[b]1 Electromagnetic Interference Sources and Their Interference on the System 1.1 Interference Sources and General Classification of Interference[/b] Interference sources affecting PLC control systems, like those affecting general industrial control equipment, mostly originate from areas of drastic current or voltage change. These areas of rapid charge movement are noise sources, i.e., interference sources. Interference types are usually classified according to the cause of interference, noise interference mode, and waveform characteristics. Specifically, based on the cause of noise generation, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc.; based on the waveform and characteristics of noise, it can be divided into continuous noise, intermittent noise, etc.; and based on the noise interference mode, 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 the signal and ground, mainly formed by the common-mode (same direction) voltage drop induced on the signal line by grid interference, ground potential difference, and spatial electromagnetic radiation. Common-mode voltage can sometimes be large, especially when using power distribution equipment with poor isolation performance; the common-mode voltage of the transmitter output signal is generally high, sometimes reaching over 130 V. Common-mode interference can be converted into differential-mode voltage through asymmetrical circuits, directly affecting the measurement and control signals and causing damage to components (this is the main reason for the high failure rate of UO modules in some systems). This common-mode interference can be either DC or AC. Differential-mode interference refers to the interference voltage acting between the two poles of the signal. It is mainly formed by the coupling induction of the spatial electromagnetic field between signals and the voltage formed by the conversion of common-mode interference by unbalanced circuits. It is directly superimposed on the signal and directly affects the measurement and control accuracy. [b] 1.2 Main Sources of Electromagnetic Interference in PLC Control Systems 1.2.1 Radiation Interference from Space[/b] The radiated electromagnetic field in space is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasts, television, radar, high-frequency induction heating equipment, etc., and is usually called radiated interference. Its distribution is extremely complex. If the PLC system is placed in the radio frequency field, it will receive radiated interference. Its influence is mainly through two paths: one is direct radiation to the inside of the PLC, which generates interference through circuit induction; the other is radiation to the PLC communication network, which introduces interference through the induction of the communication line. The radiation interference is related to the layout of the equipment on site and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting up shielded cables and PLC local shielding and high voltage discharge components. 1.2.2 Interference from external leads of the system (1) Interference from the power supply. The normal power supply of the PLC system is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by electromagnetic interference from all spaces, which will induce voltage and circuit on the line. In particular, changes within the power grid, surges from switching operations, start-up and shutdown of large power equipment, harmonics caused by AC and DC transmission devices, and short-circuit impacts of the power grid are all transmitted to the power supply through the transmission lines. PLC power supplies usually use isolated power supplies. However, due to its structure and manufacturing process, its isolation performance is not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible. (2) Interference introduced from signal lines. In addition to transmitting various types of valid information, various signal transmission lines connected to the PLC control system will always be subject to external interference signals. There are two main ways to cause this interference: one is through the power grid interference introduced by the power supply of the transmitter or the power supply of the shared signal instrument, which is often overlooked; the other is the interference induced by the electromagnetic radiation of the space on the signal line, that is, the external induced interference on the signal line, which is very serious. The interference introduced by the signal will cause the UO signal to work abnormally and the measurement accuracy to be greatly reduced, and in severe cases, it will cause damage to the components. For systems with poor isolation performance, it will also cause mutual interference between signals, cause the common ground system bus to return, and cause changes in logic data, malfunctions and crashes. (3) Interference from the grounding system disorder. Grounding is one of the effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can suppress the influence of electromagnetic interference and suppress the equipment to emit interference. Incorrect grounding will cause serious interference signals and make the system unable to work normally. [b]1.2.3 Interference from inside the PLC system[/b] is mainly generated by the mutual electromagnetic radiation between the internal components and circuits of the system, such as the mutual radiation of logic circuits and its influence on analog circuits, the influence between analog ground and logic ground and the mismatch between components, etc. This all falls under the scope of electromagnetic compatibility (EMC) design within the PLC system by the manufacturer. It's quite complex and cannot be changed by the application department, but modules with extensive application experience or proven track records should be selected. 2. Anti-interference Design for PLC Control System Engineering Applications 2.1 Equipment Selection When selecting equipment, the first priority is to choose products with high-efficiency anti-interference capabilities, including EMC. Especially important is the ability to resist external interference, such as PLC systems using floating ground technology with cage isolation performance. Secondly, the anti-interference specifications provided by the manufacturer should be understood, such as common analog signal strength, differential analog signal strength, withstand voltage, and the allowable electric field strength and magnetic field frequency. Additionally, its application performance in similar applications should be examined. When selecting imported products, note that China uses a 220V high-resistance power grid, while Europe and America use a 110V low-resistance power grid. Due to the high internal resistance of China's power grid, large zero-point potential drift, and large ground potential variation, the electromagnetic interference at industrial sites is at least four times higher than in Europe and America. This places higher demands on the system's anti-interference performance. PLC products that work normally abroad may not operate reliably in China. Therefore, when using foreign products, it is necessary to select them reasonably according to Chinese standards. 2.2 Comprehensive Anti-interference Design This mainly considers various suppression measures from outside the system. The main contents include: evaluating the PLC system and external leads to prevent electromagnetic interference radiated from space; isolating and filtering external leads, especially power cables, and arranging them in layers to prevent conducted electromagnetic interference from being introduced through external leads; correctly designing grounding points and grounding devices, and improving the grounding system. In addition, software methods must be used to further improve the system's safety and reliability. [b]3 Main Anti-interference Measures 3.1 Using a high-performance power supply to suppress interference introduced by the power grid[/b] In the PLC control system, the power supply occupies a very important position. Interference from the power grid enters the PLC control system mainly through the power supply of the PLC system (such as CPU, power supply, I/O power supply, etc.), the power supply of transmitters, and the power supply of instruments directly electrically connected to the PLC system. For the power supply of transmitters and shared signal instruments, power distributors with small distributed capacitance and large suppression band (such as those using multiple isolation and shielding technologies and leakage inductance) are selected to reduce interference to the PLC system. Furthermore, to ensure uninterrupted power supply from the power grid, an online uninterruptible power supply (UPS) can be used to improve the safety and reliability of the power supply. UPS also has strong interference isolation performance, making it an ideal power supply for PLC control systems. 3.2 Cable Selection and Laying To reduce electromagnetic interference radiated from power cables, especially from the cables used for frequency converters, steel-tape armored shielded power cables are used in the project to reduce electromagnetic interference generated by the power cables. For long-distance wiring, input signal lines and output signal lines use separate cables. AC signals and DC signals use separate cables. Input and output signal lines are wired separately from high-voltage, high-current power lines. Shielded cables must be used for the input and output signal lines of integrated circuits or transistor devices. Avoid laying signal lines close to power cables in parallel to reduce electromagnetic interference. 3.3 Qualitative Analysis of I/O Modules Insulated input and output signals and internal circuits have better anti-interference performance than non-insulated ones; bidirectional thyristor and transistor-type contactless outputs generate less interference on the PLC controller side; input modules allow a large ON-OFF voltage difference for input signals, resulting in good anti-interference performance; input modules with slow input signal response times have good anti-interference performance. Therefore, from an anti-interference perspective, the following factors should be considered when selecting I/O modules: in situations with high interference, use insulated I/O modules; I/O modules installed on the controlled object side should be insulated I/O modules; in situations without external interference, non-insulated I/O modules can be used. 3.3.1 Prevention of Input Signal Interference In addition to using filters and ensuring proper grounding of the controller to suppress interference, the following measures can also be considered to prevent input interference. When there is an inductive load at the input terminal, to prevent back surge induced electromotive force, a resistor and capacitor are connected in parallel across the load terminals (for AC input signals, see Figure 1), or a freewheeling diode is connected in parallel (for DC input signals, see Figure 2). For AC input, the selection of resistors and capacitors must be appropriate to achieve better results. For load capacities below 10VA, 0.1μF and 120Ω are generally selected; for load capacities exceeding 10VA, 0.47μF and 47Ω are more suitable. If the inductive load connected in parallel with the input signal is large, using a relay for switching is more effective. Measures to prevent induced voltage: ① Connect a surge absorber in parallel at the input terminal; ② In long-distance wiring and high-current applications, the induced voltage is large, and a relay can be used for switching. [align=center] Figure 1 Figure 2 [/align] 3.3.2 Preventing Output Signal Interference Generation of output signal interference: In inductive load applications, a sudden current is generated when the output signal changes from OFF to ON, and a reverse induced electromotive force is generated when it changes from ON to OFF. All of these can generate interference. Measures to prevent interference: (1) For AC inductive loads, connect R and C in parallel across the load as surge absorbers. When the AC 220V voltage power is about 400VA, the values ​​of R and C are 47Ω and 0.47μF respectively. The closer R and C are to the load, the better their anti-interference effect, see Figure 3. (2) For DC loads, connect a freewheeling diode in parallel across the load, see Figure 4. The diode should also be close to the load. The reverse withstand voltage of the diode should be 4 times the load voltage. [align=center] Figure 3 Figure 4[/align] When the controller switches to the output, regardless of whether the controller itself has anti-interference measures, it is best to adopt the anti-interference measures shown in Figure 3 (AC load) and Figure 4 (DC load). In cases where there is a large interference during switching, AC loads can use bidirectional thyristor output modules. The method of using intermediate relays to drive the load in the control panel is very effective. For anti-interference technology of electronic equipment, the main principle is to suppress interference from the source of interference. The interference of the PLC programmable controller output signal can be introduced to the ground through good grounding, thereby reducing the impact of interference.