Type and source of interference in PLC operation and its anti-jamming design - Database & Sql Blog Articles

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Programmable Logic Controllers (PLCs) are widely used in industrial automation systems, often installed both centrally in control rooms and distributed across the production site or on various electrical equipment. These systems operate in environments with strong electromagnetic interference caused by high-power circuits and devices. To ensure reliable operation of PLC systems, it is essential for manufacturers to enhance the anti-interference capabilities of their products and for application engineers to focus on proper design, installation, and maintenance. Collaboration between these parties can significantly reduce interference and improve system performance. 1. Types of Electromagnetic Interference and Their Impact Electromagnetic interference (EMI) sources in PLC systems are similar to those affecting other industrial control systems. Most EMI originates from areas where current or voltage changes rapidly, such as switching operations or motor start-ups. These sudden changes create electromagnetic fields that can disrupt signal integrity and cause malfunctions. Interference can be categorized based on its source, noise mode, and waveform characteristics. Common classifications include: - Discharge noise - Surge noise - High-frequency oscillation noise From a waveform perspective, interference can be continuous or sporadic. In terms of noise mode, common-mode and differential-mode interference are the two primary types. Common-mode interference occurs when there's a potential difference between the signal and ground, often due to unbalanced power supplies, poor grounding, or electromagnetic radiation. This type of interference can be significant, especially in systems with low isolation, leading to damage of I/O modules. It can be either DC or AC in nature. Differential-mode interference, on the other hand, affects the signal directly by inducing voltage between the two signal lines. It typically results from unbalanced circuit configurations or coupling from nearby electromagnetic fields, which can degrade measurement accuracy and lead to operational errors. 2. Main Sources of Electromagnetic Interference 2.1 Space Radiated Interference Radiated EMI comes from power networks, transient events in electrical equipment, lightning, radio signals, radar, and high-frequency induction heating systems. When a PLC is placed within an RF field, it can be affected through direct radiation into internal circuits or via communication lines. Protection measures include using shielded cables, partial shielding, and high-voltage bleeders. 2.2 Conducted Interference Through External Leads Conducted interference enters the system through power and signal lines. In China’s industrial environment, this type of interference is particularly prevalent and falls into three main categories: First, power supply interference: Poorly isolated power supplies can introduce voltage fluctuations that affect PLC performance. In one project, replacing the power supply with a higher isolation model resolved the issue. Second, signal line interference: Signal lines can pick up noise from the power grid or external electromagnetic fields, causing I/O malfunctions and reduced accuracy. In some cases, this leads to component failure. Third, grounding issues: Proper grounding is crucial for EMI suppression. Incorrect grounding can create ground loops, causing potential differences that disrupt logic and analog circuits. For example, if both ends of a shielded cable are grounded, a current may flow through the shield, increasing susceptibility to interference during events like lightning strikes. 2.3 Internal System Interference Internal EMI arises from interactions between components within the PLC system, such as logic and analog grounds. This is primarily a concern for manufacturers during design, and users should choose systems with proven performance and good EMC compliance. 3. Anti-Interference Design To protect against internal and external EMI, three key strategies must be considered during the design phase: reducing the source of interference, blocking or attenuating propagation paths, and improving device immunity. These principles form the foundation of effective EMI mitigation. Anti-interference in PLC systems is a comprehensive engineering task. Manufacturers must produce robust systems, while users must carefully plan installations, maintenance, and operations. Specific attention should be given to equipment selection and overall system design. 3.1 Equipment Selection When choosing PLCs, prioritize models with strong anti-interference capabilities, including high common-mode rejection ratios, good isolation, and resistance to high electric and magnetic fields. Foreign products should be evaluated based on Chinese standards (e.g., GB/T 13926), as domestic conditions differ significantly from international ones. 3.2 Comprehensive Anti-Interference Measures Shielding, filtering, and proper grounding are critical. Shielding the system and external leads prevents radiated interference. Isolation and filtering of power and signal lines help block conducted interference. Grounding must be carefully planned to avoid ground loops and ensure stability. 4. Key Anti-Interference Techniques 4.1 Power Supply Optimization Use high-isolation power supplies to suppress grid-induced interference. Consider UPS systems for additional protection and reliability. 4.2 Cable Selection and Layout Choose appropriate cables and separate power and signal lines to minimize interference. Use shielded cables and avoid running them close together. 4.3 Hardware and Software Solutions Implement filters to reduce common-mode and differential-mode interference. Use software techniques like digital filtering, timing correction, and redundancy to enhance system reliability. 4.4 Grounding Practices Ensure proper grounding with single-point connections and low resistance. Avoid multi-point grounding to prevent ground loops. The grounding system should be designed to meet safety and EMI requirements. 5. Conclusion The challenge of EMI in PLC systems is complex and requires a holistic approach. Effective suppression involves careful design, proper equipment selection, and thorough implementation. Each situation may require unique solutions, and only through comprehensive planning can PLC systems operate reliably and efficiently in industrial environments.