There are many names of control products, each with a different name. Commonly used control products are DCS and PLC. We extend the concept of DCS to FCS.
DCS (Distributed Control System) is a distributed control system, also known as distributed control system.
PLC (Program Logic Control) is a programmable logic controller.
Fieldbus control system.
So far, there is no strict boundary between DCS and PLC. In the eyes of most people, the large system is called DCS and the small system is called PLC. Of course, it is not impossible to say this, but it is wrong. Now let's re-establish this concept.
First of all, what is the difference between DCS and PLC?
1, in terms of development:
DCS is developed from the traditional dashboard monitoring system. Therefore, DCS is essentially more focused on instrument control. For example, the Yokogawa CS3000 DCS system we use has no limit on the number of PID (PID, proportional differential integral algorithm, is the standard algorithm for closed-loop control of regulating valves and frequency converters, and usually the number of PID determines the number of regulating valves that can be used).
PLC is developed from the traditional relay circuit, and the original PLC didn't even have the ability to process analog signals. Therefore, PLC emphasized the ability of logical operation from the beginning.
2. In terms of system scalability and compatibility:
There are many control products in the market, both DCS and PLC are produced and sold by many manufacturers. For PLC systems, there is generally no need or little need for expansion, because PLC systems are generally used in equipment. Generally speaking, PLC rarely requires compatibility. For example, two or more systems need to share resources, which is also very difficult for PLC. Moreover, PLC generally adopts a special network structure, such as the MPI bus network of Siemens. Even if an operator station is added, it is not easy and the cost is not high.
In the process of development, DCS is also a self-contained system of various manufacturers, but most DCS systems, such as Yokogawa, Honeywell, ABB, etc. Although the communication protocols in the system (process level) are different, the network platform at the operation level adopts Ethernet without exception and adopts standard or variant TCP/IP protocol. This provides very convenient scalability. In this network, both the controller and the computer exist as a node. As long as the network reaches the place, it can increase or decrease the number of nodes and arrange the location of nodes at will. In addition, based on the open protocols such as OPC and DDE of windows system, all systems can communicate conveniently and realize resource sharing.
3. From the database:
DCS generally provides a unified database. In other words, once the data exists in the database of DCS system, it can be referenced under any circumstances, such as in configuration software, monitoring software, trend chart and report ... The database of a PLC system is usually not unified, and configuration software, monitoring software and even archiving software all have their own databases. Why is it often said that Siemens S7 400 is called DCS when it reaches 4 14? Because the PCS7 system of Siemens uses a unified database, PCS7 requires the controller to be at least S7 4 14-3.
4. In terms of schedule:
Generally, the program of PLC can't run according to the preset cycle. The PLC program is executed once from beginning to end, and then executed from the beginning. (Now some new PLCs have been improved, but the number of task cycles is still limited. ) and DCS can set the task cycle. Such as fast tasks. Similarly, sensor sampling, the change time of pressure sensor is very short, we can sample with a duty cycle of 200ms, while the lag time of temperature sensor is very large, we can sample with a duty cycle of 2 s, so that DCS can reasonably schedule the resources of the controller.
5. From the network structure:
Generally speaking, DCS usually adopts two-layer network structure, one is process-level network, and most DCS adopt their own bus protocols, such as Modbus of Yokogawa, Profibus of Siemens and ABB, CAN bus of ABB, etc. These protocols are based on standard serial port transmission protocols RS232 or RS485. The sampling data (machine code, 2 13/ scanning period) of the field IO module, especially the analog quantity, is huge, and there are many interference factors in the field, so the network standard with large data throughput and strong anti-interference ability should be adopted. The bus structure based on RS485 serial asynchronous communication mode meets the requirements of field communication.
The sampling data of IO is converted into plastic data or real data by CPU and transmitted on the operation level network (layer 2 network). Therefore, the network standard of moderate data throughput, fast transmission speed and convenient connection can be adopted in the operation level network. Because the operation level network is generally arranged in the control room, the requirements for anti-interference are relatively low. Therefore, adopting standard Ethernet is the best choice. TCP/IP protocol is a standard Ethernet protocol. Generally, we use the communication speed of100 mbit/s. ..
The task of PLC system is relatively simple, so the amount of data to be transmitted is generally not too large, so the common PLC system is a layer of network structure. Process-level network and operation-level network are either merged or simplified as internal connections between modules. PLC does not use or rarely uses Ethernet.
6. Judging from the scale of the applicants:
PLC is generally used in small automatic control places, such as equipment control or a small amount of analog control and interlocking, and DCS is generally used in large-scale applications. Of course, this concept is not accurate, but it is intuitive. Traditionally, we call the system above 600 points as DCS, and the system below 600 points as PLC. Our control systems of heat pump, QCS and horizontal products are collectively called PLC.
Having said so much about the differences between PLC and DCS, we should realize that today's PLC and DCS are actually getting closer and closer. Strictly speaking, PLC and DCS can no longer be uniform, and the concept between them has been blurred in many cases. Now, let's discuss our similarities.
1, function:
PLC already has the control function of analog quantity, and some PLC systems are even quite powerful in analog quantity processing, such as Yokogawa FA-MA3, Siemens S7 400, ABB Control Logix and Schneider's Quantum system. Moreover, DCS also has quite strong logical processing ability. For example, we have realized all possible process interlocking and equipment linkage start-stop on CS3000.
2, from the system structure:
The basic structure of PLC and DCS is the same. Today, PLC has been completely transplanted into computer system control, and the traditional programmer has long been eliminated. Small application PLC generally adopts touch screen, and large application PLC adopts computer system in an all-round way. Like DCS, the controller and IO station use field bus (generally based on RS485 or RS232 asynchronous serial communication protocol). If there is no expansion requirement between the controller and the computer, that is, only one computer is used, then this bus will also be used for communication. However, if multiple computers are used, the system structure will be the same as DCS, and the upper computer platform will adopt Ethernet structure. This is also one of the reasons why the concepts of PLC and DCS are vague after large-scale PLC.
3, the development direction of PLC and DCS:
Miniaturized PLC will develop to a more professional use angle, such as more targeted functions, more targeted application environment and so on. The boundary between large PLC and DCS is gradually weakened until it is completely integrated.
District councils will continue to develop into functional constituencies. The core of FCS is not only a more decentralized control system, but also instruments. The application of FCS abroad has developed to instrument level. The control system only needs to deal with signal acquisition, man-machine interface and logic control. The control of the whole analog quantity is distributed to the field instrument, and the instrument and the control system do not need to be connected through the traditional cable, and the whole instrument system is connected through the field bus. (At present, Yokogawa has used FCS in CNOOC Shell Petrochemical Project in China, and the instrument level is intelligent instruments such as EJX, which has the most advanced control level in the world).
How to treat PLC and DCS correctly?
Personally, I never emphasize which is better than PLC or DCS. I used a new term "control products" for them. We provide users with the most suitable control system. The vast majority of users will not use DCS because they want to use a set of DCS, and the control products must be positioned on the basis of meeting the user's process requirements. In fact, users who propose to use DCS or PLC have never been exposed to automatic control products or have some special needs. Too much emphasis on this thing will only lead to arguments.
Understand the general situation of control products from the similarities and differences between PLC and DCS. Attention, as professionals, don't define products as PLC or DCS, and don't treat products differently in this way psychologically.
Conceptually, PLC and DCS are not a logical concept. As can be seen from the name, PLC is named after function and DCS is named after architecture. PLC can form DCS! Of course, performance differences still exist, just depending on the product and demand. From the application point of view, simply distinguishing between PLC and DCS often leads to misunderstanding.
Difference between DCS control system and PLC control
1.DCS is a "distributed control system", while PLC is just a (programmable controller) control "device", which is the difference between "system" and "device". The system can realize the function and coordination of any equipment, and the PLC equipment only realizes the function of this unit.
2. In terms of network, the DCS network is the central nerve of the whole system. The system network in the MACS system of Hollysys Company adopts 100Mbps dual redundant industrial Ethernet and adopts the international standard protocol TCP/IP. This is a safe and reliable double redundant high-speed communication network, and the system has good expansibility and openness. Because PLC basically works for individuals, when it communicates with other PLCs or upper computers, the network form adopted is basically a single network structure, and the network protocol is often inconsistent with international standards. In terms of network security, PLC has no good protection measures. We adopt dual redundancy of power supply, CPU and network.
3. Considering the overall scheme of DCS, the operator station has the function of engineer station. After the operation scheme program is downloaded, the relationship between stations is closely linked. Any station, any function and any controlled device are interlocked and coordinated. However, in the system composed only of PLC, the connection between stations (PLC and PLC) is a loose connection mode, which can not achieve the function of coordinated control.
4.DCS has a large number of extensible interfaces in the whole design, and it is very convenient to connect external systems or expand systems. After the whole system is superimposed by PLC, it is difficult to add or subtract operator stations at will.
5.DCS safety: In order to ensure the safety and reliability of DCS control equipment, DCS adopts dual redundant control units. When the important control unit fails, the relevant redundant units will switch to the working unit in real time without interference, thus ensuring the safety and reliability of the whole system. The system of PLC superposition basically has no concept of redundancy, and there is no redundant control strategy. Especially when the PLC unit fails, the whole system must be stopped before replacement, maintenance and reprogramming. Therefore, DCS system is one level higher than its safety and reliability.
6. System software and updating various process control schemes are one of the most basic functions of DCS. When the scheme changes, the engineer only needs to compile the changed scheme at the engineer station and execute the download command. The downloading process is completed automatically by the system, and the operation of the original control scheme is not affected. Various control software and algorithms of the system can improve the control accuracy of the process requirements control object. For the system composed of PLC, the workload is extremely huge. First, determine which PLC to edit and update, and then compile the program with the corresponding compiler. Finally, a special machine (reader) is used to transmit the program one-to-one to this PLC. In the process of system debugging, the debugging time and cost are greatly increased, which is extremely unfavorable for future maintenance. The control accuracy varies greatly. This determines why all systems connected by PLC are basically not used in large and medium-sized control projects (above 500 points).
7. Modules: All I/O modules of DCS system are equipped with CPU, which can realize the quality judgment and scalar transformation of collected and output signals, hot plug and random replacement of faults. The PLC module is just a simple electrical conversion unit, and there is no intelligent chip. After the failure, all the corresponding units were paralyzed.
We have to talk about the origin of PLC and DCS: the development of PLC is based on the field control demand of manufacturing industry, and the development of DCS is based on the continuous process control and monitoring of chemical industry. From the classification of control requirements, control systems can be divided into: sequence control (basic function of PLC), process control (DCS), position control (CNC) and transmission control (speed regulation or synchronization), but from the perspective of development, these control methods are gradually merging. If I have to do something, I think there are the following points:
1, different object-oriented: PLC is oriented to the general industrial control field and has strong versatility. DCS emphasizes process control and is used in chemical industry.
2.DCS emphasizes the accuracy of continuous process control, and can realize complex control such as PID, feedforward, cascade, multilevel, fuzzy and adaptive. Generally, PLC only has PID function, and its control accuracy is not as high as DCS.
3.DCS system has powerful SCAND software package and formula function, and expert software (chemical process parameter configuration and control algorithm) is developed for different lines, which is more convenient than PLC application.
Finally, modern PLC has been able to complete most functions of DCS, which can be used for redundancy or hot standby, and modules can also be plugged in and out online. Choosing PLC or DCS depends on what you are facing. Economy, reliability and convenience are the most important.