Characteristics and differences of the three contr

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Characteristics and differences of PLC, DCS and FCS

Abstract: 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

key words: programmable controller (PLC), distributed control system (DCS), fieldbus control system (FCS)

1 Preface

fieldbus control system, which became practical in the 1990s, is developing rapidly with a rapid momentum. It is the latest control system in the world at present. Fieldbus control system is a hot spot in automation technology at present, and it is receiving more and more attention from automation equipment manufacturers and users at home and abroad. The emergence of fieldbus control system will bring another revolution in the field of automation, and its depth and breadth will exceed any time 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 there are countless connections and essential differences between FCS, PLC and DCS. This paper tries to analyze the characteristics and differences of PLC, DCS and FCS, and points out the origin and development direction between them

2. Basic characteristics of three control systems: PLC, DCS and FCS

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

2.1 plc

(1) from switching value control to sequence control and transportation processing, from bottom to top

(2) continuous PID control and other functions. PID is in the interrupt station

(3) one PC can be used as the master station, and multiple PLC of the same type can be used as the slave station

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

(5) PLC grid can be used as either an independent dcs/tdcs or a subsystem of dcs/tdcs

(6) large 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, Mitsubishi's melsec-net, melsec-net/mini

(8) it is mainly used for sequential control in industrial processes, and the new PLC also has the function of closed-loop control

(9) manufacturers: Gould (US), ab (US), Ge (US), Omron (Japan), Mitsubishi (Japan), Siemens (Germany), etc

2.2 DCS or tDCS

(1) DCS and tDCS are monitoring technologies that integrate 4C (communication, computer, control, CRT) technology

(2) a tree topology large-scale system from top to bottom, in which communication is the key

(3) PID in the interrupt station, the interrupt station connects the computer with the field instruments and control devices

(4) is a tree topology and parallel continuous link structure, and there are also a large number of cables from the relay station to the field instruments in parallel

(5) analog signal, a/d-d/a, mixed with microprocessor

(6) a pair of wires of one instrument are connected to the i/o and connected to the local LAN by the control station

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

(8) the disadvantage is that the cost is high, the products of various companies cannot be interchanged and interoperable, and the large DCS system is different from each other

(9) for large-scale continuous process control, such as petrochemical, etc

(10) manufacturers: Bailey (USA), Westinghouse (USA), Hitach (Japan), Leeds & northrmp (USA), Siemens (Germany), Foxboro (USA), abb (Switzerland), Hartmann & Braun (Germany), Yokogawa (Japan), Honeywell (USA), Taylor (USA), etc

2.3 fcs

(1) the basic task is: the essence (intrinsic) can realize the infrared stealth safety of devices, dangerous areas, volatile processes, and difficult to deal with extraordinary environments

(2) full digitalization, intelligence and multi-function replace analog single function instruments, meters and control devices

(3) connect the scattered field instruments, control devices, PID and control center with two wires to replace the two wires of each instrument

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

(5) multivariable, multi node, serial and digital communication systems replace single variable, single point, parallel and analog systems

(6) it is interconnected, bidirectional and open instead of unidirectional and closed

(7) replace centralized control stations with decentralized virtual control stations

(8) controlled by the field computer, it can also be connected to the upper computer and connected to the upper computer of the same bus

(9) local area, which can be connected to the Internet

(10) change the traditional signal standards, communication standards and system standards into enterprise management

(11) manufacturers: Honeywell, SMAR, Fisher Rosemount, ab/rockwell, elsag Bailey, Foxboro, Yamatake, Yokogawa, Siemens, GEC ALSTOM, Schneider, processes data, abb, etc

(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 the typical products are FF, World FIP, PROFIBUS Pa

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

3) multipoint 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 none of the three systems used for process control is developed for power stations, or in the early stage of their development, power stations are not the preferred control object of the system. In the instructions for the use of these systems, the power station is never the preferred scope of application, and some do not mention the power station at all. Now it is strange that these three control systems, especially DCS and PLC, have been widely used in power stations, and the effect is also very good

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 has taken a revolutionary step. At present, the new DCS and the new PLC have the trend of moving closer to each other. The new DCs has strong sequence control function; 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 next section compares DCs with FCS only. In the previous chapters, the differences between DCs and FCS have actually been involved. The following will describe the architecture, investment, design, use and other aspects

3.1 key points of difference


the key of DCS system is communication. It can also be said that the data highway is the backbone of DCS. Because its task is to provide communication networks between all components of the system, the design of the data highway itself determines the overall flexibility and security. The media of data highway can be a pair of twisted wires, coaxial cables or optical fiber cables

through the design parameters of the data highway, we can basically understand the relative advantages and disadvantages of a specific DCS system

(1) how much i/o information can the system process

(2) how much control loop information related to control can the system process

(3) how many users and devices (CRT, control station, etc.) can be adapted

(4) how to thoroughly check the integrity of transmission data

(5) what is the maximum allowable length of the data highway

(6) how many branches can the data highway support

(7) whether the data highway can support the hardware (programmable controller, computer, data recording device, etc.) produced by other manufacturers

to ensure the integrity of communication, most DCS manufacturers can provide redundant data highway

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

at present, two kinds of communication means are generally used in 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 clock free reporting system


there are three key points of FCS

(1) the core of FCS system is bus protocol, that is, the previous chapter of bus standard

has described that as long as its bus protocol is determined, the relevant key technologies and related equipment will be determined. In terms of the basic principle of its bus protocol, all kinds of buses are the same, which are based on solving the two-way serial digital communication transmission. However, due to various reasons, there are great differences in the bus protocols of various buses

in order to make the fieldbus meet the interoperability requirements and become a real open system, in the IEC international standard, the fieldbus communication protocol module needs special attention, including changing the user layer of the operating procedure type of the experimental machine, which clearly stipulates that the user layer has the function of device description. In order to realize interoperability, each fieldbus device is described by device description DD. DD can be regarded as a driver of the device, which includes all necessary parameter descriptions and operation steps required by the master station. Since DD includes all the information needed to describe the device communication and is independent of the master station, it can make the field devices realize real interoperability

whether the actual situation is consistent with the above, the answer is No. At present, the fieldbus international standard adopted includes eight types, while the original IEO international standard is only one of the eight types, and its status is equal to that of the other seven types of buses. For the other seven kinds of 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 support or hardware support. Therefore, it is almost impossible to realize the mutual compatibility and interoperability of these buses in the current state

from the above, can we get such an image: the interoperability of open fieldbus control system, for a specific type of fieldbus, as long as the bus protocol of this type of fieldbus is followed, it is open to its products and has interoperability. In other words, no matter what manufacturer's products, no one 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 foundation of the FCS system is the digital intelligent field device

the digital intelligent field device is the hardware support and foundation of the FCS system. 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 the unified bus protocol, that is, the relevant communication protocol, and does not have the function of digital communication, then the so-called dual

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