Industrial control system is a requirement for large data volume and high-speed transmission such as images and voice signals, which has led to the combination of Ethernet and control networks that are currently popular in the commercial field. This wave of industrial control system networking integrates a variety of today's popular technologies such as embedded technology, multi-standard industrial control network interconnection, wireless technology, etc., thereby expanding the development space in the field of industrial control and bringing new development opportunities.
With the development of computer technology, communication technology and control technology, the traditional control field is experiencing an unprecedented change and begins to develop in the direction of networking.
The structure of the control system has developed from the original CCS (computer centralized control system), to the second generation of DCS (distributed control system), to the current popular FCS (field bus control system).
1. Development Trend of Industrial Control System
Computer and network technology is closely related to the development of control systems. As early as the mid-to-late 1950s, computers have been applied to control systems. In the early 1960s, there appeared a control system that completely replaced analog control by computer, which was called Direct Digital Control (DDC). In the mid-1970s, with the emergence of microprocessors, computer control systems entered a new period of rapid development. In 1975, the world's first distributed computer control system based on microprocessors came out. It is called distributed control system (DCS).
After entering the 1980s, people use microprocessors and some peripheral circuits to form digital instruments to replace analog instruments. This DDC control method improves the control accuracy and control flexibility of the system, and it is used in multi-loop circuit sampling and control. The control has a performance-price ratio that traditional analog instruments can't match.
In the mid-to-late 1980s, with the increasing complexity of industrial systems and the further increase of control loops, a single DDC control system can no longer meet the requirements of on-site production control and production management. greatly improved. Therefore, the layered control system, which is composed of small and medium-sized computers and microcomputers, has been widely used.
After entering the 1990s, due to the rapid development of computer network technology, the DCS system has been further developed, and the reliability and maintainability of the system have been improved. In today's industrial control field, DCS still occupies a dominant position, but DCS does not have openness. , the wiring is complex, the cost is high, and the integration of products from different manufacturers is very difficult.
Since the late 1980s, due to the development of large-scale integrated circuits, many field devices such as sensors, actuators, and drives have become intelligent, and people have begun to seek to use a communication cable to connect field devices with a unified communication protocol communication interface. Up, it is no longer I/O (4 ~ 20mA/24VDC) signal, but digital signal, which is transmitted in the device layer, which is the field bus. Because it solves the problems of reliability and openness of the network control system, the fieldbus technology has gradually become the development trend of the computer control system. Since then, some developed industrial countries and multinational industrial companies have launched their own fieldbus standards and related products, forming a trend of competition.
2. The relationship between industrial control system and information network
From the development process, the development of the information network architecture is similar to the development of the control system structure. The development of enterprise information network has generally gone through the following stages of development:
2.1 Host-based centralized mode
Almost all computing and processing tasks are done by a powerful mainframe, with little interaction between the user and the mainframe.
2.2 Hierarchical structure based on workgroups
The development of microcomputer and local area network technology makes people with similar work nature form groups to share some common resources, and the communication and cooperation between users has been strengthened.
2.3 Networked Enterprise Organization Based on Internet/Intranet/Extranet
The development of computer network technology has made it the mainstream of modern information technology, especially the development and popularization of the Internet has made it the prototype of the recognized future global information infrastructure. Using the mature technology and standards of the Internet, people put forward the concepts of Intranet and Extranet, which are respectively used for the realization of enterprise intranet and enterprise extranet, so a new generation based on Intranet, supplemented by Extranet and relying on the Internet has been formed. Enterprise Information Infrastructure (Enterprise Network).
The computer control system has also experienced several development stages such as centralized control, layered control, and network control based on fieldbus, and their development process is very similar.
With the in-depth application and improvement of enterprise information network, it is an inevitable trend for on-site control information to enter the information network to realize real-time monitoring. In order to improve the social and economic benefits of enterprises, many enterprises are trying their best to establish a comprehensive management information system, which must include real-time data information on the production site to ensure real-time grasp of the operation status of the production process and make enterprise management decision-making scientific, To achieve the optimal state of production, operation and management. Information-control integration will create favorable conditions for the realization of enterprise integrated automation CIPA (computerintegratedplantautomation) and enterprise informatization.
The similarity in the architectural development process of enterprise information network and control system is not accidental. In the development process of computer control system, the emergence of control system of each structure always lags behind the development of corresponding computer technology. In fact, in most cases, it is after the emergence of a new technology in the computer field that people begin to study how to apply this new technology to the control field. In view of the difference between the two application environments, the technical details have been appropriately modified and supplemented, but they have many common points in the principle and implementation of key technologies. It is precisely because of the relationship between the two in the development process that it is possible to realize the integration of information and control.
3. Research status of industrial control system
In the 1940s, process control was based on pneumatic standard signals of 3 to 15 PSI. Since then, due to the use of 4-20mA analog signal, the analog controller has been widely used, but not all sensing instruments and driving devices use a unified 4-20mA signal. In the 1970s, due to the pioneering use of computers in the fields of detection, analog control and logic control, centralized control was created. In the 1980s, due to the emergence of microprocessors, industrial instruments entered the era of digitization and intelligence. The 4-20mA analog signal transmission was gradually replaced by digital communication. In addition, the rapid development of distributed control and network technology promoted control, The integrated development of scheduling, optimization, decision-making and other functions. However, most of the detection, transmission, execution and other agencies use analog signal connections, and the transmission method is a one-to-one structure, which makes the wiring complex, the engineering cost is high, and the maintenance is difficult. Poor performance, which hinders the function of the upper system. On the other hand, because the functions of smart instruments far exceed those of on-site analog instruments, such as remote setting of range and zero point, self-diagnosis of instrument working status, multi-parameter measurement and compensation for environmental impact, etc. It can be seen that the development of intelligent instruments and control systems requires digital communication between upper-level systems and field instruments.
In order to overcome the technical bottleneck of the DCS system and further meet the needs of the field, the fieldbus technology came into being. Field bottom equipment control network (INFRANET). Different from the Internet, Intranet and other types of information networks, the control network is directly oriented to the production process, so it requires high real-time, reliability, data integrity and availability. In order to meet these characteristics, the fieldbus simplifies the standard network protocol, omits some intermediate layers, and only includes three layers in the ISO/OSI7 layer model: physical layer, data link layer and application layer.
At the beginning of the development of fieldbus, each company proposed its own fieldbus protocol. The IEC organization voted on December 31, 1999 and determined 8 major buses as the international field bus standards, including CANBus, ProfitBus, InterBus-S, ModBus, FOUNDA-TIONFieldbus and so on. On this basis, a new field bus control system (FieldbusControlSystemFCS) is formed. It integrates digital communication technology, computer technology, automatic control technology, network technology and intelligent instrumentation and other technical means, and fundamentally breaks through the limitations of traditional "point-to-point" analog signal or digital-analog signal control. , constitute a fully decentralized, fully digital, intelligent, two-way, interconnected, multi-variable, multi-contact communication and control system. The corresponding control network structure has also undergone great changes. The typical structure of FCS is divided into three layers: device layer, control layer and information layer.
Although the fieldbus technology develops very rapidly, there are still many problems, which restrict the further expansion of its application range.
3.1 The first is the choice of field bus. Although the IEC organization has reached an international bus standard, there are still too many types of buses, and each field bus has its own most suitable application field. It is still a difficult problem for users to choose the most suitable field bus for each part of the device.
3.2 System integration issues. Since a system is likely to use various forms of fieldbus in practical applications, how to seamlessly integrate the industrial control network with the data network, so as to realize the integration of management and control in the entire system, is the key link. When designing the network layout of the fieldbus system, not only the distance of each field node, but also the functional relationship between the field nodes and the flow of information on the network should be considered. Due to the powerful functions of intelligent field instruments, many instruments have the same function block, which function block should be carefully considered during configuration; the information flow on the network should be minimized. At the same time, the configuration of communication parameters is also very important, and a balance should be made between the real-time performance of the system and the network efficiency.
3.3 There are technical bottlenecks. Mainly in:
a. When the bus cable is cut off, the whole system may be paralyzed.
Users hope that the performance of the system can be reduced at this time, but it cannot crash, which many fieldbuses cannot guarantee.
b. Restriction of intrinsically safe explosion-proof theory. Existing explosion protection regulations limit the length of the bus and the number of loads on the bus. This limits the cable-saving advantage of the fieldbus. All countries are strengthening the research on Fieldbus Intrinsic Safety Concept (FISCO) theory, striving for a breakthrough.
c. The system configuration parameters are too complicated. Fieldbus configuration
There are many parameters and it is not easy to master, but the quality of the configuration parameters has a great impact on the system performance.
4. Ethernet for Industrial Control Systems
The development of control network, its basic trend is to gradually tend to open, transparent communication protocol. The root cause of the above problems is that the openness of fieldbus is conditional and incomplete. Ethernet has the advantages of high transmission speed, low power consumption, easy installation and good compatibility. Because it supports almost all popular network protocols, it is widely used in commercial systems. In recent years, with the development of network technology, Ethernet has entered the field of control, and a new type of Ethernet control network technology has been formed. This is mainly due to the development of industrial automation systems towards distributed and intelligent control, and an open and transparent communication protocol is an inevitable requirement. Due to the wide variety of fieldbuses and incompatibility with each other, they cannot yet meet this requirement. The openness of the TCP/IP protocol of Ethernet makes the key link of communication in the field of industrial control have an unparalleled advantage.
5. Problems with industrial control systems
Usually we consider networking the control system, mainly linking networking with fieldbus. In the field of control, the more influential field bus systems are: FF, LonWorks, Profibus, CAN, HART, and the bus network of RS485. The unified standard (FF) has been formulated by the Fieldbus Foundation. Its slow bus standard H1 has been adopted as an international standard, and its high-speed bus standard H2 is still being formulated. However, due to commercial profits, technical monopoly and other reasons, the field bus Products are still in a situation where a hundred flowers bloom, which has an adverse effect on reducing system costs and expanding the scope of applications.
Ethernet has been widely used, the speed of mainstream products has reached 100Mbps, Gigabit Ethernet has also been put into use, and its network products and software are developing rapidly. Ethernet has been widely recognized for its low cost, convenient networking, rich hardware and software, and high reliability.
The main reason for the rapid development of the Internet is the wide application of Ethernet and TCP/IP protocols. TCP/IP protocols are extremely flexible, and almost all network underlying technologies can be used to transmit TCP/IP communications. Ethernet using TCP/IP has become the most popular packet-switched local area network technology, and it is also the most open network technology.
Therefore, we consider integrating the Internet and its related technologies into the existing control system, and use the open and mature technologies on the Internet to upgrade the existing control system to speed up the information-control integration of industrial enterprises. process is a more feasible solution to the problem.
From the trend point of view, there is no doubt that industrial Ethernet will enter the field control level. But at least for now, it is difficult to completely replace fieldbus as a single standard for real-time control communication. The existing fieldbus will continue to exist, most likely to develop a hybrid control system.
