YAH2460型圓振動篩設(shè)計【全套含CAD圖紙】

資源目錄里展示的全都有，所見即所得。下載后全都有,請放心下載。原稿可自行編輯修改=【QQ：401339828 或11970985 有疑問可加】 畢業(yè)設(shè)計（論文）任務(wù)書 I、畢業(yè)設(shè)計(論文)題目： YAH-2460圓振動篩設(shè)計 II、畢 業(yè)設(shè)計(論文)使用的原始資料(數(shù)據(jù))及設(shè)計技術(shù)要求： YAH-2460圓振動篩是用來對礦石進(jìn)行篩分的設(shè)備，它可以減輕體力勞動、提高勞動生產(chǎn)率或在生產(chǎn)過程中進(jìn)行某些特殊的工藝操作，實現(xiàn)機械化和現(xiàn)代化。 1. 物料名稱： 礦石 2. 入料粒度： ≤250 mm 3．有效篩面： 14.4 m2 4. 篩面傾角： 20 0 5. 篩孔尺寸： 12X60 mm 6. 處理量: 250 t/h III、畢 業(yè)設(shè)計(論文)工作內(nèi)容及完成時間： 1. 查閱相關(guān)資料，外文資料翻譯（6000字符以上），撰寫開題報告。 第1周—第2周 2．運動及動力參數(shù)計算 第3周—第4周 3．總裝圖設(shè)計 第5周—第8周 4. 主要零、部件強度及選用計算 第9周—第11周 5．繪制零、部件圖 第12周—第16周 6. 整理畢業(yè)論文及答辯準(zhǔn)備 第17周 Ⅳ 、主 要參考資料： 【1】孫桓等主編.機械原理. 北京：高等教育出版社，2001 【2】濮良貴等主編.機械設(shè)計. 北京：高等教育出版社，2001 【3】孫時元. 《中國選礦設(shè)備手冊》（上冊）. 北京：科學(xué)出版社，2006 【4】:嚴(yán)峰主編. 篩分機械. 北京: 中國鐵道出版社，2001 【5】任德樹主編. 粉碎篩分原理與設(shè)備.北京：北京科技出版社，1988 【6】徐灝主編.機械設(shè)計手冊（第四版）.北京：機械工業(yè)出版社.1991 【7】Shigley J E,Uicher J J.Theory of machines and mechanisms.New York:McGraw-Hill Book Company,1980 學(xué)院 專業(yè) 班 學(xué)生（簽名）： 日期： 自 20** 年 2 月 23 日至 20** 年 月 日 指導(dǎo)教師（簽名）： 助理指導(dǎo)教師(并指出所負(fù)責(zé)的部分)： 系（室）主任（簽名）： 一、 選題依據(jù) YAH-2460圓振動篩是用來對礦石進(jìn)行篩分的設(shè)備，它可以減輕體力勞動、提高勞動生產(chǎn)率或在生產(chǎn)過程中進(jìn)行某些特殊的工藝操作，實現(xiàn)機械化和現(xiàn)代化。 對我們學(xué)機械設(shè)計制造及其自動化專業(yè)的學(xué)生來說，大學(xué)所學(xué)的課程非常的廣泛，這個設(shè)計的課題“YAH-2460圓振動篩設(shè)計”需要用到我們所學(xué)的大部分知識，還有我們沒學(xué)過的，要求我們對大學(xué)四年所學(xué)的知識能夠非常熟練的掌握，并且會應(yīng)用到實際中去，對我們的實際運用能力是一個很好的鍛煉。 二、 國內(nèi)外研究概況及發(fā)展趨勢 1. 國外研究現(xiàn)狀 國外從16 世紀(jì)開始篩分機械的研究與生產(chǎn)，在18 世紀(jì)歐洲工業(yè)革命時期，篩分機械得到迅速發(fā)展，到本世紀(jì)，篩分機械發(fā)展到一個較高水平。德國申克公司可提供260 多種篩分設(shè)備，STK 公司生產(chǎn)的篩分設(shè)備系列品種 較全，技術(shù)水平較高，KHD公司生產(chǎn)200多種篩分設(shè)備，通用化程度較高，KUP 公司和海因勒曼公司都研制了雙傾角的篩分設(shè)備。美國RNO公司新研制DF11 型雙頻率篩，采用了不同速度的激振器。DRK公司研制成三路分配器給料，一臺高速電機驅(qū)動。日本東海株式會社和RXR 公司等合作研制了垂直料流篩，把旋轉(zhuǎn)運動和旋回運動結(jié)合起來，對細(xì)料一次分級特別有效。英國為解決從濕原煤中篩出細(xì)粒末煤，研制成功旋流概率篩。前蘇聯(lián)研制了一種多用途兼有共振篩和直線振動篩優(yōu)點的自同步直線振動篩。 篩分設(shè)備在國外的發(fā)展已有300多年的歷史，在此之前，物料的篩分主要采用人力篩分，動力篩分最早也是搖動篩。大約100多年前就出現(xiàn)了慣性篩，最早的慣性篩是采用柴油機帶動的，主要用于物料的分級作業(yè)。 比較完善的振動慣性篩出現(xiàn)在19世紀(jì)初，主要是用于分級的圓振動篩（單軸振動篩），隨著選煤、選礦業(yè)的發(fā)展用于脫水的直線振動篩（雙軸振動篩）逐漸發(fā)展起來。 單軸振動篩的發(fā)展經(jīng)歷了簡單慣性式向自定中心式的發(fā)展過程。直線振動篩經(jīng)歷了箱式振動器到雙電機拖動的筒式振動器（自同步技術(shù)），目前為箱式振動器與側(cè)幫式偏心塊單元體振動器（自同步技術(shù)）的并存時代。 現(xiàn)在振動篩軸承普遍采用了振動設(shè)備專用軸承，篩框的主要聯(lián)接件采用了虎克鉚釘或高強螺栓，篩面采用了不銹鋼篩面、聚鞍脂篩面等。篩框結(jié)構(gòu)逐漸趨于合理，篩框受力設(shè)計上逐步由靜態(tài)動力設(shè)計向以模態(tài)分析為基礎(chǔ)的現(xiàn)代動態(tài)設(shè)計階段發(fā)展。 在振動篩的制造方面，主要焊接結(jié)構(gòu)件均采用了去應(yīng)力和噴丸處理，對篩框的形狀誤差、主要構(gòu)件的形位公差、粗糙度控制等方面的要求越來越嚴(yán)。 雖然篩分機的結(jié)構(gòu)形式在發(fā)展過程中出現(xiàn)了許多種新型結(jié)構(gòu)及篩分方法，但通過實踐證明，許多看似理想的結(jié)構(gòu)型式被無情淘汰。因此，國際上一些篩分機制造廠家生產(chǎn)的振動篩結(jié)構(gòu)形式逐漸趨于近似，機型趨于穩(wěn)定，人們已不在追求新、奇結(jié)構(gòu)型式，而把追求篩分機的可靠性指標(biāo)放在首位，因此篩分機壽命普遍提高，正常使用壽命普遍達(dá)到5年以上。 振動篩噪聲指標(biāo)是影響工人身體健康的一個主要指標(biāo)。過去箱式振動器由于采用齒輪傳動，噪聲通常達(dá)到90分貝以上，后來逐漸采用了自同步技術(shù)，噪聲由原來的90多分貝下降到85分貝左右。但自同步技術(shù)存在拋射角不穩(wěn)定，工作頻率不能有效調(diào)整等因素，使得箱式振動器的振動篩不但沒有被淘汰，甚至通過不斷改進(jìn)結(jié)構(gòu)形式，提高齒輪加工精度，改善齒面嚙合狀態(tài)等方法，而重新發(fā)展起來，噪聲也從過去的90多分貝下降至85分貝左右。 2.我國振動篩的發(fā)展概況 國內(nèi)振動篩的發(fā)展經(jīng)歷了五個階段： 1.引進(jìn)設(shè)備階段：20世紀(jì)50年代左右，國內(nèi)振動篩主要靠引進(jìn)原蘇聯(lián)、波蘭等國的設(shè)備，面積一般在10平方米以下，如BHN、TYN-IIL、SXG-1（WK型）等。 2.初步開發(fā)階段：從20世紀(jì)60年代，我國技術(shù)人員在引進(jìn)國外振動篩的基礎(chǔ)上，研究開發(fā)了類似50年代進(jìn)口的產(chǎn)品，如SZZ、SSZ圓、直線振動篩（單、雙軸振動篩）系列。 3.研究設(shè)計階段：20世紀(jì)70年代，我國技術(shù)人員對選煤廠仍在使用的進(jìn)口設(shè)備進(jìn)行了系統(tǒng)的調(diào)查研究，分析論證，并獨立研制出了單軸，雙軸系列振動篩，如DD、ZD、DS、ZS系列圓、直線振動篩（單、雙軸振動篩），并在選煤廠廣泛使用，最大規(guī)格12。 4.新產(chǎn)品開發(fā)與引進(jìn)技術(shù)階段：20世紀(jì)80年代，我國振動篩發(fā)展進(jìn)入了一個全新時期，相繼開發(fā)的新型振動篩有ZD型等厚篩、旋轉(zhuǎn)概率篩和概率篩等新品種。同時，原鞍山礦山機械廠引進(jìn)了美國RS公司的圓振動與直線振動篩系列產(chǎn)品，最大面積14.4m2,基本滿足了中小型選煤廠的生產(chǎn)需要，并在國內(nèi)大量推廣應(yīng)用，唐山煤科院參考德國KHD公司技術(shù)，研制開發(fā)了ZK、YK系列振動篩。85年左右，洛陽礦山機械廠也引進(jìn)了日本神戶制鋼的技術(shù)開始生產(chǎn)大型篩。 5.大型振動篩開發(fā)研制階段：20世紀(jì)90年代，隨著大型選煤廠生產(chǎn)需要，原來的中小規(guī)格振動篩已滿足不了生產(chǎn)需要，雖然洛礦引進(jìn)了日本神戶制鋼大型篩技術(shù)，但并沒有成功推廣應(yīng)用，許多研究單位與制造單位也相繼開發(fā)超過3米寬的大型振動篩，但事故率高，不能被用戶認(rèn)可。說明大型篩的研制存在一定難度。為此，原煤炭部把“大型直線振動篩的可靠性研究”列入國家“九五”科研攻關(guān)項目。原平頂山選煤設(shè)計院承擔(dān)了該項目，并首次研究成功2ZKP3660型大型直線振動篩，并于2000年投入使用，可靠性指標(biāo)達(dá)到了引進(jìn)產(chǎn)品的水平。目前該系列產(chǎn)品已在國內(nèi)大量推廣，將逐步替代進(jìn)口產(chǎn)品。 2000年，平頂山選煤設(shè)計院研制出的自同步型2ZKZ3660大型直線振動篩也成功應(yīng)用于兗礦集團東灘煤礦選煤廠；2002年，山西賽德篩選技術(shù)設(shè)備有限公司研制開發(fā)了JR3072香蕉篩，并形成了系列，投入實際運用，為取代大量進(jìn)口的香蕉篩產(chǎn)品奠定了技術(shù)基礎(chǔ)。 我國的振動篩技術(shù)從無到有、從小到大。目前品種型號繁多，絕大部分中小型產(chǎn)品基本能滿足了用戶要求，大型產(chǎn)品技術(shù)已趨于成熟，尚需在振動篩制造方面更進(jìn)一步提高。相信在不遠(yuǎn)的將來，振動篩大量進(jìn)口的局面將結(jié)束。 目前我國各種選煤廠使用的設(shè)備中,振動篩是問題較多、維修量較大的設(shè)備之一。這些問題突出表現(xiàn)在篩箱斷梁、裂幫,稀油潤滑的箱式振動器漏油、齒輪打齒、軸承溫升過高、噪聲大等問題,同時伴有傳動帶跳帶斷帶等故障。這類問題直接影響了振動篩的使用壽命,嚴(yán)重影響了生產(chǎn)。 三、 研究內(nèi)容及實驗方案 1. 研究內(nèi)容 主要的研究方面有以下內(nèi)容，振動篩篩面物料運動理論；振動篩的工作原理及結(jié)構(gòu)組成；振動篩動力學(xué)基本理論；振動篩參數(shù)計算；主要零件的設(shè)計與計算；振動篩的安裝維護及潤滑；設(shè)備的環(huán)保、可靠性和經(jīng)濟評價等。 其中，振動篩篩面物料運動理論包括：篩上物料的運動分析、正向滑動、反向滑動、跳動條件的確定、物料顆粒跳動平均運動速度；振動篩的工作原理及結(jié)構(gòu)組成包括：圓振動篩的工作原理、振動篩基本結(jié)構(gòu)；振動篩參數(shù)計算又包括以下內(nèi)容：運動學(xué)參數(shù)的確定、振動篩工藝參數(shù)的確定、動力學(xué)參數(shù)、電動機的選擇；主要零件的設(shè)計與計算又要研究軸承的選擇與計算、皮帶的設(shè)計、軸的設(shè)計、支承彈簧設(shè)計驗算；振動篩的安裝維護及潤滑包括：振動篩的安裝及調(diào)試、操作要點、維護與檢修、振動篩的軸承潤滑的改進(jìn)；設(shè)備的環(huán)保、可靠性和經(jīng)濟評價：設(shè)備的環(huán)保、設(shè)備的可靠性、設(shè)備的可靠性。 2. 實驗方案 （1） 先查閱先查閱相關(guān)資料，掌握該機構(gòu)的大體機構(gòu)； （2） 對其運動及受力參數(shù)進(jìn)行分析計算； （3） 再進(jìn)行總裝圖進(jìn)行設(shè)計，并繪制出圖紙； （4） 用CAD畫出其零件圖及部件圖； （5） 對其性能、可靠性及經(jīng)濟價格進(jìn)行評比。 四、 目標(biāo)、主要特色及工作進(jìn)度 1.目標(biāo) 在規(guī)定的時間內(nèi)完成YAH—2460圓振動篩的設(shè)計，并達(dá)到答辯時所要的要求。 2.主要特色 ? YA系列圓振動篩篩箱運動軌跡為圓，適用于煤、石灰石、碎石、砂礫、金屬或非金屬礦石及其他物料的篩分。 從井下或露天采礦開采出來的或經(jīng)過破碎的物料，是以各種大小不同的顆?；旌显谝黄鸬?。在選礦廠、選煤廠和其它的工業(yè)部門中，物料在使用或進(jìn)一步處理前，常常需要分成粒度相近的幾種級別。物料通過篩面的過孔分級稱為篩分。篩分所用的機械稱為篩分機械。 在選礦廠和選煤廠中應(yīng)用的篩分機械有很多種結(jié)構(gòu)型式，如固定格篩、弧形篩、旋流篩，滾軸篩，簡篩、搖動篩，慣性振動篩和共振篩等。目前，由于慣性振動篩具有構(gòu)造簡單、生產(chǎn)能力大，篩分效率高等優(yōu)點，因而在選礦廠、選煤廠及其它工業(yè)部門中已被廣泛用于分級、脫水、脫介和脫泥作業(yè)。共振篩在生產(chǎn)實踐中也取得較好的效果，但因具有較大的沖擊裁荷，故其機件(如橫梁與側(cè)板)容易損壞，須進(jìn)一步研究和改進(jìn)。隨著煤礦開采能力和入洗原煤量的提高,作為物料分級篩選的主要設(shè)備——振動篩也不斷向大型化發(fā)展。 3. 工作進(jìn)度 1. 查閱相關(guān)資料，外文資料翻譯，撰寫開題報告。 第1周—第2周 2．運動及動力參數(shù)計算 第3周—第4周 3．總裝圖設(shè)計 第5周—第8周 4. 主要零、部件強度及選用計算 第9周—第11周 5．繪制零、部件圖 第12周—第16周 6. 整理畢業(yè)論文及答辯準(zhǔn)備 第17周 五、 參考文獻(xiàn) [1]、孫桓等主編.機械原理. 北京：高等教育出版社，2001 [2]、濮良貴等主編.機械設(shè)計. 北京：高等教育出版社，2001 [3]、孫時元. 《中國選礦設(shè)備手冊》（上冊）. 北京：科學(xué)出版社，2006 [4]、嚴(yán)峰主編. 篩分機械. 北京: 中國鐵道出版社，2001 [5]、任德樹主編. 粉碎篩分原理與設(shè)備.北京：北京科技出版社，1988 [6]、劉奎勝，譚兆衡主編.《篩分機械的應(yīng)用和發(fā)展》[M] .礦山機械，1998，第7期 [7]、Shigley J E,Uicher J J.Theory of machines and mechanisms.New York:McGraw-Hill Book Company,1980 5 A A A A PracticalPracticalPracticalPractical ApproachApproachApproachApproach totototo VibrationVibrationVibrationVibration DetectionDetectionDetectionDetection andandandand MeasurementMeasurementMeasurementMeasurementPhysicalPhysicalPhysicalPhysical PrinciplesPrinciplesPrinciplesPrinciples andandandandDetectionDetectionDetectionDetection TechniquesTechniquesTechniquesTechniquesBy: John Wilson, the Dynamic Consultant, LLCThis tutorial addresses the physics ofvibration; dynamics of a spring masssystem; damping; displacement, velocity,and acceleration; and the operatingprinciples of the sensors that detect andmeasure these properties.Vibration is oscillatory motion resultingfrom the application of oscillatory orvarying forces to a structure. Oscillatorymotion reverses direction. As we shall see,the oscillation may be continuous duringsome time period of interest or it may beintermittent. It may be periodic ornonperiodic, i.e., it may or may not exhibita regular period of repetition. The nature ofthe oscillation depends on the nature of theforce driving it and on the structure beingdriven.Motion is a vector quantity, exhibitinga direction as well as a magnitude. Thedirection of vibration is usually describedin terms of some arbitrary coordinatesystem (typically Cartesian or orthogonal)whose directions are called axes. Theorigin for the orthogonal coordinate systemof axes is arbitrarily defined at someconvenient location.Most vibratory responses of structurescan be modeled assingle-degree-of-freedom spring masssystems, and many vibration sensors use aspring mass system as the mechanical partof their transduction mechanism. Inaddition to physical dimensions, a springmass system can be characterized by thestiffness of the spring, K, and the mass, M,or weight, W, of the mass. Thesecharacteristics determine not only the staticbehavior (static deflection, d) of thestructure, but also its dynamiccharacteristics. If g is the acceleration ofgravity:F = MAW = MgK = F/d = W/dd = F/K = W/K = Mg/KDynamicsDynamicsDynamicsDynamics ofofofof a a a a SpringSpringSpringSpring MassMassMassMass SystemSystemSystemSystemThe dynamics of a spring mass system canbe expressed by the systems behavior infree vibration and/or in forced vibration.FreeFreeFreeFree VibrationVibrationVibrationVibration. Free vibration is the casewhere the spring is deflected and thenreleased and allowed to vibrate freely.Examples include a diving board, a bungeejumper, and a pendulum or swing deflectedand left to freely oscillate.Two characteristic behaviors shouldbe noted. First, damping in the systemcauses the amplitude of the oscillations todecrease over time. The greater thedamping, the faster the amplitudedecreases. Second, the frequency or periodof the oscillation is independent of themagnitude of the original deflection (aslong as elastic limits are not exceeded).The naturally occurring frequency of thefree oscillations is called the naturalfrequency, fn:ForcedForcedForcedForced VibrationVibrationVibrationVibration. Forced vibration isthe case when energy is continuouslyadded to the spring mass system byapplying oscillatory force at some forcingfrequency, ff. Two examples arecontinuously pushing a child on a swingand an unbalanced rotating machineelement. If enough energy to overcome thedamping is applid, the motion willcontinue as long as the excitation continues.Forced vibration may take the form ofself-excited or externally excited vibration.Self-excited vibration occurs when theexcitation force is generated in or on thesuspended mass; externally excitedvibration occurs when the excitation forceis applied to the spring. This is the case, forexample, when the foundation to which thespring is attached is moving.TransmissibilityTransmissibilityTransmissibilityTransmissibility. When the foundationis oscillating, and force is transmittedthrough the spring to the suspended mass,the motion of the mass will be differentfrom the motion of the foundation. We willcall the motion of the foundation the input,I, and the motion of the mass the response,R. The ratio R/I is defined as thetransmissibility, Tr:Tr = R/IResonanceResonanceResonanceResonance. At forcing frequencieswell below the systems natural frequency,R I, and Tr 1. As the forcing frequencyapproaches the natural frequency,transmissibility increases due to resonance.Resonance is the storage of energy in themechanical system. At forcing frequenciesnear the natural frequency, energy is storedand builds up, resulting in increasingresponse amplitude. Damping alsoincreases with increasing responseamplitude, however, and eventually theenergy absorbed by damping, per cycle,equals the energy added by the excitingforce, and equilibrium is reached. We findthe peak transmissibility occurring whenfffn. This condition is called resonance.IsolationIsolationIsolationIsolation. If the forcing frequency isincreased above fn, R decreases. Whenff=1.414 fn, R = I and Tr = 1; at higherfrequencies R I and Tr 1. At frequencieswhen R 0.1 in., to make them practical.The change in intensity or angle of alight beam directed onto a reflectivesurface can be used as an indication of itsdistance from the source. If the detectionapparatus is fast enough, changes ofdistance can be detected as well.The most sensitive, accurate, andprecise optical device for measuringdistance or displacement is the laserinterferometer. With this apparatus, areflected laser beam is mixed with theoriginal incident beam. The interferencepatterns formed by the phase differencescan measure displacement down to 1 MHz insome PR shock accelerometers.Most contemporary PR sensors aremanufactured from a single piece of silicon.In general, the advantages of sculpting thewhole sensor from one homogeneous blockof material are better stability, less thermalmismatch between parts, and higherreliability. Underdamped PRaccelerometers tend to be less rugged thanPE devices. Single-crystal silicon can haveextraordinary yield strength, particularlywith high strain rates, but it is a brittlematerial nonetheless. Internal friction insilicon is very low, so resonanceamplification can be higher than for PEtransducers. Both these features contributeto its comparative fragility, although ifproperly designed and installed they areused with regularity to measure shockswell above 100,000 g. They generally havewider bandwidths than PE transducers(comparing models of similar full-scalerange), as well as smaller nonlinearities,zero shifting, and hysteresis characteristics.Because they have DC response, they areused when long-duration measurements areto be made.In a typical monolithic silicon sensingelement of a PR accelerometer, the 1 mmsquare silicon chip incorporates the entirespring, mass, and four-arm PR strain gaugebridge assembly. The sensor is made froma single-crystal silicon by means ofanisotropic etching and micromachiningtechniques. Strain gauges are formed by apattern of dopant in the originally flatsilicon. Subsequent etching of channelsfrees the gauges and simultaneouslydefines the masses as simply regions ofsilicon of original thickness.The bridge circuit can be balanced byplacing compensation resistor(s) in parallelor series with any of the legs, correctingfor the matching of either the resistancevalues and/or the change of the values withtemperature. Compensation is an art;because the PR transducer can havenonlinear characteristics, it is inadvisableto operate it with excitation different fromthe conditions under which it wasmanufactured or calibrated. For example,PR sensitivity is only approximatelyproportional to excitation, which is usuallya constant voltage or, in some cases,constant current, which has someperformance advantages. Because thermalperformance will in general change withexcitation voltage, there is not a preciseproportionality between sensitivity andexcitation. Another precaution in dealingwith voltage-driven bridges, particularlythose with low resistance, is to verify thatthe bridge gets the proper excitation. Theseries resistance of the input lead wiresacts as a voltage divider. Take care that theinput lead wires have low resistance, orthat a six-wire measurement be made (withsense lines at the bridge to allow theexcitation to be adjusted) so the bridge getsthe proper excitation.Constant current excitation does nothave this problem with series resistance.However, PR transducers are generallycompensated assuming constant voltageexcitation and might not give the desiredperformance with constant current. Thebalance of the PR bridge is its mostsensitive measure of health, and is usuallythe dominant feature in the totaluncertainty of the transducer. The balance,sometimes called bias, zero offset, or ZMO(zero measurand output, the output with 0g), can be changed by several effects thatare usually thermal characteristics orinternally or externally induced shifts instrains in the sensors. Transducer casedesigns attempt to isolate the sensors fromexternal strains such as thermal transients,base strain, or mounting torque. Internalstrain changes, e.g., epoxy creep, tend tocontribute to long-term instabilities. Allthese generally low-frequency effects aremore important for DC transducers thanfor AC-coupled devices because they occurmore often in the wider frequency band ofthe DC-coupled transducer.Some PR designs, particularlyhigh-sensitivity transducers, are designedwith damping to extend frequency rangeand overrange capability. Dampingcoefficients of 0.7 are considered ideal.Such designs often use oil or some otherviscous fluid. Two characteristics dictatethat the technique is useful only atrelatively low frequencies: damping forcesare proportional to flow velocity, andadequate flow velocity is attained bypumping the fluid with large displacements.This is a happy coincidence for sensitivetransducers in that they operate at the lowacceleration frequencies wheredisplacements are adequately large.Viscous damping can effectively eliminateresonance amplification, extend theoverrange capability, and more than doublethe useful bandwidth. However, becausethe viscosity of the damping fluid is astrong function of temperature, the usefultemperature range of the transducer issubstantially limited.VariableVariableVariableVariable CapacitanceCapacitanceCapacitanceCapacitance. VCtransducers are usually designed asparallel-plate air gap capacitors in whichmotion is perpendicular to the plates. Insome designs the plate is cantilevered fromone edge, so motion is actually rotation;other plates are supported around theperiphery, as in a trampoline. Changes incapacitance of the VC elements due toacceleration are sensed by a pair of currentdetectors that convert the changes intovoltage output. Many VC sensors aremicromachined as a sandwich ofanisotropically etched silicon wafers with agap only a few microns thick to allow airdamping. The fact that air viscositychanges by just a few percent over a wideoperating temperature range provides afrequency response more stable than isachievable with oil-damped PR designs.In a VC accelerometer, ahigh-frequency oscillator provides thenecessary excitation for the VC elements.Changes in capacitance are sensed by thecurrent detector. Output voltage isproportional to capacitance changes, and,therefore, to acceleration. Theincorporation of overtravel stops in the gapcan enhance ruggedness in the sensitivedirection, although resistance to overrangein transverse directions must rely solely onthe strength of the suspension, as is true ofall other transducer designs withoutovertravel stops. Some designs can surviveextremely high acceleration overrangeconditions-as much as 1000 full-scalerange .The sensor of a typicalmicromachined VC accelerometer isconstructed of three silicon elementsbonded together to form a hermeticallysealed assembly. Two of the elements arethe electrodes of an air dielectric,parallel-plate capacitor. The middleelement is chemically etched to form arigid central mass suspended by thin,flexible fingers. Damping characteristicsare controlled by gas flow in the orificeslocated on the mass.VC sensors can provide many of thebest features of the transducer typesdiscussed earlier: large overrange, DCresponse, low-impedance output, andsimple external signal conditioning.Disadvantages are the cost and sizeassociated with the increased complexityof the onboard conditioning. Also,high-frequency capacitance detectioncircuits are used, and some of thehigh-frequency carrier usually appears onthe output signal. It is generally not evennoticed, being up to three orders ofmagnitude (i.e., 1000 ) higher infrequency than the output signals.ServoServoServoServo (Force(Force(Force(Force Balance)Balance)Balance)Balance). Althoughservo accelerometers are usedpredominantly in inertial guidance systems,some of their performance characteristicsmake them desirable in certain vibrationapplications. All the accelerometer typesdescribed previously are open-loop devicesin which the output due to deflection of thesensing element is read directly. Inservo-controlled, or closed-loop,accelerometers, the deflection signal isused as feedback in a circuit that physicallydrives or rebalances the mass back to theequilibrium position. Servo accelerometermanufacturers suggest that open-loopinstruments that rely on displacement (i.e.,straining of crystals and piezoresistiveelements) to produce an output signal oftencause nonlinearity errors. In closed-loopdesigns, internal displacements are keptextremely small by electrical rebalancingof the proof mass, minimizing nonlinearity.In addition, closed-loop designs are said tohave higher accuracy than open-loop types.However, definition of the termaccuracyvaries. Check with the sensormanufacturer.Servo accelerometers can take eitherof two basic geometries: linear (e.g.,loudspeaker) and pendulous (metermovement).Pendulous geometry is most widelyused in commercial designs. Until recently,the servo mechanism was primarily basedon electromagnetic principles. Force isusually provided by driving currentthrough coils on the mass in the presenceof a magnetic field. In the pendulous servoaccelerometer with an electromagneticrebalancing mechanism, the pendulousmass develops a torque proportional to theproduct of the proof mass and the appliedacceleration. Motion of the mass isdetected by the position sensors (typicallycapacitive sensors), which send an errorsignal to the servo system. The error signaltriggers the servo amplifier to output afeedback current to the torque motor,which develops an opposing torque equalin magnitude to the acceleration-generatedtorque from the pendulous mass. Output isthe applied drive current itself (or across anoutput resistor), which, analogous to thedeflection in the open-loop transducers, isproportional to the applied force andtherefore to the acceleration.In contrast to the rugged springelements of the open-loop transducers, therebalancing force in the case of theclosed-loop accelerometer is primarilyelectrical and exists only when power isprovided. The springs are as flimsy in thesensitive direction as feasible and mostdamping is provided through theelectronics. Unlike other DC-responseaccelerometers whose bias stabilitydepends solely on the characteristics of thesensing element(s), it is the feedbackelectronics in the closed-loop design thatcontrols bias stability. Servoaccelerometers therefore tend to offer lesszero drifting, which is the major reason fortheir uses in vibration measurements. Ingeneral, they have a useful bandwidth of1000 Hz and are designed for use inapplications with comparatively lowacceleration levels and extremely lowfrequency components.ReferencesReferencesReferencesReferences1.A. Chu.Zero Shift of PiezoelectricAccelerometers in PyroshockMeasurements, Endevco TP No. 293.2. Shock & Vibration MeasurementTechnology. 1987. Endevco.3. Measuring Vibration. 1982. Bruel &Kjaer.4. C. Harris. 1995. Shock and VibrationHandbook, 4th Ed., McGraw Hill.5. General Guide to ICP Instrumentation.March 1973. PCB Piezotronics, #G-0001.6. Introduction to Piezoelectric Sensors.March 1985. PCB Piezotronics, #018.7. Application of Integrated-CircuitElectronics to Piezoelectric Transducers.March 1967. PCB Piezotronics, #G-01.8. Isotron Instruction Manual. 1995.Endevco, IM 31704.9. Instruction Manual for EndevcoPiezoresistive Accelerometers. 1978.Endevco, #121.10. Entran Accelerometer Instruction andSelection Manual. 1987. Entran Devices.11. R. Sill. Testing Techniques Involvedwith the Development of High ShockAcceleration Sensors. Endevco, TP 284.12. R. Sill. Minimizing MeasurementUncertainty in Calibration and Use ofAccelerometers. Endevco, TP 299.13. P.K. Stein. The Constant CurrentConcept for Dynamic StrainMeasurement. Stein Engineering Services,Inc., Lf/MSE Publication 46.14. B. Link. Shock and VibrationMeasurement Using VariableCapacitance. Endevco, TP 296.