All kinds of signal transmission lines connected to the PLC control system, in addition to transmitting various types of effective information, there will always be external interference signals intrusion. There are two main ways for this interference: one is the grid interference through the power supply of the transmitter or the power supply of the shared signal instrument, which is often ignored; external inductive interference, which is very serious. The interference introduced by the signal will cause the abnormal operation of the I/O signal and the measurement accuracy will be greatly reduced, and will cause component damage in severe cases. For a system with poor isolation performance, it will also lead to mutual interference between signals, causing the backflow of the common ground system bus, resulting in logical data changes, malfunctions and crashes. PLC control system due to the introduction of signal interference caused the damage of the I/O module is quite serious, resulting in a lot of system failures.
Interference from the chaos of the grounding system
Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only restrain the influence of electromagnetic interference, but also restrain the equipment from emitting external interference; but wrong grounding will introduce serious interference signals, which will make the PLC system unable to work normally.
The ground wire of the PLC control system includes system ground, shield ground, AC ground and protection ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven potential distribution of each grounding point and the ground potential difference between different grounding points, which causes ground loop current and affects the normal operation of the system. For example, the shielding layer of the cable must be grounded at one point. If both ends A and B of the shielding layer of the cable are grounded, there will be a ground potential difference, and a current will flow through the shielding layer. When an abnormal state such as a lightning strike occurs, the ground current will be larger.
In addition, the shielding layer, the ground wire and the ground may form a closed loop. Under the action of the changing magnetic field, there will be an induced current in the shielding layer, which will interfere with the signal loop through the coupling between the shielding layer and the core wire. If the system ground is confused with other grounds, the resulting ground circulation may produce unequal potential distribution on the ground wire, affecting the normal operation of logic circuits and analog circuits in the PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of the logic ground potential can easily affect the logic operation and data storage of the PLC, resulting in data confusion, program runaway or crash. The distribution of the analog ground potential will lead to a decrease in the measurement accuracy, causing serious distortion and malfunction of the signal measurement and control.
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