1. The definition of fieldbus

Fieldbus is a network for digital communication between measurement and control machines in a factory. That is, the communication between sensors, various operating terminals and controllers and the communication between controllers are specialized network. The original main wiring between these machines is ON/OFF, contact signals and analog signals, through the digitalization of communication, so that time segmentation, multiple, multi-point become possible, so as to achieve high performance, high reliability, maintenance of simple, saving wiring (wiring sharing).

In short, fieldbus is a digital communication to replace the traditional 4-20MA analog signal and ordinary switching signal transmission.

2, the technical characteristics of fieldbus

● Openness of the system

Open system refers to the communication protocol is open, the equipment of different manufacturers can be interconnected and realize the exchange of information, fieldbus pioneer is committed to the establishment of a unified factory bottom network open system. Openness here refers to the consistency and openness of relevant standards, emphasizing the consensus and compliance with standards. An open system that can be connected to any other device or system that complies with the same standards. A fieldbus network system with bus function is open, and the open system gives the right of system integration to the user. Users can organize products from different suppliers into systems of arbitrary size according to their own needs and objects.

● Interoperability and interoperability

Interoperability here refers to the realization of information transmission and communication between interconnected devices and systems, and the implementation of point-to-point and point-to-multipoint digital communication. Interoperability means that devices of similar performance from different manufacturers can be interchanged to achieve interoperability.

● Field equipment intelligence and functional autonomy

It scatters 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 equipment, and the running state of the equipment can be diagnosed at any time.

● Highly decentralized system structure

Since the field equipment itself can complete the basic function of automatic control, fieldbus has formed a new architecture of fully distributed control system. Fundamentally changed the existing DCS centralized and distributed control system system, simplified the system structure, improve the reliability.

● Adaptability to the field environment

Front working equipment at the scene, as the factory network at the bottom of the field bus, is specially designed for working in the field environment, it can support twisted-pair cable, coaxial cable, optical cable, rf, infrared, power lines, etc., with strong anti-interference ability, can use two wire system to realize transmission and communication, and can meet the demands of the nature of explosion-proof.

3, the technical advantages of fieldbus

● Save hardware quantity and investment

Due to the fieldbus system in scattered in front of smart devices can perform a variety of sensors, control, alarm and directly computing functions, thus can reduce the number of transmitter, no longer requires a separate controller, cell, etc., also no longer requires the DCS system of signal disposal, transformation and isolation technique function unit and its complex wiring, Can also use industrial PC as the operation station, thereby saving a large amount of hardware investment, due to the reduction of control equipment, but also reduce the area of the control room.

● Save installation costs

The wiring of the fieldbus system is very simple, because a pair of twisted pairs or a cable can usually be connected to more than one device, so the amount of cable, terminal, slot box, bridge is greatly reduced, and the workload of wire design and joint proofreading is also greatly reduced. When onsite control devices need to be added, you do not need to add new cables, but can connect them to the existing cables nearby, which saves investment and reduces design and installation workload. According to the calculation data of typical test works, the installation cost can be saved by more than 60%.

● Save maintenance costs

Because the field control equipment has the ability of self-diagnosis and simple fault handling, and sends the relevant diagnosis and maintenance information to the control room through digital communication, users can query the operation of all the equipment, diagnosis and maintenance information, so as to analyze the cause of the fault early and quickly eliminate it. The maintenance downtime is shortened, and the maintenance workload is reduced due to the simplified system structure and simple connection.

4. Fieldbus network structure

Star: Each site is connected to an intermediate node.

Loop type: The stations are connected in a closed loop.

Bus: All stations share one transmission link.

Tree: An extended form of the bus structure.

5. Development trend

From the perspective of fieldbus technology itself, there are two obvious trends:

The first is to seek a unified fieldbus standard

Second, Industrial Ethernet to Industrial control network

The development of industrial Ethernet

Unified and open TCP/IP Ethernet is more than 20 years of successful development of network technology, in the past has been believed that Ethernet is developed for IT field applications, IT and industrial network in real-time, environmental adaptability, bus feeding and many other aspects of the requirements of the gap, in the field of industrial automation can only be limited application. With the successful development of FF HSE and the promotion and application of PROFInet, it can be predicted that Ethernet technology will be very quickly into the industrial control system at all levels of the network.

The formation of industrial Ethernet technology on Earth four camps:

Mainly used in discrete manufacturing control systems are:

●Modbus-IDA Industrial Ethernet

●Ethernet/IP Industrial Ethernet

●PROFInet Industrial Ethernet

Mainly used in process control systems are:

●Foundation Fieldbus HSE Industrial Ethernet

6, bus control composition

The fieldbus control system consists of measurement system, control system and management system, and the hardware and software of the communication part are the more characteristic parts.

● Fieldbus control system:

Its software is an important part of the system. The software of the control system includes configuration software, maintenance software, simulation software, equipment software and monitoring software. First choose to develop the configuration software, control and operate the man-machine interface software MMI. Through the configuration software, the connection between function blocks is completed, and the parameters of function blocks are selected to perform network configuration. In the process of network operation, the system real-time data collection, data processing, calculation. Optimization control and logic control alarm, monitoring, display, report, etc.

● Fieldbus measurement system:

It is characterized by multi-variable high performance measurement, so that the measuring instrument has more functions such as computing ability, because of the use of digital signal, with high resolution, high accuracy, anti-interference, anti-distortion ability, but also has the state information of the instrument equipment, can be adjusted to the processing process.

● Equipment management system:

You can provide diagnosis information, management information, device running status information (including smart meters), and device manufacturing information provided by the manufacturer. For example, Fisher-Rosemoune introduced AMS management system, which is installed in the main computer and can complete the management function by it. It can constitute a comprehensive management system information base of field equipment, on which the reliability analysis and predictive maintenance of equipment can be realized. Changing the passive management mode to the predictable management and maintenance mode AMS software is a T-shaped structure based on the field server. It supports modularity on the field server, and the application software with rich functions provides users with a graphical interface.

● Bus system computer service mode:

Client/server mode is a popular network computer service mode. The server represents the data source (provider), and the application client represents the data consumer, which retrieves the data from the data source for further processing. The room machine runs on a PC or workstation. A server runs on a minicomputer or mainframe and uses intelligence, resources, and data from both sides to accomplish tasks.

● Database:

It can organize and dynamically store a large number of data and applications, realize full data sharing, cross access, with a high degree of independence. The parameters of industrial equipment change continuously in the process of operation, the amount of data is large, the operation and control of real-time requirements are very high. Therefore, it has formed a database system that can exchange visits and operate in a distributed relationship and real-time manner. The mature database systems available in the market are Orad, Sybas, Informix and SQL Server. Infoplus, PI, ONSPEC, etc.

● Hardware and software of network system:

Network system hardware: system management host, server, gateway, protocol converter, hub, user computer and other underlying intelligent instruments. Network system software includes network operating software such as NetWarc, LAN Mangger, Vines, and server operating software such as Lenix, OS /2, Window NT. Application software database, communication protocol, network management protocol, etc. [1]

7. Introduction to mainstream fieldbus

● FoundationFieldbus (FF)

The ISP agreement made by Fisher Rousemount and 80 other companies including Yokogawa, ABB, Siemens and Invensys and the WorldFIP agreement made by Honeywell and more than 150 other companies in Europe were merged in September 1994. The bus has been widely used in the field of process automation and has a good development prospect.

The foundation fieldbus uses the open system of the national standard organization ISO to interconnect the simplified OSI model (1,2,7 layers), namely the physical layer, data link layer, application layer, and additionally adds the user layer. FF can be divided into two communication rates: low speed H1 and high speed H2. The former has a transmission rate of 31.25Kbit/ s and a communication distance of up to 1900m. It can support bus power supply and explosion-proof environment. The latter transmission rate of 1Mbit/ s and 2.5Mbit/ s, communication distance of 750M and 500m, support twisted pair, optical cable and wireless transmission, protocol symbol IEC1158-2 standard. The transmission signal of FF's physical medium uses Manchester coding.

●CAN (ControllerAreaNetwork Controller LAN)

It is widely used in the field of discrete control. Its bus specification has been formulated as national standard by ISO, and has been supported by Intel, Motorola, NEC and other companies.

The CAN protocol is divided into two layers: physical layer and data link layer. The signal transmission of CAN adopts short frame structure, short transmission time, with automatic closing function, has strong anti-interference ability. CAN supports multi-master working mode and adopts non-destructive bus arbitration technology to avoid conflicts by setting priorities. The communication distance CAN reach 10KM/5Kbps/s, the communication rate CAN reach 40M /1Mbp/s, and the actual number of network nodes CAN reach 110. At present, many companies have developed communication chips conforming to CAN protocol.

Low Lonworks

It is developed by Echelon Corporation and co-advocated by Motorola and Toshiba Corporation.

It adopts the ISO/OSI model of all seven layers of communication protocol, uses the object-oriented design method, through the network variables to simplify the network communication design into parameter setting. Support twisted pair, coaxial cable, optical cable and infrared and other communication media, communication rate from 300bit/s to 1.5M/s, direct communication distance up to 2700m (78Kbit/s), known as the universal control network. The LonTalk protocol used in Lonworks technology is packaged into the Neuron chip and implemented. Lonworks technology and neuron chip products are widely used in building automation, home automation, security systems, office equipment, transportation, industrial process control and other industries.

Low DeviceNet

DeviceNet is a low-cost communication connection and a simple network solution with open network standards. The direct interconnectivity of DeviceNet not only improves the communication between devices but also provides important device-level positional functions. DebiceNet is based on CAN technology. The transmission rate ranges from 125Kbit/s to 500Kbit/s, and each network has 64 nodes. The communication mode of DebiceNet is as follows: Producer/Consumer. Devices on the DeviceNet network can be freely connected or disconnected without affecting other devices on the network. In addition, the installation and cabling costs of devices on the DeviceNet network are low. The organization structure of DeviceNet bus is Open DeviceNet Vendor Association (ODVA).

Low PROFIBUS

PROFIBUS is the field bus standard of German standard (DIN19245) and European standard (EN50170). It consists of PROFIBUS--DP, PROFIBUS-FMS, and PROFIBUS-PA series. DP is used for high-speed data transmission among distributed peripherals and is suitable for processing automation. FMS is suitable for textile, building automation, programmable controller, low voltage switch and so on. PA is a type of bus used for process automation. It complies with IEC1158-2 standard. PROFIBUS supports several transmission modes such as master-slave system, pure master station system, multi-master multi-slave hybrid system. The transmission rate of PROFIBUS is 9.6kbit /s to 12Mbit/s, and the transmission distance is 1200m at 9.6kbit /s, and 200m at 12Mbit/s. It can be extended to 10km by repeater. The transmission medium is twisted pair cable or optical cable, which can be connected to 127 stations.

Low HART

HART Transducer is short for Highway Addressable Remote Transducer, which was developed by Rosemount company in the early days.

It is characterized by the realization of digital signal communication on the existing analog signal transmission line and belongs to the transition product from analog system to digital system. The communication model adopts three layers: physical layer, data link layer and application layer, and supports point-to-point master-slave reply mode and multipoint broadcast mode. It is difficult to develop a general communication interface chip because it uses a mixture of analog and digital signals. HART can use bus power supply, can meet the requirements of explosion-proof, and can be used by the handheld programmer and management system host as the main equipment of the dual main equipment system.

Low CC - Link

Cc-link, which stands for Control&Communication Link (Control&Communication Link system), was launched in November 1996 by a group of companies led by mitsubishi electric, and it has grown rapidly, gaining a large share in Asia.

In the system