1, Overview
With the development of science and technology, PLC in industrial control is more and more widely used. the reliability of the PLC control system directly affects the safe production and economic operation of industrial enterprises, the system's ability to resist interference is related to the reliable operation of the entire system is the key. Automation system used in various types of PLC, some are centrally installed in the control room, some are installed in the production site and a variety of motor equipment, most of them are in the strong electrical circuit and strong electrical equipment formed by the harsh electromagnetic environment. To improve the reliability of the PLC control system, the designers only pre-understanding of the various interferences in order to effectively ensure the reliable operation of the system.
2, electromagnetic interference sources and interference with the system
Impact on the PLC control system interference from the general impact of industrial control equipment, the same source of interference, mostly generated in the parts of the current or voltage drastic changes in the parts of the charge to move violently is the source of noise, i.e., the source of interference.
Types of interference are usually generated by the cause of the interference, noise interference patterns and noise waveform nature of the different divisions. Among them: according to the different reasons for noise generation, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; according to the different waveforms and properties of the noise, it is divided into continuous noise, episodic noise, etc.; according to the different modes of acoustic interference, it is divided into common-mode interference and differential-mode interference. Common mode interference and differential mode interference is a more commonly used classification method. Common mode interference is the signal to the ground potential difference, mainly by the power grid string into the ground potential difference and space electromagnetic radiation in the signal line induced by the common-state (same direction) voltage sent to add the formation. Common mode voltage is sometimes larger, especially with poor isolation performance of electrical power supply room, transmitter output signal common mode voltage is generally higher, some can be as high as 130V or more. Common mode voltage through the asymmetrical circuit can be converted to differential mode voltage, directly affecting the measurement and control signals, resulting in component damage (this is the reason why some system I / O module damage rate is high), this common mode interference can be DC, can also be AC. Differential mode interference refers to the signal between the poles of the interference voltage, mainly by the electromagnetic field in the space between the signal coupling inductance and unbalanced circuitry to convert the common mode interference formed by the voltage, which allows the direct superposition of the signal, which directly affects the measurement and control accuracy.
3, what are the main sources of electromagnetic interference in the PLC control system?
(1) Radiation interference from space:
Space radiation electromagnetic field (EMI) is mainly generated by the power network, electrical equipment, transient processes, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually referred to as radiation interference, its distribution is extremely complex. If the PLC system is placed in the radio frequency field, it is recycled to the radiation interference, its impact is mainly through two paths; one is directly to the PLC internal radiation, by the circuit inductance of the interference; but rather to the PLC communication within the network of radiation, by the inductance of the communication line introduced by the interference. Radiation interference with the field equipment layout and equipment generated by the size of the electromagnetic field, especially the frequency, generally through the setting of shielded cables and PLC local shielding and high-voltage relief components for protection.
(2) from the system outside the lead interference:
Mainly through the power supply and signal lines introduced, usually called conduction interference. This kind of interference is more serious in our industrial site.
(3) interference from the power supply:
Practice has shown that the interference caused by the introduction of power supply PLC control system failure in many cases, I have encountered in a project debugging, and then replace the isolation performance of the higher PLC power supply, the problem is solved.
The normal power supply of the PLC system are powered by the power grid. Due to the wide coverage of the power grid
Will be subject to all the space electromagnetic interference and the induction of voltage and circuit on the line. Especially the changes within the grid, switching operation surge, large power equipment start and stop, AC and DC rotating devices caused by harmonics, grid short-circuit transient shock, etc., through the transmission line to the power side. PLC power supply is usually used to isolate the power supply, but its mechanism and manufacturing process factors make its isolation is not ideal. In fact, due to the presence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.
(4) from the introduction of the signal line interference:
Connected with the PLC control system of various types of signal transmission lines, in addition to the transmission of all types of effective signals, there will always be external interference signal intrusion. This interference mainly has two ways: one is through the transmitter or common signal instrumentation power supply string into the grid interference, which is often ignored; the second is the signal line by the space electromagnetic radiation induced interference, that is, the signal line on the external induction interference, which is very serious. Interference introduced by the signal will cause I / O signal work abnormality and measurement accuracy is greatly reduced, and in serious cases will cause component damage. For poor isolation performance of the system, will also lead to mutual interference between the signals, resulting in a common ground system bus backflow, resulting in changes in logic data, error and crash. PLC control system due to the introduction of signal interference caused by the number of damage to the I / O module is quite serious, resulting in system failure is also a lot of cases.
(5) interference from the grounding system confusion:
Grounding is to improve the electromagnetic compatibility of electronic equipment (EMC), one of the effective means. Correct grounding, both to inhibit the impact of electromagnetic interference, but also to inhibit the equipment to send out interference; and wrong grounding, but will introduce serious interference signals, so that the PLC system will not be able to work properly. PLC control system ground including the system ground, shielded ground, AC ground and protective ground and so on. Grounding system confusion on the PLC system interference is mainly uneven distribution of the potential of each grounding point, there is a ground potential difference between different grounding points, resulting in ground loop current, affecting the normal operation of the system. For example, the cable shield must be grounded at one point, if the cable shield ends A, B are grounded, there is a ground potential difference, there is a current flowing through the shield, when anomalies occur plus lightning strikes, the ground current will be greater.
In addition, the shielding layer, grounding wire and the earth may constitute a closed loop, under the action of the changing magnetic field, there will be induced currents in the shielding layer, through the shielding layer and the coupling between the core wire, interfering with the signal loop. If the system ground and other ground processing confusion, the resulting ground loop may produce unequal potential distribution in the ground, affecting the normal operation of the logic circuitry and analog circuitry within the PLC. PLC logic voltage interference tolerance is low, the logic of the distribution of ground potential interference is likely to affect the PLC's logic operations and data storage, resulting in data confusion, program run or crash. Analog ground potential distribution will lead to a decline in measurement accuracy, causing serious distortion of the signal measurement and control and false action.
(6) interference from within the PLC system:
Mainly by the system components and circuits within the mutual electromagnetic radiation generated, such as logic circuits and mutual radiation and its impact on analog circuits, analog and logic ground and the mutual influence of components and the use of mutual mismatch. This all belongs to the PLC manufacturer of the system within the electromagnetic compatibility design content, more complex, as the application of the department is unable to change, may not have to consider too much, but to choose to have more application achievements or after the test of the system.
4, the system is disturbed, often encountered the following major interference phenomena:
(1) When the system sends commands, the motor rotates irregularly;
(2) When the signal is equal to zero, the digital display meter value jumps randomly;
(3) sensor work, PLC collected signals and the actual parameters of the corresponding signal value does not match, and the error value is random, irregular;
(4) and AC servo systems share the same power supply (such as monitors, etc.) is not working properly.
5, how to better and simpler to solve the PLC system interference?
(1) Ideally, the use of isolation performance is better equipment, the use of good power supply, power line and signal line alignment, power grounding to be more reasonable, etc., but the need for different equipment manufacturers to work together in order to complete, it is difficult to do, and the cost is higher.
(2) the use of analog signal isolator, known as signal transmitter, belongs to the category of signal conditioning. Its main anti-interference role. Because it has a strong anti-interference ability so in the automation control system is very widely used. Especially for the complex industrial site, the control program is becoming more and more complex, signal isolator for a variety of analog signals for input, output, power triple isolation, is indeed one of today's automation control systems in the anti-jamming effective measures.
6, why solve the PLC system interference preferred signal isolator?
(1) the use of simple and convenient, reliable, low-cost, and can simultaneously solve a variety of interference.
(2) can greatly reduce the designers, system commissioning staff workload, even if the complex system in the hands of ordinary designers, will become very stable and reliable.
7,What is the working principle of signal isolator?
First of all, the signal received by the PLC, through the semiconductor device modulation transformation, and then through the optical or magnetic sensing device isolation conversion, and then demodulation transformation back to the original signal before isolation or different signals, and at the same time the isolation of the signal after the isolation of the power supply for isolation processing. Ensure the absolute independence between the transformed signal, power supply and ground.
8, now the market has so many brands of isolators, prices vary, how to choose?
Isolator is located between the two system channels, so the first thing to determine the selection of isolator input and output functions, at the same time to make the isolator input and output modes (voltage, current, loop power supply type, etc.) to adapt to the front and rear end channel interface mode. In addition, there are still precision, power consumption, noise, insulation strength, bus communication functions and many other important parameters related to product performance, for example: noise and precision, power consumption and reliability of heat, these users need to be carefully selected. In short, applicable, reliable, cost-effective products is the main principle of the choice of isolator.
If you are not too clear about the above sharing, then the next blackboard for everyone to highlight (11 kinds of analog interference solutions):
1, plus 1:1 signal isolator;
2, plus magnetic ring;
3, PLC power supply plus isolation transformer;
4, switching signals and analog signals go separately;
5, analog signals are best to use a separate shielded wire. Signal type is best used 4-20mA;
6, analog signal load is solenoid valve type, it is best to choose 1.5 line;
7, analog and digital signals can not be combined with the same multi-core cable, not to mention the power line and the common cable;
8, PLC input and output signal lines, must use shielded cables, in the input and output side of the suspension, and in the PLC side of the ground;
9, the signal cable should be away from strong interference sources, such as frequency converters, high-power silicon rectifier and large power equipment;
10, analog and digital signals can not be combined with the same multi-core cable, not to mention the power line and the common cable;
11, in order to reduce electronic interference for analog signals should use twisted shielded cable analog signal cable shield should be grounded at both ends, but if there is a potential difference between the two ends of the cable will be generated in the shielding layer, such as wire connection currents resulting in interference with analog signals in this case, you should let the cable shielding layer at one end of the ground.
Translated with DeepL.com (free version)