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黑龍江科技學院
畢業(yè)設(shè)計（或論文）說明書
附錄1
離心泵的故障分析
摘要
離心泵故障按其產(chǎn)生的原因可以分成泵本身的機械故障、泵和管道組成的工藝系統(tǒng)存在的缺陷導致的泵出現(xiàn)異常振動、噪聲等故障。后類故障原因比較隱蔽，不易查明。通過工作中遇到的幾個實例，對工藝和管路系統(tǒng)設(shè)計問題導致的離心泵故障進行了分析，并提出了相應(yīng)的對策。
Abstract:Leadership water pump break down to press its output reason and can be divided into the mechanical trouble of oneself pump and pump to appear with the pump that blemish cause that the craft system that piping constitute exsits abnormality vibration, voice etc. break down. The empress type breaks down the reason more concealment, find out not easily.Passes a few and solid example met in the work, to craft with take care of the road system the design the problem cause of leadership water pump break down proceeded the analysis, and put forward the homologous counterplan.
關(guān)鍵詞：離心泵 機械故障 分析 真空度
Keywords: Leadership water pump Mechanical trouble Analysis
1離心泵吸入管路進氣
由于氣體密度遠小于液體，氣體通過葉輪流道時，所能得到的壓頭遠小于液體通過葉輪流道時所得到的壓頭。在葉輪流道中的不同位置，壓力分布不同，當液體中混有氣體時，氣泡在這種不均勻的壓力作用下，先膨脹后壓縮，產(chǎn)生了類似汽蝕的沖擊，最后有可能會被壓潰或破滅。葉輪受到激振力作用會劇烈振動并發(fā)出噪聲泵出口壓在密閉系統(tǒng)中與液體一起循環(huán)流動，無法排出系統(tǒng)，如果系統(tǒng)中夾帶的氣體的量比較多，泵就會出現(xiàn)異常振動。密閉系統(tǒng)中氣體來源主要有兩個方面:
(l)系統(tǒng)本身設(shè)計不合理存在難以排氣的死角，每次向系統(tǒng)中注入液體時這些死角區(qū)域殘留有大量空氣，而在循環(huán)時這些空氣有可能被帶入泵中。
(2)系統(tǒng)工作液體在長期工藝循環(huán)中產(chǎn)生不凝性氣體由于系統(tǒng)缺少氣液分離、排放措施不凝性氣體在系統(tǒng)中積聚。如加熱系統(tǒng)中的熱媒等有機物在長期循環(huán)使用中會因氧化等原因產(chǎn)生氣態(tài)物質(zhì)。
在密閉系統(tǒng)中出現(xiàn)上述兩種情況時，首先要盡快想辦法排放系統(tǒng)中的氣體，其次要準確判斷氣體的來源杜絕氣體在系統(tǒng)中的存在;不能杜絕的，要在系統(tǒng)中增加氣液分離裝置并定期進行排放。顯然對于密閉循環(huán)系統(tǒng)，在系統(tǒng)的合適部位裝設(shè)氣體收集(存系統(tǒng)壓力轉(zhuǎn)低點、和(或、排放裝置(如系統(tǒng)管路的最高點)加強巡檢對系統(tǒng)進行定期排氣是必不可少的
某干燥轉(zhuǎn)鼓熱媒加熱系統(tǒng)(如圖l)中的1臺B41 5 H M 0506衛(wèi)型屏蔽泵在試車中發(fā)現(xiàn)泵體振動嚴重噪聲大出口壓力劇烈波動。排除泵本身有機械和電氣故障后對整個熱媒系統(tǒng)進行研究，發(fā)現(xiàn)該系統(tǒng)中的錦輪轉(zhuǎn)鼓(設(shè)備l)無排氣口系統(tǒng)充注熱媒時，轉(zhuǎn)鼓加熱夾套中的空氣排不出來。開車時，空氣被吸入屏蔽泵中造成故障。于是在錦輪轉(zhuǎn)鼓上加裝排氣閥，在系統(tǒng)補加熱媒時進行高位排氣再起動屏蔽泵泵運轉(zhuǎn)正常。
。
對于開放式循環(huán)系統(tǒng)，系統(tǒng)中產(chǎn)生的氣體可以直接排入大氣氣體在系統(tǒng)中不會積聚。所以進入泵中的氣體主要是由泵的進口管路從外界吸入的。因此發(fā)生這類故障時應(yīng)著重對吸入管路進行檢查。
如某冷凍水循環(huán)系統(tǒng)是一個開放系統(tǒng)，如圖2所示其中的循環(huán)泵(型號151 50一125一400，揚程為45m，流量為190m3/h)是1臺單級單吸離心泵。運行中發(fā)現(xiàn)泵剛起動時并無異常起動約Zmin后開始出現(xiàn)周期性振動，出口壓力表指針大輻擺動，嚴重時系統(tǒng)管架也隨之晃動。經(jīng)多次試驗，均重復出現(xiàn)同樣現(xiàn)象。經(jīng)檢查該泵運轉(zhuǎn)部件沒有問題。而且該泵供水的冷凍水箱液位高于泵體，吸入管路上水力損失很小，泵不會發(fā)生氣蝕。排除諸因素后，考慮可能是泵運行中吸入了氣體，遂對泵吸入管路進行重點檢查，檢查冷凍水箱時發(fā)現(xiàn)水箱DN250的回水管管口正好位于泵吸入管管口上方，離水箱液面0.3m，離吸入管管口lm。高速沖入池中的回水從液面夾帶大量空氣直沖泵
吸入口，使泵吸入大量空氣，造成泵體振動。為此采取將泵吸入管向池內(nèi)延伸1 .sm，避開水池回水口，結(jié)果消除了故障。
2離心泵出口管路存在的氣堵
在循環(huán)管路系統(tǒng)中，管路的較高處或較大的工藝閥門上部易產(chǎn)生氣體的聚集。離心泵運行時在這些部位會形成氣囊，液體流經(jīng)這些區(qū)域時流動阻力增大，局部壓力升高，壓縮氣體，氣體體積減小，又使局部壓力下降，周而復始，造成液體壓力劇烈波動形成系統(tǒng)管路水擊導致泵體振動。
這類故障多出現(xiàn)于系統(tǒng)剛開始運行，系統(tǒng)排氣不充分的時候。合理地設(shè)計系統(tǒng)管路可以減少產(chǎn)生氣堵。
3工藝參數(shù)變化與泵的汽蝕
常見的引起泵汽蝕的因素主要有泵的安裝福度不合理、吸入管路的阻力損失太大或泵選型可適當、工作點不合理等。但在復雜的工藝系統(tǒng)中一臺原本選型正確、工作穩(wěn)定的離心泵也會因跳工藝參數(shù)的極端變化發(fā)生汽蝕。
3·1吸入壓力變化引發(fā)汽蝕
從泵的吸入液面到葉輪流道低壓區(qū)列伯努利方程，可以看到當吸入液面上的壓力減小時，葉輪入口的壓力就降低，反之則上升。也就是說泵的抗汽蝕能力隨液面壓力增大而提高，隨液面壓力減小而降低。
由表1可以看出cq跨臨界循環(huán)系統(tǒng)在制冷系數(shù)、制熱系數(shù)均偏低的前提下，cq空調(diào)+熱水禍合系統(tǒng)方案與傳統(tǒng)工質(zhì)空調(diào)系統(tǒng)+電熱水器方案相比全年總耗電量減少了32.1%全年綜合性能系數(shù)提高了47 .4%。
4結(jié)論
(l)cq跨臨界循環(huán)空調(diào)+熱水禍合系統(tǒng)結(jié)構(gòu)緊湊，而且能夠滿足制冷、制冷+熱水、熱水、制熱、制熱+熱水5種工況需求，可以有效提高系統(tǒng)利用率;
(2)該禍合系統(tǒng)可以回收和利用cq跨臨界循環(huán)的排氣熱量，系統(tǒng)總體性能較高，在能源利用、環(huán)境安全和經(jīng)濟運行等方面都具有優(yōu)勢和潛力，具有廣闊的應(yīng)用前景。
附錄2
數(shù)控技術(shù)
先進制造技術(shù)中的一個最基本的概念是數(shù)字控制（NC）。在數(shù)控技術(shù)出現(xiàn)之前，所有的機床都是人工操縱和控制的。在與人工控制的機床有關(guān)的很多局限性中，操作者的技能大概是最突出的問題。采用人工控制時，產(chǎn)品的質(zhì)量直接與操作者的技能有直接的關(guān)系。數(shù)字控制代表了從人工控制機床做出來的第一步。
數(shù)字控制技術(shù)意味著采用預先錄制的、存儲的符號指令，控制機床和其他制造系統(tǒng)。一個數(shù)控技師的工作不是去操縱機床，而是編寫能夠發(fā)出機床操縱指令的程序。對于一臺數(shù)控機床，其上必須有一個被稱為閱讀機的界面裝置，用來接受和解譯編程指令。
發(fā)展數(shù)控技術(shù)是為了克服人類操作者的局限性，而且它確實完成了這項工作。數(shù)字控制的機器比人工操縱的機器的精度更高、生產(chǎn)零件的一致性更好、生產(chǎn)速度更快、而且長期的工藝裝備成本更低。數(shù)控技術(shù)的發(fā)展導致制造工藝中其他幾項新發(fā)明的產(chǎn)生。
●電火花加工技術(shù)
●激光切割
●電子束焊接
數(shù)字控制還使得機床比他們采用人工操縱的前輩們的用途更為廣泛。一臺數(shù)控機床可以自動產(chǎn)生很多種類的零件，每個零件都可以有不同和復雜的加工過程。數(shù)控可使生產(chǎn)廠家承擔那些對于采用人工控制的機床和工藝來說，在經(jīng)濟上是不 劃算的產(chǎn)品的生產(chǎn)任務(wù)。
與許多先進技術(shù)一樣，數(shù)控誕生于麻省理工學院的實驗室中。數(shù)控這個概念是20世紀50年代初在美國空軍的資助下提出來的。在其最初階段，數(shù)控機床可以經(jīng)濟和有效地進行直線切割。
然而，曲線軌跡成為機床加工的一個問題，在編程時應(yīng)采用一系列的水平與豎直的臺階來生成曲線。構(gòu)成臺階的每個線段越短，曲線就越光滑。臺階中的每個線段都必須經(jīng)過計算。
在這個問題促使下，與1959年誕生了自動編程工具（ATP）語言。這是個專門適用于數(shù)控的編程語言，使用類似于英語的語句來定義零件的幾何形狀，描述切削刀具的形狀和規(guī)定必要的運動。ATP語言的研究和發(fā)展是在數(shù)控技術(shù)進一步發(fā)展過程中 的一大進步。最初的數(shù)控系統(tǒng)與今天應(yīng)用的數(shù)控系統(tǒng)是有很大差別的。在那時的機床中，只有硬線邏輯電路。指令程序?qū)懺诖┛准垘希ê髞硭凰芰霞垘〈?，采用帶閱讀機將寫在紙帶或磁帶上的指令給機器翻譯出來。所有這些共同構(gòu)成了機床數(shù)字控制方面的巨大進步。然而，在數(shù)控發(fā)展的這個階段還存在著許多問題。
一個主要問題是穿孔紙帶的易損壞性。在機械加工過程中，載有編程指令信息的紙帶斷裂和被撕壞是常見的事情。在機床上每加工一個零件，都需要將載有指令的紙帶放入閱讀機中重新運行一次。因此，這個問題變得很嚴重。如果需要制造100個某種零件，則應(yīng)該將紙帶分別通過閱讀機100次。、易損壞的紙帶顯然不能承受嚴酷的車間環(huán)境和這種重復使用。
這就導致了一種專門的塑料磁帶的研制。在紙帶上通過采用一系列的小孔來載有編程指令，而在塑料帶上通過采用一系列的磁點來載有編程指令。塑料帶強度比紙帶強度要高很多，這就可以解決常見的撕壞和斷裂問題。然而，它仍然存在著兩個問題。
其中最重要的一個問題就是，對輸入帶中的指令進行修改是非常困難的，或者是根本不可能的。即使對指令程序進行最微小的調(diào)整，也必須中斷加工，制作一條新帶。而且?guī)ㄟ^閱讀機的次數(shù)還必須與需要加工的零件個數(shù)相同。幸運的是，計算機技術(shù)的實際應(yīng)用很快解決了數(shù)控技術(shù)中與穿孔紙帶有關(guān)的問題。
在形成直接數(shù)字控制（DNC）這個概念之后，可以不再采用紙帶或塑料帶作為編程指令的載體，這樣就解決了與之有關(guān)的問題。在直接數(shù)字控制中，幾臺機床通過數(shù)據(jù)傳輸線路連接到一臺主計算機上。操縱這些機床所需要的程序都存儲在這臺主計算機中。當需要時，通過數(shù)據(jù)傳輸線路提供給每臺機床。直接數(shù)字控制是在穿孔紙帶和塑料帶基礎(chǔ)上的一大進步。然而，它也有著與其他依賴于主計算機的技術(shù)一樣的局限性。當主計算機出現(xiàn)故障時，由其控制的所有機床都將停止工作。這個問題促使了計算機數(shù)字控制技術(shù)的產(chǎn)生。
微處理器的發(fā)展為可編程邏輯控制器和微型計算機的發(fā)展做好了準備。這兩種技術(shù)為計算機數(shù)控的發(fā)展打下了良好的基礎(chǔ)。采用CNC技術(shù)后，每臺機床上都有一個可編程邏輯控制器或者微機對其進行數(shù)字控制。這可以使得程序被輸入和存儲在每臺機器內(nèi)部。它還可以在機床以外編制程序，并且將其下載到每臺機床中。計算機數(shù)控解決了主計算機發(fā)生故障所帶來的問題，但是它產(chǎn)生了另一個被稱為數(shù)據(jù)管理的問題。同一個程序可能要分別裝入十個相互之間沒有通信聯(lián)系的微機中。這個問題正在解決之中，它是通過采用局部區(qū)域網(wǎng)絡(luò)將各個微機連接起來，以利于更好地進行數(shù)據(jù)管理。
數(shù)控是可編程自動化技術(shù)的一種形式，通過數(shù)字、字母和其它符號來控制加工設(shè)備。數(shù)字。字母和符號用適當 格式編碼為一個特定工件定義指令程序。當工件改變時，指令程序就改變。這種改變程序的能力使
數(shù)控適合于中，小批量生產(chǎn)，寫一段新程序遠比對加工設(shè)備做大的改動容易得多。
控機床有兩種基本形式：點控制和連續(xù)控制。點控制機床采用異步電動機，因此，主軸的定只能通過完成一個運動或一個電動機的轉(zhuǎn)動來實現(xiàn)。這種機床主要用于直線切削或鉆孔、鏜孔等場合。圖20-1表明了
一個點位控制運動從X、Y、Z坐標2,0,0(點a)到0,1,3(點d)的典型順序。在本倒中，三個伺服電動機同時式作，當?shù)竭_各自軸的適當位置時，就分別停機。運動從坐標2,0,0(點a)開始，三個電動機一起動作，將主軸帶
到1,1,1(點b)符近，此時y軸電動機停轉(zhuǎn)。主軸續(xù)運動到0,1,2(點c)此時軸x電動機停轉(zhuǎn)。最后，主軸在軸電動機帶動下運動到0,1,3(點d),完成運動.點控制的數(shù)控機床是最簡單、最便宜的。
圖20-2對點位控制運動和連續(xù)控制運動進行了比較。在連續(xù)控制時，三個電動機按一定的速度比倒連續(xù)
運轉(zhuǎn)，形成從A到D的一條直線。連續(xù)控制的數(shù)控機床通常由計算機控制。
數(shù)控系統(tǒng)由下列組件組成：數(shù)據(jù)輸入裝置，帶控制單元的磁帶閱讀機，反饋裝置和切削機床或其它形式
的數(shù)控設(shè)備。
數(shù)據(jù)輸入裝置，也稱“人機聯(lián)系裝置”，可用人工或全自動主法向機床提供數(shù)據(jù)，人工方法作為輸入數(shù)據(jù)唯一方法時，只限于少量輸入，人工輸入裝置有鍵盤、撥號盤，按鈕，開關(guān)或撥輪選擇開關(guān)，這些都位于機床附近的一個控制臺上。撥號盤通常到一個同步解析器或電位訂的模擬裝置上。在大多數(shù)情況下，按鈕、開關(guān)、和其他類似的旋鈕是數(shù)據(jù)輸入元件。人工輸入需要操作者控制每個操作，這是一個既慢又單調(diào)的過程， 除了簡單加工場合或特殊情況，已很少使用。
幾乎所有情況下，信息都是通過卡片、穿孔紙帶或磁帶自動提供給控制單元。在傳統(tǒng)的數(shù)控系統(tǒng)中，八信道空孔紙帶是最常用的數(shù)據(jù)輸入形式，紙帶上的編碼指令由一系列稱為程序塊的空孔組成。每一個程序塊代表一種加工功能、一種操作或兩者的組合。紙帶上的整個數(shù)控程序由這些連續(xù)數(shù)據(jù)單元連接而成，帶有程序的長帶子像電影膠片一樣繞在盤子上，相對較短的帶子上的程序可通過將紙帶兩端連接形成一個循環(huán)而連續(xù)不斷地重復使用。帶子一旦安裝好，就可反復使用而無需進一步處理。此時，操作者只是簡單地上、下工件。穿孔紙是在帶有特制穿孔符件的打字機或直接連到計算機上的紙帶穿孔裝置上做成的。紙帶制造很少不出錯，錯誤可能由編程、卡片穿孔或編碼、紙帶穿孔時的物理損害等開成。通常，必須要試走幾次來排除錯誤，才能得到一個可用的工作紙帶。
雖然紙帶上的數(shù)據(jù)自動進給的，但實際編程卻是手工完成的，在編碼紙帶做前，編程者經(jīng)常要和一個計劃人員或工藝工程師一起工作，選擇合適的數(shù)控機床，決定加工材料，計算切削速度和進給速度，決定所需刀具類型，仔細閱讀零件圖上尺寸，定下合適的程序開始的零參考點，然后定出程序清單，其上記載有描述加工順序的編碼數(shù)控指令，機床按順序加工工件到圖樣要求。
控制單元接受和儲存編碼數(shù)據(jù)，直到形成一個完整的信息程序塊，然后解釋數(shù)控指令，并引導機床得到所需運動。
為更好理解控制單元的作用，可將它與撥號電話進行比較，即每撥一個數(shù)字，就儲存一個，當整個數(shù)字撥好后，電話就被激活，也就完成了呼叫。
裝在控制單元里的紙帶閱讀機，通過其內(nèi)的硅光二極管，檢測到穿過移動紙帶上的孔漏過的光線，將光束轉(zhuǎn)變成電能，并通過放大來進一步加強，然后將信號送到控制單元里的寄存器，由它將動作信號傳到機床驅(qū)動裝置。
有些光電裝置能以高達每秒1000個字節(jié)的速度閱讀，這對保持機床連續(xù)動作是必須的，否則，在輪廓加工時，刀具可能在工件上產(chǎn)生劃痕。閱讀裝置必須要能以比控制系統(tǒng)處理數(shù)據(jù)更快的速度來閱讀數(shù)據(jù)程序塊。
反饋裝置是用在一些控設(shè)備上的安全裝置，它可連續(xù)補償控制位置與機床運動滑臺的實際位置之間的誤差。裝有這種直接反饋檢查裝置的數(shù)控機床有一個閉環(huán)系統(tǒng)裝置。位置控制通過傳感器實現(xiàn)，在實際工作時，記錄下滑臺的位置，并將這些信息送回控制單元。接受到的信號與紙帶輸入的信號相比較，它們之間的任何偏差都可得到糾正。
在另一個稱為開環(huán)的系統(tǒng)中，機床僅由響應(yīng)控制器命令的步進電動機驅(qū)動定位，工作的精度幾乎完全取決于絲杠的精度和機床結(jié)構(gòu)的剛度。在這個系統(tǒng)中，沒有信息反饋到控制單元的自矯正過程。出現(xiàn)誤動作時，控制單元繼續(xù)發(fā)出電脈沖。比如，一臺數(shù)控銑床的工作突然過載，阻力矩超過電機轉(zhuǎn)矩時，將沒有響應(yīng)信號送回到控制器。因為，步進電機對載荷變化不敏感，所以許多數(shù)控系統(tǒng)設(shè)計允許電機停轉(zhuǎn)。然而，盡管有可能損壞機床結(jié)構(gòu)或機械傳動系統(tǒng)，也有使用帶有特高轉(zhuǎn)矩步進電機的其他系統(tǒng)，此時，電動機有足夠能力應(yīng)付系統(tǒng)中任何偶然事故。
最初的數(shù)控系統(tǒng)采用開環(huán)系統(tǒng)。在開、閉兩種系統(tǒng)中，閉環(huán)更精確，一般說來更昂貴。起初，因為原先傳統(tǒng)的步進電動機的功率限制，開環(huán)系統(tǒng)幾乎全部用于輕場合，最近出的電液步進電動機已越來越多地用于較重的加工領(lǐng)域。
Numerical Control
One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical controlled (NC).Prior to the advent of NC; all machine tools were manually operated and controlled. Among the many limitation of operator skills. With manual control, the quality of the product is directly related to and limited to the skills of the operator. Numerical control represents the first major stop away from human control of machine tools.
Numerical control means the control means the control of machine tools and other manufacturing systems through the use of prerecorded, written symbolic instructions. Rather than operational instructions to the machine tool. For a machine tool to be numerically controlled, it must be interfaced with a device error accepting and decoding the programmed instructions, known as a reader.
Numerical control was developed to overcome the limitation of human operators, and it has done so Numerical control machines are more accurate the manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:
1. Electrical discharge machining
2. Laser cutting.
3. Electrin beam welding.
Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.
Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U.S. Air Force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively.
However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter is the straight lines making up the steps, the smoother is the curve. Each line segment in the steps had to be calculated.
This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry , describe the cutting tool configuration, and specify the necessary motions . The development of the APT language was a major step forward in the further development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.
A major problem was the fragility of the punched paper tape medium. It was common for the paper tape containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader , If it was necessary to produce 100 copies of a given part , it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shop floor environment and this kind of repeated use.
This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and breakage. However, it still left two other problems.
The most important of these was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new tape, It was also still necessary to run the tape through the reader as may times as there were parts to be produced, Fortunately, computer technology became a reality and soon solved the problems of NC associated with punched paper and plastic tape.
Te development of a concept known as direct numerical control (DNC) solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions. In direct numerical control, machine tools are tide, via a data transmission link, to a host computer. Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend on a host computer. When the host computer goes down, the machine tools also experience downtime. This problem led to the development of computer numerical control.
The development of the microprocessor allowed for the development of programmable logic controllers (PLCs) and microcomputers. These two technologies allowed for the development of computer numerical control (CNC). With CNC, each machine tool has a PLC or a microcomputer that serves the same purpose. This allows programs to be input and stored at each individual machine tool. It also allows programs to be developed off-line and downloaded at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced another problem known as data management .The same program might be loaded in ten different microcomputers with no communication among them. This problem is in the process of being solved by local area networks that connect microcomputers for better data management.
Numerical control (n/c) is a form of programmable automation in which the processing equipment is controlled by means of numbers, letters, and other symbols. The numbers, letters, and symbols are coded in an appropriate format to define a program of instructions for a particular workpants or job .When the job changes, the program of instructions is changed .The capability to change the program is what makes N/C suitable for low-and medium-volume production. It is much easier to write programs than to make major alterations of the processing equipment.
There are two basic types of numerically controlled machine tools: point-to-point and continuous-path (also called contouring). Point-to-point machines use unsynchronized motors, with the result that the position of the machining head can be assured only upon completion of a moment, or while only one motor is running. Machines of this type are principally used for straight-line cuts or for drilling or boring.fig.20-1 illustrates a typical sequence of a point-to-point movement from xyz coordinates 2,0,0(point A)to 0,1,3(point D).In this example, all three servomotors would begin operating and then each would shut off as it reached the proper station for this axis. the action would start at coordinate 2,0,o(point A).The three motors operating together would carry the machining head to the vicinity of 1,1,1(point B),where the y motor would stop. The head would then continue to 0, 1, and 2(point C), where the x motor would stop. Finally, the head would complete its movement to 0,1,3(point D) under the action of motor Z. Machine tools with point-to-point system controls are the simplest and least expensive.
A comparison of continuous-path motion to point-to-point motion is illustrated in Fig.20-2.In this example, the motors would run continuously at proportional speeds. A straight line would be generated from A to D .Machine tools equipped with continuous-path capabilities are normally operated by computers.
The N/C system consists of the following components: data input, the tape reader with the control unit, feedback devices, and the metal-cutting machine tool or other type of N/C equipment.
Data input, also called "man-to-control link", may be provided to the machine tool manually or entirely by automatic means. Manual methods when used as the sole source of input data are restricted to a relatively small number of inputs. Examples of manually overstated devices are keyboard dials, pushbuttons, switches, or thumbwheel selectors. These are located on a comps; e mere tie , caromed. Dials are analog devices usually connected to a synchronic-type resolved or potentiometer. In most cases, pushbuttons, switches, and other similar controls for each operation. It is a slow and tedious process and is seldom justified except in elementary machining applications or in special cases.
In practically all cases, information is automatically supplied to the control unit and the machine tool by cards, punched tapes, or by magnetic tape. Eight-channel punched paper tape is the most commonly used form of data input for conventional N/C systems .The coded instructions on the tape consist of sections of punched holes called blocks .Each block represents a machine function, a machining operation, or a combination of the two .The entire N/C program on a tape is made up of an accumulation of these successive data blocks. Programs resulting in long tapes are wound on reels like motion-picture film. Programs on relatively short tapes may be continuously repeated by joining the two ends of the tape to form a loop .Once installed, the tape is used again and again without further handling. In this case, the operator simply loads and unloads the parts. Punched tapes are prepared on typewriters with special tape-punching attachments or in tape punching units connected directly to a computer system. Tape production is rarely error-free, Errors may be initially caused by the part programmer, in card punching or compilation, or as a result of physical damage to the tape during handling, etc. Several trial runs are often necessary to remove all errors and produce an acceptable working tape.
While the data on the tape is fed automatically, the actual programming steps are done manually. Before the coded tape may be prepared, the programmer, often working with a planner or a process engineer, must select the appropriate N/C machine tool, determine the kind of material to be machined, calculate the speeds and feeds, and decide upon the type of tooling needed. The dimensions on the part print are closely examined to determine a suitable zero reference point from which to start the program. Aerogram manuscript is then written which gives coded numerical instructions describing the sequence of operations that the machine tool is required to follow to cut the part to the drawing specifications.
The control unit receives and stores all coded data until a complete block of information has been accumulated. It then interprets the coded instruction and directs the machine tool through the required motions.
The function of the control unit may be better understood by comparing it to the action of a dial telephone, where, as each digit is dialed, it is stored. When the entire number has been dialed, the equipment becomes activated and the call is completed.
Silicon photo diodes, located in the tape reader head on the control unit, detect light as it passes through the holes in the moving tape. The light beams are converted to electrical energy, which is amplified to further strengthen the signal .The signals are then sent to registers in the control unit, where actuation signals are relayed to the machine tool drives.
Some photoelectric devices are capable of reading at rates up to 1000 characters per second. High reading rates are necessary to maintain continuous machine-tool motion; other-wise dwell marks may be generated by the cutter on the part during contouring operations .the reading device must be capable of reading data blocks at a rate faster than the control system can process the data.
A feedback device is a safeguard used on some N/C installations to constantly compensate for errors between the command position and the actual location of the moving slides of the marching tool. An N/C machine equipped with this kind of a direct feedback checking device has what is known as a closed-loop system. Positioning control is accomplished by a sensor which, during the actual operation, records the position of the slides and relays this information back to the control unit. Signals thus