外文翻譯--可編程邏輯控制器

1、<p>　　畢業(yè)設(shè)計(論文)外文資料翻譯</p><p>　　系　　部： 機(jī)械工程系 </p><p>　　專 業(yè)： 機(jī)械工程及自動化 </p><p>　　姓 名： </p><p>　　學(xué) 號：

2、 </p><p>　　外文出處： Process automation </p><p>　　instrumentation </p><p>　　附 件： 1.外文資料翻譯譯文；2.外文原文。 </p>

3、<p>　　注：請將該封面與附件裝訂成冊。</p><p>　　附件1：外文資料翻譯譯文</p><p><b>　　可編程邏輯控制器</b></p><p><b>　　1 PLC介紹</b></p><p>　　PLCs（可編程邏輯控制器）是用于各種自動控制系統(tǒng)和過程的可控網(wǎng)絡(luò)集線器

4、。他們包含多個輸入輸出，輸入輸出是用晶體管和其它電路，模擬開關(guān)和繼電器來控制設(shè)備的。PLCs用軟件接口，標(biāo)準(zhǔn)計算器接口，專門的語言和網(wǎng)絡(luò)設(shè)備編程。</p><p>　　可編程邏輯控制器I/O通道規(guī)則包括所有的輸入觸點(diǎn)和輸出觸點(diǎn)，擴(kuò)展能力和最大數(shù)量的通道。觸點(diǎn)數(shù)量是輸入點(diǎn)和輸出點(diǎn)的總和。PLCs可以指定這些值的任何可能的組合。擴(kuò)展單元可以被堆?；蚧ハ噙B接來增加總的控制能力。最大數(shù)量的通道是在一個擴(kuò)展系統(tǒng)中輸入和輸出

5、通道的最大總數(shù)量。PLCs系統(tǒng)規(guī)則包括掃描時間，指令數(shù)量，數(shù)據(jù)存儲和程序存儲。掃描時間是 PLC需要的用來檢測輸入輸出模塊的時間。指令是用于PLC軟件（例如數(shù)學(xué)運(yùn)算）的標(biāo)準(zhǔn)操作。數(shù)據(jù)存儲是存儲數(shù)據(jù)的能力。程序存儲是控制軟件的能力。</p><p>　　用于可編程邏輯控制器的輸入設(shè)備包括DC，AC，中間繼電器，熱電偶，RTD，頻率或脈沖，晶體管和中斷信號輸入；輸出設(shè)備包括DC，AC，繼電器，中間繼電器，頻率或脈沖，

6、晶體管，三端雙向可控硅開關(guān)元件；PLC的編程設(shè)備包括控制面板，手柄和計算機(jī)。</p><p>　　可編程邏輯控制器用各種軟件編程語言來控制。這些語言包括IEC61131-3，順序執(zhí)行表（SFC），動作方塊圖（FBD），梯形圖（LD），結(jié)構(gòu)文本（ST），指令序列（IL），繼電器梯形圖（RIL），流程圖，C語言和Basic語言。IEC61131-3編程環(huán)境能支持五種語言，用國際標(biāo)準(zhǔn)加以規(guī)范，分別為SFC，F(xiàn)BD，LD

7、，ST和IL。這便允許了多賣主兼容性和多種語言編程。SFC是一種圖表語言，它提供了編程順序的配合，就能支持順序選擇和并列選擇，二者擇其一即可。FBD用一種大的運(yùn)行庫，以圖表形式建立了一些復(fù)雜的過程。標(biāo)準(zhǔn)數(shù)學(xué)和邏輯運(yùn)行可以與用戶交流和接口運(yùn)行相結(jié)合。LD是適用于離散控制和互鎖邏輯的圖表語言。它在離散控制上與FBD是完全兼容的。ST是一種文本語言，用于復(fù)雜的數(shù)學(xué)過程和計算，不太適用于圖表語言。IL是與組合編碼相似的低級語言。它用在相對比較簡

8、單的邏輯指令。繼電器梯形圖或梯形圖是適用于可編程邏輯控制器的重要的編程語言。梯形圖編程是設(shè)計成繼電器邏輯程序的圖表表示法。流程圖是一種圖表語言，用于在一個控制器或應(yīng)用軟件中描述順序操作，它用于建立有標(biāo)準(zhǔn)組件的可循環(huán)使用的運(yùn)行庫。C語言是一種高</p><p>　　可編程邏輯控制器也規(guī)范了許多計算機(jī)接口設(shè)備，網(wǎng)絡(luò)規(guī)則和特色。PLC能源設(shè)備和運(yùn)行環(huán)境也是非常重要的。</p><p><b

9、>　　2 指令</b></p><p>　　對于簡單的編程，繼電器型PLC是有效的。隨著功能的復(fù)雜化，復(fù)雜的VonNeaman型PLC就必須被采用。一個VonNeaman計算機(jī)一次只能執(zhí)行一個指令，他們是這樣運(yùn)行的，盡管許多計算機(jī)看上去一次在做許多事情。正如圖1所示的計算機(jī)組成。</p><p>　　圖1 簡化個人計算機(jī)結(jié)構(gòu)圖</p><p>　

10、　輸入是通過鍵盤和鼠標(biāo)得到的。輸出被送到屏幕。磁盤和存儲器用于輸入和輸出存儲（注意：這些箭頭的方向?qū)τ谠O(shè)計者是非常重要的，要注意表明信息是流向哪里的。）這個圖表可以像圖2那樣能被重新擬訂來闡明輸入設(shè)備和輸出設(shè)備的作用。</p><p>　　圖2 輸入輸出方向結(jié)構(gòu)圖</p><p>　　在這個圖表中數(shù)據(jù)通過輸入設(shè)備進(jìn)入左邊。（注意：大多數(shù)設(shè)計圖表都是左邊輸入，右邊輸出的。）在進(jìn)入CPU之前，

11、它穿過緩沖電路。CPU通過其他回路輸出數(shù)據(jù)。存儲器和磁盤用語存儲要輸出的數(shù)據(jù)。如果我們把個人計算機(jī)看作一個控制器，它通過在屏幕上輸出激勵和輸入來自鼠標(biāo)和鍵盤的響應(yīng)來控制用戶。PLC也是一個控制過程的計算機(jī)。當(dāng)與應(yīng)用程序完全結(jié)合起來時，類似之處變成：</p><p>　　輸入設(shè)備—鍵盤與接近開關(guān)相類比。</p><p>　　輸入電路—連續(xù)輸入芯片就像一個直流24V的輸入卡。</p>

12、;<p>　　計 算 機(jī)—686CPU就像一個PLC的CPU模塊。</p><p>　　輸出電路—圖形卡就像一個三相開關(guān)輸出卡。</p><p>　　輸出設(shè)備—監(jiān)控器就像指示燈。</p><p>　　存 儲 器—PLC的存儲器與個人計算機(jī)的存儲器相似。</p><p>　　用普通個人計算機(jī)可以運(yùn)行PLC，雖然則并不被提倡做。就P

13、LC來說，輸入和輸出</p><p>　　設(shè)備設(shè)計得更加可靠，更加粗糙，更適合惡劣的制造環(huán)境。</p><p><b>　　3 運(yùn)行順序</b></p><p>　　所有的PLC系統(tǒng)有每秒鐘重復(fù)多次的四種基本運(yùn)行階段。最初被第一次接通時，它會檢測它的硬件和軟件是否有錯誤。如果沒有錯誤，它會把所有輸入和輸入值復(fù)制到存儲器，這叫輸入掃描。只用復(fù)制

14、了輸入值的存儲器，梯形邏輯圖將被解決一個，這叫邏輯掃描。在解決梯形圖期間，輸出值只在臨時存儲器中被改變。當(dāng)梯形圖掃描完成后，輸出將用存儲器中臨時值修正，這叫做輸出掃描。PLC此時將從自我檢測開始重新啟動這個過程，這個過程很明顯地每秒鐘重復(fù)10到100次，正如圖3所示。</p><p>　　圖3 PLC掃描循環(huán)</p><p>　　自我檢測—檢測是否所有的卡沒有錯誤，把時間繼電器復(fù)零等。（如

15、果在很小一段時間內(nèi)沒有復(fù)零，時間繼電器會引起錯誤，關(guān)閉PLC系統(tǒng)。—這會表明梯形圖沒有被正常掃描。）</p><p>　　輸入掃描—從芯片上的輸入卡讀取輸入值，并把輸入值復(fù)制到存儲器，這能使PLC更快速地運(yùn)行，并且避免從程序開始到結(jié)束輸入變化。（例如：意外停止）有一些特殊的PLC功能，能直接讀取輸入值，避免了輸入表格。</p><p>　　邏輯處理/掃描—基于存儲器的輸入表格，程序被一次執(zhí)

16、行一步，同時輸出值也被修正，這是其它節(jié)的集中。</p><p>　　輸出掃描—輸出表格從存儲器復(fù)制到輸出芯片，這些芯片然后驅(qū)動輸出儀器。</p><p>　　輸入輸出掃描經(jīng)常會令初學(xué)者感到迷惑，但是他們是很重要的。輸入掃描是輸入值的快照，并且解決邏輯關(guān)系。在一個梯形圖掃描期間，如果一個輸入在梯形圖的多個地方被用到，它就會起變化，潛在問題就可能發(fā)生，而輸入掃描卻避免了這些問題。這個邊境效應(yīng)是

17、如果在一段持續(xù)時間內(nèi)如果一個輸入變化太短，它可能在輸入掃描之間會減少或者丟失。</p><p>　　當(dāng)PLC最初被啟動時，通常的輸出會被關(guān)閉，這不會影響輸入值。</p><p>　　3．1 輸入輸出掃描</p><p>　　輸入值被掃描到PLC時，自然輸入值被復(fù)制到存儲器。當(dāng)輸出值被掃描到PLC時，他們將從存儲器復(fù)制到自然輸出設(shè)備。當(dāng)梯形圖被掃描時，它將用存儲器中

18、的值，并不是實(shí)際的輸入輸出值。這樣做的主要原因是如果一個程序在多個地方用一個輸入值，那么輸入值的變化將使其邏輯關(guān)系無效。而且，如果隨著每塊的變化，輸出模塊也變化，在掃描結(jié)束時PLC的運(yùn)行速度將大大減慢。</p><p><b>　　3．2 邏輯掃描</b></p><p>　　梯形邏輯程序圖是模仿繼電器邏輯圖的。在繼電器邏輯圖中，程序的每個元件將盡可能快地開關(guān)。但是

19、在一個程序中，元件只能按固定的順序一次檢測一個。如圖4所示，梯形圖將按從左到右，從上到下的順序被解釋。在圖中，梯形邏輯掃描將從最高層開始。在底層，它將先解釋高層輸出，然后輸出它下面的分支。在第二層，沿著梯形邏輯圖移動之前，將先解釋分支。</p><p>　　圖4 梯形圖邏輯執(zhí)行順序</p><p>　　解決梯形邏輯程序時，邏輯掃描順序會變得非常重要。梯形圖輸出作為輸入，考慮輸出應(yīng)用時，它也

20、變得非常重要。如圖5所示，梯形圖第一行將檢測輸入并把輸出X置1，得到相同的值。第二行將檢測輸入B并把輸出X置1，得到相反的值。因此，直到梯形圖的第二行被掃描時X值才能與A相等。在邏輯掃描期間，輸出值只能在存儲器中被改變，只有當(dāng)梯形邏輯掃描完成時，實(shí)際的輸出才能被修正。因此，在第二行的基礎(chǔ)上，輸出掃描將修正實(shí)際輸出值。并且梯形圖的第一行將無效。</p><p><b>　　圖5 錯誤提示</b>

21、;</p><p>　　4 PLC狀態(tài)顯示</p><p>　　在一個PLC中，缺少鍵盤和其他的輸入輸出設(shè)備是非常值得注意的。在PLC前端通常有一定數(shù)量的狀態(tài)指示燈。通常指示燈表明：</p><p>　　電源啟動—只要PLC帶電，它將被啟動。</p><p>　　程序運(yùn)行—這將指示是否程序正在運(yùn)行或是否沒有程序正在運(yùn)行。</p>

22、<p>　　錯誤顯示—當(dāng)PLC有大的硬件或軟件錯誤時，這將有顯示。</p><p>　　這些燈通常用于調(diào)試。一定數(shù)量的按鈕也將提供給PLC的硬件。最普通的按鈕是一個運(yùn)行/編程選擇開關(guān)，當(dāng)在保持狀態(tài)時，它將被調(diào)到編程；當(dāng)在生產(chǎn)狀態(tài)時，它將被調(diào)到運(yùn)行。一個PLC系統(tǒng)幾乎沒有一個啟動關(guān)閉開關(guān)或復(fù)位開關(guān)在前面。這需要被設(shè)計到系統(tǒng)剩余部分。</p><p>　　PLC的狀態(tài)也能被梯形邏

23、輯圖檢測。檢測程序是否第一次被執(zhí)行是非常普遍的。如圖6所示?！暗谝淮螔呙琛陛斎朐谔菪螆D被第一次掃描時，將是對的，而在其余的每次掃描時是錯誤的。這種情況下，PLC—5的“第一次掃描”的地址是“S2：1/14”。根據(jù)例子中的邏輯關(guān)系，第一次掃描將封上“變亮”，直到“清除”被啟動。因此燈將在PLC被啟動之后變亮，但在“清除”被啟動之后，它將關(guān)閉并且保持在關(guān)閉狀態(tài)?！暗谝淮螔呙琛蹦K在“第一次掃描”模塊中被提到。</p><

24、p>　　圖6 核驗(yàn)PLC第一次掃描的程序</p><p><b>　　5 存儲器類型</b></p><p>　　有幾種基本的現(xiàn)在經(jīng)常使用的計算機(jī)存儲器類型：</p><p>　　RAM（隨機(jī)存儲器）—這種存儲器速度很快，但是當(dāng)沒電時，它的內(nèi)容將被丟失。這是一種不穩(wěn)定存儲器，每個PLC在運(yùn)行時，都用這種存儲器作為中央處理器 。</

25、p><p>　　ROM（只讀存儲器）—這種存儲器是永久性的不可擦除的。它通常用于存放PLC的操作系統(tǒng)。</p><p>　　EPROM（可擦除可編程只讀存儲器）—這是一種像ROM一樣可編程的存儲器，但是它能用紫外線光擦除并且可以重新編程。</p><p>　　EEPROM（電可擦除可編程只讀存儲器）—這種存儲器能像ROM一樣存放程序。它能被編程并且用電壓擦除，因此它正變

26、得比EPROM更加普遍。</p><p>　　所有的PLC系統(tǒng)都用RAM做CPU，用ROM存儲PLC的基本操作系統(tǒng)。當(dāng)有電時，RAM的內(nèi)容被保存，但是問題在于當(dāng)供給存儲器的電源失去時會發(fā)生什么。原先PLC賣主用帶有電池的RAM，這樣如果不失電，存儲器的內(nèi)容就不會丟失。這種方法現(xiàn)在仍被使用，但變得不那么受歡迎。EPROMS也是PLC編程的比較好的選擇。EPROM在PLC外部編程，然后被放入PLC。當(dāng)PLC被啟動時，

27、在EPROM上的梯形邏輯程序被下載PLC并且運(yùn)行。這種方法非?？煽?，但是擦除和編程技術(shù)都是很消耗時間的。EEPROM存儲器是PLC的永久部分，程序能EPROM一樣被存放在他們中。存儲器的價錢一直在下降，新類型正變得可被利用，這些變化將繼續(xù)對PLC系統(tǒng)發(fā)生影響。</p><p>　　6 檢驗(yàn)和優(yōu)化一個PLC控制時間表</p><p>　　混合的系統(tǒng)是快速地成長的重要一個研究領(lǐng)域。在系統(tǒng)中出

28、現(xiàn)不連續(xù)的和連續(xù)的現(xiàn)象，激發(fā)了我們對特殊系統(tǒng)技術(shù)挑戰(zhàn)和對我們能力的分析。這導(dǎo)致新的表達(dá)的模型發(fā)展，例如，計時的混合自動機(jī)械裝置和新的方法，最特別地做模型檢查技術(shù)包括即時方面?；旌?嵌入的)系統(tǒng)的一個重要的例子是處理控制計劃，包括處理系統(tǒng)的數(shù)傳控制，舉例來說，化學(xué)藥品廠的控制系統(tǒng)是相當(dāng)?shù)闹匾牡湫蛻?yīng)用的程序控制器系統(tǒng)。令人失望的是PLC和他們組成的系統(tǒng)語言都沒有好標(biāo)準(zhǔn)，不能用正式的模型來設(shè)計復(fù)雜可靠的控制器。歐盟研究計劃VHS(混合的系統(tǒng)

29、)有許多個案研究。其中一個研究為一個實(shí)驗(yàn)的化學(xué)工廠與PLC控制流程設(shè)計有關(guān)。在這個文章中，我們在兩個樣板的檢驗(yàn)員檢測的基礎(chǔ)上報告為給定的系統(tǒng)和最佳的控制引出預(yù)定。它是程序控制器的正確設(shè)計上的濃縮語。這個工具是一個很自然的備選者中支持即時的環(huán)境最佳的控制時間表引出的。這再一次給與我們實(shí)行個案研究的最佳化部份的機(jī)會，也是它自己功能的一種練習(xí)。收集數(shù)據(jù)并解釋與之相比較后獲得的結(jié)果。這篇文章開始部分介紹PLC性質(zhì)。</p><

30、;p>　　7 基于軟件的PLC系統(tǒng)</p><p>　　個人計算機(jī)持續(xù)下降的價格增加了他們在控制系統(tǒng)中的應(yīng)用，包括PLC的替代品。安裝了軟件就能用個人計算機(jī)解決梯形圖邏輯.從傳感器中讀取輸入，修改輸出送到激勵。這些對于維持是很重要的，因?yàn)樗麄儾挥米袷匾郧暗挠嫊r模式.例如，計算機(jī)正運(yùn)行一個游戲，就可能減慢或停止計算機(jī).這個以及其它問題現(xiàn)在正被研究，好的解決方案不久就會出現(xiàn)。</p><p

31、><b>　　8 概要</b></p><p>　　（1）PLC系統(tǒng)和計算機(jī)與輸入設(shè)備，輸出設(shè)備,存儲器等很相似。</p><p>　　（2）PLC系統(tǒng)不斷地執(zhí)行系統(tǒng)檢查，輸入掃描,邏輯掃描和輸出掃描這個循環(huán)。</p><p>　　（3）當(dāng)邏輯圖被掃描時，輸入的變化沒有被發(fā)現(xiàn),輸出也沒有被修正。</p><p>

32、　?。?）PLC系統(tǒng)用RAM，有時用EPROM存放永久程序。</p><p><b>　　9 實(shí)際問題</b></p><p>　　（1）一個PLC系統(tǒng)通常包括RAM，ROM，EPROM和/或電池嗎？</p><p>　?。?）PLC的指示燈用于什么？</p><p>　?。?）為什么一個PLC系統(tǒng)每秒鐘只能掃描梯形圖

33、幾次？</p><p>　?。?）如果一個PLC系統(tǒng)的掃描時間比輸入脈沖長，會發(fā)生什么?為什么？</p><p>　?。?）一個PLC系統(tǒng)與一部臺式計算機(jī)的不同是什么？</p><p>　　（6）為什么PLC系統(tǒng)每次掃描要做自我檢查？</p><p>　?。?）PLC檢測時間會比簡單程序所需時間長嗎？</p><p>

34、<b>　　圖7 梯形示意圖</b></p><p>　　10 實(shí)際問題解答</p><p>　　（1）每個PLC系統(tǒng)包括RAM和ROM，但是他們也包括EPROM或電池。</p><p><b>　?。?）診斷和保持。</b></p><p>　　（3）盡管程序是空的，PLC系統(tǒng)仍需掃描輸入和輸出

35、，做自我檢測。</p><p>　?。?）如果在兩次輸入掃描之間發(fā)生，脈沖就會丟失。</p><p>　?。?）主要的區(qū)別包括輸入設(shè)備輸出設(shè)備和應(yīng)用。PLC系統(tǒng)是為工廠設(shè)計的，因此它沒有鼠標(biāo)鍵盤之類的輸入設(shè)備。(雖然，一些較新型PLC能夠達(dá)到)他們也沒有屏幕聲音之類的輸出設(shè)備，.取而代之，他們有電壓,電流這樣的輸入設(shè)備和輸出設(shè)備。PLC使用戶為專門的任務(wù)設(shè)計程序,然而在個人計算機(jī)上給系統(tǒng)編

36、程是不常見的。</p><p>　　（6）這能幫助檢測硬件和軟件錯誤。如果一個錯誤發(fā)生了，PLC還繼續(xù)運(yùn)行,控制器就可能以一種不可預(yù)見的方式運(yùn)行，這對人和機(jī)器是非常危險的。自我檢測則幫助檢查出這些錯誤，并且安全地關(guān)閉系統(tǒng)。</p><p>　?。?）是的，在許多PLC系統(tǒng)中，自檢大約需要1ms，但一個單一程序需1mms。</p><p><b>　　附件2

37、：外文原文</b></p><p>　　Programmable Logic Controllers (PLCs)</p><p>　　1 About Programmable Logic Controllers (PLCs)</p><p>　　PLCs (programmable logic controllers) are the control

38、 hubs for a wide variety of automated systems and processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They are programmable v

39、ia software interfaced via standard computer interfaces and proprietary languages and network options.</p><p>　　Programmable logic controllers I/O channel specifications include total number of points, numbe

40、r of inputs and outputs, ability to expand, and maximum number of channels.  Number of points is the sum of the inputs and the outputs. PLCs may be specified by any possible combination of these values.  Expand

41、able units may be stacked or linked together to increase total control capacity.  Maximum number of channels refers to the maximum total number of input and output channels in an expanded system.</p><p>

42、;　　Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs.  Outputs for PLCs include DC, AC, relay, analog, frequency or p

43、ulse, transistor, and triac.  Programming options for PLCs include front panel, hand held, and computer.</p><p>　　Programmable logic controllers use a variety of software programming languages for contr

44、ol.  These include IEC 61131-3, sequential function chart (SFC), function block diagram (FBD), ladder diagram (LD), structured text (ST), instruction list (IL), relay ladder logic (RLL), flow chart, C, and Basic.

45、60; The IEC 61131-3 programming environment provides support for five languages specified by the global standard: Sequential Function Chart, Function Block Diagram, Ladder Diagram, Structured Text, and </p><p&

46、gt;　　code. It is used in relatively simple logic instructions.  Relay Ladder Logic (RLL), or ladder diagrams, is the primary programming language for programmable logic controllers (PLCs). Ladder logic programming i

47、s a graphical representation of the program designed to look like relay logic.  Flow Chart is a graphical language that describes sequential operations in a controller sequence or application. It is used to build mo

48、dular, reusable function libraries.  C is a high level programming language</p><p>　　Programmable logic controllers can also be specified with a number of computer interface options, network specificati

49、ons and features.  PLC power options, mounting options and environmental operating conditions are all also important to consider.</p><p>　　2 INTRODUCTION</p><p>　　For simple programming th

50、e relay model of the PLC is sufficient. As more complex functions are used the more complex VonNeuman model of the PLC must be used. A VonNeuman computer processes one instruction at a time. Most computers operate this w

51、ay, although they appear to be doing many things at once. Consider the computer components shown in Figure 1.</p><p>　　Figure1 Simplified Personal Computer Architecture</p><p>　　Input is obtaine

52、d from the keyboard and mouse, output is sent to the screen, and the disk and memory are used for both input and output for storage. (Note: the directions of these arrows are very important to engineers, always pay atten

53、tion to indicate where information is flowing.) This figure can be redrawn as in Figure 2 to clarify the role of inputs and outputs.</p><p>　　Figure2 An Input-Output Oriented Architecture</p><p>

54、;　　In this figure the data enters the left side through the inputs. (Note: most engineering diagrams have inputs on the left and outputs on the right.) It travels through buffering circuits before it enters the CPU. The

55、CPU outputs data through other circuits. Memory and disks are used for storage of data that is not destined for output. If we look at a personal computer as a controller, it is controlling the user by outputting stimuli

56、on the screen, and inputting responses from the mouse and the k</p><p>　　A PLC is also a computer controlling a process. When fully integrated into an application the analogies become;</p><p>　　

57、inputs - the keyboard is analogous to a proximity switch</p><p>　　input -circuits - the serial input chip is like a 24Vdc input card</p><p>　　computer - the 686 CPU is like a PLC CPU unit</p&

58、gt;<p>　　output - circuits - a graphics card is like a triac output card</p><p>　　outputs - a monitor is like a light</p><p>　　storage - memory in PLCs is similar to memories in personal

59、computers</p><p>　　It is also possible to implement a PLC using a normal Personal Computer, although this is not advisable. In the case of a PLC the inputs and outputs are designed to be more reliable and ru

60、gged for harsh production environments.</p><p>　　3 OPERATION SEQUENCE</p><p>　　All PLCs have four basic stages of operations that are repeated many times per second. Initially when turned on th

61、e first time it will check it’s own hardware and software for faults. If there are no problems it will copy all the input and copy their values into memory, this is called the input scan. Using only the memory copy of th

62、e inputs the ladder logic program will be solved once, this is called the logic scan. While solving the ladder logic the output values are only changed in temporary m</p><p>　　Figure3 PLC Scan Cycle</p>

63、;<p>　　SELF TEST - Checks to see if all cards error free, reset watch-dog timer, etc. (A watchdog timer will cause an error, and shut down the PLC if not reset within a short period of time - this would indicate t

64、hat the ladder logic is not being scanned normally).</p><p>　　INPUT SCAN - Reads input values from the chips in the input cards, and copies their values to memory. This makes the PLC operation faster, and av

65、oids cases where an input changes from the start to the end of the program (e.g., an emergency stop). There are special PLC functions that read the inputs directly, and avoid the input tables.</p><p>　　LOGIC

66、 SOLVE/SCAN - Based on the input table in memory, the program is executed 1 step at a time, and outputs are updated. This is the focus of the later sections.</p><p>　　OUTPUT SCAN - The output table is copied

67、 from memory to the output chips. These chips then drive the output devices.</p><p>　　The input and output scans often confuse the beginner, but they are important. The input scan takes a snapshot of the inp

68、uts, and solves the logic. This prevents potential problems that might occur if an input that is used in multiple places in the ladder logic program changed while half way through a ladder scan. Thus changing the behavio

69、rs of half of the ladder logic program. This problem could have severe effects on complex programs that are developed later in the book. One side effect of the</p><p>　　When the PLC is initially turned on th

70、e normal outputs will be turned off. This does not affect the values of the inputs.</p><p>　　3.1 The Input and Output Scans</p><p>　　When the inputs to the PLC are scanned the physical input va

71、lues are copied into memory. When the outputs to a PLC are scanned they are copied from memory to the physical outputs. When the ladder logic is scanned it uses the values in memory, not the actual input or output values

72、. The primary reason for doing this is so that if a program uses an input value in multiple places, a change in the input value will not invalidate the logic. Also, if output bits were changed as each bit was changed, in

73、</p><p>　　3.2 The Logic Scan</p><p>　　Ladder logic programs are modelled after relay logic. In relay logic each element in the ladder will switch as quickly as possible. But in a program elemen

74、ts can only be examines one at a time in a fixed sequence. Consider the ladder logic in Figure 4, the ladder logic will be interpreted left-to-right, top-to-bottom. In the figure the ladder logic scan begins at the top r

75、ung. At the end of the rung it interprets the top output first, then the output branched below it. On the second rung it solv</p><p>　　Figure4 Ladder Logic Execution Sequence</p><p>　　The logic

76、scan sequence become important when solving ladder logic programs which use outputs as inputs. It also becomes important when considering output usage. Consider Figure 5, the first line of ladder logic will examine input

77、 A and set output X to have the same value. The second line will examine input B and set the output X to have the opposite value. So the value of X was only equal to A until the second line of ladder logic was scanned. R

78、ecall that during the logic scan the outputs are o</p><p>　　Figure5 A Duplicated Output Error</p><p>　　4 PLC STATUS</p><p>　　The lack of keyboard, and other input-output devices is

79、 very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate;</p><p>　　power on - this will be on whenever the PLC has power;</p><p>　　prog

80、ram running - this will often indicate if a program is running, or if no program is running.</p><p>　　fault - this will indicate when the PLC has experienced a major hardware or software problem</p>&

81、lt;p>　　These lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is being cond

82、ucted, and back to run when in production. This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset bu

83、tton on the front. This needs to be designed into the remainder of the</p><p>　　The status of the PLC can be detected by ladder logic also. It is common for programs to check to see if they are being execute

84、d for the first time, as shown in Figure 6. The ’first scan’ input will be true on the very first time the ladder logic is scanned, but false on every other scan. In this case the address for ’first scan’ in a PLC-5 is ’

85、S2:1/14’. With the logic in the example the first scan will seal on ’light’, until ’clear’ is turned on. So the light will turn on after the PLC has been</p><p>　　Figure6 An program that checks for the first

86、 scan of the PLC</p><p>　　5 MEMORY TYPES</p><p>　　There are a few basic types of computer memory that are in use today.</p><p>　　RAM (Random Access Memory) - this memory is fast, b

87、ut it will lose its contents when power is lost, this is known as volatile memory. Every PLC uses this memory for the central CPU when running the PLC.</p><p>　　ROM (Read Only Memory) - this memory is perman

88、ent and cannot be erased. It is often used for storing the operating system for the PLC.</p><p>　　EPROM (Erasable Programmable Read Only Memory) - this is memory that can be programmed to behave like ROM, bu

89、t it can be erased with ultraviolet light and reprogrammed.</p><p>　　EEPROM (Electronically Erasable Programmable Read Only Memory) – This memory can store programs like ROM. It can be programmed and erased

90、using a voltage, so it is becoming more popular than EPROMs.</p><p>　　All PLCs use RAM for the CPU and ROM to store the basic operating system for the PLC. When the power is on the contents of the RAM will b

91、e kept, but the issue is what happens when power to the memory is lost. Originally PLC vendors used RAM with a battery so that the memory contents would not be lost if the power was lost. This method is still in use, but

92、 is losing favor. EPROMs have also been a popular choice for programming PLCs. The EPROM is programmed out of the PLC, and then placed in the P</p><p>　　6 Verification and optimization of a PLC control sche

93、dule</p><p>　　The verification of hybrid systems is a research area of rapidly growing importance in the formal methods community. The presence of both discrete and continuous phenomena in such systems poses

94、 an inspiring challenge for our specification and modelling techniques, as well as for our analytic capacities. This has led to the development of new, expressive models, such as timed and hybrid automata [3, 16], and ne

95、w verification methods, most notably model checking techniques involving a symbolic tre</p><p>　　7 SOFTWARE BASED PLCS</p><p>　　The dropping cost of personal computers is increasing their use i

96、n control, including the replacement of PLCs. Software is installed that allows the personal computer to solve ladder logic, read inputs from sensors and update outputs to actuators. These are important to mention here b

97、ecause they don’t obey the previous timing model. For example, if the computer is running a game it may slow or halt the computer. This issue and others are currently being investigated and good solutions should be </

98、p><p>　　8 SUMMARY</p><p>　　A PLC and computer are similar with inputs, outputs, memory, etc.</p><p>　　The PLC continuously goes through a cycle including a sanity check, input scan, l

99、ogic scan, and output scan.</p><p>　　While the logic is being scanned, changes in the inputs are not detected, and the outputs are not updated.</p><p>　　PLCs use RAM, and sometime EPROMs are use

100、d for permanent programs.</p><p>　　9 PRACTICE PROBLEMS</p><p>　?。?）Does a PLC normally contain RAM, ROM, EPROM and/or batteries?</p><p>　?。?）What are the indicator lights on a PLC

101、used for?</p><p>　　（3）A PLC can only go through the ladder logic a few times per second. Why?</p><p>　?。?）What will happen if the scan time for a PLC is greater than the time for an input pulse?

102、 Why?</p><p>　　（5）What is the difference between a PLC and a desktop computer?</p><p>　?。?）Why do PLCs do a self check every scan?</p><p>　　（7）Will the test time for a PLC be long c