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系統(tǒng)辨識與實時控制斗輪式裝載機的液壓系統(tǒng)土方設(shè)備
摘要
土方設(shè)備行業(yè)的迅速整裝待發(fā)，準備在近期實現(xiàn)數(shù)字化控制技術(shù)在其產(chǎn)品的快速部署的效率，性能，安全性和操作舒適巨大收益。世界上主要有兩種類型的移動設(shè)備操作的最多:挖掘機和輪式裝載機。現(xiàn)在挖掘機已受到業(yè)界的關(guān)注。輪式裝載機產(chǎn)品在本文研究的是另一種高容量多功能機在配置頻譜的另一端的例子。一個先進的電液開放中心的非壓力補償閥控制系統(tǒng)的狀態(tài)進行了研究，以評估通過實施數(shù)字化速度伺服控制的潛在收益?？刂颇繕耸菨M足運營商自覺響應(yīng)要求，滿足運營商認為光滑度要求，創(chuàng)建一個子系統(tǒng)，可以接受命令弗勒曼自治區(qū)高層次的規(guī)劃控制。
數(shù)字化速度的閉環(huán)控制是成功實施的一個貨架輪式裝載機采用標準比例積分（PI）和閥的動態(tài)變換算法的議案。動態(tài)轉(zhuǎn)換閥是液壓流量功能，是一種發(fā)動機轉(zhuǎn)速和氣缸壓力桿端功能。魯棒性的性能進行了驗證，通過廣泛的系統(tǒng)建模，驗證，并在大卡特彼勒輪式裝載機型號的硬件測試。
簡介
汽車行業(yè)已經(jīng)在效率，性能，安全性巨大收益和乘客由最近在其產(chǎn)品的數(shù)字化控制技術(shù)的廣泛和快速部署的舒適度。地球發(fā)展的行業(yè)正在迅速整裝待發(fā)，準備在短期內(nèi)實現(xiàn)類似的收益。主要有兩種類型的推土設(shè)備：挖掘機和輪式裝載機。長遠目標是建立一個獨立的產(chǎn)品，不再操作技巧和耐力依靠最大限度地提高性能。衡量其性能的噸/小時，該材料的處理最小化的運作在成本/噸的形式加工材料成本的形式。我們的目標是開發(fā)控制子系統(tǒng)，改善經(jīng)營人/機的性能，降低運營成本，這將作為較低級別控制子在自治區(qū)控制器的等級制度。
輪式裝載機（WTL中）有許多大小。工作重量范圍從15000-350000和馬力馬力范圍從100-1200。小到中型機器的應(yīng)用程序（例如，施工和材料處理應(yīng)用）最廣泛而較大的機器往往是用于礦山的應(yīng)用為主。 WTL的一個常見的功能是利用卡車裝載?？ㄜ囇b載周期是一個重復(fù)的四個步驟，其中一些類型的材料是從股票樁運到卡車上。這個過程開始時，運營商的股票樁公羊和命令的聯(lián)系，以提升負載，而在同一時間向后搖桶（步驟1：旅行，股票樁和挖）。當水桶已滿載，逆向操作的變化和旅行轉(zhuǎn)向時向后一個位置，使他有足夠的空間，然后轉(zhuǎn)移到前進，前往卡車。經(jīng)營者繼續(xù)這次旅行期間提出的部分，以便它清除車床上時，他減慢，達到卡車（步驟2：前往卡車）負載。運轉(zhuǎn)員接著命令桶傾倒從而釋放負載到卡車床（步驟3：轉(zhuǎn)儲）。最后，操作命令把水桶架回其水平位置。同時，旅游經(jīng)營者進入反向變化和命令的聯(lián)系，以降低再挖周期（步驟4：行程開始位置）返回地面。
本研究的重點是數(shù)字化速度的閉環(huán)控制的執(zhí)行情況在執(zhí)行子一個先進的輪式裝載機移動設(shè)備的接地電流狀態(tài)系統(tǒng)。這項技術(shù)的應(yīng)用不僅限于WTL以及實際上已經(jīng)到了地球的各種移動設(shè)備的廣泛應(yīng)用。挖掘機
液壓子系統(tǒng)的控制問題已引起廣泛關(guān)注最近有液壓子在一般系統(tǒng)。
描述輪式裝載機的子系統(tǒng)
土方設(shè)備可細分為4個子系統(tǒng)。（1）動力裝置，（2）制動系統(tǒng)，（三）工作裝置，（4）液壓傳動器。上電列車由一個電源，通常是一個柴油發(fā)動機。電力傳輸?shù)揭环N通過液力變矩器然后連接到差距機械傳動，驅(qū)動器和最后輪胎。這通常是對WTL案件。挖掘機有水文靜態(tài)驅(qū)動列車（即，液壓泵和馬達）連接到一個軌道。幾個發(fā)動機功率起飛通過泵提供動力轉(zhuǎn)向液壓系統(tǒng)運行，制動系統(tǒng)是典型的液壓\和液壓執(zhí)行系統(tǒng)。該液壓驅(qū)動系統(tǒng)包含地面從事工具，提供了力量和運動，從事土壤或其他材料需要處理。
2.1．自由車度
出租車經(jīng)營者被認為有六個自由度：三線性（前，后部，橫向，縱向）和三個角（偏航，俯仰，roll0。鏟斗有兩個自由度。督導(dǎo)是一種額外的自由程度。因此，車輛有著9年的自由程度。為了簡化即將舉行的分析，兩
限制將在系統(tǒng)規(guī)定。首先，前幀運動不會允許旋轉(zhuǎn)相對于后車架。第二，后車架議案將限制在一個平面上。第一個約束消除了單自由度（轉(zhuǎn)向）學(xué)位從分析中。第二個約束消除了三自由度：運營商橫向直線運動，偏航角和滾動角運動的議案。因此，我們認為只有五個自由度在我們的模型：電梯和傾斜落實桶議案和操作前，尾部和垂直直線運動和俯仰角的議案。
2.2. 連鎖
有幾種類型的WTL的實現(xiàn)使用目前的聯(lián)系。一個非常普遍的例如，所謂的Z型連桿。它是自由的兩個學(xué)位四體聯(lián)動（提升臂，杠桿，鏈接，桶）和兩個組成的非對稱液壓缸（升降機及傾斜），由九個旋轉(zhuǎn)腳關(guān)節(jié)連在一起。
2.3. 主要液壓系統(tǒng)
一個普通液壓系統(tǒng)通常用來實現(xiàn)控制流量的電梯WTL的傾斜和一缸采用開放中心非壓力補償閥芯型閥。這個系統(tǒng)包含一個水泵/釋壓閥的流體傳送到主
換向閥從而分區(qū)到液壓缸和坦克流。
組件和它們的運作是最好的形容按照液壓流體通過各種運行條件下的電路。最簡單的條件是在沒有命令從流腦放大器電流。該電梯線軸和傾斜，因為將集中在E/小時閥電磁鐵將不會被激活。在這種情況下，通過發(fā)送泵單向閥的負載流量（最大操作設(shè)置壓力）的傾斜閥芯。由于這閥芯為中心，流動收益上的電梯閥芯，這也是為中心，返回循環(huán)水箱。命名為“開放中心“的事實，即當閥門在中立的立場來，流體循環(huán)從通過到罐的閥門泵。
如果傾斜閥芯中心\及電梯閥芯右移，一開口就罐區(qū)稱為泵罐區(qū)（星期三）而受到限制。泵的壓力建立起來，克服了負載止回閥發(fā)送流向頭端（HE）的對在另一口的升降油缸稱為至氣缸區(qū)（PC）的泵。在同時，流體從桿端（重新）電梯在另一口缸流量所謂的氣缸罐區(qū)（CT）和再循環(huán)，再進行坦克。因此，
電梯活塞桿延伸提高一個可能在桶中的負載。如果電梯閥芯左移相反的情況。從他流體流經(jīng)對坦克的CT，從泵流體流動以及整個閥芯如果沒有足夠的轉(zhuǎn)移完全關(guān)閉的PT太平洋橫跨可再生能源的電腦。因此，電梯活塞桿縮回桶降低到地面。如果電梯閥芯居中和傾斜閥芯被激活時，傾斜油缸的行為類似于電梯缸。
在WTL車輛，電梯管道與閥芯系列傾斜。此配置被稱為傾斜優(yōu)先事項，因為電路的傾斜流量需求可以通過預(yù)約和關(guān)閉電梯電路。此外，氣缸安全閥可能被添加到每個RE和他如果每個部分各不相同的最高工作壓力的主要救濟
結(jié)構(gòu)壽命或安全的關(guān)注?；瘖y閥往添加到這些系統(tǒng)很好。這些止回閥提供缸罐流向稀土在事件或作真空操作過程中創(chuàng)建。這樣，空化可以顯著減少。這是如電梯武裝以重力驅(qū)動的功能降低或問題桶中的CT傾倒區(qū)已設(shè)計提供限制產(chǎn)量快速缸速度。在這種情況下，泵的流量與氣缸無法比擬從氣缸流向低壓槽和空化創(chuàng)造。這是非常以來的液壓系統(tǒng)可控的閉環(huán)控制是不可取空蝕過程中有效地失去了。
2.4. 電液壓先導(dǎo)
一個先導(dǎo)泵供應(yīng)流到壓力調(diào)節(jié)閥，它維護一個穩(wěn)定供應(yīng)的壓力，一個E/小時閥這也是連接到油箱。細胞外基質(zhì)中的驅(qū)動程序發(fā)送到電磁鐵電流其中移動一個控制閥芯。由于這閥芯移動，一米的連接口供應(yīng)壓力端口和一個節(jié)流孔連接到油箱比例開放保持或接近控制壓力。這種壓力作用在主閥芯造成它移位，打開主孔區(qū)（即，電腦斷層，鉑，電腦）。在某些情況下，位置主閥芯是用來提供反饋閉環(huán)位置控制閥芯。
2.5．數(shù)字化控制系統(tǒng)
基本低成本的組件將被選為這項研究將與一致目前的做法在這個行業(yè)。通常情況下，7位微處理器，其中使用大會編碼須達到20毫秒循環(huán)時間。旋轉(zhuǎn)式電位器用于傳感器反饋以及參考輸入信號。
3） 動態(tài)模式：平面傾斜和車輛動態(tài)電路
傾斜的動態(tài)電路模型，從投入產(chǎn)出關(guān)系點的觀點。輸入是傾斜電路閥芯的位置。的輸出是：（1）由于角速度斗缸位移傾斜，（2）平面運動（X,?，θ）的車輛。
下面的假設(shè)和近似作出的模型：
（1）電梯電路以象征式升降機固定角度。因此，我們期望獲得不同面值不同傾斜位置電梯電路電路模型。
（2）車輛被建模為一個在二維空間質(zhì)量彈簧阻尼器系統(tǒng)，有三個自由度：的x，y，美國這是一個相當不錯的，因為車身逼近和輪胎像一個大眾彈簧阻尼系統(tǒng)。（3）軟管量和水力損失包括在水力模型。
（4）電液閥是一個二階濾波器，包括動態(tài)0.707阻尼比。這是符合實際的英/ H閥使用行為是一致的在WTL車輛。
（5）電子/小時閥芯區(qū)（稱為計量）不是閥芯位置的線性函數(shù)。閥芯面積幾何精確建模為非線性函數(shù)的閥芯位置。
（6）標準孔板方程來描述流程之間的關(guān)系率（Q）和閥芯位置（因此測光區(qū)域），壓差時，ΔP。
（7）靈活性，由于石油是考慮到壓縮為體積模量液壓油。閥芯位置之間的水桶和角速度的輸入輸出關(guān)系
獲得三個逼近階段：
（1）穩(wěn)態(tài)之間的閥芯位移和傾斜缸輸入輸出功能傾斜速度被稱為調(diào)制。該模型的非線性靜態(tài)捕捉死區(qū)的幾何關(guān)系，包括與閥門缸增益。請注意有效的死區(qū)和增益是（1）流量的功能（這是一個功能發(fā)動機轉(zhuǎn)速在WTL的情況），和（2）外部負載。換句話說，直流增益?zhèn)鬟f函數(shù)是一個流量，外部負載的非線性函數(shù)，名義電梯電路的位置。
（2）線速度之間的傾斜缸和鏟斗幾何線性關(guān)系速度是描述聯(lián)動雅可比。這是表示為一系列針對不同的電梯位置曲線（插表作為實時實現(xiàn)的）。
（3）最后，我們從后臺處理模型的液壓電路的動態(tài)過濾效果位置傾斜缸速度。穩(wěn)態(tài)增益在獲取信息的調(diào)制和傾斜傾斜運動學(xué)模型。因此，在特區(qū)動態(tài)模型的傾斜收益將約為團結(jié)（0分貝）。此外，這將是電梯的位置和外部負載的函數(shù)。因此，所有三個模型塊組件方面的評估，預(yù)計全掃一電梯的位置和負載值。é/小時閥是一個開放的中心型閥。由于閥芯命令轉(zhuǎn)變的閥芯，鉑（泵罐）區(qū)開始關(guān)閉，電腦（泵缸）面積開始打開一樣的C- T的（缸罐）區(qū)。泵的壓力開始建立自的P- T是限制其流動油箱。當泵的壓力超過了何氣缸壓力，負載單向閥泵的流量持久性有機污染物允許進入氣缸他說。這種流動延伸圓柱的部隊從各地稀土流缸在C - T區(qū)油箱。由于外部負載變化時，閥芯的最低金額需要匹配通過泵壓變化，以及不同加載命令。這結(jié)果有效的死區(qū)和增益變化。如果外部負載相反（即超過運行負荷狀態(tài)），延長了氣缸率將主要取決于除兩端的壓降壓降的CT區(qū)的CT是這樣的：流量比泵進入氣缸的何流更少。接著率將延長泵驅(qū)動。
（4）閉環(huán)控制系統(tǒng)的性能目標
液壓系統(tǒng)元件（泵，油缸，伺服閥）的大小，以便它們能提供必要的功率級（流量和壓力）為移動的WTL是設(shè)計用來處理負載范圍內(nèi)桶。在這里，我們將不再重復(fù)液壓控制系統(tǒng)元件尺寸分析，但是，我只想說這它們大小，以滿足下列議案的電源要求：電梯電路在全面提高10秒，電梯的電路全部在3.5降低，傾斜在4.0的電路全機架，傾斜電路充分潮濕條件下，最大負荷2.5秒。最大負載能力是斗1.2錳。
（5）結(jié)果：仿真和實驗
該實驗?zāi)Ｐ蜕线M行了卡特彼勒WTL的889種類的車輛。為傾斜電路的控制算法實現(xiàn)用微型控制器，對船上的8位分辨率的A / D，D / A轉(zhuǎn)換。有效的死區(qū)是負載和發(fā)動機轉(zhuǎn)速的功能。在實時實現(xiàn)不同低估死區(qū)值使用，以避免過沖。不準確的死區(qū)補償?shù)捻憫?yīng)是在Fig.14.It顯示延時效果觀察到，在響應(yīng)時間減少到低于0.2的時候賠償范圍內(nèi)，其實際有效價值1.0毫米的。圖15顯示了上升時間敏感閥改造的收益。據(jù)觀察，為了滿足時間要求提高0.5秒的增益必須相當準確。圖16shows一兩個實驗和分析比較下一個完整的電梯，完全轉(zhuǎn)儲，無負荷，高怠速架步反應(yīng)的條件。造成這種狀況的性能標準的0.5秒和5％上升時間小于0.2秒的延遲就形成一個關(guān)心過沖閥變換的動態(tài)執(zhí)行中遇到的壓力是再反饋級過濾反應(yīng)所需的穩(wěn)定。據(jù)推測，壓力應(yīng)低通濾波在或低于閉環(huán)系統(tǒng)帶寬，以一個臨界值。在這樣的閉環(huán)控制能夠響應(yīng)增益的變化，就好像他們是死區(qū)對系統(tǒng)干擾。實驗和分析都反映了這種過濾。雖然沒有顯示，在穩(wěn)定性上WTL的模型進行的實驗都保持990.Therefore，這樣一種高血壓控制用PI型制度的有效控制閉環(huán)控制加閥變換模型為基礎(chǔ)的補償要求非常精確的模型。
(6) 總結(jié)
一個先進的電液開放為中心的非壓力補償?shù)膶崿F(xiàn)系統(tǒng)狀態(tài)進行了研究，以評估執(zhí)行速度伺服控制，同時滿足運營商的響應(yīng)和平滑性能規(guī)格的潛力，并建立一個模塊化的子系統(tǒng)，將接受從命令自治區(qū)高層次上的WTL的車輛規(guī)劃控制器。閉環(huán)速度控制成功地實施了傾斜電路貨架使用標準的比例積分控制器，具有動態(tài)轉(zhuǎn)換閥偶函數(shù)。閥的動態(tài)變換是發(fā)動機轉(zhuǎn)速（液壓流量）和桿端氣缸壓力的作用。閥門共享的動態(tài)變換在實時控制算法提供的自由化效果類似壓力補償?shù)呢摵蓚鞲幸簤合到y(tǒng)和元件成本降低的結(jié)果。強大的性能進行了驗證，通過廣泛的系統(tǒng)建模與一個 WTL model990車試驗。
Modeling identification and real time control of bucket hydraulic system for a wheel type loader earth moving equipment
Abstract
The earth moving equipment industry is quickly gearing up to achieve great gains in efficiency, performance, safety, and operator comfort by the rapid deployment of recent digital control technology in its products.There are two major types of earth moving equipment operating in large numbers: excavators and wheel type loaders. Excavators have received much attention by the industry recently. The wheel type loader product studied in this paper is another example of a high volume versatile machine at the opposite end of the configuration spectrum.A state of the art electro-hydraulic open centered non-pressure compensated valve control system is studied to evaluate the potential gains by implementing digital velocity servo control. The control objectives are to meet operator perceived response requirements, meet operator perceived smoothness requirements, create a sub-system that could accept commands froman autonomous high level planning controller.
Closed loop digital velocity control is successfully implemented in the racking motion of a wheel loader using a standard proportional-integral (PI) and a dynamic valve transform algorithm. The dynamic valve transform is a function of hydraulic flow rate which is a function of engine speed and rod end cylinder pressure. Robustness of performance was verified through extensive system modeling, validation, and hardware tests on a large Caterpillar wheel loader model.
Introduction
The automotive industry has made great gains in efficiency, performance, safety
and passenger comfort by the extensive and rapid deployment of the recent digital control technologies in its products. The earth moving industry is quickly gearing up to achieve similar gains in the short term. There are two major types of earth moving equipment: excavators and wheel type loader . The long term goal is to develop an autonomous product that no longer relies on the operator skill and endurance to maximize performance. The performance is measured in the form of tons/h of the material processed and minimizing the cost of the operation in the form of cost/ton of material processed. The goal is to develop controlled sub-systems that improve operator/machine performance, reduce operation cost, and that would serve as lower level control sub-system in the autonomous controller hierarchy.
Wheel type loaders (WTL) come in many sizes. Operating weight ranges from 15000-350000ib and horsepower ranges from 100-1200 hp. The small to mid-size machines have the broadest spectrum of applications (e.g., construction and material handling applications) while the larger machines tend to be used mostly in mining applications. One common function WTL are utilized for is truck loading.
The truck loading cycle is a repetitive four step process by which some type of material is transported from a stock pile to a truck. The process starts when the operator rams the stock pile and commands the linkage to lift load while at the same time the bucket is racked backwards (step 1: travel, to stock pile and dig).When the bucket has a full load, the operator shifts into reverse and travels backwards while steering to a position that allows him enough room to then shift into forward and travel to the truck. The operator continues to raise the load during this travel portion so that it clears the truck bed when he slows down and reaches the truck (step 2: travel to truck). The operator then commands the bucket to dump thus releasing the load to the truck bed (step 3: dump). Finally, the operator commands the bucket to rack back to its level position. At the same time, the operator shifts into reverse travel and commands the linkage to lower back to the ground for another dig cycle (step 4:travel to start position). This study focuses on the implementation of closed loop digital velocity control on the implement sub-system of a current state of the art wheel type loader earth moving equipment. The application of this technology is not limited to WTL and in fact has broad applications to a variety of earth moving equipment .The excavator hydraulic sub-system control problem has received much attention recently as have hydraulic sub-systems in general.
Description of the wheel type loader sub-system
Earth moving equipment can be broken down into four sub-systems. (1) power-train,(2) brakes,(3) steering, and (4) hydraulic actuators. The power-train consists of a power source which is typically a diesel engine. Power is transmitted to a mechanical transmission via a torque converter which then connects to differentials, drives and finally tires. This is typically the case for WTL. Excavators have a hydro-static drive train (i.e., hydraulic pumps and motors) that connects to a track. Several engine power take-offs provide power via pumps to run the steering hydraulic system, the brake system which is typically hydraulic，and the hydraulic actuator system. The hydraulic actuation system contains the ground engaging tool that provides the force and motion to engage the soil or other material that needs to be processed.
2.1. Vehicle degrees of freedom
The operator cab is considered to have six degrees of freedom: three linear (fore-aft, lateral, vertical) and three angular (yaw, pitch, roll). The bucket has two degrees of freedom. Steering is one additional degree of freedom. Therefore, the vehicle has nine degrees of freedom. To simplify the upcoming analysis, two constrains will be imposed on the system. First, the front frame motion will not be allowed to rotate relative to the rear frame. Second, the rear frame motion will be constrained to a plane. The First constraint eliminates one degree of freedom (steering) from the analysis. The second constraint eliminates three degrees of freedom: operator lateral linear motion, yaw angular motion and roll angular motion. Thus, we consider only five degrees of freedom in our model: lift and tilt implement motion of the bucket and operator fore-aft and vertical linear motion and pitch angular motion.
2.2. Linkage
There are several types of WTL implement linkages currently in use. A very common example, called the Z-bar linkage. It is a two degrees of freedom linkage consisting of four bodies (lift arm, lever, link, bucket) and two asymmetric hydraulic cylinders (lift and tilt), all connected together by nine revolute pin joints.
2.3. Main hydraulics
A common hydraulic implement system often used to control the flow to the lift and tilt cylinders of a WTL uses the open center non-pressure compensated spool type valve. This system contains a pump/relief valve which sends fluid to a main directional valve which in turn partitions the flow to hydraulic cylinders and tank.
The components and their operation are best described by following the hydraulic fluid through the circuit under various operating conditions. The simplest condition is when there is no command current from the ECM amplifiers. The lift and tilt spools will be centered since the E/H valve solenoids will not be activated. In this case, the pump sends flow across a load check valve (set at maximum operating pressure) to the tilt spool. Since this spool is centered, the flow proceeds on to the lift spool, which is also centered, and back to tank for recirculation. The name "open center" comes from the fact that when valve is in neutral position, fluid circulates from the pump through the valve to the tank.
If the tilt spool is centered\ and the lift spool is shifted to the right, an orifice to tank area called the pump to tank area (P-T) becomes restricted. The pump pressure builds up and overcomes the load check valve sending flow to the head end (HE) of the lift cylinder across another orifice called the pump to cylinder area (P-C). At the same time, fluid from the rod end (RE) of the lift cylinder flows across another orifice called the cylinder to tank area (C-T) and on to tank for recirculation. As a result, the lift cylinder rod extends raising a load that may be contained in the bucket. If the lift spool is shifted to the left the opposite happens. Fluid from the HE flows across the C-T to tank, fluid from the pump flows across the P-C to the RE as well as across the P-T if the spool is not shifted enough to completely close the P-T. As a result, the lift cylinder rod retracts lowering the bucket to the ground. If the lift spool is centered and the tilt spool is activated, the tilt cylinder will behave similar to the lift cylinder.
In WTL vehicles, the lift spool is piped in series with the tilt spool. This configuration is called tilt priority, since the flow requirement of tilt circuit can over-ride and shut off the lift circuit. Additionally, cylinder relief valves may be added to each RE and HE if the maximum operating pressures for each section varies from main relief for structural life or safety concerns. Make-up valves are often added to these systems as well. These check valves provide tank flow to the cylinder RE or HE in the event that a vacuum is created during operation. In this way, cavitation can be significantly reduced. This is a problem for gravity driven functions such as lift arm lower or bucket dump in which the C-T area has been designed to provide restrictions that yield fast cylinder velocities. In this case, the pump flow to the cylinder can not match the flow from cylinder to tank creating low pressures and cavitation. This is highly undesirable in closed loop control since controllability of the hydraulic system is effectively lost during cavitation.
2.4. Electro Hydraulic pilot valve
A pilot pump supplies flow to a pressure regulation valve that maintains a constant supply pressure to an E/H valve which is also connected to tank. A driver in the ECM sends a current to the solenoid which moves a control spool. As this spool moves, a meter-in orifice connected to the supply pressure port and a meter-out orifice connected to tank proportionally open or close to maintain a control pressure. This pressure acts on the main spool causingit to shift and open the main orifice area (i.e., C-T, P-T, P-C). In some cases, position feedback of the main spool is used to provide closed loop position control of the spool.
2.5. Digital control system
Basic low cost components will be chosen for this study to be consistent with the current practice in this industry. Typically, 7-bit microprocessors are used in which assembly coding is required to achieve loop times of 20 ms. Rotary potentiometers are used for sensor feedback as well as reference input signals.
Dynamic model: tilt circuit and vehicle planar dynamics
Dynamic model of the tilt circuit is developed from an input-output relations point of view. The input is the spool position of the tilt circuit valve. The outputs are (1)bucket angular velocity due to tilt cylinder displacement, and (2) planar motion (x, y, θ）of the vehicle.
The following assumptions and approximations are made for the model:
(1) Lift circuit is stationary at a nominal lift angle. Therefore, we expect to obtain different tilt circuit models for different nominal lift circuit positions.
(2) Vehicle is modeled as a mass-spring-damper system in 2-D space which has three degrees of freedom: x, y, u. This is a fairly good approximation since vehicle body and tires act like a mass-spring-damper system.
(3) Hose volume and hydraulic losses are included in the hydraulic model.
(4) Electro-hydraulic valve dynamics is included as a second order filter with 0.707 damping ratio. This is consistent with the behavior of the actual E/H valves used in WTL vehicles.
(5) E/H valve spool areas (called metering) are not linear functions of spool positions.
The spool area geometry is accurately modeled as a nonlinear function of spool position.
(6) Standard orifice equations are used to describe the relationship between the flow rate (Q) and the spool position (hence the metering area), and pressure differential, Δ p
(7) Flexibility due to the oil compressibility is taken into account as bulk modulus of the hydraulic fluid.
The input-output relationship between spool position and bucket angular velocity is obtained in three stages of approximation:
(1) Steady state input-output function between spool displacement and tilt cylinder velocity is called the tilt modulation. This model captures the nonlinear static relation including the geometric dead band and valve-cylinder gain. Notice that effective deadband and gain are functions of (1) flow rate (which is a function of engine speed in WTL case), and (2) external load. In other words, the DC gain of the transfer function is a nonlinear function of the flow rate, external load, and nominal lift circuit position.
(2) The geometric relation between tilt cylinder linear velocity and bucket linear velocity is described by the linkage jacobian. This is represented as a series of curves (implemented in real time as interpolated table) for different lift positions.
(3) Finally, we model the dynamic filtering effect of the hydraulic circuit from spool position to tilt cylinder velocity .The steady state gain information is captured in tilt modulation and tilt kinematic models. Therefore the d.c. gain of the tilt dynamic model will be approximately unity (0 dB). Furthermore, it will be function of lift position and external load. Therefore, all three block components of the model are evaluated for a full sweep of expected lift position and load values. E/H valve is an open-center type valve. As the spool command shifts the spool, the P-T (pump to tank) area starts to close, the P-C (pump to cylinder) area starts to open as does the C-T (cylinder to tank) area. The pump pressure starts to build up since the P-T is restricting its flow to tank. When the pump pressure exceeds the HE cylinder pressure, the load check valve pops allowing pump flow to enter the cylinder HE. This flow extends the cylinder which forces flow from the RE of cylinder across the C-T area to tank. As the external load varies, the minimum amount of spool command required to match the load via varying pump pressure varies as well. This results in effective deadband and gain changes. If the external load is reversed (i.e., over running load condition), the rate of cylinder extension will be primarily determined by the pressure drop across the C-T area unless the pressure drop is such that the C-T flow is less than the pump flow entering the HE of the cylinder. Then the rate of extension will be pump driven.
(4) Closed loop control system performance objectives
The hydraulic system components (pumps, cylinders, servo valves) were sized so that they can provide the necessary power levels (flow rate and pressure) to move the bucket for the range of loads the WTL is designed to handle. Here we will not repeat the hydraulic control system component sizing analysis, however, suffice it to say that they were sized to meet the power requirements for the following motions: lift circuit full raise in 10 s, lift circuit full lowering in 3.5 s, tilt circuit full rack in 4.0 s, tilt circuit full damp in 2.5s under maximum load condition. Maximum bucket load capacity is 1.2 MN.
(5)Results: simulations and experiments
The experiments were conducted on a Caterpillar model WTL 889 class vehicle. The control algorithm for the tilt circuit was implemented using a micro-controller which has 8-bit resolution A/D, D/A converters on board. Effective deadband is a function of load and engine speed. In real-time implementation different under-estimated deadband values were used in order to avoid overshoots. The effect of inaccurate deadband compensation on the delay of the response is shown in Fig.14.It is observed that the delay in response reduces to below 0.2 s when deadban compensation is within 1.0mm of its actual effective value. Figure 15 shows the rise time sensitivity to valve transform gains. It is observed that in order to meet the 0.5 s raise time requirement the gain must be fairly accurate. Figure 16shows a comparison step response for both experiment and analysis under the conditions of a full lift, full dump, no load, high idle rack. The performance criteria for this situation are met in the form of 0.5s rise time and 5%overshoot with less than 0.2s delay A concern regarding implementation of the dynamic valve transform is the level of RE pressure feedback filtering required for stable response. It was postulated that the pressure should be low pass filtered to a cutoff value at or below the bandwidth of the closed loop system. In this way the closed loop control could respond to the changes in gain and deadband as if they were disturbances on the system. Both experiment and analysis reflect this filtering. Though not shown, stability was maintained during all of the experiments conducted on WTL model 990.Therefore, the effective control of such an EH control system using a PI type closed loop control plus model based valve transform compensation requires very accurate models.
Conclusions
A state of the art electro hydraulic open centered non-pressure compensated implement system was studied to evaluate the potential of implementing velocity servo control to meet both operator response and smoothness performance specifications, and to create a modular sub-system that would accept commands from an autonomous high level planning controller on a WTL vehicle. Closed loop velocity control was successfully implemented on the racking function of the tilt circuit using a standard proportional-integral controller coupled with a dynamic valve transform. The dynamic valve transform is a function of engine speed (hydraulic flow) and rod end cylinder pressure. Inclusion of the dynamic valve transform in real-time control algorithm provides the liberalizing effect similar to a pressure compensated load sensing hydraulic system and results in lower component cost. Robust performance was verified through extensive system modeling and tests on aWTLmodel990 vehicle.