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 monitoring system consisting of electronic signals and automation instruments based on analog signals replaced the traditional electromechanical control system. 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 controlâ€. 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 “hookâ€. 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-to-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.
1. Problems in fieldbus applications1.1 standard questions
Fieldbus control systems still have some problems to be solved in practical applications. The most prominent problem is the lack of uniform standards. At the beginning of 2000, the IEC61158 international standard published by IEC produced eight kinds of IEC fieldbus international standard subsets such as H1 (FF), ControlNet, Profibus, P-Net, HSE (FF), SwiftNet, WorldFIP and Interbus. The conclusion of the IEC fieldbus international standard indicates that for a long period of time, a variety of fieldbus coexistence situations will occur, and it will be difficult to integrate the system integration and information integration of the control network segment. Both end users and engineering integrators, including manufacturers, are looking for high-performance, low-cost solutions. Eight types of fieldbus use different communication protocols, and it is almost impossible to achieve mutual compatibility and interoperability of these buses. Each fieldbus has its own most suitable application field. How to combine different levels of fieldbus according to the application object in practice, so that all parts of the system select the most suitable field bus, for the user, it is still More difficult problems.
1.2 system integration issues
In practical applications, a large system is likely to use a variety of fieldbus, especially those high-growth end users in China. It is almost impossible to unify the frontier of technology in different stages of development of enterprises and international multinational manufacturing equipment procurement. bus. How to integrate the enterprise's industrial control network with the management's data network seamlessly, so that the entire enterprise to achieve management and control integration is very critical. When designing the network layout, the fieldbus system should consider not only the distance of each site node, but also the functional relationship between the site nodes and the flow of information on the network. Due to the strong function of intelligent field instruments, many instruments will have the same function block. The configuration should carefully consider the selection of function blocks to minimize the flow of information on the network. 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.
1.3 There is a technical bottleneck
There are still some technical bottlenecks in the application of fieldbus, mainly in the following aspects.
(1) When the bus cable is disconnected, the entire system may be defective. The user hopes that the performance of the system can be reduced at this time, but it cannot be collapsed. This is currently not guaranteed by many fieldbuses.
(2) The constraints of intrinsic safety explosion protection theory. Existing explosion protection regulations limit the length of the bus and the amount of load on the bus. This is to limit the advantages of fieldbus saving cable.
(3) System configuration parameters are too complicated. There are many configuration parameters of the fieldbus, which are not easy to master, but the configuration parameters are set well or badly, which has a great impact on system performance.
Therefore, the adoption of a unified fieldbus standard is of particular importance for the development of fieldbus technology. In order to accelerate the development and application of the new generation of control systems, major manufacturers have sought other ways to solve the problem of scalability and compatibility. The industry has turned their attention to the simple and low-cost, successful in commercial LAN. Ethernet technology that is easy to install, has high transmission speed, low power consumption, rich hardware and software resources, good compatibility, high flexibility, easy integration with the Internet, and supports almost all popular network protocols. 3, Ethernet and TCP / IP
Ethernet (Ethernet) was originally derived from the network system built by Xerox in 1973. It is a bus-type LAN with baseband Coaxial Cable as the transmission medium and CSMA/CD protocol. The Ethernet built by Xerox was very successful. In 1980, Xerox, DEC and Intel jointly drafted the Ethernet standard. In 1985, the IEEE 802 committee absorbed Ethernet as the IEEE 802.3 standard and modified it. The main difference between the Ethernet standard and the IEEE802.3 standard is that the Ethernet standard only describes a bus LAN that uses 50 ohm coaxial cable and a data transmission rate of 10 Mbps, and the Ethernet standard includes the entire contents of the ISO data link layer and the physical layer. The IEEE802.3 standard describes all LANs that use CSMA/CD protocols with data rates from 1 Mbps to 10 Mbps running on various media, and the IEEE 802.3 standard only defines the data link layer in the ISO reference model. One sublayer (ie, medium access control MAC sublayer) and physical layer, and the logical link control LLC sublayer of the data link layer is described by IEEE 802.2. The specification specifies Carrier Sense MulTIple Access/Collision Detect using carrier sense multiple access/collision detection. The signal is transmitted over the coaxial cable at 10 Mbps.
According to ISO's OSI seven-layer structure, the Ethernet standard defines only the data link layer and the physical layer as a complete communication system. After becoming the protocol for the data link and the physical layer, Ethernet is tightly bundled with TCP/IP. Since the Internet has adopted Ethernet and TCP/IP protocols, people even put together TCP/IP protocol groups such as hypertext connection HTTP, which is called Ethernet technology; the simple and practical TCP/IP has been accepted by users. Ethernet technology is widely used not only in the field of office automation, but also in the management network and monitoring layer network of each enterprise, and begins to extend to the field device layer network. Today, the TCP/IP protocol becomes the most popular Internetworking protocol, and has evolved from a pure TCP/IP protocol to a series of IP-based TCP/IP protocol suites.
In the TCP protocol, the core protocol of the network layer is IP (Internet Protocol), and protocols such as ARP (Address ResoluTIon Protocol), RARP (Reverse Address ResoluTIon Protocol), and ICMP (Internet Control Messages Protocol) are also provided. The main functions of this layer include processing packet transmission requests from the transport layer (ie assembling IP datagrams and sending them to the network interface), processing incoming datagrams, forwarding datagrams or extracting packets from datagrams, and handling error and control messages ( Including processing routing, flow control, congestion control, etc.).
The function of the transport layer is to provide inter-application (end-to-end) communication services. It provides two protocols: User Datagram Protocol (UDP) and TCP (Transfer Control Protocol). UDP is responsible for providing efficient services for transmitting a small number of messages, and almost no reliability measures are required. Applications using UDP need to perform reliable operations themselves; TCP is responsible for providing highly reliable data transfer services, mainly for transmitting large amounts of data. Messages and guarantee the reliability of data transmission.
2, the advantages of EthernetThe transmission medium supported by Ethernet is thick coaxial cable, thin coaxial cable, twisted pair cable, optical fiber, etc. The biggest advantage is that it is simple, economical and practical, and easy to be mastered by people, so it is well received by users. Compared with fieldbus, Ethernet has the following advantages:
(1) Good compatibility and extensive technical support
TCP/IP-based Ethernet is a standard open network, which is suitable for solving the compatibility and interoperability of devices from different vendors in the control system. Devices from different vendors are easy to interconnect and can realize office automation network and industrial control network. The information is seamlessly integrated. Ethernet is currently the most widely used computer network technology and is widely supported by technology. Almost all programming languages ​​support Ethernet application development, such as VB, Java, VC, and so on. Ethernet is used as the field bus to ensure a variety of development tools and development environments to choose from. The industrial control network adopts Ethernet, which can avoid its development from the mainstream of the development of computer network technology, so that the industrial control network and information network technology can promote each other, develop together, and ensure the sustainable development of technology.
(2) Easy to connect with the Internet
Ethernet supports almost all popular network protocols, enabling you to monitor your business from anywhere on the Internet, easily accessing remote systems, and sharing/accessing multiple databases.
(3) low cost
Using Ethernet can reduce costs, including technician training, maintenance, and initial investment. Because Ethernet is the most widely used, it is widely supported by hardware development and manufacturers. It has rich hardware and software resources, a variety of hardware products for users to choose, and the hardware price is relatively low. At present, the price of Ethernet network card is only a fraction of that of fieldbus, and with the development of integrated circuit technology, its price will further decline. People have a lot of experience in the design and application of Ethernet, and they are also very familiar with their technology. A large amount of software resources and design experience can significantly reduce system development and training costs, and no separate research input is required for technology upgrades, which can significantly reduce the overall cost of the system and greatly accelerate the development and promotion of the system.
(4) Great potential for sustainable development
Due to the wide application of Ethernet, its development has been widely valued and attracted a large amount of technical investment. Moreover, in the era of rapid changes in information, the survival and development of enterprises will depend to a large extent on a fast and effective communication management network. The development of information technology and communication technology will be more rapid and mature, ensuring the technology of Ethernet. Continue to develop.
(5) High communication rate
At present, Ethernet communication speeds of 10M or 100M, 1000M, and 10G Fast Ethernet are also beginning to be applied. Ethernet technology is also mature, and its rate is much faster than current fieldbus. Ethernet can meet higher bandwidth requirements. .
3. Problems with Ethernet when applied to controlHowever, 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 supply 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. 5, Industrial Ethernet
4, demandIn 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) It is easy to install and adapt to the installation requirements of industrial environment, such as DIN rail installation.
5, ways to improve the practicality of EthernetWith the development of related technologies, the development of Ethernet has also made an essential leap, and with the help of related technologies, the practicality of Ethernet in industrial control can be improved as a whole.
5.1 using switching technology
Traditional Ethernet uses a shared hub. Its structure and function are just a multi-port physical layer repeater. All stations connected to the shared hub share a bandwidth and send and receive data according to the CSMA/CD protocol. The switched hub can be thought of as a controlled multi-port switch matrix. The flow of information between the ports is isolated, providing a direct and fast point-to-point connection between the source and the target of the switching device. Different ports can form multiple data channels, and the data input and output between ports are no longer constrained by CSMA/CD. With the development of modern switch technology, the transfer rate between switch ports is larger than the transfer rate between the entire device layer Ethernet ports, thus reducing the collision rate of Ethernet and providing caching for conflict data. Of course, the working mode of the switch must be the store-and-forward mode, so that there is only a point-to-point connection in the system, and no collision occurs. Multiple exchanges decompose the entire Ethernet into many independent areas. Ethernet data collisions exist only in their respective collision domains. There is no conflict between different domains, which can greatly increase the bandwidth of each site on the network, thus improving the exchange. Network performance and determinism.
Switched Ethernet does not change the original Ethernet protocol, and can directly use a common Ethernet card, which greatly reduces the cost of networking, and fundamentally solves the problem of uncertainty in Ethernet communication transmission delay. Studies have shown that when the communication load is below 10%, the transmission delay caused by Ethernet collision is almost negligible. In the industrial control network, the information transmitted is mostly periodic measurement and control data, the message is small, the amount of information is small, and the length of information transmitted is small. The information includes the measured values ​​of the operating parameters of the production equipment, the control amount, the working position of the switch and the valve, the alarm status, the resource and maintenance information of the equipment, the system configuration, the parameter modification, the zero point and the range adjustment information. The length is generally small, usually only a few to tens of bytes, and the throughput of network transmission is not high. Studies have shown that in a typical industrial control system with 6000 I / O, the communication load is about 5% of 10M Ethernet, even if there is operator information transmission (such as changes in set values, download of user applications, etc.), The load of 10M Ethernet can also be kept below 10%.
5.2 using high speed Ethernet
With the rapid development of network technology, high-speed Ethernet (100M) and Gigabit Ethernet products and international standards have been produced, and 10G Ethernet products have also been available. By improving the communication speed and combining the switching technology, the overall performance of the communication network can be greatly improved.
5.3 Adopt full duplex communication mode
One port in switched Ethernet is a collision domain, and it is still not possible to send and receive data simultaneously in the case of half-duplex. If full-duplex mode is adopted, two stations in the same data link can receive data while transmitting data, which solves the problem that half-duplex needs to wait in this case, and theoretically can double the transmission rate. . Full-duplex communication technology allows two pairs of twisted pairs (or two fibers) to receive and transmit message frames simultaneously between device ports, so that they are no longer constrained by CSMA/CD, so that any node sends a message frame. There will be no more collisions and the conflict domain will cease to exist. For emergency information, it can be based on IEEE802.3p
5.4 using virtual local area network technology
The emergence of virtual local area networks (VLANs) has broken many of the inherent concepts of traditional networks, making the network structure more flexible and convenient. In fact, a VLAN is a broadcast domain. It is not restricted by geographical location. Network users in different geographical locations can be divided into logical network segments according to factors such as department functions, object groups, and applications. Each port of a LAN switch can only mark one VLAN. All stations in the same VLAN have one broadcast domain. The broadcast information between different VLANs is isolated from each other, thus avoiding broadcast storms. In the process control of industrial process, the control layer unit should be distinguished from the common unit in terms of real-time data transmission and security. The virtual local area network is used for logical division on the open platform of Industrial Ethernet to distinguish different functional layers and different departments. Open, thereby achieving the goal of improving the overall security of the network and simplifying network management. Generally, virtual LANs are classified into three types: static port allocation, dynamic virtual network, and multiple virtual network port configurations. Static port allocation refers to the network management personnel using the port of the network management software or device switch to directly subordinate to a virtual network. These ports will maintain such dependencies unless the network administrator resets them; dynamic virtual network refers to support dynamics. The ports of the virtual network can automatically determine their dependencies by means of intelligent management software; the multiple virtual network port configuration supports one user or one port to access multiple virtual networks at the same time, so that one control layer computer can be configured to be accessed by multiple departments at the same time. It is also possible to access resources of multiple virtual networks at the same time.
5.5 Introducing Quality of Service (QoS)
IP QoS refers to the quality of service of IP, that is, the performance of IP data streams when they pass through the network. Its purpose is to provide end-to-end service quality assurance to users. QoS has a set of metrics including service availability, latency, variable latency, throughput, and packet loss rate. QoS networks can distinguish between real-time and non-real-time data. QoS technology is used in industrial Ethernet to identify higher priority data from the control layer and prioritize them in response delay, transmission delay, throughput, and reliability. Sex, transmission failure rate, priority, etc., make Industrial Ethernet meet the requirements of industrial automation real-time control. In addition, the QoS network can also prevent illegal use of the network, such as illegal access control layer field control unit and monitoring unit terminals.
In addition, there have been improvements in industrial Ethernet application standards and related protocols supported by large companies. Introducing Industrial Ethernet into the underlying network not only facilitates the integration of the field layer, the control layer and the management layer at the vertical level, but also reduces the integration cost of devices of different manufacturers at the horizontal level. The extension of Ethernet to the underlying network is inevitable. Therefore, famous manufacturers have supported industrial Ethernet and formulated different industrial application standards. For example, Rockwell, OMRON and other companies support Ethernet/IP. IP refers to the industrial protocol. It provides the Producer/Consumer model, and the application layer of ControlNet and Devicenet control and information protocols is ported to the high-speed Ethernet protocol HSE developed by TCP.FF. The publisher/orderer, object and other models are mainly used in the field of engineering control and are supported by some large companies such as Foxboro and Honeywell. The Modbus/TCP protocol released by Schneider bundles the Modbus protocol on the TCP protocol, which is easy to implement and enables interconnection.
In order to improve real-time performance, the Ethernet protocol has also made some improvements. A completely software-based protocol, RETHER (Real Time Ethernet), ensures real-time performance without changing the existing hardware of Ethernet. It uses a hybrid operation mode to reduce the impact on non-real-time data transmission performance in the network. A non-competitive admission control mechanism and an efficient token delivery scheme can prevent token loss due to node failure. Networks that adhere to the RETHER protocol operate in both CSMA and RETHER modes. During the live conversation, the network will transparently transition to RETHER mode, returning to CSMA mode after the real-time conversation is over. There is also an Ethernet protocol called RTCC (Real Time Communication Control), which provides a good foundation for distributed real-time applications. RTCC is a layer of protocol over Ethernet that provides high-speed, reliable, real-time communication. It does not require changes to existing hardware devices, and uses a novel mechanism of command/response multiplexing and bus tables to allocate channels. All nodes are divided into bus controller (BC) and remote terminal (RT) in the RTCC protocol. There is only one BC, and the rest are RT. The initiation and management of information transmission are undertaken by BC, access arbitration process and transmission control. The process is implemented by BC. Through the integration and synchronization of the two processes, not only the sending time of the node is determined, but also the time of the node using the bus is also controllable. Experiments on 10 Mbps Ethernet show that the RTCC has satisfactory certainty. A third way to improve real-time performance is traffic balancing, which is to add a traffic balancer between UDP or TCP/IP and Ethernet MAC. As an interface between them, it is installed on each network node. At the local node, it gives priority to real-time data packets to eliminate competition between real-time information and non-real-time information, while balancing non-real-time information to reduce collisions with real-time information of other nodes. To ensure the throughput of non-real-time information, the traffic balancer can also adjust the data stream generation rate according to the load of the network. This method does not require any changes to the existing standard Ethernet MAC protocol and TCP or UDP/IP.
Therefore, in response to the uncertainty of Ethernet queuing delay, real-time performance is improved by adopting appropriate flow control, switching technology, full-duplex communication technology, information priority, etc., and fault-tolerant technology, system design technology, and redundant structure are improved. Ethernet can be used in industrial control networks. In fact, in the mid-to-late 1990s, major industrial and commercial companies at home and abroad adopted Ethernet in their control systems, and launched Ethernet-based DCS, PLC, data collectors, and Ethernet-based field instruments and display instruments. And other products.
With the maturity of network and information technology, Ethernet and TCP/IP protocols are adopted as the main communication interfaces and means in industrial communication and automation systems. The development of network, standardization and openness will be various control systems. The main trend of technology development. 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 the control system.
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