On the hottest Fieldbus

2022-08-13
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Introduction it is only in recent years that fieldbus has emerged in the field of control. However, its coming trend is fierce and has a growing trend. So, what is fieldbus? What is fieldbus control system? What are the characteristics and advantages of fieldbus control system compared with the distributed control system (DCS), which is still dominant in the industrial control industry? Can we adopt the fieldbus standard before it is unified? If you want to use more than 40 kinds of fieldbus, which one should you choose

this paper will discuss the above problems, and discuss the causes of Fieldbus from the perspective of the development history of control system; The concept, architecture and characteristics of fieldbus are described, and the development status of fieldbus is given; Put forward the countermeasures that should be taken in the face of the current situation of the contending development of the products industry, such as plastic parts, plastic film, plastic building materials, plastic artificial leather, synthetic leather and plastic braiding; The development and application method of fieldbus is illustrated with specific examples

second, the emergence of Fieldbus

throughout the development history of the control system, it is not difficult to find that the introduction of each new generation of control system is a solution to the defects of the old generation of control system, and ultimately occupy the dominant position in the market driven by the two external factors of user demand and market competition. The emergence of fieldbus and fieldbus control system is no exception

1. Analog instrument control system

analog instrument control system dominated in the 1960s and 1970s. Its obvious disadvantage is that the analog signal accuracy is low and easy to be disturbed

2. Centralized digital control system

centralized digital control system dominated in the 1970s and 1980s. Single chip microcomputer, PLC, SLC or microcomputer are used as the controller, and the internal transmission of the controller is digital signal. Therefore, the defect of low accuracy of analog signal in the analog instrument control system is overcome, and the anti-interference ability of the system is improved. The advantage of centralized digital control system is that it is easy to calculate and judge the control according to the overall situation, and it can be uniformly scheduled and arranged in the selection of control mode and control machine time; The disadvantage is that the controller itself requires high processing capacity and high reliability. When the system task increases, the efficiency and reliability of the controller will decline sharply

3. Distributed control system (DCS)

distributed control system (DCS) dominated in the 1980s and 1990s. Its core idea is centralized management and decentralized control, that is, management and control are separated, the upper computer is used for centralized monitoring and management function, several lower computers are decentralized to the site to realize distributed control, and the upper and lower computers are interconnected with control networks to realize mutual information transmission. Therefore, this distributed control system architecture effectively overcomes the defects of the centralized digital control system that requires high processing capacity and reliability of the controller. In the distributed control system, the realization of distributed control idea benefits from the development and application of network technology. Unfortunately, different DCS manufacturers adopt their own special closed forms for their control communication networks in order to achieve the purpose of monopoly management. It is difficult to realize network interconnection and information sharing between DCS systems of different manufacturers and between DCs and upper intranet and Internet information networks, Therefore, from this point of view, the essence of DCS is a closed, dedicated, non interoperable distributed control system, and DCS is expensive. In this case, users put forward the urgent requirements of openness and cost reduction for the network control system

4. Fieldbus control system (FCS)

FCS is born in line with the above trend. It uses fieldbus, an open and interoperable network, to interconnect all controllers and instruments on the site, forming a fieldbus control system. At the same time, the control function is completely decentralized to the site, reducing the installation cost and maintenance cost. Therefore, FCS is essentially an open, interoperable and completely decentralized distributed control system, which is expected to become the mainstream product of control system in the 21st century

III. fieldbus and fieldbus control system

1. The concept of Fieldbus

fieldbus is a real-time control communication network that interconnects the field controller at the bottom of automation and field intelligent instrument equipment, and follows all or part of the communication protocol of ISO's OSI open system interconnection reference model. FCS is a real-time network control system that uses an open fieldbus control communication network to interconnect the field controller at the bottom of automation and the field intelligent instrument equipment

2. The difference between fieldbus and local area

(1) according to the functional comparison, fieldbus connects the field controller at the bottom of automation and the field intelligent instrument equipment, and the online transmission is a small batch of data information, such as detection information, status information, control information, etc, The transmission rate is low, but the real-time performance is high. In short, fieldbus is a real-time control network. The local area is used to connect computers in the local area. A large number of digital information, such as text, sound, image, etc., are transmitted online. The transmission rate is high, but real-time is not required. In this sense, local area is a high-speed information network

(2) according to the implementation method, the fieldbus can adopt various communication media, such as twisted pair, power line, optical fiber, wireless, infrared, etc., and the implementation cost is low. The local area needs special cables, such as shaft cables, optical fibers, etc., and the implementation cost is high

3. Fieldbus control system architecture

as shown in Figure 1, the lowest level of Infranet control is FCS, and each controller node is decentralized to the site to form a complete distributed control architecture. The network topology is arbitrary, which can be bus shaped, star shaped, ring shaped, etc. the communication medium is unlimited, and can be twisted pair, power line, wireless, infrared and other forms. The intranet control formed by FCS is easy to interconnect with intranet and internet global information, forming a complete three-level enterprise network architecture

4. Characteristics of fieldbus control system

(1) openness and interoperability

openness means that FCS will break the monopoly of large DCS manufacturers and bring equal competition opportunities to the development of small and medium-sized enterprises. Interoperability realizes the "plug and play" function of control products, so that users have more choices for industrial control products from different manufacturers

(2) complete decentralization

complete decentralization means that the system has high reliability and flexibility, and the system is easy to reorganize and expand, and easy to maintain

(3) low cost

measure the overall cost of a control system, not only considering its cost, but also considering the total investment in the whole life cycle of the system from installation and commissioning to operation and maintenance. Compared with DCS, FCS' open architecture and OEM technology will greatly shorten the development cycle and reduce the development cost, and the completely decentralized distributed structure will change the 1-to-1 analog signal transmission mode into 1-to-N digital signal transmission mode, saving a lot of a/d, d/a conversion devices, wiring installation costs and maintenance costs in the process of analog signal transmission. Therefore, on the whole, the cost of FCS is much lower than that of DCS

it can be said that openness, decentralization and low cost are the three most remarkable characteristics of fieldbus. Its emergence will bring epoch-making changes to the traditional automatic control system. The depth and breadth of this change will exceed any change in history, and will surely usher in a new era of automatic control

IV. development status of Fieldbus

fieldbus is developing rapidly, and now it is in the stage of a group of heroes rising together and a hundred schools of thought contending. At present, more than 40 kinds of fieldbuses have been developed. For example, although the company achieved a revenue of 21.65 billion yuan in 2018, INTERBUS, BITBUS, DeviceNet, MODBUS, ARCNET, p-net, FIP, ISP, etc., among which the most influential are FF, profitbus, Hart, can and LonWorks (see table 1 for performance comparison)

1, FF (Foundation Fieldbus fieldbus) was launched by the American Instrument Association (ISA) 1994, The representative companies are Honeywell and Fisher Rosemount, which are mainly used for instruments in petrochemical and continuous industrial process control. The feature of FF is that its communication protocol adds a user layer on the OSI physical layer, data link

layer and application layer of ISO, and realizes interoperability through object dictionary OD (object dictionary) and Device Description Language DDL (device description language). At present, FF based fieldbus products include pressure and temperature transmitters produced by SMAR company in the United States, processlogix system launched by honeywell&rockweill, and PlantWeb launched by Fisher Rosemount

2. PROFIBUS (process fieldbus) was introduced by Siemens in Germany in 1987 and is mainly used in PLC. There are three types of products: FMS is used for communication between master stations; DP is used for communication between slave stations in manufacturing industry; PA is used for communication between slave stations in the process industry. As the fieldbus products developed and produced by PROFIBUS were developed as early as ten years ago, limited to the level of computer network at that time, most of them were based on the IT network standard. With the continuous expansion of application fields and the increasing requirements of users, the fieldbus products can only be partially modified and supplemented on the original it protocol framework, resulting in the addition of many conversion units (such as various couplers) in the control system, This brings certain limitations to the further development of the product in the future

3. Hart (highway addressable remote transducer data path) was introduced by Rosemount company in the United States in 1989 and is mainly used in intelligent transmitters. Hart is a transitional standard. It transmits digital signals by superimposing sine waves of different frequencies (2200Hz table "0", 1200Hz table "1") on the Ma power signal line, thus ensuring the compatibility between digital systems and traditional analog systems. Its life cycle is expected to be the last 20 years

4. Can (controller area network) was launched by Bosch 6 company in Germany in 1993 and is applied to automobile monitoring, switching value control, manufacturing, etc. The media access method is non-destructive bit arbitration, which is suitable for small networks with high real-time requirements, and the development tools are cheap. Motorola, Intel and Philips all produce independent can chips and 80C51 chips with can interfaces. Can bus products include DeviceNet of AB company and Adam data acquisition products of Taiwan Advantech

5. LonWorks (lon local operating system) was launched by echelon company in the United States in 1991 and is mainly used in building automation, industrial automation and power industry. All 7-layer protocols of lontalk adopt P-P CSMA (predictive p-persistent carrier sense multiplexing) as the medium access mode, and adopt the network logical address addressing mode. The priority mechanism ensures that if the power consumption of communication is too high, the loss is not small for a long time, and the real-time performance is not small. The security mechanism adopts the confirmation mode, so it can build a large network control system. The neuron neuron chip launched by echelon company is essentially a network microcontroller. The powerful network communication processing function of the chip, coupled with the object-oriented network communication mode, greatly reduces the time and cost of developers in constructing application network communication, and can concentrate on the application layer they are good at to compile control strategies, Therefore, many experts in the industry believe that LonWorks bus is a promising field bus. The bus products based on LonWorks include flexnet&flexlink of American action company

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