What are the reasons for PLC's reliable operation and strong anti-interference ability?

Anti-interference of PLC control system

1 overview

With the development of science and technology, PLC is more and more widely used in industrial control. The reliability of 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 whole system. Various types of PLC used in the automation system are installed in the control room, and some are installed in the production site and various motor equipment. Most of them are in the harsh electromagnetic environment formed by high-voltage circuits and high-voltage equipment. In order to improve the reliability of PLC control system, on the one hand, PLC manufacturers are required to improve the anti-interference ability of equipment; On the other hand, it is necessary to attach great importance to engineering design, installation, operation and maintenance, and multi-party cooperation can improve the problem and effectively enhance the anti-interference performance of the system.

2 electromagnetic interference sources and interference to the system

2. 1 General classification of interference sources and interferences

The interference sources that affect the PLC control system, like the interference sources that generally affect industrial control equipment, are mostly generated at the parts where the current or voltage changes dramatically, and the parts where these charges move violently are noise sources, that is, interference sources.

The types of interference are usually classified according to the cause of interference, noise interference mode and noise waveform characteristics. Among them, according to the different causes of noise, it can be divided into discharge noise, surge noise, high-frequency oscillation noise and so on. According to the waveform and nature of noise, it can be divided into persistent noise and sporadic noise. According to the different ways of noise interference, it can be divided into common mode interference and differential mode interference. Common mode interference and differential mode interference are commonly used classification methods. Common-mode interference is the potential difference between signal and ground, which is mainly formed by the superposition of power grid series, ground potential difference and common-mode (same direction) voltage induced by 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 high, and some of them can be as high as130V. The common-mode voltage can be converted into differential-mode voltage through asymmetric circuits, which directly affects the measurement and control signals and causes component damage (this is the main reason for the high damage rate of I/O modules in some systems). This common-mode interference can be DC interference or AC interference. Differential mode interference refers to the interference voltage acting between the two poles of signals, which is mainly caused by the coupling induction of spatial electromagnetic field between signals and the voltage formed by the conversion of common mode interference by unbalanced circuits. This interference is directly superimposed on the signal, which directly affects the measurement and control accuracy.

2.2 the main source of electromagnetic interference in PLC control system

2.2. 1 space radiation interference

The radiated electromagnetic field in space is mainly generated by the transient process of power grid, electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment and so on. , usually called radiation interference, its distribution is extremely complex. If the PLC system is placed in the RF field, it will receive radiation interference, and its influence mainly goes through two paths: one is to directly interfere with the radiation inside the PLC, which is generated by circuit induction; But in PLC communication, it will radiate to the network, and the induction of communication lines will introduce interference. Radiation interference is related to the layout of field equipment and the electromagnetic field generated by the equipment, especially the frequency. Generally, it is protected by setting shielded cables and PLC local shielding and high-voltage pressure relief components.

2.2.2 Interference of wires outside the system

Mainly introduced through power supply and signal line, usually called conducted interference. This kind of interference is more serious in China's industrial field.

(1) power supply interference

Practice has proved that the author encountered many failures of PLC control system caused by power supply interference in an engineering debugging, and the problem was solved only after replacing the PLC power supply with higher isolation performance.

The normal power supply of PLC system is provided by power grid. Due to the wide coverage of the power grid, it will be interfered by all space electromagnetic interference, and the voltage and circuit will be induced on the line. Especially the change of power grid, switching surge, start and stop of large power equipment, harmonics caused by AC and DC drive, short-circuit transient impact of power grid, etc. Are transmitted to the primary side of the power supply through transmission lines. PLC power supply usually adopts isolated power supply, but due to its mechanism and manufacturing process factors, isolation is not ideal. In fact, absolute isolation is impossible because of the existence of distributed parameters, especially distributed capacitance.

(2) Interference introduced by signal lines

All kinds of signal transmission lines connected with PLC control system, in addition to transmitting all kinds of effective information, there will always be external interference signals invading. There are two main ways of this interference: one is the power grid interference through the series connection of transmitter power supply or common signal instrument power supply, which is often ignored; Secondly, the signal line is induced by space electromagnetic radiation, that is, the signal line is induced by the outside world, which is very serious. Interference caused by signals will cause abnormal operation of I/O signals, greatly reduce the measurement accuracy, and even cause damage to components. For the system with poor isolation performance, it will also lead to mutual interference between signals, resulting in bus backflow of the shared system, resulting in logical data change, misoperation and crash. In the PLC control system, the number of I/O module damage caused by signal interference is quite serious, and there are also many system failures.

(3) Interference of chaotic grounding system

Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can not only restrain the influence of electromagnetic interference, but also restrain the interference from equipment; However, the wrong grounding will introduce serious interference signals, which will make the PLC system unable to work normally.

The grounding wire of PLC control system includes system grounding, shielding grounding, AC grounding and protective grounding. The interference of grounding system chaos to PLC system is mainly that the potential distribution of each grounding point is uneven, and there is ground potential difference between different grounding points, which causes ground loop current and affects the normal work of the system. For example, the cable shield must be grounded at one point. If both terminal A and terminal B of the cable shield are grounded, there will be a ground potential difference and current will flow through the shield. When lightning strikes and other abnormal conditions occur, the grounding current will be greater.

In addition, the shielding layer, the grounding wire and the earth can form a closed loop. Under the action of changing magnetic field, there may be induced current in the shielding layer, which will interfere with the signal loop through the coupling between the shielding layer and the core wire. If the system is confused with other grounding treatments, the generated ground circulation may produce uneven potential distribution on the grounding wire, which will affect the normal work of logic circuits and analog circuits in PLC. The tolerance of PLC's logic voltage interference is low, and the distribution interference of logic ground potential can easily affect PLC's logic operation and data storage, resulting in data confusion, program out of control or crash. The distribution of analog ground potential will lead to the decline of measurement accuracy, resulting in serious distortion and misoperation of signal measurement and control.

2.2.3 interference from inside the PLC system.

It is mainly produced by the mutual electromagnetic radiation between components and circuits in 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 components. This is a part of the EMC design of the system by PLC manufacturers, which is complicated and cannot be changed as an application department, so you don't need to consider too much, but you should choose a system with more application results or tests.

Anti-interference design of PLC control system engineering application

In order to avoid or reduce internal and external electromagnetic interference in industrial electromagnetic environment, three suppression measures must be taken from the design stage: suppressing interference sources; Cut off or attenuate the propagation path of electromagnetic interference; Improve the anti-interference ability of equipment and system. These three points are the basic principles to suppress electromagnetic interference.

The anti-interference of PLC control system is a systematic project, which requires manufacturers to design and produce products with strong anti-interference ability, and depends on the comprehensive consideration of engineering design, installation, operation and maintenance by the user department, and the comprehensive design is carried out in combination with the actual situation to ensure the electromagnetic compatibility and operational reliability of the system. In the anti-interference design of specific projects, we should mainly pay attention to the following two aspects.

3. 1 equipment selection

When choosing equipment, we should first choose products with strong anti-interference ability, including electromagnetic compatibility (EMC), especially products with strong anti-external interference ability, such as PLC system with floating technology and good isolation performance; Secondly, we should know the anti-interference indexes given by the manufacturer, such as common-mode analog ratio, differential-mode analog ratio, voltage withstand ability, allowable electric field strength and high-frequency magnetic field strength environment. The second is to investigate its application performance in similar work. Attention should be paid when selecting imported products: China adopts 220V high internal resistance power grid system, and Europe and America adopt 1 10V low internal resistance power grid system. Due to the large internal resistance of China's power grid, large zero potential drift and large ground potential change, the electromagnetic interference of industrial enterprises is at least four times higher than that of Europe and America, and the anti-interference performance of the system is higher. PLC products that can work normally in foreign countries may not run reliably in domestic industries. When adopting foreign products, it is necessary to choose them reasonably according to Chinese standards (GB/T 13926).

3.2 Comprehensive anti-jamming design

This paper mainly considers several intermediate frequency suppression measures from outside the system. The main contents include: shielding PLC system and external wires to prevent electromagnetic interference from space radiation; The external leads are isolated and filtered, especially the main power cables are arranged in layers to prevent the introduction of conducted electromagnetic interference through the external leads; Correctly design grounding points and grounding devices, and improve the grounding system. In addition, software means must be used to further improve the security and reliability of the system.

4 Main anti-interference measures

4. 1 adopt a power supply with excellent performance to suppress the interference introduced by the power grid.

In the PLC control system, the power supply occupies a very important position. The power grid interference series-connected PLC control system is mainly coupled by the power supply of PLC system (such as CPU power supply, I/O power supply, etc.). ), transmitter power supply and instrument power supply directly connected with PLC system. At present, for the power supply of PLC system, the power supply with good isolation performance is generally used, but the transmitter power supply and the instrument power supply directly connected with PLC system have not been paid enough attention. Although some isolation measures have been taken, they are generally not enough, mainly because the isolation transformer used has large distribution parameters and poor interference suppression ability, and common-mode interference and differential-mode interference are connected in series through power coupling. Therefore, for the power supply of transmitters and common signal instruments, the distributor with small distributed capacitance and large suppression band (such as multiple isolation shielding and leakage inductance technology) should be selected to reduce the interference to PLC system.

In addition, this location ensures that the power grid feed point will not be interrupted, and online uninterruptible power supply (UPS) can be used for power supply, which improves the safety and reliability of power supply. Moreover, UPS has strong interference isolation performance and is an ideal power supply for PLC control system.

4.2 Cable Selection and Design

In order to reduce the electromagnetic interference radiated by power cables, especially the feed cables of frequency conversion devices. In a project, the author used copper tape armored shielded power cable, thus reducing the electromagnetic interference in power line production. After the project was put into production, satisfactory results were achieved.

Different types of signals are transmitted by different cables, and signal cables should be installed in different layers according to the types of transmitted signals. It is forbidden to use different wires of the same cable to transmit power and signals at the same time, so as to avoid parallel installation of signal wires and power cables and reduce electromagnetic interference.

4.3 Hardware Filtering and Software Anti-IF Measures

Before the signal is connected to the computer, a capacitor is connected in parallel between the signal line and the ground to reduce common-mode interference; Adding a filter between the two poles of the signal can reduce the differential mode interference.

Due to the complexity of electromagnetic interference, it is impossible to eliminate the influence of interference. Therefore, in the software design and configuration of PLC control system, anti-interference treatment should be carried out in the software to further improve the reliability of the system. Some common measures: digital filtering and power frequency shaping sampling can effectively eliminate periodic interference; Regularly correcting the reference point potential and adopting dynamic zero point can effectively prevent potential drift; Using information redundancy technology, the corresponding software flag bits are designed; Using indirect jump and setting software traps to improve the reliability of software structure.

4.4 Correct selection of grounding point and improvement of grounding system.

Grounding usually has two purposes, one is for safety, and the other is to suppress interference. Perfect grounding system is one of the important measures to prevent electromagnetic interference of PLC control system.

There are three grounding modes in the system: floating grounding, direct grounding and capacitive grounding. For PLC control system, it is a high-speed and low-level control equipment, which should be directly grounded. Due to the influence of distributed capacitance of signal cables and filtering of input devices, the signal exchange frequency between devices is generally lower than 1MHz, so the grounding wire of PLC control system adopts one-point grounding and one-point grounding in series. Centralized PLC system is suitable for one-point grounding in parallel, and the central grounding point of each equipment cabinet is led to the grounding electrode with a separate grounding wire. If the equipment spacing is large, one point in series should be used for grounding. Connect the central grounding point of each equipment cabinet with a large section copper bus (or insulated cable), and then connect the grounding bus directly to the grounding electrode. The grounding wire shall be made of copper wire with a cross section greater than 22mm2, and the bus bar with a cross section greater than 60mm2 shall be used. The grounding resistance of grounding electrode is less than 2Ω, and the grounding electrode should be buried at a distance of 10 ~ 15m from the building, and the grounding point of PLC system must be more than 10m from the grounding point of high-voltage 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 multi-point grounding must be avoided; When the shielded twisted pair of multiple measuring point signals is connected with the main shielded cable of multi-core twisted pair, the shielding layers should be connected and insulated. Select the appropriate grounding point for single point contact.

5 concluding remarks

Interference in PLC control system is a very complicated problem. In anti-interference design, various factors should be considered comprehensively to restrain interference reasonably and effectively. Some interference situations need to be analyzed in detail, and the right medicine method can make the PLC control system work normally.