Fieldbus is a digital, two-way transmission and multi-branch communication network connecting intelligent field devices and automation systems. It is the intersection of process technology, automation instrumentation technology and computer network technology. It will bring control system. A major change.
1 Introduction With the development of technologies such as computer, control, communication, and network, FieldBus technology, which is a typical representative of digital, intelligent, and networked industrial control, has also developed rapidly and has a great impact, causing the engineering and technology community. Universal interest and attention, the computer control system gradually moved from the distributed control system (Distributed Control System dcs) to the fieldbus-based distributed fieldbus control system (FCS), which is regarded as a revolution in the field of industrial automation. New technology. Fieldbus is one of the hotspots in the development of technology in the field of automation today.
2 Computer LAN, known as the field of automation
2.1 Fieldbus and its characteristics (1) What is the field bus?
According to the definition of the International Electrotechnical Commission (IEC) and the American Instrument Association (ISA): Fieldbus is a digital, two-way transmission, multi-branch communication network connecting intelligent field devices and automation systems. Its key symbol is to support two-way multi-node. The bus-type all-digital communication has the characteristics of high reliability, good stability, strong anti-interference ability, fast communication speed, system security, low cost and low maintenance cost.
The SP50 committee of the International Electrotechnical Commission (IEC) has the following three requirements for fieldbus:
(1) The process control unit (PCU), plc, etc. are interconnected with the digital 1/0 device on the same data link;
(2) The field bus controller can access data of multiple operating stations, sensors and actuators on the bus;
(3) Communication media installation costs are low.
The two fieldbus architecture models proposed by the SP50 committee are:
â—Star-type bus replaces analog signal transmission line with short-distance, low-cost, low-rate cable. â— Bus-type bus data transmission distance is long, high rate, using point-to-point, point-to-multipoint and broadcast communication
2.2 Fieldbus technical characteristics Fieldbus completely realizes the integration of control technology, computer technology and communication technology, and has the following technical features.
(1) Field devices have become digital devices with microprocessor as the core, and are connected to each other through a transmission medium (dual line, coaxial cable or optical fiber) in a bus topology;
(2) Network data communication uses baseband transmission (ie digital data digital transmission), and the data transmission rate is high (Mbit/s or 10 Mbit/s level), with good real-time performance and strong anti-interference ability;
(3) exhausting the I/O control station in the distributed control system (DCS), assigning this level of function to the communication network to complete;
(4) Decentralized functional modules to facilitate system maintenance, management and expansion, and improve reliability;
(5) An open interconnection structure, which can be connected to the same layer network, or connected to the control level network or the management information level network through the network interconnection device;
(6) Interoperability, under the premise of observing the same communication protocol, the field device products of different manufacturers can be uniformly configured to form the required network.
3 Fieldbus control technology
3.1 Fieldbus Control System Fieldbus Control System FCS is a state-of-the-art computer control technology that integrates today's computer technology, network technology and control technology. It is a fully distributed, all-digital, fully open control system. It is suitable for industrial process control, manufacturing and building automation, and will become the mainstream of modern computer control systems.
3.2 The impact of FCS on computer control systems 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 FCS-based control system completely overcomes these shortcomings.
(1) FCS relies on fieldbus technology, 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, only one cable (two Line or four lines can connect all the substations in the field, so that the on-site signal can be 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 very 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 IEC11314 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 high. 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.
(4) 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, data report configuration, history library configuration, graphic configuration, control algorithm configuration, data report configuration, real-time data display, historical data display, graphic display, parameter list, data printing. Output, data input and parameter modification, control operation adjustment, alarm processing, fault handling, communication control and human-machine interface, etc., and truly achieve control concentration, dangerous dispersion, data sharing, completely open control requirements. #p#分页头#e#
3.3 Eight kinds of buses and their characteristics stipulated by IEC61158 international standard (1) Profibus is mainly supported by Siemens of Germany and is the German national standard for fieldbus according to the ISO/OSI reference model. Profibus consists of three parts, Profibus-FMS, Profibus-DP and Profibus-PA. Among them, FMS is mainly used for the transmission of non-control information, PA is mainly used for signal acquisition and control of process automation. Profibus-DP is the protocol content of the main application of manufacturing automation, and is the best solution to meet the fast communication of users. The transmission speed is 12Mb/s. The time to scan 1000 I/O points is less than 1ms.
(2) FOUNDATION fieldbus FF (H1, IEC Technical Report) is designed for process automation. It is connected to the field device by digital, serial, and bidirectional communication methods. Instead of a simple digital 4-20 mA signal, FF communication uses a complex communication protocol that connects to field smart devices that can perform simple closed-loop algorithms such as PID. A communication segment can be configured with 32 field devices with a communication speed of 31.25 kb/s and a maximum communication distance of 1900 m per segment.
(3) Interbus is mainly supported by Phoenix Contact, Germany, and its network is simple and easy to install and configure. The nodes on the network are similar to simple shift registers, and the bit stream is flipped through the node. Interbus-S starts the network and is automatically configured and does not require a node address. Use a cable that complies with IEEE802.5. There can be up to 64 devices on a network with a speed of 500K/s, a maximum distance of 25600m, and a distance of 400m between mobile nodes.
(4) ControlNet is mainly supported by Rockwell Corporation of the United States, with very high real-time performance, between device-level bus (like DeviceNet) and factory-level bus (usually based on Ethernet). Providing bandwidth, real-time interlocking response, peer-to-peer information, and program transfer suitable for I/O control devices on the same communication link, providing deterministic and repetitive functionality for intermittent and continuous process control system applications; Allows multiple controllers to handle I/O control devices, providing multicast between input data and peer-to-peer data, redundancy of communication transmission media and intrinsic security options, flexible network topology selection (bus, Tree, star) and media transmission media (coaxial cable, fiber optics, etc.).
(5) WorldFIP is mainly supported by Alstom Company of France, and supports dual redundant bus operation mode. The bus can be connected to PLC, I/O field device, controller, hmi system and so on. Through the double redundant bus, it is possible to completely ensure that the control system will not be forced to stop due to control cable damage.
There are also three other types: FF HSE, which is mainly supported by Fisher-Rosemount, swift Net, which is supported by Boeing, and Process by Denmark.
3.4 Several other types of buses widely used in the industrial control field (1) CANBUS (Controller Area Network): It was developed by Bosch in Germany for automotive applications in 1983 to effectively support distributed control and real-time control. The serial communication network belongs to the fieldbus (FieldBus) category. In November 1993, ISO officially promulgated the CAN International Standard for Controller Area Network (ISO11898). The communication medium of CAN bus can be twisted pair, coaxial cable and optical fiber. The communication distance is related to the wave holding rate. The maximum communication distance can reach 10km, and the maximum communication wave holding rate can reach 1Mdps. CAN bus arbitration uses 11-bit (CAN2.0A protocol) and 29-bit (CAN2.0B protocol) identification and non-destructive bit arbitration bus structure mechanism to determine the priority of data blocks and ensure the highest priority point when network nodes conflict. No conflict waiting is required. The CAN structure model takes the Layer 1, 2, and 7 protocols of the ISO/OSI model, namely the physical layer, the data link layer, and the application layer. The CAN bus adopts a multi-master competitive bus structure, which has the characteristics of multi-master operation and distributed arbitration serial bus and broadcast communication. Any node on the CAN bus can actively send information to other nodes on the network at any time without dividing the primary and secondary, so that free communication can be achieved between the nodes. The CAN bus protocol has been certified by the International Organization for Standardization, and the technology is relatively mature. The controlled chip has been commercialized and cost-effective, and is especially suitable for digital communication between distributed measurement and control systems.
(2) CC-Link is the abbreviation of Control&Communication Link (Control and Communication Link System). It is an open field bus introduced by Mitsubishi Electric in 1996. Its data capacity is large, communication speed can be selected in multiple stages, and it is a compound. The open, adaptable network system can be applied to different management networks to different areas of the lower sensor layer network. CC-Link is a device-based network. In general, CC-Link's entire layer network can consist of one master station and sixty-four slave stations. CC-Link has high-speed data transmission speed up to 10Mbps, and its underlying communication protocol follows RS-485. 2 CC-Link data communication methods can be divided into two modes: cyclic communication and instantaneous transmission. Information is passed from the primary station to the secondary station, and the information data is split in units of 150 bytes and passed in 150 bytes per batch. If the slave passes to the master or other slave, the maximum amount of information per batch is 34 bytes. Instantaneous transfer needs to be done by dedicated instructions. Instantaneous transmission does not affect the time of cyclic communication.
(3) Leo Works bus: It is a fieldbus technology developed by ECHELON Corporation of the United States and jointly promoted by Motorola and Toshiba. It uses the entire seven-layer protocol structure of the OSI reference model. At the heart of Lon Works technology is the Neuron chip with communication and control functions. The Neuron chip implements the complete Lon Works Lon Talk communication protocol. It has three 8-bit CPUs integrated on it. A CPU completes the functions of the first and second layers of the OSI model, called the media access processor. A CPU is an application processor that runs the operating system and user code. There is also a CPU for the network processor, as the intermediary of the former two, it performs network variable addressing, update, path selection, network communication management and so on. Peer-to-peer communication can be performed between nodes composed of neural chips. Lon Works supports multiple physical media and supports multiple topologies with flexible networking. Lon Works's application range mainly includes building automation, industrial control, etc., and has superior performance in building distributed monitoring networks.
3.5 Communication standards for fieldbus control systems
The FCS adopts the first, second, and seventh layers of the ISO OSI model 7-layer architecture, namely, the physical layer (Physical Layer), the link layer (Data Link Layer), the application layer (Application Layer), and an eighth layer. The User Layer constitutes its bus communication standard. The user layer is a layer added to the fieldbus standard beyond the OSI layer model. It makes the standard more than a communication standard and becomes a system standard, which is the key to FCS openness and interoperability. The user layer defines 29 standard function modules for data processing and composition control algorithms. The number of standard function modules is less than the general DCS, which allows users to define their own algorithm modules. In addition, two tools are defined, namely device description language. DDL (Device Description Language) and object dictionary OD (Object Dictionary), used to register "visible objects" on the network to achieve interoperability. #p#分页头#e#
4 Summary Currently, various forms of fieldbus protocols coexist in the control field. In the field of building automation, Lon works and CAN have certain advantages; in the field of process automation, there are mainly CAN, FOUNDATION fieldbus FF and PROFIBUS protocol. Considering the long-term and arduous nature of unified open fieldbus protocol standards, the exit of traditional DCS will be a gradual process. In a period of time, several fieldbus coexistences will occur, and there will be several heterogeneous network interconnection communications at the same production site. However, it is a general trend to develop a unified standard of common compliance and truly form an open interconnection system.
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