Isolator/isolated transmitter is how to solve the PLC, DCS system interference

Isolator/isolated transmitter is how to solve the PLC, DCS system interference

This paper detailed describes isolator/isolated transmitter is how to solve the PLC, DCS system interference problems, Also illustrates isolation transmitter/isolators play an important role in isolationin industrial control, industrial areas, signal acquisition system, long-distance transmission, harsh industrial field andall kinds of interference environments.
Now let's work together to understand and learn the applications of isolation transmitter/isolators in PLC, DCS System.Any inadequacies, please advise.

1、General description:
With the development of modern science and technology, PLC, DCS ,PLC and DCS system transported more widely in industrial control and industrial automation.
The reliability of PLC, DCS control system directly affects the industrial safety and stable operation of the system, the system's Anti-jamming capability is is the key to the whole system  reliable operation.
Various types of PLC, DCS systems used in industrial automation systems, some are centrally installed in the control room, while some others installed on the production site,a variety of electrical equipment and PLC/DCS system,most of them are in the harsh electromagnetic environment formed by strong electric circuits and strong electric equipment.To improve PLC, DCS control system reliability, designers must advance knowledge in all kinds of interference to guarantee the system stable and reliable operation.

2、Electromagnetic interference sources and interference on the system
Interference sources of affectting PLC, DCS control system is basically the same as the interference sources of general industrial control equipment Most caused by the parts of current or voltage strenuous change, these strenuous Electric charges moving parts are noise source,that is the source of interference.
Type of interference is usually divided as diffences of the causes of interference, interference patterns of noise and the nature of noise waveform .
According to the noise generated by different reasons,divided into discharge noise, surge noise, high-frequency oscillation noise, etc.;
According to the diffences of noise waveform and nature, divided into continuous noise, occasional noise, etc.;
According to the diffences of interference patterns, divided into common mode interference and differential mode interference.

Common mode interference and differential mode interference is a general common classification method. Common-mode interference is the potential difference between  the signal and ground, mainly formed by the power grid string,ground potential difference,spatial electromagnetic radiation and the common mode produced in the signal line (in the same direction,formed by Voltage plus delivery). Common mode voltage is sometimes higher, especially using poor performance electrical power supply room or control room, the transmitter outputting signal common mode voltage is generally higher, some up to 130V above.
Common mode voltage can be converted into differential mode voltage by asymmetric circuit.Direct impact on the control signal, causing the components or equipment damage (this is the reason of some system I/O modules damage rate become higher),this common-mode interference may be DC or AC.
Differential mode interference is the interference voltage using in the signal between the poles, mainly formed by the space electromagnetic inductive coupling between signals and voltage formed by the unbalanced circuit convert into common mode interference,which directly superimposing on the signal directly affects the signal measurement and accuracy.

(1)Radiated interference from space

Radiated electromagnetic radiation fields(EMI) maily produced by the electricity network, transient of transient electrical equipment, lightning, radio, television, radar, high frequency induction heating equipment, often referred to as radiation interference, and its distribution is extremely complex.it recovered to the radiation interference If the PLC, DCS system in RF field ,the effect mainly through two paths; One is directly to the PLC, DCS internal radiation interference acaused by the circuit induction;introduce the interference  by the Induction of communication lines Radiated interference related to the field equipment layout and size of the electromagnetic field generated by the equipment,especially frequency.usually by setting the shielded cable,PLC, DCS system partial shielding and high pressure relief device to protect.

(2)The interference from outside the system leads mainly through the power supply and signal
lines, often referred to as "conducted interference." This interference in China industrial site is more serious.

(3)The interference from  power system
Practice has proved that interference caused by the power supply introduction leadl the PLC, DCS control system failure is very much , in some commissioning projects,after replacing higher isolation performance power supply of PLC or DCS , the problem was fully resolved.the power grid prodides  normal power supply to PLC, DCS system's. Since the power grid covering a wide range suffer electromagnetic interference by all the space on the line induced voltage and circuits. Especially changes within the grid, the surge generated when switching operation, a large power equipment starts and stops, AC or DC rotating device caused harmonics, Grid Short-circuit transient shocks, etc.all of them transmitted through the transmission line to the power supply side. PLC, DCS power supply usually adoptting isolated power supply, but its institutions and manufacturing process Etc.factors lead to isolational effect is not very satisfactory. In fact, absolutely isolation is impossible since the distribution parameters particular distributed capacitance.

(4)Interference from signal line introducing
Various types of signal transmission lines which connecting with  PLC, DCS control system, except transmitting various types of effective signal always there is some external interference signal string into. This interference main has two ways: first, from the signal transmitter or the grid disturbance from shared signal instrumentation power supply string into.which is often ignored.second, signal lines is interferenced by space electromagnetic radiation induction , that is to say external inductive interference from signal line, which is very serious. Interference introduced by signal can cause I/O signal abnormal working and accuracy greatly reduced,even damage the components. For the poor isolation system will also lead to mutual interference between signals, causing common ground system bus back, resulting in logical data changes, misuse and freezes.PLC, DCS control system due to signal introducting interference causing the I/O module damage very serious ,
The situation caused a lot of system failure.

(5) Interference caused by the disordering grounding system 
Ground is one of an effective means to improve the electronic equipment Electromagnetic compatibility (EMC) . Proper grounding can suppress electromagnetic interference and devices outgoing interference , interference will make the PLC, DCS system can't work properly. PLC, DCS control system ground lines including the system ground, shield, the exchange ground and protected ground, etc.Confusing grounding system caused PLC, DCS system interference is mainly every grounding piont potential uneven distribution , different ground potential difference exists between locations.causing ground loop currents, affecting the system work properly.
Such as a cable shield must be grounded at one end,if both the cable shield ends A, B are grounded, there is potential difference.Current flows through the cable shield, the ground current will be greater when an abnormal state occurs and lightning strikes.In addition, the shield, the ground wire and the earth may constitute a closed loop,Under the effect of the changing magnetic field, within the shielding layer will appear the induced current,Interference signal circuit via the coupling between shielding layer and core wire.If the system ground with other ground handling confusion arising grounding circulation may produce unequal potential distribution in the ground line ,affecting PLC, DCS internal logic circuits and analog circuits normal operation.PLC, DCS operating logic voltage interference tolerance is low,Distributing interference of logic ground potential is likely to influence the of PLC, DCS logic operations and data storage, data confusion, the program runaway or crash. Analog ground potential distribution will result in measurement accuracy decline, causing the signal control serious distortion and malfunction.

(6) from the PLC, DCS system internal interference  
Mainly caused by the system's internal components and mutual electromagnetic radiation btween circuits,Such as logic circuits mutual radiation and its effects on analog circuits, mutual influence of analog ground and logic ground and between the components used do not match each other. Which all belong to the PLC manufacturer's design content to the system internal electromagnetic compatibility, it's complex. the applications department can not change, you need not consider too much, but to select application with more practice or proven system.

4. How can we better, simpler solve PLC, DCS system disturbances?
(1)Selection better isolation performance equipment, high quality power supply, reasonable power lines and signal lines wiring , etc.Be able to resolve the interference, but more cumbersome and difficult to operate and the cost is relatively high.
(2) Using the signal isolation transmitter solve the interference problem. As long as plusing the isolation transmitter product in the middle of input and output terminals of interference,can effectively solve the interference problem.

5. Why solve PLC, DCS system disturbances preferred signal isolation transmitter ?
(1) Easy to use, reliable and low cost.
(2) Can greatly reduce the workload of designer and system debugging personnel, even very complex systems in the hands of ordinary designer, also become very simple.

6. How to the signal isolation transmitter work?
First,by semiconductor devices modulate and converse the PLC, DCS system received signals, then by the optical sense or magnetic sense components isolate and transform, and then demodulate and converte back to the original signal before isolation or different signals,While processing the isolated power supply of signal to isolate.
Ensure the converted signal, power, ground absolutely independent.

7. Now ,there are so many brands of isolated transmitter, the price varies, how to choose it?
Isolation transmitter located two systems between channels,So choose isolation transmitter/isolator must first determine the input and output functions.
Meanwhile make the isolator input and output model(Voltage, current, loop-powered, etc.) adapt front and rear channel interface mode. Additionally, there are accuracy、power lost、 noise 、insulation strength ﹑ bus communication capabilities and many other important parameters related to product performance, such as PLC,DCS system using isolation transmitter with anti-EMC electromagnetic interference, high isolation voltage, high accuracy level,long-distance transmission without distortion, small size, less power lost, easy installation, suitable and reliable isolation products,Shenzhen Jie Sheng Technology Co., Ltd. developed  isolation transmitter products are completely has these features. it's  one of the dispensable quality products in PLC, DCS system used to isolate, transform and enlarge.
high-precision passive 4-20mA isolated transmitter/high-precision passive 4-20mA isolation amplifier/high-precision analog isolation transmitter widely used in industrial control field and control room, more isolation transmitter/isolation amplifier technology information please visit : http://www.jsd-iae.com  
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Isolator/isolated transmitter physical picture: