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中文譯文
隨車液壓起重機的控制
摘 要：本文主要是描述隨車液壓起重機的控制過程。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，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控制系統(tǒng)的改進方法
隨車汽車起重機可以看成是一種大型柔性控制機械結(jié)構(gòu) 。這種控制系統(tǒng)把操作人員的命令由機械結(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)管破裂保護和超載保護有同等的地位。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)性能要求就可以進一步確定。機械結(jié)構(gòu)決定了起重機的可操作性。機械機構(gòu)是隨車起重機中可以往復(fù)轉(zhuǎn)動固有頻率低的大型柔性結(jié)構(gòu)。
為了防止起重機振動，必須使起重機在固有頻率下工作，或者提高起重機的固有頻率。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)計一種新的起重機之前，研究目前起重機存在的問題是很有必要的。當(dāng)前液壓隨車起重機主要存在以下三個問題：
1.不穩(wěn)定性
2.不經(jīng)濟性
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)雜的機構(gòu)。如圖1所示為一種起重機，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)運行穩(wěn)定，有時必須降低次要的參數(shù)值。
2.2.2 不經(jīng)濟性
目前的液壓系統(tǒng)是純液壓的機械系統(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和升縮機構(gòu)組成。這種離合閥可使這種更加高效穩(wěn)定的執(zhí)行控制機構(gòu)得以實現(xiàn)。微型計算機可以對閥進行柔性控制?？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)域進行分析。不同的區(qū)域?qū)⒂貌煌姆椒ㄌ接懀貌煌牡毒呶恢每刂啤?
3. 實驗設(shè)備
本文的中心是研究發(fā)展中的經(jīng)濟型機械控制方案的可實現(xiàn)問題，更多重點是先進的實驗結(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實驗臺獲得，第二種是通過研究一臺由丹麥一家起重機廠送給英國的一所軍校的起重機獲得。如圖1所示
圖1系統(tǒng)實驗臺 左：單自由度起重機模型 右：隨車起重機實物
雖然目前這種升縮分離機構(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行程序來控制液壓閥的。由數(shù)字信號處理器運行控制代碼，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償。壓力補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2升縮分離機構(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。
圖3起重機工作的不同情形
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動仍然不能改變，這種方式需要改進。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)。
圖4減壓控制器
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，由于起重機上安裝了載荷單向閥，所以穩(wěn)定性沒有達到要求。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)超載保護和卸載保護兩種功能。由于在這種控制方法中使用伸縮閥機構(gòu)對卸載保護很起作用，因此在起升機構(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的傾。安裝了這種單向閥，起重機操作人員不需要再增加更復(fù)雜的外力來防止起重機產(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，液壓隨車起重機的控制模將會被修改。
隨車液壓起重機的軌跡控制
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控制問題提出來的。控制隨車起重機要求操作人員技術(shù)相當(dāng)高，它的操作機動范圍很小。如果可以讓現(xiàn)代的起重機實現(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。
吊具總成
圖1所示為一臺隨車液壓裝載起重機部分液壓系統(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。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。
圖2測試起重機圖片
多數(shù)隨車液壓起重機的結(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)軌跡偏差補償控制系統(tǒng)在起重機上進一步發(fā)展，起重機的裝載能力將可以大大得到提高。當(dāng)這種在起重機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)的機會。這將使起重機生產(chǎn)商和用戶節(jié)省一大筆費用。
方案內(nèi)容
現(xiàn)以一臺如圖2所示的HMF 680-4型隨車液壓起重機來分析這些問題。在這臺起重機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性能，所以可以通過精確的模型來測試起重機的性能。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?；谙到y(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)的表達方法。機械子系統(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出來，同時使系統(tǒng)的動作在控制過程中能夠十分精確?；谶@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行補償時，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，即對機械結(jié)構(gòu)的擺動進行分析?；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)運動結(jié)構(gòu)
起重機相對于底部有一個可以操作的特定空間，即吊具總成能達到的范圍。這是公認(rèn)的起重機工作范圍。有的部位要通過不同的路線才可以達到。因此有必要在這些區(qū)域確定最佳的運動結(jié)構(gòu)。有不同的參數(shù)標(biāo)準(zhǔn)，習(xí)慣上用起重機上總負(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)這個運算法則的控制系統(tǒng)將進一步得到發(fā)展。
5.載荷判斷方案的發(fā)展
為了實現(xiàn)起重機結(jié)構(gòu)偏轉(zhuǎn)補償，需要知道起重機承受的有效載荷。因此，有必要進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)最終的運算法則。
6. 控制運算法則的發(fā)展
基于這種機械液壓子系統(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\動軌跡運行，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。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)進行控制。
外文文獻
CONTROL OF MOBILE HYDRAULIC CRANES
Marc E. MüNZER
Aalborg University
Institute of Energy Technology
Pontoppidanstr?de 101
DK-9220 Aalborg. Denmark
Email: mmun@iet. 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 can’t 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 don’t 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 (Backé; 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 can’t 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 holdin