I. Introduction
Industrial Ethernet is an Ethernet technology used in the field of industrial control and is technically compatible with commercial Ethernet (ie IEEE802.3 standard). During product design, it can meet the needs of industrial sites in terms of material selection, product strength, applicability, real-time, interoperability, reliability, anti-interference, and intrinsic safety.
Ethernet used to be regarded as a "non-deterministic" network. As the basis of information technology, it was developed for IT applications. It can only be used in limited applications in the field of industrial control. This is due to:
The medium access control (MAC) layer protocol of Ethernet uses carrier sense multiple access (CSMA / CD) with collision detection. When the network load is heavy, the certainty of the network cannot meet the real-time requirements of industrial control;
The connectors, hubs, switches and cables used for Ethernet are designed for office applications and do not meet the harsh environmental requirements of industrial sites;
In the factory environment, Ethernet anti-interference (EMI) performance is poor, if it is used in dangerous occasions, Ethernet does not have intrinsically safe performance;
The problem that Ether-net cannot supply power to field devices through signal lines.
With the development and popularization of Internet technology, the improvement of Ethernet transmission rate and the development of Ethernet switching technology, the above-mentioned problems are being quickly solved in industrial Ethernet.
Second, the key technology of Ethernet used in industrial scene
(1) Communication certainty and real-time
The biggest characteristic of the industrial control network that is different from the ordinary data network is that it must meet the real-time requirements of the control function, that is, the signal transmission must be fast enough and meet the certainty of the signal. Real-time control often requires accurate and regular refreshing of data for certain variables. Because Ethernet adopts CSMA / CD mode, when the network load is heavy, the uncertainty of network transmission cannot meet the real-time requirements of industrial control. Therefore, traditional Ethernet technology is difficult to meet the real-time requirements of accurate timing communication required by the control system, and has been regarded as "Non-deterministic" networks.
Industrial Ethernet has taken the following measures to basically solve this problem:
Use Fast Ethernet to increase network bandwidth
The communication rate of Ethernet has increased from 10, 100 Mb / s to 1, 10 Gb / s today. In the case of the same data throughput, the increase in communication rate means that the network load is reduced and the network transmission delay is reduced, that is, the probability of network collision is greatly reduced, thereby improving its real-time performance.
Using full-duplex switched Ethernet
Replace the original bus-type CSMA / CD technology with switching technology to avoid collisions caused by multiple sites sharing and competing for channels, reducing the waste of channel bandwidth, while also enabling full-duplex communication and improving channel utilization .
Reduce network load
The industrial control network is different from the commercial control network. The amount of real-time data transmitted by each node is very small, usually a few bits or bytes, and the sudden large amount of data transmission rarely occurs, so it can be passed through the restriction network. The number of segment sites reduces network traffic and further improves the real-time performance of network transmission.
4. Application of message priority technology
In smart switches or hubs, the real-time performance of transmission is improved by designing the priority of messages.
(2) Security
In the industrial production process, flammable, explosive or toxic gases are inevitably present in many sites. For the smart devices and communication equipment used in these industrial sites, certain explosion-proof technical measures must be taken to ensure the safe production of industrial sites .
Under the current technical conditions, it is feasible to adopt explosion-proof and explosion-proof measures for the Ethernet system. That is, by adopting explosion-proof measures such as increased safety, airtightness, and potting for the Ethernet field devices, the ignition energy generated by the failure of the field device itself No leakage, to ensure the safety of system operation. For non-hazardous occasions without strict intrinsic safety requirements, complex explosion-proof measures may not be considered.
Network security of industrial systems is another security issue that industrial Ethernet applications must consider. Industrial Ethernet can integrate the traditional three-layer network system of the enterprise, namely the information management layer, process monitoring layer, and field device layer, so that the data transmission rate is faster, the real-time performance is higher, and it can be seamlessly integrated with the Internet. Realize the sharing of data and improve the operation efficiency of the factory.
But at the same time, it also attracts a series of network security issues. Industrial networks may be threatened by network security including virus infection, hacker's illegal invasion and illegal operation.
Under normal circumstances, a gateway or firewall can be used to isolate the industrial network from the external network, and the security management of the network can be strengthened through various security mechanisms such as authority control and data encryption.
(3) Stability and reliability
Traditional Ethernet is not designed for industrial applications and does not consider the adaptability needs of industrial field environments. Because the mechanical, climatic, and dust conditions on the industrial site are very harsh, the Control Engineering Network reserves all rights, so it puts forward higher requirements on the industrial reliability of the equipment. In a factory environment, industrial networks must have good reliability, recoverability, and maintainability.
In order to solve the problem that the network can work steadily under extreme conditions in uninterrupted industrial applications, companies such as US SynergeTIc Microsystems, Germany Hirschmann, and Jetter AG have developed and produced rail-type hubs and switches, which are installed in The DIN rail is provided with redundant power supply, and the connector adopts a solid DB-9 structure.
In addition, in practical applications, the backbone network can use optical fiber transmission, the connection of field devices can use shielded twisted pair, and redundant network technology can also be used for important network segments to improve the anti-interference ability and reliability of the network .
(4) Bus power supply problem
Bus power supply (or bus feed) means that the cable connected to the field device not only transmits data signals, but also provides working power to the field device. The following methods can be adopted for the power supply of field devices:
On the basis of the current Ethernet standard, the technical specifications of the physical layer are appropriately modified, and the Manchester signal of the Ethernet is modulated to a DC or low-frequency AC power supply, and the two signals are separated at the field device side.
It does not change the structure of the current physical layer, but provides power to the field devices through the free cable in the connection cable.
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