Introduction
The fieldbus control system, which became practical in the 1990s, is rapidly developing with great momentum and is currently the latest type of control system in the world. The fieldbus control system is a hot topic in automation technology, attracting increasing attention from domestic and international automation equipment manufacturers and users. The emergence of the fieldbus control system will bring another revolution to the automation field, with a depth and breadth that will surpass any previous historical instance, thus ushering in a new era of automation.
In some industries, FCS has developed from PLC, while in others, FCS has developed from DCS. Therefore, there are intricate connections between FCS, PLC, and DCS, as well as essential differences. This article will analyze the characteristics and differences of the three major control systems: PLC, DCS, and FCS, pointing out their origins and development directions.
The Basic Characteristics of PLC, DCS, and FCSCurrently, in continuous process production automatic control (PA), commonly referred to as industrial process control, there are three major control systems: PLC, DCS, and FCS. Their basic characteristics are as follows:2.1 PLC
(1) Developed from switch control to sequential control and transport processing, it is bottom-up.(2) Multi-functional capabilities such as continuous PID control, with PID in the interrupt station.(3) A single PC can serve as the master station, with multiple identical PLCs as slave stations.(4) A single PLC can also serve as the master station, with multiple identical PLCs as slave stations, forming a PLC network. This is more convenient than using a PC as the master station: when user programming is involved, there is no need to know the communication protocol; just follow the manual format.(5) The PLC grid can serve as an independent DCS/TDCS or as a subsystem of DCS/TDCS.(6) Large systems are similar to DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI, MOD300.
(7) PLC networks such as Siemens’ SINEC—L1, SINEC—H1, S4, S5, S6, S7, GE’s GENET, Mitsubishi’s MELSEC—NET, MELSEC—NET/MINI.
(8) Mainly used for sequential control in industrial processes, new PLCs also have closed-loop control functions.(9) Manufacturers: GOULD (USA), AB (USA), GE (USA), OMRON (Japan), MITSUBISHI (Japan), Siemens (Germany), etc.
2.2 DCS or TDCS
(1) The Distributed Control System (DCS) and the Total Distributed Control System (TDCS) integrate 4C (Communication, Computer, Control, CRT) technologies into a monitoring technology.(2) A top-down tree topology large system, where communication is key.(3) PID is in the interrupt station, connecting computers with field instruments and control devices.(4) It features a tree topology and parallel continuous link structure, with many cables running from relay stations to field instruments.(5) Analog signals, A/D—D/A, mixed with microprocessors.(6) Each instrument connects to I/O with a pair of wires, linked to the local area network (LAN) by the control station.
(7) DCS has a three-level structure: control (engineer station), operation (operator station), and field instruments (field measurement and control station).(8) The downside is high costs, incompatibility between products from different companies, and non-interoperability; DCS systems vary by manufacturer.(9) Used for large-scale continuous process control, such as petrochemicals.(10) Manufacturers: Bailey (USA), Westinghouse (USA), HITACHI (Japan), LEEDS & NORTHRMP (USA), SIEMENS (Germany), Foxboro (USA), ABB (Switzerland), Hartmann & Braun (Germany), Yokogawa (Japan), Honeywell (USA), Taylor (USA), etc.2.3 FCS
(1) The primary task is: intrinsic safety, hazardous areas, variable processes, and challenging environments.(2) Fully digital, intelligent, and multifunctional, replacing analog single-function instruments and control devices.(3) Connects distributed field instruments, control devices, PID, and control centers with two wires, replacing two wires for each instrument.(4) On the bus, PID and instruments, control devices are equal.
(5) Multivariable, multi-node, serial, digital communication systems replace single-variable, single-point, parallel, analog systems.(6) Interconnected, bidirectional, and open, replacing unidirectional and closed systems.(7) Uses decentralized virtual control stations instead of centralized control stations.(8) Operated by field computers, which can also connect to upper-level computers on the same bus.(9) Local area networks can also connect to the internet.
(10) Changes traditional signal standards, communication standards, and system standards into enterprise management networks.
(11) Manufacturers: Honeywell (USA), Smar, Fisher—Rosemount, AB/Rockwell, Elsag—Bailey, Foxboro, Yamatake, Yokogawa (Japan), Siemens (Europe), GEC—Alsthom, Schneider, Process—Data, ABB, etc.(12) Three types of FCS: 1) Continuous process automatic control, such as petrochemicals, where “intrinsic safety” technology is critically important, with typical products being FF, World FIP, Profibus—PA; 2) Discrete process automatic control, such as automotive manufacturing robots, with typical products being Profibus—DP, CANbus; 3) Multi-point control, such as building automation, with typical products being LON Work, Profibus—FMS.
From the above basic points, have we noticed that none of the three major systems for process control were developed specifically for power plants? In their early development, they were not intended as the primary control objects for power plants. Moreover, in their usage instructions, power plants are not mentioned as the primary applicable range; some do not mention power plants at all. Interestingly, these three major control systems, especially DCS and PLC, are now widely used in power plants with very good results.
Differences Between the Three Major Control SystemsWe already know that FCS has developed from both DCS and PLC, and FCS not only possesses the characteristics of both DCS and PLC but has also taken a revolutionary step forward. Currently, new types of DCS and PLC are trending towards each other. New DCS already has strong sequential control capabilities, while new PLCs are also competent in handling closed-loop control, and both can form large networks, with significant overlap in the applicability of DCS and PLC. The next section will compare DCS and FCS. The previous sections have already touched on the differences between DCS and FCS; below, we will discuss aspects such as architecture, investment, design, and usage.3.1 Key Differences DCS
The key to DCS systems is communication. One could say that the data highway is the backbone of the distributed control system 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 safety. The media for the data highway can be: a pair of twisted wires, coaxial cables, or fiber optic cables.
Through the design parameters of the data highway, one can generally understand the relative advantages and disadvantages of a specific DCS system.
(1) How much I/O information can the system handle? Source:(2) How much control loop information related to control can the system handle?(3) How many users and devices (CRT, control stations, etc.) can it accommodate?(4) How thoroughly is the integrity of transmitted data checked?(5) What is the maximum allowable length of the data highway?(6) How many branches can the data highway support?(7) Can the data highway support hardware produced by other manufacturers (programmable controllers, computers, data recording devices, etc.)?
To ensure communication integrity, most DCS manufacturers can provide redundant data highways.
To ensure system safety, complex communication protocols and error detection technologies are used. Communication protocols are a set of rules to ensure that transmitted data is received and understood as the same as the sent data.
Currently, two types of communication methods are generally used in DCS systems: synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate data transmission and reception, while asynchronous networks use a report system without a clock…
FCS The key points of FCS are three:
(1) The core of the FCS system is the bus protocol,which has been discussed in previous chapters. Once the bus protocol of a type of bus is determined, the related key technologies and devices are also determined. In terms of the basic principles of bus protocols, all types of buses are similar, based on solving bidirectional serial digital communication transmission. However, due to various reasons, there are significant differences in bus protocols among different types of buses.
To meet interoperability requirements for fieldbus systems and to become a truly open system, the IEC international standard specifies that the user layer of the fieldbus communication protocol model must have device description functionality. To achieve interoperability, each fieldbus device is described using a Device Description (DD). DD can be considered a driver for the device, including all necessary parameter descriptions and operational steps required by the master station. Since DD includes all the information needed to describe device communication and is independent of the master station, it allows field devices to achieve true interoperability.
There are eight types of protocols, while the original IEC international standard is just one of the eight types, with the other seven types of buses being equal in status. Regardless of their market share, each bus protocol has a set of software and hardware support. They can form systems and products, while the original IEC fieldbus international standard is a framework without software or hardware support. Therefore, achieving mutual compatibility and interoperability among these buses is currently nearly impossible.
From the above, can we conclude that the interoperability of open fieldbus control systems, for a specific type of fieldbus, is open and interoperable as long as it follows the bus protocol of that type of fieldbus? In other words, regardless of the manufacturer, as long as the product adheres to the bus protocol of that fieldbus, the products 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 of the FCS system and its foundation. The reason is simple: the FCS system executes bidirectional digital communication fieldbus signals between automatic control devices and field devices. If the field devices do not adhere to a unified bus protocol, i.e., the relevant communication protocols, and lack digital communication capabilities, then the so-called bidirectional digital communication is just an empty phrase, and it cannot be called a fieldbus control system. Furthermore, a significant feature of the fieldbus is to enhance the control function at the field level. If the field devices are not multifunctional intelligent products, then the characteristics of the fieldbus control system do not exist, and the advantages of simplifying systems, facilitating design, and aiding maintenance are also illusory.
(3) The essence of the FCS system is localized information processing
For a control system, whether using DCS or fieldbus, the amount of information that the system needs to process is at least the same. In fact, using a fieldbus can yield more information from the field. The amount of information in the fieldbus system has not decreased; it has even increased, while the cabling for transmitting information has significantly decreased. This requires, on one hand, a substantial increase in the capacity of cables to transmit information, and on the other hand, to allow a large amount of information to be processed on-site, reducing the back-and-forth information flow between the field and the control room. It can be said that the essence of the fieldbus is localized information processing.
Reducing information back-and-forth is an important principle in network design and system configuration. Reducing information back-and-forth often brings benefits in improving system response time. Therefore, when designing networks, nodes with high information exchange should be placed on the same branch.
Reducing information back-and-forth and reducing system cabling can sometimes contradict each other. In such cases, the principle of saving investment should guide the choice. If the selected system’s response time allows, the cabling-saving scheme should be chosen. If the selected system’s response time is tight, slightly reducing the amount of information transmission may suffice, and the scheme that reduces information transmission should be chosen.
Currently, some field instruments with fieldbus have many functional blocks built-in. Although different products may have slight performance differences in the same functional block, it is objectively true that many similar functional blocks exist on a single network branch. The choice of which functional block to use on which field instrument is a problem that system configuration must solve.
The principle for considering this issue is to minimize information back-and-forth on the bus. Generally, one can choose the functional block with the most relevant information output from the instrument.
3.2 Typical System Comparison
By using fieldbus, users can significantly reduce field wiring, achieve multivariable communication with a single field instrument, and ensure complete interoperability between devices produced by different manufacturers, enhancing control functions at the field level. System integration is greatly simplified, and maintenance is very convenient. In traditional process control instrument systems, each field device requires a dedicated pair of twisted wires to transmit 4~20mA signals to the control room; in fieldbus systems, the twisted wires from each field device to the junction box can still be used, but only one twisted wire is needed from the field junction box to the central control room for digital communication.
By adopting fieldbus control systems, the exact amount of cable saved has not yet been calculated by the author. However, from the cable kilometers used in power plants employing DCS systems related to automatic control, we can see the share of cables in infrastructure investment.
In a certain power plant, two 300MW coal-fired units. The thermal system is unit-based. Each unit is equipped with a centralized control building, adopting a centralized control method for machine, furnace, and electricity. The elevation of the unit control room is 12.6 meters, consistent with the operating layer elevation. DCS uses WDPF—Ⅱ, with a designed I/O point of 4500 points for each unit.
Cable laying was designed using EC software, with eight people completing the design task in 1.5 months; the number of automation-related cables in the main plant for each 300MW unit is 4038; the length of automation-related cables in the main plant for each 300MW unit is 350 kilometers; the above cable counts and lengths do not include fire alarm cables and cables for auxiliary production workshops; all cable trays, columns, and small troughs are made of galvanized steel, weighing about 95 tons for each unit. Other cable trays, including straight, bend, three-way, four-way, cover plates, terminal heads, widening pieces, and direct pieces, are made of aluminum alloy, weighing about 55 tons for each 300MW unit. Accessories such as bolts and nuts are provided with the trays.
In another power plant, a 4×MW fuel gas power station. The thermal system is unit-based. DCS uses TELEPERM-XP, with a designed I/O point count of 5804 points for each unit.
Cable laying was designed using EC software, with twelve people completing the design task in 2.5 months; the number of automation-related cables in the main plant for each 325MW unit is 4413; the length of automation-related cables in the main plant for each 235MW unit is 360 kilometers; all units use galvanized steel cable trays, weighing about 200 tons. The cables in the power station can be divided into six categories: high-voltage power cables, low-voltage power cables, control cables, thermal control cables, weak current cables (mainly referring to computer cables), and other cables. If two 300MW units are simultaneously laid with cables, the number of automation-related cables is approximately 8500. Among these, thermal control cables and weak current cables will exceed 5000, accounting for about 60% (measured by count).
3.3 Design, Investment, and Usage
The above comparison focuses on purely technical aspects; the following comparison will incorporate economic factors.
The premise of the comparison is between DCS systems and typical, ideal FCS systems. Why make such an assumption? As DCS systems have developed to today, the technical requirements proposed in the early stages of development have been met and improved, and the current situation is further enhancement, so there is no typical or ideal description. In contrast, the FCS system, which just entered practical use in the 1990s, still has many technical requirements such as compatibility, openness, bidirectional digital communication, digital intelligent field devices, and high-speed buses that are not yet ideal and need improvement. This state is not unrelated to the formulation of international standards for fieldbus. Over the past decade, various bus organizations have been busy formulating standards, developing products, and capturing more market share, aiming to gain a foothold in international standards and legally occupy a larger market. Now that the battles over international standards have come to a close, major companies have realized that to truly capture the market, they must improve systems and related products. We can predict that in the near future, a complete fieldbus system and related products must become the mainstream of global fieldbus technology.
Specific Comparison:
(1) DCS systems are large systems, with powerful controllers that play a crucial role in the system, and the data highway is the key to the system. Therefore, overall investment must be made in one go, and subsequent expansion is challenging. In contrast, FCS has thoroughly decentralized functions, with localized information processing and widespread use of digital intelligent field devices, which reduces the importance of controller functions. Thus, FCS systems have a low investment threshold and can be used, expanded, and put into operation gradually.
(2) DCS systems are closed systems, with products from different companies generally incompatible. In contrast, FCS systems are open systems, allowing users to choose various devices from different manufacturers and brands to connect to the fieldbus, achieving optimal system integration.
(3) DCS systems use binary or analog signals, requiring D/A and A/D conversion. In contrast, FCS systems are fully digital, eliminating the need for D/A and A/D conversion, achieving high integration and performance, improving accuracy from ±0.5% to ±0.1%.
(4) FCS systems can integrate PID closed-loop control functions into transmitters or actuators, shortening control cycles from 2-5 times per second in DCS to 10-20 times per second in FCS, thus improving regulation performance.
(5) DCS can control and monitor the entire process, perform self-diagnosis, maintenance, and configuration. However, due to its fatal weakness, its I/O signals use traditional analog signals, making it impossible to perform remote diagnosis, maintenance, and configuration of field instruments (including transmitters, actuators, etc.) at the DCS engineer station. FCS uses fully digital technology, with digital intelligent field devices sending multivariable information, not just single-variable information, and also has the capability to detect information errors. FCS employs bidirectional digital communication fieldbus signals, allowing for remote diagnosis, maintenance, and configuration of field devices (including transmitters, actuators, etc.). This advantage of FCS is unmatched by DCS.
(6) Due to localized information processing, FCS can eliminate a significant number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files, and I/O cabinets compared to DCS, saving space and area for I/O devices and device rooms. Some experts believe that up to 60% can be saved.
(7) For the same reasons as (6), FCS can significantly reduce the amount of cabling and the cable trays used for laying cables, thus saving design, installation, and maintenance costs. Some experts believe that savings can reach 66%. Regarding points (6) and (7), it should be noted that while the investment savings from adopting FCS systems are unquestionable, whether they reach the 60-66% claimed by some experts is still uncertain. These figures have appeared in multiple articles, and the author believes they are results of mutual citation; thus, readers should be cautious when quoting these figures.
(8) FCS is simpler to configure than DCS, as its structure and performance are standardized, making installation, operation, and maintenance easier.
(9) Key design and development points for FCS used in process control. This point is not a comparison with DCS but rather highlights issues that should be considered in the design and development of FCS for process control or continuous process applications: 1) The bus must have intrinsic safety and explosion-proof functions, which are of utmost importance. 2) Basic monitoring of changes in flow, level, temperature, pressure, etc., is slow and has a lag effect; therefore, node monitoring does not require fast electronic response times but does require complex analog processing capabilities. This physical characteristic determines that the system mainly adopts a centralized polling system between master and slave, which is technically reasonable and economically beneficial. 3) The measurement of parameters such as flow, level, temperature, and pressure is based on classical physical principles, but sensors, transmitters, and controllers should develop towards digital intelligence. 4) FCS developed for continuous process applications and their instruments should focus on improving the design of low-speed bus H1.
Prospects for PLC and DCS
We already know that some FCS have developed from PLC, while others have developed from DCS. Now that FCS has become practical, what are the prospects for PLC and DCS?PLC and DCS Prospects?
PLC first appeared in the late 1960s in the USA, intended to replace relays and execute logic, timing, counting, and other sequential control functions, establishing flexible program control systems. It was officially named in 1976 and defined as a digital control-specific electronic computer that uses programmable memory to store instructions, executing functions such as logic, sequencing, timing, counting, and calculations, controlling various machines or work processes through analog and digital input and output components. After more than 30 years of development, PLC has matured and improved, developing closed-loop control functions for analog signals.
The position of PLC in FCS systems seems to be established with little debate. PLC acts as a station on the high-speed bus, fully leveraging its advantages in handling switch quantities. Additionally, in auxiliary workshops of thermal power plants, such as water treatment, circulating water, ash removal, and coal transportation, the process is primarily sequential control. PLC has unique advantages for sequential control. The author believes that the control systems for auxiliary workshops should preferably use PLCs that adhere to fieldbus communication protocols or can exchange information with FCS.
(Source: Network, copyright belongs to the original author)