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譯文和原文
譯文和原文
譯文部分：
2.4 旋轉(zhuǎn)電機(jī)的振動監(jiān)測和數(shù)據(jù)處理系統(tǒng)
旋轉(zhuǎn)電機(jī)的振動監(jiān)測和診斷，從實現(xiàn)基本換能器到機(jī)械系統(tǒng)的轉(zhuǎn)換開始。振動傳感器提供了發(fā)生在機(jī)器內(nèi)部的動態(tài)過程的重要信息。額外的傳感器測量機(jī)器的其他重要物理參數(shù)，比如過程流體參數(shù)和/或電磁特性。接下來，處理從傳感器中獲得的原始數(shù)據(jù)，從而分離出關(guān)于機(jī)器操作和健康狀況的最重要的信息。這些信息用于機(jī)器故障診斷和校核工作。
本節(jié)簡要討論最常用的振動傳感器在旋轉(zhuǎn)機(jī)械中的應(yīng)用，和振動數(shù)據(jù)處理的有用格式。
2.4.1 振動傳感器
2.4.1.1 加速計
加速計依據(jù)加速度來測量機(jī)械振動。最常用的加速計包含一個安裝在力傳感元件上的慣性質(zhì)量，比如壓電晶體（圖 2.4.1）。后者產(chǎn)生一個與施加在慣性質(zhì)量上的力成比例的輸出信號，這一與機(jī)械部件加速度成比例的信號轉(zhuǎn)而傳遞到與之相連接的傳感器上。典型加速器的敏感度是0.1V/g（g=重力加速度）。加速計是小巧輕量的傳感器，它可以在很寬的頻帶和溫度帶內(nèi)工作。它們能承受較高的振動級。加速計不需要電源供給，并且是外部安裝。然而，它們對連接方法和表面狀況很敏感。它們也對聲音和寄生振動（一些模型包含一個整體安裝放大器）也很敏感。加速計在范圍為1500cpm至1200kcpm的高頻振動測量中工作最佳。
加速計是結(jié)構(gòu)力學(xué)測量中最傳統(tǒng)的傳感器。堅固，相對便宜，同時由于它安裝在結(jié)構(gòu)或機(jī)械的外殼上，所以使用簡單。對于評估高頻振動，這是完美的工具。在機(jī)械振動監(jiān)測中，加速計對于診斷齒輪傳動齒問題，滾動元件軸承，和/或觀察葉片通過行動，是非常有價值的工具。這些例子中產(chǎn)生的振動具有高頻的特征，加速計也是為此而設(shè)計的。
2.4.1.2 速度傳感器
最常用的速度換能器的工作原理，是基于重物用彈簧懸掛振動體以保持靜止的慣性性能（地震的）。在電磁傳感器的設(shè)計中，重物攜帶線卷并用軟簧彈性懸掛，中間包含一個永久磁鐵（圖 2.4.2）。它和振動結(jié)構(gòu)相連接（比如軸承蓋或是機(jī)器外殼），傳輸機(jī)構(gòu)的振動。線圈和磁鐵之間的相對運動，生成一個與振動瞬時速度成比例的輸出電壓。這一傳感器是自生的，不需要外部動力源。速度傳感器有很好的靈敏度，特別是在0.1到1V/in/sec（4到40mV/mm/s）。它們的頻帶是從500cpm到1000kcpm。速度傳感器是外部安裝，并且可以完成通用機(jī)械和機(jī)械結(jié)構(gòu)的全部振動測量。速度傳感器的缺點包括，難于校準(zhǔn)檢查，對磁干擾、安裝方向很敏感，以及十字軸振動。移動件的毀損風(fēng)險會由于突然的沖擊或是疲勞過程而增加。
更多不包含移動件的現(xiàn)代速度傳感器，是基于與加速計相同的原理，它們包括一個電子積分電路。
加速計和速度傳感器在正確的校準(zhǔn)下，都提供加速度或振動速度的絕對值。
2.4.1.3 加速計和速度傳感器在旋轉(zhuǎn)機(jī)械中的適用性
旋轉(zhuǎn)電機(jī)主要的振動問題出現(xiàn)在從0到200Hz的低頻范圍內(nèi)。加速計也不能探測特低頻，或是提供低頻帶中的微弱分辨率的信號。因此，加速計和速度傳感器都無法閱讀出轉(zhuǎn)子的低速輥振動（有時被稱作“回轉(zhuǎn)裝置轉(zhuǎn)子”，意為低角速度）或它的中心線靜態(tài)定位（通常被稱為“直流間隙”，意為轉(zhuǎn)子中心線的位置），它們都是很重要的數(shù)據(jù)。加速計和速度轉(zhuǎn)換器都不能探測旋轉(zhuǎn)機(jī)械的主要故障系統(tǒng)，比如流體漩渦的低頻次同步振動（參見第4章）。
在旋轉(zhuǎn)電機(jī)中，轉(zhuǎn)子通過回轉(zhuǎn)運動來實現(xiàn)傳遞能量的主要運作功能。因為各種原因，作為轉(zhuǎn)子完成有用功的副作用，一部分轉(zhuǎn)動能將轉(zhuǎn)化為各種模式的振動能量。轉(zhuǎn)子本身，作為整個機(jī)械結(jié)構(gòu)中相對柔軟的部分，最有振動的傾向。如第一章提到的，轉(zhuǎn)子的振動模式可能是橫向的，軸向的，和/或扭轉(zhuǎn)的，它所產(chǎn)生的應(yīng)力和形變疊加在旋轉(zhuǎn)的、扭轉(zhuǎn)的、傳輸能量的轉(zhuǎn)子上。轉(zhuǎn)子振動通常是整個機(jī)械結(jié)構(gòu)中的最低模式。轉(zhuǎn)子的振動最終傳遞給機(jī)器的其他部分和外部環(huán)境。轉(zhuǎn)子常常是機(jī)器振動的根源。顯然，從根源來測量振動對正確評價機(jī)器的健康狀況至關(guān)重要。利用加速計或速度傳感器測量外殼的振動，是一個簡單但是間接的評價機(jī)器狀況的方法，它僅能提供“不可接受的”或是“可接受的”這一級別的振動的信息。振動級不可接受時，它不能診斷出導(dǎo)致振動的原因。如果振動級可接受，它也不能評定機(jī)械能無故障持續(xù)工作多久。因此，加速計和速度感應(yīng)器僅推薦在非緊急的，易于更換的旋轉(zhuǎn)電機(jī)上使用，或是作為對關(guān)鍵設(shè)備的補(bǔ)充傳感器。
2.4.1.4 位移傳感器
對旋轉(zhuǎn)電機(jī)健康狀況的準(zhǔn)確診斷和預(yù)測，必須以對整臺機(jī)器的振動源，即轉(zhuǎn)子的狀況在線連續(xù)監(jiān)測為基礎(chǔ)。渦流非接觸式接近傳感器是實現(xiàn)這一任務(wù)最好的基本工具。它們是測量轉(zhuǎn)子相對于機(jī)器固定部分的振動的最可信賴和有效的傳感器（圖 2.4.3）。接近傳感器的操作原則是基于對電磁場的修正，這是由于傳導(dǎo)器尖端附近的導(dǎo)電性固體材料會產(chǎn)生渦流。輸出電壓與傳感器和被觀測材料表面的距離成比例。典型的靈敏度是0.2V/mil（8mV/um）。高靈敏度（2V/mil）接近傳感器用于滾動體的故障診斷：當(dāng)一個滾動體通過與傳感器相連接的外環(huán)時，傳感器觀測外環(huán)每一次在彈性變形中的特定模式。這些特定模式被及時確認(rèn)為與磨損有關(guān)的損傷，由滾動體或是軸承套環(huán)中的裂縫發(fā)展而來。
接近傳感器不僅提供轉(zhuǎn)子運動的動力分量，即振動，還提供準(zhǔn)靜態(tài)和靜態(tài)數(shù)據(jù)：特低頻“低俗輥”數(shù)據(jù)，和寶貴的轉(zhuǎn)子中心線的零頻率位置（“直流間隙”）。這些傳感器覆蓋了從0到大約600kcpm（10kHz）的頻率范圍。接近傳感器需要一個外部電源來支持（通常-18到-24V 直流）。為了信號精度，轉(zhuǎn)子表面有一定要求。接近傳感器最好是用于測量旋轉(zhuǎn)電機(jī)轉(zhuǎn)子側(cè)向和軸向的振動和位置。這對簡單的標(biāo)定檢驗有極高的價值，可靠，在工業(yè)環(huán)境中較穩(wěn)定。在最近二十年中，接近傳感器成功取代了過時的和不可靠的抽車手。
美國石油組織曾采用一項名為“振動，軸向位置，及軸承溫度監(jiān)控”（RP#670）的操作規(guī)程建議。它囊括了在壓縮機(jī)上安裝XY配置接近傳感器和驅(qū)動系統(tǒng)以觀察中心線橫向運動的系統(tǒng)需求（圖2.4.3和2.4.4）。除了這些橫向傳感器外，這一操作規(guī)程建議要求兩個軸向的導(dǎo)向無接觸接近傳感器。他們用于監(jiān)控和警告機(jī)器的推力問題，并常常與自動跳閘相連接，以應(yīng)對危險情況的出現(xiàn)。這些傳感器的安裝方法也適用于檢測和保護(hù)渦輪式發(fā)電機(jī)，泵，風(fēng)扇，以及其他旋轉(zhuǎn)電機(jī)。
在某一些機(jī)器上，接近感應(yīng)器不可能安裝在正交方向。在這種情況下，兩個接近感應(yīng)器的角度除180°外都有一定價值，最好是接近90°。合適的軟件可以減少獲得的數(shù)據(jù)與真實90°時數(shù)據(jù)之間的誤差。
安裝在正交方向的接近傳感器，檢測轉(zhuǎn)子后提供兩方面的振動信號，通過簡單地處理，以軌道的形式生成轉(zhuǎn)子中心線實際橫向運動軌跡的放大影像。消除時間的影響后，通過一個示波器，從兩個接近傳感器接收到的信息可以在屏幕上顯示出時間基準(zhǔn)波形（X和/或Y時間軸）或是軌道模型。注意，接近傳感器通常根據(jù)“峰間值”提供振幅信息（縮寫為“pp”），而不是如數(shù)學(xué)模型中使用的“零-峰值”。
圖2.4.5顯示了三個基本的振動測量傳感器對應(yīng)的振動頻率的典型特征。能夠看出，位移傳感器與頻率有線性常數(shù)關(guān)系，并且從零位頻率到略高于10kHz都可靠。加速計的特點是與頻率的平方成正比。加速計能完美適用在始于20Hz的高頻振動的測量。速度傳感器的頻率刻度在其他兩個傳感器之間。速度傳感器的特點是與振動頻率成比例。
圖2.4.6提供了一個加速計和接近傳感器的靈敏度的例子。它展現(xiàn)了機(jī)器啟動階段的摩擦電子振動響應(yīng)的四個頻譜圖。在“暗”摩擦（（A）和（B））和“亮”摩擦（（C）和（D））情況下，轉(zhuǎn)子橫向振動用位移傳感器（（A）和（C））和加速計（（B）和（D））測量（見第5章）。注意，加速計數(shù)據(jù)能很好地測量所有摩擦導(dǎo)致的基本低頻率振動的高次諧波，但是幾乎不能“看到”與摩擦有關(guān)振動的原始分諧波，特別是在非線性很少并且不形成顯著高諧波的“亮”摩擦區(qū)域。
2.4.1.5 雙傳感器
速度和接近傳感器的結(jié)合，設(shè)計用于測量空間中轉(zhuǎn)子的絕對運動，和它相對于機(jī)房的運動（圖 2.4.7）。這一傳感器也提供機(jī)房的絕對運動。如果后者的運動超過了轉(zhuǎn)子運動的30%，那么，為了充分評定機(jī)器的健康狀況，需要知道轉(zhuǎn)子的絕對振動。在雙傳感器中，速度傳感器的信號代表機(jī)房的絕對運動。這一信號是電子集成的，它與接近傳感器的信號相結(jié)合，提供轉(zhuǎn)子的絕對位移。雙傳感器通常安裝在流體潤滑軸承中，用以觀測相對和絕對的橫向運動。
2.4.1.6 鍵相傳感器
在推薦使用的傳感器中，有一款非常重要，RP#670，即鍵相傳感器。它提供轉(zhuǎn)子每轉(zhuǎn)一次的信號。鍵相代表一個放射狀安裝的接近傳感器，用以觀測鍵，鍵槽，或是其它間斷性的每轉(zhuǎn)一次的電極表面。轉(zhuǎn)子旋轉(zhuǎn)過程中，傳感器生成每轉(zhuǎn)一次的開/閉型信號，通過另兩個安裝在XY結(jié)構(gòu)上的轉(zhuǎn)子觀測接近傳感器，疊加形成時基波形和軌道。因此，這些轉(zhuǎn)子的時基波形或軌道在示波器上顯示為一系列空白/明亮或是明亮/空白的間斷點。鍵相每轉(zhuǎn)一次的開/閉信號供應(yīng)給示波器的Z軸，射束強(qiáng)度軸，最后產(chǎn)生一個在波形或軌道上的光點，隨后信號“抑郁”（黑點）。空白/明亮點的順序取決于鍵相的安裝，所觀測的是凹口、鍵槽或是轉(zhuǎn)子突出，以及詳細(xì)的儀器協(xié)定。在使用示波器顯示由XY傳感器產(chǎn)生的時基波形之前，這些問題應(yīng)該仔細(xì)的檢查。
鍵相信號提供兩項非常重要的數(shù)據(jù)項：角速度測量和誤差測量過濾振動相位的參考。
汽車智能燈的研究與開發(fā)
摘要：通過對車輛燈產(chǎn)品的廣泛市場調(diào)查和需求分析，一種車輛智能燈通過將計算機(jī)、機(jī)械、光學(xué)原理有機(jī)地結(jié)合起來而進(jìn)行設(shè)計。經(jīng)過測試和逐漸提高性能，事實證明，這種燈可以降低駕駛員的疲勞強(qiáng)度，提高行車安全。
1、序言：
隨著現(xiàn)代汽車技術(shù)的發(fā)展和生活水平的改善，現(xiàn)代汽車使得生活方式更安全，更舒服，更舒適，更經(jīng)濟(jì)，更智能的[1]。特別是在安全，情報，大量的人力和物力資源都被消耗在生產(chǎn)安全帶，全球定位系統(tǒng)，地理信息系統(tǒng)，ABS，安全的空氣輔助，電子導(dǎo)航系統(tǒng)等。計算機(jī)的發(fā)展為汽車行業(yè)帶來了機(jī)會，它被認(rèn)為是完全安全的，智能汽車將在不久的將來會誕生。
從公安機(jī)關(guān)的統(tǒng)計量顯示出每年在世界上產(chǎn)出超過1000萬輛汽車。保有量超過約6億。僅2002年，現(xiàn)有統(tǒng)計數(shù)據(jù)的交通事故量超過77萬，造成109381人死亡，超過560,000人受傷。直接經(jīng)濟(jì)損失超過33億元。為了解決車輛的安全性問題，安全帶，安全氣囊，ABS，GPS的使用逐漸增加，以及防眩目眼鏡，變光機(jī)，大、小燈按鈕等等為了專門的夜間使用都安裝在汽車上。道路狀況信息和對操作開關(guān)（打開和關(guān)閉燈，變光）的操作使得駕駛員太忙了。一旦司機(jī)不小心，很容易導(dǎo)致嚴(yán)重的交通事故的發(fā)生。司機(jī)有太大的的勞動強(qiáng)度和紊亂的設(shè)施是非常不方便的[2]。但是在路上設(shè)置的防暈設(shè)備是一種土地和道路成本的浪費。現(xiàn)在計算機(jī)的廣泛應(yīng)用在汽車上，例如：電子燃料噴射系統(tǒng)，ABS，電子導(dǎo)航系統(tǒng)等。光智能是技術(shù)發(fā)展的趨勢和必然的。在通過國內(nèi)的市場調(diào)查以及查詢外文資料之后，人們發(fā)現(xiàn)，根據(jù)環(huán)境而自動改變的光產(chǎn)品目前是不存在的。因此，這個問題在世界上仍是一個空白。
2、實驗：
2.1整體設(shè)計方案
前照燈的功能是在夜間提供照明，這保證的空間范圍距離小于100米的距離，高度為2-2.5米可以得到良好的照明。?其發(fā)光強(qiáng)度是大于10000坎德拉，這樣司機(jī)在夜間就可以觀看到100米范圍內(nèi)的路面狀況。前照燈的另一個要求是，它必須不能導(dǎo)致對面司機(jī)的炫目。它要求在照射到對面司機(jī)處的發(fā)光強(qiáng)度低于1000cd / m和照明距離不大于40 m。從實驗分析結(jié)果表明，駕駛員的視野范圍在看到明亮燈光的影響是很大的。影響因素是兩車之間的距離L和垂直的水平距離S，特別是水平距離。
考慮到開關(guān)自動光的安全可靠性、目前的技術(shù)條件和性價比，在產(chǎn)品設(shè)計中，我們提出了幾種方案的設(shè)計：
1）普通光電測量方案：
前照燈被用作測量的光源。測量回路采用了一些，如光敏三極管、光電池、光電開關(guān)等，電子電路與它相匹配的。
2）紅外測量方案：
在汽車前照燈附近安裝一個紅外線發(fā)射管作為光源。測量電路應(yīng)該設(shè)計紅外電路。
3）調(diào)制方案
通過調(diào)制汽車前照燈的光源，從而可以防止其他可見光的干擾。測量電路也應(yīng)做相應(yīng)改變。
第一個方案是簡單又實用，有沒有必要特別設(shè)計光源。測量電路也很簡單。但缺點是，它可以很容易地受到其他光源的影響。
第二個和第三個方案可以克服第一種方案的缺點。但光源仍然需要額外設(shè)計，其測量電路也較為復(fù)雜的。
綜合上述因素，我們通過了第一個方案。為了克服這個缺點，我們采用的單片機(jī)和軟件，以判斷分析、進(jìn)行清除干擾。
2.2詳細(xì)設(shè)計
光也可能會嚴(yán)重地影響視力。當(dāng)駕駛員從光明大道駛?cè)牒诎档乃淼赖臅r候，他會覺得在那一刻都看不清楚，一段時間后就可以逐漸清楚看清周圍環(huán)境的物體。這種突然從光明進(jìn)入黑暗的地方，他的視力恢復(fù)逐漸現(xiàn)象被稱為暗適應(yīng)。與此對應(yīng)，如果突然從黑暗的地方進(jìn)入光明（如駛出隧道），開始時將看不清楚，然后視力漸漸地恢復(fù)，這種現(xiàn)象被稱為光適應(yīng)。
光適應(yīng)和暗適應(yīng)的時間與強(qiáng)度差是有關(guān)系的。強(qiáng)度差越大，其適應(yīng)時間持續(xù)的時間就越長。通常情況下，強(qiáng)光照來車駕駛員的視力急劇下降，甚至什么也看不見。這種現(xiàn)象被命名為炫目。
不止在夜間，而且還在白天和晚上（黃昏）駕駛員的視覺功能是壞的。通過研究發(fā)現(xiàn)，視力最不敏感的時候是在 一天24小時陰沉的黃昏時刻。路面及其他亮度迅速降低（特別是在秋季）。在這一刻閃耀的前大燈具有相同的周圍環(huán)境的亮度，使司機(jī)看不到車輛、道路上的行人，使得司機(jī)不能清楚的看見。此外，在這一刻，司機(jī)和行人都覺得累了。因此，事故發(fā)生的容易。大多數(shù)司機(jī)都有切身體會。
在簡單、實用、可靠、經(jīng)濟(jì)的原則下，我們設(shè)計了如下的電路。電路的框圖如圖1中所示。
2.3調(diào)試
進(jìn)行調(diào)試有兩個步驟：
1）在實驗室中進(jìn)行調(diào)試
前照燈檢查設(shè)備是一個必要的設(shè)備，它用于檢查前照燈的發(fā)光亮度和發(fā)光的方向是否符合標(biāo)準(zhǔn)。其基本原理是：它通過光電池將光轉(zhuǎn)換成電能，并根據(jù)前照燈燈光在光電池上產(chǎn)生的電流來測量發(fā)光強(qiáng)度和前照燈光軸傾斜。該儀器是由接受光機(jī)、為了ALM和汽車對正的準(zhǔn)直機(jī)、以坎德拉計的指示裝置和前照燈傾斜指示（根據(jù)前照燈前10米的偏斜角度計算）等組成。
以下圖是板上的連接回路的電路原理圖（圖2）。在實驗室中使用普通光源照射光電三極管3 DU。再次調(diào)整光源的發(fā)光強(qiáng)度和照射距離。調(diào)整放大器R2和R3的電阻值使放大器輸出值約為2.5 V。調(diào)整RV的電阻值。當(dāng)光照強(qiáng)度足夠大，由8031進(jìn)行處理，P1.1接口具有高電平。使得9013二極管導(dǎo)通。繼電器開關(guān)閉合。通過反復(fù)調(diào)試，結(jié)果就完美了。
2）在實際應(yīng)用中的調(diào)試
第二調(diào)試階段：設(shè)計電路，選擇合適原件，焊接，用電池進(jìn)行供電，在汽車上調(diào)試。在現(xiàn)代汽車有近光、遠(yuǎn)光的光學(xué)系統(tǒng)。[8]在對面沒有來車時，前照燈的遠(yuǎn)光燈工作，在汽車前以保證有足夠的照明距離和光照強(qiáng)度。當(dāng)對面有車相向行駛時，前照燈的近光工作，以防止對方司機(jī)眼睛炫目而看不清道路，并保證本車司機(jī)有足夠的視野。通過大量的調(diào)試，系統(tǒng)可以良好地運行。
3、安全性分析
安全性和可靠性是至關(guān)重要的，因此，我們采取以下步驟：
1）在電路設(shè)計中的單片機(jī)掃描并及時檢查接受的靈敏性。
（a）改進(jìn)的靈敏度，使該設(shè)備對于各種汽車燈都更加的靈敏。
（b）在車輛的行駛狀態(tài)及道路狀況是復(fù)雜多變的，這樣的設(shè)計應(yīng)用軟件根據(jù)各種光源的檔次和梯度來檢測光源。進(jìn)一步提高設(shè)備的可靠性。
（c）汽車在運行時可以有效地使用該程序?qū)彶?，判斷，分析，過濾，干涉光，由此進(jìn)一步提高了設(shè)備的抗干擾。
2）在電路設(shè)計,第二個程序控制被用來放大電路,提高可靠性和抗干擾性。
3）電子狗軟件，被用來防止系統(tǒng)運行時的突然停止。當(dāng)它發(fā)現(xiàn)，該程序進(jìn)入無限循環(huán)時，它將根據(jù)情況重新執(zhí)行初始程序。
4）使用收集光設(shè)備，以避免環(huán)境干擾。
4結(jié)論：
經(jīng)過市場的測量，分析，整體設(shè)計，電路設(shè)計，計算機(jī)仿真，實驗室測試，產(chǎn)品設(shè)計，大量的修改和不斷提高，汽車前照燈自動控制器已經(jīng)從實驗室的調(diào)試試驗階段轉(zhuǎn)向商業(yè)化階段。該產(chǎn)品是可靠的，廉價的和實用的。通過它，可以降低疲勞強(qiáng)度，提高安全性。它已達(dá)到了設(shè)計要求，具有廣闊的應(yīng)用前景。
原文部分：
2.4 ROTATING MACHINE VIBRATION MONITORING AND DATA PROCESSING SYSTEMS
Rotating machine vibration monitoring and diagnostics starts with implementation of basic transducers into machine systems. Vibration transducers provide important information about the dynamic process taking place within the machine. Additional transducers measure other specific important physical parameters of the machine, such as process fluid parameters and/or electromagnetic characteristics. The basic data acquired from the transducers is then processed, in order to pull out the most important information regarding machine operation and health. This information is used in machine malfunction diagnostics and corrective actions.
This section briefly discusses application of most popular vibration transducers in rotating machinery, as well as useful formats for vibration data processing.
2.4.1 Vibration Transducers
2.4.1.1 Accelerometers
Accelerometers measure mechanical vibration signals in terms of acceleration. The most popular accelerometer consists of an inertial mass mounted on a force-sensing element, such as a piezoelectric crystal (Figure 2.4.1). The latter produces an output proportional to the force exerted on the inertial mass, which is, in turn, proportional to the acceleration of a machine component, to which the transducer is attached. Typical sensitivity of an accelerometer is 0.1 V/g (g=ravity acceleration). Accelerometers are small, lightweight transducers that operate over a broad frequency range, as well as temperature range. They can withstand high vibration levels. Accelerometers do not require power supplies, and they are externally installed. They are, however, sensitive to method of attachment and the surface condition. They are also sensitive to noise and spurious vibrations (some models contain an integrally mounted amplifier). Accelerometers serve the best for high frequency vibration measurements in the ranges from 1500 cpm to 1200 kcpm.
Accelerometer is the most traditional transducer in structural mechanical vibration measurements. Sturdy, relatively inexpensive, and easy to use due to its external mounting on the structure or machine casing, it is a perfect tool for assessing high frequency vibrations. In machine vibration monitoring, accelerometers are invaluable tools for diagnosing problems in gear train teeth, rolling element bearings, and/or in observing blade-passing activities. The vibrations generated in these cases are characterized by high frequencies, for which the accelerometers are designed.
2.4.1.2 Velocity Transducer
The principle of operation of the most popular velocity transducer is based on inertia (seismic) property of a heavy mass suspended by springs on a vibrating body to remain motionless. In the electromagnetic transducer design, the mass carries a coil of wire and is elastically suspended by soft springs in a case containing, in the middle, a permanent magnet (Figure 2.4.2). The case, attached to a vibrating structure (such as a bearing cap or machine casing), transmits its vibration. The relative motion between the coil and the magnet generates an output voltage proportional to the instantaneous velocity of vibration. The transducer is self-generating, and does not require external power supplies. Velocity transducers have good sensitivity, typically 0.1 to 1 V/in/sec (4 to 40mV/mm/s). Their frequency range is from 500 cpm to 1000 kcpm. Velocity transducers are externally installed, and serve for overall vibration measurements of general-purpose machinery and mechanical structures. Disadvantages of velocity transducers comprise difficulties in calibration checks, sensitivity to magnetic interference, sensitivity to mounting orientation, and crossaxis vibration. The moving parts increase the risk of damage by a sudden shock or by fatigue process.
More modern velocity transducers, which do not contain moving parts, are based on the same principle as accelerometers, and they include an electronic integration circuit.
Both accelerometers and velocity transducers provide absolute values of acceleration or velocity of vibration, when properly calibrated.
2.4.1.3 Applicability of Accelerometers and Velocity Transducers on Rotating Machinery
The major vibration problems in rotating machines occur at low frequencies, in the range from zero to 200 Hz. Accelerometers either cannot detect the very low frequencies atall, or provide signals of poor resolution in the low-frequency range. Both accelerometers and velocity transducers are, therefore, unable to read the slow roll vibrations of a rotor (called sometimes “turning gear of the rotor”, meaning low rotational speed) or its centerline static position (colloquially called “dc gap”, meaning a position of the rotor centerline at rest), both of which are invaluable data. The accelerometers and velocity transducers cannot detect major malfunction symptoms of rotating machines, such as, for instance, thelow-frequency subsynchronous vibration of fluid whirl (see Chapter 4). In rotating machines, the rotors fulfill the major operational functions of transmitting energy through their rotational motion. For numerous reasons, as a side effect of the useful work performed by the rotor, a part of the rotational energy becomes converted into vibrational energy of various modes. The rotor itself, as a relatively soft element of the entire machine structure, is most prone to vibrate. As mentioned in Chapter 1, and deformations superposed on the rotating, torsionally stressed, power-transmitting rotor. The modes of rotor vibrations are usually the lowest modes of the entire machine structure. Vibrations of the rotor are eventually transmitted to other parts of the machine and to the environment. The rotor is always the source of machine vibration. It is evident that measuring vibrations “at the source” becomes vital for correct evaluation of the machine health. Measuring casing vibrations by using accelerometers or velocity transducers, as a simple, but indirect, way to assess the machine condition, brings nothing more than information on an “unacceptable” or “acceptable” level of vibration. They provide neither the possibility to diagnose what are the causes of vibration, if the vibration level is unacceptable, nor assessment on how long the machine would continue to operate without failure, if the vibration level is acceptable. Accelerometers and velocity transducers are, therefore, recommended only for noncritical, easily replaceable rotating machines, or as supplemental transducers for critical machines.
2.4.1.4 Displacement Transducer
The true diagnosis and prognosis of rotating machine health must be based on online continuous monitoring of the rotor behavior, the vibration source for the entire machine. The eddy current noncontacting proximity transducers are the best basic tools to fulfill the task. They are the most reliable and useful transducers to measure rotor vibrationsrelative to stationary elements of the machine (Figure 2.4.3). The principle of proximitytransducer operation is based on a modification of electromagnetic field, due to eddy currents induced in a conductive solid material in the proximity of the transducer tip. The output voltage is proportional to the gap between the transducer and the observed material surface. Typical sensitivity is 0.2 V/mil (8 mV/mm). High sensitivity (2 V/mil) proximity transducers are used in rolling element malfunction diagnostics: the transducer observes outer ring specific patterns in elastic deformations every time when a rolling element passes through the outer ring where the transducer is installed. In time, these specific patterns, recognized as wear-related damages, develop due to flaws in rolling elements or in bearing rings.
Proximity transducers provide not only dynamic components of the rotor motion, namely the vibrations, but also the quasi-static and static data: the very low frequency “slow roll” data, and invaluable zero frequency position of the rotor centerline (“dc gap”). These transducers cover the frequency range from zero to about 600 kcpm (10 kHz). The proximity transducer requires an external power supply (usually _18 to _24V dc) for operation. For signal accuracy, the rotor surface must be conditioned. The proximity transducer isconsidered the best for measuring rotor lateral and axial vibrations and positions on rotating machinery. It is prized for easy calibration check, reliability, and robustness in the industrial environment. During the last two decades, the proximity transducers successfully replaced obsolete and unreliable shaft riders.
The American Petroleum Institute has adopted a recommended practice (RP) entitled “Vibration, Axial Position, and Bearing Temperature Monitoring System” (RP#670). It outlines the system requirements for installing proximity transducers in the XY configuration on compressors and their driving systems to observe rotor centerline lateral motion (Figures 2.4.3 and 2.4.4). In addition to these lateral transducers, this recommended practice calls for two axially oriented noncontacting proximity transducers. They are used to monitor and warn about machine thrust problems, and are often tied to automatic trip, when a dangerous condition occurs. Both these transducer installation practices are also appropriate for the monitoring and protection of turbogenerators, pumps, fans, and other rotating machines.
On some machines, the installation of proximity transducers in orthogonal orientation is not possible. In this case, the angle between two transducers may have any value excluding 180_; best being close to 90_. Appropriate software will reduce the obtained data to signals corresponding to true 90_ difference.
The proximity transducers, mounted in orthogonal (XY) configuration, observing the rotor, provide two unilateral vibration signals, which by simple processing produce a magnified image of the rotor centerline actual lateral motion path, in the form of an orbit. Using an oscilloscope, the signals from two proximity transducers may be displayed on the screen in the time-base waveforms (x and/or y versus time) or in the orbital mode, when time is eliminated. Note that the proximity transducers usually provide amplitude signals in terms of “peak-to-peak” (abbreviation “pp”), not “zero-to-peak”, as used in mathematical models.
Figure 2.4.5 presents typical characteristics of three basic vibration-measuring transducers versus vibration frequency. It can be seen that the displacement transducer provides a linear constant relationship with frequency and is reliable from zero frequency to slightly over 10 kHz. Accelerometer characteristic is proportional to frequency squared. Accelerometers are perfect for high frequency vibration measurements, starting at about 20 Hz. Velocity transducers place between the other two transducers on the frequency scale. Velocity transducer characteristics are proportional to vibration frequency.
Figure 2.4.6 provides a sample of sensitivity of accelerometers and proximity transducers. It presents four spectrum cascade plots of rubbing rotor vibration response during start-up of the machine. Rotor lateral vibrations were measured by a displacement transducer ((A) and (C)) and an accelerometer ((B) and (D)) in the case of “heavy” rub ((A) and (B)) and”light” rub ((C) and (D)) conditions (see Chapter 5). Note that the accelerometer datameasures well all higher harmonics of basic rub-caused low frequency vibrations, but almost does not “see” these original subharmonic rub-related vibrations, especially in the case of “l(fā)ight rub”, where nonlinear effects are small and do not generate pronounced higher harmonics.
2.4.1.5 Dual Transducer
A combination of velocity and proximity transducers are designed to measure the absolute motion of the rotor in space, as well as its motion relative to the machinehousing (Figure 2.4.7). These transducers provide also the measurement of absolute motionof the housing. If the movement of the latter is larger than 30% of the rotor motion, the absolute vibration of the rotor should be known to adequately assess the machine health. In the dual transducer, the velocity transducer signal represents the housing absolute motion. This signal is electronically integrated and summed with the signal from the proximity transducer to provide the rotor absolute displacement. Dual transducers are often mounted inside fluid-lubricated bearings to observe the relative and absolute lateral motion of the journal.
2.4.1.6 Keyphasor Transducer
One of the very important transducers called for in the Recommended Practices, RP#670, is the Keyphasor_ transducer, which provides a rotor once-per-turn signal. The Keyphasor represents a radially mounted proximity transducer that observes a key, keyway, or other once-per-turn discontinuity on the rotor surface. During rotor rotation, the transducergenerates a once-per-turn on/off-type signal (Figures 2.4.8 to 2.4.10), which is superimposedon the time-base waveforms and orbits produced by two other rotor-observing proximity transducers mounted in XY configuration. These rotor time-base waveforms or orbits displayed on the oscilloscope have, therefore, a sequence of blank/brigh