Analysis of electromagnetic interference sources in PLC control systems

(1) Radiation interference from space

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

(2) Interference from external leads of the system

It is mainly introduced through power and signal lines, usually called conducted interference. This kind of interference is more serious in my country's industrial sites.

(3) Interference from power supply

Practice has shown that there are many cases where PLC control system failures are caused by interference introduced by the power supply. The author encountered this during the debugging of a certain project. The problem was solved after replacing the PLC power supply with higher isolation performance.

The normal power supply of the PLC system is supplied by the power grid. Due to the wide coverage of the power grid, it will be affected by all spatial electromagnetic interference and induce voltage and circuit on the line. In particular, changes within the power grid, switching operation surges, start-up and stop of large power equipment, harmonics caused by AC and DC rotating devices, transient impact of power grid short circuit, etc., all pass through the transmission line to the power supply side. PLC power supply usually uses isolated power supply, but its structure and manufacturing process factors make its isolation not ideal. In fact, due to the existence of distributed parameters, especially distributed capacitance, absolute isolation is impossible.

(4) Interference introduced from signal lines

In addition to transmitting effective signals, all kinds of signal transmission lines connected to the PLC control system will always have external interference signals intruding. There are two main ways of this interference: one is the interference of the power grid connected through the power supply of the transmitter or the shared signal instrument, which is often ignored; the other is the interference of the signal line induced by electromagnetic radiation in space, 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 in severe cases, it will cause damage to components. For systems with poor isolation performance, it will also cause mutual interference between signals, causing the common ground system bus to reflux, causing logical data changes, malfunctions and crashes. The PLC control system has caused serious damage to the I/O modules due to the interference introduced by the signal, and there are also many cases of system failures caused by this.

(5) Interference from chaotic grounding system

Grounding is one of the effective means to improve the electromagnetic compatibility (EMC) of electronic equipment. Correct grounding can not only suppress the impact of electromagnetic interference, but also suppress the interference emitted by the equipment; while incorrect grounding will introduce serious interference signals, making the PLC system unable to work properly. The ground wires of the PLC control system include system ground, shielding ground, AC ground and protective ground. The interference of the chaotic grounding system on the PLC system is mainly due to the uneven distribution of potentials at each grounding point. There is a ground potential difference between different grounding points, which causes 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 current flows through the shielding layer. When an abnormal state and lightning strike occur, the ground wire current will be greater.

In addition, the shielding layer, grounding wire and the earth may form a closed loop. Under the action of the changing magnetic field, an induced current will appear 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 and other grounding processes are messed up, the generated ground loop may produce unequal potential distribution on the ground wire, affecting the normal operation of the logic circuit and analog circuit in the PLC. The logic voltage interference tolerance of the PLC is low, and the distribution interference of the logic ground potential is likely to affect the logic operation and data storage of the PLC, causing data confusion, program runaway or crash. The distribution of the analog ground potential will lead to a decrease in measurement accuracy, causing serious distortion and malfunction of signal measurement and control.

(6) Interference from within the PLC system

It is mainly caused by the mutual electromagnetic radiation between components and circuits within the system, such as logic circuits, mutual radiation and its impact on analog circuits, the mutual impact between analog ground and logic ground, and the mismatch between components. These are all part of the electromagnetic compatibility design of the system by the PLC manufacturer, which is relatively complex and cannot be changed as an application department. You don’t need to consider it too much, but you should choose a system with more application performance or tested.