Analysis of Development Trend of Fieldbus

Since its birth in the 70s, fieldbus technology has attracted people’s attention due to its advantages in reducing system cables, simplifying system installation, maintenance, and management, reducing system investment and operating costs, and enhancing system performance and other advantages. The extension of the scope has led to a revolution in the field of automatic control.

First, the development of fieldbus will not be replaced by computer communication technology

In the early days of fieldbus technology, its main function was to connect the programmable logic controller (PLC) at the time in a more concise manner. With the introduction of computer technology into PLC, computer communication technology was introduced into the fieldbus; the enhancement of PLC function puts higher requirements on the fieldbus. The introduction of computer communication technology has greatly enhanced the function of the fieldbus and become the main trend of fieldbus technology development. .

In the development process of the distributed control system (DCS), the local area network (LAN) technology was adopted earlier in the inter-station communication. With the development of electronic technology, the functions of many stations can already be realized in the field, so the communication is gradually extended to the scene.

In the field of process control, many communication protocols have been used. As the local area communication in the field of commercial computers is gradually monopolized by the ethernet, the communication in the upper layer in the field of process control is gradually unified to Ethernet and Fast Ethernet. Due to the rapid development of the Internet, people have a stronger desire to remotely diagnose, maintain, and service through the Internet access control system. Therefore, the TCP/IP protocol also enters the field of process control. In fact, we can now see examples of accessing field instruments via the Internet.

Here we see two trends. The first is that more and more information needs to be sent upwards on the site, and the second is that the computer communication technology is extending downwards. People can not help but ask: Will modern computer communication technologies, including Internet technologies, eventually reach the scene and replace the fieldbus?

We believe that modern computer communication technology has the ability to be extended to the scene, and field bus technology will continue to integrate computer communication technology, but computer communication technology will not replace the field bus. Because the field bus communicates with the general computer in terms of function, requirements and structure.

1, function

The basic function of computer communication is to reliably transmit information. The function of the fieldbus is: 1Transfer information economically, safely and reliably; 2Use the transmitted information correctly; 3Process the information transmitted in time. The economics require the fieldbus to transmit information, solve the problem of power supply of the field device, and require the transmission medium to be cheaper. Safety requires fieldbus to solve the explosion problem. Reliability requires field bus to solve the environmental adaptability problems, including the adaptability of the electromagnetic environment (do not interfere with others during transmission, do not interfere with others), climate and environmental adaptability (to be temperature, waterproof, dustproof), mechanical environment adaptability ( To be shock and vibration resistant).

Proper use of information requires devices from different manufacturers to understand each other's information. This is the interoperability requirement of the fieldbus. Timely processing of information requires that field devices do not pass information back and forth over the network and that information should be processed as much as possible. The requirements for the timely processing of information are mainly directed at high-level fieldbuses and smart meters, but this requirement reflects the development trend of fieldbus technology in the most concentrated way - on-site information processing.

2, requirements

The main requirement for computer communications is fast. The field bus requires not only fast transmission, but also fast response in the field of process control, ie real-time requirements. This "fast" has three meanings.

(1) Fast transmission: It refers to more information transmitted in unit time, usually measured by baud rate. This requirement is consistent with ordinary computer communications.

(2) Short response time: It refers to the time when the instrument transmits the event to the network or the actuator receives the information immediately. This time is determined by four aspects: 1 The ability of the instrument or actuator to control the interruption; 2 The transmission time of the information between the application layer and the physical layer of the communication protocol; 3 The time waiting for the network to be idle; Avoiding the collision of information on the network time. Since this time is a random number for most communication protocols, most communication protocols do not give this parameter. The process control system does not usually require this time to be the shortest, but it requires that the maximum value is known in advance and is less than a certain value.

(3) Short tour time The time required for the system and all communication partners to complete at least one communication. This time can generally be adjusted by the system configuration. For preemptive communication systems that rely solely on priority to solve real-time problems, when high-priority events occur more frequently, low-priority events will not respond for a long time. For such communication protocols, the round-trip time is a random amount. Agnostic. The process control system expects the longest tour time to be predictable and less than a certain value.

Response time and tour time reflect the real-time nature, and real-time and communication protocols are closely related. Fieldbus uses two technologies to achieve real-time performance.

One is to simplify the technology. Simplify the form of the network into a linear form (in fact, it has not become a "net"); the communication model is simplified to only one or two layers; the node information is reduced to only a few bits. After the above simplification, the node's access is very fast. This can also shorten the node access time by greatly increasing the communication transfer speed. At this time, although there is some uncertainty in the node access time of some field buses theoretically, the probability of recurring indefinite events is very low. Some non-critical parts use this fieldbus. Node access is faster, which simplifies management of the system; in this case, the master-slave polling access is used. As long as the size of the network polling is limited, the response can be controlled within a specified time. The use of this technology can greatly reduce the cost of the bus, and most bit-digital switching field buses use this technology.

The other is the use of network management and data link scheduling technology to achieve real-time, which is a very complex technology. It is generally believed that the response of a time-sharing real-time system is predictable, but the resource utilization is low; preemptive real-time system resources have high utilization rate, but often the response is unpredictable. Today's fieldbuses are often managed and scheduled using a combination of the two to achieve a certain balance.

With the continuous development of multimedia computer communication systems, the real-time transmission of voice and images puts new demands on the response time of the network. The requirement for real-time performance of multimedia transmission is several tens of milliseconds. The real-time requirements of process control for the system are several ms to ten ms. Multimedia's requirement for real-time performance is “soft”, that is, as long as it meets the requirements most of the time, it does not matter if it does not respond in time. The requirement of process control for real-time performance is "hard" because it often involves security and must be responded to at any time without uncertainty.

Improving the real-time nature of fieldbus and reducing the uncertainty of response time is an important development trend of fieldbus.

3, structure

The structure of the computer communication system is network-like, and the communication path from one point to another may not be fixed.

Most of the fieldbus structures are linear, although the topology of the fieldbus can be bus, star, ring, loop, etc.; but in most field buses, the communication path from one point to another point is relatively fixed of.

The advantages of the linear structure are: 1 It is easier to solve the network power supply; 2 It is easier to solve the intrinsically safe explosion; 3 It is possible to eliminate several layers related to the path in the communication protocol, which is beneficial to improve the real-time performance.

Obviously, the power load of a fieldbus branch is determined in a line-like configuration, and variations along the bus supply voltage are also predictable. Multiple power supply conditions must occur in the mesh structure, load balancing of each power supply, and voltage drop at each node in the network are all more difficult to predict.

Our current intrinsically safe explosion-proof dominant theory still holds that the cable's distributed inductance and capacitance increase with the length of the cable, so the spark energy generated by electromagnetic induction also increases with the length of the cable. In this case, it is difficult to solve the intrinsically safe explosion problem of the mesh structure.

The current state of the explosion-proof explosion theory has great restrictions on the popularization and application of fieldbus. Because it limits the length of the cable and the amount of load on the bus. The main advantages of fieldbuses have disappeared under these restrictions. Therefore, the field bus requires the development of the explosion-proof theory. At present, all countries are strengthening the field bus intrinsical safety concept (FISCO) theory and strive for breakthroughs.

Overcoming the limitation of the field bus by explosion-proof explosion is the development trend of the field bus in application theory.

The advantages of the fieldbus line structure are described above. Its main disadvantage is that when a bus branch cable breaks, the branch circuit collapses. For a mesh structure, one or two branches are broken, and information can be transmitted through other paths; the performance of the system will decline, but it will not collapse. This issue is one of the main reasons that Fieldbus has not been used in the most critical situations. It is also one of the issues that fieldbus manufacturers are currently looking to solve.

The above situation shows that the fieldbus is not just a communication technology, it is a combination product of communication technology, instrument intelligent technology and automatic control technology. Although not all fieldbuses meet these requirements, these requirements are the goals pursued by fieldbuses for process control. Although we have seen some examples of direct access to field instruments through the Internet, these are some inspection systems that do not have stringent requirements for control and real-time performance. We believe that computer communication technology will not replace fieldbus.

Second, the development trend of domestic field bus

The development trend of domestic fieldbuses is: a variety of fieldbuses are fiercely competing in China, and the focus of competition is application engineering; 2 domestic fieldbus products developed by the company are put into the market; 3 fieldbus application projects in various industries in the country are rapidly developing.

Fieldbus technology has been introduced to China for several years. In the past few years, we mainly learned about learning and publicity, and then started to develop and apply it. As China’s economy is taking off and the market has great potential, the main support companies of all kinds of field buses are optimistic about the Chinese market. They have launched fierce competition in China. The concentrated manifestation of competition is the dispute that caused the fieldbus protocol in China. The focus of the debate is on which field bus is better.

We believe that this kind of argument is beneficial to the promotion of fieldbus in the early days. It encourages more people to understand the fieldbus and prepares for the promotion of the fieldbus in the future. However, it is impossible to figure out which field bus is better through this argument.

For the current debate, our view is:

From the perspective of domestic standardization, we should keep up with the trend of international standardization, increase our efforts in studying and propagating IEC standards, and enable more people to understand the development trend of international field buses.

From the point of view of fieldbus product development, limited funds should be concentrated on limited goals, and it is not appropriate to engage in too many fieldbuses. For a company, which type of bus it has invested in should be maintained; it should not be changed too much.

From the perspective of fieldbus application, we support the promotion and application of various fieldbuses in China. A variety of bus competitions will help reduce the price of products and help accelerate the promotion of field buses in China.

The author believes that each type of field bus has its own scope of application. Within its own scope of application, it is the best. Out of this range, it is not the best. At the same time, fieldbus is a technology that is evolving, and so far there is no fieldbus that can be said to be perfect. Each type of fieldbus is in the process of continuous improvement, and today's problems may be overcome tomorrow.

From the "Ninth Five-Year Plan" period, the Chinese government began investing to support the development of fieldbus. During this period, the National Machinery Industry Bureau, the Ministry of Education, and the Chinese Academy of Sciences organized the research and development of field buses such as HART and FF. Up to now, phased results have been achieved. HART meters have begun batch trials, and FF meters will soon be put into trials. In addition, there are some companies that raise funds to develop field buses. But in general, for the time being it still cannot meet the requirements of a large number of users.

The main difficulties for domestic enterprises to promote fieldbus products are as follows: 1 The products are not yet mature; 2 The development forces (funds and talents) needed for expansion and specification of different specifications are insufficient; 3 The investment for market development is insufficient. Therefore, domestic enterprises should welcome foreign companies to develop markets and promote their applications in China. After the fieldbus market is opened, domestic companies can sell products much more easily.

Now we must use field bus and objectively say we must use foreign products. However, the endless debate about the bus has made it impossible for users to determine which field bus should be used. Therefore, this kind of dispute has now begun to hinder the development of field buses in China.

We believe that the shortest way to promote the development of field bus in China is to help the strength of foreign companies and vigorously promote the application of fieldbus.

For which fieldbus is chosen, we recommend selecting the fieldbus that does reduce the system cost. It is a clear indication of whether the field bus is mature and suitable for the targeted object because it can really reduce the system cost.

Since there is no universal fieldbus now, we recommend selecting different fieldbuses in different parts of the system, ie selecting the most appropriate fieldbus for each part of the system. As mentioned earlier, the bit fieldbus is fairly inexpensive and is very suitable for transferring binary information. Therefore, when there are more switches in the process control system, a bit field bus should be added to the system.

III. Development Trend of Fieldbus Application Engineering

1, the use of technology to play the advantages of fieldbus

Field bus system has many advantages, mainly highlighting which of its advantages

We believe that we must first highlight the advantages of fieldbus in reducing system investment costs and operating costs. This is very important. With this basic idea, there will be a clear direction in the selection of bus types and network design.

However, whether this advantage of fieldbus can be used depends on whether the user uses the fieldbus rationally and gives full play to its potential.

2, different types of fieldbus combinations are more conducive to reducing costs

Table 1 (slightly) lists the simple comparison of different types of fieldbuses. Selecting different buses for different situations can minimize the system cost.

It can be seen from Table 1 that the bit-bus capability is very limited, and it is impossible to use the bit-bus as the main information transmission body of large-scale systems. However, its advantages in terms of cost, speed, etc., cannot be replaced by other high-level buses. Therefore it is often mixed with other buses.
The development of fieldbus is very fast. China has closely followed the world trend in the development and application of fieldbus, and its development speed exceeds expectations. We hope that the government and companies should put more money into this area. This is a promising cause.