TRICONEX-3501E How to solve the anti-interference problem of PLC control system
1. Overview
TRICONEX-3501E With the development of science and technology, the application of PLC in industrial control is more and more extensive. The reliability of PLC control system directly affects the safe production and economic operation of industrial enterprises, and the anti-interference ability of the system is the key to the reliable operation of the whole system. The various types of PLC used in the automation system, some are centrally installed in the control room, some are installed on the production site and various motor equipment, most of them are in the harsh electromagnetic environment formed by the strong current circuit and the strong current equipment. In order to improve the reliability of PLC control system, designers can effectively ensure the reliable operation of the system only by understanding various disturbances in advance.
2. What are the sources of electromagnetic interference and the interference to the system?
The interference that affects the PLC control system stems from 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.
TRICONEX-3501E Interference types are usually divided according to the cause of interference, the interference mode of noise and the waveform properties of noise. 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 nature of noise, it can be divided into continuous noise, accidental noise, etc. According to the different modes of sound interference, 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 power grid, the ground potential difference and the common state (same direction) voltage induced by space electromagnetic radiation on the signal line. The common mode voltage is sometimes large, especially when the electrical power supply room with poor isolation performance is used, the common mode voltage TRICONEX-3501E 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 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.
3. What are the main sources of electromagnetic interference in the PLC control system?
(1) Radiation interference from space
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 radio frequency field, the radiation interference is recovered, and its influence is mainly through two paths; One is the direct radiation inside the PLC, which interferes with 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) Interference from leads outside 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.
(3) Interference from the power supply
Practice has proved that the interference caused by the power supply caused a lot of PLC control system failure, the author encountered in a project debugging, after replacing the PLC power supply with higher isolation performance, the problem was solved.