This article introduces the trend of control systems developing towards decentralization, networking, and intelligence, discusses the important role of fieldbus technology, and explores the impact of the combination of control networks and data networks on automation technology.
With the continuous development of network technology, the Internet is gradually integrating computer and communication systems worldwide, forming an information superhighway and a public data network. The rise of fieldbus technology has changed the structure of control systems, driving them towards networking and creating another type of network with significant impact on human production and life—the control network. Because fieldbus technology adapts to the development trend of control systems towards intelligence, networking, and decentralization, it has demonstrated strong vitality and developed rapidly, becoming a hot technology in the control field. With the further development of fieldbus control networks, the integration of control networks and data networks has become a priority, providing new areas for the further development of open, fully distributed control and network systems.
Fieldbus serves as the link for digital communication between control devices, connecting communication-capable measurement and control instruments as network nodes to form an open, digital, multi-point communication low-level control network. This network then connects the nodes to form a control system. Because it resides at the bottom layer of the network structure, it is called a low-level network, or Infranet. In some contexts, Infranet has become synonymous with fieldbus control networks. Its most prominent characteristics as a network system are its open communication protocols and its role in measurement and control tasks.
The connection between the control network (Infranet) and the Internet and enterprise intranet integrates the control network and the data network, allowing control information to flow into the data network. This further broadens the scope and vision of the control system and enriches the information content and application areas of the network system.
1 Data Network
Data networks refer to computer networks widely used in office and communication fields, through which people can directly access information. These networks are characterized by large data communication volumes, frequently transmitting documents, reports, graphics, and even larger amounts of multimedia data such as audio and video. Data networks have evolved from initial Local Area Networks (LANs) to interconnected LANs, leading to the emergence of the Internet. When enterprises recognized the enormous commercial value of the Internet, they successfully combined LANs with the Internet to create Intranets, ushering in a new stage in the development and application of data networks.
An Intranet, also known as a corporate intranet or enterprise network, successfully combines the advantages of a Local Area Network (LAN) and the Internet.
It can be seen as a product of applying Internet technology to local area networks (LANs). In other words, an Intranet is based on Internet and Web technologies.
Information service system. It can be considered that:
Intranet＝Internet＋Web＋InformationTechnology
Intranets utilize Internet communication technologies and feature open interconnection standards, providing robust communication capabilities. Employing B/S (Browser/Server) web technology, Intranets enable users to dynamically access distributed network databases, significantly enhancing information sharing capabilities. By employing authentication, network data encryption, and firewall technologies, Intranets add a protective barrier between the enterprise intranet and the public network. Therefore, Intranets retain the openness of the Internet while offering the security and reliability of an enterprise network against external information attacks.
Data network communication technology is quite mature. It uses the TCP/IP protocol for communication, and its remote access services include Telnet, FTP, and World Wide Web (WWW). Among them, WWW services can provide the transmission of large amounts of text, images, and even media data such as audio and video, and also support the development of various embedded applications.
The combination of the Internet and Intranet can provide enterprises with a comprehensive environment for sharing data and information resources. It can further enhance information exchange between enterprises and the outside world on the basis of completing internal management, thereby creating more economic benefits for enterprises.
2 Control Network
A control network is a network system used to accomplish automated tasks. Its network nodes, in addition to ordinary computers and workstations, consist primarily of measurement and control instruments with computing and communication capabilities. These automated control devices are distributed in factories, buildings, and homes, used in various aspects of production and daily life.
Fieldbus systems use easily connectable twisted-pair cables as the transmission medium to connect multiple measurement and control instruments with digital computing and communication capabilities into a network system. Following open and standardized communication protocols, data transmission and information exchange are achieved between multiple microcomputer-based measurement and control devices located in the field, and between field instruments and remote monitoring computers, forming an automatic control system. This system achieves comprehensive automation functions including basic control, compensation calculation, parameter modification, alarm, display, monitoring, optimization, and integrated control and management. In short, it uses individual, distributed measurement and control devices as network nodes, connected by a fieldbus, to form a network and control system that can communicate with each other and jointly complete automatic control tasks.
Compared with data networks, control networks have the following main characteristics:
① Control networks are mainly used for controlling production and living equipment, detecting, monitoring, and controlling the status of the production process, or realizing "home automation," etc. Data networks are mainly used for communication and office work, providing data information such as text, sound, and images.
② Control networks and data networks have their own technical characteristics: Control networks require high real-time performance, security, and reliability. However, their data transmission volume is generally small, while data networks need to adapt to the transmission and processing of large amounts of data.
3. Fieldbus systems are an inevitable trend in the development of control networks.
Distributed control systems (DCS) and fieldbus systems both belong to control networks. Most DCS systems are hybrid analog-digital systems and do not form a complete network from measurement and control equipment to the operating control computer. The computer network of its host unit uses proprietary and closed communication protocols, causing many inconveniences for users' system integration and application, and significantly hindering its technological development.
One reason for the development trend of fieldbus is that it has changed the structure of traditional control systems, forming a new type of network-integrated fully distributed control system. This is a new generation of control systems following base-based instrument control systems, electric unit-combined analog instrument control systems, centralized digital control systems, and distributed control systems (DCS). Fieldbus adopts fully digital communication and features openness, full distribution, and interoperability, forming a fully digital communication network from measurement and control equipment to the operation and control computer. It possesses the conditions for connecting and communicating with digital networks, meeting the requirements of control networks. Therefore, it has become the development trend of control networks.
Figure 1 compares the network structures of DCS and fieldbus systems. Besides employing open communication protocols, the fieldbus system, as shown in the figure, simplifies the multi-level hierarchical network structure used in DCS and facilitates connection and integration with the Internet and Intranet.
Figure 1. Network structure of DCS and fieldbus system
Fieldbus technology endows automated control equipment with digital computing and communication capabilities. This improves the accuracy of signal measurement, control, and transmission, while also enriching the content of control information and creating conditions for remote transmission. In a fieldbus environment, leveraging the computing and communication capabilities of devices, many complex calculations can be performed on-site, forming a truly distributed and complete control system, thus improving the reliability of the control system. Remote automatic control can be achieved through fieldbus network segments and other network segments with communication connections, such as operating electrical switches hundreds of kilometers away. It can also provide information that traditional instruments cannot provide, such as the number of valve opening and closing actions and fault diagnosis, facilitating operators and managers to better and more deeply understand the production site and the operating status of automated control equipment.
Meanwhile, the Fieldbus Control System (FCS) has revolutionized the control structure of the Distributed Control System (DCS), integrating control algorithms into intelligent instruments. In FCS, both computer workstations and field intelligent instruments are treated as nodes within the system, with different nodes performing different functions. FCS control functions are entirely distributed among the field intelligent instruments, allowing them to control the production process on-site. Therefore, even if a computer malfunctions, the control system will not "paralyze." Furthermore, workstations and intelligent instruments communicate digitally via a control network, enabling configuration and management functions, as well as control of the production process based on complex, advanced control calculations when necessary. The closed, centralized, and inflexible systems of the past are being replaced by the open, networked, and flexible systems of the future. Networked control systems are far superior to PLCs and DCS systems.
4. Fieldbus facilitates the integration of control networks and data networks.
Fieldbus technology has enabled control systems to develop towards decentralization, networking, and intelligence, and has brought control technology more closely together with computer and network technologies, injecting new vitality into the technological development of the automation field.
In fieldbus-based control systems (FCS), control devices possess network communication capabilities and are connected to form the control system via a fieldbus network. This transforms the closed, proprietary communication solutions of DCS systems into open, standardized solutions. Because fieldbus is based on open and unified communication protocol standards, it facilitates the connection between control and data networks, thus positively promoting their integration. As traditional control systems gradually move towards fieldbus control networks (Infranet), it paves the way for building an Internet-Intranet-Infranet network structure and forming a coordinated whole.
Figure 2 shows the architecture of Internet-Intranet-Infranet.
Figure 2 Internet-Intranet-Infranet Architecture
Infranet enables interconnection with the Internet and Intranet, enriching the information content of the network and facilitating the comprehensive advantages of data and control information.
5. The integration of control networks and data networks will have a profound impact on the development of the field of automation.
The implementation of the Internet-Intranet-Infranet architecture plays an important role in people's production and life. For example, remote monitoring systems are one application of this architecture in the field of production control.
Remote monitoring refers to the monitoring and control of remote production processes by a local computer through a network system (mainly the Internet). The computer hardware and software system capable of remote monitoring is called a remote monitoring system.
Fieldbus systems, when connected to the Internet, can, under certain conditions, monitor and control the operation and various parameters of production systems and field equipment via the Internet. This eliminates the need for on-site presence, enabling remote monitoring and control of controlled objects through the network, thus saving significant amounts of transportation and manpower. Furthermore, regional, national, and even global monitoring centers can be established as needed, facilitating the overall management of factories by large manufacturing groups and improving economic efficiency. For manufacturers of control systems and field equipment, this allows for timely maintenance of their products and continuous monitoring of product performance for further design improvements.
To realize a remote monitoring system, firstly, there must be a control system capable of stably operating a fieldbus; secondly, there must be a data communication system capable of remote access via the Internet; and thirdly, there must be a virtual communication relationship established between the various components of the control system, connecting to remote control networks via the Internet, and utilizing the abundant hardware and software resources of public data networks to monitor and control the production site via the Internet.
For security reasons related to the fieldbus system, the remote monitoring software can be set up and run on a server or another computer, separate from the "fieldbus workstation" shown in Figure 3.
Figure 3 Schematic diagram of remote monitoring system
Of course, remote monitoring systems are relative. They can monitor remote locations from a local location, or vice versa, and can even provide bidirectional monitoring between both local and remote locations.
Many foreign companies are actively researching and developing remote monitoring systems, establishing a standard for connecting fieldbus networks to the Internet, and utilizing existing WWW technologies as much as possible. Collected production data is directly placed into HTML documents, allowing local users to access the production process via control networks or LANs, while remote users can access it via the Internet. For example, a large foreign petrochemical company has essentially achieved a comprehensive overview of operational and production data and curves from its headquarters to its geographically dispersed subsidiaries, monitoring the operational status of factory production units. In the field of home automation, Echelon has developed a small experimental system that allows Internet users worldwide to access and control intelligent devices in a laboratory. These devices include curtains with adjustable opening degrees (0-100%), table lamps with adjustable brightness (0-100%), and switchable wall lights. Of course, these devices have minimal impact on their surroundings regardless of their state; therefore, until network security issues are fully resolved, this remote monitoring system is unlikely to encounter major problems.
With the development of control network operating systems, control networks and data networks are gradually merging and becoming integrated. When operational information in the control network and management decision-making information in the data network can be better combined, the functionality of the network system will be greatly enhanced. The emergence and rapid development of the Internet has already brought us immense convenience, driving and changing people's production and lifestyles. In the future, the integration of data networks and control networks will undoubtedly bring people a better life and lead humanity towards a more advanced society.
