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Status and development of fieldbus control system

Abstract: This paper expounds the development direction of Fieldbus in industrial control system, introduces the advantages of fieldbus, such as full digital communication, decentralized control, open interconnection and simple design and installation, and analyzes the application status and Prospect of fieldbus

Keywords: Fieldbus distributed control system industrial control

current situation and Prospect of fieldbus control system

Electric Power Research Institute, State Power Corporation of China Beijing, 100085 China

Abstract: the current situation and Prospect of fieldbus control system (FCS) is expanded in this paper, its advances such as digital communication, distributed control, open interoperability and the simplicity of design and installation are presented. The current application situation and the prospect of FCS are analysed.

key words:field bus, DCS, industrial control

distributed control system (DCS) has been dominant in power plant automation for many years. The development of computer control system has experienced base pneumatic instrument control system, electric unit combined analog instrument control system, centralized digital control system and distributed control system (DCS). In the future, it will develop in the direction of fieldbus control system. Field bus refers to a fully digital, bidirectional and multi station communication system between the input and output of field instruments and digital control system. In recent years, fieldbus technology has become a focus in the field of computer industrial control, because it shows people a new idea of system integration

1. The characteristics of Fieldbus

in traditional DCS, it embodies the idea of centralized management and decentralized control. Its structural system can be described as "three stations and one line", namely engineer station, operator station, i/o station and communication network [1]. The engineer station is responsible for system management, control configuration, system generation and installation; The operator station realizes the control operation of the control system, process status display, historical data collection, alarm status display, trend display, report generation and printing, etc; The on-site i/o station realizes the collection and processing of various signals, the calculation of circuits and the output of control results; The communication network is responsible for providing data communication and liaison between various functional stations. The outstanding feature of fieldbus is that it overcomes the defects in the realization of communication in DCS system by closed dedicated network system, and turns the solution based on closed dedicated network into the solution based on open standardization; At the same time, the DCS distributed control structure, which combines centralization and decentralization, is transformed into a new type of fully distributed structure. The control function is completely decentralized to the site, and the basic control function is realized by the on-site intelligent equipment itself. The characteristics of fieldbus are mainly shown in the following aspects:

(1) digital signals completely replace the 4 ~ 20mA analog signals of traditional DCS, and transmit signals in both directions. A pair of twisted pairs or a cable can usually be connected with multiple devices, so the consumption of cables, terminals, slots and bridges is greatly reduced, and the workload of wiring design and joint calibration is also greatly reduced. At the same time, the communication bus extends to the field sensors, transmitters, controllers and servo mechanisms, and the operators can realize the monitoring, diagnosis, verification and parameter setting of the field equipment by the main control system in the control room, so as to improve the accuracy, monitorability and anti-interference ability of the system, and save the number of hardware and investment

(2) compared with the traditional DCS three-tier structure, the fieldbus has only two levels of structure: field equipment and operation management station. The i/o control station of the traditional DCS is incorporated into the field intelligent equipment, and the i/o module is cancelled. The field instruments are all built-in microprocessors, which carry out sampling, a/d conversion, linearization or correction operation processing, alarm judgment, control algorithm functions, etc, The output result is directly sent to the adjacent regulating valve, which does not need to go through the main control system of the control room at all. It fundamentally changes the structural system of the traditional DCS combining centralization and decentralization, and realizes the complete decentralization of the structure

(3) the bus network system is open. Give the power of system integration to users, who can form systems of different sizes with products from different suppliers according to their own needs and considerations. Therefore, fieldbus is called the open interconnection system in the field of automation. Its openness and interoperability are reflected in that it can replace the field instrument of another manufacturer with the field instrument of different manufacturers. After replacement, the communication can return to normal, and at the same time, it can query the self information of the new field instrument

the topology of Fieldbus usually adopts tree structure [2]. In this form, each field device can only be connected to a point. In most applications, the point can be a junction box or the wiring area of one of the field devices. It uses the same pair of wires to communicate with and supply power to field devices, which can significantly reduce the wiring and installation costs

2. The current fieldbus standards and products

fieldbus technology is a network communication technology born in the late 1980s. After more than ten years of development, several representative fieldbus standards and several series of products have emerged in the world. The more popular ones are:

(1) Foundation Fieldbus

in the process of researching and formulating fieldbus standards, a variety of enterprise groups or organizations have appeared, Through continuous competition, by 1994, there were basically two camps in the world, and the middle was gradually weakened by its influence. One was the ISP agreement led by Fisher Rosemount and jointly formulated by Foxboro, Yokogawa, abb, Siemens and other 80 companies; The other is the World FIP agreement, led by Honeywell and jointly formulated by 150 companies in Europe. The two groups merged in 1994 to establish the Fieldbus Foundation (FF), which is committed to developing an internationally unified fieldbus protocol. The architecture of FF refers to the layer 1, 2 and 7 protocols of iso/osi model, namely the physical layer, data link layer and application layer, and the user layer is added. FF provides two physical standards: H1 and H2. H1 is a low-speed bus for process control, with a rate of 31.25kbps and transmission distances of 200m, 400m, 1200m and 1900m. The transmission rate of H2 can be 1Mbps and 2.5mbps, and its communication distance is 750m and 500m respectively. The physical transmission medium can support twisted pair, coaxial cable and optical fiber, and the protocol conforms to IEC standards

(2) PROFIBUS Fieldbus

it is a fieldbus standard as a German national standard and a European national standard. This technology is jointly launched by more than a dozen German companies and research institutes dominated by Siemens. It adopts the physical layer and data link layer of OSI model. The decentralized peripheral device (DP) type hides the third layer to the seventh layer, and adds direct data connection fitting as the user interface; The field bus information specification (FMS) only hides the third to sixth layers and adopts the application layer. The process automation (PA) standard is still in the process of formulation. The maximum transmission rate is 12mbps, the transmission distance is 100m and 400m, and the transmission medium can be twisted pair or optical cable, which can be connected to 127 stations at most

(3) LonWork (local operating network) fieldbus

it was officially launched by echelon company in the United States in 1990. It adopts all 7-layer protocols of iso/osi model, adopts object-oriented design method, and simplifies the network communication design to parameter setting through network variables. Its maximum transmission rate is 1.5mbps, transmission distance is 2700m, and the transmission medium can be twisted pair, optical cable, radio frequency, infrared line, power line, etc. The lontalk protocol adopted is encapsulated in the neuron chip and contains three 8-bit microprocessors, one for media access control, one for network processing, and one for application processing

(4) control area network (can) control network

was first introduced by Bosch Company in Germany for data communication between internal measurement and execution parts of vehicles. The can structure model adopts the layer 1, 2 and 7 protocols of iso/osi model, namely the physical layer, data link layer and application layer. The maximum communication rate can reach 1Mbps, and the maximum communication distance can reach 10000m. The physical transmission medium can support twisted pair, and up to 110 devices can be connected. The application of can in China "was earlier, and China Huakong technology company developed SDS intelligent distributed system based on CAN protocol."; The internal network of HS2000 system developed by Hollysys company is the application of can [3]

in addition, it is worth mentioning that the addressable remote transducer (Hart) protocol is a fieldbus protocol compatible with 4 ~ 20 mA analog signals and modulated digital signals, which was first introduced by American Rosemount company. Due to the modulation/demodulation mode, its digital communication is a transitional product in the process of the transformation from analog system to digital system, so it has strong competitiveness in the current transitional period and has been developed rapidly

3. The future of Fieldbus

the general line of Fieldbus will become a revolutionary leap in the development of industrial control. In recent years, reports on Fieldbus have emerged in endlessly, of which the focus of attention is on the following two aspects:

(1) whether there can be a unified general line standard of Fieldbus in the world

this hot issue involves why there are several fieldbuses coexisting at present. In addition to the technical competition, the fierce competition in commercial interests is the main reason. Automation manufacturers, which have a great impact on fieldbus, are unwilling to give up their fieldbus because they want to maintain their own status and interests as much as possible. Thousands of nodes of these buses have been installed on the site, which has become a de facto standard. For example, making echelon abandon LonWork means that it will lose its products and customers. In a long period of time in the future, the dispute over fieldbus standards will become more and more intense. Different fieldbuses have different functions and have their own applications. The idea of relying on one Fieldbus to win the world seems unrealistic. Of course, on the other hand, a considerable part of the fieldbus is similar and very close in technology. In the environment of survival of the fittest, similar buses will eventually tend to be unified. Therefore, in the next few years, it is difficult to say whose standard can dominate the fieldbus, but who should be said to become the mainstream bus technology. It can be said that for the future industrial control system, who can command the fieldbus standard, its role and influence in the industrial control industry will be equivalent to the position of Microsoft in the computer field

(2) can fieldbus completely replace the DCS system that is currently popular in the world

fieldbus is still in the development stage. Generally speaking, industrial control system will develop in the direction of field network integrated automation system architecture. Because fieldbus has great advantages, people all think that it is an inevitable trend to replace the traditional DCS system. Of course, in terms of technology, DCs has experienced more than 20 years of development and improvement since Honeywell launched the world's first distributed control system in 1975. Relying on its flexible, convenient and functional configuration functions and reliable network communication capabilities, DCs has been widely used in industrial control. At present, several famous companies leading the development of DCS in the world

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