Along with the industrial equipment automation control technology’s development, programmable controller (PLC) is getting more and more widespread in the industrial equipment control’s application. The PLC control system’s reliable immediate influence to enterprise’s safety in production and the economical movement, system’s antijamming ability is relates the overall system reliability service the key. This article introduced in detail affects the PLC movement the disturbance type and the origin, and proposes the antijamming design implementation strategy.
The automated system uses in each type PLC, some are the centralism installs in the control room, some are installs in produces the scene and on various electrical machinery equipment, they mostly occupy in the bad electromagnetic environment which the strong electricity electric circuit and the strong electricity equipment form. Must enhance the PLC control system reliability, on the one hand requests the PLC Manufacturer to sharpen equipment’s antijamming ability, on the other hand requests using the department, in the engineering design, the installment construct and use in the maintenance to bring to highly the attention, coordinates to be able in every way to consummate solves the problem, strengthens system’s resistance to interference effectively.
1. Electromagnetic interference type and influence
Affects the PLC control system’s noise source and the general influence industrial control equipment’s noise source is the same, produces mostly in the electric current or the voltage fierce change spot, these electric charge fierce migration’s spot is a noise source.
The disturbance type usually the reason which, the noise jamming pattern and the noise profile nature produces according to the disturbance divides. Is different according to noise production’s reason, divides into the electric discharge noise, the surge noise, the higher mode of oscillation noise and so on; According to the noise profile, the nature are different, may divide into continues the noise, the accidental noise and so on; Is different according to the noise jamming pattern, divides into the syntype disturbance and the bad mold disturbance.
The syntype disturbance and the difference mold disturbance is one quite commonly used taxonomic approach. The syntype disturbance is the signal to the place potential difference, mainly enters, the terrestrial electricity potential difference and the space electromagnetic radiation by the electrical network string induces altogether condition (same direction) on the holding wire the voltage iteration to form. The syntype voltage is sometimes big, specially uses when the isolation performance bad distributor supplies power, the transmitting instrument output signal’s syntype voltage is generally high, some may reach as high as above 130V. The syntype voltage rotatable changes into the bad mold voltage through the bilateral circuit, affects the observation signal, creates the primary device damage (this is some system I/O module damage rate high primary cause), this kind of syntype disturbance may for the cocurrent, also be possible to be the exchange. The bad mold disturbance is refers to the function disturbing voltage in the signal two-pole, mainly the coupling induction and transforms by the spatial electromagnetic field in the signal between by the balanced circuit, this kind of disturbance superimposition which the syntype disturbance forms on signal, immediate influence survey and control precision.
2. Electromagnetic interference important source
2.1 come from spatial the radiated interference
Space radiation electromagnetic field (EMI) is mainly by the electric power network, electrical equipment’s transition process, the thunder and lightning, the radio broadcasting, the television, the radar, the high frequency induction heating equipment and so on produces, usually is called the radiated interference, its distribution is extremely complex. If the PLC system puts in its radio frequency field, will receive the radiated interference, its will influence mainly adopt two ways: First, to the PLC internal radiation, induces directly by the electric circuit has the disturbance; Second, to the PLC communication network’s radiation, induces the introduction disturbance by the communication link. The radiated interference and the field apparatus arrangement and the equipment produce the electromagnetic field size is specially the frequency related, generally carries on the protection through the establishment shielded cable and the PLC partial shield and the high-pressured draining part
2.2 come from outside the system the lead wire disturbance
Mainly through the power source and the holding wire introduction, usually is called the conducted interference. This kind of disturbance is more serious in our country industry scene, mainly has the following three kinds:
The first kind comes from power source’s disturbance. The practice proved that because the power source introduces the disturbance causes the PLC control failure the situation to be many, the author has met in some project debugging, latter replaces the isolation performance higher PLC power source question only then to be solved.
PLC system’s normal electric power supply by electrical network power supply, because the electrical network coverage area is broad, it will receive all space electromagnetic interference, but on the line the induced tension and the electric current, the electrical network internal change, on-off operation surge, the large-scale power equipment gets up particularly stops the overtone which, the electrical network short circuit transition condition impact, the alternating and direct transmission device causes and so on, passes to the power source primary side through the transmission line. The PLC power source usually uses isolates the power source, but because its factors and so on organization and fabrication technology cause its isolation not to be ideal. In fact, because the distribution parameter is distributed capacity’s existence specially, the absolute quarantine is impossible.
The second kind comes from the disturbance which the holding wire introduces. With the PLC control system connection’s each kind of signal transmission line, besides the transmission effective each kind of information, the general meeting has the exterior unwanted signal invasion. This disturbance mainly has two ways: First, or uses in common the electrical network disturbance which through the transmitting instrument electric power supply the signal measuring appliance’s electric power supply strings together into, this is often neglected; Second, holding wire disturbance which is induced the space electromagnetic radiation, namely on holding wire’s exterior induction interference, this kind is often serious.
Can cause the I/O signal work by the signal introduction’s disturbance exceptionally to reduce greatly with the measuring accuracy, is serious when will cause the primary device damage. Regarding isolation performance bad system, will also cause between the signal to disturb mutually, causes altogether the system bus backflow, causes the logical data change, to move by mistake and to halt. Because the PLC control system the signal introduction disturbance causes the I/O module damage number to be quite serious, from this causes the system failure situation to be also many.
The third order comes from the earth system chaotic disturbance. The earth enhances one of electronic installation electromagnetic compatibility (EMC) effective measures, the correct earth both can suppress the electromagnetic interference the influence, and can the locking device send out the disturbance outward; But the wrong earth instead will introduce the serious unwanted signal, will cause the PLC system to be unable the normal work.
The PLC control system’s grounding including, the shield, the exchange and the protectorate and so on, the earth system confusion is mainly systematically each ground point potential distribution is uneven to the PLC system’s disturbance, during the different ground point has the terrestrial electricity potential difference, causes the ring circuit electric current, interferes with the system normal work. As soon as for example the cable shielding level must accept after checking, if cable shielding level both sides A, B earth, the existence terrestrial electricity potential difference, has the electric current to wind through the shielding layer. When has the abnormal state like thunder stroke, the grounding electric current will be bigger.
In addition, the shielding layer, meets the grounding and the earth possibly constitutes the closed-loop, in changes the magnetic field under the function, in the shielding layer will present the induced current, through between shielding layer and core coupling, unwanted signal return route. If with other earth processing confusion, produces systematically the place circulation possibly does not have the equipotential distribution in the grounding, affects in PLC the logic circuit and analogous circuit’s normal work. The PLC work’s logical voltage jamming margin is low, the logical electric potential’s distribution disturbance easy to affect PLC the logic operation and the data storage, causes the data confusion, the procedure to run flies or halts. The simulation electric potential’s distribution will cause the measuring accuracy to drop, causes to the signal observation serious distortion and the misoperation.
2.3 come from the PLC system internal disturbance
Mainly mutual electromagnetic radiation production by between the system interior primary device and electric circuit’s, like the logic circuit radiates, the simulation mutually does not match the use mutually with between the logical place mutual influence and the primary device and so on. This belongs to the PLC Manufacturer to carry on the electromagnetic compatibility design to the system interior the content, is quite complex, takes is unable using the department to change, but does not need excessively to consider, but must choose has the many application actual accomplishments or undergoes the test the system.
3. Antijamming design
For the guarantee system is exempt from or reduces the inside and outside electromagnetic interference in the industry electromagnetic environment, must start from the design stage then to take three aspect braking measure: Suppresses the noise source, the cut-off or the weaken electromagnetic interference dissemination way, the enhancement installment and system’s antijamming ability. These three spots are suppress the electromagnetic interference the basic principle.
The PLC control system’s antijamming is a systems engineering, the request manufacture unit designs produces has the strong antijamming ability product, and depends on the User departments in the engineering design, the installment construction and the running attention gives the comprehensive consideration, and unifies the special details to carry on the integrated design, can guarantee system’s electromagnetic compatibility and the movement reliability. When carries on the concrete project the antijamming design, should mainly pay attention to the following two aspects.
3.1 equipment shapings
When choice device, must first choose has the high antijamming ability product, it has included the electromagnetic compatibility, particularly anti-exterior disturbance ability, like uses floats technical, the isolation performance good PLC system; Next should also understand the Manufacturer gives antijamming target, like syntype rejection ratio, difference mold rejection ratio, pressure resistance ability, permission in big electric-field intensity and in multi-high-frequency magnetic field strength environment work and so on; Moreover is depending on examines it in the similar work application actual accomplishments.
Must pay attention in the choice overseas imported product, our country uses the 220V high interface resistance electrical network service pattern, but the European and American area is the 110V low internal resistance electrical network. Because our country electrical network interface resistance is big, a zero electric potential drifts in a big way, the potential shift is big, the Industrial enterprise scene’s electromagnetic interference must compared to European and American area high 4 times above, be higher at least to the system resistance to interference request. In overseas can the normal work PLC product in the home industry not necessarily be able the reliability service, this must when uses the foreign product, according to our country’s standard (GB/T13926) reasonable choice.
3.2 comprehensive antijamming designs
The main consideration from system’s outside several kind of braking measure, the content includes: And outside the lead wire carries on the shield to the PLC system to radiate the electromagnetic interference by the air defense; The foreign lead wire carries on the isolation, the filter, specially the power electric cable should the lamination arrangement, by against through the outside lead wire introduction conduction electromagnetic interference; The correct design ground point and the grounding, consummates the earth system. In addition must use the software method, further enhances system’s security reliability.
4. Main antijamming measure
4.1 use the performance fine power source, suppresses disturbance which the electrical network introduces
In the PLC control system, the power source holds the extremely important status. The electrical network disturbance string enters the PLC control system mainly through the PLC system’s electric power supply (for example the CPU power source, the I/O power source and so on), the transmitting instrument electric power supply and has direct electrical connection couplings and so on measuring appliance electric power supply with the PLC system to enter. Now regarding the PLC system power supply’s power source, uses the isolation performance good power source generally, but has the direct electrical connection measuring appliance electric power supply regarding the transmitting instrument electric power supply as well as with the PLC system, has not received the enough value. Although has taken certain quarantine measures, but generally also insufficient, is mainly the use insulating transformer distributed parameter is big, barrage jamming ability is bad, after power source coupling, but strings together enters the syntype disturbance, the bad mold disturbance. Therefore and uses in common the signal measuring appliance power supply regarding the transmitting instrument to choose the distributed capacity small, suppresses raises (for example uses isolates and shield and leakage inductance technology many times) the distributor, reduces the PLC system’s disturbance.
In addition, to guarantee that the electrical network feed does not interrupt, may use the online -like uninterrupted electric power supply (UPS) power supply, enhances the power supply the security reliability. Moreover UPS also has the strong disturbance isolation performance, is one kind of PLC control system’s ideal power source.
4.2 choose the electric cable and the implementation placing correctly
In order to reduce the power electric cable particularly frequency conversion installment feeder cable’s radiation electromagnetic interference, the author has used the copper tape armor shield power cable in some project, reduced the electromagnetic interference which the power line produces, after this project production, has made the satisfactory progress.
The different type’s signal transmits separately by the different electric cable, signal
The electric cable should according to the transmission signal type lamination placing, refuse with the identical electric cable’s different wire simultaneously to transmit the power power source and the signal, avoids the holding wire and the power electric cable approaches the parallel placing, reduces the electromagnetic interference.
4.3 hardware filter and software antijamming measure
The signal before turning on the computer, connects in parallel fashion the electric capacity between the holding wire and the place, reduces the syntype disturbance; Installs the filter in the signal two-pole between to be possible to reduce the bad mold disturbance.
As a result of electromagnetic interference’s complexity, wants the radical interference elimination influence is impossible, therefore in time PLC control system’s software design and configuration, but should also carry on antijamming processing in the software aspect, further enhances system’s reliability. Commonly used some enhance the software architecture reliable measure to include: Digital filtering and power frequency reshaping sampling, but eliminates the periodic disturbance effectively; Fixed time the adjustment reference point electric potential, and uses the dynamic zero spot, may prevent the electric potential drifting; Uses the information redundant technique, designs the corresponding software flag bit; Uses indirect skipping, establishes the software protection and so on.
4.4 correct choice ground points, consummate the earth system.
The earth goal usually has two, one, for safe; second, for barrage jamming. The consummation earth system is one of PLC control system anti-electromagnetic interference important measures.
The system earth has floats, the direct earth and the capacity ground three ways. Speaking of the PLC control system, it is the high speed low level control device, should select the direct earth method. Because the signal cable distributed capacity and the input device filter and so on influence, between the installment signal exchange frequency is lower than 1MHz generally, as soon as soon as therefore the PLC control system meets the grounding to use accepts after checking and the series accepts after checking the way. As soon as the centralism arrangement’s PLC system is suitable for to accept after checking parallel the way, each installment’s cabinet body center ground point by independent meets the grounding to course the earthed pole. If the installment spacing is big, as soon as should use the series to accept after checking the way, (or insulated cable) connects each installment with a big section copper bus bar the cabinet body center ground point, then ground strap direct connection earthed pole. Meets the grounding to use the section to be bigger than 22mm2 the copper wire, the total bus bar net section is bigger than 60mm2 the copper bar. The earthed pole earth resistance is smaller than 2Ω, the earthed pole is best buries is being apart from the building 10? 15m distant place, moreover the PLC system ground point must be distanced above 10m with the strong electricity equipment ground point.
When supply oscillator earth, the shielding layer should in the signal side joint place; When does not earth, should in the PLC side joint place; Among when the holding wire has the attachment, the shielding layer should connect and carry on insulation processing reliably, certainly must avoid accepting after checking. Many measuring point signal’s shielded twisted pair to twists with the multi-cores when the total shielded cable connects, each shielding layer should the interconnection, and after insulation processing, chooses suitably meets the attendance single-point earth.
5. This article subtotal
The PLC control system’s disturbance is a very complex question, therefore in the antijamming design should the overall evaluation various aspects factor, the barrage jamming, also probably make the concrete study effectively reasonably to some disturbed condition, adopts method which acts appropriately to the situation, only then can cause the PLC control system normal work, the guarantee industrial equipment safe highly effective movement.
