步進(jìn)電機(jī)伺服系統(tǒng)控制.doc

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步進(jìn)電機(jī)和伺服電機(jī)的系統(tǒng)控制 只要有軟件的支持，這里將不再有猜測性的工作。 運動的控制者---軟件：只要有了軟件，它可以幫助我們配置改裝、診斷故障、 調(diào)試程序等。 數(shù)控電動機(jī)的設(shè)計者會是一個微軟窗口——基于構(gòu)件的軟件開發(fā)工具， 可以為6000系列產(chǎn)品設(shè)置代碼，同時可以控制設(shè)計者與執(zhí)行者的運動節(jié)目，并創(chuàng)造 一個定制運營商的測試小組。運動建筑師的心臟是一個空殼，它可以為進(jìn)入以下模 塊提供一個綜合環(huán)境。 1. 系統(tǒng)配置——這個模塊提示您填寫所有相關(guān)初成立信息啟動議案。配置向具體 6000 系列產(chǎn)品的選擇，然后這些信息將用于產(chǎn)生實際的 6000 - 語言代碼，這 是你的開始計劃。 2. 程序編輯器——允許你編輯代碼。它也有可行的“幫助”命令菜單。A用戶指 南提供了相關(guān)的磁盤指南。 3. 終端模擬器——本模塊，可讓您直接與 6000 系列產(chǎn)品互動。他所提供的“幫 助”是再次參考所有命令和定義。 4. 測試小組——你可以使用本模塊，模擬程序，調(diào)試程序，并跟蹤檢測程序。 由于它的對話窗口，你能很容易的知道怎么使用它。 運動建筑師已經(jīng)將所有的 6000 系列產(chǎn)品都運用在了步進(jìn)電機(jī)和伺服電機(jī)的技術(shù) 上。由于豐富的對話窗口和6000系列語言，使得你能夠從簡單到復(fù)雜的解決問題。 運動建筑師的6000系列產(chǎn)品的標(biāo)準(zhǔn)配置工具，能夠使得這些控制器更加簡單， 相當(dāng)大的縮短項目開發(fā)時間。它的另外一個增值特點是使用 6000 伺服控制器的調(diào) 諧助手。 基于調(diào)諧價值觀， 這個額外的模塊可以以圖形化的方式為你展示各種參數(shù)。 看看這些參數(shù)是如何讓變化的。用運動的建筑師，你可以一次性打開多個窗口。舉 例來說，無論是程序編輯器和終端模擬器窗口，你都可以打開運行程序， 得到信 息，然后改變這一程序。運動建筑師可以利用在線幫助，在整個互動接觸內(nèi)容中為 數(shù)控電機(jī)6000系列軟件做參考指南。 從簡單到復(fù)雜的解決應(yīng)用 伺服控制是你用伺服調(diào)諧器軟件控制。 數(shù)控電機(jī)與6000系列伺服控制器相結(jié)合 并應(yīng)用伺服調(diào)諧器軟件。伺服調(diào)諧器是一個新增功能模塊，它擴(kuò)展和提高運動建筑 師的能力。議案建筑師與伺服調(diào)諧器結(jié)合起來，以提供圖形化的反饋方式，反饋實 時運動信息并提供簡便環(huán)境設(shè)置微調(diào)收益及相關(guān)制參數(shù)以及提供文件操作，以保存 并記得微調(diào)會議。 請你用運動工具箱軟件解決自己的運動控制。運動工具箱實際上是一個為數(shù)控 電機(jī)和6000系列運動控制器而設(shè)計的廣泛應(yīng)用的虛擬圖標(biāo)式編程儀器。 當(dāng)使用運動工具箱與虛擬編程儀時， 編程6000 系列控制器實質(zhì)上是完成連接圖 形圖標(biāo)，或加上形成框圖使之可見。 運動工具箱中包含了1500多條命令，狀態(tài)欄， 實例等。所有的命令、狀態(tài)欄、實例都包括可視的來源圖表，使您可以修改他們， 如果有必要，可以滿足您的特殊的需要。運動工具箱同時還具有一個可視窗口，基 于安裝程序和一個全面的用戶手冊，可以幫助您運行得更好更快。 軟件電腦輔助運動應(yīng)用軟件 compucam compucam是基于微軟的編程包，它能從 CAD 程序、示波器文檔、數(shù)控程序和產(chǎn)生 6000系列數(shù)控電機(jī)密碼相兼容的運動控制器中輸入幾何圖形。購買數(shù)控電機(jī)是可行 的，因為 compucam 是一個附加模塊，是運動建筑師的菜單欄，它是作為公用部分 而被引用的。程序從compucam開始運行CAD 軟件包。一旦程序被起草創(chuàng)作，它就 會被保存為DXF文件，或惠普-吉爾段文檔，或G代碼數(shù)控程序。這些幾何圖形然 后輸入compucam中，產(chǎn)生6000系列代碼。在程序運行之后，你可使用的運動建筑 師功能塊，如編輯或下載代碼等執(zhí)行程序。 運動執(zhí)行者軟件可輕松編程 6000 系列 運動執(zhí)行者革命性控制運動編程。這一具有創(chuàng)新意義的軟件允許程序員以他們 所熟悉的- 流程圖式的方法編程。 運動執(zhí)行者降低了學(xué)習(xí)曲線，并使運動控制編程 變得相當(dāng)容易。運動執(zhí)行者是一套微軟軟件，基于圖形化窗口的發(fā)展，讓專家和新 手程序員容易學(xué)習(xí)計劃6000系列產(chǎn)品新的編程語言。 簡單地拖放代表議案職能的 視覺圖標(biāo)，你可以隨時的進(jìn)行你所需要的操作。運動執(zhí)行者是一個完整的應(yīng)用開發(fā) 環(huán)境的軟件。除了視覺編程6000 系列產(chǎn)品，用戶還可以配置，調(diào)試，下載， 策劃 和執(zhí)行的議案計劃。 伺服與步進(jìn)...您需要了解的 電機(jī)類型及其應(yīng)用 下一節(jié)將會給你介紹一些的適用特別場合的電機(jī)，而某些應(yīng)用是最好避免。應(yīng) 當(dāng)強(qiáng)調(diào)說，在一個廣范的應(yīng)用范圍，電機(jī)是可同樣滿足一個以上的汽車類型， 而選 擇往往是由客戶偏好、以往經(jīng)驗或與現(xiàn)有的設(shè)備的兼容性決定的。一個非常有用的 工具箱，可供你選擇適當(dāng)?shù)倪\動，為你選擇電機(jī)與選擇軟件包是 compumotor 軟件 包。使用這個軟件，使用戶可以輕松找出適當(dāng)?shù)碾姍C(jī)大小和類型。 高扭矩，低轉(zhuǎn)速 連續(xù)脈沖適宜于步進(jìn)電機(jī)時，在低速時，就相對于扭矩輸出規(guī)模和輸入功率而 言，它是非常高效率。 微步，在低速應(yīng)用，可以用來提高平滑度。如可作為計量泵 驅(qū)動非常精確的流量控制。 高扭矩，高轉(zhuǎn)速 連續(xù)脈沖適應(yīng)于伺服電機(jī)時，其實步進(jìn)電機(jī)應(yīng)避免使用在這種情況下。這是因 為高速可導(dǎo)致負(fù)荷。 簡捷，快速，重復(fù)性動作 僅是自然域的步進(jìn)電機(jī)由于其在低速時高轉(zhuǎn)矩，因而存在慣性比例大，及缺乏 折算的問題。直流電動機(jī)的電刷可限制其潛在的頻繁開始，停止和方向的改變。 低速，高光滑的應(yīng)用 這是最適合于微步或直接驅(qū)動伺服電機(jī)。 適用于危險環(huán)境或在真空中可能不能夠使用電刷電機(jī)。步進(jìn)或無刷電機(jī)是無所 謂的，靠的是對負(fù)荷的需求。牢記當(dāng)負(fù)載過高時，熱耗散可能是個問題。 選擇適合你的電機(jī) 導(dǎo)言 運動控制，在其最廣泛的意義上說，可能與任何移動式起重機(jī)中焊接機(jī)器人 液壓系統(tǒng)有關(guān)。在電子運動控制領(lǐng)域，我們的主要關(guān)切系統(tǒng)范圍內(nèi)的有限功率的大 小， 通常高達(dá)約10hp （ 7千瓦），并要求在一個或多個方面有嚴(yán)格精密。這可能 涉及精確控制的距離或速度，但很多時候是雙方的，有時還涉及其它參數(shù)如轉(zhuǎn)矩或 加速率。在以下所舉的兩個例子中，焊接機(jī)器人，需要精確的控制雙方的速度和距 離;吊臂液壓系統(tǒng)采用驅(qū)動作為反饋系統(tǒng)，因此，它的準(zhǔn)確度會隨著操作者的技能的 不同而不同。在嚴(yán)格意義上來說，這將不會被視為一項運動控制系統(tǒng)。 我們的標(biāo)準(zhǔn) 運動控制系統(tǒng)由以下三個基本要素組成： 圖 1 運動控制系統(tǒng)的組成元件 電機(jī)，可能是一個步進(jìn)電機(jī)（要么旋轉(zhuǎn)或線性） ，也可能是直流無刷電機(jī)或無刷 伺服馬達(dá)。電機(jī)必須配備一些種回饋裝置，除非它是一個步進(jìn)電機(jī)。 圖 2顯示了一個完善地反饋控制電機(jī)轉(zhuǎn)速的系統(tǒng)。這樣一個具有閉環(huán)控制系 統(tǒng)的速度伺服系統(tǒng)。 圖 2典型的閉環(huán)（速度）伺服系統(tǒng) 驅(qū)動器是一個電子功率放大器，以提供電力操作電動機(jī)來回應(yīng)低層次的控制信 號。一般來說，驅(qū)動器將特別設(shè)計，其操作與特定電機(jī)類型相配合。例如，你不能 用一個步進(jìn)驅(qū)動器來操作直流無刷電機(jī)。 不同電機(jī)適應(yīng)的不同領(lǐng)域 步進(jìn)電機(jī)： 步進(jìn)電機(jī)的好處。 驅(qū)動器 電機(jī) 控制器 主計算機(jī) 或 PLC 控制器/索 引 驅(qū)動 電機(jī) 步進(jìn)電機(jī)有以下好處： （1）成本低廉（2）堅固耐用（3）結(jié)構(gòu)簡單（4）高可靠性（5）無維修（6） 適用廣泛（7）穩(wěn)定性很高（8）無需反饋元件（8）適應(yīng)多種工作環(huán)境（9）相對伺 服電機(jī)更具有保險性。 因此， 幾乎沒有任何可以想象的失敗使步進(jìn)驅(qū)動模塊出錯。 步進(jìn)電機(jī)驅(qū)動簡單， 并且驅(qū)動和控制在一個開放的閉環(huán)系統(tǒng)內(nèi)。他們只需要4個驅(qū)動器。低速時，驅(qū)動 器提供良好的扭矩，是有刷電機(jī)同一幀大小5 倍連續(xù)力距，或相當(dāng)于無刷電機(jī)一倍 扭矩。這往往不再需要變速箱。步進(jìn)驅(qū)動系統(tǒng)遲緩，在限定的范圍內(nèi)，可以更好的 減少動態(tài)位置誤差。 步進(jìn)電機(jī)弊端。 步進(jìn)電機(jī)有下列缺點： （1）共振效應(yīng)和相對長的適應(yīng)性（2）在低速，表現(xiàn)粗糙，除非微驅(qū)動器來驅(qū) 動（3）開環(huán)系統(tǒng)可能導(dǎo)致未被查覺的損失（4）由于過載，他們消耗過多電流。因 此傾向于過熱運行。（5）虧損速度比較高，并可產(chǎn)生過多熱量因此，他們噪音很大 （尤其是在高速下） 。（6）他們的滯后現(xiàn)象導(dǎo)致振蕩，這是很難抑制的。對他們的可 行性，這兒有一個限度，而他們的大小，定位精度主要依靠的是機(jī)器（例如，滾珠 絲杠的精確度） 。許多這些缺點是可以克服的，通過使用一個閉環(huán)控制方案。 注： compumotor系列能很多的減小或降低了這些不同的步進(jìn)電機(jī)不利之處。 主要有3類步進(jìn)電機(jī)： （1）永磁式步進(jìn)電機(jī) ，（2）可變磁阻式步進(jìn)電動機(jī)，（3） 混合式步進(jìn)電機(jī)汽車。 當(dāng)電動機(jī)驅(qū)動，在其全步模式，給兩個繞組通電時或"2 相"通電的時候（見圖 1.8 ） ，扭矩可于每一個步將是相同（除極少數(shù)的變異和傳動特性）。在半步模式 下，我們交替改變兩相電流，如圖 1.9 所示。假設(shè)該驅(qū)動器在每種情況下提供了相 同的繞組電流，再通電時，這將導(dǎo)致更大的轉(zhuǎn)矩。換句話說，交替的步進(jìn)距將時強(qiáng) 時若。對電動機(jī)表現(xiàn)來說，這并不代表著一個重大的威懾。扭矩明顯受制于較弱的 一步，但在全步模式時，低速平滑有一個顯著的改善。 顯然，我們想在每一個步驟實現(xiàn)約相等扭矩對時，這扭矩應(yīng)該在水平較強(qiáng)的一 步。們可以實現(xiàn)這個，當(dāng)只有一個繞組通電時，通過用高電流水平。這并不過度消 耗電機(jī)，因為該電機(jī)的額定電流假定兩個階段被激活（目前的評級是基于許可的情 況溫度） 。只有一相通電，如果目前是增加了40 ％的功率，同樣的總功率將會消 散 。利用這種更高的電流在一相中產(chǎn)生大致相等的扭矩，在交替的步進(jìn)距中。 （見 圖1.10 ）。 我們已經(jīng)看到，給兩相都通與平等電流產(chǎn)生的一個中間步進(jìn)，居于每一相的中 間位置。如果兩相電流是不平等的， 轉(zhuǎn)子位置將轉(zhuǎn)向更強(qiáng)的一極。這種作用是利用細(xì) 分驅(qū)動，其中細(xì)分的大小基于兩個繞組中的電流的大小。以這種方式，步長是減少了， 而低速平滑度得到大幅度提高。高細(xì)分驅(qū)動電動機(jī)細(xì)分整步步進(jìn)到多達(dá)500 個細(xì)分 步 ， 轉(zhuǎn)一圈可細(xì)分十萬步。 在這種情況下， 繞組中的電流極為相似的兩個正弦波有90 相移。 （圖1.11 ） 電機(jī)被驅(qū)動好像轉(zhuǎn)換成了交流同步電機(jī)。事實上，步進(jìn)電機(jī)可被驅(qū) 動，從60赫茲美（50赫茲-歐洲）正弦波源頭起，包括電容器系列的一相。它將旋轉(zhuǎn) 72轉(zhuǎn)。 圖 1.11 步進(jìn)電機(jī)的相電流 標(biāo)準(zhǔn)步進(jìn)電機(jī)運行在同就如同我們的簡單模式，但有一個更大的數(shù)目齒數(shù)在轉(zhuǎn) 子和定子中，從而有了一個較小的基本步長。轉(zhuǎn)子有2部分，但每部分有50個齒。 該半齒位于兩部分之間。定子每5個齒有 8個極，完整的共有40個齒（見圖1.12 ） 圖 1.12 200步混合標(biāo)準(zhǔn)電機(jī) 如果我們想象一個齒， 是擺在2個定子極點每一齒隙中， 假設(shè)定子共有48個齒， 少于轉(zhuǎn)子齒數(shù)兩個。因此，如果轉(zhuǎn)子和定子的齒排列一整圈，他們同樣也可以排列 半圈。1/4和3/4圈也同樣可以排列。 然而，由于轉(zhuǎn)子齒排列位置，在另一端的轉(zhuǎn) 子，排列將發(fā)生在1/4和3/4位置處。 繞組4個一組，并對角線方向的極性相反。如圖1.12所示 ，北極在轉(zhuǎn)子前面 的12點和 6點位置，吸引著在在背面 3時和 9 時的南極。通過開關(guān)第二組線圈的電 流，定子場模式旋轉(zhuǎn)45 。不過，要配合這個新的領(lǐng)域，轉(zhuǎn)子只轉(zhuǎn)過1.8 。相當(dāng) 于轉(zhuǎn)子，這只轉(zhuǎn)過了四分之一齒間距，每一次旋轉(zhuǎn)要200個全步。 注意到，每一次旋轉(zhuǎn)全部時這兒有很多定位點位置，通常是200個 。該定位點 的位置與轉(zhuǎn)子齒全面接軌定子齒時相對應(yīng)。當(dāng)通電給步進(jìn)驅(qū)動器時， 它通常是"零 階段"狀態(tài)時最活躍，也就是兩套繞組都通電。因此產(chǎn)生的轉(zhuǎn)子位置并不符合轉(zhuǎn)子自 然定位點的位置。因此，空載時，一旦通電電機(jī)將至少步進(jìn)半步。當(dāng)然，如果系統(tǒng) 關(guān)機(jī)，或在零相位位置，電機(jī)一旦通電將步進(jìn)一大步。 另一點要注意的是，對于一個給定電流的繞組，有很多穩(wěn)定的位置，正如轉(zhuǎn)子 齒（200步進(jìn)電機(jī)有50個齒）。如果電機(jī)是同步電機(jī)，導(dǎo)致位置誤差將永遠(yuǎn)是一個 整體倍轉(zhuǎn)子齒或能被7.2 整除 。電機(jī)不能"細(xì)分"，如個別一個或兩個位置誤差，是 由于噪聲，錯誤脈沖或控制器故障造成的。 圖 2.19 數(shù)字伺服驅(qū)動 圖2.19顯示為伺服電機(jī)的數(shù)控驅(qū)動。所有的主控制功能是微處理器，驅(qū)動為D A 模擬轉(zhuǎn)換器，以產(chǎn)生一個模擬扭矩需求信號。從這個角度上，這臺機(jī)器非常很像 一個模擬伺服放大器。 反饋的信息是來自隸屬該電機(jī)軸的一個編碼器。編碼器生成脈沖流可確定傳輸 路程，并通過計算脈沖頻率，是可以測定轉(zhuǎn)速的。 數(shù)碼驅(qū)動通過求解一系列的方程式，履行同樣類似的功能。微處理器是與數(shù)學(xué) 模型（或“算法" ）的等效的編程模擬系統(tǒng)。這模型預(yù)測系統(tǒng)的行為。它響應(yīng)一個 給定輸入的信號并產(chǎn)生速度。它同樣也考慮到額外信息如輸出速度，速率轉(zhuǎn)變中的 投入和各種調(diào)校設(shè)定。 解決所有方程需數(shù)額需有限的時間，即使是一個快速的處理器一次處理通常也 是100 ms和2 ms 之間 。在此之間，在改變輸入或輸出，先前的計算值將有沒有回 應(yīng)時，扭矩要求必須保持恒定。因此更新時間成為數(shù)字伺服和一臺高性能系統(tǒng)關(guān)鍵 的因素，它必須保持及時更新。 調(diào)試數(shù)字伺服電機(jī)可按鈕或從一個計算機(jī)或終端調(diào)試。電位器調(diào)整是涉及的。 調(diào)試數(shù)據(jù)是設(shè)置在伺服算法的各種系數(shù)，因此，它決定了系統(tǒng)的性能。即使如果調(diào) 諧進(jìn)行使用按鈕，終值也可以上傳到終端，讓其進(jìn)行簡單的重復(fù)。 在某些應(yīng)用中，因負(fù)載慣量各異，例如一個機(jī)器手臂卸載后又帶有沉重的負(fù) 荷。改變慣性可能是一個系數(shù)為20或以上，而這樣的變化需要該驅(qū)動器重新調(diào)整， 以保持其穩(wěn)定。這只不過是在操作系統(tǒng)的適當(dāng)點通過發(fā)送新的調(diào)試參數(shù)來實現(xiàn)的。 (此文轉(zhuǎn)載自 一覽 電機(jī)英才網(wǎng)) Step Motor&Servo MotorSystems and Controls WITH SUPPORT SOFTWARE, THERE’S NO MORE GUESS WORK Motion Architect Software Does the Work for You... Configure ,Diagnose, Debug Compumotor’s Motion Architect is a Microsoft Windows?-based software development tool for 6000Series products that allows you to automatically generate commented setup code, edit and execute motion control programs, and create a custom operator test panel. The heart ofMotion Architect is the shell, which provides an integrated environment to access the following modules. ? System Configurator—This module prompts you to fill in all pertinent set-up information to initiate motion. Configurable to the specific 6000 Series product that is selected, the information is then used to generate actual 6000-language code that is the beginning of your program. ? Program Editor—This module allows you to edit code. It also has the commands available through “Help” menus. A user’s guide is provided on disk. ? Terminal Emulator—This module allows you to interact directly with the 6000 product. “Help” is again available with all commands and their definitions available for reference. ? Test Panel—You can simulate your programs, debug programs, and check for program flow using this module. Because Its Windows, You Already Know How to Use It Motion Architect has been designed for use with all 6000 Series products—for both servo and stepper technologies. The versatility of Windows and the 6000 Series language allow you to solve applications ranging from the very simple to the complex. Motion Architect comes standard with each of the 6000 Series products and is a tool that makes using these controllers even more simple—shortening the project development time considerably. A value-added feature of Motion Architect, when used with the 6000 Servo Controllers, is its tuning aide. This additional module allows you to graphically display a variety of move parameters and see how these parameters change based on tuning values. Using Motion Architect, you can open multiple windows at once. For example, both the Program Editor and Terminal Emulator windows can be opened to run the program, get information, and then make changes to the program. On-line help is available throughout Motion Architect, including interactive access to the contents of the Compumotor 6000 Series Software Reference Guide. SOLVING APPLICATIONS FROM SIMPLE TO COMPLEX Servo Control is Yours with Servo Tuner Software Compumotor combines the 6000 Series servo controllers with Servo Tuner software. The Servo Tuner is an add-on module that expands and enhances the capabilities of Motion Architect. Motion Architect and the Servo Tuner combine to provide graphical feedback of real-time motion information and provide an easy environment for setting tuning gains and related systemparameters as well as providing file operations to save and recall tuning sessions. Draw Your Own Motion Control Solutions with Motion Toolbox Software Motion Toolbox? is an extensive library of LabVIEW virtual instruments (VIs) for icon-based programming of Compumotor’s 6000 Series motion controllers. When using Motion Toolbox with LabVIEW, programming of the 6000 Series controller is accomplished by linking graphic icons, or VIs, together to form a block diagram. Motion Toolbox’s has a library of more than 150 command,status, and example VIs. All command and status VIs include LabVIEW source diagrams so you can modify them, if necessary, to suit your particular needs. Motion Toolbox als user manual to help you gut up and running quickly. comprehensiveM Software for Computer-Aided Motion Applications CompuCAM is a Windows-based programming package that imports geometry from CAD programs, plotter files, or NC programs and generates 6000 code compatible with Compumotor’s 6000 Series motion controllers. Available for purchase from Compumotor, CompuCAM is an add-on module which is invoked as a utility from the menu bar of Motion Architect. From CompuCAM, run your CAD software package. Once a drawing is created, save it as either a DXF file, HP-GL plot file or G-code NC program. This geometry is then imported into CompuCAM where the 6000 code is generated. After generating the program, you may use Motion Architect functions such as editing or downloading the code for execution. Motion Builder Software for Easy Programming of the 6000 Series Motion Builder revolutionizes motion control programming. This innovative software allows programmers to program in a way they are familiar with—a flowchart-style method. Motion Builder decreases the learning curve and makes motion control programming easy. Motion Builder is a Microsoft Windows-based graphicaldevelopment environment which allows expert and novice programmers to easily program the 6000 Series products without learning a new programming language. Simply drag and drop visual icons that represent the motion functions you want to perform. Motion Builder is a complete application development environment. In addition to visually programming the 6000 Series products, users may configure, debug, download, and execute the motion program. SERVO VERSUS STEPPER... WHAT YOU NEED TO KNOW Motor Types and Their Applications The following section will give you some idea of the applications that are particularly appropriate for each motor type, together with certain applications that are best avoided. It should be stressed that there is a wide range of applications which can be equally well met by more than one motor type, and the choice will tend to be dictated by customer preference, previous experience or compatibility with existing equipment. A helpful tool for selecting the proper motor for your application is Compumotor’s Motor Sizing and Selection software package. Using this software, users can easily identify the appropriate motor size and type. High torque, low speed continuous duty applications are appropriate to the step motor. At low speeds it is very efficient in terms of torque output relative to both size and input power. Microstepping can be used to improve smoothness in lowspeed applications such as a metering pump drive for very accurate flow control. High torque, high speed continuous duty applications suit the servo motor, and in fact a step motor should be avoided in suchapplications because the high-speed losses can cause excessive motor heating. Short, rapid, repetitive moves are the natural domain of the stepper due to its high torque at low speeds, good torque-to-inertia ratio and lack of commutation problems. The brushes of the DC motor can limit its potential for frequent starts, stops and directionchanges. Low speed, high smoothness applications are appropriate for microstepping or direct drive servos. Applications in hazardous environments or in a vacuum may not be able to use a brushed motor. Either a stepper or a brushless motor is called for, depending on the demands of the load. Bear in mind that heat dissipation may be a problem in a vacuum when the loads are excessive. SELECTING THE MOTOR THAT SUITS YOUR APPLICATION Introduction Motion control, in its widest sense, could relate to anything from a welding robot to the hydraulic system in a mobile crane. In the field of Electronic Motion Control, we are primarily concerned with systems falling within a limited power range, typically up to about 10HP (7KW), and requiring precision in one or more aspects. This may involve accurate control of distance or speed, very often both, and sometimes other parameters such as torque or acceleration rate. In the case of the two examples given, the welding robot requires precise control of both speed and distance; the crane hydraulic system uses the driver as the feedback system so its accuracy varies with the skill of the operator. This wouldn’t be considered a motion controlsystem in the strict sense of the term.Our standard motion control system consists ofthree basic elements: Fig. 1 Elements of motion control system The motor. This may be a stepper motor (either rotary or linear), a DC brush motor or a brushless servo motor. The motor needs to be fitted with some kind of feedback device unless it is a stepper motor. Fig. 2 shows a system complete with feedback to control motor speed. Such a system is known as a closed-loop velocity servo system. Fig. 2 Typical closed loop (velocity) servo system The drive. This is an electronic power amplifier thatdelivers the power to operate the motor in response to low-level control signals. In general, the drive will be specifically designed to operate with a particular motor type – you can’t use a stepper drive to operate a DC brush motor, for instance. Application Areas of Motor Types Stepper Motors Stepper Motor Benefits Stepper motors have the following benefits: ? Low cost ? Ruggedness ? Simplicity in construction ? High reliability ? No maintenance ? Wide acceptance ? No tweaking to stabilize ? No feedback components are needed ? They work in just about any environment ? Inherently more failsafe than servo motors. There is virtually no conceivable failure within the stepper drive module that could cause the motor to run away. Stepper motors are simple to drive and control in an open-loop configuration. They only require four leads. They provide excellent torque at low speeds, up to 5 times the continuous torque of a brush motor of the same frame size or double the torque of the equivalent brushless motor. This often eliminates the need for a gearbox. A stepper-driven-system is inherently stiff, with known limits to the dynamic position error. Stepper Motor Disadvantages Stepper motors have the following disadvantages: ? Resonance effects and relatively long settling times ? Rough performance at low speed unless a microstep drive is used ? Liability to undetected position loss as a result of operating open-loop ? They consume current regardless of load conditions and therefore tend to run hot ? Losses at speed are relatively high and can cause excessive heating, and they are frequently noisy (especially at high speeds). ? They can exhibit lag-lead oscillation, which is difficult to damp. There is a limit to their available size, and positioning accuracy relies on the mechanics (e.g., ballscrew accuracy). Many of these drawbacks can be overcome by the use of a closed-loop control scheme. Note: The Compumotor Zeta Series minimizes or reduces many of these different stepper motor disadvantages. There are three main stepper motor types: ? Permanent Magnet (P.M.) Motors ? Variable Reluctance (V.R.) Motors ? Hybrid Motors When the motor is driven in its full-step mode, energizing two windings or “phases” at a time (see Fig. 1.8), the torque available on each step will be the same (subject to very small variations in the motor and drive characteristics). In the half-step mode, we are alternately energizing two phases and then only one as shown in Fig. 1.9. Assuming the drive delivers the same winding current in eachcase, this will cause greater torque to be produced when there are two windings energized. In other words, alternate steps will be strong and weak. This does not represent a major deterrent to motorperformance—the available torque is obviously limited by the weaker step, but there will be a significant improvement in low-speed smoothness over the full-step mode. Clearly, we would like to produce approximately equal torque on every step, and this torque should be at the level of the stronger step. We can achieve this by using a higher current level when there is only one winding energized. This does not over dissipate the motor because the manufacturer’s current rating assumes two phases to be energized the current rating is based on the allowable case temperature). With only one phase energized, the same total power will be dissipated if the current is increased by 40%. Using this higher current in the one-phase-on state produces approximately equal torque on alternate steps (see Fig. 1.10). Fig. 1.8 Full step current, 2-phase on Fig. 1.9 Half step current Fig. 1.10 Half step current, profiled We have seen that energizing both phases with equal currents produces an intermediate step position half-way between the one-phase-on positions. If the two phase currents are unequal, the rotor position will be shifted towards the stronger pole. This effect is utilized in the microstepping drive, which subdivides the basic motor step by proportioning the current in the two windings. In this way, the step size is reduced and the low-speed smoothness is dramatically improved. High-resolution microstep drives divide the full motor step into as many as 500 microsteps, giving 100,000 steps per revolution. In this situation, the current pattern in the windings closely resembles two sine waves with a 90 phase shift between them (see Fig. 1.11). The motor is now being driven very much as though it is a conventional AC synchronous motor. In fact, the stepper motor can be driven in this way from a 60 Hz-US (50Hz-Europe) sine wave source by including a capacitor in series with one phase. It will rotate at 72 rpm. Fig. 1.11 Phase currents in microstep mode Standard 200-Step Hybrid Motor The standard stepper motor operates in the same way as our simple model, but has a greater number of teeth on the rotor and stator, giving a smaller basic step size. The rotor is in two sections as before, but has 50 teeth on each section. The half-tooth displacement between the two sections is retained. The stator has 8 poles each with 5 teeth, making a total of 40 teeth (see Fig. 1.12). Fig. 1.12 200-step hybrid motor If we imagine t