(1) Radiative dry disturbance from space
The radiated electromagnetic field (EMI) in space is mainly caused by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, frequency induction heating equipment, etc., generally known as radiation disturbance, and its dispersion is extremely chaotic. If the PLC system is placed in the RF field, the radiation disturbance will be recovered, and its impact is mainly through two paths: one is directly to the radiation inside the PLC, which is induced by the circuit to cause disturbance; Rather, it is the radiation of the network in the PLC communication, which is disturbed by the induction of the communication line. Radiated disturbance is related to the layout of the field equipment and the magnitude of the electromagnetic field generated by the equipment, especially the frequency, and is generally maintained by setting shielded cables and PLC local shielding and pressure relief components.
(2) Disturbance from the external leads of the system
It is mainly introduced through power and signal lines, and is generally referred to as conducted disturbance. This kind of disturbance is more severe in China's industrial sites.
aDisturbance from the power supply
The normal power supply of the PLC system is powered by the power grid. Due to the wide range of the power grid, it will be subjected to all space electromagnetic disturbances that induce voltages and circuits on the line. In particular, changes within the power grid, such as the surge of incoming and switching operations, the start and stop of large power equipment, the harmonics caused by AC and DC transmission devices, and the transient impact of the power grid short circuit, are all transmitted to the primary side of the power supply through the transmission line. PLC power supply is generally used as a barrier power supply, but its mechanism and manufacturing process elements make its barrier not ideal. In fact, due to the dispersion parameters, especially the dispersion capacitance, it is definitely not possible.
bDisturbance from the introduction of signal lines
In addition to transmitting all kinds of useful information, there will always be external disturbance signal intrusion in all kinds of signal transmission lines connected to the PLC control system.
There are two main ways of this disturbance: one is the disturbance of the power grid connected through the power supply of the transmitter or the power supply of the common signal instrument, which is often overlooked; The second is that the signal line is disturbed by the induction of electromagnetic radiation in space, that is, the external induction disturbance on the signal line, which is very serious. The introduction of disturbance by the signal will cause abnormal I/O signal operation and greatly reduce the measurement accuracy, and in severe cases, it will cause component damage. As for the system with poor barrier function, it will also lead to mutual disturbance between signals, causing the bus backflow of the common-ground system, resulting in logical data change, malfunction and crash. The number of I/O module damage caused by signal interference in the PLC control system is quite severe, and there are many cases of system failure.
c Disturbances from the grounding system when it is disturbed
Grounding is one of the useful means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the influence of electromagnetic disturbance, but also inhibit the equipment from emitting disturbance; The wrong grounding will introduce severe disturbance signals, so that the PLC system will not be able to operate normally.
The ground wire of the PLC control system includes the system ground, the shielding ground, the AC ground and the maintenance ground. The disturbance of the grounding system to the PLC system is mainly due to the uneven distribution of the potential of each grounding address, and the ground potential difference between different grounding addresses, which causes the ground loop current and affects the normal operation of the system. For example, the cable shield must be grounded at one point, if both ends of the cable shield A and B are grounded, there is a ground potential difference, and there is a current flowing through the shield, when an abnormal situation such as lightning strikes, the ground wire current will be greater.
In addition, the shield, the ground wire, and the ground may form a closed loop, and when the magnetic field is changed, an induced current will occur in the shield, which will disturb the signal loop through the coupling between the shield and the core. If the system is disordered with other grounding treatments, the unequal potential dispersion of the ground circulation may occur on the ground wire, which will affect the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage disturbance tolerance of PLC operation is low, and the dispersion disturbance of logical ground potential can easily affect the logic operation and data storage of PLC, resulting in data disorder, program flight or crash. The dispersion of imitating ground potential will lead to the decrease of measurement accuracy, and cause severe distortion and misoperation of signal measurement and control.
(3) Disturbance from within the PLC system
It is mainly caused by the mutual electromagnetic radiation between the components and circuits within the system, such as the mutual radiation of logic circuits and their influence on the imitation circuit, the interaction between the imitation ground and the logical ground and the mutual mismatch between components. This belongs to the content of the PLC manufacturer's electromagnetic compatibility design inside the system, which is relatively messy, and cannot be changed as a part of use, so you don't need to think too much, but you should choose a system that has more performance or passed the test.
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