Source: Thermosphere
In some industries, FCS is developed from PLCs; In other industries, FCS is developed from DCS, so FCS is inextricably linked with PLC and DCS, and there are essential differences. This paper analyzes the characteristics and differences of the three major control systems of PLC, DCS and FCS one by one, and points out the origin and development direction between them.
01
The basic characteristics of the three major control systems of PLC, DCS and FCS
At present, there are three major control systems in continuous process production automatic control (PA) or commonly known as industrial process control, namely PLC, DCS and FCS.
◎
(1) From switching control to sequence control and delivery processing, it is from the bottom up.
(2) Continuous PID control and other multi-functions, PID in the interrupt station.
(3) One PC can be used as the main station, and multiple PLCs of the same type can be used as slaves.
(4) A PLC can also be the master station, and multiple PLCs of the same type can be slave stations to form a PLC network. This is more convenient than using a PC as a master station: when there is a user programming, you don't need to know the communication protocol, just write it in the format of the manual.
(5) The PLC grid can be used as an independent DCS/TDCS, or as a subsystem of DCS/TDCS.
（6）大系统同DCS/TDCS，如TDC3000、CENTUMCS、WDPFI、MOD300。
（7）PLC网络如Siemens公司的SINEC—L1、SINEC—H1、S4、S5、S6、S7等，GE公司的GENET、三菱公司的MELSEC—NET、MELSEC—NET/MINI。
(8) It is mainly used for sequence control in industrial processes, and the new PLC also has closed-loop control functions.
(9)制造商:GOULD(美)、AB(美)、GE(美)、OMRON(日)、MITSUBISHI(日)、Siemens(德)等。
◎
（1）分散控制系统DCS与集散控制系统TDCS是集4C（Communication，Computer， Control、CRT）技术于一身的监控技术。
(2) A large topological system with a top-down tree, in which communication is the key.
(3) The PID is in the interrupt station, and the interrupt station is connected to the computer and the field instrumentation and control device.
(4) It is a tree-like topology and parallel continuous link structure, and there are also a large number of cables parallel from the relay station to the field instrumentation.
(5) Analog signal, A/D—D/A, hybrid with microprocessor.
（6）一台仪表一对线接到I/O，由控制站挂到局域网LAN。
(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, the products of each company are not interchangeable, can not be interoperable, and the DCS system is different.
(9) It is used for large-scale continuous process control, such as petrochemical, etc.
(10)制造商:Bailey(美)、Westinghous(美)、HITACH(日)、LEEDS & NORTHRMP(美)、SIEMENS(德)、Foxboro(美)、ABB (瑞士)、Hartmann & Braun(德)、Yokogawa(日)、Honewell(美国)、Taylor(美)等。
◎
(1) The basic tasks are: intrinsic (intrinsic) safety, dangerous areas, volatile processes, and extraordinary environments that are difficult to deal with.
(2) Fully digital, intelligent and multi-functional to replace analog single-function instruments, meters and control devices.
(3) Use two wires to connect the scattered field instruments, control devices, PIDs and control centers, replacing two wires for each instrument.
(4) On the bus, the PID is equal to the instrument, meter and control device.
(5) Multivariate, multi-node, serial, digital communication systems replace univariate, single-point, parallel, analog systems.
(6) It is interconnected, two-way, and open instead of one-way and closed.
(7) Replace centralized control stations with decentralized virtual control stations.
(8) It is operated by the on-site computer, and can also be hung up to the upper computer and connected to the upper computer of the same bus.
(9) LAN, which can be connected to the Internet.
(10) Change the traditional signal standards, communication standards and system standards into the enterprise management network.
（11）制造商：美Honeywell 、Smar 、Fisher— Rosemount、 AB/Rockwell、Elsag— Bailey、Foxboro 、Yamatake 、日Yokogawa、欧 Siemens、 GEC—Alsthom 、Schneider、 proces—Data、 ABB等。
(12) Typical of Class 3 FCS: 1) Continuous process automatic control, such as petrochemical industry, in which "intrinsically safe explosion-proof" technology is absolutely important, and typical products are FF, World FIP, Profibus-PA; 2) Discrete automatic control of process actions, such as automobile manufacturing robots, automobiles, typical products are Profibus-DP, CANbus; 3) Multi-point control such as building automation, typical products are LON Work, Profibus-FMS.
From the description of the above basic points, do we notice that none of the three major systems used for process control were developed for power stations, or that in the early stage of their development, none of them were the preferred control objects for power plants. In the instructions for the use of these systems, the power station is never regarded as the first choice of application, and some do not mention the power station at all in the scope of application. Now the strange thing is that these three major control systems, especially DCS and PLC, have been widely used in power stations, and the effect is also very good.
02
Differences between the three major control systems
As we already know, FCS is developed from DCS and PLC, which not only has the characteristics of DCS and PLC, but also takes a revolutionary step. At present, the new DCS and the new PLC have a tendency to move closer to each other.
The new DCS already has a strong sequential control function; The new PLC is not bad in dealing with closed-loop control, and both can form a large network, and the scope of application of DCS and PLC has been greatly crossed. In the next section, we will compare DCS with FCS only. In the previous chapters, the differences between DCS and FCS have actually been touched, and the architecture, investment, design, and usage are described below.
◎
The key to a DCS system is communication. It can also be said that the data highway is the spine of the distributed control system DCS. Since its mission 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 pair of stranded wires, coaxial cables, or fiber optic cables.
The design parameters of the data highway basically give an idea of the relative strengths and weaknesses of a particular DCS system.
(1) How much I/O information can be processed by the system. Source:
(2) How much information about the control loop related to control can be processed by the system.
(3) How many users and devices can be adapted (CRT, control station, etc.).
and (4) how the integrity of the transmitted data is thoroughly checked.
(5) What is the maximum allowable length of the data highway.
(6) How many branch roads can be supported by the data highway.
(7) Whether the data highway can support hardware (programmable controllers, computers, data logging devices, etc.) produced by other manufacturers.
In order to ensure the integrity of communication, most DCS manufacturers can provide redundant data highways.
In order to ensure the security of the system, complex communication protocols and error detection techniques are used. A communication protocol is a set of rules that guarantee that the transmitted data is received and understood in the same way as the data sent.
At present, two types of communication means are generally used in the DCS system, namely synchronous and asynchronous, synchronous communication relies on a clock signal to regulate the transmission and reception of data, and asynchronous network adopts a reporting system without a clock......
The key takeaways from FCS FCS are threefold:
(1) The core of the FCS system is the bus protocol, that is, the bus standard has been described in the previous chapter, a type of bus, as long as the bus protocol is determined, the relevant 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, and they are all based on solving the two-way serial digital communication transmission. However, for a variety of reasons, there are significant differences in the bus protocols of the various types of buses.
In order for the fieldbus to meet the interoperability requirements and make it a truly open system, the user layer of the IEC international standard, the fieldbus communication protocol model, clearly stipulates that the user layer has the function of device description.
To enable interoperability, each fieldbus device is described by a device description DD. The DD can be thought of as a drive for the unit, which includes all the necessary parameter descriptions and the required operating steps of the master. Since DD includes all the information needed to describe the communication of the device and is independent of the master, it enables true interoperability of field installations.
There are 8 types of quasi-buses, and the original IEO international standard is only one of the 8 types, and the status of the other 7 types of buses is equal. The other 7 buses, regardless of their market share, each bus protocol is supported by a set of software and hardware. They can form systems and products, while the original IEC fieldbus international standard is an empty shelf without software or hardware support.
Therefore, it is almost impossible to achieve mutual compatibility and interoperability of these buses in their current state.
From the above, can we get the impression that the interoperability of an open fieldbus control system, in the case of a particular type of fieldbus, is open to its products and interoperable, as long as the bus protocol of that type of fieldbus is followed?
In other words, no matter what manufacturer's product is the product of the fieldbus company, as long as the bus protocol of the bus is followed, the products are open and interoperable, and the bus network can be formed.
(2) The basis of the FCS system is the digital intelligent field device
The digital intelligent field device is the hardware support of the FCS system, is the foundation, the reason is very simple, the FCS system implements the two-way digital communication fieldbus signal system between the automatic control device and the field device.
If the field device does not follow a unified bus protocol, i.e. the relevant communication protocol, and does not have a digital communication function, then the so-called two-way digital communication is just an empty word, and it cannot be called a fieldbus control system. Again, one of the main features of the fieldbus is the addition of field-level control functions. If the field device is not multi-functional
Intelligent products, then the characteristics of the fieldbus control system do not exist, the so-called simplified system, convenient design, conducive to maintenance and other advantages are also virtual.
(3) The essence of the FCS system is on-site information processing
For a control system, whether it is a DCS or a fieldbus, the system needs to process at least as much information. In fact, with the adoption of fieldbuses, much more information can be obtained from the field.
The amount of information in the fieldbus system has not decreased, but has even increased, while the number of cables that transmit the information has been significantly reduced. This requires that on the one hand, the ability of the cable to transmit information should be greatly improved, and on the other hand, a large amount of information should be processed on the spot, so as to reduce the information round-trip between the site and the control room. It can be said that the essence of fieldbus is the on-site processing of information.
Reducing information round-tripping is an important principle in network design and system configuration. Reducing information round-trips often provides the benefit of improved system response time. Therefore, in the network design, the nodes with a large amount of information exchange between each other should be preferentially placed in the same branch.
Reducing information round-trips and reducing the system's cables are sometimes contradictory. At this time, the choice should still be based on the principle of saving investment. If the response time of the selected system allows, you should choose a cable-saving option. If the response time of the selected system is relatively tight, and a slight reduction in information transmission is sufficient, then a solution to reduce information transmission should be chosen.
Now some field instruments with fieldbus are equipped with many function blocks, although the same kind of function blocks of different products will be slightly different in performance, but the situation that there are many similar function blocks on a network branch is objective. Which function block on the field instrument to choose is the problem to be solved by the system configuration.
The principle of thinking about this is to minimize the round-trip of information on the bus. In general, you can choose the function block on the instrument with the most information output related to this function.
◎
By using fieldbuses, users can significantly reduce field wiring, achieve multivariable communication with a single field instrument, fully interoperate between devices produced by different manufacturing plants, increase field-level control functions, greatly simplify system integration, and simplify maintenance.
The traditional process control instrumentation system needs to use a pair of dedicated twisted pair cables from each field device to the control room to transmit 4~20mA signals; In a fieldbus system, the twisted pair from each fieldunit to the junction box can still be used, but only one twisted pair is used for digital communication from the field junction box to the central control room.
The exact amount of cable savings that can be achieved by using a fieldbus control system has not yet been calculated. However, we can see the share of cables in infrastructure investment in power plants with DCS systems in terms of the number of cable kilometers used in relation to automatic control systems.
A power plant, 2×300MW coal-fired units. The thermal system is a unit system. Each unit is equipped with a centralized control building, which adopts the centralized control mode of machine, furnace and electrical unit. The elevation of the unit control room is 12.6 meters, which is the same as the elevation of the operating level. The DCS adopts WDPF-II., and the I/O points designed for each unit are 4500 points.
The cable laying adopts EC software, and 8 people take 1.5 months to complete the design task of cable laying; the number of cables in the main plant of each 300MW unit automation professional is 4038; the length of the cable of each 300MW unit automation professional in the main plant is 350 kilometers; the number and length of the above cables do not include the factory supply cable of the whole plant fire alarm and the cable of the auxiliary production workshop of the whole plant; the column, bridge and small slot box of the cable tray are all made of steel galvanized, and each unit is about 95 tons.
Other cable trays, including straight-through, bending, tee, cross, cover, terminal head, widening piece, direct sheet, etc., are made of aluminum alloy, and each 300MW unit is about 55 tons. Accessories are supplied with the bridge (e.g. bolts, nuts).
A power plant, 4×MW oil-fired power station. The thermal system is a unit system. The DCS adopts TELEPERM-XP, and the design I/O points of each unit are 5804 points.
EC software is used for cable laying, and 12 people take 2.5 months to complete the design task of cable laying, the number of cables in the main plant is 4413 for each 325MW unit, the length of the cables for each 235MW unit in the main plant is 360 kilometers, and each unit is made of steel galvanized cable tray, which weighs about 200 tons.
The cables of 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 referring to computer cables), and other cables. If two 300MW units are laid at the same time, the number of automated professional cables is about 8,500. Among them, there will be more than 5,000 thermal control cables and weak current cables, that is, about 60% (measured by the number of cables).
◎
The above comparison is more purely technical, and the following comparison is intended to include economic factors.
The premise of the comparison is that the DCS system is compared to a typical, ideal FCS system. Why make such assumptions. As the DCS system has developed to this day, the technical requirements put forward in the early stage of development have been met and improved, and the current situation is to further improve, so there is no typical and ideal statement.
As an FCS system, it has just entered practical use in the 90s, and as a technical requirement in the early stage of development: compatible and open, two-way digital communication, digital intelligent field device, high-speed bus, etc., is not ideal and needs to be improved. This state of affairs cannot be said to be unrelated to the development of international standards for fieldbuses. In the past ten years, various bus organizations have been busy formulating standards, developing products, and occupying more markets, with the aim of squeezing into international standards and legally occupying a larger market.
Now that the battle for international standards has come to an end, major companies and organizations have realized that in order to truly capture the market, they must improve the system and related products. We can make such a prediction that in the near future, perfect fieldbus systems and related products must become the mainstream of world's fieldbus technology.
Specific comparison:
(1) The DCS system is a large system, its controller function is strong and the role in the system is very important, the data highway is the key to the system, so the overall investment must be in place in one step, and the expansion is difficult afterwards. However, the decentralization of FCS functions is relatively thorough, the information processing is on-site, and the wide adoption of digital intelligent field devices makes 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 at the same time.
(2) The DCS system is a closed system, and the products of each company are basically incompatible. The FCS system is an open system, and users can choose various devices from different manufacturers and brands to connect to the fieldbus to achieve the best system integration.
(3) The information of the DCS system is all formed by binary or analog signals, and there must be D/A and A/D conversion. And the FCS system
It is fully digital, which eliminates the need for D/A and A/D conversion, and is highly integrated and high-performance, so that the accuracy can be improved from ±0.5% to ±0.1%.
(4) The FCS system can install the PID closed-loop control function into the transmitter or actuator, which shortens the control cycle, and can be increased from 2~5 times per second of DCS to 10~20 times per second of FCS, so as to improve the regulation performance.
(5) DCS can control and monitor the whole process of the process, and diagnose, maintain and configure itself. However, due to its Achilles' heel, its I/O signal uses traditional analog signals, so it cannot remotely diagnose, maintain, and configure field instruments (including transmitters, actuators, etc.) on the DCS engineer station.
FCS adopts fully digital technology, and the digital intelligent field device sends multi-variable information, not just single-variable information, and also has the function of detecting information errors. FCS uses a two-way digital communication fieldbus signaling system. As a result, it enables remote diagnostics, maintenance, and configuration of field devices including transmitters, actuators, etc. This advantage of FCS is unmatched by 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 due to the on-site information processing, and also save the space and floor space of I/O devices and device rooms. 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, etc., while also saving design, installation, and maintenance costs. Some experts believe that 66% can be saved.For (6) and (7), it should be added that the effect of saving investment by using the FCS system is not doubtful, but whether it is as if some experts say that it reaches 60~66%.These figures appear in many articles, and the editor believes that this is the result of mutual extraction, and the original source of these figures has not yet been found, so readers should be cautious when quoting these figures.
(8) FCS is simple to configure compared with DCS, and is easy to install, operate and maintain due to the standardization of structure and performance.
(9) Key points of FCS design and development for process control. This is not intended as a comparison with DCS, but rather as an indication of the key issues that should be considered in the design and development of FCS for process control or for simulating continuous processes.
1) The intrinsically safe explosion-proof function of the bus is required, and it is the most important.
2) The changes of basic monitoring such as flow, material level, temperature, pressure, etc. are slow, and there is a lag effect, therefore, node monitoring does not require fast electronic response time, but requires complex analog processing capabilities. This physical characteristic determines that the system basically adopts the centralized round-robin system between master and slave, which is technically reasonable and economically advantageous.
3) The physical principle of the measurement of flow, material level, temperature, pressure and other parameters is classical, but the sensors, transmitters and controllers should develop towards digital intelligence.
4) As an FCS developed for continuous processes and their instrumentation, the focus should be on the design of the low-speed bus H1.
03
The future of PLC vs. DCS
We already know that some FCS is developed from PLC, and some FCS is developed from DCS, so what will be the prospect of PLC and DCS when FCS has been put into practical use today?
PLC first appeared in the late 60s in the United States, the purpose is to replace relays, perform logic, timing, counting and other sequential control functions, and establish a flexible program control system.
It was officially named and defined in 1976: PLC is a kind of digitally controlled special electronic computer, which uses programmable memory to store instructions, perform functions such as logic, sequence, timing, counting and calculus, and control various mechanical or working programs through analog and digital inputs, outputs and other components. After more than 30 years of development, PLC has been very mature and perfect, and has developed analog closed-loop control function.
The place of the PLC in the FCS system seems to have been determined, and there is not much debate. The PLC is hung on the high-speed bus as a station. Give full play to the advantages of PLC in handling switching quantities. In addition, the auxiliary workshops of thermal power plants, such as make-up water treatment workshops, circulating water workshops, ash and slag removal workshops, coal conveying workshops, etc., are mainly sequenced in the process of these workshops. PLCs have their own unique advantages for sequence control.
The control system of the auxiliary workshop should be preferably a PLC that follows the fieldbus communication protocol or a PLC that can communicate and exchange information with the FCS.