The Development and Current Status of Network Control Systems

0 Introduction

With the development of computer technology, communication technology and control technology, the traditional control field is undergoing an unprecedented transformation and is beginning to develop in the direction of networking. The structure of the control system has evolved from the initial CCS (computer centralized control system) to the second-generation DCS (distributed control system) and now the popular FCS (fieldbus control system) [1]. The requirements for large amounts of data and high-speed transmission such as images and voice signals have given rise to the combination of Ethernet and control networks that are currently popular in the commercial field. This wave of industrial control system networking has also integrated a variety of popular technologies such as embedded technology, multi-standard industrial control network interconnection, and wireless technology, thereby expanding the development space in the field of industrial control and bringing new development opportunities.

1 Development of computer control systems

Computer and network technology are closely related to the development of control systems. Computers were first used in control systems in the mid-to-late 1950s. In the early 1960s, control systems that completely replaced analog control with computers appeared, which were 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 microprocessor-based distributed computer control system was launched. It uses multiple microprocessors for distributed control and realizes centralized management through data communication networks. It is called a distributed control system (DCS).

After entering the 1980s, people used 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 has a performance-price ratio that traditional analog instruments cannot match in multi-loop sampling and control.

In the mid-to-late 1980s, with the increasing complexity of industrial systems and the further increase in control loops, a single DDC control system could no longer meet the production control requirements and management requirements of production work on site. At the same time, the performance-price ratio of small and medium-sized computers and microcomputers has greatly improved. As a result, hierarchical control systems that work together with small and medium-sized computers and microcomputers have been widely used.

After entering the 1990s, due to the rapid development of computer network technology, the DCS system has been further developed, improving the reliability and maintainability of the system. In today's industrial control field, DCS still occupies a dominant position. However, DCS is not open, 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 drive devices have become intelligent. People have begun to seek to use a communication cable to connect field devices with a unified communication protocol and communication interface. The signal transmitted at the device layer is no longer an I/O (4-20 mA/24VDC) signal, but a digital signal. This is the fieldbus. Because it solves the reliability and openness problems of the network control system itself, fieldbus technology has gradually become the development trend of computer control systems. Since then, some developed industrial countries and multinational industrial companies have launched their own fieldbus standards and related products, forming a trend of competition among the heroes.

2 Relationship between information network and control system

From the perspective of development history, the development of information network architecture is similar to the development of control system structure. The development of enterprise information network has generally gone through the following stages:

① Host-based centralized mode

Almost all computing and processing tasks are completed by a powerful host, and there is very little interaction between the user and the host.

② Hierarchical structure based on workgroup

The development of microcomputers and local area network technology has enabled people with similar work natures to form groups and share certain public resources, thereby strengthening communication and collaboration among users.

③Networked 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 recognized prototype of the future global information infrastructure. Using the mature technology and standards of the Internet, people have proposed the concepts of Intranet and Extranet, which are used to implement corporate intranets and corporate extranets respectively. Thus, a new generation of corporate information infrastructure (enterprise network) based on the Internet has been formed, with Intranet as the center and Extranet as the supplement.

Computer control systems have also gone through several development stages, including centralized control, hierarchical control, and fieldbus-based network control. Their development processes are very similar.

With the in-depth application and improvement of enterprise information network, it is an inevitable trend for field control information to enter the information network for 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 of the production site to ensure real-time grasp of the operation status of the production process, make enterprise management decisions scientific, and achieve the optimal state of production, operation and management. Information-control integration will create favorable conditions for the realization of enterprise comprehensive automation CIPA (computer integrated plant automation) and enterprise informatization.

The similarity between enterprise information network and control system in the process of architecture development is not accidental. In the process of computer control system development, the emergence of each structure of control system always lags behind the development of corresponding computer technology. In fact, in most cases, it is only after a new technology appears in the computer field that people begin to study how to apply this new technology to the control field. In view of the differences in the two application environments, the technical details have been appropriately modified and supplemented, but they have many similarities in the principles and implementation of key technologies. It is precisely because of this relationship between the two in the development process that it is possible to achieve information-control integration.

3 Research status of fieldbus technology

In the 1940s, process control was based on pneumatic standard signals of 3 to 15 PSI. Later, due to the use of 4 to 20 mA analog signals, analog controllers were widely used, but not all sensors and drive devices use a unified 4 to 20 mA signal. In the 1970s, the use of computers in the fields of detection, analog control and logic control led to centralized control. In the 1980s, the emergence of microprocessors prompted industrial instruments to enter the era of digitalization and intelligence. 4 to 20 mA analog signal transmission was gradually replaced by digital communication. In addition, the rapid development of distributed control and network technology promoted the development of integrated functions such as control, scheduling, optimization, and decision-making. However, since most detection, transmission, and execution mechanisms use analog signal connections, the transmission method is a one-to-one structure, which makes wiring complex, engineering costs high, and maintenance difficult. The signal transmission accuracy is low, it is susceptible to interference, and the instrument interchangeability is poor, which hinders the function of the upper system. On the other hand, the functions of intelligent 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 on-site instruments.

In order to overcome the technical bottleneck of DCS system and further meet the needs of the site, fieldbus technology came into being. It is actually a two-way serial, digital, multi-node communication network connecting field intelligent devices and automation control equipment, also known as the field underlying equipment control network (INFRANET). Unlike information networks such as the Internet and Intranet, the control network is directly oriented to the production process, so it requires high real-time performance, 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 3 layers in the ISO/OSI 7-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. On December 31, 1999, the IEC organization voted to determine 8 major buses as international fieldbus standards, including CAN Bus, Profit Bus, InterBus-S, Mod Bus, FOUNDA-TION Fieldbus, etc. On this basis, a new fieldbus control system (Fieldbus Control System FCS) was formed. It integrates a variety of technical means such as digital communication technology, computer technology, automatic control technology, network technology and intelligent instruments, fundamentally breaking through the limitations of traditional "point-to-point" analog signals or digital-analog signal control, and forming a fully decentralized, fully digital, intelligent, bidirectional, interconnected, multi-variable, multi-contact communication and control system. The corresponding control network structure has also undergone major changes. The typical structure of FCS is divided into three layers: device layer, control layer and information layer.

Although fieldbus technology develops very rapidly, there are still many problems that restrict the further expansion of its application scope.

(1) The first is the selection of fieldbus. Although the IEC organization has reached an international bus standard, there are still too many types of buses, and each fieldbus has its own most suitable application field. How to combine fieldbuses of different levels according to the application object in practice so that each part of the system can choose the most suitable fieldbus is still a difficult problem for users.

(2) System integration issues. Since a system may use multiple forms of fieldbus in actual applications, how to seamlessly integrate the industrial control network with the data network so that the entire system can achieve integrated management and control is a key link. When designing the network layout of the fieldbus system, it is necessary to consider not only the distance between the field nodes, but also the functional relationship between the field nodes, the flow of information on the network, etc. Since the functions of intelligent field instruments are very powerful, many instruments will have the same functional blocks. When configuring, it is necessary to carefully consider which functional block to choose; the flow of information on the network should be minimized. At the same time, the configuration of communication parameters is also very important, and a balance should be struck between the real-time performance of the system and the network efficiency.

(3) There are technical bottlenecks[2]. They are mainly manifested in:

a. When the bus cable is cut, the entire system may be paralyzed.

Users hope that the system performance can be reduced at this time, but it cannot crash, which is something that many fieldbuses cannot guarantee at present.

b. Constraints of intrinsic safety explosion-proof theory. Existing explosion-proof regulations limit the length of the bus and the number of loads on the bus. This limits the advantages of fieldbus in saving cables. Currently, all countries are strengthening research on the fieldbus intrinsic safety concept (FISCO) theory and striving for breakthroughs.

C. System configuration parameters are too complex. Fieldbus configuration

There are many parameters, which are not easy to master, but the quality of configuration parameter settings has a great impact on system performance.

4 Ethernet Control Network

The basic trend of the development of control networks is to gradually move towards open and transparent communication protocols. The fundamental reason for the above problems is that the openness of the fieldbus is conditional and incomplete. Ethernet has the advantages of high transmission speed, low consumption, easy installation and good compatibility. Since 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 control field and formed a new type of Ethernet control network technology. This is mainly due to the development of industrial automation systems towards distributed and intelligent control, and open and transparent communication protocols are an inevitable requirement. The current fieldbus cannot meet this requirement due to its wide variety and incompatibility. The openness of Ethernet's TCP/IP protocol makes it have an incomparable advantage in the key link of communication in the field of industrial control.

5 Current Problems

Usually we consider networking the control system, mainly linking networking with fieldbus. At present, the fieldbus systems with greater influence in the control field include: FF, LonWorks, Profibus, CAN, HART, and RS485 bus networks. The Fieldbus Foundation has formulated a unified standard (FF), its slow bus standard H1 has been passed 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, fieldbus products are still in a situation of flourishing, which has an adverse effect on reducing system costs and expanding the scope of application.

Ethernet has been widely used. The speed of mainstream products has reached 100Mbps. Gigabit Ethernet has also been put into use. Its network products and software are developing rapidly. Ethernet has been widely recognized for its low cost, convenient networking, rich software and hardware, and high reliability.

The main reason for the rapid development of the Internet is the widespread application of Ethernet and TCP/IP protocol. TCP/IP protocol is 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 switching LAN technology, and it is also the most open network technology.

Therefore, we consider integrating the Internet and its related technologies into the existing control systems, using the open and mature technologies on the Internet to upgrade and transform the existing control systems, and accelerating the information-control integration process of industrial enterprises, which is a more feasible solution to the problem.

6 Conclusion

Judging from the current trend, there is no doubt that industrial Ethernet will enter the field control level. However, at least for now, it is difficult for it to completely replace the fieldbus as the single standard for real-time control communication. The existing fieldbus will continue to exist, and the most likely development is a hybrid control system [6].