Discussion on unstable factors of the hottest PROF

2022-08-14
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Discussion on unstable factors of Fieldbus PROFIBUS

fieldbus PROFIBUS is an industrial communication system. It uses a kind of transmission medium (such as copper cable, optical fiber or wireless with bit string transmission) to improve its chemical resistance and heat resistance, and uses bit string transmission to connect scattered field devices (such as sensors, actuators, drivers, transmitters, etc.) to the central control or management system. Although the manufacturer has taken some measures to improve its reliability, there are many external factors that will also cause instability, thus affecting the reliable transmission of data, which will cause the sudden stop of the equipment. To improve the reliability of Fieldbus PROFIBUS, on the one hand, the quality of Fieldbus Profibus and the anti-interference ability of related equipment are required; On the other hand, it is required to attach great importance to engineering design, installation, construction, use and maintenance, and multi-party cooperation can improve and solve the problem, and effectively enhance the reliability of fieldbus. Its application improves the productivity and flexibility of automation process, and establishes the basic preconditions for the construction of distributed automation system. During the establishment and operation of its complete system, it can reduce the total cost of users, enhance performance and improve quality. At present, it has been applied in many tobacco factories in the tobacco industry. It really realizes the transformation of process control from result control to process control, and from control index to control parameter; From manual control experience decision-making to automatic control scientific decision-making. However, with the increasingly extensive application, its instability is also becoming increasingly important. This paper puts forward some possible factors for those who are committed to fieldbus technology to discuss

I. The special grounding of the whole system is not designed, or the grounding cannot meet the requirements

the transmission medium of Fieldbus PROFIBUS shares the transmission data and power supply current. Its working voltage is low (about 5V), and its withstand voltage is also very low (-7v~+12v). Once overvoltage is introduced, it will be broken down. When there is a strong surge energy, you can even see the phenomenon that the transceiver bursts and the circuit board is burnt. Therefore, the grounding resistance of the whole system should not be greater than 4 Ω or less. When the author was building a factory, he had seen that the design only designed lightning protection grounding according to the GBJ provisions of the code for design of lightning protection of buildings, but did not design special grounding for electrical equipment. The units and equipment manufacturers that installed the electrical auxiliary facilities also did not raise objections or requirements. As a result, all grounding is shared with lightning protection grounding. As a result, during the commissioning, a group of PLC in the electric control cabinet was burned, and then realized the importance of special grounding and made up for it. Some have designed special grounding, but the owner or construction unit has buried unqualified materials or insufficient depth in order to save; Make the grounding fail to meet the requirements (less than 4 Ω or less). The author believes that this is one of the most important factors that may affect the instability of Fieldbus PROFIBUS

II. Do not pay attention to the following grounding

generally, low-voltage equipment has the following types of grounding: ① enclosure ground is also called shielding ground. Its enclosure and ground are at the same potential, which can protect personal safety, prevent static electricity, magnetic induction and shield external interference. ② AC ground, N line of AC power supply. This ground wire is the main source of noise. ③ DC ground is the starting point of the standard voltage of DC power supply. In the non floating DC power supply, it is regarded as the ground wire, and it is the connecting line of grounding. ④ Signal ground usually refers to the ground of the sensor. ⑤ Digital ground, also known as logical ground, is the equipotential point of the digital circuit and the zero potential of the digital signal. ⑥ Analog ground is the zero potential of various analog signals. The grounding requirements between them are different. Therefore, special attention should be paid. For example, AC ground and signal ground cannot be shared. There will be several millivolts or even several volts between two points of a section of power ground, which is a very serious interference to the low-level signal circuit. It must be cut off and prevented

III. according to the code for electrical installation, the signal line and power line are required to be separated, but it is impossible to be completely separated in specific practice

the transmission medium of Fieldbus jointly transmits data and power supply current. According to the code for electrical installation, when the inertia is reduced to the square of the reduction ratio, the double-layer bridge is adopted, with the power bridge laid on one layer and the fieldbus laid on the other layer; But in practical application, they are inevitably mixed together. Especially when entering the electric control cabinet, the electric control cabinet is currently not designed with double-line slots, but with the same first-line slot, so it is inevitable to mix together. Therefore, the author suggests that the manufacturer should consider whether to design a double groove in the electric control. Completely separate the power line from the fieldbus

III. mismatch between distance and rate setting

the relationship between transmission rate and distance of fieldbus. As shown in the table: 1

the above table shows the theoretical values of transmission rate and distance. In practical application on site, factors such as site environment and equipment stability should be considered. The pretreatment section of Sichuan Sanxin Tobacco Co., Ltd. is composed of master PLC (s), 18 i/o boxes, two nuclear scales, three moisture meters, a set of enzyme adding system and three repeaters. When initially configured, the transmission rate of the network is set to 1.5mbps, and the system can stabilize the development of the extruder industry, which still has a large market space. But after a period of time, the system appears unstable, and check that its actual distance is less than the maximum theoretical distance. But later, the author changed the transmission rate from 1.5mbps to 187.5kbps, and this phenomenon never occurred again

IV. instability of components

the instability of fieldbus system is closely related to the stability of components. During on-site maintenance, a Siemens MM440 frequency converter was often disconnected. Check that the parameter settings of the frequency converter were normal, reset the parameters to the factory settings, and the fault still existed. After the card was finally replaced, there was no disconnection. If the connector is broken, the network data cannot be collected. From the stability of the whole system, the stability of components is also one of the important factors affecting the stability of the system

v. surrounding environment

the instability of Fieldbus PROFIBUS system is related to the surrounding environment. Siemens PLC, frequency converter and other components have specific requirements for the environment (humidity, temperature, altitude, dust, etc.). If these environments are not satisfied for a long time, it will cause the dissatisfaction of components

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