帶鋼跑偏機的分析設(shè)計【含CAD圖紙、說明書】

帶鋼跑偏機的分析設(shè)計【含CAD圖紙、說明書】,含CAD圖紙、說明書,帶鋼,跑偏機,分析,設(shè)計,cad,圖紙,說明書,仿單 修改參數(shù)后得到的方塊圖 1-開環(huán)頻率特性；2-閉環(huán)頻率特性 圖5-2 修改參數(shù)后系統(tǒng)的博德圖 圖5-3 修改參數(shù)后系統(tǒng)的階躍特性 跑偏系統(tǒng)方塊圖 原系統(tǒng)的簡化方塊圖 帶壓力反饋電液伺服閥系統(tǒng)原理圖 帶壓力反饋的跑偏系統(tǒng)方塊圖 電液控制閥 電液閥是機械運動的組成部分，其運動直接受電子電路的影響。對于液油壓系統(tǒng)設(shè)計者來說，它已成為了一個流行的部分。電子閥門的主要類型是開關(guān)，比例，伺服閥和數(shù)字閥。根據(jù)1995年美國科學(xué)作家沙利文估計，電子IC與常規(guī)液壓閥需求預(yù)計年均增長速度在5%左右。銷售比例和伺服閥據(jù)估計會在3個主要應(yīng)用領(lǐng)域增加，這3個領(lǐng)域分別是移動機械、機械工業(yè)、航天設(shè)備.一般而言，有液壓閥的綜合電子電路會增加機器的準(zhǔn)確性和速度。運用電子系統(tǒng)也會有一些缺點。在一些應(yīng)用中，由于熱，部分效率會喪失。Vickers公司已經(jīng)設(shè)計出了能代表未來電子系統(tǒng)的。它是專門的機器，其主要組成部分是比例閥、缸和數(shù)碼控制。 1993在美國獲專利的機械工程師jun是液油壓系統(tǒng)設(shè)計者。他們工作時是否使用機器人或自行移動機器和對于它們已經(jīng)開始使用電腦和廉價的電子元件，這會改善液壓系統(tǒng)性能還可以省錢。 電子閥是機械的組成部分，其運動直接受電子線路影響。它已成為系統(tǒng)設(shè)計者設(shè)計的一個流行的部分了。液壓控制是反過來受電子部分控制的。象扭矩馬達(dá)或螺線管，通常是用電流通過一個有距離的比例閥來使它的線圈閥運動的 電子閥的主要類型是開關(guān)、比例閥、伺服閥和數(shù)字閥. 該裝置是固定在流體之間來提供動力和驅(qū)動源泉的，如扶輪摩托以及控制流體的壓力、方向和曲率的。它能使機器控制驅(qū)動器組成部分的加速、速度、位置和力量的。 在 Bethlehem, Pa.Rexroth公司的產(chǎn)品開發(fā)部門的經(jīng)理 Paul Stavrou說：“電子控制液壓系統(tǒng)在20世紀(jì)40年代已經(jīng)有了，但是直到80年代初期具有微型化,降低成本和高可靠性優(yōu)點的它才蓬勃運用于工業(yè)。 主要生產(chǎn)電子閥的公司包括Rexroth，Vickers公司，總部設(shè)在Troy, Mich；Parker Hannifin Corp. of Cleveland, Ohio; and Eaton Corp. of Eden Prairie等等這些地方。每閥有截然不同的作用，取決于它的類型。 最簡單的就是開關(guān)閥。它的開關(guān)轉(zhuǎn)換是通過前后來回轉(zhuǎn)換線圈來實現(xiàn)的。在"on"的位置上，線圈于閥位置的排成一條直線通道，它允許一定量流體流動通過裝置。在“off”位置上，線圈隔開連通渠道。通過調(diào)節(jié)電子控制螺線管流通速度可以使同一個閥的開關(guān)有不同的速率。這類型的控制有時也叫做“bangbang控制”，因為高強度的閥會過度震蕩產(chǎn)生噪音。 一般而言，比例閥會比較精確的按照流體的速度和壓力來控制并對電子輸入信號迅速做出反應(yīng)。Stavrou說道：“電子控制已經(jīng)給予比例閥精確度和信號反應(yīng)時間，所以它適合應(yīng)用于大多數(shù)工業(yè)?！? 比例閥運行時,線圈運動是靠一個有距離的直接成正比的有電螺線管來實現(xiàn)的。比如說在Rexroth公司生產(chǎn)的一個比例閥中，電動放大器接收到的信號為9伏特,它轉(zhuǎn)換成當(dāng)前高達(dá)1.5安培的電流.電流到達(dá)了一個有可以往返移動活塞的電動螺線管，被發(fā)送到螺線形電導(dǎo)管的定量電流控制著活塞移動線圈閥移動一段距離。 隨著時間的推移，流動量也改變。閥有變化地控制速度和驅(qū)動的力量。簡單的說，一個比例閥可以改變流體的流動方向，也可以在一個驅(qū)動構(gòu)件中控制輸出部件的方向。 比例閥通常被用在工作時無感應(yīng)反饋的中開放性系統(tǒng)。不過,他們有時也被納入封閉性的系統(tǒng)中。封閉性系統(tǒng)就是在其中一個傳感器,通常是有差別的可變線性變壓器傳送可以告知控制者在每一個反復(fù)運動中螺線管活塞的位置的信號的。傳感器也可以發(fā)出衡量驅(qū)動構(gòu)件輸出的信號。 現(xiàn)在包含比例閥的封閉性系統(tǒng)，曾經(jīng)是伺服閥的領(lǐng)域。伺服閥通常比比例閥制作的更精確，通常有兩三個階段。 通常第一階段，在試驗電路中螺線管或扭矩馬達(dá)控制流體流動。（一些扭矩馬達(dá)在不是試驗電路中有足夠的力量去直接控制閥。）第二階段中，試驗電路控制能調(diào)節(jié)流體流動到驅(qū)動構(gòu)件的活塞閥的運動。 當(dāng)使用扭矩發(fā)動機時, 電流被輸送到其旋管使轉(zhuǎn)子轉(zhuǎn)動。轉(zhuǎn)子與一個可在孔中來回轉(zhuǎn)動的擋板連接。在實驗電路中，這些孔是到達(dá)兩個相分離渠道的通道。 另外，通向線圈相反端的渠道控制閥的第二階段。在接到電流時,扭矩馬達(dá)會調(diào)整flapper板的位置。不同壓力情況下，流體會流向線圈相反的方向。然后線圈會移動一段距離，這個距離與不同的壓力和輸送到發(fā)動機的電流有關(guān)。在線圈位置上的反饋通常是由一個線性的變數(shù)差別的變壓器提供的。 比例閥和伺服閥通常是由放大器和滑向系統(tǒng)控制架的控制卡來控制的。當(dāng)一些還在被傳統(tǒng)的軟件和硬件控制時，最先進(jìn)的伺服閥已經(jīng)與規(guī)劃和電腦數(shù)值控制相聯(lián)系了。安裝閥的時間應(yīng)該保持到最小值，而且機器空間是緊的，控制卡應(yīng)直接固定到閥的位置。 應(yīng)用及濫用 伺服閥比比例閥性能跟高, 但他們是更加昂貴的。根據(jù)美國Frost & Sullivan公司的市場調(diào)查，與它的復(fù)雜性有關(guān)，每個伺服閥的價格大約在$1000 到 $2000之間。電子控制器價格可能會增加超過$500。然而，比例閥平均會是那些花費的一半。 據(jù)Frost & Sullivan公司.1995年估計，美國對電子和常規(guī)水力閥的需求預(yù)計以每年5%的年率增長。比例閥和伺服閥的銷售量預(yù)計會在三個主要應(yīng)用領(lǐng)域增加: 移動機械、工業(yè)機械, 和航空航天設(shè)備。 例如,在移動機械市場上,電動液壓閥必須有多功能而且堅固。他們必須適合機器, 包括垃圾車, 鏟車, 和挖掘機。閥經(jīng)常在戶外使用，這樣會導(dǎo)致振動。 電動液壓的閥門的一個好處是, 他們在操作重設(shè)備時能增加安全性。例如，大多數(shù)起重機,禁止了一些區(qū)域。這些區(qū)域是機器不能設(shè)法運載重的裝載或它也許下落的區(qū)域。 然而在過去五年里, 為了使操作人員更好的操作機器以及限定危險的區(qū)域，起重機制造商開始對他們的常規(guī)液壓系統(tǒng)增加傳感器。根據(jù)Vickers公司的先進(jìn)技術(shù)經(jīng)理弗雷德菲利普所估計，如果起重機制作商最終決定使用微處理器控制, 安全系統(tǒng)將是更加有效的。他說："微處理器使設(shè)計一個系統(tǒng)成為可能。這個系統(tǒng)就是能夠自動使起重機在被禁止的區(qū)域外部的系統(tǒng)。" Vickers 公司把電動液壓閥投入到新用途的使用中。例如在Burbank和Calif.的 娛樂業(yè), 他們把那個公司的電液比例閥運用在行動模擬器上。這個機器包括震動模仿運動譬如加速度、減速, 和自轉(zhuǎn)。 電動液壓閥也使用在轉(zhuǎn)臺式行動控制上。Eaton 流體分裂制作了一個電子速度控制系統(tǒng)來提高使用在建筑業(yè)攪拌混凝土的運輸攪拌器的耐久性。由于電液閥為混合鼓自動速度控制提供獨立發(fā)動機速度，因此機器的使用壽命被提高了。 引擎驅(qū)動鼓通過一個水力泵、馬達(dá), 和齒輪還原劑。當(dāng)混凝土攪拌時，鼓每分鐘轉(zhuǎn)動在1 和17 轉(zhuǎn)之間。在泵內(nèi)，二個伺服閥控制流體流入一臺流體伺服機。二條螺線管致力不同的流程方向，使每個閥都接收到從計算機控制器發(fā)出的信號并允許液壓機液體轉(zhuǎn)動的流動以使混合鼓以渴望的速度轉(zhuǎn)動。計算機接收來自泵輸出軸的一個霍爾效應(yīng)傳感器的鼓速度的連續(xù)的測量。 沒有電子控制, 在機器內(nèi)當(dāng)速度增加和減少時，攪拌器也跟著加速和減速。發(fā)動機的轉(zhuǎn)動速度的比例會影響攪拌器的耐久性, 主要取決于在它的使用期間混凝土鼓轉(zhuǎn)動多少次。鼓的轉(zhuǎn)動也會影響引擎消耗的燃料量。 使用電子速度控制系統(tǒng), 機器操作員能使混合鼓的速度降低到1 轉(zhuǎn)每分鐘, 有效地打破攪拌器的和那引擎之間速度的連接。因而，混合鼓在工作的地方會更多的減少轉(zhuǎn)動次數(shù)。在這個過程中節(jié)省下來的機器能量可以更好的利用到它工作的地方。 Eaton 聲稱電子系統(tǒng)會使攪拌器的使用壽命增加一年。而且這個公司的研究表明由于引擎產(chǎn)生較少馬力，對于混凝土的每一次裝載，電子系統(tǒng)會減少引擎0.8 加侖的燃料消耗量。 在眾多優(yōu)點中, 速度控制系統(tǒng)會自動地控制混凝土的混合速度。計算機系統(tǒng)也會存儲以往每批混凝土的資料, 包括在混合期間鼓的轉(zhuǎn)動數(shù)和自鼓轉(zhuǎn)動開始消耗的時間多少。這些數(shù)據(jù)對于評估已完成的混凝土結(jié)構(gòu)的完整性的房屋檢查員是有價值。 是否要使用電液 通常,電子線路和流體閥的綜合會增加機器的運轉(zhuǎn)的速度和精確度。譬如執(zhí)行一項特殊任務(wù),在挖掘機里轉(zhuǎn)動桶。電液會比傳統(tǒng)無電的閥控制使用較少能量和有更好的精確度。傳統(tǒng)方法包括直接給螺線管通電而使機械和自動控制連接，從而手工閥。 在許多應(yīng)用中，機電系統(tǒng)可以代替電液系統(tǒng)。例如, ac 馬達(dá)會給予退彈管道系統(tǒng)的機器工具軸以能量。在許多情況下，機電系統(tǒng)與電動液壓相比較， 它們不會漏油, 通常更加安靜,較不昂貴，對時尚模式和數(shù)字控制的反應(yīng)更加直接。 而且使用電動液壓的系統(tǒng)也會有一些缺點。例如, 在某些應(yīng)用中,由于熱構(gòu)件的效率會流失。另外, 對電子使用會使設(shè)計,應(yīng)用和維護的液壓機構(gòu)的復(fù)雜性增加。例如,一位電液系統(tǒng)設(shè)計師必須使電子元件在苛刻的操作條件譬如到處溫度和過份振動中，受到保護。 電動液壓的系統(tǒng)經(jīng)常用于控制移動機械的行動。這里, 液壓構(gòu)件的優(yōu)點就是依照設(shè)計者的意愿，它們能用機器相對小的空間來移動高而且重的裝載。 設(shè)計怎樣去控制機器的行動經(jīng)常取決于系統(tǒng)設(shè)計師是否了解應(yīng)用和維護各個系統(tǒng)的細(xì)微差異。因為用戶可能更了解傳統(tǒng)液壓系統(tǒng)的操作, 他們害怕無法改正電子問題，這是電液制造商面對的一個障礙。 所以，電液構(gòu)件制作商決定設(shè)計容易使用和維護的產(chǎn)品。Eaton 公司的運輸攪拌器有一個診斷的特點，它允許卡車司機精確定位和確定電子失誤的原因。當(dāng)有問題時, 代碼會出現(xiàn)在小室的一個診斷盤區(qū)，司機可以跟蹤代碼查明故障以及根據(jù)提示作出處理。 對電液系統(tǒng)的采納是用戶關(guān)于構(gòu)件的性能怎樣與一種特殊應(yīng)用匹配，這是很重要的。根據(jù)電液系統(tǒng)制作商所說, 隨著時間的推移用戶會發(fā)現(xiàn)有流體構(gòu)件的連接電子的優(yōu)點。但是, 一些用戶對于由控制傳統(tǒng)方法的轉(zhuǎn)換持有懷疑態(tài)度。Vickers公司機械系統(tǒng)經(jīng)理Paul Smith說：“我們的顧客應(yīng)該肯定對液壓系統(tǒng)增加復(fù)雜性的價值。他們會認(rèn)識到產(chǎn)品會回報他們投資在它上面的額外金錢?；貓髞碜愿嗟胤?例如，減少在設(shè)施和維修方面的費用。" Smith說：“發(fā)展趨向是首先為用戶實施一個開放的環(huán)形系統(tǒng)。然后, 當(dāng)他們與它相適應(yīng)時,再使他們轉(zhuǎn)向更加復(fù)雜但更加準(zhǔn)確的閉環(huán)系統(tǒng)?！? 發(fā)展中的改善 Vickers公司的先進(jìn)技術(shù)小組正致力于對電液系統(tǒng)的一系列改善。例如, 工程師正對用于特定產(chǎn)品和多用途產(chǎn)品的控制器進(jìn)行改善。另外, 他們正設(shè)法提高能量利用率，減少噪聲和挖掘電液系統(tǒng)的潛力。 進(jìn)一步,這個公司設(shè)計了他們認(rèn)為能代表未來的電液系統(tǒng):一臺專業(yè)的發(fā)動機。他的主要構(gòu)件是比例閥、圓筒和一位數(shù)字式控制器。除了減少液壓系統(tǒng)的構(gòu)件數(shù)量以外, 這種專業(yè)的發(fā)動機還有其它的優(yōu)點。例如,在設(shè)計一個液壓系統(tǒng)時, 為了符合應(yīng)用要求，許多工作會進(jìn)行優(yōu)化。在這種專業(yè)的發(fā)動機里，為了一系列應(yīng)用，獲取到的它的工作情況會被編程到控制器里。因此, 系統(tǒng)設(shè)計師就節(jié)省了花費在調(diào)整控制系統(tǒng)的時間。沒有一臺專門的發(fā)動機，系統(tǒng)設(shè)計師設(shè)置的在電子控制器里的決定它效率工作情況的獲取將會是錯誤的。 發(fā)動機里開發(fā)軟件的應(yīng)用也使記錄運轉(zhuǎn)控制的工作情況變得容易了。最具代表性的就是程序員用一臺個人計算機就可以連接發(fā)動機的控制器。軟件和它的幫助菜單隨后被一步步用到審閱編程處理中, 省去了在操作指南中的查找。 Vickers公司的Phillips說：“系統(tǒng)設(shè)計師的一個目標(biāo)就是選擇能在運用中帶來良好結(jié)果的構(gòu)件。”設(shè)計師必須記住邏輯上什么零件在一起運作得最好。同時, 他們設(shè)法使零件數(shù)量減到最少而仍然達(dá)到必需的效果。使系統(tǒng)變的更靈活，要想達(dá)到那樣的目的才更容易。 Electrohydraulic valves take control Copyright American Society of Mechanical Engineers Jun 1993 The electrohydraulic valve, a mechanical component whose movements are directly influenced by electronic circuits, has become a popular part for system designers of hydraulic fluid systems. The main types of electrohydraulic valves are on-off, proportional, servo, and digital valves. The demand for electronic and conventional hydraulic valves is expected to grow at an annual rate of 5% in the US through 1995, according to Frost & Sullivan. Sales of proportional and servo valves are expected to increase in the 3 primary areas of application: mobile machinery, industrial machinery, and aerospace equipment. Generally, the integration of electronic circuits with hydraulic valves increases the precision and speed of a machine's motion. There are also a few drawbacks associated with using electrohydraulic systems. In some applications, component efficiency is lost because of heat. Vickers Inc. has designed what it feels represents the electrohydraulic system of the future: an expert actuator whose primary components are a proportional valve, a cylinder, and a digital controller. Designers of hydraulic fluid systems, whether they are working with robots or mobile earth movers, are beginning to use compact and inexpensive electronic components to both improve the performance of the hydraulic systems and save money. The electrohydraulic valve, a mechanical component whose movements are directly influenced by electronic circuits, has become a popular part for system designers. This regulator of fluids is in turn regulated by an electric component, such as a solenoid or torque motor, that typically moves the spool of the valve through a distance proportional to an electric current. The main types of electrohydraulic valves are on-off, proportional, servo, and digital valves. The devices are mounted between the fluid supply source and a driven actuator, such as a rotary motor, and control a fluid's pressure, direction, and Bow rate. They enable the actuator to control the acceleration, velocity, position, and force of the driven machine component. "Electronic controls for hydraulic systems have been around since the 1940s, but it was not until the early '80s, with their miniaturization, lower costs, and improved reliability that they began to flourish in industry," said Paul Stavrou, manager of systems and product development at Rexroth Corp's. servo and proportional controls group in Bethlehem, Pa. The leading producers of electrohydraulic valves include Rexroth; Vickers Inc., based in Troy, Mich.; Parker Hannifin Corp. of Cleveland, Ohio; and Eaton Corp. of Eden Prairie, Minn. Each valve has a distinctly different level of performance depending on its type (see table on page 56 ). (Table omitted) The simplest is the on-off valve, which turns the valve on and off by shuttling a spool back and forth. In the "on" position, channels in a spool align with ports in the valve housing and allow the flow of a defined volume of fluid through the device. In the "off" position, the spool blocks off the port. The same valve may be turned on and off at varying rates b an electronically controlled solenoid to modulate the rate of flow. This style of regulation is sometimes called bang-bang control because at high power the valves often vibrate excessively and are noisy. The proportional valve is generally more precise in terms of the rate and pressure of flow that it controls, and faster in its response to electronic input signals. "Electronic controls have given proportional valves accuracy and signal-response times that are adequate in most industrial applications," Stavrou said. When a proportional valve is operating, its spool moves through a distance directly proportional to the current received by a solenoid. In one of Rexroth's proportional valves, for example, an electric amplifier receives a signal of up to 9 volts, which it converts into a current of up to 1.5 amps. The current reaches an electric solenoid, which has a plunger that moves back and forth. The plunger moves the valve spool through a distance defined by the measure of current sent to the solenoid. By changing the volume of flow over time, the valve variably controls the speed and force of a driven unit. Similarly, a proportional valve that changes the direction of flow also controls the direction of output components on a driven unit. Proportional valves are often used in an open-loop system that works without sensor feedback. However, they are sometimes integrated into closed-loop systems in which a sensor, usually a linear variable differential transformer, sends signals that tell the controller where the solenoid plunger is during each stroke. The sensor can also send signals that measure the output of the driven unit. Closed-loop systems, which now incorporate proportional valves, were once the domain of servo valves. The servo devices are machined more precisely than proportional valves and usually have two or three stages. Typically, in the first-stage a solenoid or torque motor controls the flow of fluid in a pilot circuit. (Some torque motors are powerful enough to directly control valves without a pilot circuit.) In the second stage, the pilot circuit controls the movement of the valve spool, which itself regulates fluid flowing to the driven unit. When a torque motor is used, an electrical current is sent to its coils to move an armature. The armature is connected to a flapper plate that moves back and forth between orifices. These orifices are entry ports to two separate channels within the pilot circuit. Separately, the channels run to the opposite ends of a spool that controls the second stage of the valve. Upon receiving the electric current, the torque motor adjusts the position of the flapper plate. A difference in pressure emerges between the fluid flowing to opposite ends of the spool. Then the spool moves through a distance that is related to the pressure difference and electric current sent to the motor. Feedback on the position of the spool is often provided by a linear variable differential transformer. Proportional and servo valves are usually controlled from amplifier and control cards that slide into system control racks. The most sophisticated servo valves are often linked to programmable and computer numerical controllers, while some are controlled by custom software and hardware. In applications where time spent installing the valve needs to be kept to a minimum and machine space is tight, control cards are mounted directly into the valve's housing. USE AND ABUSE Servo valves hare higher performance than proportional valves, but they are more expensive. The average servo valve costs between $1000 and $2000, depending on its complexity according to the research firm Frost & Sullivan Market Intelligence in New York. Electronic controllers can add more than $500 to the price. The average proportional valve, however, can cost half that much. The demand for electronic and conventional hydraulic valves is expected to grow at an annual rate of 5 percent in the United States through 1995, according to Frost & Sullivan. Sales of proportional and servo valves are expected to increase in the three primary areas of application: mobile machinery, industrial machinery, and aerospace equipment. In the mobile machinery market, for example, electrohydraulic valves must be versatile and rugged. They have to fit in a range of machines, including garbage trucks, forklifts, and excavators. The valves are often used outdoors and in conditions that induce vibrations. One benefit of electrohydraulic valves is that they can increase safety in operating heavy equipment. Most large cranes, for example, have forbidden zones into which the machine must not try to carry heavy loads or it may fall over. Within the past five years, however, crane makers have begun adding sensors to their conventional hydraulic systems to keep the operator better informed of the outline and limits of the danger zone. If crane makers eventually decide to use microprocessor controls, the safety system will be even more effective, according to Fred Phillips, manager of advanced technology at Vickers. "A microprocessor makes it possible to design a system that automatically keeps the crane outside the forbidden zone," he said. Vickers' electrohydraulic valves are being put to novel uses. For example, Iwerks Entertainment in Burbank, Calif., uses the company's electrohydraulic proportional valves in a motion simulator used in the entertainment industry. This machine consists of a seat that shakes to simulate movements such as acceleration, deceleration, and rotation. Electrohydraulic valves are also used in the control of rotary motion. The hydraulics division of Eaton makes an electronic speed-control system that improves the durability of transit mixers used in the construction industry to mix concrete. The lifetime of the machine is improved because electrohydraulic valves provide for the automatic control of speed of the mixing drum, independent of engine speed. The engine drives the drum through a hydraulic pump, motor, and gear reducer. The drum rotates at between 1 and 17 rpm when concrete is mixing. Two servo valves control the flow of fluid into a hydraulic servo in the pump. Two solenoids dedicated to different flow directions and mounted to each valve take signals from a computer controller and allow the hydraulic fluid to flow at a rate and pressure that rotates the mixing drum at the desired speed. The computer receives continuous measurements of the drum's speed from a Hall-effect sensor mounted at the output shaft of the pump. Without electronic controls, the mixer accelerates and decelerates as speed increases and decreases in the engine. Rotating in proportion to engine speed influences the mixer's durability, a characteristic that largely depends on how many times the concrete drum turns during a lifetime. The drum's rotation also influences the amount of fuel the engine consumes. With the electronic speed-control system, the machine operator can reduce the speed of the mixing drum to 1 rpm, effectively breaking the link between the speed of the mixing machine and that of the engine. The mixing drum, consequently, turns far fewer times on its way to a work site. Mechanical energy saved on the journey is put to better use at the work site. Eaton claims the electronic system adds a year to the life of a transit mixer. Further, a study conducted by the company shows that the electronic system reduces the engine's fuel consumption by 0.8 gallon for each load of concrete due to the engine generating less horsepower. Among other benefits, the speed-control system automatically controls the speed at which concrete is mixed. The system's computer also stores historical information on each batch of concrete, including the number of drum turns during mixing and the length of time that has elapsed since the procedure took place. These data are valuable to building inspectors, who evaluate the integrity of the finished concrete structure. USING ELECTROHYDRAULICS OR NOT Generally, the integration of electronic circuits with hydraulic valves increases the precision and speed of a machine's motion. A particular task, such as rotating the bucket on an excavator, can be performed using less energy and with greater precision than by traditional nonelectronic methods of valve control. The traditional methods include manual control of a valve through mechanical links and automatic control by directly energizing a solenoid. Electromechanical systems can take the place of electrohydraulic systems in many applications, however. For example, ac motors often power machine-tool axes with ball-screw feed systems. Electromechanical systems in many cases are competitive with their electrohydraulic counterparts; they cannot leak oil, are often quieter, can be less expensive, and respond in a more linear fashion to analog and digital control. There are also a few drawbacks associated with using electrohydraulic systems. For example, in certain applications, component efficiency is lost because of heat. Additionally, the use of electronics may increase the complexity of designing, applying, and maintaining hydraulic systems. An electrohydraulic system designer, for example, may have to shield electronic components from harsh operating conditions, such as high and low temperatures and excessive vibration. Electrohydraulic systems are often used to control the motion of mobile machinery. Here, the defining advantage of hydraulic systems, their ability to move high loads using a relatively small volume of the machine's space, weighs heavily in the designer's decision. The decision of how to control machine motion often depends on whether the system designer is familiar with the nuances of applying and maintaining each system. Since the user is likely to be more familiar with the operation of traditional hydraulic systems, the fear of being unable to correct electronic problems is one obstacle faced by makers of electrohydraulic systems. As a result, makers of electrohydraulic components are designing products that are easy to use and maintain. Eaton's transit mixer has a diagnostic feature that allows a truck driver to pinpoint the location and reason for an electronic failure. When there is a problem, a code appears on a diagnostic panel in the cab and the driver can track the code to a troubleshooting procedure in a manual. Just as important to the acceptance of electrohydraulic systems is the user's knowledge of how the performance of a component matches a particular application. According to makers of electrohydraulic systems, over time users become more aware of the benefits of linking electronics with hydraulic components. Still, some users are skeptical of switching from traditional methods of control. "Our customers have to determine the value of adding sophistication to the hydraulic system," said Paul Smith, a manager of systems engineering at Vickers. "They have to see that the product will pay back the extra money they have invested in it. Payback may come, for example, from more parts per hour or reductions in installation and maintenance costs." The trend, Smith said, is for users to first implement an open-loop system. Then, when they become comfortable with it, they move to the more complex but more accurate closed-loop system. DEVELOPING IMPROVEMENTS The advanced technology group at Vickers is working on a number of improvements to electrohydraulic systems. For example, engineers are developing controllers that are dedicated to specific products and applications. Additionally, they are trying to improve energy efficiency and reduce noise and the potential for leakage in electrohydraulic systems. Further, the company has designed what it believes represents the electrohydraulic system of the future: an expert actuator whose primary components are a proportional valve, a cylinder, and a digital controller. In addition to reducing the number of components in a hydraulic system, the expert actuator has other bene