自動化外文翻譯---自動化制造系統(tǒng)與plc關系

1、<p>　　自動化制造系統(tǒng)與PLC關系</p><p>　　控制工程隨著時間的演變。過去的人們主要致力于控制方面研究。最近電力已被應用于控制，早期電氣控制是基于繼電器的。這些繼電器使其可以在沒有機械開關的情況下被開動和關閉。這是通常使用繼電器進行簡單的邏輯控制的方法。低成本計算機的發(fā)展帶來了新的革命，可編程邏輯控制器（PLC）出現(xiàn)于70年代，它已成為制造控制的最常見選擇。PLC的功能受到越來越多的工廠歡

2、迎并可能作為主要控制手段再今后的一段時間內。而這其中絕大部分原因是因為PLC它的優(yōu)點很多。</p><p><b>　　1.1梯形邏輯</b></p><p>　　梯形邏輯編程法是主要的PLC編程方法。正如之前所說，梯形邏輯已發(fā)展到模仿繼電器邏輯。通過選擇簡單的梯形邏輯編程法，培訓工程師和商人所需要的金錢極大的減少?，F(xiàn)代控制系統(tǒng)仍然包括繼電器，但這些都是很少的邏輯使用

3、。字母a繼電器是一個簡單的裝置，它使用一個磁場來控制開關，如圖圖1.1。當電壓作用于輸入線圈產(chǎn)生的磁場，產(chǎn)生電流領域。拉起磁場的金屬開關，再實現(xiàn)它的接觸和聯(lián)系，關閉開關。</p><p>　　圖1.1 簡單的布局和繼電器電路圖</p><p>　　繼電器的工作方式，讓一個電源開關關閉另一（通常是高電流）電源，同時保持他們孤立。一個簡單的例子，控制繼電器應用，見圖1.2。在這方面，左邊第一

4、個接力是通常使得系統(tǒng)關閉，并允許電流流動，直到電壓加到輸入端甲，第二個中繼器通常是開放的，不會允許目前的速度，目前的輸入二是流經(jīng)前兩個繼電器然后電流流通過在第三繼電器線圈，并關閉輸出C.此電路的開關會通常應用在制定階梯邏輯形式。這可以被解釋為將C邏輯作用，如果A關閉B合上的話。</p><p>　　圖1.2一個簡單的繼電器控制器</p><p>　　圖1.2中的例子沒有顯示整個控制系統(tǒng)，只

5、有邏輯。當我們考慮一個PLC有輸入，輸出，和邏輯。圖1.3顯示的更全面。這里有兩個按鈕的輸入。我們可以想像激活24V直流在PLC繼電器線圈的輸入。反過來驅動器是一個輸出繼電器，開關115伏交流電，結果打開了一盞燈。請注意，在實際情況下PLC的輸入繼電器，常常又是輸出繼電器。PLC梯形圖邏輯其實一種計算機程序，用戶可以輸入和更改它。注意，兩個輸入的推按鈕常開，但里面的PLC梯形圖邏輯有一個常開觸點，和一個常閉觸點。在PLC梯形邏輯圖不需要

6、匹配輸入或輸出。許多初學者會被抓住這點試圖使階梯邏輯匹配它的輸入類型。</p><p>　　圖1.3繼電器PLC的簡圖</p><p>　　許多繼電器也有多個輸出（拋出），這允許輸出繼電器可以同時輸入。圖1.4所示的電路是一個例子，它是在電路里稱為印章。此電路的電流流過兩個電路的分支，通過接觸標簽A或B的輸入端，B只相對乙輸出。如果B是關閉的，而A是通電，那么B將打開。如果B打開，然后輸入

7、，B將打開。打開后，乙在輸出，乙將不會關閉。</p><p><b>　　圖1.4電路</b></p><p><b>　　1.2編程</b></p><p>　　第一個是PLC的編程，一個基礎技術的繼電器邏輯接線示意圖。雖然這就不需要教電工，技術員和工程師電腦編程 - 但是，這種方法一直是被認可的，這是現(xiàn)在最常見的PLC

8、的編程技術。梯形邏輯的一個例子，圖1.5。為了解釋這個圖，想像左手垂直線方向，我們稱之為熱鐵路。在右邊是中立軌道。圖中有兩個人物，每個梯級有輸入（2垂直線）和組合輸出（圓圈）。如果輸入是打開，或正確的組合可以關閉熱流量通過鐵路的輸入，使得電力輸出，最后中立鐵路。輸入來自于一個傳感器，開關，或任何其他類型的傳感器。輸出會有是一些外圍的PLC設備，開啟或關閉則是如燈光或馬達之類的。在發(fā)出指令后，有常開和常閉2種出點。這意味著，如果輸入A和B

9、是關閉，然后將輸出并激活它。任何其他組合輸入值將導致輸入被關閉。</p><p>　　圖1.5一個簡單的梯形邏輯圖</p><p>　　第二個梯級圖1.5更復雜，其中有多種組合的輸入，輸出Y將開機。在最左邊的部分發(fā)出聲響，流過頂端，如果C和D是關閉的。電流也可以（和同時）流經(jīng)底部，如果E和F都為真。這將使得大部分響起，然后，如果是G或H輸出y的話，我們將在后面的章節(jié)解釋這些。</p&

10、gt;<p>　　還有其他的PLC編程方法。最早的一個技術涉及的記憶指令。這些指令由階梯邏輯圖編寫，并輸入到PLC的編程，通過簡單的終端。圖1.6是一個記憶法的例子。在這個例子中，讀取指令一次一行從上到下的時間。第一行00000的指令LDN（輸入負載而不是輸入答）這將檢查輸入到PLC，如果將它關閉記得1 1（或真），如果它會記住一個0（或假）。下一行使用一個（輸入負載）語句看看輸入。如果輸入的是一個0，如果輸入記得它是1（

11、注意：這是相反的）。該聲明回顧與最后兩個數(shù)字記住，如果都真正的結果是1，否則結果是0。這一結果現(xiàn)在取代了兩個數(shù)字，只有一個數(shù)字記憶中。這個過程重復行00003和00004，但是，當這些完成現(xiàn)在有三個數(shù)字的記憶中。 </p><p>　　最古老的數(shù)字是從與，較新的數(shù)字是從兩個工作點處顯示的，并且符合00005，結合從最后的結果和指示工作點處，現(xiàn)在有兩個數(shù)字的記憶中。指令采用現(xiàn)在剩下的兩個數(shù)字，如果一方是1

12、的結果是1，否則結果是0。這一結果可替代兩個數(shù)字，現(xiàn)在有一個數(shù)字在這。最后一個指令是存儲量，則看最后一個值儲存，如果是1，輸出將被打開，如果是0輸出將被關閉。</p><p>　　圖1.6的一個助記符和等效梯形邏輯實例</p><p>　　圖1.6梯形邏輯程序，相當于記憶程序。即使你有梯形邏輯編程，PLC的，將被轉換為記憶形式使用前由PLC。在過去的記憶節(jié)目是最共同的，但現(xiàn)在是常見的用戶甚

13、至看到記憶程序。順序功能圖（SFCs）已經(jīng)制定，以適應規(guī)劃更先進的系統(tǒng)。這是類似于流程圖，但更強大。在圖1.7中看到的例子是做兩件不同的事情。要閱讀圖表，頂部是說，地方開始啟動。下面這存在著雙重的水平線，上面寫著遵循兩個路徑。因此，臨立會開始跟隨在左，右支另一方面，同時雙方分開。在左邊有兩個功能，第一個是拉功能的權力。此函數(shù)將運行至決定這樣做，和電力下來后功能會。在右邊是閃光功能，這將運行直到它完成?？纯催@些職能不明，但每個例如啟動功能

14、，將一個小梯形邏輯程序。這種方法有很大不同的流程圖因為它沒有按照流程圖通過一個單一的路徑。</p><p>　　圖1.7的一個順序功能圖例子</p><p>　　結構化文本編程已經(jīng)發(fā)展成為一個更現(xiàn)代的編程語言。這是很相似，如BASIC語言。一個簡單的例子所示圖1.8。此示例使用一個PLC的內存位置島該內存位置為整數(shù)，也將在后面解釋這本書。該計劃的第一行設置值為0。下一行開始一個循環(huán)，并將在

15、循環(huán)返回。下一行回顧我珍惜的位置，給它加1，并返回到相同的位置。下一行檢查是否應該退出循環(huán)。如果我是大于或等于10，那么循環(huán)將退出，否則計算機將返回到重復的聲明繼續(xù)從那里。每次程序通過這個循環(huán)時，i去將增加1至值達到10。</p><p>　　圖1.8一個結構化文本程序范例</p><p>　　2.1 PLC的連接</p><p>　　當一個進程被控制的PLC，它使

16、用傳感器的輸入作出決定和更新輸出，可驅動器，如圖2.1所示。這個過程是一個真正的進程將隨時間而改變。執(zhí)行器將驅動系統(tǒng)，以新的國家（或模式操作）。這意味著，該控制器是由傳感器提供，如果輸入有限不可用時，控制器將無法檢測的條件。</p><p>　　圖2.1控制器和分離過程</p><p>　　控制回路是臨立會讀的投入不斷循環(huán)，解決了階梯邏輯，然后更改輸出。如同任何電腦不會發(fā)生即時。圖2.2顯

17、示了PLC的基本操作周期。當電源開啟最初的PLC做了快速完整性檢查，以確保硬件正常工作。如果有問題，臨立會停止，并說明有錯誤。例如，如果PLC的功率下降，即將引爆這將導致故障類型之一。如果臨立會通過的完整性檢查，然后將掃描（讀取）所有的投入。輸入值后，存儲在內存中的階梯邏輯將掃描（解決）使用存儲的值 - 不是當前值。這樣做是為了防止當輸入邏輯問題期間更改梯子邏輯掃描。當梯子邏輯掃描完成的產(chǎn)出將掃描（輸出值將被更改）。之后系統(tǒng)將可以追溯到

18、做完整性檢查，和循環(huán)繼續(xù)下去。不同于一般的計算機，整個程序將被每次掃描運行。對每個階段的典型是時代的毫秒秩序。</p><p>　　圖2.2 PLC的掃描周期</p><p><b>　　2.2梯形邏輯輸入</b></p><p>　　PLC的輸入很容易代表梯形邏輯。在圖2.3有三個類型的顯示的投入。前兩個是常開和常閉投入，討論以前。 IIT的

19、（立即輸入）函數(shù)允許后才能投入讀輸入掃描，而梯形邏輯被掃描。這使得梯形邏輯研究輸入值往往超過一個周期。 （注：本指令是不可用在ControlLogix處理器，但仍然可以用舊型號的。）</p><p>　　圖2.3梯形邏輯圖輸入</p><p><b>　　2.3梯形邏輯輸出</b></p><p>　　在梯形邏輯有多種類型的產(chǎn)出，但這些都不是一

20、貫可在所有的PLC。產(chǎn)出部分將外部連接的設備以外PLC的，但它也可以用在PLC內部存儲器位置。 6種輸出顯示在圖2.4。第一個是正常的輸出，輸出時活力會打開，和激勵輸出。用斜線通過圓是正常在輸出。當通電輸出將關閉。這種類型的輸出上沒有所有的PLC類型。當最初活力的OSR（一炮接力）指令將打開一個掃描，但后來被掃描后，就所有關閉，直到它關閉。的L（鎖）和U（解鎖）指令可以用來鎖定輸出。當一個L輸出帶旺輸出會變成無限期，即使輸出線圈deen

21、ergized。輸出可只有關閉使用的U輸出。最后一個指令是互操作性測試（立即輸出）這將允許產(chǎn)出，而不必為梯形邏輯等待掃描更新為完成。</p><p><b>　　3.1輸入和輸出</b></p><p>　　在投入和產(chǎn)出，到PLC是必要的監(jiān)測和控制的過程。輸入和輸出都可以分為兩種：基本類型的邏輯或連續(xù)。考慮一個燈泡的例子。如果它只能打開或關閉，這是合乎邏輯的控制。如果

22、光線可以使變暗淡不同層次，它是連續(xù)的。連續(xù)價值觀似乎更直觀的，但邏輯值是首選，因為它們讓更多的確定性和簡化控制。</p><p>　　因此，大多數(shù)控件的應用程序（和PLC）和邏輯投入使用輸出對于大多數(shù)應用。因此，我們將討論邏輯I / O和休假連續(xù)的I / O后。對執(zhí)行器輸出使PLC在導致一些事情發(fā)生的過程。字母a執(zhí)行器的流行短名單如下，以相對受歡迎。電磁閥 - 邏輯輸出，可以切換液壓或氣動流。燈 - 這通常可以采

23、用直接從PLC輸出邏輯輸出板。馬達起動器 - 電機常常引起人們的電流時，開始大量的，因此他們需要電動機起動器，基本上大的繼電器。伺服電機 - 從PLC的連續(xù)輸出可以命令變速或立場。從PLC的產(chǎn)出常常繼電器，但它們也可以固體電子學例如DC輸出或輸出的雙向交流晶體管。連續(xù)輸出要求特別輸出卡與數(shù)字到模擬轉換器。輸入來自傳感器轉化為電信號的物理現(xiàn)象。</p><p>　　傳感器典型的例子是下面列出的普及相對順序。接近開關

24、 - 使用電感，電容或光線來檢測對象的邏輯。開關 - 機械機制，將打開或關閉電接觸的邏輯信號。電位器 - 不斷措施角位置，使用性。LVDT（線性可變差動變壓器） - 線性位移的措施不斷用磁耦合。從PLC的產(chǎn)出常常繼電器，但它們也可以固體電子學例如DC輸出或輸出的雙向交流晶體管。連續(xù)輸出要求特別輸出卡與數(shù)字到模擬轉換器。輸入來自傳感器轉化為電信號的物理現(xiàn)象。傳感器典型的例子是下面列出的普及相對順序。接近開關 - 使用電感，電容或光線來檢測

25、對象的邏輯。開關 - 機械機制，將打開或關閉電接觸的邏輯信號。電位器 - 不斷措施角位置，使用性。LVDT（線性可變差動變壓器） - 線性位移的措施不斷用磁耦合。</p><p>　　3.1.1 PLC的輸入</p><p>　　在較小的投入通常是內置在購買時指定的PLCPLC的。對于較大的PLC的投入是作為模塊或信用卡購買，8或16投入的每張卡上同一類型。為了便于討論，我們將討論所有的投

26、入如果他們已經(jīng)購買的卡。下面的列表顯示了典型的輸入電壓范圍，大約是為了普及。PLC的輸入卡很少供電，這意味著一個外部電源需要提供的投入和傳感器等。圖3.1中的例子顯示了如何連接到一個AC輸入卡。</p><p>　　圖3.1 AC輸入卡和梯形邏輯</p><p>　　在這個例子中有兩個輸入，一個是常開按鈕，和第二個是一個溫度開關，或熱繼電器。 （注意：這些符號是標準稍后將討論在這一章。）的

27、開關都采用了積極/炎熱的輸出器24VAC電源 - 這就像在一個直流電源正端的。權力是提供給對兩個開關的左側。當開關打開有傳遞到輸入卡沒有電壓。如果任一電源開關，將關閉提供給輸入卡。在這種情況下投入1和3是使用 - 通知，開始投入在0。輸入卡比較這些電壓的共同。如果輸入電壓范圍內一個給定的容差范圍的投入，將開關。梯形邏輯圖中顯示為的投入。在這里它使用的ControlLogix艾倫布拉德利符號。在頂部的標簽（變量名）為在機架上。輸入卡（&#

28、39;我'）是在插槽3，因此該卡的地址是鮑勃：3.I.Data.x，其中，'x'是輸入位的數(shù)字。這些地址也可以給定別名標簽，使較少的階梯邏輯混亂。</p><p>　　許多初學者成為混淆在連接電路中需要上面的。關鍵是要記住單詞的電路，這意味著有一個完整的循環(huán)電壓必須能夠遵循。在圖3.1之后，我們可以啟動電路（循環(huán)）在電源。路徑穿過交換機，通過輸入卡，再回到電源回流的地方通過向啟動。在實施全

29、面的PLC那里將是每一個都必須完成許多電路。第二個重要的概念是共同的。在這里，中立的電力供應是共同的，或參考電壓。實際上，我們選擇了這是我們?yōu)?V參考，和所有其他電壓測量相對于它。如果我們有一個第二個電源，我們還需要連接的中性，使這兩個中立國將連接到同樣普遍。通常的共同和地面也會無所適從。常見的是一個參考，或基準電壓，用于為0V使用，但地面是用于防止沖擊和損害到設備。地面連接到下一個金屬管道或在網(wǎng)格建設地面。這是連接到建筑物的電氣系統(tǒng)，

30、對電源插座，在電氣設備的金屬案件有關。當電源流經(jīng)地上是壞的。不幸的是許多工程師，制造混淆的地面和共同的。這是很常見找到一個與地面和共同錯誤標簽供電。</p><p>　　最后一個概念，初學者往往陷阱是每個輸入卡是孤立的。這意味著，如果你有一個共同的連接只有一個卡，那么其他卡未連接。當發(fā)生這種情況的其他卡將無法正常工作。您必須連接共同為每個輸出卡。</p><p>　　3.1.2 輸出模塊&

31、lt;/p><p>　　正如輸入模塊，輸出模塊很少提供任何權力，而是作為開關。外部電源連接到輸出卡和卡開關電源或關閉每個輸出。典型的輸出電壓下面列出，并大致排序受歡迎。</p><p>　　這些卡通常有8至16個相同類型的輸出，就可以買到不同的額定電流。一種常見的選擇，采購卡的繼電器輸出，晶體管或可控硅。繼電器是最靈活的輸出設備。他們有能力開關交流和直流輸出。但是，他們會更慢（約10ms的切換

32、是典型），他們是笨重的，他們花費更多，而且會磨損周期后，數(shù)百萬人。繼電器輸出通常被稱為干觸點。晶體管是有限的直流輸出，和雙向的僅限于交流輸出。晶體管和可控硅輸出稱為切換輸出。</p><p>　　一個獨立的繼電器，致力于每個輸出。這使得混合電壓（AC或DC和電壓水平，直至最高），以及絕緣輸出以保護其他產(chǎn)出和PLC。響應時間往往大于10毫秒。此方法是最不敏感的電壓變化和尖峰。交換產(chǎn)出 - 電壓提供給PLC的卡，卡切

33、換到使用固態(tài)電路（晶體管，晶閘管等）雙向不同的輸出適合交流設備需要小于1A。晶體管輸出NPN或使用PNP晶體管高達1A典型。它們的反應時間大大低于1毫秒。</p><p>　　Automating Manufacturing Systems with PLCs</p><p>　　Control engineering has evolved over time. In the past

34、humans were the mainmethod for controlling a system. More recently electricity has been used for control andearly electrical control was based on relays. These relays allow power to be switched on</p><p>　　a

35、nd off without a mechanical switch. It is common to use relays to make simple logicalcontrol decisions. The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC).

36、 The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls.PLCs have been gaining popularity on the factory floor and will probably remainpredominant for some time to come.

37、 Most of this is because of the advantages they offer.</p><p>　　1.1 Ladder logic</p><p>　　Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been de

38、veloped to mimic relay logic. logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.</p>&

39、lt;p>　　Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 1.1. When a voltage is applied

40、to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch. </p><p>　　Figure 1.1 Simple Relay

41、Layouts and Schematics</p><p>　　Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control ap

42、plication is shown in Figure 1.2. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and wil

43、l not allow current to flow until a voltage is applied to the input B. If current is flowing through the </p><p>　　Figure 1.2 A Simple Relay Controller</p><p>　　The example in Figure 1.2 does no

44、t show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure 1.3 shows a more complete representation of the PLC. Here there are two inputs from push butto

45、ns.We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs ar

46、e often relays. The ladder </p><p><b>　　.</b></p><p>　　Figure 1.3 A PLC Illustrated With Relays</p><p>　　Many relays also have multiple outputs (throws) and this allows

47、an output relay to also be an input simultaneously. The circuit shown in Figure 1.4 is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, throug

48、h the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if inp<

49、/p><p>　　Figure 1.4 A Seal-in Circuit</p><p>　　1.2 Programming</p><p>　　The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminate

50、d the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. An example of ladder logic can be see

51、n in Figure 1.5. To interpret this diagram imagine that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the</p><p>　　Figure 1.5 A Simple Ladder L

52、ogic Diagram</p><p>　　The second rung of Figure 1.5 is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power c

53、ould flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the pow

54、er will be delivered to output Y. In later chapters we will examine how to interpret and constr</p><p>　　There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instruc

55、tions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure 1.6. In this example the instruc

56、tions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input A. . This will examine the input to the PLC and </p><p>　　The oldest number i

57、s from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numb

58、ers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that

59、will look at the last value stored and if</p><p>　　Figure 1.6 An Example of a Mnemonic Program and Equivalent Ladder Logic</p><p>　　The ladder logic program in Figure 1.6, is equivalent to the m

60、nemonic program. Even if you have programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for u

61、sers to even see mnemonic programs.</p><p>　　Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powe

62、rful. The example seen in Figure 1.7 is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC

63、 will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are</p><p>　　Figure 1.7 An Example of a Sequential Function Chart</p><p>　　Stru

64、ctured Text programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC. A simple example is shown in Figure 1.8. This example uses a PLC memory location i. This m

65、emory location is for an integer, as will be explained later in the book. The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop returns to. The next line recalls the v

66、alue in location i, adds 1 to it and returns it to the s</p><p>　　Figure 1.8 An Example of a Structured Text Program</p><p>　　2.1 PLC Connections</p><p>　　When a process is control

67、led by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in Figure 2.1. The process is a real process that will change over time. Actuators will drive the system to new s

68、tates (or modes of operation). This means that the controller is limited by the sensors available, if an input</p><p>　　is not available, the controller will have no way to detect a condition.</p><

69、;p>　　Figure 2.1 The Separation of Controller and Process</p><p>　　The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any comput

70、er this does not happen instantly. Figure 2.2 shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly.If there is a pr

71、oblem the PLC will halt and indicate there is an error. For example, if the PLC power is dropping and about to go off this will result in one </p><p>　　Figure 2.2 The Scan Cycle of a PLC</p><p>

72、　　2.2 Ladder Logic Inputs</p><p>　　PLC inputs are easily represented in ladder logic. In Figure 2.3 there are three types of inputs shown. The first two are normally open and normally closed inputs, discuss

73、ed previously. The IIT (Immediate InpuT) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle.

74、 (Note: This instruction is not available on the ControlLogix processors, but is still available on older mod</p><p>　　Figure 2.3 Ladder Logic Inputs</p><p>　　2.3 Ladder Logic Outputs</p>

75、;<p>　　In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possibl

76、e to use internal memory locations in the PLC. Six types of outputs are shown in Figure 2.4. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line t

77、hrough is a normally on output. When energized the output will turn off. T</p><p>　　3.1 INPUTS AND OUTPUTS</p><p>　　Inputs to, and outputs from, a PLC are necessary to monitor and control a pro

78、cess. Both inputs and outputs can be categorized into two basic types: logical or continuous. Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to

79、 different levels, it is continuous. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control. </p><p>　　As a result most controls appli

80、cations (and PLCs) use logical inputs and outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O for later. Outputs to actuators allow a PLC to cause something to happen in a process.

81、A short list of popular actuators is given below in order of relative popularity. Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow. Lights - logical outputs that can often be powered direct

82、ly from PLC output boards.Mot</p><p>　　Typical examples of sensors are listed below in relative order of popularity.Proximity Switches - use inductance, capacitance or light to detect an object logically. Sw

83、itches - mechanical mechanisms will open or close electrical contacts for a logical signal. Potentiometer - measures angular positions continuously, using resistance. LVDT (linear variable differential transformer) - mea

84、sures linear displacement continuously using magnetic coupling. Inputs for a PLC come in a few basic varieties, </p><p>　　3.1.1 Inputs</p><p>　　In smaller PLCs the inputs are normally built in a

85、nd are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. For discussion purposes we will discuss all inputs as if they ha

86、ve been purchased as cards. The list below shows typical ranges for input voltages, and is roughly in order of popularity. PLC input cards rarely supply power, this means that an external power supply is needed to supply

87、 power for</p><p>　　Figure 3.1 An AC Input Card and Ladder Logic</p><p>　　In the example there are two inputs, one is a normally open push button, and the second is a temperature switch, or ther

88、mal relay. (NOTE: These symbols are standard and will be discussed later in this chapter.) Both of the switches are powered by the positive/ hot output of the 24Vac power supply - this is like the positive terminal on a

89、DC supply. Power is supplied to the left side of both of the switches. When the switches are open there is no voltage passed to the input card. If either of the</p><p>　　Many beginners become confused about

90、where connections are needed in the circuit above. The key word to remember is circuit, which means that there is a full loop that the voltage must be able to follow. In Figure 3.1 we can start following the circuit (loo

91、p) at the power supply. The path goes through the switches, through the input card, and back to the power supply where it flows back through to the start. In a full PLC implementation there will be many circuits that mus

92、t each be complete. A s</p><p>　　One final concept that tends to trap beginners is that each input card is isolated. This means that if you have connected a common to only one card, then the other cards are

93、not connected. When this happens the other cards will not work properly. You must connect a common for each of the output cards.</p><p>　　3.1.2.Output Modules</p><p>　　As with input modules, out

94、put modules rarely supply any power, but instead act as switches. External power supplies are connected to the output card and the card will switch the power on or off for each output. Typical output voltages are listed

95、below, and roughly ordered by popularity. </p><p>　　These cards typically have 8 to 16 outputs of the same type and can be purchased with different current ratings. A common choice when purchasing output car

96、ds is relays, transistors or triacs. Relays are the most flexible output devices. They are capable of switching both AC and DC outputs. But, they are slower (about 10ms switching is typical), they are bulkier, they cost

97、more, and they will wear out after millions of cycles. Relay outputs are often called dry contacts. Transistors are limited t</p><p>　　This allows mixed voltages (AC or DC and voltage levels up to the maximu

98、m), as well as isolated outputs to protect other outputs and the PLC. Response times are often greater than 10ms. This method is the least sensitive to voltage variations and spikes. Switched outputs - a voltage is suppl

99、ied to the PLC card, and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are well suited to AC devices requiring less than 1A. Transistor outputs use N</p>