How to eliminate interference in PLC systems?

There is also electromagnetic interference and pulse interference, all of which need to be eliminated through shielding or by using capacitors for physical suppression, as well as through digital filtering

(1) Interference introduced by resistive coupling (conductive introduction): ① When several signal lines are transmitted together, leakage current due to the aging of insulating materials affects other signals, thereby introducing interference into them. ②In some control systems that use electrical energy as the actuating mechanism (such as electric heating furnaces, electrolyzers, etc.), leakage from signal sensors and contact with live parts can also cause significant interference. ③In some older instruments and actuators, the field side is powered by 220V; sometimes the equipment gets damaged, resulting in a short circuit between the power supply and the signal lines, which can also cause significant interference. ④Due to improper grounding, such as grounding at both ends of the signal line, a significant amount of interference is introduced due to the difference in ground potential, as shown in Figure 3.4.9. Both ends of the signal line are grounded simultaneously; as a result, if the distance between points A and B is large, a significant potential difference eN may arise. This potential difference can induce a large circulating current in the signal line running between A and B. (2) Interference introduced by capacitive and inductive coupling: Since many signals often reach the computer at the controlled site simultaneously, and these signal wires are either routed through cable trays or cable ducts, it is certain that numerous signals travel together along the same pathways. There is distributed capacitance between these signals, and interference is introduced onto other signal lines through this distributed capacitance. At the same time, an alternating magnetic flux is generated around the alternating signal lines, and this alternating magnetic flux creates electromotive forces between parallel conductors, which also leads to interference in the circuits. 3) Interference introduced into the computer power supply lines. In some industrial facilities (especially power plants, metallurgical enterprises, and large machinery manufacturing plants), large electrical equipment is started frequently, and large switching devices also operate often. The sparks generated by the starting of these motors and the closing of switches create strong alternating magnetic fields in their vicinity. These alternating magnetic fields can cause interference either by coupling to the signal lines or by generating high-frequency interference on the power supply lines; if such interference exceeds the allowable levels, it can affect the operation of the computer system as well. (4) Interference caused by lightning strikes Lightning strikes can generate significant electromagnetic interference around the system, and they can also introduce interference through various grounding wires. This post was last edited by wopale3 on 2009-3-3 15:31]

Interference that is difficult to eliminate can be addressed by adding a signal isolator for isolation

As stated in the book \"PLC Technology\", for reference: Main anti-interference measures for PLCs 4.1 Use power supplies with excellent performance to suppress interference introduced from the power grid. In PLC control systems, the power supply plays an extremely important role. Grid interference intruding into PLC control systems enters primarily through the power supplies of the PLC system (such as the CPU power supply, I/O power supplies, etc.), the power supplies of transmitters, and the power supplies of instruments that are in direct electrical connection with the PLC system. Currently, for the power supplies that supply energy to PLC systems, power supplies with good isolation properties are generally used. However, insufficient attention is paid to the power supplies that supply energy to transmitters and to the instruments that are electrically connected directly to the PLC system. Although certain isolation measures are taken, they are often not sufficient; this is mainly because the isolation transformers used have large distributed parameters, resulting in poor ability to suppress interference, allowing common-mode and differential-mode interference to be introduced through power coupling. Therefore, for the power supply of transmitters and instruments that share signals, it is necessary to use power distribution units with low distributed capacitance and a wide suppression band (such as those that employ multiple isolation and shielding methods as well as leakage inductance techniques) in order to reduce interference in the PLC system. Furthermore, to ensure uninterrupted power supply to the grid feed points, an online uninterruptible power supply (UPS) can be used to enhance the safety and reliability of the power supply. Furthermore, UPS also has strong interference isolation capabilities, making it an ideal power supply for PLC control systems. 4.2 Installation considerations for cable selection: To reduce electromagnetic interference generated by power cables, especially those used for feeding frequency conversion devices. In a certain project, the author used copper tape-armed shielded power cables, which reduced the electromagnetic interference generated by the power lines; satisfactory results were achieved after the project was put into operation. Different types of signals are transmitted via separate cables. Signal cables should be arranged in layers according to the type of signal they transmit. It is strictly prohibited to use different wires within the same cable to transmit both power supply and signals simultaneously. Signal cables should also not be laid close to power cables in order to reduce electromagnetic interference. 4.3 Hardware filtering and software countermeasures: Before the signal is fed into the computer, a capacitor is connected in parallel between the signal line and ground to reduce common-mode interference ； Installing a filter between the two poles of the signal can reduce differential mode interference. Due to the complexity of electromagnetic interference, it is impossible to completely eliminate its effects. Therefore, when designing and configuring the software for PLC control systems, anti-interference measures must also be implemented at the software level in order to further improve the reliability of the system. Some commonly used measures include digital filtering and power-frequency shaping sampling, which can effectively eliminate periodic interference ； Regularly adjusting the reference point potential and using a dynamic zero point can effectively prevent potential drift ； Utilize information redundancy techniques to design corresponding software flag bits ； Use indirect jumps and set up software traps to improve the reliability of the software structure. 4.4 Proper selection of grounding points and improvement of the grounding system. There are usually two purposes for grounding: one is for safety, and the other is to suppress interference. A proper grounding system is one of the important measures to protect PLC control systems from electromagnetic interference. The system grounding methods include floating ground, direct grounding, and capacitive grounding. For PLC control systems, which are high-speed low-level control devices, direct grounding should be employed. Due to factors such as the distributed capacitance of signal cables and the filtering in input devices, the signal exchange frequency between devices is generally below 1 MHz; therefore, PLC control systems use either single-point grounding or series single-point grounding for their grounding wires. A centrally arranged PLC system is suitable for a parallel single-point grounding scheme, with the grounding points at the center of each device’s cabinet being connected to the ground electrode via separate grounding wires. If the distance between devices is large, a series single-point grounding method should be used. Connect the center grounding points of the cabinets of each device using a large-cross-section copper busbar (or insulated cable), and then connect the grounding busbar directly to the grounding electrode. The grounding wire is made of copper conductors with a cross-sectional area greater than 22 mm2, while the main busbars are made of copper bars with a cross-sectional area greater than 60 mm2. The grounding resistance of the grounding electrode should be less than 2Ω. It is advisable to bury the grounding electrode at a distance of 10 to 15 meters from the building, and the grounding point of the PLC system must be at least 10 meters away from the grounding points of high-voltage equipment. When the signal source is grounded, the shielding layer should be grounded on the signal side ； When not grounded, grounding should be done on the PLC side ； When there are connectors in the middle of the signal cable, the shielding layer must be securely connected and insulated; multiple grounding points must be avoided at all costs ； When the shielded twisted-pair wires of multiple measurement points are connected to a multi-core twisted shielded cable, the respective shielding layers should be properly connected to one another and insulated. Select an appropriate grounding point as a single contact point.

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I also want to give it a try; there’s a lot of interference in the live environment as well