2 PLC interference analysis
With the development of science and technology, the application in industrial control of PLC is more and more extensive. The dependability of PLC control system influences the safety in production and economical operation of the industrial enterprise directly; systematic anti-interference ability is the key to concerning the whole system and running reliably. All kinds of PLC used in the automated system, some install concentrated in the control room , some install in production scene and every electrical machinery equipment, they in strong electric circuit and strong electric abominable electromagnetic environment that equipment form mostly. Should improve PLC dependability of control systems, on one hand require PLC manufacturer to improve the anti-interference ability of the equipment; On the other hand, require engineering design, installation construct and use of maintaining causing paying close attention to, cooperate and perfect the problem of solving in many ways, strengthen systematic anti-interference performance effectively.
2.1 Sources of interference and interference with the general classification
It is like generally influencing the interference source of the industrial control equipment to influence the interference source of PLC control system, mostly producing the position in the electric current or violent change of voltage, these violent position that move of electric charge is the noise source, namely interfere the source.
Interfere type interfere the division different from wave form nature of the noise of the mode according to reason, noise produced to interfere usually. Among them: According to reason that noise produce being different, is it discharge noise, wave well up noise, high frequency shake noise, etc. to divide into; Different according to the wave form, nature of the noise, divided into a lasting noise, accidental noiseless.; It is different to interfere the mode according to the noise, is divided into mould interfering and difference mould interference altogether. It is a kind of commonly more used categorized method that the mould interferes interfering with the difference mould altogether. It is signal electric potential difference to place for mould not to interfere altogether, bunch enter, ground electric potential difference and space electromagnetic radiation reaction attitude (with direction) voltage is it is it form to add to change altogether at signal line by electric wire netting mainly. Mould voltage sometimes relatively heavy altogether, is it isolate performance mixing electric apparatus supply power in the room bad to adopt, changer output voltage generally relatively high mould altogether of signal, some can up to 130V to be the above-mentioned. Mould voltage through asymmetric circuit getting convertible model's voltage of difference, influence the signal of observing and controlling directly altogether, cause the components and parts to be damaged (this is some systematic I/O moulds a main reason of higher spoilage), this kind mould can interfere for direct current, can for exchange also altogether. The difference mould interferes referring to acting on the voltage of interference between the two poles of the signal, coupling reaction change voltage that mould's interfering form altogether by uneven circuit among signal by space electric magnetic field mainly, this kind superposes it on the signal directly, influence measurement and control the precision directly.
2.2 PLC control system electromagnetic interference the main source
1) The radiation from space interference
mainly produced by temporary attitude course , thunder and lightning , radio broadcasting , TV,radar, high-frequency reaction heating device of the electric network , electric equipment ,etc., usually called radiation to interfere, it is extremely complicated that it is distributed. Whether if PLC system put on radio frequency in the field is it reaches radiation interfere to retrieve, it influences and mainly passes two routes. First, produced and interferes by the reaction of the circuit to the radiation within PLC directly; But the radiation of the network, is introduced and interfered by the reaction of the communication line while communicating to PLC. Radiation is interfered with the on-the-spot equipment assigning and size of electric magnetic field that the equipment produce , especially frequency have something to do with, generally through set up shielding cable and PLC some shielding and high pressure discharge component protect.
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