Fieldbus (Fieldbus) was developed internationally in the late 1980s and early 1990s for field intelligent device interconnection communication networks in the fields of process automation, manufacturing automation, and building automation. As the basis of the factory's digital communication network, it communicates the relationship between the production process site and the control equipment and its relationship with higher control management. It is not only a grassroots network, but also an open, new fully distributed control system. This integrated technology with intelligent sensing, control, computer, digital communication and other technologies has attracted worldwide attention and become a hot spot in the development of automation technology, and will lead to profound changes in the structure and equipment of automation systems. Many internationally powerful and influential companies have developed fieldbus technologies and products to varying degrees. The working environment of the fieldbus device is at the bottom of the process equipment. As the factory equipment level basic communication network, it requires the characteristics of simple protocol, strong fault tolerance, good security and low cost.
It has certain time determinism and high real-time requirements. It also has stable network load, most of which are characterized by short frame transmission and frequent information exchange. Due to the above characteristics, the field bus system has the characteristics of different upper layer high speed data communication networks from network structure to communication technology.
The fieldbus system is generally referred to as the fifth-generation control system, also known as FCS-fieldbus control system. People generally refer to the PCS as the first generation of the Pneumatic Signal Control System before the 1950s. The electric analog signal control system such as 4-20 mA is called the second generation, and the digital computer centralized control system is called the third generation. The distributed distributed control system dcs since the middle of the period is called the fourth generation. As a new generation control system, the fieldbus control system FCS breaks through the limitations of the DCS system using a communication-dedicated network. It adopts an open and standardized solution to overcome the defects caused by the closed system. On the other hand, the DCS The centralized and decentralized system of distributed systems has become a new fully distributed structure, and the control functions have been completely decentralized to the scene. It can be said that openness, dispersion and digital communication are the most prominent features of the fieldbus system.
After the initial stage of the decentralization of the fieldbus technology, the fieldbus technology has not yet formed a complete unified international standard despite a certain scope of consultation and merger. Among them, there are strong strengths and influences: FoudationFieldbus (FF), LonWorks, Profibus, HART, CAN, Dupline, etc. They have their own characteristics and have formed their own advantages in different application areas. This paper will briefly describe the characteristics of fieldbus technology, closely related to the reliability and practicability of the system, and introduce the key technologies such as fieldbus network structure and architecture, and the current status of several popular fieldbus technologies. Finally, the development trend and technology prospect of fieldbus are explained.
First, the technical characteristics of the field bus
1. The openness of the system. An open system means that the communication protocol is open, and devices of different manufacturers can be interconnected and exchange information. The fieldbus developer is committed to establishing a unified open system of the factory underlying network. Openness here refers to the consistency and openness of relevant standards, emphasizing consensus and compliance with standards. An open system that can be connected to any other device or system that adheres to the same standards. A bus-enabled fieldbus network system must be open, and the open system gives the user the right to system integration. Users can make products from different suppliers into random systems according to their own needs and objects.
2. Interoperability and interoperability. Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems. It can implement point-to-point, point-to-multipoint digital communication. Interoperability means that devices of similar performance from different manufacturers can be interchanged for interoperability.
3. Intelligent and functional autonomy of field devices. It distributes the functions of sensing measurement, compensation calculation, engineering quantity processing and control into the field equipment. The basic functions of automatic control can be completed only by the field device, and the running status of the equipment can be diagnosed at any time.
4. Highly dispersed system structure. Since the field device itself can complete the basic functions of automatic control, the field bus has formed a new architecture of a fully distributed control system. It fundamentally changed the system of distributed control system combining the existing DCS concentration and dispersion, which simplified the system structure and improved the reliability.
5. Adaptability to the on-site environment. Working at the front end of the field equipment, as the field bus on the bottom of the factory network, it is designed for working in the field environment. It can support twisted pair, coaxial cable, optical cable, RF, infrared, power line, etc., with strong resistance. The interference capability can be realized by two-wire system for power transmission and communication, and can meet intrinsic safety explosion-proof requirements.
Second, the advantages of the field bus Due to the above characteristics of the field bus, especially the simplification of the structure of the field bus system, the design, installation, commissioning of the control system to the normal production operation and its maintenance and repair, all show superiority.
1. Save hardware quantity and investment. Since the smart devices scattered in the front end of the device in the fieldbus system can directly perform a variety of sensing, control, alarm and calculation functions, the number of transmitters can be reduced, eliminating the need for separate controllers, computing units, etc. In addition, DCS system signal conditioning, conversion, isolation technology and other functional units and their complex wiring are required. Industrial PCs can also be used as operating stations, saving a large amount of hardware investment. Due to the reduction of control equipment, the control room can be reduced. The footprint.
2. Save installation costs. The wiring of the fieldbus system is very simple. Since a pair of twisted pairs or one cable can usually be connected to multiple devices, the amount of cables, terminals, slots, and bridges is greatly reduced, and the workload of wiring design and joint proofing is also decrease very much. When it is necessary to add on-site control equipment, it is not necessary to add a new cable, and it can be connected to the original cable nearby, which saves investment and reduces the workload of design and installation. According to the calculation data of typical test projects, the installation cost can be saved by more than 60%.
3. Save maintenance overhead. Because the field control equipment has the ability of self-diagnosis and simple fault handling, and sends relevant diagnostic maintenance information to the control room through digital communication, the user can query the operation of all equipment and diagnose the maintenance information, so as to analyze the cause of the fault early and quickly eliminate it. Maintenance downtime is reduced, and maintenance is reduced due to simplified system structure and simple wiring.
4. The user has a high degree of system integration initiative. Users are free to choose the devices provided by different vendors to integrate the system. Avoiding the choice of a certain brand of products is "framed dead" equipment, will not be incompatible with the incompatible protocols and interfaces in the system integration, so that the initiative in the system integration process is completely in the hands of users.
5. Improve the accuracy and reliability of the system. Due to the intelligence and digitization of the fieldbus device, it fundamentally improves the accuracy of measurement and control and reduces the transmission error compared with the analog signal. At the same time, due to the simplified structure of the system, the equipment and wiring are reduced, and the internal functions of the field instrument are strengthened: the round-trip transmission of signals is reduced, and the operational reliability of the system is improved. In addition, due to its equipment standardization and functional modularity, it also has the advantages of simple design and easy reconstruction.
Third, the introduction of typical fieldbus
1. FOUNDATION Fieldbus Foundation Fieldbus, or Foudation Fieldbus, referred to as FF, is a technology that has been widely supported in the field of process automation and has good development prospects. Its predecessor is the United States Fisher-Rousemount company, combined with Foxboro, Yokogawa, ABB, Siemens and other 80 companies to develop the ISP agreement and the Honeywell company, combined with 150 companies in Europe and other places to develop the WordFIP agreement. Under the pressure of users, the two groups merged in September 1994 and established the Fieldbus Foundation to develop an internationally unified fieldbus protocol. It is based on the ISO/OSI open system interconnection model, taking its physical layer, data link layer and application layer as the corresponding layers of the FF communication model, and adding a user layer on the application layer.
FOUNDATION fieldbus is divided into two speeds: low speed H1 and high speed H2. H1 has a transmission rate of 3125Kbps and a communication distance of 1900m (can be extended with repeaters). It can support bus power supply and support intrinsically safe explosion-proof environment. H2 has a transmission rate of 1 Mbps and 2.5 Mbps, and its communication distance is 750 m and 500 m. The physical transmission medium can support specific stranding, fiber optic cable and wireless transmission, and the protocol complies with the IEC1158-2 standard. The transmission signal of the physical medium adopts Manchester coding, and the center position of each transmitted data is either a positive transition or a negative transition. The positive transition represents 0 and the negative transition represents 1, so that there is sufficient positioning information in the serial data bit stream to keep the time synchronization of both parties. The receiver can judge the "1" and "0" status of the data according to the polarity of the hopping, and can also accurately locate according to the center position of the data.
In order to meet the needs of users, Honeywell, Ronan and other companies have developed dedicated chips that can complete the physical layer and some data link layer protocols. Many instrument companies have developed products that comply with the FF protocol. The 1 bus has passed the a test and the beta test. Completed the FF fieldbus factory test system consisting of equipment supplied by 13 different manufacturers. 2 bus standards have also been formed. In October 1996, at the ISA96 exhibition in Chicago, organized by the Fieldbus Foundation, the company displayed more than 70 FF-compliant products from more than 40 manufacturers and distributed them on different floors of the exhibition hall. The FF exhibits on different booths are interconnected into a seven-segment fieldbus demonstration system with eye-catching orange-red cables. The field devices of each booth can be field-interoperated on-site, demonstrating the achievements and technical strength of FOUNDATION fieldbus.
2, LonWorks
LonWorks is another powerful fieldbus technology, which was launched by Ecelon in the United States and was jointly promoted by Motorola and Toshiba in 1990. It adopts all seven layers of communication protocol of ISO/OSI model, adopts object-oriented design method, and simplifies network communication design to parameter setting through network variables. Its communication speed ranges from 300bps to 15Mbps, and direct communication distance can reach 2700m. (78kbps, twisted pair), supports a variety of communication media such as twisted pair, coaxial cable, optical fiber, RF, infrared, power cable, etc., and developed the corresponding intrinsically safe explosion-proof products, known as the universal control network.
The LonTalk protocol used by LonWorks technology is packaged in a chip called Neuron and implemented. There are three 8-bit CPUs in the integrated chip; one is used to complete the functions of Layers 1 to 2 in the Open Interconnect Model, called the Media Access Control Processor, which implements control and processing of media access; the second is used to complete the 3 to 6 layers of functions, called network processors, addressing, processing, background diagnostics, function path selection, software metering, network management, and network communication control, sending and receiving data packets, etc.; The application processor executes the operating system services and user code. The chip also has a storage information buffer to implement information transfer between the CPUs and acts as a network buffer and an application buffer. For example, Motorola's neuron integrated chip MC143120E2 contains 2KRAM and 2K EEPROM.
The continuous promotion of LonWorks technology has contributed to the low cost of neuron chips (about 5 to 9 US dollars per piece), and the low cost of the chip has promoted the promotion and application of LonWorks technology, forming a good cycle, according to Ecelon. According to the data, by July 1996, 5 million neuron chips had been produced. LonWorks' technology strategy is to encourage OEM developers to use LonWorks technology and neuron chips to develop their own applications. It is said that more than 2,600 companies have been involved in LonWorks technology to varying degrees: more than 1,000 companies have launched LonWorks products, and further organized the LonWark Interoperability Association to develop and promote LonWorks technology and products. It is widely used in building automation, home automation, security systems, office equipment, transportation equipment, industrial process control and other industries. To support the interconnection and interoperability between LonWorks and other protocols and networks, the company is developing various gateways to interconnect LonWorks with Ethernet, FF, Modbus, DeviceNet, Profibus, Serplex, etc. as systems.
In addition, LonWorks neuron chips have unique advantages in developing intelligent communication interfaces and smart sensors. LonWorks technology has been designated by the American Society of Heating Engineers ASRE as a standard for the Building Automation Protocol BACnet. According to the news just received, the Consumer Electronics Manufacturers Association of the United States has passed a resolution to develop the EIA-709 standard based on LonWorks technology.
In this way, LonWorks has established a complete set of development, manufacturing, promotion and application architectures from protocol development, chip design, chip manufacturing, control module development and manufacturing, OEM control products, final control products, distribution, system integration, etc. It has attracted tens of thousands of companies to participate in this work, which has greatly promoted the promotion and application of a technology.
3, Profibus
Profibus is a fieldbus as the German national standard DIN 19245 and the European standard prEN50170. The ISO/OSI model is also its reference model. The Profibus series is composed of Profibus -Dp, Profibus -FMS and Profibus-PA. The DP type is used for high-speed transmission between distributed peripherals and is suitable for applications in the field of processing automation. FMS means on-site information specification for general automation such as textiles, building automation, programmable controllers, low-voltage switches, etc., while PA type is a bus type for process automation that complies with the IEC1158-2 standard. The technology was jointly launched by more than a dozen German companies and research institutes led by Siemens. It adopts the physical layer and data link layer of the OSI model. The two parts form a subset of the first part of the standard. The DP type hides the 3-7 layers, and the direct data connection fitting is added as the user interface. The FMS type only hides the third to sixth layers, and uses the application layer as the second part of the standard. The PA type standard is still in the process of development, and its transmission technology complies with IEC1158-2 #p#page title#e#
(1) Standard, which can realize bus power supply and intrinsically safe explosion-proof.
Porfibus supports several transmission modes such as master-slave system, pure master station system, multi-master multi-slave hybrid system. The master station has control over the bus and can actively send information. For the multi-master system, the master station uses the token method to transfer information, and the site that obtains the token can have the bus control right within a predetermined time, and the token is preset in the main station for one week. The longest time. According to Profibus's communication specification, tokens are transmitted in the upstream direction in the order of address numbers between the primary stations. When the master station gains control, it can send or request information to the slave station in a master-slave manner to achieve peer-to-peer communication. The primary station can broadcast to all stations (no answer required) or selectively broadcast to a group of sites.
Profibus has a transmission rate of 96 to 12 kbps and a maximum transmission distance of 1000 m at 12 kbps and 400 m at 15 Mbps, which can be extended to 10 km with repeaters. The transmission medium can be twisted pair or optical cable, and can be attached at most.
127 sites.
4, CAN
CAN is the abbreviation of ControlAreaNetwork, which was first introduced by BOSCH in Germany, and is used for data communication between internal measurement and execution components of automobiles. Its bus specification has been developed into an international standard by the ISO International Standards Organization and has been supported by Motorola, Intel, Philips, Siemens, NEC and other companies. It has been widely used in the field of discrete control.
The CAN protocol is also based on the open system interconnection model of the International Standards Organization. However, its model structure has only three layers, and only the physical layer, the data link layer and the top-layer application layer of the OSI underlying layer are taken. The signal transmission medium is twisted pair, the communication speed is up to 1Mbps/40m, the direct transmission distance is up to 10km/kbps, and the maximum number of attachable devices is up to 110. The signal transmission of CAN adopts a short frame structure, and the number of valid bytes per frame is eight, so the transmission time is short and the probability of interference is low. When the node is seriously wrong, it has the function of automatically shutting down to cut off the connection between the node and the bus, so that other nodes on the bus and their communication are not affected, and have strong anti-interference ability.
CAN supports multi-master mode operation. Any node on the network actively sends information to other nodes at any time, and supports point-to-point, point-to-multipoint and global broadcast mode to receive/send data. It uses bus arbitration technology. When several nodes transmit information on the network at the same time, the node with higher priority can continue to transmit data, while the node with lower priority actively stops sending, thus avoiding bus conflict. A number of companies have developed and produced communication chips that comply with the CAN protocol, such as 82 52 of Intel Corporation, MC68HC05X4 of Motorola, and 82C2 50 of Philips. There is also a CAN bus interface card inserted in the PC, which has the advantages of simple interface, convenient programming and cheap development system.
5, HART
HART is an abbreviation of HighwayAddressableRemoteTransduer. Originally developed by Rosemout and supported by more than 80 famous instrument companies, the HART Communication Foundation was established in 1993. This open communication protocol, called the Addressable Remote Sensing High-Speed ​​Channel, is characterized by the realization of digital communication on existing analog signal transmission lines, which is a transitional product of industrial process control in the transition from analog systems to digital systems. The current transition period has strong market competitiveness and has achieved good development.
The HART communication model consists of three layers: the physical layer, the data link layer, and the application layer. The physical layer uses FSK (Frequency Shift Keying) technology to superimpose a frequency signal on the 4-20 mA analog signal. The frequency signal adopts the Bell202 international standard; the data transmission rate is 1200 bps, the logic "0" signal frequency is 2200 Hz, and the logic "1" signal The transmission frequency is 1200 Hz. The data link layer is used to establish the HART information format in accordance with the HART communication protocol rules. The information composition includes a start code, a display terminal and field device address, a number of bytes, a field device status and communication status, data, parity, and the like. Its data byte structure is 1 start bit, 8 data bits, 1 parity bit, and 1 stop bit. The role of the application layer is to put the HART instruction into effect, that is, to convert the communication state into the corresponding information. It specifies a series of commands; works by command. It has three types of commands, the first is called a generic command, which is the command that all devices understand and execute; the second is called the general behavior command, which provides functions that can be implemented in many, if not all, of the field devices. Such commands include the library of functions of the most commonly used field devices; the third category is called special device commands to implement special functions in some devices, which can be used both in the Foundation and in development. The order of the company is unique. These three types of commands can usually be found in one field device. HART supports point-to-point master-slave response and multicast mode. The data update rate when working in response mode is 2 to 3 times/s, and the data update rate when working in broadcast mode is 3 to 4 times/s. It can also support two communication master devices. There are up to 15 devices on the bus, each field device can have 256 variables, and each message can contain up to 4 variables. The maximum transmission distance is 3000m, and HART adopts the unified device description language DDL. Field device developers use this standard language to describe device characteristics. The HART Foundation is responsible for registering and managing these device descriptions and compiling them as device description dictionaries. The master device uses DDL technology to understand the device's characteristic parameters without having to A dedicated interface for device development. However, due to this analog-digital mixed-signal system, it is difficult to develop a communication interface chip that can meet the requirements of various companies. HART can be powered by the bus to meet the requirements of intrinsically safe explosion protection.
6, RS-485
Although RS-485 cannot be called a fieldbus, as the originator of fieldbus, there are many devices that continue to use this communication protocol. The use of RS-485 communication has the advantages of simple equipment and low cost, and still has certain vitality. The RS-485-based OPTO-22 command set has also been widely used in many systems.
IV. Fieldbus technology prospects and development trends The development of fieldbus technology has become a focus of attention in the field of industrial automation. The research and development of fieldbus in the world has enabled the measurement and control system to break through the long-term closed system and embark on open development. The journey, which is an excellent opportunity for the development of China's fieldbus control system, is also a serious challenge. Fieldbus technology is the intersection and integration of control, computer and communication technologies. The content involved is very extensive. The author believes that we should seize the opportunity to do a good job in the research and development of China's fieldbus technology and products. The networking of automation systems is a major trend in development. Fieldbus technology is deeply influenced by computer network technology. Nowadays, network technology is changing with each passing day, and the development is very rapid. Some new network technologies with significant influence will be further integrated into the fieldbus technology. These fieldbus technologies with promising prospects include: software and hardware technologies for smart meter and network equipment development; State of the art, including network topology, network equipment, network segment interconnection, etc.; network management technology, including network management software, network data operation and transmission; human-machine interface, software technology; fieldbus system integration technology. Fieldbus belongs to the technology that is still developing. China has just started in this technology field. It understands the current situation and development trend of this technology in the world. It has undoubtedly has the development of related industries in China and the updating of automation technology and equipment. Important role. Overall, automation systems and equipment will move in the direction of the fieldbus architecture, and this trend is positive. Since it is a bus, it has to be developed in a direction that is open and unified, and has become a standard specification that everyone follows. However, because this technology involves a wide range of applications, it covers almost all continuous and discrete industrial fields, such as process automation. Manufacturing process automation, building semi-automation, home automation, and more. There are many different fields and needs in different worlds. A fieldbus system may not only accommodate a single standard. In addition, it can be seen from the above introduction that several major technologies have their own characteristics and have formed their own advantages in different application fields. Coupled with the driving of commercial interests, they are each seeking to develop in a very fierce market competition. There is reason to believe that in the next 10 years from now, several bus standards may coexist, and even within a fieldbus system, several bus standard devices realize information sharing through routing gateway interconnection. In the field of continuous process automation, in the next 10 years, FF Foundation fieldbus will become the mainstream development trend, LonWorks will become a strong competitor, HART as a transitional product can also have a certain market. These three technologies are based on the industrial needs of this field. The various functions of the user layer are designed for professional continuous processes, and fully consider the use environment of continuous industry, such as supporting bus power supply, which can meet the requirements of intrinsic safety explosion protection. . In addition, the FF Foundation has concentrated almost all of the world's major automated instrument manufacturers; LonWorks has formed a comprehensive division of labor cooperation system. These factors are critical to becoming the mainstream technology in this field. Because HART is built on the currently widely used analog system, it can fully take into account the benefits of existing equipment and existing investments, and technically consider the needs of the continuous process environment. At present, it has a certain market share, and its technology itself is constantly improving and updating, such as increasing the transmission rate. At present, the market share of foreign HART instruments is still growing, showing an upward trend, but it is a transitional product after all, and its life cycle will not be very long. In China, since many projects are new projects, there are fewer considerations for compatibility, and there are more considerations for advanced nature. It is believed that HART's domestic market share will not be large. There will be big differences between the domestic market and foreign markets. On the one hand, the products that are dominant in the foreign market will continue to penetrate into the country; on the other hand, because the scale of domestic manufacturers is relatively small, the research and development capabilities are poor, and more dependent on the support of technology suppliers, it is more susceptible to fieldbus technology. The impact of suppliers (chip manufacturers, etc.) on domestic support and marketing efforts. At present, only LonWorks technology in the country has substantial market activities, so most domestic manufacturers will first accept LonWorks technology. Although FF is known as the future standard of the instrumentation industry, market share will be greatly affected by the lack of clear market strategies and aggressive market activities in the country. Moreover, the facts show that all Fieldbus Foundation (FF) members are developing applications that use LonWorks technology while developing FF-compliant standards. This shows that the life of LonWorks technology is very tenacious. In the field of discrete manufacturing and processing, due to the characteristics of industry applications and historical reasons, there will be some differences in mainstream technology. Profibus and CAN are highly competitive in this area. They have formed their own advantages in this field.
LonWorks has unique advantages in building automation, home automation, and intelligent communications products. Due to the characteristics of LonWorks technology, there will be greater development in the application of diverse control systems.
The rise of fieldbus technology has opened up a new world of factory underlying networks. It will promote the rapid development of enterprise networks, bring new benefits to enterprises, and thus will be widely used and promote the development of automation related industries.
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