(1) Radiated interference from space

Radiated electromagnetic field (EMI) in space is mainly generated by power network, transient process of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc. It is usually called radiated interference, and its distribution is extremely complex. If the PLC system is placed in the radio frequency field, the radiation interference will be recovered, and its impact mainly passes through two paths: one is the radiation directly inside the PLC, and the interference will be generated by circuit induction; It is the radiation of the network in the PLC communication, and the interference is introduced by the induction of the communication line. The radiation interference is related to the field equipment layout and the size of the electromagnetic field generated by the equipment, especially the frequency. It is generally protected by setting shielded cables, PLC local shielding and high-voltage relief elements.

(2) Interference from external leads of the system

It is mainly introduced through power and signal lines, which is usually called conducted interference. This kind of interference is severe in China’s industrial scene.

1. Interference from power supply

The normal power supply of Siemens PLC system is supplied by the power grid. Because the power grid covers a wide range, it will be subject to all spatial electromagnetic interference and induce voltage and circuit on the line. In particular, changes in the power grid, switching surges, startup and shutdown of large power equipment, harmonics caused by AC and DC drive devices, and transient impact of power grid short-circuit are all transmitted to the power source through transmission lines. PLC power supply usually adopts isolated power supply, but its isolation is not ideal due to its mechanism and manufacturing process factors. In fact, because of the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.

2. Interference from signal line

All kinds of signal transmission lines connected with the PLC control system, in addition to transmitting various kinds of effective information, will always be invaded by external interference signals.

There are two main ways of this interference: one is the power grid interference connected in series by the transmitter power supply or the power supply of the shared signal instrument, which is often ignored; The second is that the signal line is interfered by the induction of space electromagnetic radiation, that is, the external induction interference on the signal line, which is very serious. The interference introduced by the signal will cause the I/O signal to work abnormally and the measurement accuracy to be greatly reduced, and will cause damage to the components in severe cases. For the system with poor isolation function, it will also cause mutual interference between signals, cause common ground system bus backflow, and cause logic data change, misoperation and crash. The number of I/O module damage caused by signal introduction interference in PLC control system is relatively serious, and there are many system failures caused by it.

3. Interference from chaotic grounding system

Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Accurate grounding can not only suppress the influence of electromagnetic interference, but also suppress the outward interference of equipment; On the contrary, the wrong grounding will introduce severe interference signals, which will make the PLC system unable to work normally.

The ground wire of Siemens PLC control system includes system ground, shielding ground, AC ground and protection ground. The disturbance of the grounding system chaos to the PLC system is mainly caused by the uneven distribution of potential at each grounding point, and the existence of ground potential difference between different grounding points, which causes the ground loop current and affects the normal operation of the system. For example, the cable shielding layer must be grounded at one point. If both ends A and B of the cable shielding layer are grounded, there is a ground potential difference, and there is current flowing through the shielding layer. When an abnormal condition occurs, such as lightning strike, the ground wire current will be greater.

In addition, the shielding layer, the grounding wire and the earth may form a closed loop. Under the effect of changing magnetic field, there will be 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 grounding is confused with other grounding, the generated ground circulation may produce unequal potential distribution on the ground wire, affecting the normal operation of logic circuit and analog circuit in PLC. The logic voltage interference tolerance of PLC operation is low, and the distribution interference of logic ground potential easily affects the logic operation and data storage of PLC, resulting in data confusion, program flight or crash. The distribution of simulated ground potential will lead to the decline of measurement accuracy, resulting in severe distortion and misoperation of signal measurement and control.

(3) Interference from the PLC system

It is mainly generated by the mutual electromagnetic radiation between components and circuits in the system, such as the mutual radiation of logic circuits and their influence on analog circuits, the mutual influence between analog and logic circuits and the mutual mismatch between components. These are all the contents of the electromagnetic compatibility design carried out by the PLC manufacturer for the internal system, which is relatively complex. As an application part, it cannot be changed. It is not necessary to consider too much, but it is necessary to select a system with more application achievements or tested.