On the advantages and disadvantages of DCS, PLC, and FCS

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In some industries, FCS evolved from PLCs； In other industries, FCS evolved from DCS; therefore, FCS is closely related to PLCs and DCS, yet it also has fundamental differences from them. 1. Basic characteristics of the three major control systems: PLC, DCS, and FCS. Currently, in the automatic control of continuous process production (PA), or what is commonly referred to as industrial process control, there are three major control systems, namely PLC, DCS, and FCS. Their respective basic characteristics are as follows: 1.1. PLC (1) It has evolved from on/off control to sequence control and transfer processing, progressing from bottom to top. (2) Multiple functions such as continuous PID control, with the PID located in the interrupt station. (3) A single PC can be used as the master station, with multiple identical PLCs serving as slave stations. 　 (4) A single PLC can also serve as the master station, with multiple identical PLCs acting as slave stations, to form a PLC network. The advantage of this over using a PC as the master station is that when programming, users do not need to know the communication protocol; they just need to write according to the instructions provided. 　 (5) The PLC grid can function as an independent DCS/TDCS or as a subsystem of a DCS/TDCS. 　 (6) Large-scale systems are the same as DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI, MOD300. 　 (7) PLC networks such as Siemens’ SINEC—L1, SINEC—H1, S4, S5, S6, S7, etc., GE’s GENET, and Mitsubishi’s MELSEC—NET, MELSEC—NET/MINI. 　 (8) It is mainly used for sequential control in industrial processes, and modern PLCs also have closed-loop control capabilities. 　 (9) Manufacturers: GOULD (USA), AB (USA), GE (USA), OMRON (Japan), MITSUBISHI (Japan), Siemens (Germany), etc. 1.2 DCS or TDCS (1) The Distributed Control System DCS and the Distributed Control and Data Acquisition System TDCS are monitoring technologies that integrate 4C technologies (Communication, Computer, Control, CRT). 　 (2) A top-down tree-like large-scale system, in which communication is key. 　 (3) The PID is located in the interrupt station, which connects the computer with the field instruments and control devices. 　 (4) It features a tree topology and a parallel, continuous link structure, with a large number of cables running from the relay stations to the field instruments. (5) Analog signals, A/D–D/A, microprocessor-based hybrids. 　 (6) One instrument is connected to I/O via a pair of wires, and the control station connects it to the local area network LAN. 　 (7) DCS is a three-level structure consisting of control (engineer station), operation (operator station), and field instruments (field measurement and control station). 　 (8) The disadvantages are high costs, products from different companies cannot be interchanged or interoperable, and large DCS systems vary from one company to another. (9) Used for large-scale continuous process control, such as in the petrochemical industry. 　 (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. 1.3, FCS 　 (1) The basic tasks are: intrinsic safety, hazardous areas, variable processes, and challenging extreme environments. 　 (2) Fully digital, intelligent, and multifunctional devices replace analog single-function instruments, meters, and control devices. 　 (3) Connect the dispersed field instruments, control devices, PIDs to the control center using two wires, replacing the two wires required for each instrument. 　 (4) On the bus, PID is equal to instruments, meters, and control devices. 　 (5) Multivariable, multi-node, serial digital communication systems are replacing single-variable, single-point, parallel analog systems. 　 (6) It is interconnected, bidirectional, and open, replacing the one-way, closed approach. (7) Replace centralized control stations with decentralized virtual control stations. 　 (8) It can be controlled by a on-site computer, and can also be connected to a higher-level computer on the same bus. (9) A local area network, which can also be connected to the Internet. 　 (10) Change traditional signal standards, communication standards, and system standards to be integrated into the enterprise management network. 　 (11) Manufacturers: American Honeywell, Smar, Fisher—Rosemount, AB/Rockwell, Elsag—Bailey, Foxboro, Yamatake, Japanese Yokogawa, European Siemens, GEC—Alsthom, Schneider, Proces—Data, ABB, etc. (12) Examples of Category 3 FCS 　1) Automatic control of continuous manufacturing processes such as in the petrochemical industry, where \"intrinsically safe\" technology is absolutely essential; typical products include FF, World FIP, Profibus—PA ； 　 2) Automatic control of discrete process actions such as automotive manufacturing robots and vehicles; typical protocols include Profibus—DP and CANbus ； 　 3) Multi-point control, such as in building automation; typical products include LON Work and Profibus-FMS. 　 From the description of the above key points, have we noticed that none of the three major systems used for process control were developed for power plants? In other words, at the initial stage of their development, power plants were not the primary target systems for these controls. Moreover, the user manuals for these systems also never consider power plants as the preferred application area; in some of them, power plants are not even mentioned at all in the list of applicable areas. Strangely enough, these three major control systems—especially DCS and PLC—are widely used in power plants, and they perform extremely well. 2. Differences among the three control systems As we already know, FCS evolved from DCS and PLC; it not only possesses the characteristics of DCS and PLC but also represents a revolutionary advancement. Currently, both new types of DCS and new types of PLC are showing a tendency to converge with each other. The new DCS already has strong sequential control capabilities ； The new type of PLCs also perform well in handling closed-loop control, and both can be used to form large networks. The application areas of DCS and PLCs have significantly overlapped. The next section will compare only DCS and FCS. The differences between DCS and FCS were actually covered in the previous chapters; below, we will discuss aspects such as architecture, investment, design, and usage. 2.1 Key Differences: The key to a DCS system is communication. It can also be said that the data highway is the backbone of the Distributed Control System DCS. Since its task is to provide a communication network among all components of the system, the design of the data highway itself determines the overall flexibility and security. The media for data highways can be: twisted pair, coaxial cable, or fiber optic cable. 　 Through the design parameters of the data highway, it is possible to understand the relative advantages and disadvantages of a specific DCS system. (1) How much I/O information can the system handle. (2) How much information related to control circuits can the system process? (3) How many users and devices (CRTs, control stations, etc.) can it accommodate? (4) How is the integrity of the transmitted data thoroughly checked? (5) What is the maximum allowable length of a data highway? (6) How many branches can a data highway support? (7) Can the data highway support hardware produced by other manufacturers (programmable controllers, computers, data recording devices, etc.)? 　 To ensure the integrity of communication, most DCS manufacturers can provide redundant data paths. 　 To ensure the security of the system, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules designed to ensure that the data transmitted is received and understood in the same way as the data sent. 　 Currently, two types of communication methods are generally used in DCS systems: synchronous and asynchronous. Synchronous communication relies on a clock signal to regulate the transmission and reception of data, while asynchronous networks use a clock-free reporting system. 2.2, FCS: There are three key points regarding FCS. (1) The core of an FCS system is the bus protocol, that is, the bus standard. As explained in previous sections, once the bus protocol for a particular type of bus is determined, the related key technologies and devices are also determined. In terms of the basic principles of their bus protocols, all types of buses are the same; they all are fundamentally based on facilitating bidirectional serial digital communication and transmission. However, for various reasons, the bus protocols of different types of buses vary greatly. 　 To enable fieldbuses to meet interoperability requirements and become true open systems, international IEC standards specify that the user layer of the fieldbus communication protocol model must have a device description function. To achieve interoperability, each fieldbus device is described using a device description DD. DD can be considered a driver for the device, encompassing all necessary parameter descriptions and the operational steps required by the master station. Since DD includes all the information required to describe device communication and is independent of the master station, it enables true interoperability among field devices. 　 The actual situation is not consistent with what was mentioned above; the answer is no. The currently adopted international standards for fieldbuses include 8 types, whereas the original IEO international standard was just one of those 8 types, holding an equal status to the other 7 bus types. For the other 7 buses, regardless of their market share, each bus protocol comes with its own set of software and hardware support. They can form systems and products; whereas the original IEC fieldbus international standard is nothing but an empty framework lacking any software or hardware support. Therefore, achieving mutual compatibility and interoperability among these buses is almost impossible in the current state of things. 　 From the above, can we draw the following conclusion: regarding the interoperability of open fieldbus control systems, for a specific type of fieldbus, as long as its products comply with the bus protocol of that fieldbus type, they are open and interoperable? In other words, regardless of the manufacturer of the products, and as long as none of them are produced by that fieldbus company, as long as they comply with the bus protocol of that bus, the products can be interconnected to form a bus network, offering interoperability. (2) The foundation of the FCS system is digital intelligent field devices. Digital intelligent field devices serve as the hardware backbone of the FCS system and constitute its foundation; the reason for this is simple: the FCS system relies on a two-way digital communication system based on field bus signals between automatic control devices and field devices. If the field devices do not follow a unified bus protocol, that is, the relevant communication standards, and lack digital communication capabilities, then so-called two-way digital communication is nothing but empty talk, and such systems cannot be called field bus control systems. Lastly, a major characteristic of fieldbuses is to enhance control functions at the field level. If the field devices are not multi-functional and intelligent products, then the advantages of field bus control systems no longer exist; benefits such as simplified systems, easier design, and improved maintenance also become meaningless. (3) The essence of an FCS is the on-site processing of information. For any control system, whether it uses a DCS or a fieldbus, the amount of information that needs to be processed is at least the same. In fact, by using fieldbuses, more information can be obtained from the field. The amount of information in the fieldbus system has not decreased; in fact, it has increased, while the cables used to transmit this information have **decreased**. This requires, on the one hand, **improving the capacity of cables to transmit information, and on the other hand, processing large amounts of information on-site to reduce the back-and-forth of information between the field site and the control room.** It can be said that the essence of field buses is the localization of information processing. 　 Reducing information back-and-forth is an important principle in network design and system configuration. Reducing information back-and-forth often brings the benefit of improving system response time. Therefore, when designing a network, nodes that exchange a large amount of information with each other should be placed on the same branch as a priority. 　 Reducing information back-and-forth and reducing the cables in a system can sometimes be contradictory to each other. At this point, the principle of saving investment should still guide the choices. If the response time of the selected system permits, a cable-saving solution should be chosen. If the response time of the selected system is quite limited, and slightly reducing the amount of information transmitted is sufficient, then the option to reduce information transmission should be chosen. 　 Currently, many field instruments equipped with fieldbuses come pre-installed with numerous function blocks. Although the performance of function blocks performing the same task may vary slightly among different products, it is a fact that there are many function blocks with similar functionalities on a single network branch. Which functional block from the field instrument to select is a problem that needs to be resolved in system configuration. 　 The principle behind considering this issue is to minimize the back-and-forth of information on the bus. Generally, it is possible to choose the function block on the instrument that provides the most information related to that function. 3.1 Comparison of typical systems By using fieldbuses, users can significantly reduce the number of on-site wiring connections; multiple variables can be communicated through a single field instrument. Devices manufactured by different manufacturers can operate together seamlessly, control functions at the field level are enhanced, system integration is simplified, and maintenance becomes much easier. 3.2 Design, Investment, and Use The premise of the comparison is to compare DCS systems with typical, ideal FCS systems. Why make such an assumption? As the DCS system has evolved to its current stage, the technical requirements set at the beginning of its development have already been met and further improved; the goal now is to achieve even greater advancements, so there is no such thing as a typical or ideal configuration. As an FCS system, it became practical in the 1990s. The technical requirements at the initial stage of its development included compatibility and openness, two-way digital communication, digital intelligent field devices, and high-speed buses; however, these requirements are not yet ideal and require further improvement. This situation is certainly related to the development of international standards for fieldbuses. Over the past decade or so, various bus organizations have been busy formulating standards, developing products, and capturing more markets, with the goal of gaining a foothold in international standards and legally accessing larger markets. Now that the battle over international standards has come to an end, major corporate 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, sophisticated fieldbus systems and related products will inevitably become the mainstream of world fieldbus technology. Specific comparisons: (1) The DCS system is a large-scale system; its controllers have powerful functions and play a very important role within the system. The data highway is also a key component of the system. Therefore, it is necessary to make a one-time, comprehensive investment; subsequent expansion is quite difficult. With the thorough delegation of FCS functions, on-site information processing, and the widespread use of digital intelligent field devices, the functions and importance of controllers have relatively diminished. Therefore, the FCS system requires a low initial investment; it can be used, expanded, and put into operation gradually. 　 (2) DCS systems are closed systems; products from different companies are basically incompatible. The FCS system is an open system, allowing users to select various devices from different manufacturers and brands to connect to the field bus, thereby achieving optimal system integration. 　 (3) All information in the DCS system is in the form of binary or analog signals, so D/A and A/D conversions are necessary. The FCS system is fully digital, thus eliminating the need for D/A and A/D conversion. It features high integration and superior performance, enabling the accuracy to be improved from ±0.5% to ±0.1%. 　 (4) The FCS system can incorporate PID closed-loop control functions into transmitters or actuators, reducing the control cycle time. Currently, the control frequency can be increased from 2–5 times per second in DCS systems to 10–20 times per second in FCS systems, thereby improving the regulation performance. 　 (5) The DCS can control and monitor the entire process, as well as perform self-diagnosis, maintenance, and configuration. However, due to its inherent fatal flaw—that its I/O signals use traditional analog signals—it is unable to perform remote diagnosis, maintenance, and configuration of field instruments (including transmitters, actuators, etc.) from a DCS engineer station. FCS employs fully digital technology; the digitally intelligent field devices transmit multivariable information, rather than just single-variable information, and also have the capability to detect information errors. FCS uses a bidirectional digital communication fieldbus signaling system. Therefore, it enables remote diagnosis, maintenance, and configuration of field devices (including transmitters, actuators, etc.). This advantage of FCS is incomparable to that of DCS. 　 (6) Due to on-site information processing, FCS can eliminate a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files, and I/O cabinets compared to DCS, thereby saving space and floor area for I/O devices and device rooms as well. Some experts believe that 60% can be eliminated. 　 (7) For the same reasons as (6), FCS can reduce the need for a large number of cables and cable trays used for laying them, thereby saving on design, installation, and maintenance costs. Some experts believe that 66% can be saved. 　 Regarding points (6) and (7), it should be added that with the FCS system, the savings in investment are undoubtedly significant, but whether they reach 60–66% as some experts claim remains to be seen. These figures appear in multiple articles; the editor believes this is the result of mutual citation. The original source of these figures has not yet been identified, so readers should be cautious when citing them. 　 (8) Compared to DCS, FCS has a simpler configuration; due to its standardized structure and performance, it is easy to install, operate, and maintain. 　 (9) Key points in the design and development of FCS for process control. This is not intended as a comparison between W and DCS; it merely illustrates the issues that should be given priority consideration in the design and development of FCS, which are used for process control or for simulating continuous processes. 1) The intrinsic safety and explosion-proof functionality of the bus is required; it is of utmost importance. 　 2) Changes in basic monitoring parameters such as flow rate, level, temperature, and pressure occur slowly, and there is also a lag effect; therefore, node monitoring does not require the fast response times of electronics, but rather sophisticated capabilities for processing analog signals. This physical characteristic dictates that the system predominantly employs a centralized polling mechanism between a master and slaves, which is technically reasonable and economically beneficial. 　 3) The measurement of parameters such as flow rate, level, temperature, and pressure relies on classical physical principles, but sensors, transmitters, and controllers should evolve toward digital intelligence. 　 4) As an FCS developed for continuous process systems and their instruments, emphasis should be placed on the improved design of the low-speed bus H1.