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本科畢業(yè)論文（設(shè)計(jì)） 翻譯資料 論文題目 某乘用車齒條助力式轉(zhuǎn)向系統(tǒng)設(shè)計(jì) 班 級(jí) 姓 名 院（系） 導(dǎo) 師 - 12 - 電動(dòng)助力轉(zhuǎn)向系統(tǒng) 電動(dòng)助力轉(zhuǎn)向系統(tǒng)是什么 電動(dòng)助力轉(zhuǎn)向系統(tǒng)是通過一個(gè)電動(dòng)機(jī)來驅(qū)動(dòng)動(dòng)力方向盤液壓泵或直接驅(qū)動(dòng)轉(zhuǎn)向聯(lián)動(dòng)裝置。電子動(dòng)力轉(zhuǎn)向的功能由于不依賴于發(fā)動(dòng)機(jī)轉(zhuǎn)速，所以能節(jié)省能源 電動(dòng)助力轉(zhuǎn)向系統(tǒng)是怎樣運(yùn)行的 傳統(tǒng)的動(dòng)力方向盤系統(tǒng)使用一條引擎輔助傳送帶駕駛泵浦，提供操作在動(dòng)力方向盤齒輪或作動(dòng)器的一個(gè)活塞協(xié)助司機(jī)的被加壓的流體。在電動(dòng)液壓的指點(diǎn)，一個(gè)電子動(dòng)力方向盤概念使用一臺(tái)電動(dòng)機(jī)駕駛的一個(gè)高效率泵浦。 泵浦速度是由一個(gè)電控制器調(diào)控的變化泵浦壓力和流程，提供被剪裁的指點(diǎn)努力為不同的駕駛的情況。 泵浦可以跑在低速或關(guān)閉提供節(jié)能在大多時(shí)間在多數(shù)世界市場上)直向前的駕駛期間(直接電指點(diǎn)使用一臺(tái)電動(dòng)機(jī)附加指點(diǎn)機(jī)架通過齒輪機(jī)構(gòu)(沒有泵浦或流體)。 各種各樣的馬達(dá)類型和齒輪驅(qū)動(dòng)是可能的。 微處理器控制指點(diǎn)動(dòng)力學(xué)和司機(jī)努力。 輸入包括車速和指點(diǎn)、輪子扭矩，角位和轉(zhuǎn)動(dòng)率。 工作運(yùn)行時(shí)的具體細(xì)節(jié) A “指點(diǎn)傳感器”位于它進(jìn)入傳動(dòng)箱住房的輸入軸。 指點(diǎn)傳感器實(shí)際上是在一個(gè)的二個(gè)傳感器： 那“扭矩的傳感器”轉(zhuǎn)換指點(diǎn)扭矩輸入和它的方向成電壓信號(hào)，并且那“自轉(zhuǎn)的傳感器”轉(zhuǎn)換轉(zhuǎn)動(dòng)速度和方向成電壓信號(hào)。 分享同一套住房的“接口”電路轉(zhuǎn)換從扭矩傳感器和自轉(zhuǎn)傳感器的信號(hào)成控制電子學(xué)可能處理的信號(hào)。從指點(diǎn)傳感器的輸入由那微處理器的控制單元消化也監(jiān)測從車速傳感器的輸入。 傳感器輸入然后被比較確定多少機(jī)械化根據(jù)一張被預(yù)編程序的“力量地圖”需要在控制單元的記憶。 控制單元然后派出適當(dāng)?shù)拿顚θ缓蠊┙o電動(dòng)機(jī)以潮流的“電源裝置”。 馬達(dá)推擠機(jī)架在右邊或左根據(jù)哪個(gè)方式電壓流動(dòng)(扭轉(zhuǎn)潮流扭轉(zhuǎn)方向馬達(dá)旋轉(zhuǎn))。 增加潮流對馬達(dá)增加功率協(xié)助。系統(tǒng)有三種操作方式： 左邊或右邊機(jī)械化提供以回應(yīng)從指點(diǎn)扭矩和自轉(zhuǎn)傳感器的輸入的輸入的“正?！笨刂品绞? 被用于在完成輪以后協(xié)助指點(diǎn)回歸的“回歸”控制方式; 并且改變與車速改進(jìn)路感受和挫傷傭金的“更加潮濕的”控制方式。如果方向盤被轉(zhuǎn)動(dòng)，并且舉行在充分鎖位置和指點(diǎn)協(xié)助到達(dá)最大值，控制單元使潮流降低到電動(dòng)機(jī)防止也許損壞馬達(dá)的超載情況。 控制單元也被設(shè)計(jì)保護(hù)馬達(dá)以防止電壓浪涌免受一個(gè)有毛病的交流發(fā)電機(jī)或充電的問題。 電子轉(zhuǎn)向控制單位有能力在自我診斷的缺點(diǎn)通過監(jiān)測系統(tǒng)輸入和產(chǎn)品和電動(dòng)機(jī)的激勵(lì)電流上。 如果問題發(fā)生，控制單元通過開動(dòng)在電源裝置的一個(gè)故障自動(dòng)保險(xiǎn)的中轉(zhuǎn)關(guān)閉系統(tǒng)。 這消滅所有機(jī)械化，造成系統(tǒng)恢復(fù)回到手工指點(diǎn)。 破折號(hào)EPS警告燈也被闡明警告司機(jī)。 要診斷問題，技術(shù)員跳服務(wù)檢查連接器的終端并且讀出問題代碼。 當(dāng)前發(fā)明與提供的供給動(dòng)力的援助一電子功率驅(qū)動(dòng)器馬達(dá)關(guān)連給車操縱機(jī)構(gòu)。根據(jù)當(dāng)前發(fā)明的一個(gè)方面，那里為提供供給動(dòng)力的援助提供一個(gè)電子功率驅(qū)動(dòng)器機(jī)制給有車的操縱機(jī)構(gòu)一名手動(dòng)地可旋轉(zhuǎn)的成員為操作操縱機(jī)構(gòu)、傳動(dòng)機(jī)構(gòu)包括可行扭矩的傳感器感覺手動(dòng)地被申請于可旋轉(zhuǎn)的成員的扭矩，一個(gè)電子功率驅(qū)動(dòng)器馬達(dá)操縱著被連接到可旋轉(zhuǎn)的成員和安排控制主驅(qū)動(dòng)電動(dòng)機(jī)自轉(zhuǎn)速度和方向以回應(yīng)從扭矩傳感器收到的信號(hào)的控制器，扭矩傳感器包括為與可旋轉(zhuǎn)的成員的連接適應(yīng)的傳感器軸形成引伸因此，以便扭矩通過前述傳感器軸被傳送，當(dāng)時(shí) 可旋轉(zhuǎn)的成員被轉(zhuǎn)動(dòng)，并且應(yīng)變儀在導(dǎo)致的信號(hào)傳感器軸手動(dòng)地登上表示通過前述軸被傳送的相當(dāng)數(shù)量扭矩。 傳感器軸不旋轉(zhuǎn)更好地登上在一個(gè)軸向末端在第一名聯(lián)結(jié)成員和不旋轉(zhuǎn)地登上在它的相反軸向末端在第二名聯(lián)結(jié)成員，第一和第二名聯(lián)結(jié)成員相互允諾允許有限的自轉(zhuǎn)之間連接，以便在一個(gè)被預(yù)先決定的極限之下的扭矩由僅傳感器軸傳送，并且，以便在前述被預(yù)先決定的極限之上的扭矩通過第一和第二名聯(lián)結(jié)成員被傳送。 更適宜地安排第一和第二名聯(lián)結(jié)成員作為操縱的連接的第一和第二個(gè)部分的一座橋梁互相的旋轉(zhuǎn)式成員。合適的傳感器軸是通常在多數(shù)的長方形橫斷面它的長度中。應(yīng)變儀包括一個(gè)或更多的適應(yīng)地看見了諧振器綁到傳感器軸上。好的馬達(dá)操縱的被連接到可旋轉(zhuǎn)的成員通過傳動(dòng)器。馬達(dá)更好地包括一個(gè)工具箱和同心地被安排相對可旋轉(zhuǎn)的成員。當(dāng)前發(fā)明的許多方面此后將描述，關(guān)于伴隨的圖畫 ：圖1是一個(gè)車操縱機(jī)構(gòu)的一個(gè)圖表看法包括一個(gè)電子功率驅(qū)動(dòng)器機(jī)制根據(jù)當(dāng)前發(fā)明，圖 2是說明在圖顯示的傳動(dòng)機(jī)構(gòu)的各種各樣的組分的之間流程圖互作用1上，圖 3是一個(gè)軸截面通過在圖顯示的傳動(dòng)機(jī)構(gòu)1，圖4上是一張截面圖被采取沿著線IV-IV在表3，圖5是在圖顯示的輸入推進(jìn)聯(lián)結(jié)的一張更加詳細(xì)的分解圖3上，和圖 6是顯示在表3.的傳動(dòng)器的一張更加詳細(xì)的分解圖。 圖1的最初Referring，那里顯示一個(gè)車操縱機(jī)構(gòu)10操縱的被連接到一個(gè)對易操縱的路輪子12。這個(gè)顯示的操縱機(jī)構(gòu)包括一個(gè)齒條和齒輪匯編14被連接到路輪子12通過聯(lián)接15。 鳥翼末端(沒顯示)匯編14可旋轉(zhuǎn)地駕駛一名手動(dòng)地可旋轉(zhuǎn)的成員以駕駛桿18的形式哪些由方向盤19手動(dòng)地轉(zhuǎn)動(dòng)。這個(gè)駕駛桿18包括包括一臺(tái)電主驅(qū)動(dòng)電動(dòng)機(jī)的一個(gè)電力的傳動(dòng)機(jī)構(gòu)30 (沒顯示在駕駛的鳥翼末端圖1)上以回應(yīng)在駕駛桿18的扭矩裝貨為了為機(jī)械人員提供力量援助，當(dāng)轉(zhuǎn)動(dòng)方向盤19時(shí)。如概要地被說明在表2，電力的傳動(dòng)機(jī)構(gòu)包括測量駕駛桿申請的扭矩18，當(dāng)駕駛鳥翼末端時(shí)并且提供信號(hào)給控制器40的扭矩傳感器20?？刂破?0被連接到主驅(qū)動(dòng)電動(dòng)機(jī)50并且控制電流被提供給馬達(dá)50控制馬達(dá)50和它的自轉(zhuǎn)的方向扭矩引起的相當(dāng)數(shù)量。馬達(dá)50 操縱的更適宜地被連接到駕駛桿18通過工具箱60，更適宜地一個(gè)周轉(zhuǎn)齒輪箱子和傳動(dòng)器70。 在一定條件下傳動(dòng)器70在正常運(yùn)行時(shí)更適宜地永久地接合并且是有效的隔絕從馬達(dá)50的驅(qū)動(dòng)使鳥翼末端通過傳動(dòng)機(jī)構(gòu)30手動(dòng)地被駕駛。 這是使機(jī)制的安全特點(diǎn)起作用在試圖的馬達(dá)50情形下駕駛太快速的駕駛桿并且/或者在錯(cuò)誤的方向或在案件 電動(dòng)機(jī)和工具箱占領(lǐng)了。 扭矩傳感器20更適宜地是一個(gè)匯編包括在扭矩應(yīng)用達(dá)到的傳感器軸登上應(yīng)變儀能夠準(zhǔn)確測量張力在一個(gè)被預(yù)先決定的范圍之內(nèi)的一個(gè)短的傳感器軸。被測量扭矩的被預(yù)先決定的范圍是0-lONm; 更好是關(guān)于l-5Nm。被測量的扭矩的范圍更好地對應(yīng)于大約0-1000微指令，并且傳感器軸的建筑更好被選擇這樣5Nm扭矩比在軸的2導(dǎo)致較少的轉(zhuǎn)彎，少于1 。好的應(yīng)變儀是鋸諧振器，在WO91/13832被描述的一臺(tái)適當(dāng)?shù)匿徶C振器。 類似在圖顯示的那WO91/13832 3上更好地運(yùn)用配置，二看見諧振器被安排在對軸軸的45和在90對互相。諧振器經(jīng)營與在200-400 MHz之間共鳴頻率和被安排導(dǎo)致信號(hào)到控制器1 MHz 40 500 KHz根據(jù)傳感器軸的自轉(zhuǎn)方向的自我調(diào)節(jié)。 因此，當(dāng)傳感器軸不被扭轉(zhuǎn)的歸結(jié)于缺乏扭矩時(shí)，它導(dǎo)致一個(gè)1 MHz信號(hào)。當(dāng)它導(dǎo)致在1.0到1.5 MHz之間的一個(gè)信號(hào)的傳感器軸在一個(gè)方向被扭轉(zhuǎn)。 當(dāng)傳感器軸在相反方向時(shí)被扭轉(zhuǎn)它導(dǎo)致在1.0到0.5 MHz之間的一個(gè)信號(hào)。 因而同樣傳感器能導(dǎo)致信號(hào)表示程度扭矩并且傳感器軸的自轉(zhuǎn)的方向。好的馬達(dá)扭矩引起的相當(dāng)數(shù)量以回應(yīng)在0-10Nm之間被測量的扭矩是0-40Nm，并且為在l-5Nm之間被測量的扭矩是0-25Nm。反饋電路提供自我調(diào)節(jié)，借以馬達(dá)使用的電流由控制器40測量并且比較保證馬達(dá)在正確方向運(yùn)行并且提供期望功率協(xié)助。 控制器更好地行動(dòng)使被測量的扭矩降低到零和如此控制馬達(dá)增加它的扭矩產(chǎn)品減少被測量的扭矩。 (沒顯示)更適宜地提供車速傳感器哪些寄發(fā)一個(gè)信號(hào)表示車速到控制器。 控制器使用這個(gè)信號(hào)修改程度力量協(xié)助提供以回應(yīng)被測量的扭矩。將提供在低車速最大力量協(xié)助的，因而，并且將提供高車速極小的力量協(xié)助。更適宜地是邏輯順序器有一個(gè)現(xiàn)場可編程序的門數(shù)組例如XC 4005如Xilinx供應(yīng)這個(gè)控制器。 這樣控制器不依靠軟件和，因此能更起作用可靠地在汽車車環(huán)境里。 被想象也許使用有邏輯的序列一個(gè)現(xiàn)場可編程序的列陣。 一個(gè)電力傳動(dòng)機(jī)構(gòu)10的A具體建筑在表3.被說明。 電子動(dòng)力轉(zhuǎn)向系統(tǒng)（英文簡稱EPS）， 與液壓動(dòng)力轉(zhuǎn)向系統(tǒng)（HPS）相比，EPS具有很多優(yōu)點(diǎn)。即EPS的優(yōu)勢在于： 1）效率高。HPS效率很低，一般為60%~70%；而EPS與電機(jī)連接，效率高，有的可高達(dá)90%以上。 2）耗能少。汽車在實(shí)際行駛過程中，處于轉(zhuǎn)向的時(shí)間約占行駛時(shí)間的5%，對于HPS系統(tǒng)，發(fā)動(dòng)機(jī)運(yùn)轉(zhuǎn)時(shí)，油泵始終處于工作狀態(tài)，油液一直在管路中循環(huán)，從而使汽車燃油消耗率增加4%~6%；而EPS僅在需要時(shí)供能，使汽車的燃油消耗率僅增加0.5%左右。 3）“路感“好。由于EPS內(nèi)部采用剛性連接，系統(tǒng)的滯后特性可以通過軟件加以控制，且可以根據(jù)駕駛員的操作習(xí)慣進(jìn)行調(diào)整。 4）回正性好。EPS結(jié)構(gòu)簡單內(nèi)部阻力小，回正性好，從而可得到最佳的轉(zhuǎn)向回正特性，改善汽車操縱穩(wěn)定性。 5）對環(huán)境污染少。HPS液壓回路中有液壓軟管和接頭，存在油液泄露問題，而且液壓軟管不可回收，對環(huán)境有有一定污染；而EPS對環(huán)境幾乎沒有污染。 6）可以獨(dú)立于發(fā)動(dòng)機(jī)工作。EPS以電池為動(dòng)力元件，只要電池電量充足，不論發(fā)動(dòng)機(jī)出于何種狀態(tài)，都可以產(chǎn)生助力作用。 7）應(yīng)有范圍廣。 8）裝配性好易于布置。 現(xiàn)在，動(dòng)力轉(zhuǎn)向系統(tǒng)已成為一些轎車的標(biāo)準(zhǔn)設(shè)置，全世界約有一半的轎車采用動(dòng)力轉(zhuǎn)向。隨著汽車電子技術(shù)的發(fā)展，目前一些轎車已經(jīng)使用電動(dòng)助力轉(zhuǎn)向器，使汽車的經(jīng)濟(jì)性、動(dòng)力性和機(jī)動(dòng)性都有所提高。電動(dòng)助力轉(zhuǎn)向裝置是汽車上一種新的助力轉(zhuǎn)向系統(tǒng)裝置,近年來在國內(nèi)外發(fā)展迅速,由于它采用了可編程電子控制裝置,在帶來靈活性的同時(shí)也存在著安全隱患.在分析這種產(chǎn)品特殊性的基礎(chǔ)上,筆者結(jié)合電子控制裝置的特點(diǎn),指出了事關(guān)安全性的因素,提出了處理安全性的措施,并討論了幾個(gè)事關(guān)安全性的具體問題.研究結(jié)果表明:現(xiàn)有標(biāo)準(zhǔn)不能夠滿足電動(dòng)助力轉(zhuǎn)向裝置安全性的需要;并提出了對電動(dòng)助力轉(zhuǎn)向裝置進(jìn)行安全性測評(píng)的思想.研究工作對電動(dòng)助力轉(zhuǎn)向裝置的開發(fā)以及評(píng)價(jià)具有參考意義。 Electronic power steering system What it is Electrically powered steering uses an electric motor to drive either the power steering hydraulic pump or the steering linkage directly. The power steering function is therefore independent of engine speed, resulting in significant energy savings. How it works : Conventional power steering systems use an engine accessory belt to drive the pump, providing pressurized fluid that operates a piston in the power steering gear or actuator to assist the driver. In electro-hydraulic steering, one electrically powered steering concept uses a high efficiency pump driven by an electric motor. Pump speed is regulated by an electric controller to vary pump pressure and flow, providing steering efforts tailored for different driving situations. The pump can be run at low speed or shut off to provide energy savings during straight ahead driving (which is most of the time in most world markets). Direct electric steering uses an electric motor attached to the steering rack via a gear mechanism (no pump or fluid). A variety of motor types and gear drives is possible. A microprocessor controls steering dynamics and driver effort. Inputs include vehicle speed and steering, wheel torque, angular position and turning rate. Working In Detail: A "steering sensor" is located on the input shaft where it enters the gearbox housing. The steering sensor is actually two sensors in one: a "torque sensor" that converts steering torque input and its direction into voltage signals, and a "rotation sensor" that converts the rotation speed and direction into voltage signals. An "interface" circuit that shares the same housing converts the signals from the torque sensor and rotation sensor into signals the control electronics can process. Inputs from the steering sensor are digested by a microprocessor control unit that also monitors input from the vehicles speed sensor. The sensor inputs are then compared to determine how much power assist is required according to a preprogrammed "force map" in the control units memory. The control unit then sends out the appropriate command to the "power unit" which then supplies the electric motor with current. The motor pushes the rack to the right or left depending on which way the voltage flows (reversing the current reverses the direction the motor spins). Increasing the current to the motor increases the amount of power assist. The system has three operating modes: a "normal" control mode in which left or right power assist is provided in response to input from the steering torque and rotation sensors inputs; a "return" control mode which is used to assist steering return after completing a turn; and a "damper" control mode that changes with vehicle speed to improve road feel and dampen kickback. If the steering wheel is turned and held in the full-lock position and steering assist reaches a maximum, the control unit reduces current to the electric motor to prevent an overload situation that might damage the motor. The control unit is also designed to protect the motor against voltage surges from a faulty alternator or charging problem. The electronic steering control unit is capable of self-diagnosing faults by monitoring the systems inputs and outputs, and the driving current of the electric motor. If a problem occurs, the control unit turns the system off by actuating a fail-safe relay in the power unit. This eliminates all power assist, causing the system to revert back to manual steering. A dash EPS warning light is also illuminated to alert the driver. To diagnose the problem, a technician jumps the terminals on the service check connector and reads out the trouble codes. Electric power steering systems promise weight reduction, fuel savings and package flexibility, at no cost penalty. Europes high fuel prices and smaller vehicles make a fertile testbed for electric steering, a technology that promises automakers weight savings and fuel economy gains. And in a short time, electric steering will make it to the U.S., too. "Its just just a matter of time," says Aly Badawy, director of research and development for Delphi Saginaw Steering Systems in Saginaw, Mich. "The issue was cost and thats behind us now. By 2002 here in the U.S. the cost of electric power steering will absolutely be a wash over hydraulic." Today, electric and hybrid-powered vehicles (EV), including Toyotas Prius and GMs EV-1, are the perfect domain for electric steering. But by 2010, a TRW Inc. internal study estimates that one out of every three cars produced in the world will be equipped with some form of electrically-assisted steering. The Cleveland-based supplier claims its new steering systems could improve fuel economy by up to 2 mpg, while enhancing handling. There are true bottom-line benefits as well for automakers by reducing overall costs and decreasing assembly time, since theres no need for pumps, hoses and fluids. Another claimed advantage is shortened development time. For instance, a Delphi group developed E-TUNE, a ride-and-handling software package that can be run off a laptop computer. "They can take that computer and plug it in, attach it to the controller and change all the handling parameters -- effort level, return ability, damping -- on the fly," Badawy says. "It used to take months." Delphi has one OEM customer that should start low-volume production in 99.Electric steering units are normally placed in one of three positions: column-drive, pinion-drive and rack-drive. Which system will become the norm is still unclear. Short term, OEMs will choose the steering system that is easiest to integrate into an existing platform. Obviously, greater potential comes from designing the system into an all-new platform."We have all three designs under consideration," says Dr. Herman Strecker, group vice president of steering systems division at ZF in Schwaebisch Gmuend, Germany. "Its up to the market and OEMs which version finally will be used and manufactured.""The large manufacturers have all grabbed hold of what they consider a core technology," explains James Handysides, TRW vice president, electrically assisted steering in Sterling Heights, Mich. His company offers a portfolio of electric steering systems (hybrid electric, rack-, pinion-, and column-drive). TRW originally concentrated on what it still believes is the purest engineering solution for electric steering--the rack-drive system. The system is sometimes refered to as direct drive or ball/nut drive.Still, this winter TRW hedged its bet, forming a joint venture with LucasVarity. The British supplier received $50 million in exchange for its electric column-drive steering technology and as sets. Initial production of the column and pinion drive electric steering systems is expected to begin in Birmingham, England, in 2000. "What we lack is the credibility in the steering market," says Brendan Conner, managing director, TRW/LucasVarity Electric Steering Ltd. "The combination with TRW provides us with a good opportunity for us to bridge that gap." LucasVarity currently has experimental systems on 11 different vehicle types, mostly European. TRW is currently supplying its EAS systems for Ford and Chrysler EVs in North America and for GMs new Opel Astra. In 1995, according to Delphi, traditional hydraulic power steering systems were on 7596 of all vehicles sold globally. That 37-million vehicle pool consumes about 10 million gallons in hydraulic fluid that could be superfluous, if electric steering really takes off. The present invention relates to an electrically powered drive mechamsm for providing powered assistance to a vehicle steering mechanism. According to one aspect of the present invention, there is provided an electrically powered driven mechanism for providing powered assistance to a vehicle steering mechanism having a manually rotatable member for operating the steering mechanism, the drive mechanism including a torque sensor operable to sense torque being manually applied to the rotatable member, an electrically powered drive motor drivingly connected to the rotatable member and a controller which is arranged to control the speed and direction of rotation of the drive motor in response to signals received from the torque sensor, the torque sensor including a sensor shaft adapted for connection to the rotatable member to form an extension thereof so that torque is transmitted through said sensor shaft when the rotatable member is manually rotated and a strain gauge mounted on the sensor shaft for producing a signal indicative of the amount of torque being transmitted through said shaft.Preferably the sensor shaft is non-rotatably mounted at one axial end in a first coupling member and is non-rotatably mounted at its opposite axial end in a second coupling member, the first and second coupling members being inter-engaged to permit limited rotation there between so that torque under a predetermined limit is transmitted by the sensor shaft only and so that torque above said predetermined limit is transmitted through the first and second coupling members.The first and second coupling members are preferably arranged to act as a bridge for drivingly connecting first and second portions of the rotating member to one another.Preferably the sensor shaft is of generally rectangular cross-section throughout the majority of its length.Preferably the strain gauge includes one or more SAW resonators secured to the sensor shaft.Preferably the motor is drivingly connected to the rotatable member via a clutch.Preferably the motor includes a gear box and is concentrically arranged relative to the rotatable member.Various aspects of the present invention will hereafter be described, with reference to the accompanying drawings, in which :Figure 1 is a diagrammatic view of a vehicle steering mechanism including an electrically powered drive mechanism according to the present invention,Figure 2 is a flow diagram illustrating interaction between various components of the drive mechanism shown in Figure 1 ,Figure 3 is an axial section through the drive mechanism shown in Figure 1, Figure 4 is a sectional view taken along lines IV-IV in Figure 3,Figure 5 is a more detailed exploded view of the input drives coupling shown in Figure 3, and Figure 6 is a more detailed exploded view of the clutch showing in Figure 3. Referring initially to Figure 1 , there is shown a vehicle steering mechanism 10 drivingly connected to a pair of steerable road wheels The steering mechanism 10 shown includes a rack and pinion assembly 14 connected to the road wheels 12 via joints 15. The pinion(not shown) of assembly 14 is rotatably driven by a manually rotatable member in the form of a steering column 18 which is manually rotated by a steering wheel 19.The steering column 18 includes an electric powered drive mechanism 30 which includes an electric drive motor (not shown in Figure 1) for driving the pinion in response to torque loadings in the steering column 18 in order to provide power assistance for the operative when rotating the steering wheel 19.As schematically illustrated in Figure 2, the electric powered drive mechanism includes a torque sensor20 which measures the torque applied by the steering column 18 when driving the pinion and supplies a signal to a controller 40. The controller 40 is connected to a drive motor 50 and controls the electric current supplied to the motor 50 to control the amount of torque generated by the motor 50 and the direction of its rotation.The motor 50 is drivingly connected to the steering column 18 preferably via a gear box 60, preferably an epicyclic gear box, and a clutch 70. The clutch 70 is preferably permanently engaged during normal operation and is operative under certain conditions to isolate drive from the motor 50 to enable the pinion to be driven manually through the drive mechanism 30. This is a safety feature to enable the mechanism to function in the event of the motor 50 attempting to drive the steering column too fast and/or in the wrong direction or in the case where the motor and/or gear box have seized. The torque sensor 20 is preferably an assembly including a short sensor shaft on which is mounted a strain gauge capable of accurately measuring strain in the sensor shaft brought about by the application of torque within a predetermined range.Preferably the predetermined range of torque which is measured is 0-lONm; more preferably is about l-5Nm.Preferably the range of measured torque corresponds to about 0-1000 microstrain and the construction of the sensor shaft is chosen such that a torque of 5Nm will result in a twist of less than 2 in the shaft, more preferably less than 1 .Preferably the strain gauge is a SAW resonator, a suitable SAW resonator being described in WO91/13832. Preferably a configuration similar to that shown in Figure 3 of WO91/13832 is utilised wherein two SAW resonators are arranged at 45 to the shaft axis and at 90 to one another.Preferably the resonators operate with a resonance frequency of between 200-400 MHz and are arranged to produce a signal to the controller 40 of 1 MHz 500 KHz depending upon the direction of rotation of the sensor shaft. Thus, when the sensor shaft is not being twisted due to the absence of torque, it produces a 1 MHz signal.When the sensor shaft is twisted in one direction it produces a signal between 1.0 to 1.5 MHz. When the sensor shaft is twisted in the opposite direction it produces a signal between 1.0 to 0.5 MHz. Thus the same sensor is able to produce a signal indicative of the degree of torque and also the direction of rotation of the sensor shaft.Preferably the amount of torque generated by the motor in response to a measured torque of between 0-10Nm is 0-40Nm and for a measured torque of between l-5Nm is 0-25Nm.Preferably a feed back circuit is provided whereby the electric current being used by the motor is measured and compared by the controller 40 to ensure that the motor is running in the correct direction and providing the desired amount of power assistance. Preferably the controller acts to reduce the measured torque to zero and so controls the motor to increase its torque output to reduce the measured torque.A vehicle speed sensor (not shown) is preferably provided which sends a signal indicative of vehicle speed to the controller. The controller uses this signal to modify the degree of power assistance provided in response to the measured torque.Thus at low vehicle speeds maximum power assistance will be provided and a high vehicle speeds minimum power assistance will be provided.The controller is preferably a logic sequencer having a field programmable gate array for example a XC 4005 as supplied by Xilinx. Such a controller does not rely upon software and so is able to function more reliably in a car vehicle environment. It is envisaged that a logic sequence not having a field programmable array may be used. Electronic power steering system (English as EPS), and hydraulic power steering system (HPS) compared to, EPS has many advantages. The advantage is that the EPS: 1) high efficiency. HPS efficiency is very low, generally 60% to 70%, while EPS and electrical connections, high efficiency, and some can be as high as 90 percent. 2) less energy consumption. Automobile traffic in the actual process, at the time to about 5 percent of the time travelling, the HPS system, engine running, the pumps will always be in working condition, the oil pipeline has been in circulation, so that vehicle fuel consumption rate by 4 % To 6%, while EPS only when needed for energy, vehicle fuel consumption rates only increased by 0.5 percent. 3) "Road sense of" good. Because EPS internal use of rigid, system of the lag c