Summary:Automation control system products are mainly divided into DCS and PLC, they have no strict boundaries in the process of application, the industry does not have a strict distinction, the article is mainly to re-examine these points, respectively, from the development, scalability, compatibility, database From the point of view to discuss!
I. Introduction Automation Control System Products With the rapid development of the IT industry, intelligent control products have many names but they can be summarized into two major categories: DCS (computer technology + communication technology) and PLC (relay technology + computer technology). DCS (DistributedContorlSystem), distributed control system, also known as distributed control system, such as Westinghouse's WDPF; BAILEY's NETWORK-90 and so on. PLC (ProgramLogicControl) is also called a programmable logic controller, such as PLC5 of AB company; 90-30 of GE company; Schneider MODICON and so on. Up to now, there has been no strict boundary between DCS and PLC in the application of process control. In most people's view, large systems use DCS, and small systems use PLC. Of course, it is not impossible to say so, but there is a great one-sidedness. Let us now re-examine these points.
Second, what is the difference between DCS and PLC? 2.1 From the aspect of development, DCS has developed from the traditional dashboard monitoring system. Therefore, DCS focuses on the control of the instrument from the congenital point of view. For example, the YOKOGAWACS3000DCS system we use does not even have a PID number limit (PID, Proportional Calculus of Integrals, which is the standard algorithm for the closed-loop control of valves and frequency converters, usually the number of PIDs. Determines the number of regulators that can be used).
PLC evolved from the traditional relay loop. The original PLC did not even have analog processing power. Therefore, the PLC emphasized the logic computation ability from the beginning.
2.2 From the aspects of system scalability and compatibility, there are many kinds of control products on the market. No matter DCS or PLC, there are many manufacturers in production and sales. For PLC systems, there is generally no or little extended requirement because PLC systems are generally used for equipment. In general, PLCs rarely have compatibility requirements. For example, two or more systems require resource sharing, which is also more difficult for PLCs. Moreover, PLCs generally use a dedicated network structure, such as Siemens MPI bus network, and even adding an operator station is not easy or costly. In the process of development, DCS is also a self-contained system, but most of the DCS systems, such as Yokogawa, Honeywell, ABB, etc., although the communication protocol within the system (process level) is not the same, but the operation level The network platform invariably chose Ethernet, using standard or variant TCP/IP protocols. This provides a very convenient scalability. In this kind of network, the controller and the computer all exist as one node. As long as the network arrives, the number of nodes and the position of the nodes can be arbitrarily increased or decreased. In addition, based on the windows system's open protocols such as OPC, DDE, etc., each system can also easily communicate to achieve resource sharing.
2.3 DCS generally provides a unified database from the database. In other words, once a data exists in the database in the DCS system, it can be referenced in any situation, such as in the configuration software, in the monitoring software, in the trend graph, in the report... and in the PLC system. Databases are usually not uniform, and configuration software and monitoring software and even archiving software have their own databases. Why often say that Siemens S7400 will only be called 414 DCS? Since Siemens' PCS 7 system uses a unified database, PCS 7 requires the controller to be at least S7414-3.
2.4 From the time scheduling point of view, the PLC program can not generally run in accordance with the preset cycle. The PLC program is executed once from beginning to end and from the beginning. (Now some new PLCs have improved, but there are still limits on the number of mission cycles) and DCS can set mission cycles. For example, quick tasks. The same is the sampling of the sensor, the change time of the pressure sensor is very short, we can use 200ms duty cycle sampling, and the temperature sensor lag time is very large, we can use 2s task cycle sampling. In this way, the DCS can reasonably schedule controller resources.
2.5 In terms of network structure development Generally speaking, DCS routinely uses two layers of network structure. One layer is a process-level network. Most DCSs use their own bus protocols, such as Yokogawa's Modbus, Siemens and AB's Profibus, and ABB's CANbus. Etc., these protocols are based on the standard serial port transmission protocol RS232 or RS485 protocol. The on-the-spot IO module, especially the analog sampling data (machine code, 213/scan cycle) is very large, and there are many on-site interference factors, so a network standard with large data throughput and strong anti-interference capability should be used. The bus structure based on the RS485 serial port asynchronous communication method meets the requirements of field communication. The sampled data of the IO is converted into shaped data or real data by the CPU, and transmitted on the operation level network (Layer 2 network). Therefore, operation-level networks can adopt network standards with moderate data throughput, fast transmission speed, and easy connection. At the same time, operation-level networks are generally deployed in control rooms, and the requirements for combating interference are relatively low. Therefore, using standard Ethernet is the best choice. The TCP/IP protocol is a standard Ethernet protocol. Generally, we use 100 Mbit/s communication speed.
The task of the PLC system is relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a layer of network structure. Process-level networks and operation-level networks are either merged together, or the process-level networks are simply reduced to internal connections between modules. PLCs will not or rarely use Ethernet.
2.6 From the scale of the application object, PLC is generally used in small-scale self-control places, such as equipment control or a small amount of analog control and interlocking, while large-scale applications are generally DCS. Of course, this concept is not very accurate, but it is intuitive. We used to use DCS systems for systems larger than 1,000 points, and PLC control for smaller systems. Our heat pump and QCS, horizontal product supporting control system is generally called PLC.
Third, the development status of DCS and PLC said that the difference between so many PLCs and DCS, but we should realize that, PLC and DCS development to today, in fact, are moving closer to each other, strictly speaking, the current PLC and DCS can not A knife cut, the concept between many times has been blurred. Now let's discuss the same (like) aspects of each other.
3.1 From the function point of view PLC has already possessed the analog quantity control function, some PLC system analog quantity processing capacity even is quite formidable, for example Yokogawa FA-MA3, Siemens S7400, AB ControlLogix and Schneider's Quantum system. The DCS also has a very strong logic processing capabilities, such as the CS3000; I / A can be achieved on all possible process interlocks and equipment start and stop linkage.
3.2 From the system structure, the basic structure of PLC and DCS is the same. PLC has been fully transplanted to the computer system control since its development. Traditional programmers have long been eliminated. The PLC of small-scale application generally uses the touch screen, and PLC of large-scale application fully uses the computer system. Like the DCS, the controller and IO station use fieldbus (usually bus based on RS485 or RS232 asynchronous serial communication protocol). If there is no requirement for expansion between the controller and the computer, it means that only one computer is used. In this case, this bus communication will also be used. However, if there is more than one computer, the system structure will be the same as that of the DCS. The upper computer platform uses the Ethernet structure. This is one of the reasons why the PLC has become large and the DCS concept has been blurred.
3.3 The development direction of PLC and DCS The miniaturized PLC will develop to a more specialized use point of view, such as a more professional function, a more targeted application environment, and a high reliability and low cost. The boundary between large-scale PLC and DCS is gradually reduced until it is fully integrated. DCS will continue to develop in the direction of FCS (FieldBusContorlSyestem) fieldbus control system. In addition to the more decentralized control system at the heart of the FCS, instrumentation is particularly important. The application of FCS in foreign countries has been developed to the instrument level. The control system only needs to deal with signal acquisition and provide man-machine interface and logic control. The control of the entire analog quantity is dispersed to the field instrument. No traditional cable connection is required between the instrument and the control system. The entire instrument system is connected using the field bus. (Currently, Yokogawa has used FCS in the Sinopec Shell petrochemical project. The instrumentation class uses smart meters such as EJX and the world's most advanced control level).
IV. How to treat PLC and DCS correctly? We never emphasize the superiority of PLC and DCS. I have defined a new term "control products" for them. What we offer to our users is the control system that best suits the user. Most users do not use DCS because they want to use a DCS. Control products must be based on the user's process requirements. In fact, most of the users who use DCS or PLCs have never contacted self-controlled products or have some special needs. Too much emphasis on this thing will only get caught up in the debate. From the differences and commonalities between PLC and DCS, we understand the general situation of control products. As an automatic control professional, we can be responsible for the current whether it is PLC or DCS products, they have entered the mature period, has been more and more widely used in the field of automation control.
I. Introduction Automation Control System Products With the rapid development of the IT industry, intelligent control products have many names but they can be summarized into two major categories: DCS (computer technology + communication technology) and PLC (relay technology + computer technology). DCS (DistributedContorlSystem), distributed control system, also known as distributed control system, such as Westinghouse's WDPF; BAILEY's NETWORK-90 and so on. PLC (ProgramLogicControl) is also called a programmable logic controller, such as PLC5 of AB company; 90-30 of GE company; Schneider MODICON and so on. Up to now, there has been no strict boundary between DCS and PLC in the application of process control. In most people's view, large systems use DCS, and small systems use PLC. Of course, it is not impossible to say so, but there is a great one-sidedness. Let us now re-examine these points.
Second, what is the difference between DCS and PLC? 2.1 From the aspect of development, DCS has developed from the traditional dashboard monitoring system. Therefore, DCS focuses on the control of the instrument from the congenital point of view. For example, the YOKOGAWACS3000DCS system we use does not even have a PID number limit (PID, Proportional Calculus of Integrals, which is the standard algorithm for the closed-loop control of valves and frequency converters, usually the number of PIDs. Determines the number of regulators that can be used).
PLC evolved from the traditional relay loop. The original PLC did not even have analog processing power. Therefore, the PLC emphasized the logic computation ability from the beginning.
2.2 From the aspects of system scalability and compatibility, there are many kinds of control products on the market. No matter DCS or PLC, there are many manufacturers in production and sales. For PLC systems, there is generally no or little extended requirement because PLC systems are generally used for equipment. In general, PLCs rarely have compatibility requirements. For example, two or more systems require resource sharing, which is also more difficult for PLCs. Moreover, PLCs generally use a dedicated network structure, such as Siemens MPI bus network, and even adding an operator station is not easy or costly. In the process of development, DCS is also a self-contained system, but most of the DCS systems, such as Yokogawa, Honeywell, ABB, etc., although the communication protocol within the system (process level) is not the same, but the operation level The network platform invariably chose Ethernet, using standard or variant TCP/IP protocols. This provides a very convenient scalability. In this kind of network, the controller and the computer all exist as one node. As long as the network arrives, the number of nodes and the position of the nodes can be arbitrarily increased or decreased. In addition, based on the windows system's open protocols such as OPC, DDE, etc., each system can also easily communicate to achieve resource sharing.
2.3 DCS generally provides a unified database from the database. In other words, once a data exists in the database in the DCS system, it can be referenced in any situation, such as in the configuration software, in the monitoring software, in the trend graph, in the report... and in the PLC system. Databases are usually not uniform, and configuration software and monitoring software and even archiving software have their own databases. Why often say that Siemens S7400 will only be called 414 DCS? Since Siemens' PCS 7 system uses a unified database, PCS 7 requires the controller to be at least S7414-3.
2.4 From the time scheduling point of view, the PLC program can not generally run in accordance with the preset cycle. The PLC program is executed once from beginning to end and from the beginning. (Now some new PLCs have improved, but there are still limits on the number of mission cycles) and DCS can set mission cycles. For example, quick tasks. The same is the sampling of the sensor, the change time of the pressure sensor is very short, we can use 200ms duty cycle sampling, and the temperature sensor lag time is very large, we can use 2s task cycle sampling. In this way, the DCS can reasonably schedule controller resources.
2.5 In terms of network structure development Generally speaking, DCS routinely uses two layers of network structure. One layer is a process-level network. Most DCSs use their own bus protocols, such as Yokogawa's Modbus, Siemens and AB's Profibus, and ABB's CANbus. Etc., these protocols are based on the standard serial port transmission protocol RS232 or RS485 protocol. The on-the-spot IO module, especially the analog sampling data (machine code, 213/scan cycle) is very large, and there are many on-site interference factors, so a network standard with large data throughput and strong anti-interference capability should be used. The bus structure based on the RS485 serial port asynchronous communication method meets the requirements of field communication. The sampled data of the IO is converted into shaped data or real data by the CPU, and transmitted on the operation level network (Layer 2 network). Therefore, operation-level networks can adopt network standards with moderate data throughput, fast transmission speed, and easy connection. At the same time, operation-level networks are generally deployed in control rooms, and the requirements for combating interference are relatively low. Therefore, using standard Ethernet is the best choice. The TCP/IP protocol is a standard Ethernet protocol. Generally, we use 100 Mbit/s communication speed.
The task of the PLC system is relatively simple, so the amount of data that needs to be transmitted is generally not too large, so the common PLC system is a layer of network structure. Process-level networks and operation-level networks are either merged together, or the process-level networks are simply reduced to internal connections between modules. PLCs will not or rarely use Ethernet.
2.6 From the scale of the application object, PLC is generally used in small-scale self-control places, such as equipment control or a small amount of analog control and interlocking, while large-scale applications are generally DCS. Of course, this concept is not very accurate, but it is intuitive. We used to use DCS systems for systems larger than 1,000 points, and PLC control for smaller systems. Our heat pump and QCS, horizontal product supporting control system is generally called PLC.
Third, the development status of DCS and PLC said that the difference between so many PLCs and DCS, but we should realize that, PLC and DCS development to today, in fact, are moving closer to each other, strictly speaking, the current PLC and DCS can not A knife cut, the concept between many times has been blurred. Now let's discuss the same (like) aspects of each other.
3.1 From the function point of view PLC has already possessed the analog quantity control function, some PLC system analog quantity processing capacity even is quite formidable, for example Yokogawa FA-MA3, Siemens S7400, AB ControlLogix and Schneider's Quantum system. The DCS also has a very strong logic processing capabilities, such as the CS3000; I / A can be achieved on all possible process interlocks and equipment start and stop linkage.
3.2 From the system structure, the basic structure of PLC and DCS is the same. PLC has been fully transplanted to the computer system control since its development. Traditional programmers have long been eliminated. The PLC of small-scale application generally uses the touch screen, and PLC of large-scale application fully uses the computer system. Like the DCS, the controller and IO station use fieldbus (usually bus based on RS485 or RS232 asynchronous serial communication protocol). If there is no requirement for expansion between the controller and the computer, it means that only one computer is used. In this case, this bus communication will also be used. However, if there is more than one computer, the system structure will be the same as that of the DCS. The upper computer platform uses the Ethernet structure. This is one of the reasons why the PLC has become large and the DCS concept has been blurred.
3.3 The development direction of PLC and DCS The miniaturized PLC will develop to a more specialized use point of view, such as a more professional function, a more targeted application environment, and a high reliability and low cost. The boundary between large-scale PLC and DCS is gradually reduced until it is fully integrated. DCS will continue to develop in the direction of FCS (FieldBusContorlSyestem) fieldbus control system. In addition to the more decentralized control system at the heart of the FCS, instrumentation is particularly important. The application of FCS in foreign countries has been developed to the instrument level. The control system only needs to deal with signal acquisition and provide man-machine interface and logic control. The control of the entire analog quantity is dispersed to the field instrument. No traditional cable connection is required between the instrument and the control system. The entire instrument system is connected using the field bus. (Currently, Yokogawa has used FCS in the Sinopec Shell petrochemical project. The instrumentation class uses smart meters such as EJX and the world's most advanced control level).
IV. How to treat PLC and DCS correctly? We never emphasize the superiority of PLC and DCS. I have defined a new term "control products" for them. What we offer to our users is the control system that best suits the user. Most users do not use DCS because they want to use a DCS. Control products must be based on the user's process requirements. In fact, most of the users who use DCS or PLCs have never contacted self-controlled products or have some special needs. Too much emphasis on this thing will only get caught up in the debate. From the differences and commonalities between PLC and DCS, we understand the general situation of control products. As an automatic control professional, we can be responsible for the current whether it is PLC or DCS products, they have entered the mature period, has been more and more widely used in the field of automation control.