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 DCS and the new PLC have a tendency to move closer to each other. The new DCS has Very strong sequence control function; and the new PLC is not bad in dealing with closed-loop control, and both can form a large network. The scope of application of DCS and PLC has been greatly crossed.
The difference between PLC, DCS and FCS1, PLC
PLC (Programmable Logic Controller) is a digital arithmetic operation electronic system specially designed for application in industrial environments. It uses a programmable memory that stores instructions for performing operations such as logic operations, sequence control, timing, counting, and arithmetic operations, and controls various types of mechanical equipment or production through digital or analog input and output. process.
(1) High reliability. Since most PLCs use a single-chip microcomputer, the integration is high, and the corresponding protection circuit and self-diagnosis function improve the reliability of the system.
(2) Programming is easy. PLC programming mostly uses relay control ladder diagrams and command statements, the number of which is much less than that of microcomputer instructions. In addition to medium and high-end PLCs, there are only about 16 small PLCs in general. Due to the simplicity of the ladder image, it is easy to grasp, easy to use, and even requires no computer expertise to program.
(3) Flexible configuration. Since the PLC adopts a modular structure, the user can flexibly change the function and scale of the control system simply by combining, so that it can be applied to any control system.
(4) The input/output function modules are complete. One of the biggest advantages of PLC is that for different field signals (such as DC or AC, switching, digital or analog, voltage or current, etc.), there are corresponding templates for industrial devices (such as buttons and switches). , sensor current transmitter, motor starter or control valve, etc.) are directly connected and connected to the CPU board through the bus.
(5) Easy to install. Compared with computer systems, PLC installation requires neither a dedicated machine room nor strict shielding measures. When you use it, you only need to connect the detection device to the I/O interface terminal of the actuator and PLC correctly, and it can work normally.
(6) The running speed is fast. Since the control of the PLC is controlled by the program, regardless of its reliability or operating speed, the relay logic control cannot be compared.
In recent years, the use of microprocessors, especially with the large number of single-chip microcomputers, has greatly enhanced the capabilities of PLCs, and the difference between PLCs and microcomputer control systems has become smaller and smaller, especially for high-end PLCs.
2, DCS
The key to DCS systems is communication. It can also be said that the data highway is the spine of the distributed control system DCS. Because 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 twisted wires, coaxial cables, or fiber optic cables.
Through the design parameters of the data highway, you can basically understand the relative advantages and weaknesses of a particular DCS system.
(1) How much I/O information the system can handle.
(2) How much control loop related information the system can handle.
(3) How many users and devices (CRT, control station, etc.) can be accommodated.
(4) How to completely check the integrity of the transmitted data.
(5) What is the maximum allowable length of the data highway?
(6) How many branches can be supported by the data highway.
(7) Whether the data highway can support hardware (programmable controllers, computers, data recording devices, etc.) produced by other manufacturers.
To ensure the integrity of communications, 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. The so-called communication protocol is a set of rules to ensure that the transmitted data is received and understood as the transmitted data.
At present, in the DCS system, two types of communication means are generally used, that is, synchronous and asynchronous, and synchronous communication relies on one clock to regulate data transmission and reception, and the asynchronous network adopts a reporting system without a clock.
3, FCS
The key points of the FCS are the following three points.
(1) The core of the FCS system is the bus protocol, the bus standard.
A type of bus, as long as its bus protocol is determined, the relevant key technologies and related equipment are also determined. As far as the basic principle of the bus protocol is concerned, all types of buses are the same, all solving the two-way serial Digital communication transmission is the basic basis. However, for various reasons, the bus protocols of various types of buses vary greatly.
In order to make the fieldbus meet the interoperability requirements, making it a truly open system, the original IEC international standard, the user layer simulated by the fieldbus communication protocol, clearly specifies that the user layer has a device description function. In order to achieve interoperability, each fieldbus device is described by a device description DD. The DD can be thought of as a drive for the device, which includes all the necessary parameter descriptions and the operational steps required by the primary station. Since the DD includes all the information needed to describe the communication of the device and is independent of the primary station, the field device can be made truly interoperable.
Whether the actual situation is consistent with the above, the answer is no. Currently, the fieldbus international standard has eight types, and the original IEC international standard is only one of eight types, and the status of the other seven types of buses is equal. The other seven types of buses, regardless of their market share, each bus protocol has a set of software, hardware support. They are able to form systems and form products. The original IEC fieldbus international standard is an empty shelf with neither software support nor hardware support. To achieve mutual compatibility and interoperability of these buses, it is almost impossible in the current state.
Through the above, can we come up with an image of the interoperability of an open fieldbus control system, which is open to a specific type of fieldbus as long as it follows the bus protocol of this type of fieldbus And interoperability, in other words, no matter what manufacturer's products, it is not necessarily the product of the fieldbus company, as long as the bus protocol of the bus is followed, the products are open and interoperable. It is possible to form a bus network.
(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. It is the basis and the reason is very simple. The FCS system performs the two-way digital communication field bus signal system between the automatic control device and the field device. If the field device does not follow the unified bus protocol, that is, the relevant communication protocol, and does not have digital communication function, then the so-called two-way digital communication is just an empty talk, and can not be called a field bus control system. Another point, a major feature of the fieldbus is to increase the level of control at the site. If the field device is not a multi-functional intelligent product, the characteristics of the fieldbus control system will not exist. The advantages of simplifying the system, facilitating design, and facilitating maintenance are also virtual.
(3) The essence of the FCS system is the on-site processing of information processing.
For a control system, whether it is DCS or fieldbus, the amount of information that the system needs to process is at least as much. In fact, after using the fieldbus, more information can be obtained from the site. The amount of information in the fieldbus system has not decreased or even increased, and the cable for transmitting information has been greatly reduced. This requires on the one hand to greatly improve the ability of the cable to transmit information, on the other hand, to allow a large amount of information to be processed on site, reducing the information between the scene and the control room. It can be said that the essence of fieldbus is the on-siteization of information processing.
Reducing information round trips is an important principle in network design and system configuration. Reducing information round trips often brings the benefits of improved system response time. therefore. In the network design, the nodes with large amount of information exchange should be placed in the same branch.
Cables that reduce information to and from the system are sometimes contradictory. At this time, we should still choose the principle of saving investment. If the response time of the selected system allows, you should choose a cable-saving solution. If the response time of the selected system is relatively tight, and a little reduction in the transmission of information is sufficient, then the solution for reducing the information transmission should be selected.
Now some field instruments with fieldbus are equipped with many function blocks. Although the performance of different functional blocks of different products will be slightly different, there are many functional sub-functions on a network branch. Which function on the field instrument is selected is the problem to be solved by the system configuration.
The principle of considering this problem is to minimize the round-trip of information on the bus. It is generally possible to select the function block on the meter that outputs the most information related to this function.
Typical comparison between the threeBy using the field bus, users can greatly reduce the field wiring. Multi-variable communication can be realized with a single field instrument. The devices produced by different manufacturers can be fully interoperable, adding on-site control functions, system integration is greatly simplified, and maintenance is very Simple. In the traditional process control system, each field device to the control room needs to use a pair of dedicated twisted pairs to transmit 4-20 mA signals. In the fieldbus system, each field device to the junction box twisted pair can still be used. However, digital communication is performed using only one twisted pair from the field junction box to the central control room.
By using a fieldbus control system, how many cables can be saved, the editor has not done this calculation. However, we can use the number of cable kilometers used in the DCS system power plant related to the automatic control system to see the share of the cable in infrastructure investment.
1. A power plant, 2&TImes; 300MW coal-fired unit
The thermal system is a unit system. Each unit is equipped with a centralized control building, which adopts centralized control of the machine, furnace and electric unit. The unit control room has an elevation of 12.6m, which is consistent with the operating level. The DCS uses WDPF-II, and each unit is designed with an I/O point of 4,500 points.
The cable was laid using Russian EC components, and 8 people completed the design task in 1.5 months.
The number of cables for automation of each 300MW unit in the main plant is 4,038.
The length of the automation professional cable for each 300MW unit in the main building is 350 kilometers.
The number and length of the above cables do not include the factory supply cable for the whole plant fire alarm and the cables for the auxiliary production workshops of the whole plant.
The column, bridge and small box of the cable tray are all made of steel galvanized, each unit is about 95t.
Other cable trays include straight through, bend through, three-way, four-way, cover plate, terminal head, width adjustment sheet, direct sheet and other aluminum alloy materials, each of which is about 55t.
Accessories are supplied with the bridge (eg bolts, nuts).
2. A power plant, 4&TImes; 325MW fuel gas power station
The thermal system is a unit system. DCS adopts TELEPERM--XP. The design I/O point of each unit is 5804 points.
The cable was laid using EC software, and 12 people completed the design task of cable laying in 2.5 months.
The number of cables for automation of each 325MW unit in the main building is 4413.
The cable length of each 325MW unit in the main plant is 360km.
Each unit is made of steel galvanized cable tray, and its mass is about 250t.
3. The power station cables can be divided into six categories: high-voltage power cables, low-voltage power cables, control cables, thermal control cables, weak electrical cables (mainly computer cables), and other cables. If two 300MW units are used for cable laying at the same time, the number of automation professional cables is about 8,500. Among them, the number of thermal control cables and weak electric cables will be more than 500, and about 60% (measured by the number of roots).
The above comparisons are biased towards purely technical comparisons, and the following comparisons are intended to incorporate economic factors.
The premise of comparison is that the DCS system is compared to a typical, ideal FCS system. Why do you want to make such an assumption? As the DCS system has developed to the present day, the technical requirements put forward at the beginning of development have been met and improved, and the current situation is further improved, so there is no typical and ideal statement. As the FCS system, it has just entered the practical use in the 1990s. As a technical requirement in the early stage of development: compatible open, two-way digital communication, digital intelligent field devices, high-speed bus, etc., it is still not ideal and needs to be improved. This state cannot be said to have nothing to do with the development of international standards for fieldbus. Over the past ten years, various bus organizations have been busy setting standards, developing products, and occupying more markets. The purpose is to squeeze into international standards and legally occupy more markets. Now the battle for international standards has come to an end. Big company organizations have realized that to truly occupy the market, they must improve the system and related products. We can make such predictions. In the near future, perfect fieldbus systems and related products will become the mainstream of the world's fieldbus technology.
Specific comparison:
(1) The DCS system is a large system. Its controller function is very important and its role in the system is very important. The data highway is the key to the system. Therefore, the overall investment must be in one step, and the expansion afterwards is difficult. The FCS function is more thoroughly decentralized, the information processing is on-site, and the digital intelligent field device is widely used, which makes the controller function and importance relatively weak. Therefore, the FCS system has a low starting point for investment and can be used while expanding, while expanding.
(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 select various devices from different manufacturers and brands to connect to the field bus to achieve optimal system integration.
(3) The information of the DCS system is all formed by binary or analog signals, and must have D/A and A/D conversion. The FCS system is fully digital, eliminating D/A and A/D conversion, and high integration and high performance, which can improve the accuracy from ±0.5% to ±0.1%.
(4) The FCS system can load the PID closed-loop control function into the transmitter or actuator, shortening the control cycle, which can be increased from 2-5 times per second of DCS to 10-20 times per second of FCS. , thereby improving the adjustment performance.
(5) The DCS system can control and monitor the whole process of the process, and diagnose, maintain and configure itself. However, due to its own pathogenic weakness, its I/O signals use traditional analog signals. Therefore, it is not possible to remotely diagnose, maintain and group field instruments (including transmitters, actuators, etc.) at the DCS engineering station. state. The FCS system uses all-digital technology, and the digital intelligent field device transmits multi-variable information, not just univariate information, but also has the function of detecting information errors. The FCS system uses a two-way digital communication fieldbus signal system. As a result, it can diagnose, maintain and configure field devices (including transmitters, actuators, etc.). This superiority of the FCS system is unmatched by the DCS system.
(6) Due to the on-site processing of information processing, the FCS system can save a considerable number of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files and I/O cabinets compared with DCS systems. At the same time, it also saves space and floor space of I/O devices and equipment rooms. Some experts believe that 60% can be saved.
(7) For the same reason as (6), the FCS system can reduce the number of cables and bridges used for laying cables, and also saves design, installation and maintenance costs. Some experts believe that 66% can be saved.
It should be added to the two points (6) and (7) that the use of the FCS system saves investment is not doubtful, but if it is said by some experts, it is 66%. These figures appear in several articles, and the editors believe that this is the result of mutual referrals, and the original source of these numbers has not yet been found. Therefore, readers should be cautious when referring to numbers.
(8) FCS is relatively simple to DCS configuration. Due to the standardized structure and performance, it is easy to install, operate and maintain.
(9) FCS design and development points for process control. This point is not a DCS comparison. It just describes the issues that should be considered in the design development of FCS for process control or for simulating continuous process classes.
1 requires the bus intrinsically safe explosion-proof function, and is of paramount importance.
2 Basic monitoring such as flow, material level, temperature, pressure and other changes are slow, and there is a hysteresis effect. Therefore, node monitoring does not require fast electronics response time, but requires complex analog processing capability. This physical feature determines that the system basically adopts a centralized polling system between master and slave, which is technically reasonable and economically advantageous.
3 The measurement of flow, material level, temperature, pressure and other parameters, the physical principle is classical, but the sensor, transmitter and controller should be developed to digital intelligence.
4 As an FCS system developed for continuous process types and their instrumentation, emphasis should be placed on the design of the low-speed bus H1.
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