90 ‘s of the last century to the practicality of the fieldbus control system is a rapid momentum of rapid development, is currently the world‘s most modern control systems. Fieldbus control system is a focus in automation technology, is growing by domestic and foreign manufacturers of automation equipment and user attention. The appearance of fieldbus control system and automation will bring another revolution, in its depth and breadth than any time, so as to open up a new era of automation.
In some industries, FCS was developed by PLC of, while in other industries, developed by DCS and FCS come from, so between PLC and DCS and FCS are inextricably linked, and there is a profound difference. This paper tries to PLC, DCS and FCS three control systems analysis the characteristics and differences, pointing out the direction between the origin and the development.
2.PLC, DCS and FCS three basic characteristics of control system
At present, in continuous process production control (PA) or used to called in industrial process control, there are three control systems, namely, PLC, DCS and FCS. their respective basic characteristics are as follows: 2.1 PLC
(1) from the control switch control to the order, shipping, from bottom to top.
(2) continuous PID control function, PID in break stations.
(3) a PC machine available, multiple identical PLC from the station.
(4) can also be a PLC, multiple identical PLC from the station, a PLC network. Than a PC as the main station is: when a user program, do not need to know the communication protocol, as long as according to the instructions written on the line.
(5) PLC grid can be used as an independent DCS/TDCS, also available as a DCS/TDCS subsystem.
(6) large-scale systems with DCS/TDCS, such as TDC3000, CENTUMCS, WDPFI, MOD300. (7) PLC networks such as Siemens SINEC-L1, SINEC-H1, S4, S5, S6, S7, GE company MELSEC-NET GENET, Mitsubishi company, MELSEC-NET/ MINI. (8) mainly used in industrial process control in the order, the new PLC is a combination of closed–loop control function.
(9) manufacturer: GOULD (USA), AB (US), GE (US), OMRON (Japan), MITSUBISHI (Japan), Siemens (Germany).
2.2 DCS or TDCS
(1) the distributed control system with distributed control system DCS TDCS is set to 4C (Communication,Computer, Control, and CRT) technologies in one monitoring technologies.
(2) from top to bottom of tree-like topology systems, including communications (Communication) is the key.
(3) the PID in break, interrupt joins computer with live instrumentation and control devices.
(4) is a tree-like topology and continuous parallel link structure, there are a large number of cables from the relay station parallel to the field instrumentation.
(5) analog signal, A/D-D/A, mixed w/microprocessor. Test Forum
(6) one instrument received a line I/O, hanging by the control station to the local area network LAN. (7) DCS control (engineering station), operational (operator station), field instruments (field station) level 3 structure.
(8) the disadvantage is the high cost, the company‘s products are not interchangeable, not interoperable, DCS system is different.
(9) for large-scale continuous process control, such as petrochemicals.
(10) the manufacturer: Bailey (US), Westinghous (US), HITACH (Japan), LEEDS and NORTHRMP (USA), SIEMENS (Germany), Foxboro (US), ABB (Switzerland), Hartmann and Braun (Germany), Yokogawa (Japan), Honewell (United States), a Taylor (US).
2.3 FCS
(1) the basic tasks are: essence (Eigen–) safety, hazardous area, easy to change, difficult to deal with the environment.
(2) digital, intelligent, multifunctional replaces analog single–function equipment, instrumentation and control devices.
(3) using two lines join scattered field instrumentation, control, PID control, replace the two lines of each instrument.
(4) the PID on the bus and equipment, instrumentation and control devices are equal.
(5) multiple variables, and multiple nodes, serial, digital communications system replaced single–variable, single, parallel, simulation systems.
(6) is connected, bi-directional, open instead of a one-way, closed.
(7) replace centralized control with distributed virtual control station.
(8) the PC, can also be hung to the PC, computers on the same bus level.
(9) the LAN and can be connected with the Internet.
(10) changes the traditional criteria of signals, communications and systems into the enterprise network.
(11) manufacturers: American Honeywell, Smar, Fisher-Rosemount, and AB/Rockwell, and Elsag-Bailey, and Foxboro, and Yamatake, day Yokogawa Europe Siemens, GEC-Alsthom, Schneider, proces-Data, ABB etc.
(12) 3 typical of such FCS 1) continuous automatic control of technological process such as Petro–chemical, in which “intrinsically safe explosion-proof“ technology is absolutely essential, a typical product is FF, World FIP and Profibus-PA;2) separation process automatic control, such as car manufacturing, car, typical products is Profibus-DP, CANbus;3) control points such as building automation, a typical product is the LON Work, Profibus-FMS.
From the above description of the basic elements, we noticed a little, the three systems for process control, none of them are developed for power plants, or, in the initial stage of their development, are not preferred control for power system object. And in the instructions for use of these systems will not plant as the preferred scope in scope, not to mention the power station at all. Now the strange thing is, the three control systems, particularly the DCS, PLC, has been widely used in power stations, and the effect is very good.
3. the differences between the three control systems
We already know that FCS was developed from DCS and PLC, and FCS not only has the characteristics of DCS and PLC, and took the revolutionary step. At present, new DCS with the new PLC, has a tendency to move closer to each other. New DCS control functions in the order has had a strong; but the new PLC, not too bad in terms of dealing with closed–loop control, and both can form a large network, scope of application of DCS and PLC, already has a lot of crossover. The next section compares only to DCS and FCS. In the previous section, actually involves the difference between DCS and FCS, the following investment, design, architecture, describes the use of.
3.1 differences. DCS
DCS system is the key to communication. Can also say the data highway is distributed control system DCS of the spine. Because of its mission is to provide a communication network between all parts of the system, so data of highway design itself determines the overall flexibility and security. Highway of data media can be: a pair of twisted pair, coaxial cable, or fiber optic cable.
Design parameters of the data highway, basically understand the relative strengths and weaknesses of a particular DCS system.
(1) how many I/O system can handle information.
(2) the system can handle and control information about the control loop.
(3) can adapt to how many users and devices (CRT, control stations, etc).
(4) how to thoroughly check the integrity of transmitted data.
(5) data the maximum allowed length of the highway is.
(6) the data highway to support branch.
(7) whether the data highway to support by other factories to produce hardware (programmable controller, computer, data logger, etc).
To ensure the integrity of communications, most of the DCS manufacturers provide redundant data highway.
In order to ensure the safety of the system, using the complex communication protocols and error–detecting technology. Called communication protocol is a set of rules to ensure the transmission of data is received and understood as well and sending data.
Currently in DCS systems generally use two types of communication methods, synchronous and asynchronous, synchronous communication relies on a clock signal to adjust the transmission and reception of data, asynchronous networks using no clock reporting system … … FCS FCS key elements of three
(1) the FCS system is the core of the Protocol, namely bus standards described in previous chapters have been, a type of bus, as long as the bus protocol is determined, the relevant key technologies and related equipment is determined. As regards its basic principles of bus protocol, various types of bus are the same, to deal with two-way serial digital transmission for the base. But for a variety of reasons, various bus–bus protocol there is a big difference.
In order for the field bus to meet interoperability requirements, making it a truly open system, IEC international standards Fieldbus Communication protocol model in the user layer, specify user–level device description. In order to achieve interoperability, each fieldbus device is described in terms of device description DD. DD can be considered to be a drive device, it includes all the necessary parameters description and steps required to master. Because DD describe devices required to communicate all of the information, and has nothing to do with the main station, you can enable field devices to achieve true interoperability.
Fact is, as above, the answer is no. The via field bus standard contains 8 types, while the IEO international standard is only one of the 8 types, with the other 7 types of bus have equal status. 7 other buses, regardless of how much market share, each bus protocol is a set of software and hardware support. They can form a system, product, and the international standards IEC fieldbus, is neither software nor hardware support of empty shelves. Therefore, compatibility and interoperability to meet the bus, on the current state, it is almost impossible.
Through the above, we can draw an image: the open interoperability of fieldbus control system and, in relation to a specific type of fieldbus, as long as we follow the type of fieldbus Protocol, its products are open, and interoperable. In other words, no matter what the manufacturers of products, nor a field bus is the company‘s product, as long as we follow the bus bus protocol, products are open, and interoperable, network bus can.
(2) the FCS system is based on digital intelligent field devices
Digital intelligent field devices is the FCS system hardware support is the Foundation, the reason is very simple, FCS systems automatic control device and two-way digital communication field bus signals between devices. If the device does not follow the same protocol, communication protocol, and does not have communication capability, the so-called two-way digital communication is just empty talk, nor called Fieldbus control system. Again, a major feature of the fieldbus is to increase controls at the field level. If the device is not work
Intelligent products, the characteristics of field–bus control system does not exist, the so-called simplified system, convenient design, to help safeguard the superiority is false.
(3) FCS systems of information processing is the essence of the scene
For a control system, whether using DCS or the fieldbus, systems need to deal with the amount of at least as much. In fact, after the bus, you can get more information from the scene. Fieldbus system without reducing the amount of, and even increased, and much less cable to transfer information. This requires on the one hand to improve cable‘s ability to transmit information, on the other hand to make a large amount of information in the field in-place finishes processing, information round-trip between reduce and control room. Can say field bus is the nature of the processing of the site.
Information to and from network design and system configuration of an important principle. Reduce round trips can often improve system response time benefits. Therefore, the network design should give priority to mutual exchange of information between nodes on the same slip.
Reduce round trips and reduce the system‘s cable can sometimes conflict with each other. Should save investment principles to make a selection. If you select the system response time permits, should be selected to save cable programmes. If the selected system response time is quite high, slightly reducing the transmission of information is enough, that programmes to reduce transmission to be elected.
Now with fieldbus instrument itself with many functional blocks, although the different products of the same function blocks on performance will be slightly different, but a network of identical road with lots of features function blocks is objective. Choose which function blocks on field instruments, is the configuration of the system to solve the problem.
This principle is: try to reduce the amount of information to and from bus. General selection and most of the information about the function of the output function blocks on that instrument.
3.2 typical systems
Through the use of field–bus users can significantly reduce field wiring, using a single instrument can achieve MIMO communication, full interoperability between the devices produced by different manufacturers, increase the level of control, systems integration is greatly simplified, and the maintenance is very simple. Traditional process control instrument system for each field unit to control room are required to use a special pair of twisted-pair cable to send 4~20mA signals; field bus systems, each twisted-pair wire field devices to the junction box can still be used, but from the junction box to the central control room only with a dual strand complete digital communications.
Through the use of field–bus control system, how much can you save cable, the editor has yet to do this calculation. However, we from the DCS system in power plant cable and automatic control system about the mileage, cable‘s share in infrastructure investment.
A power plant, 2x300MW coal-fired units. Thermodynamic system is modular. Each unit set up a central control building, machine, centralized control of furnace and electric units. Unit control room for 12.6 meters above sea level, and operation elevations. DCS adopts WDPF-II, each unit designed I/O
PLC ONE 