隨車液壓起重機(jī)的控制外文翻譯@中英文翻譯@外文文獻(xiàn)翻譯

附錄 A 譯文 隨車液壓起重機(jī)的控制 摘 要 ： 本文主要是描述隨車液壓起重機(jī)的控制過程。 這篇論文分為五個部分：需求分析，液壓系統(tǒng)以及存在的問題的分析， 不同結(jié)構(gòu)產(chǎn)生不同問題的分析，基于更加先進(jìn)復(fù)雜電液比例控制閥的新技術(shù)的發(fā)展趨勢的分析。 本文的研究工作是和實際的工業(yè)相結(jié)合的，比純粹的研究理論更有意義。 關(guān)鍵字 ： 隨車液壓起重機(jī)，控制策略，電液比例控制閥 1.引言 本文主要敘述的是對隨車起重機(jī)控制系統(tǒng)的改進(jìn)方法 隨車汽車起重機(jī)可以看成是一種大型柔性控制機(jī)械結(jié)構(gòu) 。 這種控制系統(tǒng)把操作人員的命令由機(jī)械結(jié)構(gòu)變?yōu)閳?zhí)行動作。 這樣定義這種控制系統(tǒng)是為了避免在設(shè)計它事產(chǎn)生模糊的思想這是一種通過人的命令把能量轉(zhuǎn)化成機(jī)械動作的控制系統(tǒng) 。 本文所寫的就是這種控制系統(tǒng)。以這個目標(biāo)為指導(dǎo)方針來分析怎樣設(shè)計出新的控制系統(tǒng)。 文章分為五個部分： 1.分析這種控制系統(tǒng)必須據(jù)有易操作性，高強(qiáng)度，高效性，穩(wěn)定性，安全性。 2.分析目前這種操作系統(tǒng)所存在的問題。 3.從不同的方面分析這種控制系統(tǒng)：不同的操作方式，不同的控制方法，不 同的組織結(jié)構(gòu)。 4.介紹一種適合于未來工業(yè)的比較經(jīng)濟(jì)的新的控制系統(tǒng)。 5.分析一種據(jù)有高性能，高效率，易控制等的比較 好的控制系統(tǒng)。它將成為 今后研究的比較經(jīng)濟(jì)高效的一種方案。 2. 論文部分 2.1 對控制系統(tǒng)必備條件的分析 在一種新的操作系統(tǒng)開始正式投入工作之前，對這種控制系統(tǒng)據(jù)有嚴(yán)格的要求。對控制系統(tǒng)的影響有很多因素。例如：機(jī)械結(jié)構(gòu)的可實行性因素，可操作性因素，效率因素，符合工業(yè)標(biāo)準(zhǔn)。 工業(yè)需求必須放在第一位。這與在控制系統(tǒng)中導(dǎo)管破裂保護(hù)和超載保護(hù)有同等的地位。其次穩(wěn)定性要求也很重要；系統(tǒng)不穩(wěn)定就沒法正常工作。一旦穩(wěn)定性要求得以確定，控制系統(tǒng)性能要求就可以進(jìn)一步確定。機(jī)械結(jié)構(gòu)決定了起重機(jī)的可操作性。機(jī)械機(jī)構(gòu)是隨 車起重機(jī)中可以往復(fù)轉(zhuǎn)動固有頻率低的大型柔性結(jié)構(gòu)。 為了防止起重機(jī)振動，必須使起重機(jī)在固有頻率下工作，或者提高起重機(jī)的固有頻率。如果它的固有頻率太低或者太高，操作人員將無法給它進(jìn)行操作。最后傳動效率可以在工業(yè)標(biāo)準(zhǔn)，穩(wěn)定性，執(zhí)行機(jī)構(gòu)確定的基礎(chǔ)上得到最優(yōu)的方案。 2.2 對目前這種控制系統(tǒng)的分析 在設(shè)計一種新的起重機(jī)之前，研究目前起重機(jī)存在的問題是很有必要的。當(dāng)前液壓隨車起重機(jī)主要存在以下三個問題： 1.不穩(wěn)定性 2.不經(jīng)濟(jì)性 3.低效性 2.2.1 不穩(wěn)定性 不穩(wěn)定性是一個嚴(yán)重問題，他可能會損傷操作人員或 者會是設(shè)備受到毀壞。當(dāng)一個系統(tǒng)不穩(wěn)定時通常產(chǎn)生嚴(yán)重振動。為了消除當(dāng)前系統(tǒng)的不穩(wěn)定性，設(shè)計人員既花費了很多時間來研究又花費了很多財力設(shè)計出更加復(fù)雜的機(jī)構(gòu)。如圖 1所示為一種起重機(jī)，它適合于在高速下工作。但是為了可以安全的工作必須合理控制其運(yùn)行速度。要提高它的控制速度又必須增加更加昂貴復(fù)雜的機(jī)械系統(tǒng)。 液壓系統(tǒng)的參數(shù)，如溫度或壓力同樣影響系統(tǒng)的穩(wěn)定性。一個參數(shù)合理的液壓系統(tǒng)比一個設(shè)計參數(shù)不合理的液壓系統(tǒng)穩(wěn)定，為了使整個系統(tǒng)運(yùn)行穩(wěn)定，有時必須降低次要的參數(shù)值。 2.2.2 不經(jīng)濟(jì)性 目前的液壓系統(tǒng)是純液壓的機(jī)械系統(tǒng) ，因此如果用戶想實現(xiàn)一個功能，他就必須買一個能使現(xiàn)這個功能的液壓機(jī)械組件。因為大多數(shù)用戶又不同的使用要求，要求同一個設(shè)備可以進(jìn)行升級。這就意味著這些標(biāo)準(zhǔn)設(shè)備可以人為的改造，這就增加了組件升級費用。 2.2.3 低效性 液體在液壓系統(tǒng)的兩個液壓缸之間流動時效率較低。這是因為大多數(shù)液壓閥都是用一個閥心來控制兩個節(jié)流口，由于這個鏈接不可能使閥芯兩側(cè)的壓力相等，因此在流出端就產(chǎn)生一個與液流方向相反的背壓力，同時也增加了流入端的壓力。由激勵源產(chǎn)生的這個背壓力與閥芯兩端的壓力差成正比的，給油缸的實際壓力沒有被有效的作用 在油缸上。例如，給液壓缸的壓力為 1000psi/1600psi傳到液壓缸時就只有 0psi/600 psi 了。無論如何，這樣的話，提供的電量必須高于有效電量，這些額外的電量就被白白的浪費了 2.3 控制系統(tǒng)不同的控制方法 目前主要用電液比例控制閥來控制液壓閥的運(yùn)動。然而對控制筒有不同的控制方法。 電液比例控制閥對閥的關(guān) /開，公共汽車系統(tǒng)，電源的智能激勵，泵的調(diào)節(jié)方案控制精度都較高。必須對這種系統(tǒng)的優(yōu)缺點進(jìn)行分析，找出合理的方案。 2.4 近期方案 即使這種十分新的系統(tǒng)最佳外形的布局已經(jīng)得以證明是可行的，但是起重 機(jī)制造商和配件商還不能立刻就接受這種技術(shù)。這是一個漸進(jìn)的過程，所以提出了一種臨時解決的方案。 這種方案是由微型計算機(jī)和升縮機(jī)構(gòu)組成。這種離合閥可使這種更加高效穩(wěn)定的執(zhí)行控制機(jī)構(gòu)得以實現(xiàn)。微型計算機(jī)可以對閥進(jìn)行柔性控制?？梢园堰@些變量編入軟件。這樣就消除了制造商許許多多不同的變量問題。起重機(jī)制造廠家可以根據(jù)產(chǎn)品功能選擇不同型號的液壓閥。配件商也將不得不生產(chǎn)這種型號的閥，這樣不僅降低了制造成本，而且使起重機(jī)的性能得到提高。 2.5 更高效方案的分析 這種分析依賴于不同布局結(jié)果，液壓泵控制的區(qū)域決定將要用的控制方 法，再依次對這個區(qū)域進(jìn)行分析。不同的區(qū)域?qū)⒂貌煌姆椒ㄌ接?，用不同的刀具位置控制?3. 實驗設(shè)備 本文的中心是研究發(fā)展中的經(jīng)濟(jì)型機(jī)械控制方案的可實現(xiàn)問題，更多重點是先進(jìn)的實驗結(jié)果。實驗結(jié)果由兩種方法獲得。第一種是通過研究單自由起重機(jī)實驗臺獲得，第二種是通過研究一臺由丹麥一家起重機(jī)廠送給英國的一所軍校的起重機(jī)獲得。如圖 1 所示 圖 1系統(tǒng)實驗臺 左：單自由度起重機(jī)模型 右：隨車起重機(jī)實物 雖然目前這種升縮分離機(jī)構(gòu)在生產(chǎn)商中沒有被普遍接受，但是兩分離閥將會被逐漸取代。如圖 2所示是一種幅度 -脈沖變換液壓 缸，它是通過數(shù)字信息處理器 /奔騰雙信息處理器運(yùn)行程序來控制液壓閥的。由數(shù)字信號處理器運(yùn)行控制代碼，奔騰處理器來判斷并提供圖形用戶界面。 4. 當(dāng)前工作 4.1 直線軸流控法 當(dāng)今市場常見的直線流控器都需要壓力補(bǔ)償。壓力補(bǔ)償器可以使閥芯突然受壓時保持恒定的壓力。但是新增加的壓力補(bǔ)償器會使閥的結(jié)構(gòu)比簡單的隨動閥更加復(fù)雜。另一種解決方法是用流控器測量閥的壓力降來調(diào)整閥芯的位置來實現(xiàn)。這種想法雖然簡單，但是由于壓力傳感器和微控器的費用比較高，想普遍運(yùn)用于商品上是很難的。然而目前這種利用微控器和壓力傳感器的思想對于生 產(chǎn)商來說是可以接受的。 雖然依據(jù)方程來看很簡單，但是要實現(xiàn)卻很難。流控器的位置精度取決于位置傳感器的精度壓力傳感器的精度。噪聲會影響位置傳感器和壓力傳感器的穩(wěn)定性。采用延時控制可以消除影響穩(wěn)定性的噪聲，這樣，超過閥的運(yùn)行范圍的特征值用就不能用柏努力方程計算，應(yīng)用更復(fù)雜的方程來計算。 圖 3 起重機(jī)工作的不同情形 圖 2升縮分離機(jī)構(gòu) 4.2 液壓缸控制方法 根據(jù)不同的受力方向和速度方向這種液壓缸有四種工作情形。如圖 3所示： 多數(shù)是普通的隨動液壓閥，它這種控制方法已經(jīng)在文獻(xiàn)中可以找到，依靠一般的測量 法測液壓缸的速度位移相當(dāng)復(fù)雜。它們也需要相當(dāng)復(fù)雜的運(yùn)算法則來控制。本文主要分析基于簡單的 PI控制器和沒有嚴(yán)格速度位移要求的液壓缸的控制方法。這種系統(tǒng)的控制方法比復(fù)雜的控制方法簡單得多，由于它不需要特殊的傳感器而且容易被大多數(shù)工程師理解所以比較容易被廠商采用。 在設(shè)計一種控制方法時另一種特別的控制方法也需要了解，它也是液控中常用的一種方法。移動液壓閥要求低泄漏，以前的液壓閥大們通常有很大的交迭。然而，使生產(chǎn)商能夠接受的這種線軸式液壓缸的驅(qū)動性能相當(dāng)慢。這種具有很大交迭的重合以及激發(fā)很慢的液壓閥很難滿足現(xiàn)在的 要求。交迭和較慢的驅(qū)動使壓力控制變得相當(dāng)困難。 新的控制方法可以用一個例子清楚簡單的描述出來。從入口端實行流控制，出口端就實現(xiàn)液壓力。流控制符合柏努力方程。液壓控制過程中 PI控制器圖 4 減壓控制器 維持較小的壓力來提高效率并且可以防止氣穴現(xiàn)象。這些都是為了解決大交迭和較低的驅(qū)動所做的工作，壓力控制器僅僅能排除控制中的一點問題。這就意味著如果控制人員想提高壓力，卻不能使液壓缸移動，只能夠降低控制口的開口量。這樣做的作用只能使操作人員想改變活塞的方向時使它準(zhǔn)時脫離零位。這種情況下外力方向和活塞運(yùn)動仍然不能改變，這種方式需要改進(jìn) 。既然這樣，需要壓力控制器在出口變大時提供與外力方向相反的有用壓力，當(dāng)已知入口端的壓力下降的時候，它可以增加與外力相反的壓力。這個壓力也受 PI控制器控制，如圖 4所示就是是一個這種控制系統(tǒng)的控制模型結(jié)構(gòu)。 在寫本文的時候這種控制的實驗已經(jīng)在圖 1所示的實驗臺上完成了，由于起重機(jī)上安裝了載荷單向閥，所以穩(wěn)定性沒有達(dá)到要求。然而，用液壓單向閥取代這種載荷單向閥，可以使系統(tǒng)的穩(wěn)定。在液壓系統(tǒng)中，載荷閉式閥可以實現(xiàn)超載保護(hù)和卸載保護(hù)兩種功能。由于在這種控制方法中使用伸縮閥機(jī)構(gòu)對卸載保護(hù)很起作用，因此在起升機(jī)構(gòu)中很有 必要使用有這種功能的單向閥。一個操作單向閥的駕駛員可以做這一點，沒有增加復(fù)雜的動力來阻止起重機(jī)的傾。安裝了這種單向閥，起重機(jī)操作人員不需要再增加更復(fù)雜的外力來防止起重機(jī)產(chǎn)生傾翻。 5. 結(jié)束語 即使沒有大量的實驗設(shè)施，但是實驗還是完成了，一個好的開始是成功的一半。這個論文題的大輪闊已經(jīng)確定，它是有意義而且合理的。這個工作分為需求分析、目前的系統(tǒng)分析、不同布局分析、近期的解決辦法的分析和最優(yōu)解決方案的發(fā)展趨勢分析五個部分。在本論題的最后，液壓隨車起重機(jī)的控制模將會被修改。 6.感謝語 感謝 Danfoss Fluid Power A/S為這個研究提供了部分基金。也感謝 Hjbjerg Maskinfabrik (HMF) A/S愿意為這種起重機(jī)的測試提供技 術(shù)上的支 持 隨車液壓起重機(jī)的軌跡控制 問題描述 這項方案是根據(jù)如圖 1所示的多自由度隨車液壓起重機(jī)控制問題提出來的?？刂齐S車起重機(jī)要求操作人員技術(shù)相當(dāng)高，它的操作機(jī)動范圍很小。如果可以讓現(xiàn)代的起重機(jī)實現(xiàn)遙控控制的話，操作人員只需要控制他手中的遙控器就可以控制起重機(jī)把重物放在他要求的任何地方。一個按鈕控制一個自由度方向上 的轉(zhuǎn)動。因此只需要讓操作人員得到熟練的訓(xùn)練他就可以每次控制更多的按鈕來實現(xiàn)多個自由度的轉(zhuǎn)動。 圖 1 所示為一臺隨車液壓裝載起重機(jī)部分液壓系統(tǒng)控制圖實例 這項工程的目標(biāo)是設(shè)計一臺非熟練操作人員都能夠控制的移動式液壓起重機(jī)。操作人員根據(jù)吊具總成的合成軌跡控制一根操縱桿。這樣不同的自由度就可以同時被控制。 多數(shù)隨車液壓起重機(jī)的結(jié)構(gòu)就像圖 1所示的那樣，大多數(shù)都是非常柔性化的，因此當(dāng)受載時它們就會彎曲。這樣做可以使起重機(jī)吊重比最低。事實上吊重頂端位置也是制約控制系統(tǒng)結(jié)構(gòu)偏差的因素。這種問題可以通過一個好的位置 偏差補(bǔ)償控制系統(tǒng)解決，這個系統(tǒng)還可以消除操作初期結(jié)構(gòu)上發(fā)生的擺動。 繼續(xù)使結(jié)構(gòu)軌跡偏差補(bǔ)償控制系統(tǒng)在起重機(jī)上進(jìn)一步發(fā)展，起重機(jī)的裝載能力將可以大大得到提高。當(dāng)這種在起重機(jī)里的擺動可以被控制系統(tǒng)抑制的方法能夠得到充分證明，在一個長的期限里可能有一個降低動力學(xué)安全系數(shù)的機(jī)會。這將使起重機(jī)生產(chǎn)商和用戶節(jié)省一大筆費用。 吊具總成 圖 2 測試起重機(jī)圖片 方案內(nèi)容 現(xiàn)以一臺如圖 2 所示的 HMF 680-4 型隨車液壓起重機(jī)來分析這些問題。在這臺起重機(jī)的不同位置安裝了傳感器來監(jiān)視系統(tǒng)上的不同參數(shù)值，它們都是一些起重機(jī)上很重要的不同連接位置的壓力、流量 、應(yīng)變參數(shù)值。實驗測試可以證實起重機(jī)性能，所以可以通過精確的模型來測試起重機(jī)的性能。為了使所含蓋的幾個問題能夠描述得更清楚，這些問題被簡略的表述如下： 1. 分析系統(tǒng)要求說明書 系統(tǒng)的執(zhí)行標(biāo)準(zhǔn)分析已被完成。基于系統(tǒng)的這種要求連同確保系統(tǒng)的執(zhí)行的檢驗程序?qū)⒈涣腥肭鍐巍?2. 機(jī)械子系統(tǒng)模型 許多技術(shù)模型已經(jīng)存在，因此這些部件包括研究明確的模型局部動力學(xué)的表達(dá)方法。機(jī)械子系統(tǒng)的分析與局部模型偏差的詳細(xì)分析相同。這樣做是為了使計算的有效性能夠明確表達(dá)出來，同時使系統(tǒng)的動作在控制過程中能夠十分精確。基于這種非常有前 景的用公式表示一個數(shù)學(xué)子系統(tǒng)模型的方法已經(jīng)完成，它將從起重機(jī)試驗臺的實驗結(jié)果中得到校驗。 3. 液壓子系統(tǒng)模型 跟機(jī)械子系統(tǒng)建模一樣，液壓子系統(tǒng)模型由液壓泵、不同的液壓閥、激勵源和液壓導(dǎo)管組成。然而，并不是這些都要建模，只是那些對系統(tǒng)動力學(xué)部件影響比較大的成分才建模。液壓子系統(tǒng)模型也需要用實驗的方法來證明。除此之外是否在對偏差進(jìn)行補(bǔ)償時，系統(tǒng)中用了比重比較大的電液比例控制閥都必須被分析，即對機(jī)械結(jié)構(gòu)的擺動進(jìn)行分析?；谏鲜鲂拚?，對液壓系統(tǒng)如果有必要都要做。 4.分析和標(biāo)準(zhǔn)的解決反轉(zhuǎn)運(yùn)動結(jié)構(gòu) 起重機(jī)相對 于底部有一個可以操作的特定空間，即吊具總成能達(dá)到的范圍。這是公認(rèn)的起重機(jī)工作范圍。有的部位要通過不同的路線才可以達(dá)到。因此有必要在這些區(qū)域確定最佳的運(yùn)動結(jié)構(gòu)。有不同的參數(shù)標(biāo)準(zhǔn)，習(xí)慣上用起重機(jī)上總負(fù)荷的最小值，也就是在臨界狀態(tài)點的最小壓力值。為了做這個重要的結(jié)構(gòu)壓力分析，基于實現(xiàn)這個運(yùn)算法則的控制系統(tǒng)將進(jìn)一步得到發(fā)展。 5.載荷判斷方案的發(fā)展 為了實現(xiàn)起重機(jī)結(jié)構(gòu)偏轉(zhuǎn)補(bǔ)償，需要知道起重機(jī)承受的有效載荷。因此，有必要進(jìn)行不同的載荷在線可能情況分析，這樣就可以判斷哪一個傳感器需要進(jìn)行載荷復(fù)合鑒定?；谶@種鑒定 方案分析，可以實現(xiàn)最終的運(yùn)算法則。 6. 控制運(yùn)算法則的發(fā)展 基于這種機(jī)械液壓子系統(tǒng)模型，一種吊具總成位置軌跡控制的控制規(guī)律將會得到發(fā)展。這種控制規(guī)律可以保證系統(tǒng)按照吊臂頂?shù)倪\(yùn)動軌跡運(yùn)行，并且系統(tǒng)在工作情況下保持穩(wěn)定。這包含在載荷判斷和運(yùn)動學(xué)最佳參數(shù)方案的分析中。 7. 控制系統(tǒng)的執(zhí)行 最后系統(tǒng)的控制規(guī)律已經(jīng)通過仿真試驗得出，應(yīng)該實現(xiàn)通過處理器或者數(shù)據(jù)信號處理檢驗系統(tǒng)實物了，即測試起重機(jī)。用這種測試方法將可以實現(xiàn)對系統(tǒng)制定測試，到測試結(jié)束的整個過程。這種測試技術(shù)還可以對一些典型系統(tǒng)進(jìn)行控制。 機(jī)械化和自動化 自從 18 世紀(jì)末工業(yè)革命開始，工業(yè)機(jī)械化進(jìn)程一直在不斷地發(fā)展，并且變得越來越復(fù)雜。但目前的工業(yè)自動化過程較以前的工業(yè)自動化過程有很大的不同。 20 世紀(jì)的工業(yè)自動化之所以有別于 18、 19 世紀(jì)的機(jī)械化，是因為機(jī)械化僅應(yīng)用于操縱（執(zhí)行）機(jī)構(gòu)，而自動化則涉及整個生產(chǎn)單元中的執(zhí)行和控制兩個（核心）部分。盡管不是所有的情況，但在大多數(shù)情況下，控制元件依然發(fā)揮著強(qiáng)大的力量，機(jī)械化已經(jīng)代替了手工勞動，而自動化代替了腦力勞動。 機(jī)械化程度的發(fā)展在過去和現(xiàn)在的區(qū)別不是很明顯 ，而在一端是具有強(qiáng)大辨別和控制功能的餓電子計算機(jī)，另一端是我們目前所說的“轉(zhuǎn)換機(jī)構(gòu)”正如傳輸帶一樣與其他設(shè)備簡單的連接起來。自動調(diào)整機(jī)構(gòu)能夠自動調(diào)節(jié)系統(tǒng)，也就是說，它能在沒有人干預(yù)和調(diào)整的情況下，自動對系統(tǒng)或生產(chǎn)過程進(jìn)行控制和調(diào)節(jié)?，F(xiàn)代工業(yè)技術(shù)的核心因素就是當(dāng)前人們經(jīng)常提起的反饋（控制），它是以自動調(diào)節(jié)系統(tǒng)為基礎(chǔ)，借助于系統(tǒng)偏差與期望之間的偏差來控制，可由自動檢測、測量、顯示和校正方法得到。反饋控制應(yīng)用于高速運(yùn)轉(zhuǎn)的大型數(shù)字計算機(jī)進(jìn)行復(fù)雜運(yùn)算時，對于輸入的復(fù)雜問題，計算機(jī)通常會一直運(yùn)行，直到求出與問題匹配的結(jié) 果。這或許于我們以前熟知的機(jī)器有很大的差別。同樣的，反饋是我們所熟悉的機(jī)器概念。舊式的蒸汽機(jī)安裝有離心傳感器，控制桿上的兩個小球不停的繞立軸旋轉(zhuǎn)，氣壓升高，發(fā)動機(jī)轉(zhuǎn)速變快，旋轉(zhuǎn)控制器速度增加，使立桿上升，關(guān)閉閥門，切斷蒸汽，從而發(fā)動機(jī)恢復(fù)到合適的速度。 隨著工業(yè)革命的出現(xiàn)，機(jī)械化也隨之產(chǎn)生，由于這時的機(jī)械化僅局限于單個生產(chǎn)過程。因此，需要使用人工控制每部機(jī)器及裝卸材料，并把材料從一個地方運(yùn)到另一個地方。僅僅在很少的情況下，這些生產(chǎn)過程才能夠自動地銜接起來，形成連續(xù)的產(chǎn)品生產(chǎn)線。 一般而言，從 20 世紀(jì) 20 年代 以來，盡管現(xiàn)代工業(yè)已經(jīng)實現(xiàn)了高度機(jī)械化，然而通常機(jī)械化的部分還沒有聯(lián)系在一起。機(jī)械化的工廠生產(chǎn)了光電燈泡、瓶子和大量生產(chǎn)的產(chǎn)品的元件，這些機(jī)械化工廠的自動化程度日益得到了加強(qiáng)。 20 世紀(jì) 40 年代電子計算機(jī)的發(fā)展，意味著在機(jī)械控制領(lǐng)域內(nèi)將出現(xiàn)大量比計算機(jī)更簡單、更廉價的產(chǎn)品。這些裝 置 機(jī)械裝置、氣動裝置、液壓裝置，在近些年內(nèi)已有了很大的發(fā)展，并將繼續(xù)發(fā)展下去，普通的觀點認(rèn)為這有利于自動控制的發(fā)展。當(dāng)然不僅僅電子設(shè)備對目前自動控制的發(fā)展舉足輕重，無疑在今后自動控制發(fā)展方面還繼續(xù)會發(fā)揮不可估量的作用。 液壓傳動 對于兩點之間較遠(yuǎn)的傳動，不適合用傳動帶和傳動鏈的機(jī)械系統(tǒng)，可優(yōu)先考慮采用液壓傳動，液壓傳動的優(yōu)點是：低速大力矩、機(jī)構(gòu)緊密、穩(wěn)定性高、無振動的平穩(wěn)滑動，速度和方向能靈活控制，輸出速度可實現(xiàn)無級快速變化。 由電力驅(qū)動的油泵提供有傳遞能量作用的油液，并可供給液壓馬達(dá)或油缸，從而將液壓能轉(zhuǎn)化成機(jī)械能。液壓油流動是通過控制閥進(jìn)行控制的，壓力油的作用產(chǎn)生線性的或螺旋性的機(jī)械運(yùn)動，此時的油液產(chǎn)生的動能相對低。因此，有時候使用靜壓傳動。液壓馬達(dá)與液壓泵的結(jié)構(gòu)幾乎是相同的，任何液壓泵都可以當(dāng)成馬達(dá)應(yīng)用，一定時間的流量可由調(diào)節(jié) 閥使用變量泵來控制。 一般來說液壓傳動可分為直線式的和旋轉(zhuǎn)式的，旋轉(zhuǎn)式傳動產(chǎn)生旋轉(zhuǎn)運(yùn)動，而活塞及缸體部件產(chǎn)生往返的運(yùn)動是線性運(yùn)動。 所有液壓馬達(dá)的功能基于同一個原理，壓力油被交換地擠入、擠出到油腔中，進(jìn)油循環(huán)由最小的腔體注油開始，當(dāng)油腔達(dá)到最大容積時，油腔和油路隔開，停止進(jìn)油，然后通過回油路油液返回到油箱中，同時另一個油腔開始進(jìn)油。 計算機(jī)輔助設(shè)計技術(shù) 在廣義上講，計算機(jī)輔助設(shè)計（ CAD）指的是計算機(jī)在解決設(shè)計問題中的應(yīng)用。工程技術(shù)人員可以借助于直觀顯示屏幕、鍵盤、繪圖儀和人機(jī) 接口等諸多方式與計算機(jī)通信。工程技術(shù)人員可以提出問題并能很快有計算機(jī)得到解答。更確切地說， CAD 是使工程技術(shù)人員和計算機(jī)系統(tǒng)工作，彼此發(fā)揮長處的技術(shù)。 過去，工程技術(shù)人員設(shè)計時所使用的傳統(tǒng)工具是制圖板、制圖儀、計算器和技術(shù)數(shù)據(jù)圖紙。后來，計算機(jī)的出現(xiàn)導(dǎo)致了工業(yè)中的巨大變化。隨著數(shù)字控制、計算機(jī)數(shù)字控制、機(jī)床的引入，計算機(jī)在制造業(yè)中的應(yīng)用在 20 世紀(jì) 50 年代末期首次有了實質(zhì)性進(jìn)展，通過磁帶輸入到機(jī)器中的數(shù)據(jù)控制了裝配零件的機(jī)器運(yùn)轉(zhuǎn)。這一切對工程設(shè)計者并沒有直接影響。 20 世紀(jì) 60 年代初隨著計算機(jī)輔助設(shè)計的引入產(chǎn) 生了一場重大變革。 CAD 允許設(shè)計者以圖形方式與計算機(jī)交互作用，工程技術(shù)人員能夠檢驗一個設(shè)計思想，并很快地查看到設(shè)計效果，然后對其進(jìn)行修改和重新評價。如此循環(huán)往復(fù)，直至形成一個合格的設(shè)計。每重復(fù)一次，設(shè)計方案都會得到一步的改善。因此，在時間、材料和資金允許的條件下所執(zhí)行的循環(huán)次數(shù)越多，設(shè)計效果就越好。 計算機(jī)能加快設(shè)計進(jìn)程，提高設(shè)計的精確程度。它能夠在短時間內(nèi)完成大量的、復(fù)雜的計算并得出準(zhǔn)確可靠的結(jié)果。由于在有限的時間內(nèi)某些設(shè)計所需要的大量計算不能簡單的由人來完成，計算機(jī)的上述特征證明了作為一個設(shè)計工具的作用 是無法估量的。 計算機(jī)可在磁盤或直接存儲器等永久性介質(zhì)上保存大量的信息。因此，以數(shù)字形式描述一個工程圖紙的細(xì)目或一個汽車車身的造型，并把信息存儲在存儲器中都是可以做到的。這些數(shù)據(jù)能從存儲器中檢索、快速轉(zhuǎn)換并顯示在 VDU（視頻顯示器）圖形屏幕上，或交替地利用繪圖儀繪制在圖紙上。此外，設(shè)計者還可以迅速、容易地更新或修改圖紙的任何部分。也能把修改后的圖紙數(shù)據(jù)寫回到存儲器中。 計算機(jī)輔助設(shè)計在工程技術(shù)領(lǐng)域中有著重要的作用，例如，計算機(jī)系統(tǒng)生成工程圖紙的應(yīng)用；求解復(fù)雜構(gòu)件的熱應(yīng)力問題的有限元技術(shù)的使用；機(jī)械裝置和連接 的分析及大量的輔助工程應(yīng)用。 附錄 B 外文文獻(xiàn) CONTROL OF MOBILE HYDRAULIC CRANES Marc E. MNZER Aalborg University Institute of Energy Technology Pontoppidanstrde 101 DK-9220 Aalborg. Denmark Email: mmuniet. auc. dk The goal of the thesis described in this paper is to improve the control of mobile hydraulic cranes. The thesis is split into five parts: a requirements analysis, an analysis of the current systems and their problems, an analysis of different possibiilities for system topologies, development of a new control system for the near future based on electro-hydraulic separate meter in / separate meter out valves, and finally an analysis of more advanced and complex solutions which can be applied in the more distant future. The work of the thesis will be done in cooperation with industry so the thesis will have more of an industrial focus than a purely theoretical focus. Key words: Mobile Hydraulic Cranes, Control strategies, Separate Meter-in/Separate Meter-out. 1 INTRODUCTION The goal of the thesis described in this paper is to improve the control of mobile hydraulic cranes. A mobile hydraulic crane can be thought of as a large flexible mechanical structure which is moved by some sort of control system, The control system takes its input from a human operator and translates this command into the motion of actuators which move the mechanical structure. The definition of this control system is purposely left vague in order not to impose any constraints on its design. The control system consists of actuators which move the mechanical structure, a means of controlling the actuators, a means of supplying power to the actuators, and a way of accepting inputs from the operator. It is this control system which is the target of this thesis. The goal is to analyze the requirments made on the control system and present guidelines for the gesign of new control systems. The thesis will be split into five parts: 1. Analysis of the requirements of the control system, from the perspective of the operator, the mechanical system, efficiency, stability, and safety requirements. 2. Analysis of current control systems and what their problems are. 3. Analysis of the different options for the control system: different types of actuators different types of control strategies, and different ways of organizing components. 4. Presentation of a new type of control system, which is commercially implementable. A system that will meet the needs of industry in the near future. 5. Analysis of more optimized systems, with higher performance, better efficiency, more flexible control, etc. This will be less commercially applicable but will be a starting point for more research. 2 SECTIONS OF THE THESIS 2.1 Requirements Analysis of the Control System Before starting detailed work on developing new control systems, it is important to analyze what the exact demands are on the control system. The control system is influenced by many factors.For example: the mechanical structure it is controlling, the human operator, efficiency, stability, and industry requlations. Industry regulations are the first requirements that have to be addressed. Things like hose rupture protection and runaway load protection make a lot of demands on the control system. After regulations, stability is the next most important requirement； without stability the control system cant be used. Once stability has been assured, the performance requirements of the control system have to be set. They are determined by the mechanical structure of the crane and the human operator. The mechanical structure of a mobile hydraulic crane is a very necessary to keep the speed of the control system below this natural frequency or to develop a control system which can increase this frequency. The human operator also impossible limits on the control system. If the control system is too slow or too fast then it is impossible for a human operator to give it proper inputs. And finally, once the requlations have been met, stability is assured, and the performance is at the right level, the power efficiency of the control system has to be optimized. 2.2 Analysis of Current Control Systems Before designing a new control system it is good to analyze the current control systems to find out what their problems are. Current control systems are mainly hydraulic and can suffer from three main problems: 1. Instability 2. High cost 3. Inefficiency 2.2.1 Instability Instability is a serious problem as it can cause injury to human operators or damage to equipment. When a system becomes unstable it usually starts to oscillate violently. To avoid instability in current systems, the designers either sacrifice certain functions which are desirable, or add complexity and cost. For example, in the crane shown in Figure 1, it would be desirable to have control over the speed. But due to the safety system that cranes are required to have, standard speed control is not stable. To add speed control requires a more complex and more expensive mechanical system. The parameters of a hydraulic system, such as temperature or load force, also affect stability. A system that is stable with one set of parameters might be unstable with another set. To ensure stability over the entire operating range of the system, performance must sometimes be sacrificed at one of the parameter range. 2.2.2 High cost Current systems are purely hydraulic-mechanical, so if the user wants a certain function, the user buys a certain hydraulic-mechanical component. Because most user have different requirements, there are many different variations of the same basic component. This means that many specialized components must be manufactured rather than one standard product. This drives up the cost of components. 2.2.3 Inefficiency One form of inefficiency in current systems is due to the link between the flows of the two ports of the cylinder. This is because most valves use a single spool to control the flow in both ports. Because of this link, it is impossible to set the pressure levels in the two sides of the cylinder independently. Therefore, the outlet side will develop a back pressure which acts in opposition to the direction of travel, which increases the pressure required on the inlet side to maintain motion. Since the force generated by the actuator is proportional to the pressure difference between the two sides, the actual pressures in the cylinder dont affect the action of the cylinder. For example, the action of the cylinder for 0psi/600psi would be the same as 1000psi/1600psi. However, in the second case, the power supply would have to supply much more power. This extra power is wasted. 2.3 Different Options for Control Systems Current control systems use hydraulic actuators with directional/proportional valves to control the movement. However there are many different options for controlling a cylinder. Options range from new high performance electro-hydraulic valves, to separate meter in / separate meter out (SMISMO) valves, to hydraulic bus systems, to intelligent actuators with built in power supplies, to pump based control strategies. These systems all have advantages and disadvantages which need to be analyzed if the most optimum solution is to be chosen. 2.4 Near Future Solution It is expected that even if it is proven that a completely new system topology is the optimum configuration, the crane manufacturers and component manufacturers will not accept the new technology overnight. This will most likely take time, so an interim solution will be developed. This solution will be made up of micro computer controlled Separate Meter In / Separate Meter Out (SMISMO) valves (Elfving, Palmberg 1997； Jansson, Palmberg, 1990； Mattila, Virvalo 1997). SMISMO valves will make it possible to implement new control strategies which are more efficient and stable. The micro computer will make it possible to introduce flexibility to valves. Variants can be programmed in software. This eliminates the need to manufacture hundreds of different variants. The crane manufacturer will be able to choose the exact functions he wants in his valve, while the component manufacturer will have to manufacture only one valve. This will lower the cost, even though the performance will have increased. 2.5 Analysis of Higher Performance Solutions This analysis will depend on the results of the analysis of different topologies. If it is shown that pump based control is to be the way of the future for example, then analysis will be performed in this area. Another area which will also be explored, is tool position control. 3 LABORATORY FACILITIES As the focus of this thesis is on developing control strategies that can be implemented on commercial machinery, much emphasis will be placed on experimental results. Experimental results will be obtained from two systems. The first, a simple one degree of freedom crane, was designed as an experimental platform. The second is a real crane which was donated to the University by Hojbjerg Maskinfabrik (HMF) a Danish crane manufacturer. Refer to Figure 1. Figure 1 Experimental Systems in Laboratory. Left: One DOF crane model. Right: Real Mobile Hydraulic Crane As there are currently no commercially available separate meter-in/separate meter-out valves, two separate valves will be used instead. A sample circuit of one cylinder is shown in Figure 2. The control algorithms which control the valves, will be programmed on a Digital Signal Processor (DSP)/Pentium dual processor system. The DSP will run the control code and the Pentium will do diagnostics and provide a graphical user interface. Figure 2 Separate Meter In / Separate Meter Out Setup 4 CURRENT WORK 4.1 Flow Control by Direct Actuation of the Spool Most flow control valves on the market today work with a pressure compensator (Andersen； Ayers 1997). The pressure compensator keeps a constant pressure drop across the main spool of the valve, which keeps the flow constant. However, the addition of a pressure compensator makes the valve more complicated than a simple single spool valve. Another way of doing flow control is to measure the pressure drop across the valve and adjust the spool position to account for this (Back； Feigel 1990). This is not a new idea but has not been implemented commercially because of the high cost of pressure transducers and micro controllers. However, with the current drop in cost of micro controllers and pressure transducers this idea is now commercially feasible. The concept is very simple, spool position is calculated from the Bernoulli equation using the pressure drop across the spool and reference flow. Even though this is a simple equation, it is not easy to implement. The accuracy of the flow control is dependent on the precision of the position sensors and of the pressure transducers. Noise on the pressure or the position signals can cause stability problems. Filtering the noise, introduces delays in the control which can also affect stability. In addition the Bernoulli equation is not followed exactly over the entire operating range of the valve, so it may be necessary to store the valve characteristics as a data table or develop a more complex equation. 4.2 Cylinder Control Strategy To control a hydraulic cylinder, the strategy has to be able to handle four different situations depending on the directions of the load and the velocity of the cylinder. Refer to Figure 3. Figure 3 Different Situations in Crane Operation The control strategies that have appeared in the literature are usually quite complex and depend on measurements of the cylinder position and velocity (Elfving, Palmberg 1997； Mattila； Virvalo 1997). They are also based on rather complex control algorithms. It is the goal of this thesis to start with a control strategy which is based on simple PI controllers and makes no demands for position and velocity of the cylinder. The performance of this system will be lower than a complex control strategy, but it may be easier to implement commercially because it has no need for special sensors and is easier to understand for the average engineer. Another feature which needs to be acknowledged when designing a control strategy, is the type of valve used. Mobile hydraulic valves demand low leakage and since most mobile valves are spool valves, they usually have large overlaps. In addition, to make the cost of the valve acceptable to industry, the actuation stage on the spool is usually quite slow. This combination of large overlap and slow actuation makes it hard to implement many of the strategies that have been presented. Pressure control especially becomes difficult when there is an overlap and a slow actuator. One example of a new strategy which is simple and robust is described as follows. Flow control is implemented on the inlet side and pressure control is implemented on the outlet side. The flow control is based on the Bernoulli equation. Pressure control is done by PI controller which maintains a low constant pressure to increase the efficiency and prevent cavitation. To work around large overlaps and slow actuation stage, the pressure controller only does meter out control. This means that if the controller wishes to raise the pressure, it cant add flow to the cylinder, it can only decrease the opening of the meter out port. The benefit of this is that the only time that the spool has to cross the zero position is when the operator wishes to change the direction of motion of the cylinder. For the case where the load force and the velocity are in the same direction, this strategy has to be modified. In this case, the pressure reference of the pressure controller at the outlet is increased to a value which opposes the load force. The pressure reference is increased when it is noticed that the pressure of the inlet side is dropping. The pressure reference is also controlled by a PI controller. A schematic model of the controller system for the load lowering case is shown in Figure 4. At the time of writing this paper the initial experimental tests had performed on the real crane shown in Figure 1. Stability was not achieved because the crane is equipped with a load holding valve. However, the load holding valve will be replaced with a pilot operated check valve, which should make it possible to stabilize the system. In current systems, the load holding valve serves two functions, load holding and runaway load protection. Due to the use of a SMISMO valve setup, the runaway load protection is built into the control strategy, therefore the only function which is necessary for the load holding valve to perform is load holding. A pilot operated check valve will be able to do this, without adding complex dynamics which upset the stability of the system. Figure 4 Controller Strategy for Lowering of Load 5 CONCLUSION Even though not much experimental work has been finished, a good start has been made and initial tests have been promising. The outline of the thesis has been developed and organized in a logical manner. The work is split into five parts, requirements analysis, analysis of current systems, analysis of different topologies, development of a near future solution, and development of a more optimum solution. At the end of the thesis, the control of mobile hydraulic cranes will have been improved. 6 ACKNOWLEDGEMENTS This project is being funded in part by Danfoss Fluid Power A/S. The author would also like to thank Hojbjerg Maskinfabrik (HMF) A/S for the donation of the test crane. 7 REFERENCES Andersen, B. R.； Ayres, J. L. (1997). Load Sensing Directional Valves, Current Technology and Future Development, The Fifth Scandinavian International Conference on Fluid Power Back, W.； Feigel, H. (1990). Neue Mglichkeiten Beim Elektrohydraulischen Load-Sening, O+P lhydraulik und Pneumatik 34 Elfving, M.； Palmberg, J. O. (1997). Distributed Control of Fluid Power Actuators-Experimental Verification of a Decoupled Chamber Pressure Controlled Cylinder, 4th International Conference on Fluid Power Jansson, A.； Palmberg, J. O. (1990). Separate Controls of Meter-in and Meter-Out Orifices in Mobile Hydraulic Systems, International Off-Highway and Powerplant Congress and Exposition Mattila, J.； Virvalo, T. (1997). Computed Force Control of Hydraulic Manipulators, 5th Scandinavian International Conference On Fluid Power Trajectory Control of Mobile Hydraulic Crane EMSD 9/10 - 69C Problem Description This project takes its base in the problem of controlling mobile hydraulic cranes with multiple degrees of freedom, such as the one shown in figure 1. Controlling a mobile hydraulic crane takes a highly trained operator as it is often operated in areas with little space for maneuverability. Modern cranes are sometimes fitted with radio control so that if possible, the operator can be placed close at hand of where the load must be positioned. Still only one degree of freedom is controlled per button/handle. Therefore only if the operator has been sufficiently trained he/she may control two or more degrees of freedom at a time by operating more buttons. Figure1 Drawing showing a example of a hydraulic loader crane, for mounting on lorry. Only parts of the hydraulical system is sketched. The aim of this project is to develop a control system for a mobile hydraulic crane so that less training of the operator is needed. This is incorporated through trajectory control of the tool center of the crane by operating a joystick only. In this way multiple degrees of freedom are controlled simultaneously. Mobile hydraulic crane structures like the one depicted in figure 1 are normally also very flexible, i.e. they bend when they are loaded. This is due to highly optimized constructions regarding material usage, in order to keep the weight down. As it is the position of the tool center that is controlled the control system should also compensate for this structural deflection. This way by having an adequately good control system Which compensates for deflection, the system may also eliminate the possibilities for the operator to initialize oscillations in the structure. Making use of a trajectory control system with compensation for structural deflection will therefore expand the possibility of utilising the crane to its maximum regarding loading capability. In long term this may give the opportunity to lower the crane will be damped by the control system. All together this results in advantages for both manufacturer and end user advantages through a higher cost/capacity-ratio and a more easily controlled system. Project Contents The problem described will practically be delt with using a HMF 680-4 mobile hydraulic crane, a picture of this may be seen in figure 2. The crane is mounted with sensors for monitoring different parameters in the system, which are the most important pressures, flows, strains and relative link positions of the crane. This crane will be the basis for the experimental testing and verification, and therefore also for the mathematical models derived. In order to fulfil the above described problem several subjects has to be covered, in short these are: Figure 2 Picture of the text crane. 1. Analysis and specification of the demands for the system An analysis of performance criterias for the system is to be made. Based on this demands for the system will be specified along with testing procedures for the system to ensure the system fulfil the demands. 2. Modelling of the mechanical subsystem Many different modelling techniques exist, and therefore this part includes studying formulations methods for modelling multi-body dynamics. In particular an analysis of how to model the deflections in the mechanical subsystem should be made. The purpose is to arrive at a formulation which is computational efficient, but at the same timesufficiently accurate in describing the behaviour of the mechanical system, in order to include it in the control strategy. Based on the most promising formulation a mathematical model of the subsystem will be made, and it will be verified through experimental results obtained from the test crane. 3. Modelling of the hydraulic subsystem As well as mechanical system should be modeled, so shall the hydraulic subsystem, which consists of a pump, different valves, actuators and hoses. However, not all of these will be modeled, but only the components which have significant influence on the dynamical properties of the system. Also the model of the hydraulic subsystem shall be verified experimentally. Besides this it must be analysed whether or not the bandwidth of the controlling proportional valves are sufficiently high for using these in the control system when compensating for deflections, i.e. oscillations in the mechanical structure. Based on the above modifications to the hydraulic systems must be made if it is found necessary. 4. Analysis and criterias for solving inverse kinematic configurations The crane has a given space, measured relative to its base, in which it can operate, i.e. which the tool center can reach. This is known as the workspace of the crane. Some parts of the workspace may be reached in several different ways. Therefore it is necessary to determine the optimal kinematic configuration of the crane for these areas. There may be different criterias for optimisation, here one is sued which minimises the overall load on the crane, i.e. minimizes stress at critical points. In order to do this an analysis of the stress in the structure must be made and based on this an algorithm for implementing in the control system will be developed. 5. Development of load identification scheme In order to compensate for the structural deflections in the crane, the payload carried by the crane needs to be known. Therefore an analysis of the different possibilities for online identification of the load is necessary, this includes considering which sensors are needed and how complex the load identification will be. Based on the analysis an identification scheme is to be made, which may be implemented in the final control algorithm. 6. Development of control algorithm Based on the models of the mechanical and hydraulic subsystems a control law for the position trajectory control of tool center shall be developed. This control law must ensure that the system behaves as specified through control of the toll center, and that the system will be stable under all working conditions. Included in this is also the inclusion of the load identification and the kinematic optimization schemes. 7. Implementation of the control system Finally the control law developed for the system which has been tested through simulations, should be implemented in a microprocessor of DSP and verified on the physical system, i.e. the text crane. This will be done by experimentally testing it against the demands through the specified test procedure for the system. Based on these experiments it will be determined, what is attainable with the given technology for these type of systems, regarding control possibilities. Mechanization and Automation Processes of mechanization have been developing and becoming more complex ever since the beginning of the Industrial Revolution at the end of the 18th century . The current developments of automatic processes are , however , different from the old ones . The “automation” of the 20th century is distinct from the mechanization of the 18th and 19th centuries in as much as mechanization was applied to individual operations , whereas “automation” is concerned with the operation and control of control is go great that whereas And in many ,though not all , instances the element of control is so great that whereas mechanization displaces muscle , automation displeases brain as well . The distinction between the mechanization of the past and what is happening now is , however , not a sharp one . At one extreme we have the electronic computer with its quite remarkable capacity for discrimination and control , while at the other end of the scale are “transfer machines” , as they are now called , which may be as simple as a convey or belt to another . An automatic mechanism is one which has a capacity for self-regulate ； that is , it can regulate or control the system or process without the need for constant human attention or adjustment . Now people often talk about “feedback” as being an essential factor of the new industrial techniques , upon which is based an automatic self-regulating system and by virtue of which any deviation in the system from desired conditions can be detected, measured , reported and corrected . When “feedback” is applied to the process by which a large digital computer runs at the immense speed through a long series of sums , constantly rejecting the answers until it finds one to fit a complex set of facts that have been put to it , it is perhaps different in degree from what we have previously been accustomed to machines . But “feedback”, as such , is a familiar mechanical conception . The old-fashioned steam engine was fitted with a centrifugal governor , two balls on levers spinning round and round an upright shaft . If the steam pressure rose and the engine started to go too fast , the increased speed of the spinning governor caused it to rise up the vertical rod and shut down a valve . This cut off some of the steam and thus the engine brought itself back to its proper speed . The mechanization , which was introduced with the Industrial Revolution , because it was limited to individual processes , required the employment of human labor to control each machine as well as to load and unload materials and transfer them from one place to another . Only in a few instances were processes automatically linked together and was production organized as a continuous flow . In general , however , although modern industry has been highly mechanized ever since the 1920s , the mechanized parts have not as a rule been linked together . Electric-light bulbs , bottles and the components of innumerable mass-produced articles are made in mechanized factories in which a degree of automatic control has gradually been building up . The development of the electronic computer in the 1940s suggested that there were a number of other devices less complicated and expensive than the computer which could share the field of mechanical control . These devices mechanical , pneumatic and hydraulic have been considerably developed in recent years and will continue to advance now that the common opinion is favoring the extension of “automation” . Electronic devices , of course , although not the sole cause of what is happening , are nevertheless in a key position . They are gaining in importance and unquestionably hold out exceptional promise for development in the future . Hydraulic Power Transmission Hydraulic drives are used in preference to mechanical system when power is to be transmitted between points too far apart for chains or belts ； high torque at low speed is required ； a very compact unit is needed ； a smooth transmission , free of vibration , is required ； easy control of speed and direction is necessary ； or output speed must be varied steplessly . Electrically driven oil pressure pumps establish an oil flow for energy transmission , which is fed to hydraulic motor or hydraulic cylinder , converting it into mechanical energy . The control of the oil flow is by means of valves .The pressurized oil flow produces linear or rotary mechanical motion . The kinetic energy of the oil flow is comparatively low , and therefore the term hydrostatic driver is sometimes used . There is little constructional difference between hydraulic motor and pumps . Any pump may be used as a motor . The quantity of oil flowing at any given time may be varied by means