“With the development of science and technology, PLC is more and more widely used in industrial control. The reliability of the PLC control system directly affects the safe production and economic operation of industrial enterprises. The interference capability of the system is the key to the reliable operation of the entire system. Various types of PLCs used in automation systems are installed in the control room, while others 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 electric equipment. To improve the reliability of the PLC control system, the designer can only effectively guarantee the reliable operation of the system if he knows the various interferences in advance.
With the development of science and technology, PLC is more and more widely used in industrial control. The reliability of the PLC control system directly affects the safe production and economic operation of industrial enterprises. The interference capability of the system is the key to the reliable operation of the entire system. Various types of PLCs used in automation systems are installed in the control room, while others 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 electric equipment. To improve the reliability of the PLC control system, the designer can only effectively guarantee the reliable operation of the system if he knows the various interferences in advance.
2. Sources of electromagnetic interference and interference to the system
The interference that affects the PLC control system comes from the interference sources that generally affect industrial control equipment, and most of them occur in the parts where the current or voltage changes drastically. The parts where the charge moves drastically are the noise source, that is, the interference source.
The type of interference is usually divided according to the cause of the interference, the interference mode of the noise, and the nature of the noise waveform. Among them: according to the different causes of noise, it is divided into discharge noise, occasional noise, etc.: According to different sound interference modes, divided into common mode interference and differential mode interference is a relatively common classification method. Common mode interference is the potential difference between the signal and the ground, which is mainly caused by the series connection of the power grid, the ground potential difference and the common mode voltage induced on the signal line by the space electromagnetic radiation. The common mode voltage is sometimes large, especially in the electrical power supply room with poor isolation performance. The common mode voltage of the transmitter output signal is generally high, and some can be as high as 130V or more. The common-mode voltage can be converted into a common-mode voltage through an asymmetrical circuit, which directly affects the measurement and control signal and causes damage to the components. This common-mode interference can be DC or AC. Common mode interference refers to the interference voltage used between the two levels of the signal, which is mainly caused by the coupling and induction of the spatial electromagnetic field between the signals and the voltage formed by the unbalanced circuit converting the common mode interference. This type of signal is directly superimposed on the signal and directly affects the measurement. And control accuracy.
3 What are the main electromagnetic interference in the PLC control system?
1) Radiation interference from space
The radiated electromagnetic field in space is mainly produced by the transient process of power network, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., which is usually called radiated interference. Its distribution is extremely complicated. If the plc system is placed in the set frequency field, the radiation interference will be recovered, and its impact is mainly through two paths. One is to directly interfere with the radiation inside the PLC, which is induced by the circuit, but to the PLC The radiation of the network in the communication leads to interference from the induction of the communication line. Radiation interference is related to the size of the electromagnetic dry field generated by the field equipment, especially the frequency, and it is generally protected by setting up shielded cables and PLC partial shielding and high-voltage discharge components.
2) Interference from the external leads of the system
Mainly introduced through power and signal lines, usually called conducted interference. This kind of interference is serious in our country’s industrial scene.
3) Interference from the power supply
Practice has proved that there are many cases of control system failure due to the interference introduced by the power supply, which has been encountered in the engineering adjustment, and then replaced the PLC power supply with higher isolation performance. The problem can be solved.
The normal power supply of the PLC system is powered by the grid. Due to the wide coverage of the grid, it will be subject to electromagnetic interference from all spaces and induced voltage and circuits on the line, especially the changes within the power grid, switching surges, large-scale power equipment startup and shutdown, and switching. The harmonics caused by the DC rotating device, the short-circuit transient impact of the power grid, etc., are all connected to the power supply side through the social power line. The PLC power supply usually uses an isolated power supply, but its structure and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation of the room is impossible.
4) Interference introduced when the grounding system is chaotic
The various signal transmission lines connected to the PLC control system, in addition to transmitting effective various signals, always have the influence of external interference, and can inhibit the interference from the equipment, and the wrong grounding will introduce serious interference signals. , So that the PLC control system will not work normally. The ground wire of the Plc control system includes the system ground, shield ground, AC ground, and protective ground. The interference of the grounding system to the PLC system is mainly due to the uneven distribution of grounding points. There is a ground potential difference between different grounding points, which causes ground loops. The circuit current affects the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends of the cable shielding layer are grounded, there is a ground potential difference, and current flows through the ground potential difference. When an abnormal state occurs and a lightning strike occurs, the ground current Will be bigger.
In addition, the shielding layer, the grounding wire and the ground may form a closed loop. Under the action of the changing magnetic field, induced currents will appear in the shielding layer, and the coupling between the shielding layer and the core wire will interfere with the signal loop. If the system ground is confused with other grounding treatments, the generated ground circulation current may produce unequal point distribution on the ground line, which will affect the normal operation of the logic circuit and the analog circuit in the PLC. The logic voltage interference capacity of PLC work is low. The distribution interference of the logic ground potential easily affects the logic operation and data storage of the PLC, causing data confusion, program runaway or crashes, and the distribution of analog ground potential will cause the measurement accuracy to drop and cause the signal to be disturbed. Severe distortion and misoperation of measurement and control.
5) Interference introduced from the signal line
The various signal transmission lines connected to the PLC control system, in addition to transmitting effective various signals, there will always be external interference signals invading. There are two main ways of this interference: one is the power grid interference that comes in series through the transmitter or the power supply of the shared signal instrument. This is often overlooked; the second is that the signal line is interfered by space electromagnetic radiation induction, that is, the external induction interference on the signal line, which is very serious. The interference introduced by the signal will cause the signal to work abnormally and the measurement accuracy will be greatly reduced, and it will cause component damage in severe cases. For systems with poor isolation performance, it will also cause mutual interference between signals, causing backflow of the common ground system bus, resulting in logic data changes, malfunctions and crashes. The PLC control system causes serious damage to the modules due to signal interference, and there are many cases of system failures caused by this.
6) Interference from inside the PLC
Mainly produced by the mutual electromagnetic radiation between the internal components and circuits of the system, such as the mutual radiation of logic circuits and its influence on analog circuits, the mutual influence between analog ground and logic ground, and the mismatched use of fragrance flowers between components. This part belongs to the PLC manufacturer’s internal electromagnetic compatibility design content of the system. It is more complicated and cannot be changed as an application department. You don’t need to think too much, but you should choose a system with more application performance or tested.
4 When the system is interfered, the following main interference phenomena are often encountered:
1) When the system sends instructions, the motor rotates irregularly;
2) When the signal is equal to zero, the value of the digital Display meter jumps randomly;
3) When the sensor is working, the signal collected by the PLC does not match the signal value corresponding to the actual parameter, and the error value is random and irregular;
4) It does not work properly when sharing the same power supply with the AC servo system.
5 How can we better and more simply solve the PLC system interference?
1) In an ideal state, it is necessary to select equipment with better isolation performance, select good power supplies, power lines and signal lines, and power supply grounding to be more reasonable, but it requires the cooperation of different equipment manufacturers to complete it, which is difficult to achieve. And the cost is higher.
2) The use of analog signal isolators, called signal transmitters, belongs to the category of signal conditioning, which is mainly used for anti-interference. Because of its strong anti-interference ability, it is widely used in automation control systems. Especially for complex industrial sites, the older the control program, the more complex it is. The signal isolator isolates the input, output, and power supply of various analog signals. It is indeed one of the effective anti-interference measures in today’s automation control system.
6Why is the signal isolator the first choice for solving PLC system interference?
1) It is easy to use, reliable, low cost, and can solve multiple interferences at the same time.
2) It can greatly reduce the workload of designers and system debuggers, even if the complex system is in the hands of ordinary designers, it will become very stable and reliable.
What is the working principle of a signal isolator?
First, the signal received by the PLC is modulated and transformed by semiconductor devices, and then isolated and converted by light-sensitive or magnetic-sensitive devices, and then demodulated and transformed back to the original signal before isolation or a different signal, and at the same time, the power supply of the isolated signal is isolated. . Ensure that the transformed signal, power, and ground are absolutely independent.
Now there are so many brands of isolators in the market, and the prices vary. How to choose?
The isolator is located between the two system channels, so the choice of isolator must first determine the input and output functions, and at the same time, make the isolator input and output mode (voltage type, current type, loop power supply type, etc.) adapt to the front and back channel interface modes. There are still many important parameters related to product performance, such as accuracy, power consumption, noise, insulation strength, and bus communication. For example, noise is related to accuracy, power consumption and reliability are related to the user’s careful selection. In short, trial, reliability, and product cost-effectiveness are the main principles for choosing an isolator.
Working principle: Firstly, the semiconductor device is modulated and converted, and then isolated and converted by a light sensor or a magnetic sensor, and then demodulated back to the original signal before isolation. At the same time, the power supply of the isolated signal is isolated to ensure the power supply of the converted signal The power supply is isolated. Ensure that the converted signal, power, and ground are absolutely independent.
One: Protect the lower-level control loop.
Two: Reduce the influence of environmental noise on the test circuit.
Three: Suppress the faithful protection of public grounding, inverter, solenoid valve PLC/DCS input and output, and communication interface.
The standard series guide rail structure is easy to install and can be effectively isolated; the potential between the input, output and the solenoid valve and the earth can overcome various high and low frequency pulsation interference of the noise level of the frequency converter.
What are the main types of signal isolators?
In industrial production, in order to increase the load capacity of the instrument and ensure that the instruments connected to the same signal do not interfere with each other, and improve the electrical installation performance. It is necessary to collect, amplify, calculate, and perform anti-interference processing on the input voltage, current or frequency, resistance and other signals, and then output isolated current and voltage signals, and safely send them to the secondary instrument and PLC/DCS for use.
Industrial sites generally need to adopt a two-wire transmission method, which not only provides 24V power distribution for primary instruments such as transmitters, but also collects, amplifies, calculates, and performs anti-interference processing on the input current signal before outputting. The isolated current signal and voltage signal are used by the subsequent secondary instrument or other instruments.
3) Safety barrier
Some special industrial sites not only need two-wire transmission, but also provide power distribution and signal isolation. At the same time, they also need to have the performance of safe spark and explosion-proof, which can reliably contain the power of the power supply and prevent the ignition between the power supply, the signal and the ground. , Current limiting, step-down dual limiting signal and power circuit, limiting the energy entering the dangerous place within the range of the safety rating.
What should be paid attention to when installing and maintaining signal isolator?
Due to different manufacturers, the production process and wiring definitions of the isolators are not all the same, but the use occasions are basically the same, so the protection requirements and maintenance of the products are basically the same.
1. Read the instructions carefully before use.
2. When used as signal isolation, the input end should be connected to the loop circuit in series, and the output end should be connected to the sampling loop.
3. When used as isolated power distribution, the input end should be incorporated into the power circuit, and the output end should be connected to the transmitter.
4. If it does not work normally, first check whether the wiring is correct, and pay attention to whether the power supply is available and the polarity is issued.
Why sometimes the phenomenon signal received by the PLC has large errors and poor stability?
There are many reasons for this imagination, and the potential difference between the reference points of different instrument signals is an important factor. Because of this difference, interference currents are generated between the instrument signals, resulting in large PLC errors and poor stability. Therefore, it is best to have a common reference point for the signals of different equipment and meters. The isolator completely isolates the input/output electrically. The analog interface board forms a common reference point on the PLC, and the problem of ideal mounting resistance is solved.
Isolate the 4-20mA channel, and there is no room for power supply in the cabinet. How to do?
A passive signal isolator, which can realize 4-20mA signal isolation without external power supply. PH1033 is such a product.
Now there are so many brands of isolators in the market, and the prices vary greatly. How to choose?
The isolator is located between the two system channels, so when choosing an isolator, you must first determine the input and output functions. At the same time, you must adapt the input and output mode of the isolator to the interface mode of the front and rear channels. In addition, there are still accuracy, power consumption, noise, insulation strength, Many important parameters such as bus communication function are related to product performance, for example: noise is related to accuracy, power consumption and reliability are related. These need to be carefully selected by the user. In short, applicability, reliability, and product cost-effectiveness are the main principles for choosing an isolator.
The signal of the on-site two-wire pressure transmitter received by the DCS is unstable, how to solve it?
Two-wire transmitters are frequently used in the field of industrial automation. Like other industrial field devices, two-wire transmitters also have problems with interference and anti-interference. According to the interface mode of the DCS analog board, select isolation distributors with different functions. In principle, it is required to provide isolated power to the transmitter, ensure that each transmitter has an independent power supply, and isolate the transmitter signal to the DCS. .
The PLC analog board interface is powered by a two-wire loop, and the signal needs to be isolated. How to choose a product?
Two-wire loop power supply is a common analog board interface. Products suitable for this interface are called two-wire loop isolation products. The internal isolation devices of this isolation transmitter all adopt the transformer method, which transmits the signal on the one hand, and on the other hand also transmits the electric energy of the power supply terminal to the input part, so that the various circuits of the input part can work normally. For example, ph2217 and other distributor products.
There are several commonly used two-wire distributors?
There are two series of commonly used two-wire isolation distributors to choose from. The common point of the two series of products is that both can provide an independent isolated power supply to the two-wire transmitter after isolation through the external power supply: the transmitter’s power distribution depends on the PLC/DCS analog power supplied on site.
Application of safety barrier
The safety barrier originally belonged to the auxiliary unit of the electric unit combination instrument, and its main purpose was the isolation device of the explosion-proof system. The safety barrier transmits the signal from the dangerous area through isolation, and outputs the isolated current signal to the safe area. The safety barrier is used in the intrinsically safe explosion-proof system, and limits the energy sent to the field loop through the current limiting and voltage limiting circuit, thereby preventing Dangerous energy from non-intrinsically safe circuits is cascaded into intrinsically safe circuits. Safety barriers are widely used in automation systems such as DCS/PLC/PCS in petrochemical industries.
Common safety barriers can be divided into Zener type and isolated type according to the structure.
Application of Zener safety barrier:
Fast fuses, current-limiting resistors or voltage-limiting diodes are used in the circuit to limit the input electrical energy so as to ensure the energy output to the hazardous area. However, due to the defect of the principle, the reliability of its application is affected and the scope of its application is limited. The reasons are as follows:
1. The installation location must have a very reliable grounding system. The grounding resistance of the Zener safety barrier must be less than 1 ohm, otherwise it will lose the protection and safety protection performance. Obviously such requirements are very demanding and difficult to guarantee in engineering applications.
2. The field instruments in dangerous areas must be isolated, otherwise the signal cannot be transmitted correctly after the ground terminal of the Zener safety barrier is connected to the earth, and the signal grounding directly reduces the signal anti-interference ability and affects the stability of the system.
3. The Zener safety barrier has a greater impact on the power supply, and it is also easy to cause damage to the Zener safety barrier due to power fluctuations.
Application of isolated safety barrier:
It adopts a circuit structure that electrically isolates the input, output, and power supply from each other, and at the same time meets the requirements of intrinsically safe limited energy.
1. The three-way isolation method is adopted, so there is no need for system grounding lines, which brings great convenience to design and on-site construction.
2. There is no need to use isolated instruments for field instruments in hazardous areas.
3. The signal lines do not need to share the ground, which greatly enhances the stability and anti-interference ability of the detection and control loop signals, thereby improving the reliability of the entire system.
4. Isolated safety barriers have stronger input signal processing capabilities, and can receive and process signals such as thermocouples, thermal resistances, and frequencies, which is not possible with Zener safety barriers.
5. The isolated safety barrier can output two mutually isolated signals to provide two devices using the same signal source, and ensure that the signals of the two devices do not interfere with each other, and at the same time improve the electrical safety insulation performance of the connected devices. .
Therefore, it can be seen from the characteristics and performance of Zener and isolated safety barriers that isolated safety barriers have outstanding advantages and a wider range of applications. In the engineering sites with higher requirements, isolated safety barriers are almost invariably adopted. As the main intrinsically safe explosion-proof instrument, safety barriers have gradually replaced Zener safety barriers.