Electromagnetic interference source and anti-inter

Electromagnetic interference source and anti-interference design of PLC control system

with the development of industrial equipment automation control technology, programmable controller (PLC) is more and more widely used in industrial equipment control. The reliability of PLC control system directly affects the safety production and economic operation of enterprises. The anti-interference ability of the system is the key to the reliable operation of the whole system. This paper introduces in detail the interference types and sources that affect the operation of PLC, and puts forward the implementation strategy of anti-interference design

various types of plastic tableware used in the automation system and the regulations on the use of degradable plastic products are implemented. Some PLCs are centrally installed in the control room, some are installed on the production site and various motor equipment, and most of them are in the harsh electromagnetic environment formed by strong current circuits and strong current equipment. To improve the reliability of the PLC control system, on the one hand, the PLC manufacturer is required to improve the anti-interference ability of the equipment, on the other hand, the application department is required to pay high attention to the engineering design, installation, construction, use and maintenance, and the multi-party cooperation can improve the problem-solving and effectively enhance the anti-interference performance of the system

1. Electromagnetic interference types and their effects

the interference sources that affect the PLC control system are the same as the interference sources that generally affect industrial control equipment. Most of them are generated in the parts where the current or voltage changes violently, and these parts where the charge moves violently are the interference sources

interference types are usually divided according to the cause of interference, noise interference mode and the nature of noise waveform. According to the different causes of noise, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc; According to the different waveforms and properties of noise, it can be divided into continuous noise, accidental noise, etc; According to different noise interference modes, it is 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 the signal and the ground, which is mainly formed by the superposition of electric series input, ground potential difference and common state (same direction) voltage induced by space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially when the distributor with poor isolation performance is used for power supply, the common mode voltage of the transmitter output signal is generally high, and some can be as high as more than 130v. Common mode voltage can be converted into differential mode voltage through asymmetric circuit, which affects the measurement and control signal, resulting in whether the brand and after-sales service are normal and perfect device damage (this is the main reason for the high damage rate of some system i/o modules). This common mode interference can be DC or AC. Differential mode interference refers to the interference voltage acting between the two poles of the signal, which is mainly formed by the coupling induction of space electromagnetic field between signals and the conversion of common mode interference by unbalanced circuit. This interference is superimposed on the signal and directly affects the measurement and control accuracy

2. The main source of electromagnetic interference

2.1 radiated interference from space

space radiated electromagnetic field (EMI) is mainly produced by the transient process of power networks and electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., which is usually called radiated interference, and its distribution is extremely complex. If the PLC system is placed in its RF field, it will be subject to radiation interference, which is mainly affected by two paths: one is the direct radiation inside the PLC, which generates interference by circuit induction; The second is the radiation to PLC communication network, and the interference is induced by the communication line. Radiation interference is related to the layout of field equipment and the size of electromagnetic field generated by the equipment, especially the frequency. Generally, it is protected by setting shielded cables and PLC local shielding and high-voltage discharge elements

2.2 interference from external leads of the system

is mainly introduced through power supply and signal lines, which is usually called conducted interference. This kind of interference is relatively serious in China's industrial sites, mainly including the following three categories:

the first category is the interference from the power supply. Practice has proved that there are many PLC control system failures caused by the interference introduced by the power supply. The author encountered it in the commissioning of a project, and then the problem of replacing the PLC power supply with higher isolation performance was solved

the normal power supply of PLC system is powered by electricity. Due to the wide coverage of electricity, it will be subject to all space electromagnetic interference, and the induced voltage and current on the road, especially the internal changes of electricity, switching surges, start and stop of large power equipment, harmonics caused by AC and DC drives, electrical short-circuit transient impact, etc., will be transmitted to the original side of the power supply through the transmission line. PLC power supply usually adopts isolated power supply, but its isolation is not ideal due to its mechanism, manufacturing process and other factors. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible

the second type is the interference introduced from the signal line. 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 signal intrusion. There are mainly two ways of this interference: one is the electrical interference through the power supply of the transmitter or the power supply of the common signal instrument, which is often ignored; Second, the signal line is interfered by the induction of space electromagnetic radiation, that is, the external induction interference on the signal line, which is often 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, which will cause component damage in serious cases. For systems with poor isolation performance, it will also lead to mutual interference between signals, causing common ground system bus backflow, resulting in logic data changes, maloperation and crash. The number of i/o module damage caused by signal interference in PLC control system is quite serious, which leads to a lot of system failures

the third type is the interference from the chaos of the grounding system. Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the influence of electromagnetic interference, but also suppress the outward interference of equipment; However, the wrong grounding will introduce serious interference signals, making the PLC system unable to work normally

the ground wire of PLC control system includes system ground, shielding ground, AC ground, protective ground, etc. the interference of grounding system chaos on PLC system is mainly the uneven distribution of potential at each grounding point, 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 shielding layer must be grounded at one point. If both ends a and B of the cable shielding layer are grounded, there is a ground potential difference, and current flows through the shielding layer. When abnormal conditions such as lightning strike occur, the ground current will be greater

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

2.3 interference from the PLC system

is mainly generated by the mutual electromagnetic radiation between components and circuits in the system, such as the mutual radiation of logic circuits, the mutual influence between analog and logic, and the mismatching use of components and parts. This is the content of the electromagnetic compatibility design of the system by the PLC manufacturer, which is relatively complex. As an application department, it can't be changed, so you don't have to think too much, but you should choose a system with more application achievements or tested

3. Anti interference design

in order to ensure that the system is free from or reduce internal and external electromagnetic interference in the industrial electromagnetic environment, three suppression measures must be taken from the beginning of the design stage: suppressing interference sources, cutting off or attenuating the propagation path of electromagnetic interference, and improving the anti-interference ability of devices and systems. These three points are the basic principles of suppressing electromagnetic interference

3.1 equipment selection

when selecting equipment, we must first select products with high anti-interference ability, including electromagnetic compatibility, especially anti-external interference ability, such as PLC system with floating technology and good isolation performance; Secondly, we should also know the anti-interference indicators given by the manufacturer, such as common mode rejection ratio, differential mode rejection ratio, withstand voltage ability, and how much electric field strength and high frequency magnetic field strength environment are allowed to work; In addition, it depends on its application performance in similar work

when choosing imported products from abroad, it should be noted that China adopts 220V high internal resistance system, while Europe and the United States adopts 110V low internal resistance. Due to the large internal resistance, large zero potential drift and large ground potential change in China, the electromagnetic interference on the site of industrial enterprises is at least 4 times higher than that in Europe and the United States, and the requirements for the anti-interference performance of the system are higher. PLC products that can work normally in foreign countries may not be able to operate reliably in domestic industry, so it is necessary to reasonably select foreign products according to Chinese standards (gb/t13926)

3.2 comprehensive anti-interference design

several suppression measures from the outside of the system are mainly considered, including: shielding the PLC system and external leads to prevent space radiated electromagnetic interference; The external leads shall be isolated and filtered, especially the power cables shall be arranged in layers to prevent conductive electromagnetic interference from being introduced through the external leads; Correctly design the grounding point and grounding device, and improve the grounding system. In addition, software means must be used to further improve the safety and reliability of the system

4. Main anti-interference measures

4.1 the power supply with excellent performance is used to suppress the interference introduced by electricity

in the PLC control system, the power supply plays a very important role. Electrical interference is cascaded into the PLC control system mainly through the coupling of the power supply of the PLC system (such as CPU power supply, i/o power supply, etc.), transmitter power supply and instrument power supply with direct electrical connection with the PLC system. At present, the power supply of PLC system generally adopts the power supply with good isolation performance, while the transmitter power supply and the instrument power supply with direct electrical connection with PLC system have not been paid enough attention. Although some isolation measures have been taken, they are generally not enough. The main reason is that the isolation transformer used has large distribution parameters and poor interference suppression ability. Common mode interference and differential mode interference are cascaded through power coupling. Therefore, for the power supply of transmitters and common signal instruments, the distributor with small distribution capacitance and large suppression band (such as multiple isolation, shielding and leakage inductance technology) should be selected to reduce the interference of PLC system

in addition, in order to ensure the uninterrupted power supply, UPS can be used to improve the safety and reliability of power supply. Moreover, UPS also has strong interference isolation performance, which is an ideal power supply for PLC control system

4.2 correct selection and laying of cables

in order to reduce the radiated electromagnetic interference of power cables, especially the feed cables of frequency conversion devices