中型汽車循環(huán)球液壓助力轉(zhuǎn)向系統(tǒng)設(shè)計(jì)（整體式循環(huán)球動(dòng)力轉(zhuǎn)向器）
中型汽車循環(huán)球液壓助力轉(zhuǎn)向系統(tǒng)設(shè)計(jì)（整體式循環(huán)球動(dòng)力轉(zhuǎn)向器）,中型,汽車,循環(huán),液壓,助力,轉(zhuǎn)向,系統(tǒng),設(shè)計(jì),整體,動(dòng)力,轉(zhuǎn)向器
附錄A譯文
本文以轉(zhuǎn)向幾何參數(shù)的討論為開始，包括主銷后傾角，后傾拖距，主銷內(nèi)傾角，主銷偏置量。接下來(lái)的部分討論了轉(zhuǎn)向齒輪齒條以及阿克曼轉(zhuǎn)向幾何關(guān)系。跳動(dòng)轉(zhuǎn)向和側(cè)傾轉(zhuǎn)向之間是緊密相關(guān)的，如果沒有柔性這兩種情況是等同的。最后討論了車輪的調(diào)整。與懸架幾何形狀密切相關(guān)，在設(shè)計(jì)新的底盤系統(tǒng)時(shí)，轉(zhuǎn)向和懸架幾何參數(shù)是優(yōu)先考慮的因素。轉(zhuǎn)向幾何關(guān)系（定位參數(shù)在整體式車橋上轉(zhuǎn)向節(jié)主銷是轉(zhuǎn)向時(shí)的樞軸。1932年Maurice Olley在Cadillac首次提出了現(xiàn)在的非獨(dú)立懸架，主銷因此而被兩個(gè)球絞連接定義的轉(zhuǎn)向軸線代替。因?yàn)楦鞣N原因這根軸并不是垂直的 也不在輪胎接地中心處。
圖1 主銷定位參數(shù)
Fig. 1 main pin positioning parameters
在前視圖中，主銷偏轉(zhuǎn)的角度被稱為主銷內(nèi)傾角，轉(zhuǎn)向主銷與地面的交點(diǎn)至車輪中心平面與地面相交處的距離稱之為主銷偏置量。在前軸所在水平面內(nèi)，從主銷軸心到車輪中心平面的距離稱為主銷偏距（spindle length）。在側(cè)視圖中，主銷偏轉(zhuǎn)角度稱為主銷后傾角。如果主銷軸線沒有通過(guò)車輪中心那么就有了側(cè)視的主銷偏距（side view kingpin offset），就像大部分的摩托車前輪一樣。在地平面內(nèi)測(cè)量從主銷到輪胎接地點(diǎn)中心的距離稱為主銷后傾拖距。 前視圖中的主銷定位參數(shù) 。正如在17章中提到，主銷內(nèi)傾角，主銷偏距還有主銷偏置量在裝配以及性能滿足時(shí)往往是互相妥協(xié)的。一些需要考慮的因素包括以下：?
1.當(dāng)主銷偏距是正的時(shí)（一般的車都是正主銷偏距，如圖1中一樣）那車輪轉(zhuǎn)離中心位置的時(shí)候車會(huì)有一個(gè)抬升效果。主銷內(nèi)傾角偏離豎直平面越大前輪轉(zhuǎn)向時(shí)車被抬起的效果越明顯。不管車輪往哪個(gè)方向轉(zhuǎn)都會(huì)是一個(gè)抬升的效果，除非主銷是完全垂直的。這個(gè)效果只有在主銷后傾角為零時(shí)才是兩邊對(duì)稱的。見后面關(guān)于主銷后傾角部分。對(duì)于一個(gè)給定的主銷內(nèi)傾角來(lái)說(shuō)，主銷偏距越大轉(zhuǎn)向時(shí)的抬升量也越大。?
2.主銷內(nèi)傾角和主銷偏距將車子前端抬起的效果對(duì)于自身來(lái)說(shuō)是有助于低速轉(zhuǎn)向的。在高速轉(zhuǎn)向時(shí)，只要有主銷后傾拖距就可能會(huì)掩蓋掉轉(zhuǎn)向時(shí)抬升和下落的效果。?
3.主銷內(nèi)傾角影響轉(zhuǎn)向時(shí)車輪的外傾角特性。如果主銷向內(nèi)傾斜（主銷上端傾向車輛中心）當(dāng)車輪轉(zhuǎn)向的時(shí)候，車輪上端將會(huì)向外傾斜，趨向正的車輪外傾角。左右轉(zhuǎn)向都會(huì)導(dǎo)致正的車輪外傾。如果跑道有比較緊的彎這個(gè)作用效果是比較小但卻是有重要意義的。?
4.當(dāng)車輪滾過(guò)顛簸不平的路面時(shí)，滾動(dòng)半徑是不斷變化的，將會(huì)導(dǎo)致輪速的改變。這將會(huì)增加車輪中心的縱向力。這些力的反作用與主銷偏距的大小成比例，成為反沖效果進(jìn)入轉(zhuǎn)向系統(tǒng)。如果主銷偏距為零，那么將不會(huì)有由此引起的反沖。在前面提到的一輛通用“P”型車（菲羅車）中做出設(shè)計(jì)改動(dòng)，與較早的一輛“P”型車模型相比，減小了主銷偏距，因此而減少了不平路面上的反沖。?
5.如圖1中所示的主銷偏置量是負(fù)的，正如下面這輛前輪驅(qū)動(dòng)車用的一樣。來(lái)自地面的驅(qū)動(dòng)和制動(dòng)力與主銷偏置量成比例的轉(zhuǎn)化成轉(zhuǎn)向力矩。如果左右輪的制動(dòng)或者驅(qū)動(dòng)力是不等的，那么駕駛者將會(huì)感受到的到這個(gè)轉(zhuǎn)向力矩（假設(shè)轉(zhuǎn)向器有較高的逆效率）。只有在主銷偏置量為零時(shí)才不會(huì)有這個(gè)力矩產(chǎn)生因?yàn)榇藭r(shí)制動(dòng)力或驅(qū)動(dòng)力對(duì)主銷的作用力臂為零。?如果輪胎比較寬的話輪胎力通常并不是作用在輪胎中心平面內(nèi)的，因?yàn)檩p微的外傾角變化、路面不平、輪胎有一定圓錐度、或者其他的不對(duì)稱因素存在。這些不對(duì)稱因素可能導(dǎo)致轉(zhuǎn)向反沖，即使沒有前輪的各個(gè)定位參數(shù)作用。裝配要求通常會(huì)與中心點(diǎn)轉(zhuǎn)向要求沖突因而很多賽車在較平整的賽道上是采用較大的主銷偏置量也是可以的。?
6.對(duì)于前輪驅(qū)動(dòng)來(lái)說(shuō)，一個(gè)負(fù)的主銷偏置量有兩個(gè)重要的穩(wěn)定作用：?第一，?固定方向盤，如果一個(gè)驅(qū)動(dòng)輪打滑，另外一個(gè)輪將會(huì)外張一定角度，因?yàn)檗D(zhuǎn)向系統(tǒng)內(nèi)有變形。即使兩側(cè)的牽引力不等，不同的牽引力使車輛產(chǎn)生一個(gè)偏航角，這個(gè)負(fù)的主銷偏置量作用也會(huì)使車輛回復(fù)到直線行駛。第二,有良好的反饋?zhàn)饔们闆r下駕駛員從來(lái)不會(huì)真正的固定住方向盤。在這種情況下方向盤可能在不等的車輪縱向牽引力作用下而轉(zhuǎn)動(dòng)，因此而增加了負(fù)主銷偏置量的穩(wěn)定效果。制動(dòng)的情況同樣適用。負(fù)的主銷偏置量能使車子回正，即使是在左右輪制動(dòng)力不等的情況下（左右輪的制動(dòng)情況或者路面情況不同時(shí))。?
如圖1中所示，在有后傾拖距時(shí)，側(cè)視圖中輪胎接地點(diǎn)是在主銷之后的。或許最簡(jiǎn)單的例子就是辦公室座椅上的小腳輪,不管移動(dòng)多遠(yuǎn)，輪子總會(huì)校正使其自身在樞軸之后。主銷拖距越大意味著輪胎側(cè)向力在主銷軸上作用有更大的力臂。這會(huì)產(chǎn)生更明顯的回正作用，并且是作用在主銷上最主要的回正力矩。在選擇主銷后傾角和主銷拖距時(shí)需要考慮的因素如下：
1.主銷后傾拖距越大轉(zhuǎn)向力也越大。對(duì)于所有的車來(lái)說(shuō)，小的后傾拖距都將會(huì)減小轉(zhuǎn)向力。在某些情況下，如果后傾拖距減小接近零的話，人力轉(zhuǎn)向也可能被用于重型轎車（代替助力轉(zhuǎn)向）。?
2.像主銷內(nèi)傾角一樣，主銷后傾角伴隨著轉(zhuǎn)向過(guò)程也會(huì)引起車輪的抬起和回落。與內(nèi)傾角不同的是，后傾角對(duì)兩側(cè)的影響是相反的。在有對(duì)稱定位參數(shù)時(shí)（包括左右輪有相等的正的主銷后傾角），左轉(zhuǎn)的效應(yīng)是使車向右側(cè)傾，導(dǎo)致一個(gè)對(duì)角線的重量轉(zhuǎn)移。在這種情況下，左前——右后對(duì)角線會(huì)承受更大的載荷，有一個(gè)左轉(zhuǎn)時(shí)的過(guò)度轉(zhuǎn)向效應(yīng)。使用的彈簧越硬對(duì)角線的重量轉(zhuǎn)移效果也會(huì)越明顯因?yàn)檫@個(gè)是幾何效應(yīng)。每個(gè)車輪被抬起（或者下落）的距離是恒定的但是重量抬起量和底盤側(cè)傾角是前后側(cè)傾剛度的作用結(jié)果。這個(gè)對(duì)角線的載荷轉(zhuǎn)移可以通過(guò)把車放在秤上和定位板上來(lái)測(cè)量。?記住在實(shí)際比賽中前輪并沒有轉(zhuǎn)過(guò)很大的角度，除非是非常緊的發(fā)夾彎。例如，在一個(gè)半徑是100英尺（時(shí)速在40-50英里）的彎，一個(gè)10英尺的軸距的中性轉(zhuǎn)向車輛轉(zhuǎn)彎時(shí)前輪只需要轉(zhuǎn)過(guò)5.7°（轉(zhuǎn)向傳動(dòng)比是16：1時(shí)方向盤的轉(zhuǎn)角大概在90°）。對(duì)于只往一個(gè)方向轉(zhuǎn)的車來(lái)說(shuō)，因?yàn)檎嚍榱藢で笞畹偷淖钚‰x地間隙，可以使主銷后傾角交錯(cuò)（左右主銷后傾角不同）來(lái)把車?yán)揭贿?。主銷后傾角的交錯(cuò)也會(huì)影響上面提到的對(duì)角線重量抬升效應(yīng)。?如果兩側(cè)主銷后傾角是相反的（一側(cè)為正一側(cè)為負(fù)且兩側(cè)角度大小相等）那么在轉(zhuǎn)向時(shí)車的前端只會(huì)抬升和下落，而不會(huì)有對(duì)角線的重量抬升。?
3.主銷后傾角也會(huì)影響轉(zhuǎn)向外傾角，但是不像內(nèi)傾角一樣，這個(gè)效應(yīng)是有利的。當(dāng)有正的后傾角時(shí)將會(huì)導(dǎo)致外側(cè)車輪內(nèi)傾（車輪的上部指向車的中心）同時(shí)內(nèi)測(cè)輪外傾角為正，兩輪都向彎內(nèi)傾。在側(cè)滑恢復(fù)的時(shí)候，反打方向（出彎），后傾角引起的外傾角變化會(huì)使前輪抓地力減小。而此時(shí)后輪抓地力也很小并不需要很大的前輪抓地力。?
4.輪胎本身的輪胎拖距會(huì)使實(shí)際主銷后傾拖距明顯增加（有大的側(cè)偏角時(shí)會(huì)減?。?。這個(gè)效應(yīng)并不是隨著側(cè)向力變化而線性變化的，并且會(huì)影響轉(zhuǎn)向力矩和駕駛感。特別是輪胎到極限時(shí)輪胎拖距會(huì)接近零，這時(shí)回正力矩會(huì)減小，并給車手一個(gè)信號(hào)輪胎就要側(cè)滑了。如果主銷后傾拖距相對(duì)輪胎拖距很大的話，輪胎拖距給出的這個(gè)信號(hào)會(huì)被掩蓋。?
5.有時(shí)主銷后傾拖距是在垂直于主銷軸心的方向上進(jìn)行測(cè)量的（而不是像圖1中在水平面內(nèi)測(cè)量的），因?yàn)檫@能更準(zhǔn)確的描述輪胎側(cè)向力對(duì)主銷作用的力臂。拉桿位置圖如下：?
圖2 阿克曼轉(zhuǎn)向幾何關(guān)系
Fig. 2 Ackermann turning geometry relation
圖3 阿克曼幾何，齒條在前軸后
Fig. 3 Ackerman geometry, rack behind front axle
注意在圖1中的陰影部分就是轉(zhuǎn)向拉桿的合適位置。側(cè)向力引起的外傾角是不可避免的，如果拉桿的位置如圖中示，會(huì)有不足轉(zhuǎn)向效應(yīng)。如果懸架和齒條安裝在一些柔性的副車架上，情況要比現(xiàn)在的更加復(fù)雜。
? 當(dāng)汽車前輪轉(zhuǎn)向時(shí)，轉(zhuǎn)向傳動(dòng)機(jī)構(gòu)的設(shè)計(jì)將會(huì)決定車輪是保持平行還是一個(gè)輪比另一輪轉(zhuǎn)過(guò)更多的角度。左右輪轉(zhuǎn)向的角度差不應(yīng)該被車輪前束值混淆，前束值是靜態(tài)調(diào)整時(shí)的值，他是在阿克曼幾何效應(yīng)的基礎(chǔ)上增減的。?對(duì)于橫向加速度較小的車（街車）一般使用阿克曼幾何關(guān)系。正如圖2左圖所示，這個(gè)幾何關(guān)系保證了所有輪子在沒有滑動(dòng)的情況下自由滾動(dòng)，因?yàn)樗休喿又挥幸粋€(gè)滾動(dòng)中心。需注意的是在低速時(shí)所有車輪的轉(zhuǎn)彎半徑都不同，前內(nèi)側(cè)輪必須比前外輪轉(zhuǎn)過(guò)更大的角度。一個(gè)合理的近似幾何關(guān)系可見圖3。根據(jù)Ref.99中Rudolf所說(shuō)，阿克曼在1817年獲得雙樞軸轉(zhuǎn)向系統(tǒng)的專利，Charles?Jeantaud?又提出了上段中的概念，消除了轉(zhuǎn)向時(shí)車輪的滑動(dòng)。Maurice?Olley?又提出了阿克曼轉(zhuǎn)向幾何關(guān)系的推導(dǎo)以使車輪免于鐓粗平滑的礫石車道。?
在高側(cè)向加速度下要對(duì)這個(gè)幾何模型做明顯的修改。實(shí)際輪胎都會(huì)有一個(gè)側(cè)偏角，內(nèi)側(cè)輪的載荷也要比外側(cè)輪小。回顧輪胎性能曲線可以看出負(fù)載較輕的時(shí)候獲得峰值側(cè)向力所需的側(cè)偏角較小。使用低速幾何結(jié)構(gòu)（阿克曼關(guān)系），前內(nèi)側(cè)輪會(huì)被迫超過(guò)對(duì)應(yīng)最大側(cè)向力時(shí)的側(cè)偏角，這樣，拖動(dòng)內(nèi)輪會(huì)使輪胎升溫并降低車速。對(duì)于賽車來(lái)說(shuō)，通常使用平行轉(zhuǎn)向甚至反阿克曼結(jié)構(gòu)如圖2中（b）（c）所示。?如果知道輪胎參數(shù)通常可以計(jì)算出正確的反阿克曼量。大部分情況下計(jì)算得到的幾何關(guān)系是比較極端的因?yàn)檐嚳隙〞?huì)有低速行駛的情況，如進(jìn)站加油等。?另外值得注意的一點(diǎn)是比賽時(shí)大部分彎道半徑都比較大阿克曼影響是非常小的。實(shí)際上，除非懸架、轉(zhuǎn)向系統(tǒng)結(jié)構(gòu)剛度很大，轉(zhuǎn)向載荷作用下產(chǎn)生的變形也可能使車輪轉(zhuǎn)向，會(huì)超過(guò)幾何關(guān)系上的阿克曼轉(zhuǎn)角關(guān)系（或者反阿克曼）。能產(chǎn)生阿克曼幾何關(guān)系的最簡(jiǎn)單模型見圖3，為后置轉(zhuǎn)向。這里，齒條（以及轉(zhuǎn)向器系統(tǒng)內(nèi)的橫拉桿連接）是在前軸之后的，從主銷軸心開始畫線，延伸到橫拉桿外端，并交于后軸中心。轉(zhuǎn)向節(jié)的這個(gè)角度使內(nèi)輪轉(zhuǎn)向角度大于外輪（轉(zhuǎn)向時(shí)外張）可以獲得一個(gè)較好的近似的100%阿克曼關(guān)系。?第二種獲得內(nèi)外輪轉(zhuǎn)角差的方法是通過(guò)前移或后移齒條（或拉桿）的位置，這時(shí)兩個(gè)拉桿外端球頭間的連接不再是直線連接。圖中后置梯形將齒條前移時(shí)將傾向于平行轉(zhuǎn)向（最后至反阿克曼），齒條后移將增加轉(zhuǎn)向時(shí)的前輪外張量（內(nèi)外輪轉(zhuǎn)角差更大）。?
圖4 變更的轉(zhuǎn)向幾何關(guān)系-齒條前移或后移
Fig.4 changing steering geometry-rack forward or backward
第三種獲得轉(zhuǎn)角差的方法是使兩邊轉(zhuǎn)向節(jié)臂不等長(zhǎng)。節(jié)臂（從主銷軸至拉桿外端的距離）越短轉(zhuǎn)向時(shí)轉(zhuǎn)角越大。當(dāng)然這種不對(duì)稱結(jié)構(gòu)僅會(huì)用于只向一個(gè)方向轉(zhuǎn)向的車輛——橢圓形賽道賽車。雖然上文提到的一些要求間會(huì)有沖突，筆者認(rèn)為平行轉(zhuǎn)向或者反阿克曼是一個(gè)較合理的折中方法。雖然平行轉(zhuǎn)向時(shí)進(jìn)站會(huì)有一點(diǎn)困難因?yàn)榍拜啎?huì)互相干涉。在高速時(shí)，彎道較大，轉(zhuǎn)向角較小，相對(duì)參考的轉(zhuǎn)向角度，阿克曼效應(yīng)對(duì)于車輪的側(cè)偏角影響不大。
隨著汽車電子技術(shù)的迅猛發(fā)展,人們對(duì)汽車轉(zhuǎn)向操縱性能的要求也日益提高。汽車轉(zhuǎn)向系統(tǒng)已從傳統(tǒng)機(jī)械轉(zhuǎn)向、液壓助力轉(zhuǎn)向(Hydraulic Power Steering ,簡(jiǎn)稱HPS) 、電控液壓助力轉(zhuǎn)向( Electric Hydraulic PowerSteering , 簡(jiǎn)稱EHPS) ,發(fā)展到電動(dòng)助力轉(zhuǎn)向系統(tǒng)(Electric Power Steering ,簡(jiǎn)稱EPS) ,最終還將過(guò)渡到線控轉(zhuǎn)向系統(tǒng)(Steer By Wire ,簡(jiǎn)稱SBW)。
機(jī)械轉(zhuǎn)向系統(tǒng)是指以駕駛員的體力作為轉(zhuǎn)向能源,其中所有傳力件都是機(jī)械的,汽車的轉(zhuǎn)向運(yùn)動(dòng)是由駕駛員操縱方向盤,通過(guò)轉(zhuǎn)向器和一系列的桿件傳遞到轉(zhuǎn)向車輪而實(shí)現(xiàn)的。機(jī)械轉(zhuǎn)向系由轉(zhuǎn)向操縱機(jī)構(gòu)、轉(zhuǎn)向器和轉(zhuǎn)向傳動(dòng)機(jī)械3大部分組成。
通常根據(jù)機(jī)械式轉(zhuǎn)向器形式可以分為:齒輪齒條式、循環(huán)球式、蝸桿滾輪式、蝸桿指銷式。應(yīng)用最廣的兩種是齒輪齒條式和循環(huán)球式(用于需要較大的轉(zhuǎn)向力時(shí)) 。在循環(huán)球式轉(zhuǎn)向器中,輸入轉(zhuǎn)向圈與輸出的轉(zhuǎn)向搖臂擺角是成正比的;在齒輪齒條式轉(zhuǎn)向器中,輸入轉(zhuǎn)向圈數(shù)與輸出的齒條位移是成正比的。循環(huán)球式轉(zhuǎn)向器由于是滾動(dòng)摩擦形式,因而正傳動(dòng)效率很高,操作方便且使用壽命長(zhǎng),而且承載能力強(qiáng),故廣泛應(yīng)用于載貨汽車上。齒輪齒條式轉(zhuǎn)向器與循環(huán)球式相比,最大特點(diǎn)是剛性大,結(jié)構(gòu)緊湊重量輕,且成本低。由于這種方式容易由車輪將反作用力傳至轉(zhuǎn)向盤,所以具有對(duì)路面狀態(tài)反應(yīng)靈敏的優(yōu)點(diǎn),但同時(shí)也容易產(chǎn)生打手和擺振等現(xiàn)象,且其承載效率相對(duì)較弱,故主要應(yīng)用于小汽車及輕型貨車上,目前大部分低端轎車采用的就是齒輪齒條式機(jī)械轉(zhuǎn)向系統(tǒng)。
隨著車輛載重的增加以及人們對(duì)車輛操縱性能要求的提高,簡(jiǎn)單的機(jī)械式轉(zhuǎn)向系統(tǒng)已經(jīng)無(wú)法滿足需要,動(dòng)力轉(zhuǎn)向系統(tǒng)應(yīng)運(yùn)而生,它能在駕駛員轉(zhuǎn)動(dòng)方向盤的同時(shí)提供助力,動(dòng)力轉(zhuǎn)向系統(tǒng)分為液壓轉(zhuǎn)向系統(tǒng)和電動(dòng)轉(zhuǎn)向系統(tǒng)2 種。其中液壓轉(zhuǎn)向系統(tǒng)是目前使用最為廣泛的轉(zhuǎn)向系統(tǒng)。
液壓轉(zhuǎn)向系統(tǒng)在機(jī)械系統(tǒng)的基礎(chǔ)上增加了液壓系統(tǒng),包括液壓泵、V 形帶輪、油管、供油裝置、助力裝置和控制閥。它借助于汽車發(fā)動(dòng)機(jī)的動(dòng)力驅(qū)動(dòng)液壓泵、空氣壓縮機(jī)和發(fā)電機(jī)等,以液力、氣力或電力增大駕駛員操縱前輪轉(zhuǎn)向的力量,使駕駛員可以輕便靈活地操縱汽車轉(zhuǎn)向,減輕了勞動(dòng)強(qiáng)度,提高了行駛安全性。
液壓助力轉(zhuǎn)向系統(tǒng)從發(fā)明到現(xiàn)在已經(jīng)有了大約半個(gè)世紀(jì)的歷史,可以說(shuō)是一種較為完善的系統(tǒng),由于其工作可靠、技術(shù)成熟至今仍被廣泛應(yīng)用。它由液壓泵作為動(dòng)力源,經(jīng)油管道控制閥向動(dòng)力液壓缸供油,通過(guò)活塞桿帶動(dòng)轉(zhuǎn)向機(jī)構(gòu)動(dòng)作,可通過(guò)改變缸徑及油壓的大小來(lái)改變助力的大小,由此達(dá)到轉(zhuǎn)向助力的作用。傳統(tǒng)液壓式動(dòng)力轉(zhuǎn)向系統(tǒng)一般按液流的形式可以分為:常流式和常壓式2 種類型,也可根據(jù)控制閥形式分為轉(zhuǎn)閥式和滑閥式。
隨著液壓動(dòng)力轉(zhuǎn)向系統(tǒng)在汽車上的日益普及,人們對(duì)操作時(shí)的輕便性和路感的要求也日益提高,然而液壓動(dòng)力轉(zhuǎn)向系統(tǒng)卻存在許多的缺點(diǎn):①由于其本身的結(jié)構(gòu)決定了其無(wú)法保證車輛在任何工況下轉(zhuǎn)動(dòng)轉(zhuǎn)向盤時(shí),都有較理想的操縱穩(wěn)定性,即無(wú)法同時(shí)保證低速時(shí)的轉(zhuǎn)向輕便性和高速時(shí)的操縱穩(wěn)定性;②汽車的轉(zhuǎn)向特性受駕駛員駕駛技術(shù)的影響嚴(yán)重;③轉(zhuǎn)向傳動(dòng)比固定,使汽車轉(zhuǎn)向響應(yīng)特性隨車速、側(cè)向加速度等變化而變化,駕駛員必須提前針對(duì)汽車轉(zhuǎn)向特性幅值和相位的變化進(jìn)行一定的操作補(bǔ)償,從而控制汽車按其意愿行駛。這樣增加了駕駛員的操縱負(fù)擔(dān),也使汽車轉(zhuǎn)向行駛中存在不安全隱患;而此后出現(xiàn)了電控液壓助力系統(tǒng),它在傳統(tǒng)的液壓動(dòng)力轉(zhuǎn)向系統(tǒng)的基礎(chǔ)上增加速度傳感器,使汽車能夠隨著車速的變化自動(dòng)調(diào)節(jié)操縱力的大小,在一定程度上緩和了傳統(tǒng)的液壓轉(zhuǎn)向系統(tǒng)存在的問題.
目前我國(guó)生產(chǎn)的商用車和轎車上采用的大多是電控液壓助力轉(zhuǎn)向系統(tǒng),它是比較成熟和應(yīng)用廣泛的轉(zhuǎn)向系統(tǒng)。盡管電控液壓助力裝置從一定程度上緩解了傳統(tǒng)的液壓轉(zhuǎn)向中輕便性和路感之間的矛盾,然而它還是沒有從根本上解決HPS 系統(tǒng)存在的不足,隨著汽車微電子技術(shù)的發(fā)展,汽車燃油節(jié)能的要求以及全球性倡導(dǎo)環(huán)保,其在布置、安裝、密封性、操縱靈敏度、能量消耗、磨損與噪聲等方面的不足已越來(lái)越明顯,轉(zhuǎn)向系統(tǒng)向著電動(dòng)助力轉(zhuǎn)向系統(tǒng)發(fā)展。
電動(dòng)助力轉(zhuǎn)向系統(tǒng)是現(xiàn)在汽車轉(zhuǎn)向系統(tǒng)的發(fā)展方向,其工作原理是:EPS 系統(tǒng)的ECU 對(duì)來(lái)自轉(zhuǎn)向盤轉(zhuǎn)矩傳感器和車速傳感器的信號(hào)進(jìn)行分析處理后,控制電機(jī)產(chǎn)生適當(dāng)?shù)闹D(zhuǎn)矩,協(xié)助駕駛員完成轉(zhuǎn)向操作。近幾年來(lái),隨著電子技術(shù)的發(fā)展,大幅度降低EPS的成本已成為可能,日本的大發(fā)汽車公司、三菱汽車公司、本田汽車公司、美國(guó)的Delphi 汽車系統(tǒng)公司、TRW公司及德國(guó)的ZF 公司都相繼研制出EPS。Mercedes2Benz 和Siemens Automotive 兩大公司共同投資6500萬(wàn)英鎊用于開發(fā)EPS ,目標(biāo)是到2002 年裝車,年產(chǎn)300 萬(wàn)套,成為全球EPS 制造商。到目前為止,EPS 系統(tǒng)在輕微型轎車、廂式車上得到廣泛的應(yīng)用,并且每年以300 萬(wàn)臺(tái)的速度發(fā)展。
轉(zhuǎn)向是一個(gè)專業(yè)術(shù)語(yǔ),適用于采集部件,聯(lián)系等,其中允許一艘(艦船)或汽車(轎車)按照預(yù)期的方向行駛. 一個(gè)例外的情況是鐵路運(yùn)輸由路軌組合在一起鐵路道岔提供轉(zhuǎn)向功能。
許多現(xiàn)代轎車使用齒輪齒條式轉(zhuǎn)向器,在方向盤末端有轉(zhuǎn)動(dòng)齒輪;該齒輪帶動(dòng)齒條移動(dòng),它是一種線性的齒輪緊密配合,從一邊到一邊。這種運(yùn)動(dòng)把轉(zhuǎn)矩通過(guò)轉(zhuǎn)向橫拉桿和一種叫做轉(zhuǎn)向節(jié)臂的短形臂傳遞給轉(zhuǎn)向輪的主銷。
以前的設(shè)計(jì)往往采用循環(huán)球式轉(zhuǎn)向器,而這種轉(zhuǎn)向器仍然應(yīng)用在卡車和多用途車輛。這是一種老式的螺母和齒扇設(shè)計(jì),該轉(zhuǎn)向管柱轉(zhuǎn)動(dòng)大螺絲("蝸輪"),它與一個(gè)齒扇齒輪嚙合,當(dāng)蝸輪轉(zhuǎn)動(dòng)時(shí),齒扇也隨之轉(zhuǎn)動(dòng),一個(gè)安裝在齒扇軸上且與轉(zhuǎn)向聯(lián)動(dòng)有關(guān)的搖臂帶動(dòng)轉(zhuǎn)向節(jié)臂,從而使車輪轉(zhuǎn)動(dòng). 循環(huán)球式轉(zhuǎn)向器通過(guò)安裝滾珠減少螺母和螺桿之間的摩擦;兩根導(dǎo)管和螺母內(nèi)的螺旋管狀通道組合成兩條各自獨(dú)立的封閉的鋼球“流動(dòng)”。
齒輪齒條式轉(zhuǎn)向器設(shè)計(jì)具有很大程度的反饋和直接轉(zhuǎn)向"路感";它也通常不會(huì)有任何反彈,或呆滯。缺點(diǎn)是,它是不可調(diào)的,因此當(dāng)它磨損唯一的解決辦法更換。
循環(huán)球式轉(zhuǎn)向器的優(yōu)點(diǎn)是機(jī)械優(yōu)勢(shì),因此,它被使用在較大較重的車輛,而齒輪齒條式原本僅限于較小和較輕;由于幾乎普遍采用動(dòng)力轉(zhuǎn)向系統(tǒng),不過(guò),這已不再是一個(gè)重要的優(yōu)勢(shì),導(dǎo)致越來(lái)越多地在新型汽車應(yīng)用齒輪齒條式轉(zhuǎn)向器。循環(huán)球式轉(zhuǎn)向器設(shè)計(jì)在中心也有明顯的沖擊,或"死點(diǎn)"。凡一分鐘交替方向盤出不來(lái)并不移動(dòng)轉(zhuǎn)向機(jī)構(gòu);這是很容易可調(diào)螺桿的端部來(lái)減少磨損,但它并不能完全消除或機(jī)制開始磨損很快。這項(xiàng)設(shè)計(jì)目前仍在使用中,在卡車和其他大型車輛,也應(yīng)用于迅速轉(zhuǎn)向,路感與穩(wěn)健性,可維護(hù)性,和機(jī)械的優(yōu)勢(shì)相比不太重要的場(chǎng)合。較小程度的反饋,這樣的設(shè)計(jì)也有時(shí)是一種優(yōu)點(diǎn);當(dāng)前輪碰撞時(shí),使用齒輪齒條轉(zhuǎn)向的司機(jī)只有自己的大拇指受傷,造成方向盤揭開一邊突然(因?yàn)轳{駛教練告訴學(xué)生把自己的大拇指在前面的方向盤,而非放在左右的內(nèi)邊緣). 這種效果在像卡車一樣的重型汽車更為明顯;循環(huán)球式轉(zhuǎn)向防止這種程度的反饋,只是因?yàn)樗梢栽谡Ｇ闆r下防止可取反饋。
轉(zhuǎn)向連鎖連接轉(zhuǎn)向器和車輪通常符合一個(gè)阿克曼轉(zhuǎn)向幾何的變化,它交代了一個(gè)事實(shí):當(dāng)轉(zhuǎn)向是,內(nèi)輪轉(zhuǎn)過(guò)的半徑比外輪小得多,因此適合駕駛的直路,是不適合曲折。
由于車輛已成為較重而改用前輪驅(qū)動(dòng),為了扭轉(zhuǎn)方向盤,通常的,主要的是體力。為了解決這一問題,汽車業(yè)發(fā)展的動(dòng)力轉(zhuǎn)向系統(tǒng)。有兩種類型的助力轉(zhuǎn)向系統(tǒng)-液壓和電氣/電子。還有一種液壓-電動(dòng)混合系統(tǒng)。
液壓助力轉(zhuǎn)向系統(tǒng)(hps)利用油壓供應(yīng)的一個(gè)發(fā)動(dòng)機(jī)驅(qū)動(dòng)泵,以協(xié)助將方向盤轉(zhuǎn)轉(zhuǎn)動(dòng)。電動(dòng)助力轉(zhuǎn)向系統(tǒng)(EPS)方式,是較有效率的液壓助力轉(zhuǎn)向系統(tǒng),由于電動(dòng)助力轉(zhuǎn)向汽車只需要提供協(xié)助時(shí),方向盤被轉(zhuǎn)動(dòng),而液壓泵必須不斷運(yùn)行。在EPS的幫助下是很容易調(diào)節(jié)車型,最高車速,甚至駕駛的喜好。另外一個(gè)好處是,通過(guò)泄漏和處置動(dòng)力轉(zhuǎn)向液消除對(duì)環(huán)境構(gòu)成危險(xiǎn)。
動(dòng)力轉(zhuǎn)向的分支是速度可調(diào)轉(zhuǎn)向而轉(zhuǎn)向是大量輔助以低速行駛,稍微協(xié)助高速。汽車制造商認(rèn)位,當(dāng)要停車時(shí)駕駛?cè)丝赡苄枰龀龃罅哭D(zhuǎn)向投入,但當(dāng)時(shí)高速行駛時(shí)則不然。第一輛有這特點(diǎn)的汽車,是雪鐵龍與其diravi,雖然改變了現(xiàn)代汽車轉(zhuǎn)向系統(tǒng)資金的投入,但它改變了定心凸輪的壓力,使得方向盤盡力去回到原來(lái)的位置。現(xiàn)代速度可調(diào)式動(dòng)力轉(zhuǎn)向系統(tǒng),當(dāng)速度增長(zhǎng)時(shí)減少了活塞的壓力,給予更直接的感受,這一特點(diǎn)在所有新車正逐漸成為司空見慣。
附錄B外文文獻(xiàn)
Steering??systems?Introduction??This?chapter?begins?with?a?discussion?of?steering??geometry—caster?angle?,trail?,kingpin?inclination?,and?scrub?radius?.The?next?section?discuss?Ackermann?geometry?followed?by?steering?racks?and?gears?.Ride?steer?(bump?steer?)?and?roll?steer?are?closely?related?to?each?other?;without?compliance?they?would?be?the?same?.Finally?,wheel?alignment?is?discussed?.this??chapter?is?tied?to?chapter?17 ?on?suspension?geometry?–when?designing?a?new?chassis?,steering?and?suspension?geometry?considerations?are?high?priorities?.?1?steering?geometry??The?kingpin?in?a?solid?front?axle?is?the?steering?pivot?.In?modern?independent?suspensions?,?introduced?by?Maurice?olley?at?Cadillac?in?1932,the?kingpin?is?replaced?by?two?(or?more?)?ball?joints?that?define?the?steering?axis?.This?axis?is?not?vertical?or?centered?on?the?tire?contact?patch?for?a?number?of?reason?.see?figure1?to?clarify?how?kingpin?location?is?measured?.?In?front?view?,the?angle?is?called?kingpin?inclination?and?the?offset?of?the?steering?axis?from?the?center?of?the?tire?print?measured?along?the?ground?is?called?scrub?(or?scrub?radius?).?The?distance?from?the?kingpin?axis?to?the?wheel?center?plane?,?measured?horizontally?at?axle?height?,is?the?spindle?length?.?In?side?view?the?kingpin?angle?is?called?caster?angle?;?if?the?kingpin?axis?does?not?pass?through?the?wheel?center?then?side?view?kingpin?offset?is?present?,as?in?most?motorcycle?front?ends?.The?distance?measured?on?the?ground?from?the?steering?axis?to?the?center?of?the?tire?print?is?the?trail?(called?caster?offset?in?ref?.1?)
Fig. 1 main pin positioning parameters
??Kingpin front view geometry As mentioned 7, kingpin inclination ,spindle length ,and scrub are usually a compromise between packaging and performance requirements .Some factors to consider include :
1. The effect of kingpin inclination and spindle length in raising the front end ,by itself ,is to aid centering of the steering at low speed .At high speed any trail will probably swamp out the effect that raise ad fall have on centering .
2. Kingpin inclination affects the steer –camber characteristic .when a wheel is steered ,it will lean out at the top ,toward positive camber ,if the kingpin is inclined in the normal direction (toward the center of the car at the upper end ). Positive camber results for both left– and right-hand steer .the amount of this effect is small ,but significant if the track includes tight turns.
3. When a wheel is rolling over a bumpy road ,the rolling radius is constantly changing ,resulting in changes of wheel rotation speed . This gives rise to longitudinal forces at the wheel center .The reaction of these forces will introduce kickback into the steering in proportion to the spindle length .If the spindle length is zero then there will be no kick from this source .Design changes made in the last model of the GM ―P ‖car (fiero ) shortened the spindle length and this resulted in less wheel kickback on rough roads when compared to early model ―P ‖cars.
4. The scrub radius shown in figure1 is negative ,as used on front-wheel –drive cars (see below ) . driving or braking forces (at the ground ) introduce steer torques proportional to the scrub radius . If the driving or braking force is different on left and right wheels then there will be a net steering torque felt by the driver (assuming that the steering gear has good enough rev erse efficiency ).The only time that this is not true is with zero scrub (centerpoint steering ) because there is no moment arm for the drive (or brake ) force to generate torque about the kingpin . With very wide tires the tire forces often are not centered in the wheel center plane due to slight changes in camber ,road surface irregularities ,tire nonuniformity (conicity ),or other asymmetric effects .These asymmetries can cause steering kickback regardless of the front view geometry .Packaging requirements often conflict with centerpoint steering and many race cars operate more or less okay on smooth tracks with large amounts of scrub .
6. For front drive ,a negative scrub radius has two strong stabilizing effects :first ,fixed steering wheel –if one drive wheel loses traction ,the opposing
wheel?will?toe?–out?an?amount?determined?by?the?steer?compliance?in?the?system?.This?will?tend?to?steer?the?car?in?a?straight?line?,even?though?the?tractive?force?is?not?equal?side-to?–side?and?the?unequal?tractive?force?is?applying?a?yaw?moment?to?the?vehicle?.?Second?,with?good?reverse?efficiency?the?driver’s?hands?never?truly?fix?the?steering?wheel?.?In?this?case?the?steering?wheel?may?be?turned?by?the?effect?of?uneven?longitudinal?tractive?forces?,increasing?the?stabilizing?effect?of?the?negative?scrub?radius?.?Under?braking?the?same?is?true?.Negative?scrub?radius?tends?to?keep?the?car?traveling?straight?even?when?the?braking?force?is?not?equal?on?the?left?and?right?side?front?tiresome?(due?to?differences?in?the?roadway?or?the?brakes).
?Caster?angle?and?trail??With?mechanical?trail?,shown?in?figure1,the?tire?print?follows?behind?the?steering?axis?in?side?view?.Perhaps?the?simplest?example?is?on?an?office?chair?caster?–with?any?distance?of?travel?,the?wheel?aligns?itself?behind?the?point?.More?trail?means?that?the?tire?side?force?has?a?large?moment?arm?to?act?on?the?kingpin?axis?.This?produces?more?self-centering?effect?and?is?the?primary?source?of?self-centering?moment?about?the?kingpin?axis?at?speed?.Some?considerations?for?choosing?the?caster?angle?and?trail?are?:
1.More?trail?will?give?higher?steering?force?.with?all?cars?,less?trail?will?lower?the?steering?force?.In?some?cases?,manual?steering?can?be?used?on?heavy?sedans?(instead?of?power?steering?)?if?the?trail?is?reduced?to?almost?zero?.?
2.Caster?angle?,like?kingpin?inclination?,cause?the?wheel?to?rise?and?fall?with?steer?.unlike?kingpin?inclination?,the?effect?is?opposite?from?side?to?side?.With?symmetric?geometry?(including?equal?positive?caster?on?left?and?right?wheels?)?,the?effect?of?left?steer?is?to?roll?the?car?to?the?right?,causing?a?diagonal?weight?shift?.In?this?case?,more?load?will?be?carried?on?the?LF?–RR?diagonal?,an?oversteer?effect?in?a?left-hand?turn?.?The?diagonal?weight?shift?will?be?larger?if?stiffer?springing?is?used?because?this?is?a?geometric?effect?.The?distance?each?wheel?rises?(or?falls?)?is?constant?but?the?weight?jacking?and?chassis?roll?angle?are?functions?of?the?front?and?rear?roll?stiffness.?This?diagonal?load?change?can?be?measured?with?the?car?on?scales?and?alignment?(?weaver?)?plates?.?Keep?in?mind?that?the?front?wheels?are?not?steered?very?much?in?actual?racing?,?except?on?the?very?tightest?hairpin?turns?.?For?example?,?on?a?100-ft?.radius?(a?40-50?mph?turn?),?a?10-ft.?wheelbase?neutral?steer?car?needs?only?about?0.1rad?.(5.7)of?steer?at?the?front?wheels?(with?a?16:1steering?ratio?this?is?about?90degree?at?the?steering?wheel?).?For?cars?that?turn?in?one?direction?only?,?caster?stagger?(differences?in?left?and?right?caster?)?is?used?to?cause?the?car?to?pull?to?one?side?due?to?the?car?seeking?the?lowest?ride?height?.?caster?stagger?will?also?affect?the?diagonal?weight?jacking?effect?mentioned?above?.?????If?the?caster?is?opposite?(positive?on?one?side?and?negative?the?same?number?of?degrees?on?the?other?side?)?then?the?front?of?the?car?will?only?rise?and?fall?with?steer?,?no diagonal weight jacking will occur .
3. Caster angle affects steer-camber but ,unlike kingpin inclination ,the effect is favorable . With positive caster angle the outside wheel will camber in a negative direction (top of the wheel toward the center of the car ) while the inside wheel cambers in a positive direction , again learning the turn . In skid recovery , ―opposite lock ‖ (steer out of the turn ) is used and in this case the steer–camber resulting from caster angle is in the ―wrongdirection for increased front tire grip . conveniently ,this condition results from very low lateral force at the rear so large amounts of front grip are not needed .
4. As discussed , tires have pneumatic trail which effectively adds to (and at high slip Angles subtracts from ) the mechanical trail . This tire effect is nonlinear with lateral force and affects steering torque and driver feel .In particular , the fact that pneumatic trail approaches zero as the tire reaches the limit will result in lowering the self-centering torque and can be s signal to the driver that the tire is near breakaway . The pneumatic trail ―breakaway signal‖ will be swamped out by mechanical trail if the mechanical trail is compared to the pneumatic trail .
5.Sometimes the trail is measured in direction perpendicular to the steering axis (rather than horizontal as shown ) because this more accurately describes the lever (moment ) arm that connects the tire lateral forces to the kingpin .
Fig. 2 Ackermann turning geometry relation
Fig. 3 Ackerman geometry, rack behind front axle
Tie?rod?location?Note?that?in?figure?1?a?shaded?area?is?shown?for?the?steering?tie?rod?location?.?Camber?compliance?under?lateral?force?is?unavoidable?and?if?the?tie?rod?is?located?as?noted?,the?effect?on?the?steering?will?be?in?the?understeer?(?steer?out?of?the?turn?)?direction?becomes?much?more?complex?than?can?be?covered?here?.2?Ackerman?steering?geometry??As?the?front?wheels?of?a?vehicle?are?steered?away?from?the?straight-ahead?position?,the?design?of?the?steering?linkage?will?determine?if?the?wheels?stay?parallel?or?if?one?wheel?steers?more?than?the?other?.This?difference?in?steer?Angles?on?the?left?and?right?wheels?should?not?be?confused?with?toe-in?or?toe-out?which?are?adjustments?and?add?to?(?or?subtract?from?)?Ackerman?geometric?effects?.?For?low?lateral?acceleration?usage?(street?cars)?it?is?common?to?use?Ackerman?geometry?.?as?seen?on?the?left?of?figure?2,?this?geometry?ensures?that?all?the?wheels?roll?freely?with?no?slip?Angles?because?the?wheels?are?steered?to?track?a?common?turn?center?.?Note?that?at?low?speed?all?wheels?are?on?a?significantly?different?radius?,?the?inside?front?wheel?must?steer?more?than?the?outer?front?wheel?.?A?reasonable?approximation?to?this?geometry?may?be?as?shown?in?figure?3.?ccording to ref .99, Rudolf Ackerman patented the double pivot steering system in 1817 andCharles Jeantaud added the concept mentioned above to eliminate wheel scrubbing when cornering . Another reason for Ackermann geometry ,mentioned by Maurice olley , was to keep carriage wheels from upsetting smooth gravel driveways . High lateral accelerations change the picture considerably .
Now?the?tires?all?operate?at?significant?slip?Angles?and?the?loads?on?the?inside?track?are?less?than?on?the?outside?track?.? Looking?back?to?the?tire?performance?curves?,it?is?seen?that?less?slip?angle?is?required?at?lighter?loads?to?reach?the?peak?of?the?cornering?force?to?a?higher?slip?angle?than?required?for?maximum?side?force?.?Dragging?the?inside?tire?along?at?high?slip?Angles?(?above?for?peak?lateral?force?)?raise?the?tire?temperature?and?slows?the?car?down?due?to?slip?angle?(?induced?)?drag?.For?racing?,?it?is?common?to?use?parallel?steering?or?even?reverse?Ackermann?as?shown?on?the?center?and?right?side?of?figure?2.?It?is?possible?to?calculate?the?correct?amount?of?reverse?Ackermann?if?the?tire?properties?and?loads?are?known?.?In?most?cases?the?resulting?geometry?is?found?to?be?too?extreme?because?the?car?must?also?be?driven?(or?pushed?)?at?low?speeds?,?for?example?in?the?pits?.
?Another?point?to?remember?is?that?most?turns?in?racing?have?a?fairly?large?radius?and?the?Ackermann?effect?is?very?small?.?In?fact?,?unless?the?steering?system?and?suspension?are?very?stiff?,compliance?(deflection?)?under?cornering?loads?may?steer?the?wheels?more?than?any?Ackermann?(or?reverse?Ackermann?)?built?into?the?geometry?.?The?simplest?construction?that?generates?Ackermannn?geometry?is?shown?in?figure?3?for?―rear?steer?‖?.?Here?,the?rack?(cross?link?or?relay?rod?in?steering?box?systems?)?is?located?behind?the?front?axle?and?lines?staring?at?the?kingpin?axis?,extended?through?the?outer?tie?rod?ends?,?intersect?in?the?center?of?the?rear?axle?.?The?angularit
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