Abstract: This paper introduces the application of fieldbus technology in industrial control system and the hierarchical structure of control network under fieldbus conditions. At the same time, it discusses the problems of integration between control network and control network and information network.
After the combination of computer and communication technology has created a computer network, the communication between enterprise management departments has used the network as an intermediary to realize the sharing of information and resources. At the same time, the development of information technology has also led to changes in the automation structure, and gradually formed an enterprise information system based on a network integration automation system. Fieldbus is a new technology developed in response to this situation. It is an inevitable result of the combination of computer networks and control systems. Its appearance marks the beginning of a new era in industrial control.
Fieldbus is an extension of DCS (Distributed Control System). It breaks through the relatively closed limitation of DCS. It distributes the measurement and control tasks to field devices. The host computer is only responsible for monitoring and some complex optimization and advanced control functions. The field bus is the main body of the factory floor information and data transmission. In the entire plant's control network, the field bus is in an important basic position. Since the end of the 1980s, there have been some influential fieldbus standards in foreign countries, such as Foundation Fieldbus (FF), LONWorkbus, Profibus, CAN control LAN, etc. Most of them are based on the company's standards. Gradually formed, in the coming period of time, there will be several situations where fieldbus standards coexist. Now that an international standard for a unified fieldbus communication protocol has been formed, the real open interconnection system is the general trend.
1. Characteristics of fieldbus control system Fieldbus system (FCS) has the following characteristics compared with the traditional DCS:
1.1 Bus Structure A pair of transmission lines (buses) are connected to multiple field devices to transmit multiple digital signals in both directions. This kind of structure is simpler than the one-to-one unidirectional analog signal transmission structure, the installation cost is low, and maintenance is simple.
1.2 Open Interoperability Fieldbus adopts a unified protocol standard and is an open Internet. It is transparent to users. In traditional DCS, devices of different manufacturers cannot access each other. While FCS adopts a unified standard, network products of different manufacturers can be easily connected to the same network and integrated in the same control system for interoperability, thus simplifying system integration.
1.3 Completely decentralized control Fieldbus decentralizes control functions to on-site smart meters and devices that act as network nodes, enabling complete decentralized control, improving system flexibility, autonomy, security and reliability, and alleviating the CPU of the control station. Calculation burden.
1.4 Information Integration and Configuration Flexibility Through digital transmission of field data, FCS can obtain various status and diagnostic information of field instruments, realizing real-time system monitoring and management. In addition, FCS introduced the concept of functional blocks. Through a unified configuration method, the system configuration is simple and flexible. The functional blocks in different field devices can form a complete control loop.
1.5 Multiple Transmission Media and Topology Due to its digital communications, FCS can communicate using a variety of transmission media. According to the spatial distribution of nodes in the control system, various network topologies can be applied. This variety of transmission media and network topology has brought great convenience to the construction of automated systems. According to statistics, FCS can save 40% of the cost of a cabling project compared with the traditional master-slave structure of DCS. Funds.
2. Fieldbus Network Model Fieldbus is essentially a control network, so network technology is an important foundation for fieldbus. Unlike Internet, Intranet, and other types of information networks, control networks are directly oriented to the production process and therefore require high real-time performance, reliability, data integrity, and availability. In order to meet these characteristics, the fieldbus simplifies the standard network protocol, and generally only includes three layers in the ISO/OSl7 layer model: the physical layer, the data link layer, and the application layer. In addition, the fieldbus also completes the data exchange and transmission with the upper factory information system. Integrated automation is the development direction of modern industrial automation. In the complete enterprise network architecture, the fieldbus control network model should involve the data transmission process from the underlying field device network to the upper information network.
Based on the above considerations, a unified fieldbus control network model should have a three-tier structure. From the bottom up, it should be: the field intelligent device layer, the fieldbus monitoring layer, and the remote monitoring layer.
2.1 On-site smart device layer According to the fieldbus protocol standard, the smart device adopts the structure of a functional block. Through configuration and design, various functions such as data acquisition, A/D conversion, digital filtering, temperature and pressure compensation, and PID control are completed. Smart converter digitally converts and compensates traditional instrumentation currents and voltages. In addition, there should be a PLC interface on the bus to facilitate the connection of the original system.
Field devices are connected to the fieldbus network in the form of network nodes 6g. In order to ensure real-time and reliable data transmission between nodes, the fieldbus control network must adopt a reasonable topology. The common fieldbus network topology includes:
2.1.1 The ring network is characterized by good time delay and high network efficiency during heavy load. However, waiting for tokens during light load causes unnecessary delays and transmission efficiency decreases. 2.1.2 The bus network is characterized by convenient access to nodes and low costs. The delay is light at light load, but the delay increases when the network communication load is heavy, and the network efficiency decreases. In addition, the transmission delay is uncertain.
2.1.3 The tree network is characterized by good scalability and wide frequency band, but the communication between nodes is inconvenient.
2.1.4 Token The bus network combines the advantages of a ring network and a bus network, ie, a physical bus network and a logical token network. In this way, the network transmission delay can be determined without conflict, and the nodes are convenient for access and have good reliability.
2.2 Fieldbus monitoring layer This layer acquires data from field devices and performs functions such as various controls, monitoring of operating parameters, alarms and trend analysis, as well as design and download of control configurations. The function of the monitoring layer is generally completed by the host computer. It is connected to the field bus through the network interface board in the expansion slot to coordinate the data communication between network nodes; or through a dedicated field bus interface (converter) to realize the field bus segment and the ether Network segment connection, this way makes the system configuration more flexible. This layer is in Ethernet, so its key technology is the interface between Ethernet and the underlying field device network. It is mainly responsible for the conversion of the fieldbus protocol and Ethernet protocol to ensure the correct interpretation and transmission of data packets. In addition to the above functions, the monitoring layer also provides a support environment for advanced control and remote operation optimization.
2.3 The main purpose of the remote monitoring layer is to build a secure remote monitoring system in a distributed network environment. First, the information in the database of the intermediate monitoring layer shall be transferred to the upper level of the relational database, so that the remote user can query the network running status and the working conditions of the on-site equipment at any time through the browser, and real-time remote monitoring of the production process. After granting certain authority, various device parameters and operating parameters can also be modified online so as to realize real-time transmission of underlying measurement and control information within the scope of the wide area network. At present, the way to achieve remote monitoring is through the Internet, the main way is to lease corporate private line or use the public data network. Due to the actual production process, network security must be ensured. Technologies that can be used include firewalls, user authentication, and key management. In this regard, WorldFIP fieldbus technology has advantages. WorldFIP's messages are more flexible and compatible with TCP/IP and can seamlessly connect to the Internet without affecting the transmission of real-time data. Therefore, the entire control network can adopt a unified protocol standard. In addition, WorldFIP's field devices have embedded Web servers that allow users to access information in field devices directly over the Internet without the need for intermediate protocol converters.
In the entire fieldbus control network model, the field device layer is the core of the entire network model. Only by ensuring reliable, accurate, and complete data transmission between the bus devices, the upper layer network can obtain information and achieve monitoring functions. At present, the discussion of fieldbus mostly stays in the bottom field intelligent device network segment, but starting from the complete fieldbus control network model, more consideration should be given to the data transmission between the field device layer and the middle monitoring layer and the Internet application layer. Interaction issues, and the tight integration between the control network and the information network.
3. Network Integration 3.1 Integration of Fieldbus Control Network and DCS Network Fieldbus does not exert its advantages in all control situations, such as a simple small-scale digital simulation hybrid system, especially where the distance between the site and the control room is short. Happening. Therefore, it is more convenient for the hybrid control to be performed in a centralized and centralized CPU, and the risk of the centralized control of the small system is also small. The decentralized control of the fieldbus requires several devices to implement, and it is cumbersome. In the current and future period of time, the industrial control network will face the situation that the field bus and the DCS network coexist. Therefore, in the design of the industrial control network structure, how to implement the network integration under the control of heterogeneous network segments in the network is considered. It is also very realistic.
We know that DCS is a non-open private network and the DCS host is the control and communication center of the DCS system. In the new industrial control network system, the entire DCS system will exist as one of the special subnets. The DCS host is an ordinary node. There are two different scenarios when integrating the fieldbus network:
(1) Using a gateway to connect a DCS private network to a high-speed network;
(2) Using a special gateway or communication controller to connect the fieldbus system to the DCS network;
3.2 Control Network and Information Network Integration The control network communication technology is different from the information network for the purpose of information and resource sharing. Its ultimate goal is to achieve effective control of energy and material transfer in controlled objects, and to make the system safe and stable. get on. Therefore, it is required to have the characteristics of simple protocol, safety and reliability, good error correction and low cost. Its network load is stable, mostly for short frame transmission, and information exchange is frequent. The close integration between the control network and the information network is the foundation for the establishment of an integrated enterprise real-time information base, which provides the basis for the enterprise's optimization control and scheduling decisions. By controlling the structure of the network and information network, a unified distribution database can be established to ensure all data. Integrity and interoperability; real-time communication between field devices and information networks, enabling users to understand the production situation at any time and anywhere through the standard graphical interface in the information network; tight integration of control networks and information networks also facilitates remote monitoring, diagnosis, and maintenance.
The integration of control networks and information networks can be done in the following ways:
3.2.1 Adding a Switching Interface Between the Control Network and the Information Network This mode is implemented by hardware, that is, adding special hardware devices such as repeaters, bridges, and routers between the bottom network segment and the intermediate monitoring layer to control The network is closely integrated with the expansion of the information network. A hardware device can be a dedicated computer, relying on the software it runs to complete the identification, interpretation, and conversion of data packets. For applications with multiple network segments, it can also store and forward packets between different network segments. The role. In addition, the hardware device can also be an intelligent interface network board. The Fisher-Rosemount Deltav system passes the H1 interface card in a cabinet to complete the data communication between the fieldbus intelligent device and the monitoring computer in the Ethernet.
The conversion interface has a strong integration function, but its real-time performance is poor. The information network is generally Ethernet using TCP/IP, and TCP/IP does not consider the real-time nature of data transmission. When the field device has a large number of information uploads or remote monitoring operations are frequent, the conversion interface will become the bottleneck of real-time communication.
3.2.2 Adopting DDE technology between control network and information network When there is an intermediate system or a shared memory workstation between the control network and the information network, the integration between the two can be achieved using the DDE method. Sharing memory to exchange information, an information processor in an intermediate system is a workstation that controls the network, and it is also a workstation in the information network. Which runs two programs, one is to receive and verify real-time information communication program for the information network database to provide real-time data information; the other is a data access application program interface, which receives DDE server real-time data and writes in the database server for The information network implements functions such as information processing and statistical analysis.
The DDE method has strong real-time performance and is relatively easy to implement. Standard Windows technology can be used. But when it comes to complex protocol conversions, the software cost of the DDE approach is relatively large. Therefore, this method is suitable for configuring a simple small system.
3.2.3 Controlling the network and information network Adopting a uniform protocol standard This approach will become the ultimate solution for the integration of the control network and the information network. Since the control network and information network adopt protocol standards for different applications, the integration of the two always requires some data format conversion mechanism, which will complicate the system and ensure the integrity of the data. If the protocol standard of the information network is to improve its real-time performance, and the protocol standard of the control network is to increase its transmission speed, the compatibility of the two will increase, and the two will be combined into one, so that from the bottom device to the remote monitoring system, both Uniform protocol standards can be used to ensure not only accurate, fast, and complete transmission of information, but also greatly simplify system design. The above-mentioned WorldFIP protocol can be compatible with TCP/IP. Therefore, the integration of Ethernet and the Internet can be conveniently implemented, and the control network and the information network can be tightly combined to finally realize a unified network structure. At present, a variety of bus standards coexist, and information network protocols are not the same. Therefore, to implement a unified protocol standard for controlling networks and information, there are still many problems to be solved.
4. Conclusion The network is the basis of enterprise's integrated automation. In the entire enterprise's network architecture, the fieldbus is in the basic position, so it is of great significance to build a complete fieldbus control network model. This model extends to the highest level of the control field, ie, the management decision layer. Therefore, the control network and the information network are required to be closely combined to ensure correct data transmission and data forwarding from the underlying field devices to the top-level production management. In the long run, the control network and information network will eventually be integrated, and the field bus will serve as a bridge to communicate data flow between the production process and the information network.
After the combination of computer and communication technology has created a computer network, the communication between enterprise management departments has used the network as an intermediary to realize the sharing of information and resources. At the same time, the development of information technology has also led to changes in the automation structure, and gradually formed an enterprise information system based on a network integration automation system. Fieldbus is a new technology developed in response to this situation. It is an inevitable result of the combination of computer networks and control systems. Its appearance marks the beginning of a new era in industrial control.
Fieldbus is an extension of DCS (Distributed Control System). It breaks through the relatively closed limitation of DCS. It distributes the measurement and control tasks to field devices. The host computer is only responsible for monitoring and some complex optimization and advanced control functions. The field bus is the main body of the factory floor information and data transmission. In the entire plant's control network, the field bus is in an important basic position. Since the end of the 1980s, there have been some influential fieldbus standards in foreign countries, such as Foundation Fieldbus (FF), LONWorkbus, Profibus, CAN control LAN, etc. Most of them are based on the company's standards. Gradually formed, in the coming period of time, there will be several situations where fieldbus standards coexist. Now that an international standard for a unified fieldbus communication protocol has been formed, the real open interconnection system is the general trend.
1. Characteristics of fieldbus control system Fieldbus system (FCS) has the following characteristics compared with the traditional DCS:
1.1 Bus Structure A pair of transmission lines (buses) are connected to multiple field devices to transmit multiple digital signals in both directions. This kind of structure is simpler than the one-to-one unidirectional analog signal transmission structure, the installation cost is low, and maintenance is simple.
1.2 Open Interoperability Fieldbus adopts a unified protocol standard and is an open Internet. It is transparent to users. In traditional DCS, devices of different manufacturers cannot access each other. While FCS adopts a unified standard, network products of different manufacturers can be easily connected to the same network and integrated in the same control system for interoperability, thus simplifying system integration.
1.3 Completely decentralized control Fieldbus decentralizes control functions to on-site smart meters and devices that act as network nodes, enabling complete decentralized control, improving system flexibility, autonomy, security and reliability, and alleviating the CPU of the control station. Calculation burden.
1.4 Information Integration and Configuration Flexibility Through digital transmission of field data, FCS can obtain various status and diagnostic information of field instruments, realizing real-time system monitoring and management. In addition, FCS introduced the concept of functional blocks. Through a unified configuration method, the system configuration is simple and flexible. The functional blocks in different field devices can form a complete control loop.
1.5 Multiple Transmission Media and Topology Due to its digital communications, FCS can communicate using a variety of transmission media. According to the spatial distribution of nodes in the control system, various network topologies can be applied. This variety of transmission media and network topology has brought great convenience to the construction of automated systems. According to statistics, FCS can save 40% of the cost of a cabling project compared with the traditional master-slave structure of DCS. Funds.
2. Fieldbus Network Model Fieldbus is essentially a control network, so network technology is an important foundation for fieldbus. Unlike Internet, Intranet, and other types of information networks, control networks are directly oriented to the production process and therefore require high real-time performance, reliability, data integrity, and availability. In order to meet these characteristics, the fieldbus simplifies the standard network protocol, and generally only includes three layers in the ISO/OSl7 layer model: the physical layer, the data link layer, and the application layer. In addition, the fieldbus also completes the data exchange and transmission with the upper factory information system. Integrated automation is the development direction of modern industrial automation. In the complete enterprise network architecture, the fieldbus control network model should involve the data transmission process from the underlying field device network to the upper information network.
Based on the above considerations, a unified fieldbus control network model should have a three-tier structure. From the bottom up, it should be: the field intelligent device layer, the fieldbus monitoring layer, and the remote monitoring layer.
2.1 On-site smart device layer According to the fieldbus protocol standard, the smart device adopts the structure of a functional block. Through configuration and design, various functions such as data acquisition, A/D conversion, digital filtering, temperature and pressure compensation, and PID control are completed. Smart converter digitally converts and compensates traditional instrumentation currents and voltages. In addition, there should be a PLC interface on the bus to facilitate the connection of the original system.
Field devices are connected to the fieldbus network in the form of network nodes 6g. In order to ensure real-time and reliable data transmission between nodes, the fieldbus control network must adopt a reasonable topology. The common fieldbus network topology includes:
2.1.1 The ring network is characterized by good time delay and high network efficiency during heavy load. However, waiting for tokens during light load causes unnecessary delays and transmission efficiency decreases. 2.1.2 The bus network is characterized by convenient access to nodes and low costs. The delay is light at light load, but the delay increases when the network communication load is heavy, and the network efficiency decreases. In addition, the transmission delay is uncertain.
2.1.3 The tree network is characterized by good scalability and wide frequency band, but the communication between nodes is inconvenient.
2.1.4 Token The bus network combines the advantages of a ring network and a bus network, ie, a physical bus network and a logical token network. In this way, the network transmission delay can be determined without conflict, and the nodes are convenient for access and have good reliability.
2.2 Fieldbus monitoring layer This layer acquires data from field devices and performs functions such as various controls, monitoring of operating parameters, alarms and trend analysis, as well as design and download of control configurations. The function of the monitoring layer is generally completed by the host computer. It is connected to the field bus through the network interface board in the expansion slot to coordinate the data communication between network nodes; or through a dedicated field bus interface (converter) to realize the field bus segment and the ether Network segment connection, this way makes the system configuration more flexible. This layer is in Ethernet, so its key technology is the interface between Ethernet and the underlying field device network. It is mainly responsible for the conversion of the fieldbus protocol and Ethernet protocol to ensure the correct interpretation and transmission of data packets. In addition to the above functions, the monitoring layer also provides a support environment for advanced control and remote operation optimization.
2.3 The main purpose of the remote monitoring layer is to build a secure remote monitoring system in a distributed network environment. First, the information in the database of the intermediate monitoring layer shall be transferred to the upper level of the relational database, so that the remote user can query the network running status and the working conditions of the on-site equipment at any time through the browser, and real-time remote monitoring of the production process. After granting certain authority, various device parameters and operating parameters can also be modified online so as to realize real-time transmission of underlying measurement and control information within the scope of the wide area network. At present, the way to achieve remote monitoring is through the Internet, the main way is to lease corporate private line or use the public data network. Due to the actual production process, network security must be ensured. Technologies that can be used include firewalls, user authentication, and key management. In this regard, WorldFIP fieldbus technology has advantages. WorldFIP's messages are more flexible and compatible with TCP/IP and can seamlessly connect to the Internet without affecting the transmission of real-time data. Therefore, the entire control network can adopt a unified protocol standard. In addition, WorldFIP's field devices have embedded Web servers that allow users to access information in field devices directly over the Internet without the need for intermediate protocol converters.
In the entire fieldbus control network model, the field device layer is the core of the entire network model. Only by ensuring reliable, accurate, and complete data transmission between the bus devices, the upper layer network can obtain information and achieve monitoring functions. At present, the discussion of fieldbus mostly stays in the bottom field intelligent device network segment, but starting from the complete fieldbus control network model, more consideration should be given to the data transmission between the field device layer and the middle monitoring layer and the Internet application layer. Interaction issues, and the tight integration between the control network and the information network.
3. Network Integration 3.1 Integration of Fieldbus Control Network and DCS Network Fieldbus does not exert its advantages in all control situations, such as a simple small-scale digital simulation hybrid system, especially where the distance between the site and the control room is short. Happening. Therefore, it is more convenient for the hybrid control to be performed in a centralized and centralized CPU, and the risk of the centralized control of the small system is also small. The decentralized control of the fieldbus requires several devices to implement, and it is cumbersome. In the current and future period of time, the industrial control network will face the situation that the field bus and the DCS network coexist. Therefore, in the design of the industrial control network structure, how to implement the network integration under the control of heterogeneous network segments in the network is considered. It is also very realistic.
We know that DCS is a non-open private network and the DCS host is the control and communication center of the DCS system. In the new industrial control network system, the entire DCS system will exist as one of the special subnets. The DCS host is an ordinary node. There are two different scenarios when integrating the fieldbus network:
(1) Using a gateway to connect a DCS private network to a high-speed network;
(2) Using a special gateway or communication controller to connect the fieldbus system to the DCS network;
3.2 Control Network and Information Network Integration The control network communication technology is different from the information network for the purpose of information and resource sharing. Its ultimate goal is to achieve effective control of energy and material transfer in controlled objects, and to make the system safe and stable. get on. Therefore, it is required to have the characteristics of simple protocol, safety and reliability, good error correction and low cost. Its network load is stable, mostly for short frame transmission, and information exchange is frequent. The close integration between the control network and the information network is the foundation for the establishment of an integrated enterprise real-time information base, which provides the basis for the enterprise's optimization control and scheduling decisions. By controlling the structure of the network and information network, a unified distribution database can be established to ensure all data. Integrity and interoperability; real-time communication between field devices and information networks, enabling users to understand the production situation at any time and anywhere through the standard graphical interface in the information network; tight integration of control networks and information networks also facilitates remote monitoring, diagnosis, and maintenance.
The integration of control networks and information networks can be done in the following ways:
3.2.1 Adding a Switching Interface Between the Control Network and the Information Network This mode is implemented by hardware, that is, adding special hardware devices such as repeaters, bridges, and routers between the bottom network segment and the intermediate monitoring layer to control The network is closely integrated with the expansion of the information network. A hardware device can be a dedicated computer, relying on the software it runs to complete the identification, interpretation, and conversion of data packets. For applications with multiple network segments, it can also store and forward packets between different network segments. The role. In addition, the hardware device can also be an intelligent interface network board. The Fisher-Rosemount Deltav system passes the H1 interface card in a cabinet to complete the data communication between the fieldbus intelligent device and the monitoring computer in the Ethernet.
The conversion interface has a strong integration function, but its real-time performance is poor. The information network is generally Ethernet using TCP/IP, and TCP/IP does not consider the real-time nature of data transmission. When the field device has a large number of information uploads or remote monitoring operations are frequent, the conversion interface will become the bottleneck of real-time communication.
3.2.2 Adopting DDE technology between control network and information network When there is an intermediate system or a shared memory workstation between the control network and the information network, the integration between the two can be achieved using the DDE method. Sharing memory to exchange information, an information processor in an intermediate system is a workstation that controls the network, and it is also a workstation in the information network. Which runs two programs, one is to receive and verify real-time information communication program for the information network database to provide real-time data information; the other is a data access application program interface, which receives DDE server real-time data and writes in the database server for The information network implements functions such as information processing and statistical analysis.
The DDE method has strong real-time performance and is relatively easy to implement. Standard Windows technology can be used. But when it comes to complex protocol conversions, the software cost of the DDE approach is relatively large. Therefore, this method is suitable for configuring a simple small system.
3.2.3 Controlling the network and information network Adopting a uniform protocol standard This approach will become the ultimate solution for the integration of the control network and the information network. Since the control network and information network adopt protocol standards for different applications, the integration of the two always requires some data format conversion mechanism, which will complicate the system and ensure the integrity of the data. If the protocol standard of the information network is to improve its real-time performance, and the protocol standard of the control network is to increase its transmission speed, the compatibility of the two will increase, and the two will be combined into one, so that from the bottom device to the remote monitoring system, both Uniform protocol standards can be used to ensure not only accurate, fast, and complete transmission of information, but also greatly simplify system design. The above-mentioned WorldFIP protocol can be compatible with TCP/IP. Therefore, the integration of Ethernet and the Internet can be conveniently implemented, and the control network and the information network can be tightly combined to finally realize a unified network structure. At present, a variety of bus standards coexist, and information network protocols are not the same. Therefore, to implement a unified protocol standard for controlling networks and information, there are still many problems to be solved.
4. Conclusion The network is the basis of enterprise's integrated automation. In the entire enterprise's network architecture, the fieldbus is in the basic position, so it is of great significance to build a complete fieldbus control network model. This model extends to the highest level of the control field, ie, the management decision layer. Therefore, the control network and the information network are required to be closely combined to ensure correct data transmission and data forwarding from the underlying field devices to the top-level production management. In the long run, the control network and information network will eventually be integrated, and the field bus will serve as a bridge to communicate data flow between the production process and the information network.
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