Some Problems Needing Attention in Application of Fieldbus

1、 Overview

The concept of bus has a long history, but field bus appeared in 1984. Its emergence was not accidental, but due to the needs of users at that time, the competition among global manufacturers and the constraints of national regulations or decrees on safety, health care and environmental protection; It came into being after computer technology, communication technology and control technology developed to a certain extent. Therefore, the demand of users is the power to generate fieldbus, and the above three technologies provide a technical basis for the research and development of fieldbus.

1. What is Fieldbus

Fieldbus is an open, full digital, two-way communication and multi station communication system used between field instruments and control room systems. Fieldbus is only the lowest level control system (infranet) in the enterprise network (intranet).

2. Why do users need fieldbus

Users need an open control system. DCS (Distributed Control System), which appeared in 1975, and PLC (Programmable Logic Controller), which appeared soon after, belong to closed control systems. Each DCS or PLC manufacturer's products have their own hardware and software to form their own product system. Some of them contain their own patented technology and are blocked from each other. Only through the gateway and other interfaces can they communicate with other manufacturers' systems. Because manufacturers, no matter big or small, have their own unique features, so users need an open control system to select products from different companies (transmitters, actuators and control systems) to achieve the best integration.

In order to reduce the cost of the enterprise and improve the product quality to increase the competitiveness of the enterprise, the user has to expand its production scale and strengthen its production process. At the same time, in order to ensure safe production and meet the requirements of environmental protection, the number of detection points and control loops have risen sharply. Take the blast furnace produced by iron and steel enterprises as an example, its smelting volume has reached more than 4000m3, and the number of detection and control points has also exceeded 5000. Such a large number of detection signals converge to the population of DCS, which will inevitably lead to "bottleneck" phenomenon, leading to signal congestion, reducing the safety and reliability of the system. Moreover, such a large number of point-to-point cables not only increase the difficulty of line laying design and the complexity of construction, but also increase the cost of investment and installation, commissioning and maintenance. Therefore, users are in urgent need of field bus and intelligent field instruments, so that 80% of the control functions can be transferred to the field instruments, reducing the pressure of DCS processing information, enhancing the reliability and safety of the system, and reducing the cable and related costs.

3. What are the characteristics of fieldbus

Fieldbus has the following characteristics:

(1) Openness;

(2) Interoperability and interchangeability;

(3) Fully digital, unlike DCS, which requires A/D or D/A transformation

(4) Two way communication, unlike DCS, can only be one-way communication;

(5) Intelligent field instruments.

4. What are the advantages of fieldbus

(1) It can reduce 1/2~2/3 of isolators, terminal cabinets, I/O terminals, I/O cards, I/O files and I/O cabinets, thus saving the space of I/O devices and device rooms.

(2) The accuracy can be improved from ± 0.5% to ± 0.1% due to the elimination of D/A and A/D transformation.

(3) The cable consumption can be reduced, and the cost can be saved by 66% or more.

(4) Advanced and intelligent field instruments, such as multivariable transmitters and actuators, can be generated to make maintenance prediction possible.

(5) Because PID function can be implanted into the transmitter or actuator, the control cycle is greatly shortened. At present, DCS can be adjusted 2-3 times per second to 10-20 times per second, thus improving the regulation performance.

(6) Because the "bottleneck" phenomenon at the host population is eliminated, the security and reliability of the system can be improved; At the same time, it can also make the host free to engage in optimization and other work to improve efficiency.

(7) Simple configuration, simple installation, operation and maintenance.

(8) Users can choose the best to achieve the best integration.

5. Significance of Fieldbus

The appearance of fieldbus has brought another revolution to the control field, but its depth and breadth will exceed any time in history, and will open a new era of automation. FCS (fleldbus control system) will eventually replace the traditional DCS.

6. How many types of international standards are there for fieldbus

Around the battle for the international standard of fieldbus, it lasted for more than 16 years. After repeated hardships and setbacks, after 9 votes, they failed to pass. Finally, through negotiation and compromise, at the end of 1999, IECTC65 (the 65th Standardization Technical Committee in charge of industrial measurement and control) adopted eight types of field buses as the international standard of IEC61158. The 8 types are:

Type 1 1EC Technical Report (i.e. H1 of PP)

Type 2ControlNet (supported by Rockwell, USA)

Type 3Profibus (supported by Siemens Germany)

Type 4P Net (supported by Danish ProcessData)

Type 5PPHSE (i.e. supported by the original 1V I-12Fisher Rosemount company)

Type 6 SwiftNet (supported by Boeing)

Type 7WorldFIP (supported by Alstom, France)

Type 8 Interbus (supported by Phoenix contact in Germany)

In addition, the international standards of fieldbus adopted by IECTCl7B are respectively SDS (Smart Distributed System), ASI (Atuator Sensor Interface) and Device Net. In addition, ISO also has a CAN (Control Area Network) of IS011898. So there are as many as 12 kinds.

7. Current situation

In addition to 12 international standards, there are also European standards. Other countries in the world also have their own national standards, such as ERA in the UK, FINT in Norway, etc. Some large foreign companies have also introduced their own standards, such as CC-LINK in Mitsubishi, Japan, Modb us and Ethernet in Shineider.

At the same time, some international companies and organizations are working intensively on the research and development of fieldbus using Ethernet.

8. Limitations of Fieldbus

Although fieldbus will bring benefits to users, it is still at the lowest level of the enterprise network after all. Although it is beneficial locally, sometimes it cannot bring overall benefits to the enterprise. Therefore, fieldbus must be combined with enterprise management automation (such as ERP). It is also called management and control integration.

2、 Some Problems Needing Attention in Application of Fieldbus

Since fieldbus has many advantages and has been paid more and more attention by people, some of them have started to apply. However, according to the current situation, the following 11 questions are proposed for reference.

1. About international standards

It is well known that the formulation of standards can promote the development of social productivity and protect the interests of manufacturers and users. Reviewing the international standards for signaling in the field of automation can also illustrate this point. For example, the international standard of pneumatic instruments in the 1930s and 1940s is 0.2~1.0kgf/cm2; In the 1960s, electric unit combination instruments began to prevail. At that time, the signal system was quite chaotic, including 0~1 OmA, 0~20 mA, 4-20 mA and 5~25 mA. At that time, the signal system of DDZ-I type electric unit combination instrument in China adopted the German national standard 0-10mA as the national standard of China. However, in 1975, when the international standard was 4~20mA, the national standard of China had to be changed to 4~20mA. Therefore, the DDZ-III type electric unit combination instrument had to be changed to 4~0mA. Although 0~10mA has the advantage of facilitating analog calculation, 4~20mA also has the advantage of being easy to detect when the transmitter signal line breaks. However, once the international standard is determined, the electric unit combination instrument has made great progress.

This standard change has brought great inconvenience to instrument manufacturers, design institutes and users in China, and has also suffered some losses, so the historical experience is worth noting.

At present, there are more than 12 international standards for fieldbus, and there is a trend of increasing. As for countries around the world, there are more standards from major manufacturers, including quite a few rising stars. This phenomenon is not only unprecedented, but also extremely abnormal. The result is that no matter the manufacturer or the user, they are often confused and at a loss when choosing in the face of numerous standards

2. Openness

It is popular for users to adopt an open control system, because there are no large or small factories in the world, and there are many cheap and high-quality products with superior performance and thoughtful service. Users can choose the best ones to achieve the best integration. Because the traditional DCS or PLC is a closed system, and the new generation FCS (Fieldbus Control System) fieldbus control system is an open system, it has been favored by the majority of users since its appearance. However, although various field buses are open to themselves, they are not open to each other.

The coexistence of multiple field buses is obviously against the original intention of developing field buses.

How different are the open fieldbuses from the closed DCS or PLC in the market?

3. Interoperability

One of the main characteristics of fieldbus is interoperability, but there is no interoperability between different fieldbuses. Although they can communicate through the gateway or interoperate through OPC (OLE for Process Control) protocol, such interoperability can only be carried out between their respective hosts, not directly between each other's field instruments. That is to say, field instruments of type A field bus cannot directly interoperate with field instruments of type B field bus, and field instruments of type A field bus must first pass through their own hosts, Then, the OPC protocol is used to reach the host of type B field bus, and then the destination of type B field bus field instrument is reached. vice versa.

This kind of interoperability can only be achieved through the above-mentioned tortuous ways, which obviously cannot meet the real-time requirements of real time control, and therefore has no practical significance.

4. Time certainty

The communication of various field buses is not completely the same. There are mainly two forms, namely master slave and broadcast. Of course, they have their own characteristics. But in terms of time determinacy, there are differences strictly. Of course, for those industrial process objects that change slowly, their differences are not significant, so they can be ignored. But for those special objects, they are very sensitive. For example, power plants have strict requirements for time determinacy, because once an accident occurs, the cause of the failure should be analyzed, It is necessary to determine which process parameter deviates first from the accident report, which is determined from the time when the accident occurred. Because the certainty of the master slave type fieldbus on time is insufficient in the first place, some master slave type fieldbuses are also taking measures to make up for its shortcomings, such as adopting software methods to lock the time on the parameters that deviate first.

5. Redundancy

The field bus 1V Ⅲ and Profibus PA applicable to process control have no redundancy. According to the design idea of the field bus, the low-speed part does not need redundancy, because the field bus has completely dispersed the danger, local faults will not lead to global faults, and the intelligent instrument of the field bus has the function of maintenance pre directed to prevent in advance. On the other hand, if the field instrument fails and the regulating circuit fails, the host can intervene; Once the host fails, the intelligent instrument can form its own regulating loop on the site to achieve independent regulation.

However, for some industries, the parts with special requirements for safety of some objects really need redundancy; Some enterprises, due to differences in national conditions or corporate culture, or enterprises that have had malignant accidents, often put forward redundancy requirements for different procedures. The former, such as the steam drum water level gauge, furnace pressure and other parameters of the power plant, often have redundancy at the measuring points, so two low-speed buses can be used to achieve redundancy. For FF, two transmitters can be used from two HIs to the host from different wiring paths, so as to avoid loss of redundancy due to mechanical damage or other injuries on the same path; For Profibus PA, two transmitters can be connected with redundant Profibus DP through their adapters to achieve redundancy. This, of course, will increase the cost.

6. Demonstration project

Engineering technology is different from scientific research. Scientific research is exploratory, so failure can be allowed; However, engineering technology is practical, and failure is not allowed. Therefore, demonstration projects are very important for engineering design. Of course, we can't copy mechanically in the design. We must learn to digest and absorb the process of improvement through investigation and research according to the national conditions and the actual situation of the project.

Who will do the demonstration project? There are always people who dare to eat "crabs", but in addition to courage, there are also certain backgrounds and conditions. For example, when Baoshan Iron and Steel Co., Ltd. in Shanghai started to build in 1978, No. 1 blast furnace wanted to use DCS, which was only available in 1975. At that time, Nippon Iron and Steel Co., Ltd. in Japan, although it had experience in applying DCS, finally increased its investment and used a complete set of analog instruments as the backup device of DCS. This is because Baosteel is a new large-scale iron and steel enterprise completely built on the flat ground in China, which is in line with international standards, and has strong support from the central and local governments. The 4000m3 blast furnace is also one of the few in the world. With such a model project, DCS was gradually applied to the newly built blast furnace.

In addition, it is also possible to use a gradual approach, first on the periphery or auxiliary facilities, and then on the less critical parts of the host, to take measures to test, and then gradually expand after success, but this will take time.

In a word, the demonstration project is important. Unfortunately, a decent and convincing large demonstration project of fieldbus has not been seen so far.

7. About project cost

Due to different national conditions, domestic users are more sensitive to the price of hardware at present, while they often pay less attention to the cost of software.

According to the reports of foreign pilots, the cost of FCS in hardware may be the same or slightly higher than that of DCS, but the cost of engineering design and engineering installation, commissioning and start-up is much lower, so the overall cost is lower. However, in China, the cost of imported hardware is much higher. Due to the low labor cost in China, the cost of engineering design, management, installation and commissioning is far lower than that in foreign countries. Therefore, when applying fieldbus, its cost can only remain high, which affects its popularization and application.

8. About benefits

According to the reports on the application of fieldbus in China, they are basically small systems, but the benefits are significant. But from the essence of fieldbus, the larger the system is, the more significant the benefits will be. Therefore, the benefits of these reports must be carefully analyzed. Most of the reports are often the reconstruction of old plants. Fieldbus is also mixed with old analog instruments or DCS, and often accompanied by the reconstruction of old plants, process flow The process equipment, raw materials and fuels will also be changed, and the management will also be improved. Therefore, the benefits obtained are actually comprehensive benefits. If all benefits are attributed to the fieldbus, it is unconvincing. In addition, the benefits obtained from the process and management improvement should be the main body of benefits.

9. About the use of advanced technology

The adoption of advanced technology is one of the driving forces for the development of productive forces, and after the adoption of advanced technology, it can indeed create benefits for enterprises, but the adoption of advanced technology requires some necessary preconditions. In the field of automation, there are also many advanced technologies. If there is no advanced process flow, process equipment, good raw material conditions, scientific management level, operation level and maintenance level to match them, even if the most advanced automation technology or equipment is used, it will not work and will only cause waste. The historical experience is worth noting. Since the introduction of DCS in the 1980s, most of them have only played about 30% of the functions.

We know that advanced automation technology is only a means and a tool, and its adoption is not our goal. Our real purpose is to get the maximum output with the least input. Therefore, as an automatic engineering designer, we must not simply be advanced for the sake of advanced, field bus for the sake of field bus.

10. Pay close attention to the progress of field bus and its related technologies

Although there are 12 international standards for fieldbus, it continues to develop or improve. For example, Profibus is constantly improving, and now it has introduced Profinet, Profisafe, etc. Sereos is striving to become an international standard. SDS, however, wants to "fade out of the Jianghu". At present, it is limited to continuing to support and serve 12 major companies in North America (such as Post and Boeing), and it does not want to enter such a large market as China. In today's fierce market competition, not seeking development is tantamount to shrinking and disappearing. The CC Link launched by Mitsubishi Electric in 1997 is going to the international market, while the Ethernet based fieldbus is being developed by some companies or enterprises at home and abroad. Although some have achieved some phased results, Ethernet was only used for office automation in the past, but now it is used for industrial field, and there are still many problems to be solved. If you want to meet the real-time requirements of industrial control, Anti electromagnetic interference, seismic resistance, high temperature resistance, high humidity resistance, dust-proof performance in industrial environment, problems requiring power supply to field instruments, and explosion-proof performance required in hazardous areas. Although there are many problems and difficulties, the development companies are confident and energetic, and have the spirit and trend of "unifying" the field bus and "the world". Others, such as the Internet of heterogeneous field buses and intelligent IlO, are also constantly developing or improving.

The collection, research and analysis of such information are conducive to our work deployment and decision-making.

11. Work hard to be professional and versatile

Fieldbus technology is the integration of control, communication and computer technology, involving three different disciplines; In addition, since DCS and PLC manufacturers, who are familiar with industrial control and production process in some industries, have combined IT (information technology), they have worked hard from the field level (basic level) of enterprise automation system to the process control level and management level, and launched the EIC integration system APC (AdvancProcess Control) advanced process control, management and control integration, total solution and other products, while some computer manufacturing companies and some software companies use their advantages in information technology to infiltrate by every means, and also launched PC based control systems, soft DCS, soft PLC and virtual instruments. The two trends above and below are bound to have an intersection point in the enterprise automation system, which is in a dynamic change in the competition. It is important, but it is not easy, to grasp this intersection point, because you must understand the electric control, instrument control, computer and communication technologies, and be familiar with the process of production objects, the dynamic characteristics of objects and management technologies when applying. After all, one's energy is limited, so it is impossible to master and be familiar with all the above technologies. However, as an excellent automation worker or a backbone of automation technology, one must be able to specialize in one thing and be versatile. On this basis, one should sum up experience and constantly improve. As a leader of a design institute or enterprise, it is necessary to make a plan, strive to cultivate and bring up all kinds of talents with expertise and versatility, and form a team with complete specialties and mutual support.

Therefore, our slogan is to learn, learn and learn again.

3、 Conclusion

To sum up, the conditions for the comprehensive promotion and application of fieldbus are not complete and mature at present, but fieldbus has many advantages, so it must exist and develop. It is expected that in the near future, a unified international standard of fieldbus will inevitably emerge, and the future of fieldbus is bright