Fieldbus should determine the development strategy of fieldbus in the long run according to the national economic development and market demand, combining the characteristics of fieldbus and China's national conditions. With the birth of a fully open control system based on fieldbus technology, it will revolutionize the traditional control system architecture. Using fieldbus can save the amount of hardware and investment, reduce the number of transmitters, and no longer need a separate controller, calculation unit, etc .; save installation and maintenance costs.
The field bus refers to a data bus that realizes bidirectional serial multi-node digital communication and completes measurement and control tasks between the field device installed in the manufacturing or process area and the automatic control device in the control room. The fieldbus control system is gradually merging electrical control and process control to achieve truly centralized control.
1. Introduction to fieldbus technology
1.1 PROFIBUS (Process Fieldbus) field bus
The PROFIBUS fieldbus protocol led by Siemens meets the requirements of an open bus system, forming a transparent communication system from sensor / actuator, field level, unit level to management level. PROFIBUS is a family. It has three members or three types, namely -FMS, -DP and -PA, the basic features are as follows:
1) PROFIBUS-FMS (Fieldbus MessageSpecification). This part is to complete the communication between the controller and the field devices and the information exchange between the controllers. It is suitable for completing the cyclic and acyclic communication tasks of medium transmission speed for larger data exchange. When using RS485, the communication rate is 9.6k-500k baud, the distance is 1.6km-4.8km, up to 122 nodes can be connected, and the medium can be twisted pair or optical cable.
2) PROFIBUS-DP (Decentralized Pe-riphery distributed peripheral equipment) is an optimized communication module, suitable for the communication between the automatic control system and the peripheral equipment, and the time-critical requirements. RS-485 twisted pair or optical cable, 9.6K-12M baud, the maximum number of stations is 126.
3) PROFIBUS-PA (Process Automa-tion process automation), used in process automation, for occasions with high safety requirements and stations powered by the bus. The communication rate is 31.25kbps, and the maximum distance is ≤1.9km.
1.2 LonWorks (Local Operating Networks) fieldbus
LonWorks technology is a set of standards introduced by Echelon Corporation of the United States in 1991 and is an open distributed control bus technology. At present, products using LonWorks technology are widely used in industrial control, building automation and other fields. The corresponding LON (LocalOperating Networks) bus has also become one of the most popular buses before, and the basic characteristics are as follows:
1) The LONWORKS protocol follows the ISO / OSI reference model, it provides all 7-layer services defined by OSI, and can form a typical network structure such as bus type, star type, ring type and mixed type, which can realize the free combination of network topology , Can realize the interconnection of different field buses through the gateway.
2) The communication rate is 78Kbps / 2700m, 1.25Mbps / 130m, the number of nodes is 32000, and the transmission medium can be twisted pair, coaxial cable, optical fiber, power line, etc.
1.3 CAN (Controller ATea Network) bus
The CAN bus was first introduced by the German BOSCH company and is used for data communication protocols between unmeasured and executive parts in automobiles. Widely used in the field of discrete control, it is currently the only fieldbus approved as an international standard. The basic features are as follows:
1) The CAN protocol follows the ISO / OSI model, using the three-layer structure of the physical layer, link layer, and application layer; the number of effective bytes per frame is 8, so the transmission time is short, and the probability of interference is low.
2) The communication rate of CAN is 5Kbps / l0km, 1Mbps / 40m, the number of nodes can reach 110, and the transmission medium can be twisted pair or optical fiber. It can realize a fully distributed multi-machine system without master and slave.
2. Application of fieldbus
2.1 Application in power automation
The following is a transformation of power automation of a 35KV substation, in which the 35KV substation's communication network of the bay layer all uses field bus technology. LonWorks field bus technology is used in the substation integrated automation system, and the microprocessor-based spacer layer equipment is directly integrated into the LonWorks field bus. With its high reliability and high capacity and high speed communication capabilities, the information of the communication system is greatly improved Throughput and data processing capabilities, thus solving the bottleneck phenomenon that often occurs when applying low-speed serial communication to sample data.
2.2 Application in smart buildings
Building automation system is the most basic and important component in intelligent building. The Intelligent Building Center of the Ministry of Construction of China has promoted the use of LonWorks technology as a recommended control network technology in intelligent buildings. According to statistics, as of 2009, there are 182 access control controllers, 50 fire alarm controllers, 350 security monitoring controllers, 87 door and window sensors, and 568 air conditioning controllers. Products still appear on a large scale at multiple levels
3. Application characteristics and development trend of fieldbus technology
The field bus method can save the amount of hardware and investment, reduce the number of transmitters, and no longer need a separate controller, calculation unit, etc .; save installation and maintenance costs, the wiring of the bus system is simple, due to a pair of twisted pairs or Multiple devices can usually be connected to a cable, so the amount of cables, terminals, slot boxes, and bridges is greatly reduced, and the workload of wiring design and joint proofing is greatly reduced. At the same time, due to the simplified system structure and simple wiring Maintenance workload; improves the accuracy and reliability of the system. Due to the intelligence and digitization of fieldbus devices, compared with analog signals, it fundamentally improves the accuracy of measurement and control and reduces transmission errors.
Fieldbus should determine the development strategy of fieldbus in the long run according to the national economic development and market demand, combining the characteristics of fieldbus and China's national conditions. With the birth of a fully open control system based on fieldbus technology, it will revolutionize the traditional control system architecture.
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