Fieldbus (Fieldbus) is an industrial data bus that has been rapidly developed in recent years. It mainly solves the digital communication between field devices such as intelligent instrumentation, controllers and actuators in industrial field, and these field control devices and advanced control systems. The problem of information transfer between. Due to the outstanding advantages of fieldbus, such as simple, reliable, economical and practical, it has been highly valued by many standards bodies and computer manufacturers.
It is an industrial data bus and is the underlying data communication network in the field of automation. Simply put, fieldbus replaces the traditional 4-20mA analog signal and ordinary switch signal transmission with digital communication. It is an all-digital, two-way, multi-station communication system that connects intelligent field devices and automation systems.
The field control device has a communication function to facilitate the construction of the plant's underlying control network. The openness and consistency of communication standards make the system open and interoperable between devices. The normalization of function blocks and structures makes interchangeability between devices of the same function. The control function is placed on the site to make the control system structure highly dispersible.
Bus advantageFieldbus enables self-control devices and systems to enter the ranks of information networks, opening up a wider field for their applications; multiple control devices can be attached to a pair of twisted pairs to save installation costs; save maintenance overhead; The reliability of the system; provides users with more flexible system integration initiative.
Bus defectThe transmission delay of data packets in network communication, the transient error of the communication system and the loss of data packets, the inconsistency between the transmission and the arrival order, etc. will destroy the determinism originally possessed by the traditional control system, making the analysis and synthesis of the control system more complicated. The performance of the control system is negatively affected.
(1) Transmission by wire, called conducted interference. In the field bus, the main performance is the ground impedance interference and interference from the power frequency power supply.
(2) Transmission through space in the form of radiation, known as radiated interference.
Fieldbus anti-interference measures (1) away from interference sourcesThe interference of the power equipment and the power cable to the fieldbus is inversely proportional to the square of the distance, that is, as the distance increases, the interference attenuation is very fast. Therefore, the field bus equipment is far away from the power equipment, and the field bus cable and the power cable layered bridge frame arrangement can play a good anti-interference effect. Keeping away from interference sources is an important measure to prevent radiation interference.
(2) Shielding of fieldbus devices and cablesThe mechanism of fieldbus shielding is that external electromagnetic waves generate eddy currents on the metal surface to cancel the original magnetic field. Second, electromagnetic waves generate reflection loss on the metal surface, and another part of the transmitted wave propagates in the metal shielding layer to attenuate the absorption loss. Fieldbus shielding uses a shield made of a conductive material in combination with ground to cut off the source of interference.
It can prevent interference from power frequency power supply.
(4) Transmission of signals using fiber optic cableThe transmission speed on the field bus is high! In the case where the transmission distance is far from interference, the optical cable is used as much as possible. After the use of fiber optic cable, many problems of radiated disturbance and conducted interference are effectively solved. If the fieldbus signal is transmitted by optical cable between two points that are not in common or when the grounding condition is very bad, interference such as ground loop can be effectively prevented. Especially the paper machine control system and paper machine drive! Between MCCs, there is a tendency to have a ground potential difference or a ground loop. Special attention should be paid to the selection of optical cables over twisted pairs.
(5) Correct selection and installation of the paper machine drive systemThe rectification section uses 12-pulse rectification to effectively eliminate the 5th and 7th harmonics. The power cable between the drive cabinet and the motor must be a well-shielded and three-core symmetrical power cable.
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