The application of Ethernet in the field of industrial control and its future prospects

Ethernet refers to the baseband LAN specification created by Xerox and jointly developed by Xerox, Intel and DEC. It is the most common communication protocol standard adopted by existing LANs. The Ethernet network uses CSMA/CD (Carrier Sense Multiple Access and Collision Detection) technology and runs on multiple types of cables at 10M/S. Ethernet is similar to the IEEE802.3 family of standards.

IEEE 802.3 specifies the content of the wiring, electrical signals, and media access layer protocols including the physical layer. Ethernet is the most popular LAN technology in use today, and it largely replaces other LAN standards. Such as Token Ring, FDDI and ARCNET. After the rapid development of 100M Ethernet at the end of the last century, Gigabit Ethernet and even 10G Ethernet are expanding the scope of application under the impetus of international organizations and leading enterprises.

Common 802.3 applications are:

10M: 10base-T (copper line UTP mode)

100M: 100base-TX (copper wire UTP mode)

100base-FX (optical fiber line)

1000M: 1000base-T (copper wire UTP mode)

In general, Industrial Ethernet is a standard Ethernet designed specifically for industrial applications. Industrial Ethernet is technically compatible with commercial Ethernet (ie IEEE802.3 standard), and the similarities and differences between Industrial Ethernet and Standard Ethernet can be compared with industrial control computers and commercial computers. To meet the needs of industrial sites, Ethernet needs to meet the following requirements.

(1) Adaptability

Including mechanical properties (vibration and shock resistance), environmental characteristics (operating temperature requirements of -40 to +85 ° C, and corrosion resistance, dustproof, waterproof), electromagnetic environment adaptability or electromagnetic compatibility EMC should comply with EN50081-2, EN50082-2 standard.

(2) Reliability

Due to the harsh environment of the industrial control site, higher requirements are placed on the reliability of industrial Ethernet products.

(3) Intrinsic safety and safety explosion-proof technology

For intelligent equipment and communication equipment used in industrial sites where flammable, explosive and toxic gases are present, certain explosion-proof measures must be taken to ensure safe production at the industrial site. Explosion-proof technologies for field devices include explosion-proof type (such as increased safety, airtightness, encapsulation, etc.) and intrinsically safe type. Compared with the explosion-proof technology, intrinsic safety technology adopts the suppression of ignition source energy as an explosion-proof means, which can bring the following technical and economic advantages: simple structure, small size, light weight, low cost; maintenance under electrification And replacement; high safety and reliability; wide application range. The key technologies for achieving intrinsic safety are low-power technologies and intrinsically safe explosion-proof technologies. Since the power consumption of the Ethernet transceiver itself is relatively large, it is generally six or seventy mA (5V working power), so low-power field devices (such as industrial field Ethernet switches, transmission media, and Ethernet-based changes) Designs such as transmitters and actuators are difficult to implement. Therefore, under the current technical conditions, it is more feasible to use explosion-proof and explosion-proof measures for Ethernet systems. On the other hand, for non-hazardous situations where strict intrinsic safety requirements are not met, complex explosion protection measures may be disregarded.

(4) Easy to install, adapt to the installation requirements of industrial environment, such as DIN rail installation.

Due to the undisputed advantages of Ethernet, the application of Ethernet in the field of industrial automation is becoming a hot spot. This paper introduces the hierarchical structure of enterprise information system, analyzes some problems that Ethernet needs to solve in industrial field, shows the current status of Ethernet industrial application, and looks forward to its development prospects.

In industrial production, with the expansion of production scale and the increase of complexity, the practical application of control systems is getting higher and higher. In the 1950s and 1960s, the analog system consisting of electronic devices and automated instruments dominated by analog signals replaced traditional electromechanical control systems. Then in the 70s and 80s, the distributed control system DCS (Distributed Control System) appeared, centralized and unified management of a large number of scattered single-loop measurement and control systems through computers, replacing the control room instruments with various I/O function modules, using computers Achieve a variety of functions such as loop regulation, working condition interlocking, parameter display, data storage, etc., thus achieving a leap in industrial control technology.

DCS generally consists of three levels: operation station level, process control level and field instrument. It is characterized by “centralized management, decentralized control”. The basic control function is in the process control level. The main function of the workstation level is supervision and management. Decentralized control makes the damage to the whole system to a low degree due to a certain partial unreliability, and various software and hardware technologies continue to mature, greatly improving the reliability of the whole system, and thus quickly becoming industrial automation. The mainstream of control systems. However, the structure of the DCS is a multi-level master-slave relationship. The information transmission between the underlying layers must pass through the host, which causes the host to be overloaded and inefficient, and once the host fails, the entire system will “hook”. Moreover, DCS is a digital-analog hybrid system. The field instrument still uses the traditional 4~20mA analog signal, which has high engineering and management cost and poor flexibility. In addition, the DCS of each manufacturer is self-contained and the communication protocol is closed, which greatly restricts the integration and application of the system.

In the 1990s, Fieldbus technology with digital communication methods, fully decentralized system architecture, open interconnection network, multiple transmission media and topology, and high environmental adaptability quickly rose and matured. The function is fully transferred to the on-site intelligent instrument, and the new fieldbus control system FCS (Fieldbus Control System) formed on this basis integrates various technical means such as digital communication technology, computer technology, automatic control technology, network technology and smart instrument. It fundamentally breaks through the limitations of traditional "point-to-point" analog signals or digital-analog signal control, and constitutes a fully distributed, fully digital, intelligent, bidirectional, interconnected, multivariable, multi-contact communication and Control System. The corresponding control network structure has also undergone major changes. The typical structure of FCS is divided into device layer, control layer and information layer. The use of fieldbus technology makes it possible to decentralize control functions to field devices. Fieldbus standards are not only communication standards, but also system standards. FCS is moving towards replacing DCS and driving another leap in industrial control technology.

Traditional Ethernet is a commercial network. There are still some problems in application to industrial control, mainly in the following aspects.

(1) There is a problem of poor real-time performance and uncertainty

The traditional Ethernet adopts the medium access control mechanism of CSMA/CD. Each node adopts the BEB (Binary ExponenTIal Back-off) algorithm to deal with conflicts, which has the defect of queuing delay uncertainty. Each network node needs to obtain the information packet through competition. Send right. The node listens to the channel during communication, and can only send information when the channel is found to be idle; if the channel is busy, it needs to wait. After the information starts to be sent, it is also necessary to check whether a collision occurs. If the information collides, it needs to exit the retransmission. Therefore, the determined queuing delay and the responsiveness of the communication response cannot be guaranteed, and the real-time requirements of the industrial process control cannot be met, even in the communication. When it is busy, there is also the danger of information loss, which limits its application in industrial control.

(2) Industrial reliability issues

Ethernet is designed for office automation and does not take into account the adaptability needs of industrial field environments, such as ultra-high or ultra-low operating temperatures, strong electromagnetic noise generated by large motors or other high-power devices that affect channel transmission characteristics, etc. . Ethernet, if applied at the bottom of the shop floor, must address reliability issues.

(3) Ethernet does not provide power and must have additional power cable

The industrial field control network not only transmits communication information, but also supplies power for field device transmission work. This is mainly due to the convenience of cable laying and maintenance, while bus power can also reduce cables and reduce wiring costs.

(4) Ethernet is not an intrinsically safe system

(5) Security issues

Because Ethernet uses the TCP/IP protocol, Ethernet may be subject to network security threats including viruses, hackers, and illegal operations. Unauthorized users may enter the control layer or management layer of the network, causing security breaches. In this regard, user security, such as user passwords, data encryption, and firewalls, can be used to enhance the security management of the network. However, solutions for industrial automation control network security issues need to be carefully studied.

(6) Integration of existing control networks and new Ethernet control networks

Among these issues, real-time, deterministic, and reliability issues are major obstacles to the long-term barrier to Ethernet entry into industrial control. In order to solve this problem, people have proposed a solution for industrial Ethernet.

As a new trend in networking and information technology, Ethernet plays a vital role in industrial communication and automation systems. Its superior performance brings huge benefits to your application: rapid assembly through simple connection. Continuous development provides continuous compatibility, thus ensuring investment security. Communication performance is virtually unlimited through switching technology. A variety of networking applications, such as the networking of office environments and production application environments.

The use of Ethernet and TCP/IP protocols as the most important communication interfaces and means in industrial communication and automation systems, and the development towards network, standardization and openness will be the main trends in the development of various industrial control system technologies. Market share is as high as 80%.

As the most widely used and fastest growing LAN technology, Ethernet has achieved extraordinary development in the fields of industrial automation and process control. At the same time, IP-based integrated addressing, standard, shared, high-speed information channel solutions for industrial production will also have a profound impact on industrial control systems.