1 Overview
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, and the anti-interference ability 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 effectively ensure the reliable operation of the system only by knowing various interferences in advance.
2. Electromagnetic interference source and interference to the system
The interference that affects the PLC control system originates from the interference sources that generally affect industrial control equipment. 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.
3. What are the main sources of electromagnetic interference in the PLC control system?
(1) Radiation interference from space:
The radiated electromagnetic field (EMI) in space is mainly generated by the transient process of power networks, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. It is usually called radiated interference, and its distribution is extremely complicated. If the PLC system is placed in the radio frequency field, the radiated interference is recovered, and its impact mainly passes through two paths; one is to directly interfere with the internal radiation of the PLC, which is induced by the circuit; but the radiation to the network in the PLC communication is caused by The induction of communication lines introduces interference. Radiation interference is related to the field equipment layout and the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting up shielded cables and PLC partial shielding and high-voltage relief 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 more serious in our country's industrial scene.
(3) Interference from the power supply:
Practice has proved that there are many cases of PLC control system failure due to the interference introduced by the power supply. The author encountered in a project debugging, and then replaced the PLC power supply with higher isolation performance, and the problem was solved.
The normal power supply of the PLC system is supplied by the power grid. Due to the wide coverage of the grid,
Will be subject to electromagnetic interference from all spaces and induce voltages and circuits on the line. Especially changes in the power grid, switching surges, start and stop of large-scale power equipment, harmonics caused by AC and DC rotating devices, and grid short-circuit transient impacts, all pass through the transmission line to the power source. PLC power supply usually uses isolated power supply, but its mechanism and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.
(4) Interference introduced from the signal line:
The various signal transmission lines connected with the PLC control system, in addition to transmitting effective various signals, will always have external interference signals invade. There are two main ways of this interference: one is the power grid interference through the transmitter or the power supply of the shared signal instrument, which is often ignored; the other is the interference of the signal line by electromagnetic radiation induction in space, that is, the external signal line Inductive interference, which is very serious. The interference introduced by the signal will cause the I/O signal to work abnormally and the measurement accuracy will be greatly reduced. In severe cases, it will cause component damage. 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 number of I/O module damages in the PLC control system due to signal interference is quite serious, and there are many cases of system failures caused by this.
(5) Interference from the chaos of the grounding system:
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Proper grounding can not only suppress the influence of electromagnetic interference, but also restrain the equipment from sending out interference; and wrong grounding will introduce serious interference signals and make the PLC system unable to work normally. The ground wire of PLC control system includes system ground, shield ground, AC ground and protection ground. The main interference of the chaos of the grounding system to the PLC system is the uneven distribution of the potential of each grounding point, and there is a ground potential difference between different grounding points, which causes ground loop currents and affects the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there will be a ground potential difference, and current will flow through the shielding layer. When an abnormal state occurs and a lightning strike occurs, the ground current will be greater.
In addition, the shielding layer, the grounding wire and the ground may form a closed loop. Under the action of the changing magnetic field, there will be induced currents in the shielding layer, which interferes with the signal loop through the coupling between the shielding layer and the core wire. If the system ground is confused with other grounding treatments, the generated ground circulating current may produce unequal potential distribution on the ground wire, which will affect the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of PLC work is low, and 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 crash. The distribution of analog ground potential will lead to a decrease in measurement accuracy, causing serious distortion and misoperation of signal measurement and control.
(6) Interference from inside the PLC system:
Mainly caused by the mutual electromagnetic radiation between the internal components and circuits of the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the mutual influence between analog ground and logic ground, and the mismatching use of components. This is the content of the electromagnetic compatibility design of the internal system by the PLC manufacturer. 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 PLC does not match the signal value corresponding to the actual parameter, and the error value is random and irregular;
(4) Sharing the same power source (such as display, etc.) with the AC servo system does not work properly.
5. How can we solve PLC system interference better and more easily?
(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, etc., 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 isolator, called signal transmitter, belongs to the category of signal conditioning. It mainly plays a role of anti-interference. Because of its strong anti-interference ability, it is widely used in automation control systems. Especially for complex industrial sites, the control procedures are becoming more and more complex. The signal isolator isolates various analog signals from input, output, and power supply. It is indeed one of the effective anti-interference measures in today's automation control system.
6. Why is the signal isolator the first choice for solving PLC system interference?
(1) It is simple, convenient, reliable, low-cost to use, 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.
7. What is the working principle of the signal isolator?
First, the signal received by the PLC is modulated and converted by semiconductor devices, and then isolated and converted by light-sensitive or magnetic-sensitive devices, and then demodulated and converted back to the original signal before isolation or a different signal, and the power supply of the isolated signal is isolated. . Ensure that the transformed signal, power supply, and ground are absolutely independent.
8. Now there are so many brands of isolators in the market, and the prices are uneven. 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. In addition, there are many important parameters related to product performance, such as accuracy, power consumption, noise, insulation strength, and bus communication function. For example, noise is related to accuracy, power consumption and heat are related to reliability, which require users to choose carefully. In short, applicability, reliability, and product cost-effectiveness are the main principles for choosing an isolator.
If you are not clear about the above sharing, then I will give everyone a key point (11 analog interference solutions):
1. Add 1:1 signal isolator;
2. Add magnetic ring;
3. PLC power supply plus isolation transformer;
4. The switch signal and the analog signal are separated;
5. It is better to use a separate shielded wire for the analog signal. The signal type is best to use 4-20mA;
6. The analog signal load is solenoid valve type, preferably 1.5 line;
7. The analog signal and digital signal cannot use the same multi-core cable, let alone share the cable with the power cord;
8. The PLC input and output signal lines must use shielded cables, which are suspended on the input and output side and grounded on the PLC side;
9. Signal cables should be far away from strong interference sources, such as frequency converters, high-power silicon rectifiers and large power equipment;
10. The analog signal and digital signal cannot be used in the same multi-core cable, let alone share the cable with the power cord;
11. In order to reduce electronic interference, a twisted-pair shielded cable should be used for analog signals. The shielding layer of the analog signal cable should be grounded at both ends. However, if there is a potential difference between the two ends of the cable, equal wire connection currents will be generated in the shielding layer, which will cause damage to the analog signal. Interference In this case you should ground one end of the cable shield.
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