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In the context of industrial control systems, minimizing electromagnetic interference (EMI) is essential for ensuring reliable and stable operation. One effective approach is to carefully select cabling that reduces radiated EMI, particularly in power cables such as those used in inverters. In a real-world project, the use of copper-tape-armored shielded power cables significantly reduced EMI from power lines, resulting in improved system performance once implemented.
Different types of signals should be transmitted via dedicated cables, with proper layering based on signal type. It is crucial to avoid combining power and signal transmission within the same cable, as this can lead to parallel connections that increase EMI. Separating these functions ensures better signal integrity and reduces potential interference sources.
Power supply design also plays a critical role in EMI suppression. In PLC control systems, the power supply is a key component that can introduce grid-based interference. To mitigate this, high-quality isolated power supplies are typically used. However, even with some isolation measures, many power supplies still lack sufficient shielding and filtering. This can allow common-mode and differential-mode interference to couple into the system through power lines. Therefore, selecting power supplies with low distributed capacitance and wide suppression bandwidth—such as those incorporating multiple isolation layers, shielding, and leakage inductance—can greatly reduce interference in the PLC system.
Additionally, using an online uninterruptible power supply (UPS) helps ensure continuous power availability and enhances the system's ability to isolate external interference. A UPS not only provides backup power but also acts as a barrier against electrical disturbances, making it an ideal choice for PLC systems.
Proper grounding is another vital aspect of EMI reduction. Grounding serves two main purposes: safety and interference suppression. A well-designed grounding system is one of the most effective methods for reducing electromagnetic interference in PLC systems. There are several grounding methods, including floating, direct, and capacitive grounding. For PLC systems, which operate at high speed and low voltage, direct grounding is generally preferred.
Due to the influence of cable capacitance and filtering at input devices, signal exchange frequencies are usually below 1 MHz. Therefore, one-point grounding is often used, where all grounding points connect to a central ground point. In centralized systems, parallel grounding is suitable, with each device’s grounding point connected directly to the main grounding pole. When devices are spaced far apart, a series one-point grounding method is more appropriate, using thick copper busbars or insulated cables to connect the central ground to each unit before connecting to the grounding pole.
The grounding wire should have a cross-section larger than 22 mm², while the main busbar should be made of copper with a cross-section greater than 60 mm². The grounding resistance must be less than 2 Ω, and the grounding pole should be placed at least 10 to 15 meters away from buildings or no more than 50 meters from the controller. Additionally, the PLC grounding point should be at least 10 meters away from strong electrical equipment grounding points.
When the signal source is grounded, the shielding layer should be grounded on the signal side. If not grounded, it should be connected to the PLC side. In cases where there are joints in the signal line, the shielding layer should be securely connected and insulated, avoiding multi-point grounding. When connecting shielded twisted pairs to multi-core cables, the shielding layers should be joined and insulated properly.
Hardware filtering and software anti-interference measures are also essential. Due to the complexity of EMI, it is impossible to eliminate all interference completely. Therefore, software design should include anti-interference techniques to enhance system reliability. Common strategies include digital filtering, power frequency shaping sampling, timing correction, dynamic zero-point adjustment, and information redundancy. Software traps and indirect jumps can also improve the robustness of the system.
Before signals reach the computer, a capacitor can be placed between the signal line and ground to suppress common-mode interference. Adding filters between the signal poles can help reduce differential-mode interference. For analog signals with low signal-to-noise ratios, which are prone to fluctuations due to transient interference, digital filtering techniques can be applied. These methods convert the analog signal into a digital format, store the data in sequence, and then process it to remove noise, improving signal accuracy.
By implementing these strategies, PLC systems can achieve higher stability and reliability in harsh industrial environments.