Abstract : This article mainly introduces the types of interference that PLC control systems may encounter in industrial applications, proposes targeted anti-interference measures, and emphasizes that a comprehensive analysis of the environment must be conducted during system design and installation to determine the nature of the interference and take corresponding anti-interference measures. Keywords : PLC; type of disturbance; anti-jamming [align=center]The Anti-jamming Technology of PLC in the Application of the Industrial Control System Zhao Lin Zhai Nuo Shen Min (Automation Department of Laiwu Iron and Steel Ltd. ShanDong 271104)[/align] Abstract : This paper mainly tells us the type of disturbance that the PLC Control System is liable to in the application of industry, and puts forward the method of the aim to anti-jamming, and emphasizes that it must be a comprehensive analysis of the environment, conform to the characteristics of disturbance, and adopt the relevant anti-jamming method in the system design and installation. Keywords : PLC; type of disturbance; anti-jamming 1 Introduction The rapid development of industry has made it increasingly dependent on control systems. Distributed control systems (DCS), programmable logic controllers (PLC), fieldbus control systems (FCS), industrial control computers (IPC), and various measurement and control instruments have become the main hardware facilities constituting industrial automation. With the development of microelectronics technology and the increasing integration of control systems, the number of components per unit area within large-scale integrated chips is increasing, the transmitted signal current is decreasing, and the system supply voltage is decreasing, now down to 5V, 3V, and even 1.8V. Therefore, chips are becoming increasingly sensitive to external noise, resulting in low anti-interference capabilities. Furthermore, compared to other electronic information systems, control systems are not only more complex, with more devices and input/output (I/O) ports, but also have numerous and long external connection cables. This acts like a highly efficient noise-picking antenna, providing ample conditions for noise coupling and making it easy for various noises to intrude into the control system. PLCs, with their advantages of simple programming, good versatility, powerful functions, and easy expansion, especially due to the use of highly integrated microelectronic devices, have high reliability and strong adaptability to harsh industrial environments, and have been widely used in industrial control. Currently, the PLCs used in industrial production line control systems are mainly installed in the main control room. These systems are mostly located in harsh electromagnetic environments created by high-voltage circuits and equipment, making them easily susceptible to interference from surrounding sources, which can cause malfunctions and affect the normal operation of the system. Therefore, it is essential to emphasize the anti-interference design of the system. To prevent interference, a combination of hardware and software anti-interference methods can be used. 2 Basic Composition Structure of a PLC System The programmable controller hardware system consists of a PLC, functional I/O units, and external devices, as shown in Figure 1. The PLC comprises a CPU, memory, basic I/O modules, I/O expansion interfaces, peripheral interfaces, and a power supply, all connected by an internal system bus. [align=center] Figure 1 Basic Composition Structure of a PLC System[/align] 3 Types of Interference Affecting the Stability of a PLC Control System 3.1 Spatial Radiation Interference Spatial radiated electromagnetic fields (EMI) are mainly generated by power networks, electrical equipment, lightning, high-frequency induction heating equipment, and large rectifier equipment, and are commonly referred to as radiated interference. The high-density use of electrical and electronic equipment has led to increasingly serious spatial electromagnetic pollution. The radiated waves generated by these interference sources have a wide frequency range and are irregular. Spatial radiated interference, through electromagnetic induction, forms a receiving circuit via the casing and wires of the detection system, causing interference to the system. If the PLC is placed within its radiation field, its signal, data, and power lines can act as antennas to receive radiated interference. This type of interference is related to the layout of field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by shielded cables, local shielding of the PLC, and high-voltage discharge components. 3.2 Power Supply Interference PLC systems are generally powered by industrial power networks. The start-up and shutdown of certain large equipment in industrial systems may cause overvoltage, undervoltage, surges, sags, and spike interference in the power supply. These voltage noises can be coupled to the PLC system circuitry through the power supply's internal resistance, causing significant damage to the system. 3.3 Interference from Signal Transmission Lines In addition to transmitting valid information, various signal transmission lines connected to the PLC system will always be subject to external interference signals. Interference introduced by signal lines can cause abnormal I/O signal operation and a significant reduction in measurement accuracy, and in severe cases, damage to components. If the system isolation performance is poor, it will also lead to mutual interference between signals, causing backflow in the common ground system bus, resulting in changes in logic data, malfunctions, or even system crashes. 3.4 Interference caused by digital circuits Although the DC current drawn by digital integrated circuits is only in the mA range, it will generate significant interference when the circuit is in high-speed switching mode. For example, a TTL gate circuit draws about 5mA of current from the DC power supply in the on state and 1mA in the off state. Its current changes by 4mA within 5ns. If there is an inductance of 0.5μH on the power distribution line, when this gate circuit changes state, the noise voltage generated on the power distribution line is: U=L x di/dt=0.5x 10-6 x 4x10-3 /5x10-9=0.4V. If this value is multiplied by the value of a large number of gates in a typical system, although the supply voltage of this gate circuit is only 5V, the resulting interference noise will be very serious. When processing pulse digital circuits, a rough concept of the spectrum contained in the pulse should be obtained. If the pulse rise time t is known, its equivalent highest frequency can be calculated using an approximate formula: fmax = 1/2πt. 1.4 Interference generated inside the PLC system. 4 Anti-interference design in PLC system 4.1 Grounding anti-interference design Grounding plays a significant role in eliminating interference. Good grounding is one of the important conditions for ensuring the reliable operation of the PLC and can avoid the damage caused by accidental voltage surges. In order to suppress interference attached to the power supply and input/output terminals, the PLC should be connected to a dedicated ground wire. The grounding wire should be separate from the grounding point of the power equipment (such as the motor). If this requirement cannot be met, it can be grounded together with other equipment. It is strictly forbidden to connect it to other equipment in series. The grounding resistance should be less than 5Ω, the grounding wire should be thick, the area should be greater than 2 square millimeters, and the grounding point should preferably be close to the PLC device, with a distance of less than 50 meters. The grounding wire should avoid high-voltage circuits. If it is impossible to avoid them, they should intersect perpendicularly and the length of parallel lines should be shortened. 4.2 Power Supply Anti-interference Design Voltage distortion or glitches caused by power supply fluctuations will adversely affect the PLC and I/O modules. Statistical analysis shows that 70% of interference in PLC systems originates from power supply coupling. To suppress interference, the PLC power supply system can adopt the following approach: the controller and I/O system are powered by their own isolation transformers, separated from the main circuit power supply. When a part of the power supply fails, it will not affect other parts. For example, if the input or output power supply is interrupted, the controller can still continue to supply power, improving system reliability. 4.3 Input/Output Signal Anti-interference Design To prevent interference to input and output signals, isolated I/O modules should be selected. 4.3.1 Input Signal Anti-interference Design Differential-mode interference between input signal lines can be reduced by using input module filtering, while common-mode interference between input lines and ground can be suppressed by grounding the controller. When there is an inductive load at the input terminal, in order to prevent the influence of induced electromotive force caused by sudden changes in circuit signals, hardware reliability, fault tolerance, and tolerance design techniques can be adopted. For AC input signals, a capacitor C and a resistor R can be connected in parallel across the load terminals. For DC input signals, a freewheeling diode D can be connected in parallel. Generally, when the load capacity is below 10VA, C should be 0.1μF and R should be 120Ω. When the load capacity is above 10VA, C should be 0.47μF and R should be 47Ω. The specific circuit is shown in Figure 2: [align=center] Figure 2 Anti-interference design of input signal[/align] 4.3.2 Anti-interference design of output circuit For PLC systems with switching outputs, there are three forms: relay output, thyristor output, and transistor output. The specific selection depends on the load requirements. If the load exceeds the output capacity of the PLC, an external relay or contactor should be connected for normal operation. If an inductive load is connected to the PLC output terminal, there will be sudden changes in electrical quantity when the output signal changes from OFF to ON or from ON to OFF, which may cause interference. Appropriate protective measures should be taken during the design to protect the PLC output contacts, as shown in Figure 3. For DC loads, a freewheeling diode D is usually connected in parallel across the coil. The diode should be as close to the load as possible, and it can be a 1A diode. For AC loads, an RC snubber circuit should be connected in parallel across the coil. Depending on the load capacity, the capacitor can be 0.1μF to 0.47μF, and the resistor can be 47Ω to 120Ω, with the RC circuit as close to the load as possible. 4.4 Anti-interference design of external wiring There are mutual inductances and distributed capacitances between external wirings, which will generate crosstalk during signal transmission. To prevent or reduce interference from external wiring, AC input and output signals and DC input and output signals should use their own cables. Shielded cables should be used for the input and output signal lines of integrated circuits or transistor devices. The shielded cables should be left floating on the input and output sides, and grounded on the controller side. For short distances of less than 30 meters, DC and AC input and output signal lines should preferably not use the same cable. If they must run in the same conduit, the input signal should use a shielded cable. For wiring distances of 30 to 300 meters, DC and AC input and output signal lines should use separate cables, and input signal lines must be shielded. For wiring distances exceeding 300 meters, intermediate relays can be used to convert signals, or remote I/O channels can be used. The controller's grounding wire should be separated from the power supply line, and input and output signal lines should be wired separately from high-voltage, high-current power lines. 4.5 Software Anti-interference Design Although hardware anti-interference can filter out most interference signals, the causes of interference signals are complex and highly random, making it difficult to guarantee that the system is completely free from interference. Therefore, software anti-interference technology is often used as a supplement to hardware anti-interference measures. Software anti-interference methods are simple to design, flexible to modify, and consume fewer resources, and have also been widely used in PLC measurement and control systems. For PLC measurement and control devices, their data input, output, and storage systems are low-voltage systems. If interference exists in the working environment, the data may be corrupted, resulting in data errors, control failure, changes in program state and the working state of certain devices, and in severe cases, system program corruption. Generally, two methods are used: instruction repetition and digital filtering. [align=center]Figure 3 Protection of PLC output contacts[/align] 4.5.1 Instruction Repetition Instruction repetition involves repeatedly executing instructions with the same function as needed. It is generally suitable for anti-interference of switch or digital inputs and outputs. When acquiring certain switch or digital quantities, multiple acquisitions can be performed until two or more consecutive acquisitions are completely identical, which is considered valid. If the signal is constantly changing after multiple acquisitions, acquisition can be stopped and an alarm signal issued. Under the premise of meeting real-time requirements, inserting a delay between each acquired signal will improve data reliability. If the system's real-time requirements are not very high, the instruction repetition cycle should be as long as possible. 4.5.2 Digital Filtering In the acquisition process of some signals, random interference may increase the random error of the measured signal. In this case, digital filtering technology can be used. This method has the characteristics of high reliability and good stability and is widely used in industrial computer measurement and control systems. In addition, the commonly used digital filtering methods include: program judgment filtering method, median filtering method, arithmetic mean filtering method, recursive average filtering method, etc. 5 Conclusion With the gradual expansion of the application scope of PLC, coupled with the harsh working environment of the system, the interference it needs to overcome will increase, even though the reliability of the PLC itself is very high. However, in the system design and installation, it is still necessary to conduct a comprehensive analysis of the environment, determine the nature of the interference, and take corresponding anti-interference measures to ensure the long-term stable operation of the system. References: [1] Zhu Bangtian, Practical Anti-interference Technology for Electronic Circuits [M], Beijing, People's Posts and Telecommunications Press, 1994. [2] Instrument Piping and Wiring Design Specification (HG/T20512—2000) (S). [3] Qu Jianchang (ed.), Electromagnetic Compatibility Design of Electronic Equipment [M], Beijing: Electronic Industry Press, 2003. [4] Jin Jianxiang, Zou Haiming, Xu Yiheng, Discussion on the CMRR and SMRR Index Values ​​of Analog Input Templates [J], Automation Instrumentation, 2004, 3, 14-18. [5] Lin Guorong, Electromagnetic Interference and Control [M], Beijing: Electronic Industry Press, 2003. About the Author : Zhao Lin (1981-), male, Bachelor's degree, Engineer, major research direction is distributed control and robust control. Contact Person: Zhao Lin Contact Address: Yinshan Rolling Mill Workshop, Automation Department, Laiwu Iron and Steel Group, Shandong Province, China Postcode: 271104 Mobile: 13863447631 E-mail: [email protected]