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. 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 centrally in the control room, and some are installed on the production site and various motor 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 guarantee the reliable operation of the system only by understanding various disturbances in advance.
2. What are the sources of electromagnetic interference and interference to the system?
The interference that affects the PLC control system is the same as the interference source that generally affects industrial control equipment. Most of them are generated in the parts where the current or voltage changes drastically. The parts where these charges move violently 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 pattern of the noise, and the waveform nature of the noise. Among them: according to different causes of noise, it is divided into discharge noise, surge noise, high-frequency oscillation noise, etc .; according to the waveform and nature of noise, it is divided into continuous noise, incidental noise, etc .; according to the different sound interference modes, it is divided into a total of Mode interference and differential mode interference. Common mode interference and differential mode interference are a commonly used classification method. Common mode interference is the potential difference between the signal and the ground. It is mainly formed by the grid connection, the ground potential difference, and the common state (in the same direction) of the voltage induced by the space electromagnetic radiation on the signal line. 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 differential-mode voltage through an asymmetric circuit, which directly affects the measurement and control signal and causes component damage (this is why some system I / O modules have a high damage rate). This common-mode interference can be DC, It can also be used for communication. Differential mode interference refers to the interference voltage used between the two poles of the signal, which is mainly formed by the coupling induction of the space electromagnetic field between the signals and the common mode interference converted by the unbalanced circuit. This superimposed directly on the signal directly affects the measurement and control Precision.
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 caused by the transient process of the power network, 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 a radio frequency field, it will be affected by radiation interference, which is mainly affected by two paths; one is direct radiation to the PLC, which is caused by circuit induction; the other is radiation to the PLC communication network, which is caused by communication Line induction introduces interference. Radiated interference is related to the layout of the field equipment and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by the installation of shielded cables and PLC local shielding and high-voltage relief components.
(2) Interference from the external leads of the system
It is mainly introduced through the power supply and signal line, and is usually called conducted interference. This interference is more serious in China's industrial scene.
(3) Interference from 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 has encountered in the debugging of a project and then replaced the PLC power supply with higher isolation performance before the problem was solved.
The normal power supply of the PLC system is powered by the power grid. Due to the wide coverage of the power grid, it will be subject to all spatial electromagnetic interference and induce voltages and circuits on the line. In particular, changes within the power grid, surges in switching operations, start and stop of large power equipment, harmonics caused by AC and DC rotating devices, and transient impact of power grid short circuit, etc., all pass through the transmission line to the power supply side. PLC power supply usually adopts 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 not possible.
(4) Interference from signal lines
All kinds of signal transmission lines connected to the PLC control system, in addition to transmitting all kinds of effective signals, there will always be external interference signals invading. There are two main ways for this interference: one is the interference of the power grid connected through the power supply of the transmitter or the common signal instrument, which is often ignored; the other is that the signal line is interfered by the induction of space electromagnetic radiation, that is, the outside of the 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 damage to the components. For systems with poor isolation performance, it will also cause mutual interference between signals, causing common system bus to return, causing logical data changes, malfunctions, and crashes. The number of I / O modules damaged by the PLC control system is very serious due to the interference caused by the signal, and there are many cases of system failure 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 suppress the equipment from emitting interference; and wrong grounding will introduce serious interference signals, which will make the PLC system unable to work normally. The ground of PLC control system includes system ground, shielded ground, AC ground and protective ground. The disturbance of the grounding system chaos to the PLC system is mainly due to the uneven distribution of the potential of each grounding point. There is a ground potential difference between different grounding points, which causes a ground loop current and 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 A and B are grounded, there is a ground potential difference, and current flows through the shielding layer. When an abnormal state occurs with a lightning strike, the ground current will be greater.
In addition, the shielding layer, the ground wire and the earth may form a closed loop. Under the action of the changing magnetic field, there will be induced currents 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 ground is confused with other grounding processes, the resulting ground loop current may produce unequal potential distribution on the ground line, affecting the normal operation of logic circuits and analog circuits in the PLC. The logic voltage interference tolerance of PLC work is low, and the distribution interference of logic ground potential easily affects the PLC's logic operation and data storage, 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 malfunctions in signal measurement and control.
(6) Interference from within the PLC system
It is mainly generated by the mutual electromagnetic radiation between components and circuits within the system, such as the mutual radiation of logic circuits and its effect on analog circuits, the mutual influence of analog ground and logical ground, and the mismatch between components. This belongs to the content of the PLC manufacturer's electromagnetic compatibility design inside the system. It is relatively complex and cannot be changed as an application department. It does not need to be considered too much, but it is necessary to choose a system with more application performance or proven performance.
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