Abstract : With the increasing use of PLC control system, the corresponding problems have emerged. For example, the anti-jamming ability of plc system directly affects the industrial production and operation problems. This article will introduce the emergence of such problems. How to solve it.
1. 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 automation systems are centralized in the control room, and are installed on the production site and 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, the designer can only effectively ensure the reliable operation of the system by understanding all kinds of disturbances in advance.
2. Electromagnetic interference sources and interference effects on the system The interference from the PLC control system originates from the interference sources that generally affect the industrial control equipment. Most of them are generated in places where the current or voltage changes drastically. The site where these charges move violently is the noise source. Source of interference.
The type of interference is usually divided according to the cause of the interference, the interference pattern of the noise, 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 the different sound interference patterns, divided into a total of Mode interference and differential mode interference. Common-mode interference and differential-mode interference are more commonly used classification methods. Common-mode interference is the potential difference between the signal and the ground. It is mainly formed by the in-line voltage of the common state (in the same direction) induced by the power line in series, the ground potential difference, and space electromagnetic radiation on the signal line. The common-mode voltage is sometimes large, especially in the power supply room with poor isolation performance. The common-mode voltage of the transmitter output signal is generally higher, and some may be up to 130V or more. The common-mode voltage can be converted into differential-mode voltage through an asymmetrical circuit, directly affecting the measurement and control signals and causing damage to the components (this is the reason that some system I/O modules have a high damage rate). This common-mode interference can be DC, It can also be an exchange. Differential mode interference refers to the interference voltage 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 the unbalanced circuit converting the common mode interference. This directly superimposes on the signal and directly affects the measurement. control precision.
3. What are the main sources of electromagnetic interference in PLC control systems?
(1) Radiated from the space Radiated electromagnetic fields (EMI) are mainly generated by the power network, the transient process of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., and are often referred to as radiation. Interference, its distribution is extremely complicated. If the PLC system is placed in the radio frequency field, radiation interference is recovered, and its influence mainly passes through two paths; the first is direct radiation to the PLC, and the circuit induces interference; 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) The interference from the outside of the system leads is mainly introduced through the power supply and the signal line, which is often referred to as conducted interference. This interference is more serious in our country's industrial scene.
(3) 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, all space electromagnetic interference will be induced and voltages and circuits will be induced on the line. In particular, changes within the power grid, surges in switching operations, start-up and shutdown of large-scale electrical equipment, harmonics caused by AC-DC rotating devices, and transient short-circuiting of power grid short circuits, etc., all pass through the power transmission lines to the power supply side. 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.
(4) All kinds of signal transmission lines that are connected with the PLC control system from the interference introduced by the signal line. In addition to the transmission of various types of signals, there will always be external interference signals. There are two main ways of this interference: First, the grid interference caused by the power supply of the transmitter or the common signal instrument is often ignored; second, the signal line is interfered by space electromagnetic radiation, ie, the signal line is external. Inductive interference, this 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, 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.
(5) Interference from the grounding system is an effective means 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 from sending out interference. However, wrong grounding will introduce serious interference signals, which will make the PLC system unable to work properly. The ground of the PLC control system includes system ground, shield 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 shields A and B are grounded, there is a difference in ground potential, and a current flows through the shield. When an abnormal state occurs and lightning strikes, 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 through the coupling between the shielding layer and the core wire. If system grounding and other grounding processes are confusing, the resulting earth current may produce unequal potential distribution on the ground, affecting the normal operation of logic circuits and analog circuits within the PLC. The logical voltage disturbance tolerance of the PLC is low. The disturbance of the distribution of the logic ground potential can easily affect the logic operation and data storage of the PLC, causing chaos in the data and running or crashing the program. 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.
(6) 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 the mutual interference of the logic circuit and its influence on the analog circuit, the interaction between the simulation ground and the logic ground, and between the components. Unmatched 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.
4, how can we better and simpler to solve the PLC system interference?
1) Use equipment with better isolation performance, better power supply, power lines, and signal lines to be more reasonable, etc. to solve the interference, but it is cumbersome, difficult to operate, and costly.
2) Use a signal isolator to solve the interference problem. As long as there is interference, adding this kind of product between the input and output can effectively solve the interference problem.
5, why solve the PLC system interference preferred signal isolator it?
1) Easy to use, reliable, and low cost.
2) The workload of designers and system debuggers can be greatly reduced, even if the complex system is in the hands of ordinary designers, it will become very simple and reliable.
6. What is the working principle of the signal isolator?
Firstly, the signal received by PLC is modulated by a semiconductor device, and then is isolated and converted by a light-sensing or magnetic-inductance device, and then demodulated and converted back to the original signal or signal before isolation, and the power supply to the isolated signal is isolated at the same time. . Ensure absolute independence between transformed signals, power supply, and ground.
7. Now that there are so many brands of isolators on the market, prices vary and how should we choose?
The isolator is located between two system channels. Therefore, the isolator must first determine the input and output functions. At the same time, the input and output modes (voltage, current, and loop-powered) of the isolator must be adapted to the front and rear channel interface modes. In addition, there are many important parameters such as accuracy, power consumption, noise, dielectric strength, and bus communication function that relate to product performance. For example, noise is related to accuracy, power consumption is related to reliability, and these require careful selection by the user. In short, the main principles for selecting isolators are their suitability, reliability, and cost-effectiveness.
1. 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 automation systems are centralized in the control room, and are installed on the production site and 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, the designer can only effectively ensure the reliable operation of the system by understanding all kinds of disturbances in advance.
2. Electromagnetic interference sources and interference effects on the system The interference from the PLC control system originates from the interference sources that generally affect the industrial control equipment. Most of them are generated in places where the current or voltage changes drastically. The site where these charges move violently is the noise source. Source of interference.
The type of interference is usually divided according to the cause of the interference, the interference pattern of the noise, 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 the different sound interference patterns, divided into a total of Mode interference and differential mode interference. Common-mode interference and differential-mode interference are more commonly used classification methods. Common-mode interference is the potential difference between the signal and the ground. It is mainly formed by the in-line voltage of the common state (in the same direction) induced by the power line in series, the ground potential difference, and space electromagnetic radiation on the signal line. The common-mode voltage is sometimes large, especially in the power supply room with poor isolation performance. The common-mode voltage of the transmitter output signal is generally higher, and some may be up to 130V or more. The common-mode voltage can be converted into differential-mode voltage through an asymmetrical circuit, directly affecting the measurement and control signals and causing damage to the components (this is the reason that some system I/O modules have a high damage rate). This common-mode interference can be DC, It can also be an exchange. Differential mode interference refers to the interference voltage 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 the unbalanced circuit converting the common mode interference. This directly superimposes on the signal and directly affects the measurement. control precision.
3. What are the main sources of electromagnetic interference in PLC control systems?
(1) Radiated from the space Radiated electromagnetic fields (EMI) are mainly generated by the power network, the transient process of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., and are often referred to as radiation. Interference, its distribution is extremely complicated. If the PLC system is placed in the radio frequency field, radiation interference is recovered, and its influence mainly passes through two paths; the first is direct radiation to the PLC, and the circuit induces interference; 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) The interference from the outside of the system leads is mainly introduced through the power supply and the signal line, which is often referred to as conducted interference. This interference is more serious in our country's industrial scene.
(3) 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, all space electromagnetic interference will be induced and voltages and circuits will be induced on the line. In particular, changes within the power grid, surges in switching operations, start-up and shutdown of large-scale electrical equipment, harmonics caused by AC-DC rotating devices, and transient short-circuiting of power grid short circuits, etc., all pass through the power transmission lines to the power supply side. 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.
(4) All kinds of signal transmission lines that are connected with the PLC control system from the interference introduced by the signal line. In addition to the transmission of various types of signals, there will always be external interference signals. There are two main ways of this interference: First, the grid interference caused by the power supply of the transmitter or the common signal instrument is often ignored; second, the signal line is interfered by space electromagnetic radiation, ie, the signal line is external. Inductive interference, this 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, 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.
(5) Interference from the grounding system is an effective means 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 from sending out interference. However, wrong grounding will introduce serious interference signals, which will make the PLC system unable to work properly. The ground of the PLC control system includes system ground, shield 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 shields A and B are grounded, there is a difference in ground potential, and a current flows through the shield. When an abnormal state occurs and lightning strikes, 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 through the coupling between the shielding layer and the core wire. If system grounding and other grounding processes are confusing, the resulting earth current may produce unequal potential distribution on the ground, affecting the normal operation of logic circuits and analog circuits within the PLC. The logical voltage disturbance tolerance of the PLC is low. The disturbance of the distribution of the logic ground potential can easily affect the logic operation and data storage of the PLC, causing chaos in the data and running or crashing the program. 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.
(6) 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 the mutual interference of the logic circuit and its influence on the analog circuit, the interaction between the simulation ground and the logic ground, and between the components. Unmatched 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.
4, how can we better and simpler to solve the PLC system interference?
1) Use equipment with better isolation performance, better power supply, power lines, and signal lines to be more reasonable, etc. to solve the interference, but it is cumbersome, difficult to operate, and costly.
2) Use a signal isolator to solve the interference problem. As long as there is interference, adding this kind of product between the input and output can effectively solve the interference problem.
5, why solve the PLC system interference preferred signal isolator it?
1) Easy to use, reliable, and low cost.
2) The workload of designers and system debuggers can be greatly reduced, even if the complex system is in the hands of ordinary designers, it will become very simple and reliable.
6. What is the working principle of the signal isolator?
Firstly, the signal received by PLC is modulated by a semiconductor device, and then is isolated and converted by a light-sensing or magnetic-inductance device, and then demodulated and converted back to the original signal or signal before isolation, and the power supply to the isolated signal is isolated at the same time. . Ensure absolute independence between transformed signals, power supply, and ground.
7. Now that there are so many brands of isolators on the market, prices vary and how should we choose?
The isolator is located between two system channels. Therefore, the isolator must first determine the input and output functions. At the same time, the input and output modes (voltage, current, and loop-powered) of the isolator must be adapted to the front and rear channel interface modes. In addition, there are many important parameters such as accuracy, power consumption, noise, dielectric strength, and bus communication function that relate to product performance. For example, noise is related to accuracy, power consumption is related to reliability, and these require careful selection by the user. In short, the main principles for selecting isolators are their suitability, reliability, and cost-effectiveness.
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