I. Overview With the development of science and technology, PLC has become more and more widely used in industrial control. The reliability of the PLC control system directly affects the safety production and economic operation of industrial enterprises. The system's anti-jamming capability is the key to the reliable operation of the entire system. Various types of PLCs used in the automation system are centralized in the control room, and some are installed on the production site and on various electrical equipment. Most of them are in the harsh electromagnetic environment formed by strong electric circuits and strong electrical equipment. To improve the reliability of the PLC control system, on the one hand, the PLC manufacturer is required to improve the anti-jamming capability of the equipment; on the other hand, it requires a great deal of attention in the engineering design, installation and construction as well as in the use and maintenance. Many parties can improve and solve the problem and effectively enhance the system. The anti-jamming performance.
Second, electromagnetic interference sources and interference to the system 1 interference sources and interference general classification Affect the PLC control system interference sources and generally affect the interference of industrial control equipment, mostly in the current or voltage changes in the site, these charges move violently The source of noise is the source of interference.
The type of interference is usually divided according to the cause of the interference, the noise interference pattern, and the waveform nature of the noise. Among them: according to the different causes of noise, divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the noise waveform, different nature, divided into continuous noise, occasional noise, etc.; according to different noise interference modes, divided into total Mode interference and differential mode interference. Common-mode interference and differential-mode interference are more commonly used classification methods. Common-mode interference is the signal-to-ground potential difference, which is mainly caused by the superposition of common-mode (in the same direction) voltage induced on the signal line by the grid string in, ground potential difference, and space electromagnetic radiation. Common-mode voltage is sometimes large. Especially in power distribution rooms with poor isolation performance, the common-mode voltage of the output signal of the transmitter is generally high, and some may reach more than 130V. The common-mode voltage can be converted to differential-mode voltage through an asymmetrical circuit, directly affecting the measurement and control signals and causing damage to the components (this is the main reason for the high breakdown rate of some system I/O modules). This common-mode interference can be DC. Can also be exchanges. Differential mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly caused by the coupling of the space electromagnetic field in the signal and the voltage formed by unbalanced circuit conversion common mode interference. This kind of direct superimposition on the signal directly affects the measurement and control. Accuracy.
2 Main sources of electromagnetic interference in PLC control systems 2.1 Radiated interference from space Radiated electromagnetic fields (EMI) are mainly caused by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. Generated, often referred to as radiated disturbances, whose distribution is extremely complex. If the PLC system is placed in the RF field, radiated interference is recovered. The impact is mainly through two paths: one is direct radiation to the PLC, and the other is induced by the circuit; it is radiation to the PLC communication network. The induction of the communication line introduces interference. The radiated interference is related to the field equipment layout and the electromagnetic field generated by the equipment, especially the frequency. Generally, shielded cables and PLC partial shields and high-pressure discharge components are used for protection.
2.2 The interference from the outside of the system leads is mainly introduced through the power supply and signal lines and is often referred to as conducted interference. This interference is more serious in our country's industrial scene.
(1) Interference from power sources Practice has proved that due to power supply interference caused by PLC control system failure in many cases, the author encountered in a project commissioning, after replacing the higher isolation performance of the PLC power supply, the problem was resolved.
The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the power grid, it will be subject to all space electromagnetic disturbances and voltages and circuits induced on the line. In particular, changes in the power grid, surges in switching operations, start-up and shutdown of large-scale power equipment, harmonics caused by AC/DC transmission devices, and transient short-circuiting of power grid short circuits are transmitted to the primary side of the power supply through the transmission lines. The PLC power supply is usually an isolated power supply, but its organization and manufacturing process factors make its isolation unsatisfactory. In fact, absolute isolation is not possible due to the presence of distributed parameters, especially distributed capacitance.
(2) Various types of signal transmission lines that are connected to the PLC control system due to the interference introduced by the signal lines. In addition to transmitting various types of effective information, there are always external interference signals. This interference mainly has two ways: First, through the power supply of the transmitter or the power supply of the common signal instrument, it is often overlooked; second, the signal line is interfered with by space electromagnetic radiation, that is, the signal line. The external inductive interference is very serious. The introduction of interference by the signal will cause abnormal I/O signal operation and greatly reduce the measurement accuracy. In severe cases, it will cause component damage. For systems with poor isolation performance, they will also cause interference between the signals, causing common-system bus backflows, causing logic data changes, misoperations, and crashes. The number of I/O module damages caused by the introduction of signal to the PLC control system is quite serious, and there are many cases of system failure.
(3) Interference from the grounding system is an effective way to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can not only suppress the influence of electromagnetic interference, but also inhibit the equipment to send out interference; and the wrong grounding, it will introduce serious interference signals, so that the PLC system will not work properly.
The ground wire of the PLC control system includes system ground, shielding ground, exchange ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of the potential of each grounding point. There is a difference in ground potential between different grounding points, which causes the ground loop current and affects the normal operation of the system. For example, the cable shield must be grounded at one point. If both ends of the cable shield A and B are grounded, there is a difference in ground potential, and a current flows through the shield. When an abnormal state such as a lightning strike occurs, the ground current will be greater.
In addition, the shielding layer, grounding wire and ground may constitute a closed loop. Under the effect of changing the magnetic field, there will be induced current in the shielding layer, and the signal loop will be disturbed by the coupling between the shielding layer and the core wire. If system grounding and other grounding processes are confusing, the resulting ground loop may produce unequal potential distribution on the ground line, affecting the normal operation of logic circuits and analog circuits within the PLC. The logic voltage disturbance tolerance of PLC work is lower, the logic ground potential distribution interference easily affects PLC logic operation and data storage, causes the data to be chaotic, the procedure runs away or crashes. The distribution of the simulated ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunction of the signal measurement and control.
2.3 The interference from the PLC system is mainly caused by the mutual electromagnetic radiation between the internal components of the system and the circuits, such as mutual radiation of logic circuits and their influence on analog circuits, interaction between analog ground and logic ground, and mutual interference between components. Matching use etc. This is all part of the PLC manufacturing plant's internal electromagnetic compatibility design. It is more complex and can't be changed as an application department. You don't have to think too much about it, but you have to choose a system with more application performance or tests.
Third, PLC control system engineering application of anti-jamming design In order to ensure that the system in the industrial electromagnetic environment to avoid or reduce electromagnetic interference from inside and outside, must be taken from the design stage to take three measures to inhibit: suppress interference source; cut off or attenuate electromagnetic interference Transmission methods; Improve the anti-jamming capability of devices and systems. These three points are the basic principles for suppressing electromagnetic interference.
The anti-jamming of the PLC control system is a systematic project that requires the manufacturing unit to design and produce a product with strong anti-jamming capability, and depends on the use department to fully consider it in the engineering design, installation construction and operation and maintenance, and in combination with the situation. Comprehensive design can ensure the system's electromagnetic compatibility and operational reliability. When carrying out anti-jamming design for specific projects, the following two main aspects should be applied.
1 Selection of equipment When choosing equipment, we must first choose a product with higher anti-jamming capability, which includes electromagnetic compatibility (EMC), especially resistance to external interference, such as the use of floating technology, good isolation performance PLC system Secondly, it should also understand the anti-jamming indicators given by the manufacturer, such as co-analog ratios, differential analog ratios, pressure resistance capabilities, and the ability to operate in a magnetic field strength environment with multiple electric field strengths and multiple high frequencies; Application performance in similar work. We must pay attention to the selection of imported products abroad: China adopts a 220V high internal resistance grid standard, while Europe and the United States use a 110V low internal resistance grid. Due to the large internal resistance of the power grid in our country, large shift in zero point potential, and large changes in ground potential, the electromagnetic interference at the industrial enterprise site is at least four times higher than that in Europe and the United States, and the requirement for system anti-jamming performance is higher. PLC products that can work normally in foreign countries The domestic industry may not be able to operate reliably. This requires a reasonable choice based on our country's standard (GB/T 13926) when adopting foreign products.
2 comprehensive anti-jamming design mainly considers several from the outside of the system if the suppression measures. The main contents include: shielding the PLC system and the outer leads from electromagnetic interference in the space; external leads are isolated and filtered, especially the principle of the power cable, layered arrangement, to prevent the introduction of conductive electromagnetic interference through the outer leads; correctly design the grounding point And grounding device, perfect grounding system. In addition, software must be used to further improve the system's safety and reliability.
Fourth, the main anti-jamming measures 1 the use of excellent power, to suppress the interference caused by the power grid in the PLC control system, the power occupies a very important position. Grid interference into the PLC control system mainly through the PLC system power supply (such as CPU power, I / O power supply, etc.), the transmitter power supply and the PLC system has a direct electrical connection to the instrument power supply, etc. coupled into. Now, for the power supply of PLC system, power supply with better isolation performance is generally used, and the power supply for the power supply of the transmitter and the power supply for the instrument directly connected to the PLC system have not received enough attention, although it has taken certain The isolation measures, but generally not enough, is mainly used in isolation transformers with large distribution parameters, poor ability to suppress interference, and common mode interference and differential mode interference through power coupling. Therefore, for the power supply of the transmitter and the common signal instrument, the distributor with small distributed capacitance and large suppression band (such as multiple isolation and shielding and leakage inductance technology) should be selected to reduce the interference of the PLC system.
In addition, to ensure that the grid feed points are not interrupted, online uninterruptible power supply (UPS) power supply can be used to improve the safety and reliability of power supply. And UPS also has strong interference isolation performance, which is an ideal power source for PLC control systems.
2 The choice and setting of the cable is set to reduce the radiated electromagnetic interference of the power cable, especially the frequency conversion device feeder cable. In the project, the author adopted a copper tape armor shielded power cable, which reduced the electromagnetic interference generated by the power line, and achieved satisfactory results after the project was put into operation.
Different types of signals are transmitted by different cables. The signal cables should be layered according to the types of transmission signals. It is forbidden to use the same cable with different wires to transmit power and signals at the same time, to avoid parallel arrangement of signal cables and power cables to reduce electromagnetic emissions. interference.
3 Hardware filtering and software anti-jamming measures Before connecting the signal to the computer, connect the capacitor between the signal line and the ground to reduce the common mode interference. Adding a filter between the two signals of the signal can reduce the differential mode interference.
Due to the complexity of electromagnetic interference, it is impossible to eliminate all interference effects at all. Therefore, in the software design and configuration of the PLC control system, the anti-jamming processing should be performed in the software to further improve the reliability of the system. Some commonly used measures: digital filtering and frequency-shaping sampling can effectively eliminate periodic interference; timing correction of reference point potential and adopting dynamic zero point can effectively prevent potential drift; using information redundancy technology, design corresponding software flag bits; Indirect jumps, software traps, etc. are used to improve the reliability of the software structure.
4 The correct selection of grounding points and the improvement of grounding system grounding usually have two purposes. One is for safety, and the other is for suppression of interference. A perfect grounding system is one of the important measures against electromagnetic interference in the PLC control system.
System grounding methods include: floating ground, direct grounding, and capacitive grounding. For the PLC control system, it is a high-speed low-level control device and should be directly grounded. Because of the influence of signal cable distribution capacitance and input device filtering, the signal exchange frequency between devices is generally lower than 1 MHz, so the ground wire of the PLC control system adopts one-point grounding and one-point grounding. The centrally-arranged PLC system is suitable for parallel grounding at one point. The central ground point of the cabinet of each device is led to the grounding electrode by a separate grounding wire. If the distance between the devices is large, a one-point grounding method should be used. Use a large cross-section copper busbar (or insulated cable) to connect the cabinet's center grounding point of each device, and then connect the grounding bus directly to the grounding electrode. The grounding wire uses a copper wire with a section of more than 22mm2, and the total busbar uses a copper wire with a section of more than 60mm2. The grounding resistance of the grounding electrode is less than 2Ω, and the grounding electrode is preferably buried 10~15m away from the building, and the grounding point of the PLC system must be more than 10m away from the grounding point of the strong electrical equipment.
When the signal source is grounded, the shielding layer should be grounded on the signal side; when it is not grounded, it should be grounded on the PLC side; when there is a connector in the middle of the signal line, the shielding layer should be firmly connected and insulated, and must be grounded at multiple points; When the shielded twisted pair of point signal is connected to the multi-core twisted pair screen cable, the shielding layers should be connected well and insulated. Select the appropriate point of contact at the grounding point.
V. Concluding remarks The interference in the PLC control system is a very complicated issue. Therefore, in the anti-jamming design, various factors should be comprehensively considered, and the anti-jamming should be reasonably and effectively suppressed. Specific analysis must be made for some interference situations. The method can only make the PLC control system work normally.
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