Reasons for interference
Issues with materials and equipment
The control signals connected to the DCS system usually use cables as transmission media to send signals to the DCS system. When the insulation material of the signal cable ages and the shielding material is damaged, it will be interfered by other electromagnetic interference sources, which will cause measurement errors. Severe interference can cause equipment damage.
Construction is not standardized
In DCS control systems, there are often many signals connected to DCS simultaneously. These signal lines either go through cable trays or cable conduits, resulting in many different types of cables being attached to the same path on site. There are distributed capacitors between these signals, which can be used to add interference to other signal lines. At the same time, alternating magnetic flux is generated in the surrounding environment of signal transmission, such as power lines, motors, generators, power transformers, and relays. These magnetic fields often cause significant interference.
Unreasonable grounding
A common grounding accident is when both ends of the signal line are grounded, which can cause significant interference due to the ground potential difference. If both ends of the signal line are grounded at the same time, there may be a large potential difference, which may generate a large circulating current on the signal line between the two ends. Generate electromagnetic waves and interference signals. There are also issues such as the total grounding resistance of the DCS system not meeting the requirements for normal operation of the DCS system.
How to suppress interference
Ensure the normal use of signal lines
During the use of signal lines, avoid entanglement of multiple signal lines, and regularly inspect the signal lines. Repair and replace signal lines with damaged insulation glue to ensure the normal use of signal lines and avoid interference with transmitted signals caused by leakage.
Lay cables in layers
Isolate cables that transmit different signals and use layered laying to regulate the cables to ensure that they do not interact with each other. For cables that transmit the same signal, it is also advisable to avoid dense entanglement of the lines as much as possible. As computer ports can perform a certain degree of precision analysis on signals when processing them, if similar signals interfere with each other, it will affect the computer’s judgment and lead to analysis errors during the computer’s operation.
Signal line isolation
During the operation of the equipment, cable isolation can be used to isolate the signals, ensuring that there is no mutual influence between the signals. But the isolation method can not only isolate the transmission signal line, but also classify and isolate the signal at the computer end. Adopting this isolation method can greatly reduce the harm caused by common mode interference to the computer. Isolation amplifiers can also be used to completely isolate the input end of the signal from the computer. This method can form a loop between the blocked interference signals, suppress the harm of interference, and ensure the normal operation of the system.
Twisting and shielding
The twisting of signal wires, which replaces parallel wires with twisted pair wires, can achieve the effect of suppressing magnetic field interference.
Shielding refers to the use of metal conductors to isolate components, assemblies, and signal lines. Shielding can effectively suppress capacitive coupling noise. The commonly used method is to connect analog signals through shielded twisted pair cables. Usually, not only electrical noise interferes with the signal, but also strong alternating magnetic fields can interfere with the signal. Therefore, not only electrical shielding but also magnetic shielding needs to be considered, and shielding measures can be achieved by using conductors with superior magnetic conductivity (such as iron, nickel, etc.).
Ensure proper grounding
For DCS grounding system, its main functions include two aspects: on the one hand, it can bear overload current and transmit it to the ground when the signal and power supply enter the DCS system or when it fails itself; On the other hand, the grounding system can provide a shielding layer to DCS, thereby isolating electronic noise interference, and can also provide a reference zero potential to the entire system. So DCS systems usually include two grounding points for operation and protection.
