This paper analyzes the characteristics and differences of PLC, DCS and FCS, and points out the origin and development direction of the three control systems.

Keywords: programmable logic controller (PLC), distributed control system (DCS) and field bus control system (FCS)

1. Introduction

Fieldbus control system, which was put into practical application in the 1990s, developed rapidly and is the latest control system in the world. Fieldbus control system is a hot spot of automation technology at present, which attracts more and more attention from domestic and foreign automation equipment manufacturers and users. The appearance of fieldbus control system will bring another revolution to the field of automation, and its depth and breadth will exceed any period in history, thus creating a new era of automation.

In some industries, FCS is developed from PLC; In other industries, FCS is developed from DCS, so FCS is inextricably linked with PLC and DCS and has essential differences. This paper attempts to analyze the characteristics and differences of PLC, DCS and FCS, and point out their sources and development direction.

The basic characteristics of 2.2. PLC, DCS and FCS.

At present, there are three main control systems in continuous process production automatic control (PA) or industrial process control, namely PLC, DCS and FCS. Their basic characteristics are as follows:

2. 1 PLC

(1) The development from on-off control to sequential control and conveying treatment is from bottom to top.

(2) Multi-functions such as continuous PID control, and the interrupt station is equipped with PID.

(3) One PC can be used as the master station, and multiple PLCs of the same type can be used as the slave stations.

(4) A PLC can also be used as a master station, and multiple PLCs of the same type can be used as slave stations to form a PLC network. This is much more convenient than using PC as the main station: when there is user programming, you don't need to know the communication protocol, just write it according to the instruction format.

(5)PLC power grid can be used as an independent DCS/TDCS or as a subsystem of DCS/TDCS.

(6) Large-scale systems are the same as DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI and MOD300.

(7)PLC networks such as SINEC-L 1 of Siemens, SINEC-H 1, S4, S5, S6, S7, GENET of GE, MELSEC-NET of Mitsubishi and MELSEC-NET/MINI.

(8) It is mainly used for sequential control in industrial process, and the new PLC also has closed-loop control function.

(9) Manufacturers: Gould (USA), AB (USA), GE (USA), Omron (Japan), Mitsubishi (Japan), Siemens (Germany), etc.

2.2 documentary credit or TDCS

(1) DCS and TDCS are monitoring technologies integrating 4C (communication, computer, control and CRT) technologies.

(2) Top-down tree topology large-scale system, in which communication is the key.

(3)PID In the interrupt station, the interrupt station is connected with computer, field instrument and control device.

(4) It is a tree topology with a parallel and continuous link structure, and a large number of cables run in parallel from the relay station to the field instrument.

(5) Analog signal, A/D-D/A, mixed with microprocessor.

(6) A pair of wires of the instrument are connected to I/O, and the instrument is connected to the local area network LAN from the control station.

(7)DCS is a three-level structure of control (engineer station), operation (operator station) and field instrument (field measurement and control station).

(8) The disadvantage is that the cost is high, and the products of various companies cannot interoperate with each other, and the large DCS system is different from company to company.

(9) Used for large-scale continuous process control, such as petrochemical industry.

(10) Manufacturers: Bailey (USA), westinghouse (USA), HITACH (Japan), Leeds &; Northrop (USA), Siemens (Germany), Foxboro (USA), ABB (Switzerland), Hartman &; Braun (Germany), Yokogawa (Japan), Honewell (America), Taylor (America), etc.

2.3 FCS

The basic tasks of (1) are: intrinsic safety, dangerous area, changeable process and difficult environment.

(2) Fully digital, intelligent and multifunctional, replacing instruments, meters and control devices that simulate a single function.

(3) Use two wires to connect scattered field instruments, control devices, PID and control centers, instead of using two wires for each instrument.

(4) PID is equal to the instruments, meters and control devices on the bus.

(5) Multivariable, multi-node, serial and digital communication systems have replaced univariate, single point, parallel and analog systems.

(6) It is interconnected, two-way and open, not one-way and closed.

(7) Replace centralized control station with decentralized virtual control station.

(8) It is controlled by the field computer, and can also be connected to the upper computer and the upper computer on the same bus.

(9) Local area network, which can be connected with the Internet.

(10) Transform the traditional signal standard, communication standard and system standard into enterprise management network.

(1 1) Manufacturers: Honeywell, Smar, Fisher— Rosemount, AB/Rockwell, Elsag— Bailey, Foxboro, Yamatake, Yokogawa, Siemens Europe, GEC—-Alstom, Schneider, Proceedings-Data, ABB, etc.

Typical (12) Class 3 Fieldbus

1) continuous process automatic control, such as petrochemical industry, in which "intrinsically safe and explosion-proof" technology is absolutely important. Typical products are FF, FIP and PROFIBUS-PA；;

2) Discrete automatic control of process actions, such as automobile manufacturing robots and automobiles. Typical products are PROFIBUS-DP and CANbus；;

3) Multi-point control such as building automation, typical products are LON Work and Profibus—FMS.

From the description of the above basic points, do we notice that the three systems used for process control were not developed for power plants, or that in the early stage of their development, power plants were not the first choice for system control? In the operating instructions of these systems, power stations are never regarded as the preferred scope of application, and some do not mention power stations at all. Strangely, these three control systems, especially DCS and PLC, have been widely used in power stations and have achieved good results.

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3. Differences among the three control systems

We already know that FCS is developed from DCS and PLC. FCS not only has the characteristics of DCS and PLC, but also takes a revolutionary step. At present, the new district boards and the new provisional legislature tend to be close to each other. The new DCS has powerful sequential control function; The new PLC is not bad at dealing with closed-loop control, and the two can form a large network. The application scope of DCS and PLC has greatly crossed. The next section only compares DCS and FCS. In the previous chapter, the difference between DCS and FCS has actually been involved, and the following will describe the architecture, investment, design and use.

3. 1 difference point

File (short for documents) Distributed Control System (Distributed control system)

The key of DCS system is communication. It can also be said that data expressway is the backbone of distributed control system DCS. Because its task is to provide a communication network between all components of the system, the design of the data expressway itself determines the overall flexibility and security. The medium of data expressway can be a pair of twisted pair, coaxial cable or optical cable.

Through the design parameters of data expressway, we can basically understand the relative advantages and disadvantages of a specific DCS system.

(1) How much I/O information can the system handle?

(2) How much control loop information can the system handle?

(3) How many users and devices (CRT, control station, etc.). ) can it adapt?

(4) How to thoroughly check the integrity of the transmitted data.

(5) What is the maximum allowable length of data expressway?

(6) How many branches can data expressway support?

(7) Can the data expressway support hardware (programmable controller, computer, data recording equipment, etc.)? ) produced by other manufacturers.

In order to ensure the integrity of communication, most DCS manufacturers can provide redundant data expressway.

In order to ensure the security of the system, complex communication protocols and error detection techniques are used. The so-called communication protocol is a set of rules to ensure that the transmitted data is received and understood as transmitted data.

At present, DCS system generally adopts two communication modes, namely synchronous and asynchronous. Synchronous communication relies on a clock signal to adjust the sending and receiving of data, and asynchronous network adopts a report system without a clock.

Fire Control System (fire control system)

Functional constituencies have three main points.

The core of (1)FCS system is bus protocol, that is, bus standard.

As mentioned in the previous chapter, once a type of bus protocol is determined, the related key technologies and related equipment are also determined. As far as the basic principle of its bus protocol is concerned, all kinds of buses are the same, which are based on solving two-way serial digital communication transmission. However, due to various reasons, the bus protocols of various buses are very different.

In order to make the Fieldbus meet the requirements of interoperability and make it a truly open system, in the user layer of IEC international standard and Fieldbus communication protocol model, it is clearly stipulated that the user layer has the function of device description. In order to achieve interoperability, each fieldbus device is described by device description DD. DD can be regarded as a driver of the equipment, which contains all necessary parameter descriptions and operation steps required by the master station. Because DD contains all the information needed to describe the communication of devices, it has nothing to do with the master station, so it can make the field devices truly interoperate.

Whether the actual situation is the same as above, the answer isno. At present, there are eight types of fieldbus international standards, and the original IEO international standard is only one of these eight types, which is equal to the other seven types of buses. The other seven buses, regardless of their market share, each bus protocol has a set of software and hardware support. They can form systems and products, but the original IEC fieldbus international standard is an empty shelf, with neither software support nor hardware support. Therefore, it is almost impossible to realize the mutual compatibility and interoperability of these buses at present.

From the above, we can get an image that the interoperability of an open fieldbus control system is open to its products and interoperable as long as it follows the bus protocol of a specific type of fieldbus. In other words, no matter what manufacturer's products are, they are not the products of Fieldbus Company. As long as the bus protocol of the bus is followed and the products are open and interoperable, a bus network can be formed.

(2)FCS system is based on digital intelligent field devices.

Digital intelligent field device is the hardware support and foundation of FCS system. There is a simple reason. FCS system realizes two-way digital communication field bus signal system between automatic control equipment and field equipment. If the field devices don't follow the unified bus protocol, that is, the related communication protocol, and don't have the digital communication function, then the so-called two-way digital communication is just empty talk and can't be called the field bus control system. In addition, a major feature of Fieldbus is the addition of field-level control function. If the field devices are not multifunctional and intelligent products, the characteristics of the field bus control system will not exist, and the advantages of simplifying the system, facilitating design and maintenance will be illusory.

The essence of FCS system is field information processing.

For a control system, whether using DCS or fieldbus, the system needs to process at least the same amount of information. In fact, more information can be obtained from the field after adopting the field bus. The amount of information in the field bus system has not decreased, or even increased, while the cables for transmitting information have been greatly reduced. This requires that on the one hand, the ability of cable to transmit information should be greatly improved, on the other hand, a large amount of information should be processed at the scene to reduce the information round trip between the scene and the control room. It can be said that the essence of fieldbus is the scene of information processing.

Reducing information roundtrip is an important principle in network design and system configuration. Reducing information roundtrip can usually bring the benefit of improving system response time. Therefore, in network design, nodes with large information exchange should be placed in the same branch.

Reducing information round-trip and reducing system cables are sometimes contradictory. At this time, the choice should be based on the principle of saving investment. If the response time of the selected system allows, the cable saving scheme should be selected. If the response time of the selected system is tight, it is enough to reduce the information transmission slightly, then the scheme of reducing the information transmission should be chosen.

At present, some field instruments with fieldbus are equipped with many functional blocks. Although the performance of the same function block of different products will be slightly different, there are many function blocks with the same function on a network branch that exist objectively. It is a problem to be solved in system configuration to choose which functional block on the field instrument.

The principle of considering this problem is to minimize the round-trip information on the bus. Usually, you can choose the function block with the most information output related to this function on the instrument.

3.2 Comparison of Typical Systems

By using field bus, users can greatly reduce field wiring, a single field instrument can realize multivariable communication, devices produced by different manufacturers can fully interoperate, field-level control functions are increased, system integration is greatly simplified, and maintenance is very simple. The block diagram of a typical fieldbus system is shown in figure 1. As can be seen from Figure 1, in the traditional process control instrument system, each field device needs to use a pair of special twisted pair to transmit 4~20mA signals. In the Fieldbus system shown in Figure 2, the twisted pair from each field device to the junction box can still be used, but only one twisted pair is used to complete the digital communication from the field junction box to the central control room.

Figure 1: traditional process control system

How many cables can be saved by using fieldbus control system has not been calculated by the editor. However, we can't see the share of cables in infrastructure investment from the kilometers of cables used in DCS power plants related to automatic control systems.

A power plant with 2×300MW coal-fired units. Thermal system is a unit system. Each unit is equipped with a centralized control building, which adopts centralized control mode of machine, furnace and electricity. The elevation of the unit control room is12.6m, which is consistent with the elevation of the operation floor. DCS adopts wdpf-ⅱ, and the design I/O point of each unit is 4500 points.

Figure 2: Fieldbus Control System

The cable laying adopts EC software, and it takes 8 people 1.5 months to complete the cable laying design task. The number of automation cables for each 300MW unit in the main workshop is 4038; The length of the automation professional cable of each 300MW unit in the main workshop is 350km. The number and length of the above cables do not include the fire alarm cables provided by the factory and the cables in the auxiliary production workshop of the whole factory. The uprights, bridges and boxes of cable bridges are made of galvanized steel, each of which is about 95 tons. Other cable trays, including straight, bent, tee, four-way, cover plate, terminal head, width adjusting plate and straight plate, are all made of aluminum alloy, and each 300MW unit is about 55 tons. The bridge is equipped with accessories such as bolts and nuts.

A power plant, 4×MW oil-fired gas-fired power station. Thermal system is a unit system. TELEPERM-XP is adopted in the distributed control system. The design I/O points of each unit are 5804 points.

The cable laying adopts EC software, and it takes 12 people 2.5 months to complete the cable laying design task; The number of automation cables for each 325MW unit in the main workshop is 4413; The length of automation cable for each 235MW unit in the main workshop is 360 kilometers; Each unit is made of galvanized steel cable trough, and its weight is about 200 tons. Cables in power stations can be divided into six categories: high voltage power cables, low voltage power cables, control cables, thermal control cables, weak current cables (mainly computer cables) and other cables. If two 300MW units are laying cables at the same time, the number of automation professional cables is about 8500. Among them, there will be more than 5000 thermal control cables and weak current cables, accounting for about 60% (measured by the number of cables).

Page 3.3 Design, Investment and Use

The above comparison is purely technical, and the following comparison is intended to add economic factors.

The premise of comparison is to compare DCS system with typical and ideal FCS system. Why do you make such an assumption? As a DCS system, the technical requirements put forward at the initial stage of development have been met and improved, and the current situation is further improved, so there is no typical and ideal statement. As a FCS system, it was just put into practical use in the 1990s. As a technical requirement in the initial stage of development, it is still not ideal and needs to be improved. This state has nothing to do with the formulation of international standards for fieldbus. In the past ten years, public transport organizations have been busy making standards, developing products and occupying more markets, with the aim of squeezing into international standards and legally occupying a larger market. Now that the international standard dispute has come to an end, major companies and organizations have realized that they must improve their own systems and related products in order to truly occupy the market. We can predict that in the near future, perfect fieldbus system and related products will become the mainstream of the world fieldbus technology.

Specific comparison:

(1)DCS system is a large system, its controller is powerful and plays an important role in the system, and data expressway is the key to the system. Therefore, the overall investment must be in place in one step, and it is difficult to expand afterwards. However, the complete decentralization of FCS function, field information processing and the wide adoption of digital intelligent field devices make the function and importance of the controller relatively weakened. Therefore, the FCS system has a low investment starting point and can be used, expanded and put into operation.

(2)DCS system is a closed system, and the products of various companies are basically incompatible. FCS system is an open system, and users can choose various devices from different manufacturers and brands to connect to the field bus to achieve the best system integration.

(3) 3) The information of DCS system is all formed by binary or analog signals, and there must be D/A and A/D conversion. The FCS system is fully digital, which saves D/A and A/D conversion, has high integration and high performance, and improves the accuracy from 0.5% to 0. 1%.

(4)FCS system can put PID closed-loop control function into transmitter or actuator, which shortens the control cycle. At present, it can be increased from 2~5 times per second of DCS to 10~20 times per second of FCS, thus improving the regulation performance.

(5)DCS can control and monitor the whole process, and can self-diagnose, maintain and configure. However, due to its own fatal weakness, its I/O signal adopts traditional analog signal, so it is impossible to remotely diagnose, maintain and configure field instruments (including transmitters and actuators) on DCS engineer station. FCS adopts full digital technology, and digital intelligent field devices send multiple information, not only univariate information, but also have the function of detecting information errors. FCS adopts two-way digital communication field bus signal system. Therefore, it can remotely diagnose, maintain and configure field devices (including transmitters and actuators). This advantage of FCS is incomparable to DCS.

(6) Compared with DCS, FCS can save a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files and I/O cabinets, and can also save the space and floor space between I/O devices. Some experts believe that 60% can be saved.

(7) For the same reason as (6), FCS can reduce a large number of cables and cable trays, and also save design, installation and maintenance costs. Some experts think it can save 66%.

For points (6) and (7), it needs to be added that the effect of saving investment by adopting FCS system is undoubted, but is it 60-66% as some experts say? These figures appear in many articles, which the editors think are the result of mutual translation. At present, the original source of these figures has not been found, so readers should be careful when quoting these figures.

(8) Compared with DCS, FCS is simple in configuration and easy to install, operate and maintain due to its standardized structure and performance.

(9) FCS design and development points of process control. This is not as a comparison with DCS, but only to explain the key issues that should be considered in the design and development of FCS for process control or continuous process simulation.

1) requires the intrinsically safe explosion-proof function of the bus, and it is the first.

2) The basic monitoring of flow, material level, temperature and pressure changes slowly, and there is a lag effect. Therefore, node monitoring does not need fast electronic response time, but needs complex analog processing ability. This physical characteristic determines that the system basically adopts the master-slave centralized polling system, which is technically reasonable and economically beneficial.

3) The physical principle of measuring flow, material level, temperature, pressure and other parameters is classic, but sensors, transmitters and controllers should develop into digital intelligence.

4) Because FCS is developed for continuous process and its instruments, we should focus on the design perfection of low-speed bus H 1.

4.4 prospects. PLC and DCS

We already know that some FCS are developed from PLC and some FCS are developed from DCS. So, what will be the future of PLC and DCS today when FCS has been put into practical use?

PLC first appeared in the United States in the late 1960s. Its purpose is to replace relays, perform sequential control functions such as logic, timing and counting, and establish a flexible program control system. 1976 formal naming definition: PLC is a special electronic computer used for digital control. It uses programmable memory to store instructions, perform functions such as logic, sequence, timing, counting and calculus, and controls various machines or working programs through analog and digital input and output components. After more than 30 years of development, PLC has been very mature and perfect, and developed the analog closed-loop control function. The position of PLC in FCS system seems to have been determined, and there is not much debate. See Figure 3: 3: Architecture of fieldbus control system recommended by IEC. As a station, PLC hangs on the high-speed bus. Give full play to the advantages of PLC in processing switching values. In addition, auxiliary workshops of thermal power plants, such as make-up water treatment workshop, circulating water workshop, ash and slag removal workshop, coal conveying workshop, etc. , mainly controlled by sequence. PLC has its unique advantages for sequential control. The editor thinks that the control system of the auxiliary workshop should be based on PLC that follows the fieldbus communication protocol or PLC that can communicate and exchange information with FCS.

Figure 3: 3: Architecture of Fieldbus Control System Recommended by IEC

Since 1973 put forward the first microprocessor-based controller, it has been gradually improved, and finally formed a fully functional, safe and reliable digital decentralized control system DCS. Its performance is far superior to any control system. It can meet the requirements of DAS, MCS, SCS and APS systems in thermal power plants. At present, the management network can be established through industrial Ethernet to meet the growing demand of strengthening management in thermal power plants. It can be said that the monitoring of DCS system can cover the whole process of large thermal power units.

However, since the appearance and practical application of FCS in 1990s, the following arguments have been published in public publications: "From now on, the new fieldbus control system FCS will gradually replace the traditional DCS"; "When the regulation function is decentralized to the site, the traditional DCS is unnecessary and will disappear automatically"; In the next decade, the traditional 4~20mA analog signal system will be gradually replaced by the bidirectional digital communication field bus signal system, and the distributed control system of analog and digital DCS will be replaced by the all-digital field bus control system FCS. These arguments can be summarized in one sentence: functional constituencies will replace district boards, and district boards will die out from now on.

All the above statements come from authoritative experts, which is really reasonable. Digital communication is a trend, representing technological progress, and no one can stop it. The two-way digital communication fieldbus signal system and its great driving force have accelerated the transformation of field devices and control instruments, and developed more and more multifunctional and perfect digital intelligent field devices. These are not available in DCS system, and the advantages and benefits brought to the design, configuration, configuration, operation, maintenance and management of thermal power plants are also beyond the reach of DCS system. Moreover, FCS is developed from DCS and PLC, which retains the characteristics of DCS, or FCS absorbs the experience of DCS development and field practice for many years, including lessons. The conclusion that "FCS will replace DCS" seems logical.

At the same time, it should be noted that DCS system has been developed for nearly 30 years and has been widely used in thermal power plants. Its design idea, configuration and function matching have reached a very perfect level (of course, DCS also needs further development, such as advanced software development to meet the requirements of information integration), which has penetrated into all fields of thermal power plant control system and is also reflected in FCS system. From this point of view, it seems that it can't be said that DCS system has died out since then. In addition, as mentioned in the previous chapters, DCS system still has its place in the field where FCS system can not give full play to its characteristics and advantages.

It seems that we don't need to argue too much about words, but we must emphasize who will replace who. Just like the current DCS and the new PLC, due to years of development and research, they have maintained their original characteristics and complemented each other to form a new system. The current DCS is not the original DCS, and the new PLC is not the PLC in the early stage of development. We can say that it is obviously inappropriate for DCS to replace PLC or PLC to replace DCS.

5. Conclusion

Through the above analysis and discussion, we can draw the following simple conclusions: with the appearance of FCS, the digital distributed control DCS will not die out, but it has moved the DCS, once the center of the control system, to a site on the field bus. In other words, the situation that DCS is in the center of control system will be broken from now on. In the future, the control system of thermal power plant will be a new control system with FCS as the center and DCS system concept.