為什么說用PLC實現(xiàn)對系統(tǒng)的控制是非常可靠的

用PLC實現(xiàn)對系統(tǒng)的控制是非常可靠的。這是因為PLC在硬件與軟件兩個方面都采取了很多措施，確保它能可靠工作。事實上，如果PLC工作不可靠，就無法在工業(yè)環(huán)境下運用，也就不成其為PLC了。

    1·在硬件方面：

    PLC的輸入輸出電路與內(nèi)部CPU是電隔離。其信息靠光耦器件或電磁器件傳遞。而且，CPU板還有抗電磁干擾的屏蔽措施。故可確保PLC程序的運行不受外界的電與磁干擾，能正常地工作。

    PLC使用的元器件多為無觸點的，而且為高度集成的，數(shù)量并不太多，也為其可靠工作提供了物質(zhì)基礎(chǔ)。

    在機械結(jié)構(gòu)設(shè)計與制造工藝上，為使PLC能安全可靠地工作，也采取了很多措施，可確保PLC耐振動、耐沖擊。使用環(huán)境溫度可高達攝氏50多度，有的PLC可高達80--90度。

    有的PLC的模塊可熱備，一個主機工作，另一個主機也運轉(zhuǎn)，但不參與控制，僅作備份。一旦工作主機出現(xiàn)故障，熱備的可自動接替其工作。

    還有更進一步冗余的，采用三取一的設(shè)計，CPU、I/O模塊、電源模塊都冗余或其中的部分冗余。三套同時工作，終輸出取決于三者中的多數(shù)決定的結(jié)果。這可使系統(tǒng)出故障的機率幾乎為零，做到*。當然，這樣的系統(tǒng)成本是很高的，只用于特別重要的場合，如鐵路車站的道叉控制系統(tǒng)。

    2．在軟件方面：

    PLC的工作方式為掃描加中斷，這既可保證它能有序地工作，避免繼電控制系統(tǒng)常出現(xiàn)的"冒險競爭"，其控制結(jié)果總是確定的；而且又能應(yīng)急處理急于處理的控制，保證了PLC對應(yīng)急情況的及時響應(yīng)，使PLC能可靠地工作。

    為監(jiān)控PLC運行程序是否正常，PLC系統(tǒng)都設(shè)置了"看門狗"（Watchingdog）監(jiān)控程序。運行用戶程序開始時，先清"看門狗"定時器，并開始計時。當用戶程序一個循環(huán)運行完了，則查看定時器的計時值。若超時（一般不超過100ms），則報警。嚴重超時，還可使PLC停止工作。用戶可依報警信號采取相應(yīng)的應(yīng)急措施。定時器的計時值若不超時，則重復(fù)起始的過程，PLC將正常工作。顯然，有了這個"看門狗"監(jiān)控程序，可保證PLC用戶程序的正常運行，可避免出現(xiàn)"死循環(huán)"而影響其工作的可靠性。

    PLC還有很多防止及檢測故障的指令，以產(chǎn)生各重要模塊工作正常與否的提示信號。可通過編制相應(yīng)的用戶程序，對PLC的工作狀況，以及PLC所控制的系統(tǒng)進行監(jiān)控，以確保其可靠工作。

    PLC每次上電后，還都要運行自檢程序及對系統(tǒng)進行初始化。這是系統(tǒng)程序配置了的，用戶可不干預(yù)。出現(xiàn)故障時有相應(yīng)的出錯信號提示。

    正是PLC在軟、硬件諸方面有強有力的可靠性措施，才確保了PLC具有可靠工作的特點。它的平均*時間可達幾萬小時以上；出了故障平均修復(fù)時間也很短，幾小時以至于幾分鐘即可。

    曾有人做過為什么要使用PLC的問卷調(diào)查。在回答中，多數(shù)用戶把PLC工作可靠作為選用它的主要原因，即把PLC能可靠工作，作為它的*指標。

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OMEGA TAC-386-KC
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NICOLET INSTRUMENT CORP 000-8481-02
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OPTO 22 001788L
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REEDHOLM 11049
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Nikon 12031-1C
Nikon 12034-2B
Nikon 12038-4
Nikon 12038-5
Nikon 12049-1
Nikon 12059-3
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Nikon 12071-1
Nikon 12072-1
Nikon 13032B
Nikon 13036-2
Nikon 13036-2
Nikon 13037-3A
Nikon 13037-3A
Nikon 13077
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138315-31
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138395-1
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Nikon 14116.W.AGC-N
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IDI 231813-001
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Nikon 27124-1
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COMPAQ 277498-001
OnTrack 28-8875-014
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BTU Engineering Corp. 3161950
BTU Engineering Corp. 3162280
348332-9012
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Kevex Instruments 51301307
Fisons Instruments 51301310
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KLA-TENCOR CORP 710-800667
LUDL ELECTRONC 73002042
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Opal 78319030200
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SVG
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ATI INDUSTRIAL AUTOMATION 109-A26000-00
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CYMER S05-05178-00
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MEGA TEST TG BOARD
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QUANTUM TN18L462
Texas Instruments TSP50C14
DELL U4575
DELL W5320
PNY VCQ4400NVS
DELL X8760
BIO-RAD MICROMEASUREMENTS LTD. Y5301267
ZIATECH ZT88CT72