1, overview

 

As the development of science and technology, the PLC application in industrial control is wider and wider. PLC controls systematic dependability to affect the safe production of industrial business and economy to move directly, interference rejection ability of the system is the key that matters to whole system to move reliably. All sorts of type PLC that use in automation system, have a plenty of concentration to install in control room, have a plenty of installation to go up with all sorts of electric machinery equipment in manufacturing site, they lie mostly in strong report circuit and the abominable electromagnetism environment that strong report equipment forms. Want to raise PLC to control systematic dependability, design personnel understands all sorts of interference ability to make sure the system moves reliably effectively beforehand only.

 

2, electromagnetism interference source and the interference to the system

 

The interference that affects PLC to control a system results from the interference source that affects industry to control device commonly is same, arise mostly in electric current or voltage the place of acuteness change, the place with these acuteness and mobile charge is noise source, violate a source namely.

 

Of the undee property of the interference pattern that violates the account that the type produces by interference normally, noise and noise different differentiate. Among them: The account that produces by noise is different, cent is noise of discharge noise, surge noise, high frequency oscillation to wait; The weaveform that presses noise, property is different, cent is continuously noise, abiogenesis noise; By sound interference mode differs, cent is disturbed to the model is disturbed and need a standard in all. The model is disturbed and differring a model to disturb in all is a kind of more commonly used classification method. In all modular interference is signal the voltage to the ground, basically string together by electrified wire netting, ground voltage and dimensional electromagnetism radiate are on line of the induction in all voice (with direction) voltage sends add place to form. In all modular voltage sometimes bigger, use the room of electric equipment power supply that keeps apart function difference especially, change send implement output signal in all modular voltage is general taller, some can be as high as 130V above. In all modular voltage can be changed into wrong standard voltage through asymmetry circuit, immediate impact is measured accuse signal, cause yuan of parts of an apparatus to damage (this is the reason with higher rate of attaint of module of I/O of a few systems) , this kind in all modular interference can be communication for dc, Yi Ke. The interference that need a standard is to point to use at signal polar a disturb voltage, basically reach in induction of the coupling between signal by dimensional electromagnetism field change by lopsided circuit in all the voltage that modular interference forms, this kind lets direct overlay be on signal, immediate impact is measured with control precision.

 

 

 

3, what does PLC control the main source that the electromagnetism in the system violates to have?

 

(1) comes from dimensional radiate to disturb:

 

Radiation report magnetic field of the space (EMI) basically wait for generation by heating installation of the transient process of power network, electric equipment, thunder, radiobroadcast, TV, radar, high frequency induction, call radiation to disturb normally, its distributing very complex. Be like field of radio frequency of place of PLC system park inside, reclaim to be disturbed to radiation, its influence basically adopts two method; It is direct in-house to PLC radiate, generate interference by circuit induction; Be opposite however the radiate of the network inside PLC communication, introduce interference by the induction of communication circuitry. Faze of spoke blackberry lily and spot equipment decorate the electromagnetism field size that reachs equipment place generation, especially frequency is concerned, pass local screen of setting screen cable and PLC and high-pressured release component to undertake protective commonly.

 

(2) comes from the interference of systematic terminal:

 

Basically introduce through power source and line, call conducted interference normally. This kind of interference is in our country industrial site is graver.

 

(3) comes from the interference of power source:

 

Carry out a proof, the situation that creates PLC to control systematic breakdown because of the interference that power source introduces is very much, the author is debugged in some project in had encountered, the PLC power source with segregation higher performance changes after, the problem just gets settlement.

 

The normal power supply source of PLC system all by power supply of electrified wire netting. Because electrified wire netting covers range wide,

 

Will be disturbed by all space electromagnetism and voltage of the induction on online road and circuit. Especially the change of interior of electrified wire netting, switch stops since facilities of operation surge, large power, concussion of transient of short circuit of the harmonic that hands in dc to turn device is caused, electrified wire netting, arrive through transmission network by the side of power source. PLC power source uses segregation power source normally, but its orgnaization and workmanship element make its segregation sex not ideal. Actually, distributing especially as a result of distributinging parameter the existence of capacitance, absolutely segregation is impossible.

 

(4) comes from the interference that line introduces:

 

Transmission line of as of all kinds as connective of PLC control system signal, besides transmit effective of all kinds signal besides, total meeting has exterior interference signal to invade. This interference basically has two kinds of ways: It is send through changing implement or common the interference of electrified wire netting that the power supply source of signal appearance strings together, this often by oversight; 2 it is the interference that line gets response of dimensional electromagnetism radiate, namely the exterior induction interference on line, this is very serious. Introduce interference to be able to cause I/O signal by signal the job is unusual and measure precision to be reduced greatly, serious when will cause loss of yuan of parts of an apparatus. To keeping apart the system with poor performance, still will cause trouble of be concerned with of the each other between signal, cause circumfluence of in all systematic bus line, cause logistic data change, be moved by accident and dead machine. PLC control system introduces interference to cause I/O module to damage number considers because of signal serious, the condition that causes systematic trouble from this is very much also.

 

(5) comes from the interference when ground connection system is troubled:

 

Ground connection is to enhance compatibility of electronic equipment electromagnetism (EMC) one of significant steps. Correct ground connection, can restrain the effect that electromagnetism disturbs already, can restrain equipment to give out interference outwards again; And wrong ground connection, can introduce serious interference signal instead, make PLC system will not work normally. The ground wire of PLC control system includes systematically, screen ground, communication ground and protective ground. Disorder of ground connection system basically is to receive place potential to distributing individually to the interference of PLC system not all, different ground connection nods an existence ground voltage, cause electric current of ground annulus road, influence system works normally. For example cable screen layer must a bit ground connection, if cable screen layer two upright A, B ground connection, with respect to existence ground voltage, electric current has shed screen layer, when producing unusual position to add be struck by lightning, ground wire electric current will be bigger.

 

In addition, screen layer, ground wire and earth are formed likely close annulus road, below the action of metabolic magnetic field, have inside screen layer can appear faradic, pass the coupling between screen layer and core line, disturb signal loop. If treat a disorder with other ground connection systematically, circumfluent likelihood is in the ground that produces the generation on ground wire is not equipotential distributing, affect the regular job of circuit of the logic inside PLC and imitate circuit. The tolerance of logistic voltage interference that PLC works is inferior, the distributinging interference of ground potential affects logic easily the logistic operation of PLC and data storage, cause data disorder, program to run to fly or die machine. Of imitate ground potential distributing will bring about measure precision to drop, cause pair of signal to measure those who accuse is serious and lack fidelity with misoperation.

 

(6) comes from the interference of PLC system interior:

 

Main by systematic interior yuan the mutual electromagnetism radiate between parts of an apparatus and circuit arises, if radiate of logistic circuit each other reachs his to be opposite the influence of imitate circuit, imitate ground and logically mutual influence reachs yuan of parts of an apparatus between do not match each other use etc. This belongs to PLC manufactory to undertake electromagnetism to systematic interior the content of compatible design, more complex, regarding application as the branch is to cannot be changed, but need not consider too much, but the system that should choose to have more application gain or course test.

 

4, when the system suffers interference, regular meeting encounters phenomenon of the following kinds of main interference:

 

(When 1) system bristle with anger makes, electric machinery does not have regular roll;

 

(2) signal is equal to zero hour, the number shows watch numerical value frisks;

 

(When 3) sensor works, the signal that PLC collects the signal that come over and correspondence of actual parameter place is worth not be identical, and error value is random, mobbish;

 

(4) and communication servo are common and same power source (wait like monitor) the job is abnormal.

 

5, how does ability solve PLC system weller, simplier to disturb?

 

(It is to choose segregation property below 1) good position finer installation, choose line of good power supply, power and line to walk along ground connection of line, power source to want more reasonable etc, but need different equipment manufacturer joint efforts ability is finished, accomplish very hard, and cost is higher.

 

(2) uses imitate signal isolator, have call signal to change send implement, the category that belongs to signature manage. Its basically have interference rejection effect. The application in because of it especially strong interference rejection capability controls a system in automation so is very wide. Especially to complex industrial spot, control program is more and more complex, signal isolator measures signal to undertake 3 end keep apart input, output, power source to all sorts of imitate, it is one of significant step of the interference rejection in system of current automation control really.

 

6, why to solve PLC system to disturb isolator of first selection signal?

 

(1) is used simple convenient, reliable, cost is low, can solve a variety of interference at the same time.

 

(2) can reduce design personnel, system to debug personnel workload in great quantities, although complex system is in ordinary design personnel hand, the extraordinary stability that also can change is reliable.

 

7, what is principle of signal isolator job?

 

The signal that receives PLC above all, pass commutation of semiconductor device modulation, undertake keeping apart changeover through parts of an apparatus of smooth feeling or magnetism feeling next, before undertaking demodulation commutation answers segregation again next former signal or different signal, undertake keeping apart handling to the power supply source of the signal after segregation at the same time. Assure to become independent absolutely between the signal after commutation, power source, ground.

 

8, the isolator that the market has so much brand now, the price is uneven, how should choose?

 

Isolator is located in between 2 systems passageway, so choice isolator should decide the input outputs a function above all, want seclude at the same time implement the input outputs mode (power supply of region of flow pattern of electric profiling, report, annulus etc) get used to around to carry passageway interface mode. In addition still a lot of important parameter such as function of communication of bus line of ﹑ of intensity of insulation of ﹑ of noise of ﹑ of precision ﹑ power comsumption involve product function, for example: Noise and precision are concerned, power comsumption quantity of heat and dependability are concerned, the person that these need are used careful anthology. Anyhow, price of sex of applicable, reliable, product compares the main principle that is choice isolator.

 

If you are right of above it is not quite clear still to share, that knocks blackboard to delimit to everybody next key (interference of quantity of 11 kinds of imitate settles way) :

 

1, add 1: 1 signal isolator;

 

2, the annulus that add magnetism;

 

3, PLC power supply adds segregation transformer;

 

4, switch quantity signal and departure of imitate quantity signal go;

 

5, imitate signal had better introduce alone screen line. Signal type had better use 4-20mA;

 

6, imitate signal load is electromagnetism a powerful person kind, best can choose the route of 1.5;

 

7, imitate signal and digital signal cannot operable and same root multicore cable, cannot mix more power supply cord is common cable;

 

8, PLC input outputs line, must use screen cable, side is outputted in the input impending, and in PLC side ground connection;

 

9, line cable should be far from strong interference source, be like device of commutate of transducer, high-power silicon and large power plant;

 

10, imitate signal and digital signal cannot operable and same root multicore cable, cannot mix more power supply cord is common cable;

 

11, should use to imitate signal to reduce an electron to disturb twinning the screen layer of cable of signal of screen cable imitate should two end earthed, but the ground connection of screen layer one aspect of the matter that the interference that if two end put cable in voltage,will wait for electrical wiring to join electric current builds didymous imitate signal in the generation in screen layer issues you to should let cable in this kind of circumstance.