has a centralized control building, employing a centralized control method for the turbine, boiler, and electrical units. The unit control room is at an elevation of 12.6m, consistent with the operating floor elevation. The DCS adopts WDPF-Ⅱ, with 4500 I/O points designed for each unit.
The cable laying used Russian EC components, and eight people completed the design task of laying the cable in 1.5 months.
The number of cables for the automation system of each 300MW unit in the main plant is 4038.
The length of the automation cables for each 300MW unit inside the main plant is 350 kilometers.
The above number and length of cables do not include the factory-supplied cables for the fire alarm system and the cables for all auxiliary production workshops in the factory.
The cable tray columns, trays, and small troughs are all made of galvanized steel, with each unit weighing approximately 95 tons.
Other cable trays, including straight sections, bends, tees, crosses, covers, terminal caps, width adjustment plates, and direct sections, are made of aluminum alloy, with each 300MW unit weighing approximately 55 tons.
Accessories (such as bolts and nuts) are provided with the cable tray.
2. A power plant, 4×325MW oil and gas-fired power plants
The thermal system is unit-based. The DCS uses Teleperm XP. Each unit is designed with 5804 I/O points.
The cable laying was carried out using EC software, and 12 people completed the cable laying design task in 2.5 months.
The number of cables for the automation system of each 325MW unit in the main plant is 4413.
The length of the cables for the automation installation of each 325MW unit in the main plant is 360km.
Each unit uses galvanized steel cable trays, which weigh approximately 250 tons.
3. Power plant cables can be divided into six categories: high-voltage power cables, low-voltage power cables, control cables, thermal control cables, low-voltage cables (mainly computer cables), and other cables. If two 300MW units are to be cabled simultaneously, the number of automation cables will be approximately 8,500. Among them, thermal control cables and low-voltage cables will exceed 500, accounting for approximately 60% (measured by the number of cables).
III. Design, Investment and Use
The above comparisons are primarily technical; the following comparisons will incorporate economic factors.
The comparison is based on the premise of comparing a DCS system with a typical, ideal FCS system. Why make such an assumption? As DCS systems have evolved to their current state, the technical requirements set forth in the early stages of development have been met and improved; the current state is one of further improvement, therefore the concept of a "typical" or "ideal" system no longer exists. However, FCS systems, which only entered practical application in the 1990s, still have unmet technical requirements from their early development stages, such as compatibility with open systems, bidirectional digital communication, digital intelligent field devices, and high-speed buses, which require further improvement. This situation is undeniably related to the development of international fieldbus standards. For the past decade or so, various fieldbus organizations have been busy developing standards, products, and capturing more market share, aiming to become part of the international standards and legally secure a larger market share. Now, the battle for international standards has subsided, and major companies and organizations have realized that to truly capture the market, they must improve their systems and related products. We can predict that in the near future, improved fieldbus systems and related products will inevitably become the mainstream of fieldbus technology worldwide.
Detailed comparison:
(1) A DCS system is a large system with powerful controllers that play a crucial role in the system, and the data highway is the key component. Therefore, a one-time investment is required for the entire system, and subsequent expansion is difficult. In contrast, FCS decentralizes functions more thoroughly, localizes information processing, and widely adopts digital intelligent field devices, which relatively reduces the function and importance of the controller. Therefore, FCS systems have a lower initial investment and can be used, expanded, and put into operation simultaneously.
(2) DCS systems are closed systems, and products from different companies are basically incompatible. In contrast, FCS systems are open systems, and users can choose various devices from different manufacturers and brands to connect to the fieldbus to achieve optimal system integration.
(3) The information in a DCS system is all formed by binary or analog signals, which requires D/A and A/D conversion. However, the FCS system is fully digital, which eliminates the need for D/A and A/D conversion. Its high integration and high performance allow the accuracy to be improved from ±0.5% to ±0.1%.
(4) The FCS system can incorporate PID closed-loop control function into the transmitter or actuator, shortening the control cycle. Currently, it can be increased from 2-5 times per second in DCS to 10-20 times per second in FCS, thereby improving the regulation performance.
(5) The DCS system can control and monitor the entire process, and perform self-diagnosis, maintenance, and configuration. However, due to its inherent weakness, its I/O signals use traditional analog signals, therefore, it cannot perform remote diagnosis, maintenance, and configuration of field instruments (including transmitters, actuators, etc.) on the DCS engineering station. The FCS system adopts fully digital technology, with digital intelligent field devices sending multi-variable information, not just single-variable information, and also has the function of detecting information errors. The FCS system uses a bidirectional digital communication fieldbus signal system. Therefore, it can diagnose, maintain, and configure field devices (including transmitters, actuators, etc.). This advantage of the FCS system is unmatched by the DCS system.
(6) Due to its on-site information processing, the FCS system can eliminate a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files, and I/O cabinets compared to the DCS system. It also saves space and floor area for I/O devices and device rooms; some experts believe it can save up to 60%.
(7) For the same reason as (6), the FCS system can reduce a large number of cables and cable trays used for laying cables, and also saves design, installation and maintenance costs. Some experts believe that it can save 66%.
Regarding points (6) and (7), it should be added that the investment savings achieved by adopting the FCS system are undeniable, but whether it reaches 66% as some experts claim remains to be seen. These figures appear in multiple articles, which the editors believe is the result of mutual referencing, and the original sources of these figures have not yet been found. Therefore, readers should be cautious when citing these figures.
(8) FCS is simpler to configure than DCS. Due to its standardized structure and performance, it is easy to install, operate and maintain.
(9) Key considerations for the design and development of FCS for process control. This point is not for comparison with DCS, but rather to illustrate the key issues that should be considered in the design and development of FCS for process control or for simulating continuous processes.
① The bus must be explosion-proof, and this is of paramount importance.
② Basic monitoring, such as changes in flow rate, material level, temperature, and pressure, is slow and has a lag effect. Therefore, node monitoring does not require fast electronic response times, but it does require complex analog signal processing capabilities. This physical characteristic determines that the system basically adopts a centralized polling system between master and slave, which is technically reasonable and economically advantageous.
③ The physical principles for measuring parameters such as flow rate, material level, temperature, and pressure are classical, but sensors, transmitters, and controllers should develop towards digital and intelligent technologies.
④ As an FCS system developed for continuous processes and their instruments, the focus should be on the design and improvement of the low-speed bus H1.
