Fieldbus technology and fieldbus control system - Solutions - Huaqiang Electronic Network

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1 Introduction

Before the 1970s, the analog system used analog quantity to convert and transmit the transmission and control signals. The accuracy was poor and the influence of the interference signal was large. Therefore, the control effect and system stability of the whole control system were very poor. At the end of the 1970s, with the advent of large-scale integrated circuits, microprocessor technology has been greatly developed. The microprocessor has strong function, small size, high reliability, and is used to control the system through appropriate interface circuits, and the control effect is improved; however, it is still a centralized control system. With the maturity and development of process control technology, automated instrumentation technology and computer network technology, another technological change has taken place in the control field. This change has made a huge leap in the traditional control system (such as the distributed control system) both in terms of structure and performance. The basis of this change is the generation of fieldbus technology.

Fieldbus is a two-way serial, digital, multi-node communication network that connects field intelligence devices and automation control devices. It is also known as the Field Underlying Device Control Network (INFRANET). Since the 1980s, various fieldbus technologies have begun to appear. People are calling for more and more changes in the traditional analog instrumentation and control systems, making the fieldbus a worldwide wave of technological change. The American Instrument Association (ISA) began to develop fieldbus standards in 1984. In Europe, there are German PROFIBUS and French FIP, and various fieldbus standards are formed. The main ones are: Foundation Fieldbus FF (Foundation Fieldbus), Control Area Network CAN (Controller Area Network), Local Operating Network (LonWorks) (Local Operating Network), Process Field Bus PROFIBUS (Process Field Bus) and HART Protocol (Highway Addressable). Remote Transducer). But no matter what, the development of a single open international fieldbus standard is inevitable.

2. Fieldbus technology and its characteristics

Fieldbus is one of the hotspots in the development of technology in the field of automation today, and is known as the computer LAN in the field of automation. Most of the existing field devices, in order to improve their performance and price ratio, use microprocessors and digital components in their internal operations, thus demanding that digital communication must be realized between digital devices in these fields.

Fieldbus is an all-digital, two-way, multi-station communication system that connects intelligent field devices and automation systems. It mainly solves the digital communication between field devices such as intelligent instrumentation, controllers and actuators in the industrial field and the information transmission between these field control devices and advanced control systems. It integrates various technical means such as digital communication technology, computer technology, automatic control technology, network technology and smart instrument, and fundamentally breaks through the limitations of traditional "point-to-point" analog signal or digital-analog signal control. A fully distributed, fully digital, intelligent, bidirectional, interconnected communication and control system. The fieldbus is a digital, two-way transmission, multi-branch communication network connecting intelligent field devices and automation systems, based on smart meters. The smart meters scattered in various industrial sites are connected by a digital field bus and together with the controllers and monitors in the control room form the fieldbus control system FSC. By following certain international standards, fieldbus products from different vendors can be integrated into the same FCS, with interchangeability and interoperability. The FCS further decentralizes the control functions of the traditional dcs to the on-site smart meters, and the functions of data acquisition, data processing, control operations and data output are performed by the on-site smart meters. The data of the field instrument (including the collected data and the diagnostic data) is transmitted to the control device of the control room through the field bus. The control device of the control room is used to monitor the running status of each field instrument, and save the data uploaded by each smart meter, and complete at the same time. Advanced control features that cannot be completed with a small number of field instruments. In addition, FCS can also be connected to the enterprise's superior management network through the gateway, so that enterprise managers can grasp the first-hand information and provide the basis for decision-making. So the fieldbus has the following outstanding features:

2.1 The open fieldbus control system (FCS) adopts an open communication protocol, and devices of different manufacturers that comply with the same communication standard can interconnect and realize information exchange. Users can flexibly choose fieldbus products from different manufacturers to form an actual control system for optimal system integration.

2.2 Interoperability Interoperability means that control devices from different manufacturers can not only communicate with each other, but also can be configured in a unified manner to achieve the same control strategy and “plug and play”. Devices with the same performance of different manufacturers can be interchanged.

2.3 Flexible network topology The fieldbus control system can form different network topologies according to complex field conditions, such as tree, star, bus and hierarchical network structure.

2.4 Highly decentralized system structure The field device itself is an intelligent device with basic functions of independent automatic control, which fundamentally changes the combination of centralized and decentralized DCS, forming a new distributed control system. The control function is completely dispersed, the reliability of the control system is improved, and the structure of the control system is simplified. After the fieldbus is disconnected from the upper-level network, the independent operation of the underlying device can be maintained, and the intelligence is greatly enhanced.

2.5 Highly intelligent field devices Traditional DCS uses a relatively centralized control station whose control station consists of a CPU unit and an input/output unit. The fieldbus control system completely distributes the functions of the DCS control station to the field control equipment. The basic functions of automatic control can be realized only by the field bus equipment, such as data acquisition and compensation, PID calculation and control, device self-checking and self-diagnosis. And other functions. The operator of the system can remotely monitor, set or adjust the operating parameters of the field devices in the control room, and can locate and diagnose faults by means of the self-diagnosis function of the field devices.

2.6 Highly adaptable to the environment The fieldbus is designed for industrial sites. It can transmit data using twisted pair, coaxial cable, fiber optic cable and wireless. It is highly resistant to interference. Commonly used data transmission lines are inexpensive twisted pairs, and allow on-site use of data communication lines for power supply, and can also meet intrinsic safety explosion-proof requirements.

3. Introduction to the main field bus

3.1 Foundation Fieldbus (Foundation Fieldbus)
FOUNDATION fieldbus is introduced by the Fieldbus Foundation. Unlike proprietary network bus protocols, the FF bus is not affiliated with any enterprise or country. Its bus architecture is a communication model established by reference to the physical layer, data link layer and application layer of the ISO OSI model and adding the user layer. FF has been supported by almost all the famous instrument manufacturers in the world, while complying with the IEC protocol planning, with the IEC fieldbus international standards and draft basics, plus its technical advantages, so there is great hope for the future The main international standard.

The FF bus provides two physical layer standards, H1 and H2. Hl is a low-speed bus for process control with a transmission rate of 31.25Kbps and transmission distances of 200m, 450m, 1200m, and 1900m (plus repeaters can be extended). It can be powered by bus and supports intrinsically safe devices and non-intrinsically safe buses. equipment. H2 is a high-speed bus with a transmission rate of 1 Mbps (the transmission distance is 750 m at this time) or 2.5 bps (the transmission distance is 500 m at this time). H1 and H2 can reach 32 nodes per segment, and 240 can be used after repeaters. Hl and H2 can be interconnected through bridges. The outstanding features of FF are equipment interoperability, improved process data, earlier predictive maintenance and reliable safety.

3.2 Process Fieldbus Profibus
Profibus was proposed by Siemens and strongly advocated. It has become the German national standard DIN19245 and the European standard EN50170. It is an open and independent bus standard, which acts as a communication network in mechanical manufacturing, industrial process control and intelligent buildings. Profibus consists of three series: Profibus-PA, Profibus-DP and Profibus-FMS. Profibus-PA (Process Automation) is a low-speed data transfer for process automation. Its basic features are the same as FF's H1 bus, which provides bus power and intrinsic security, and is supported by application-specific integrated circuits (ASICs) and software. Profibus-DP is compatible with Profibus-PA. Its basic characteristics are the same as FF's H2 bus. It can realize high-speed transmission. It is suitable for high-speed data transmission between distributed external devices and automatic control devices, and connects Profibus-PA and processing automation. Profibus-FMS is suitable for medium-speed data transmission in general automation, mainly for sensors, actuators, electric drives, plc, textiles and building automation. The latter two series use the RS485 communication standard, the transmission rate is from 9.6kbps to 12Mbps, and the transmission distance is from 1200M to 100m (depending on the transmission rate). The basic mode of media access control is the token mode between the master station and the master-slave mode between the master station and the slave station, and a combination of the two methods. Profibus is a relatively mature bus, which is widely used in engineering.

3.3 LonWorks

LonWorks technology is a network control technology implemented on the ISO OSI seven-layer protocol using Neuron Chip technology. It uses object-oriented programming methods to communicate with each other using network variables or explicit messages.

3.4CAN

Controller area network (CAN) was developed by Bosch in Germany for automotive applications in 1983. Its basic design specifications require high bit rates and high immunity to electromagnetic interference. And can detect any errors that are generated. Widely used in the automotive industry as well as in the aviation industry.

3.5 HART bus

The HART protocol is a communication protocol proposed by Rosemount in 1986. It is a protocol for communication between on-site smart meters and control room equipment. It includes the physical layer, data link layer, and application layer of the ISO/OSI model. HART communication can be a bit-point or multi-point connection mode. This protocol is an open communication protocol for addressable remote sensor high-speed channels. It is characterized by digital signal communication on existing analog signal transmission lines, which is a transitional product in the transition from analog systems to digital systems, and thus in the current transition period. With strong market competitiveness, it has a large share in the smart meter market.

4, FCS control system

Traditional computer control systems generally use a DCS structure. In DCS, point-to-point connections to the field signals are required, and the I/O terminals are placed in the control cabinet along with the PLC or automation instrument, rather than on site. This requires a large number of signal transmission cables to be laid, and the wiring is complicated, which is both time consuming and time consuming, the signal is easily attenuated and easily interfered, and it is inconvenient to maintain. The DCS is generally composed of an operator station, a control station, etc., and has a complicated structure and high cost. Moreover, DCS is not an open system, and interoperability is poor, making it difficult to share data. The PC-based FCS completely overcomes these shortcomings.

(1) In FCS, all I/O modules are placed in the industrial field, and all signals are converted to standard digital signals in the field through distributed intelligent I/O modules. All cables can be used with only one cable. The field substations are connected, and the on-site signal is transmitted to the control room monitoring equipment very simply, which reduces the cost and is easy to install and maintain. At the same time, the digital data transmission makes the system have high transmission speed and strong anti-interference. ability.

(2) FCS is open. In FCS, both software and hardware follow the same standards, with good interchangeability and easy replacement. The programming adopts IEC1131-3 five international standard programming languages. The programming and development tools are completely open, and can also utilize PC's rich hardware and software resources.

(3) The efficiency of the system is greatly improved. In the FCS, a PC can simultaneously complete the PLC and NC/CNC tasks that were originally completed with two devices. Under the multitasking Windows NT operating system, the soft PLC in the PC can execute up to a dozen PLC tasks at the same time, which improves efficiency and reduces costs. And the PLC on the PC has in-circuit debugging and simulation functions, which greatly improves the programming environment.

In FCS, the basic structure of the system is: industrial computer or commercial PC, fieldbus master interface card, fieldbus input/output module, PLC or NC/CNC real-time multitasking control software package, configuration software and application software. The main functions of the host computer include system configuration, database configuration, history library configuration, graphic configuration, control algorithm configuration, data report configuration, real-time data display, historical data display, graphic display, parameter list, data printout. , data input and parameter modification, control operation adjustment, alarm processing, fault handling, communication control and human-machine interface, and truly achieve control concentration, dangerous dispersion, data sharing, completely open control requirements.

5. Conclusion

Currently, various forms of fieldbus protocols coexist in the field of control. In the field of building automation, Lonworks and CAN networks have certain advantages; in the field of process automation, there are mainly transitional HART protocols, widely supported FF fieldbus protocols and the same competitive PROFIBUS protocol. The HART protocol will be the main communication protocol for intelligent instrumentation in the past few years; FOUNDATION fieldbus is a promising fieldbus in the field of process automation, supported by many automated instrumentation equipment manufacturers; due to the openness of Lonworks technology, domestic Many developers who have used it to develop control systems have emerged. Considering the long-term and arduous nature of unified open fieldbus protocol standards, the exit of traditional DCS will be a gradual process. With the continuous development and improvement of fieldbus technology, fieldbus control systems will be widely used.