FCP280 RH924YF FOXBORO
Electromagnetic interference sources and interference to the system
2.1 Interference sources and general classification of interference
The interference source that affects the PLC control system is the same as the interference source that generally affects the industrial control equipment, which is mostly generated in the part where the current or voltage changes dramatically, and the part where the charge moves violently is the noise source, that is, the interference source.
Interference types are usually divided according to the cause of interference, noise interference mode and noise waveform quality. Among them, according to the different causes of noise, it can be divided into discharge noise, surge noise, high-frequency oscillation noise, etc. According to the different waveform and quality of noise, it can be divided into continuous noise, accidental noise, etc. According to the different noise interference modes, it can be divided into common mode interference and differential mode interference. Common mode interference and differential mode interference are commonly used classification methods. Common-mode interference is the potential difference between the signal and the ground, which is mainly formed by the superposition of the common state (same direction) voltage induced by the power grid, the ground potential difference and the space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially when the distributor power supply room is used with poor isolation energy, the common mode voltage of the transmitter output signal is generally high, and some can be as high as 130V or more. The common mode voltage can be converted into a differential mode voltage through an asymmetric circuit, which directly affects the measurement and control signal, causing damage to components (this is the main reason for the high damage rate of some system I/O modules), and this common mode interference can be DC or AC. Differential mode interference refers to the interference voltage between the two poles of the signal, which is mainly formed by the coupling induction of the spatial electromagnetic field between the signals and the conversion of common mode interference by the unbalanced circuit. This voltage is directly superimposed on the signal and directly affects the measurement and control accuracy.
2.2 Main source of electromagnetic interference in PLC control system
2.2.1 Dry interference from space radiation
The radiation electromagnetic field (EMI) in space is mainly generated by power networks, transient processes of electrical equipment, lightning, radio broadcasting, television, radar, high-frequency induction heating equipment, etc., usually called radiation interference, and its distribution is extremely complex. If the PLC system is placed in the RF field, the radiation interference is recovered, and its impact is mainly through two paths: one is the direct radiation inside the PLC, which is interfered by the circuit induction; It is the radiation to the network within the PLC communication, and interference is introduced by the induction of the communication line. Radiation interference is related to the field equipment layout and the size of the electromagnetic field generated by the equipment, especially the frequency, and is generally protected by setting shielded cables and PLC local shielding and high voltage release elements.
2.2.2 Interference from external leads of the system
It is mainly introduced through power and signal lines and is often referred to as conducted interference. This kind of interference is more serious in the industrial field of our country.
(1) Interference from the power supply
In practice, due to the interference caused by the power supply, many PLC control system failures, the author encountered in a project debugging, after changing the PLC power supply with high isolation energy, the problem was solved.
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