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In the context of industrial control systems, electromagnetic interference (EMI) is a major concern that can significantly impact system performance and reliability. To mitigate this, several strategies are employed in the design and implementation of PLC-based systems.
First, selecting the right type of cable plays a crucial role in reducing radiated EMI, especially for power cables like those used in inverters. In one project, a copper-tape-armored shielded power cable was implemented, which effectively minimized the EMI generated by the power lines. This approach led to improved system performance after the project was deployed. It's important to differentiate between signal and power cables, as mixing them within the same cable can lead to unwanted coupling and increased interference. Therefore, signal cables should be layered according to their function, and it’s essential to avoid running power and signal wires in parallel to reduce cross-talk and EMI.
Second, the power supply is a critical component in suppressing grid-induced interference. In PLC systems, the power supply serves as a primary entry point for EMI, particularly through components like the CPU, I/O modules, and other connected devices. Although many systems use isolated power supplies, attention is often not given to the isolation of transmitter and instrument power sources. These components can still introduce common-mode and differential-mode interference, even with some isolation measures. To address this, it's recommended to use power supplies with low distributed capacitance and broad suppression bands, incorporating technologies such as multiple isolations, shielding, and leakage inductance. Additionally, an online uninterruptible power supply (UPS) can enhance power reliability and provide strong isolation against interference, making it an ideal choice for PLC systems.
Third, proper grounding is essential for both safety and EMI reduction. The PLC control system typically requires direct grounding due to its high-speed, low-level operation. Grounding methods such as single-point or parallel grounding are commonly used depending on the system layout. For centralized systems, each device's grounding point should be connected to a central ground bus, which then connects to a grounding electrode. The grounding wire should be made of copper with a cross-section larger than 22 mm², while the main busbar should be at least 60 mm². The grounding resistance must be less than 2 Ω, and the grounding electrode should be placed at least 10–15 meters away from the building or no more than 50 meters from the controller. Also, the PLC grounding point should be separated from strong electrical equipment by at least 10 meters.
When grounding shielded cables, the shield should be grounded on the signal side if the source is grounded, and on the PLC side if not. If there is a joint in the signal line, the shield must be securely connected and insulated, avoiding multi-point grounding. When connecting a shielded twisted pair to a multi-core screen cable, the shields should be connected and insulated. Choosing the correct grounding point is vital for maintaining signal integrity.
Finally, hardware filtering and software anti-interference techniques are essential to handle complex EMI scenarios. Even with physical shielding, some interference may still occur, so software-based solutions are necessary. Techniques like digital filtering, power frequency sampling, and dynamic zero-point adjustment can help eliminate periodic and transient interference. Redundancy in data and the use of software traps or indirect jumps can further improve system reliability. Before signals are processed, capacitors can be added between the signal line and ground to suppress common-mode noise, while filters can be used to reduce differential-mode interference.
For analog signals with poor signal-to-noise ratios, instantaneous interference can cause large fluctuations. Using digital filtering methods, such as averaging multiple samples over time, helps stabilize the signal. A/D conversion transforms continuous analog signals into discrete digital values, which are then stored in memory. A digital filter processes these values to remove noise, improving accuracy. These filtered signals can then be used for display or control purposes.
In harsh industrial environments, long-distance I/O signal transmission increases the risk of errors. Implementing fault-tolerant software allows the PLC to detect and correct errors, ensuring continued operation. Overall, a combination of proper cabling, grounding, power supply isolation, and software filtering is key to achieving a reliable and interference-free PLC control system.