外文翻譯--輕量級(jí)絲杠作動(dòng)器設(shè)計(jì)在便攜的機(jī)器人的應(yīng)用

1、<p>　　輕量級(jí)絲杠作動(dòng)器設(shè)計(jì)在便攜的機(jī)器人的應(yīng)用</p><p><b>　　機(jī)械設(shè)計(jì)報(bào)</b></p><p>　　凱文·W.霍蘭德·托馬斯G.唐</p><p>　　一個(gè)便攜機(jī)器人是直接與它的用戶聯(lián)系的一個(gè)受控和開(kāi)動(dòng)的設(shè)備。同樣，也要求這個(gè)設(shè)備必須也是便攜的，輕量級(jí)的，最重要的是安全的。為了達(dá)到這些目標(biāo)。標(biāo)

2、準(zhǔn)絲杠的設(shè)計(jì)通常不能很好的按要求執(zhí)行這些。典型的絲杠有很低投球角度和大的半徑，從而產(chǎn)生很低的機(jī)械效率和很大的重量。可是，使用文本中的設(shè)計(jì)程序，效率和重量是被改進(jìn)的; 因而可以產(chǎn)生一種與人的肌肉相似的絲杠系統(tǒng)。例子中的問(wèn)題說(shuō)明一個(gè)可行性的絲杠設(shè)計(jì)應(yīng)該是277 的功率質(zhì)量比，接近驅(qū)動(dòng)它的馬達(dá)，即312W/kg，并且機(jī)械效率為0.74和最大動(dòng)能到11.3 kN/kg的絲杠設(shè)計(jì)。 </p><p><b>　　

3、1引言</b></p><p>　　在美國(guó)，有五分之一的人有不同形式的殘疾，這些人當(dāng)中，61％的人患有感覺(jué)或身體殘疾。在老年人中，8％到19 ％是步態(tài)失調(diào)。許多殘疾人可以獨(dú)立的受益于某種形式機(jī)器人的協(xié)助。一個(gè)便攜機(jī)器人是一個(gè)被計(jì)算機(jī)控制和驅(qū)動(dòng)的裝置，是直接接觸用戶的。這種裝置的目的是增強(qiáng)用戶的行為能力。在病人治療期間，它可以用于訓(xùn)練，或是僅僅當(dāng)作一種協(xié)助病人完成日常生活的裝置。"便攜&quo

4、t;的含義是指機(jī)器人必須攜帶方便，重量輕，而且安全是最重要的。相比之下，工廠車(chē)間的機(jī)器人是沒(méi)有這些功能的，因此，要簡(jiǎn)單修改現(xiàn)有的技術(shù)是不可能實(shí)現(xiàn)的。設(shè)計(jì)便攜機(jī)器人的標(biāo)準(zhǔn)方法有三大局限性;</p><p><b>　　1低電池功率密度；</b></p><p>　　2電機(jī)的低強(qiáng)度質(zhì)量比；</p><p>　　3重量和安全性的機(jī)械傳動(dòng)系統(tǒng)。<

5、/p><p>　　這些工作的目的是審查絲杠驅(qū)動(dòng)器的設(shè)計(jì)過(guò)程;結(jié)果顯示在局限性第三項(xiàng)方面有了重大改進(jìn)，即，重量和安全性的機(jī)械傳動(dòng)系統(tǒng)。</p><p><b>　　2 背景</b></p><p>　　有趣的是，在便攜機(jī)器人學(xué)領(lǐng)域的研究已經(jīng)超過(guò)了過(guò)去十年的增長(zhǎng)。最近，浪涌的利益可以歸因于電子小型化、微處理器能力和無(wú)線技術(shù)擴(kuò)散的推進(jìn)。提高便攜計(jì)算機(jī)控制

6、設(shè)備的能力的可行性是可以實(shí)現(xiàn)的。</p><p>　　然而，除便攜式的計(jì)算平臺(tái)的可及性之外，必須談及物理機(jī)制的問(wèn)題。在便攜機(jī)器人發(fā)展中，主要的問(wèn)題是強(qiáng)度質(zhì)量比、重量和安全。有多少可利用的動(dòng)力可完成機(jī)械功？機(jī)器人設(shè)備有多少額外的力給人？還有，如何轉(zhuǎn)移這些動(dòng)力和怎么一直維護(hù)安全等？用戶和開(kāi)動(dòng)的機(jī)器人之間的安全互作用在便攜機(jī)器人設(shè)計(jì)中是一個(gè)首要問(wèn)題。</p><p>　　一個(gè)便攜的機(jī)器人系統(tǒng)的目

7、的是將操作員通過(guò)存貯設(shè)備獲得的努力和能量抵消，即，電池、燃料電池和空氣坦克。作動(dòng)器的效率和整個(gè)系統(tǒng)的重量沉重影響分享在操作員和機(jī)器人之間的工作負(fù)擔(dān)。在很多情況下，機(jī)器人加給用戶的額外力量，能多完成一項(xiàng)測(cè)量任務(wù)。這意味著機(jī)器人不僅必須增添操作員的能力，也必須補(bǔ)嘗它自己另外的重量。</p><p>　　2.1 作動(dòng)器的比較。</p><p>　　很多機(jī)器人作動(dòng)器被比作成人的骨骼肌的標(biāo)準(zhǔn)。設(shè)計(jì)師

8、了解他們好的功率強(qiáng)度比和優(yōu)秀的強(qiáng)制生產(chǎn)能力就是為了動(dòng)作器與骨骼肌相比擬。為了匹配骨骼肌的性能，重要的是知道其中一些措施。不幸地是，生物文學(xué)中的普遍性是:被測(cè)量的肌肉參數(shù)是變化繁多的。雖然報(bào)告參數(shù)有一個(gè)寬的變化，這些參數(shù)一直能給生物材料行為標(biāo)度的感覺(jué)。制成表的數(shù)據(jù)和幾個(gè)原始估計(jì)數(shù)據(jù)被用于描述人的肌肉表現(xiàn)屬性和結(jié)果如表1所示。</p><p>　　表1：作動(dòng)器比較：通過(guò)機(jī)械效率，勢(shì)能，和校正動(dòng)能對(duì)各種各樣的作動(dòng)器類(lèi)型

9、進(jìn)行比較：</p><p>　　允許與有效能的運(yùn)用直接進(jìn)行比較。然而，在便攜機(jī)器人作動(dòng)器的發(fā)展中這兩個(gè)參量需要得到審查。考慮到所有作動(dòng)器在100%效率中運(yùn)行，然后整個(gè)小組能直接地由他們各自的功率強(qiáng)度比進(jìn)行比較?？墒?，如果勢(shì)能中的動(dòng)力被提供給每臺(tái)作動(dòng)器，由于他們各自的效率僅僅是輸出一小部分動(dòng)力。所以，適當(dāng)?shù)乇容^上面被描述的作動(dòng)器，他們校正的勢(shì)能必須計(jì)算,即：</p><p><b>

10、;　　(1)</b></p><p>　　機(jī)械效率和Pwt是原始的功率質(zhì)量比。對(duì)各種動(dòng)作器演算的結(jié)果如表1所示</p><p>　　表1的內(nèi)容是從文獻(xiàn)或基于那些文獻(xiàn)的估計(jì)中獲得的。dc馬達(dá)的參數(shù)是:Maxon RE40馬達(dá)。 傳動(dòng)箱組合的參數(shù)在Maxon 2004編目中能夠找到。一臺(tái)電系列有彈性作動(dòng)器的參數(shù)用于估計(jì)這些參數(shù)。然而，一個(gè)一般大小的絲杠系統(tǒng)可能有更好的強(qiáng)度質(zhì)量比，因

11、為它有很高的負(fù)載能力，并且有很低的重量。對(duì)于McKibben樣式的空氣肌肉，從各種各樣文獻(xiàn)中發(fā)現(xiàn)了描述它的相關(guān)方法。</p><p>　　比較中顯然顯示的是校正功率質(zhì)量比，cP，dc馬達(dá)的參數(shù)，空氣肌肉和人的骨骼肌是都是簡(jiǎn)單匹配的。然而，馬達(dá)上一旦加上額外的硬件，它的執(zhí)行力會(huì)極大減小。基于動(dòng)作器的重量，如果能修改一個(gè)不是很大的dc馬達(dá)重量的機(jī)械傳動(dòng)系統(tǒng)，則它接近于人的骨骼肌的功能可能會(huì)實(shí)現(xiàn)。</p>

12、<p><b>　　3絲杠設(shè)計(jì)</b></p><p>　　如上所見(jiàn)，當(dāng)一個(gè)典型絲杠系統(tǒng)與其他便攜機(jī)器人作動(dòng)器在概念上進(jìn)行比較時(shí)，它的性能是有限的。產(chǎn)生這種低性能的主要原因是它的機(jī)械效率很低。如果在一個(gè)標(biāo)準(zhǔn)絲杠系統(tǒng)中使用大約是=0.36的摩擦系數(shù)，會(huì)有更好的潤(rùn)滑效果。</p><p>　　相反，典型的球螺絲系統(tǒng)有非常好機(jī)械效率。 滾珠軸承的滾動(dòng)接觸對(duì)這個(gè)系

13、統(tǒng)的摩擦作用會(huì)保持很低。然而，效率雖然有了改進(jìn)，球螺絲作動(dòng)器的cPt參數(shù)仍然低于那骨骼肌，這是因?yàn)榍蚵萁z系統(tǒng)的重量很大。如果改進(jìn)球螺絲的cP性能，那么重量的減少就可以實(shí)現(xiàn)了。</p><p><b>　　機(jī)械設(shè)計(jì)學(xué)報(bào)</b></p><p>　　圖1 絲杠外形; 主角l…在一個(gè)單一螺旋螺絲中是等效的</p><p>　　用于設(shè)計(jì)圍攏絲杠的基本數(shù)

14、學(xué)也適用于球螺絲系統(tǒng)。這兩個(gè)機(jī)械傳輸之間的主要差別是他們的摩擦系數(shù)。在以下部分會(huì)考慮影響絲杠重量和機(jī)械效率的設(shè)計(jì)參數(shù)，并且對(duì)它的cP進(jìn)行改進(jìn)。</p><p><b>　　3.1絲杠外形</b></p><p>　　在圖1顯示的是普通絲杠的基本外形。絲杠的關(guān)鍵參量是主角l，螺絲半徑r和前置角。主角l是螺絲每次改進(jìn)達(dá)到的位移數(shù)量，一個(gè)高精度螺絲有非常小或非常好的主角。在

15、圖1的正三角形顯示的螺絲的唯一一次改進(jìn)被剝開(kāi)的構(gòu)造。前置角代表螺紋的斜面或傾斜度。 三角的基礎(chǔ)是螺絲軸的圓周，三角形的右腿是它的主角，螺線螺紋的弦代表路徑長(zhǎng)度。</p><p>　　并且在正三角形中看出使螺母舉起負(fù)載的強(qiáng)大的力。負(fù)載的力量顯示為F，螺絲的扭矩強(qiáng)度是F,在螺絲螺紋上的正常反作用力是N，并且摩擦力是N。從這張圖中，舉起的扭矩的等式就可以是：</p><p><b>　

16、　(2)</b></p><p><b>　　3.2 對(duì)R。</b></p><p>　　還考慮，絲杠的外形在圖1可以顯示主角 l是由螺絲半徑r和前置角描述的。這些可改變量之間的關(guān)系是：</p><p><b>　　(3)</b></p><p><b>　　(4)</b

17、></p><p>　　公式4的意思是r、螺絲半徑和，前置角，都是需要螺絲主角l的。這意味著在r和之間存在一個(gè)連續(xù)的關(guān)系。雖然存在這個(gè)連續(xù)的關(guān)系，多數(shù)螺絲系統(tǒng)還是被設(shè)計(jì)成非常小的前置角。從首選螺絲大小的經(jīng)驗(yàn)來(lái)看，雖然各自的直徑都在變化，但前置角都小于3°。</p><p>　　在公式4種顯示對(duì)所有螺絲主角的需求，各種各樣的半徑都可以使用。這個(gè)意義在于螺絲半徑 r的變小，螺絲

18、的重量是通過(guò)r2減小的。因此，要補(bǔ)嘗小螺絲半徑，必須考慮前置角這個(gè)參數(shù)。 </p><p><b>　　前角，</b></p><p>　　圖2絲杠系統(tǒng)機(jī)械效率：遮蔽一部分的圖表多數(shù)是絲杠的典型設(shè)計(jì)區(qū)域。是小的，半徑大，重量大，并且效率是較低的。在圖表的未遮住的區(qū)域設(shè)計(jì)，是大的，暗示更小的半徑、更低的重量和更高的效率。</p><p>　　3

19、.3效率對(duì)阿爾法。</p><p>　　對(duì)于一個(gè)便攜機(jī)器人的設(shè)計(jì)，不僅絲杠作動(dòng)器的重量是一個(gè)重要問(wèn)題，而且作動(dòng)器的效率也是非常關(guān)鍵的。如上所述，螺絲半徑的減小可以使動(dòng)作器的重量大大減小。然而，要減小螺絲半徑，必須增加前置角 ，以保持恒定的主角。當(dāng)看公式2時(shí)，可以看出要求承受負(fù)載的力矩Fw，取決于兩前置角和摩擦系數(shù).</p><p>　　影響螺絲效率的是前置角和摩擦系數(shù)，圖2顯示對(duì)摩擦系數(shù)_

20、和前置角_的沖擊在于絲杠系統(tǒng)的效率</p><p><b>　　(5)</b></p><p>　　在圖2的每條線是基于摩擦系數(shù)不同的參數(shù)。幾份普通的工程材料作為例子給讀者一個(gè)在絲杠系統(tǒng)中能有不同物質(zhì)或涂層的作用的感覺(jué)。這個(gè)圖表示，當(dāng)前置角增加，機(jī)械效率就增加; 或者至少到達(dá)一個(gè)峰值。</p><p>　　理論上，選擇最大效率采摘角度是有利的。

21、一個(gè)絲杠系統(tǒng)在高效率運(yùn)行時(shí)需要使負(fù)載力矩達(dá)到最小Fw。在高峰值效率發(fā)生的角度可以取決于與角度效率有關(guān)的參數(shù)，結(jié)果是可以看到的。</p><p><b>　　(6)</b></p><p>　　雖然一個(gè)高前置角可能提高效率，但它也可能導(dǎo)致反驅(qū)動(dòng)系統(tǒng)。一個(gè)反驅(qū)動(dòng)系統(tǒng)是一種負(fù)載力矩，沒(méi)有力矩協(xié)助的情況下，螺絲可能自轉(zhuǎn)，因而允許負(fù)載自我降低。反驅(qū)動(dòng)絲杠不適合應(yīng)用于汽車(chē)起重器，

22、但是可以應(yīng)用于便攜機(jī)器人當(dāng)中。因此反驅(qū)動(dòng)的前置角是：</p><p><b>　　(7)</b></p><p>　　不管產(chǎn)生多么高的負(fù)載力量，多么低的摩擦系數(shù)系統(tǒng)，前置角和摩擦系數(shù)總是影響這些條件的，例如球螺絲，反驅(qū)動(dòng)是一個(gè)必然結(jié)果。</p><p><b>　　4 實(shí)用考慮</b></p><p&g

23、t;　　理論上，如先前的文獻(xiàn)所顯示，是希望螺絲半徑 r減小的，甚至到一個(gè)幾乎微觀尺度。然而，從設(shè)計(jì)和制造業(yè)方面來(lái)講，這不是一種實(shí)用的解決方案。雖然從重量和效率的角度來(lái)講小螺絲的直徑和高前置角是極其重要的，但他們可能不允許設(shè)計(jì)師適應(yīng)物理系統(tǒng)的力量需要。例如軸向產(chǎn)生，壓縮折和機(jī)制困境都需要被考慮?？紤]到單一的超薄的螺絲也許是輕量級(jí)的，它可能沒(méi)有一個(gè)系統(tǒng)所需要足夠的負(fù)載能力。但可以使用單一的，或幾個(gè)螺絲，就會(huì)有足夠大的負(fù)載能力。用幾個(gè)小螺絲承

24、受大載荷是沒(méi)有重量?jī)?yōu)勢(shì)的，作為因計(jì)算一個(gè)螺絲斷面產(chǎn)生的重量和壓強(qiáng)。然而，使用幾個(gè)小螺絲承受載荷可能允許對(duì)高前置角的持續(xù)使用和在高效率中運(yùn)行，甚至在很高負(fù)載。通過(guò)推擠絲杠原材料物產(chǎn)極限，可以達(dá)到軸向很高的負(fù)載。這種工作方法的好處在于一個(gè)緊張系統(tǒng)比它壓縮軸承更好運(yùn)作的系統(tǒng)。當(dāng)考慮到減小一個(gè)既長(zhǎng)又細(xì)的螺絲的負(fù)載時(shí)，類(lèi)似于McKibben作動(dòng)器甚至人的肌肉，（絲杠作動(dòng)器能被設(shè)計(jì)負(fù)擔(dān)僅緊張裝載），因而消除共折的考慮。在一個(gè)便攜機(jī)器人中創(chuàng)建緊張驅(qū)動(dòng)

25、系統(tǒng)不一定意味著需要一個(gè)對(duì)抗性。實(shí)際上，與一個(gè)協(xié)助機(jī)器人相比，殘疾人在做單一的直接動(dòng)作時(shí)，肌肉存在弱點(diǎn)，因此，這些人是非常需要?jiǎng)?lt;/p><p>　　對(duì)于那些推擠螺絲半徑和因此導(dǎo)致前置角的極限超過(guò)最大效率的設(shè)計(jì)師，摩擦極限角度多少是可以?xún)A斜的。所有這些的物理解釋是系統(tǒng)捆綁或鎖，由導(dǎo)出的公式2可以看見(jiàn)。一個(gè)由公式（2）導(dǎo)出，可以產(chǎn)生以下關(guān)系</p><p><b>　　(8)<

26、;/b></p><p>　　除被列出的實(shí)用考慮之外，還可能存在著許多其他問(wèn)題。包括扭轉(zhuǎn)力僵硬或屈服力甚至熱擴(kuò)散等。這些因素中的每一個(gè)都是重要的并且都需要我們考慮?？墒牵@個(gè)練習(xí)的目的是展示選擇一個(gè)設(shè)計(jì)或選擇螺絲系統(tǒng)的典型方法。這個(gè)選擇方法的好處是可直接適用于一個(gè)便攜機(jī)器人系統(tǒng)的設(shè)計(jì)。</p><p><b>　　5 例子中的問(wèn)題</b></p>

27、<p>　　展示一份粗糙設(shè)計(jì)報(bào)告，考慮高峰距小腿關(guān)節(jié)扭矩在到一個(gè)有能力裝載80 kg的個(gè)體并且是0.8 Hz的跨步頻率期間的連接扭矩。在步態(tài)期間的腳腕扭矩峰值大約是100毫微米。這個(gè)峰值大致發(fā)生在45%的步態(tài)周期。步態(tài)周期是指一只腳跟的停止到這支腳跟下一次停止的時(shí)間。腳趾是承受另一只腿重力和開(kāi)始搖擺的點(diǎn)。 搖擺階段的判斷是步態(tài)再次安置腳回到腳跟停止位置時(shí)，然后下一個(gè)步態(tài)周期開(kāi)始。</p><p>　　例

28、如，讓我們考慮修造一個(gè)腳腕步態(tài)協(xié)助絲杠作動(dòng)器。我們假設(shè)協(xié)助水平在30%左右和到小腿關(guān)節(jié)是12厘米的力矩臂。</p><p>　　表2作動(dòng)器問(wèn)題比較：絲杠設(shè)計(jì)I和II與人的肌肉的效率比較，對(duì)勢(shì)能的比較，校正勢(shì)能和動(dòng)能的措施的比較。</p><p>　　這些參數(shù)都可以根據(jù)自己的個(gè)人經(jīng)驗(yàn)并且在合理的范圍內(nèi)進(jìn)行修改和變化。參數(shù)和可用的參量接近于Maxon馬達(dá)，即RE40，這個(gè)例子中，主角長(zhǎng)度的范圍

29、已經(jīng)確定了;它的范圍可以是</p><p>　　解決設(shè)計(jì)兩個(gè)絲杠的問(wèn)題:第一個(gè)設(shè)計(jì)問(wèn)題是解決最大效率。假設(shè)是2 mm和=0.05，螺絲在=43.5°、半徑是0.34mm 的地方產(chǎn)生的效率是90%。這樣小的一條半徑，需要多個(gè)螺絲承受負(fù)載。即使如此，估計(jì)作動(dòng)器的勢(shì)能是280 W/kg 。通過(guò)馬達(dá)重量和預(yù)測(cè)的傳輸系統(tǒng)，劃分需要的功率峰值就可以得出勢(shì)能的大小。我們從以前的工作知道了，輔助組分的重量成比例可以減小

30、螺絲和螺釘?shù)闹亓俊?lt;/p><p>　　第二個(gè)設(shè)計(jì)，絲杠II，從商業(yè)供營(yíng)商得到可利用的維度。 螺絲的=13.6°和0.82的效率。 更大一些的維度也可行，動(dòng)作器的勢(shì)能最好是277 W/kg。為了達(dá)到比較的目的，這個(gè)例子出現(xiàn)的問(wèn)題結(jié)果制成了表格。表2顯示兩個(gè)絲杠設(shè)計(jì)方案的數(shù)字結(jié)果。這些參數(shù)與先前的dc馬達(dá)參數(shù)和人的骨骼肌的估計(jì)值進(jìn)行比較。通過(guò)例子，動(dòng)能大小是基于力的峰值進(jìn)行計(jì)算的。</p>

31、<p><b>　　6 討論</b></p><p>　　在分析解決最大效率的方案上，絲杠設(shè)計(jì)I顯示了一個(gè)單一小半徑螺絲永遠(yuǎn)不會(huì)處理所要求的負(fù)載?？墒牵鄠€(gè)螺絲同時(shí)平行執(zhí)行那項(xiàng)任務(wù)會(huì)有同樣高的效率。雖然使用典型的技術(shù)不容易制造出一個(gè)0.34 mm半徑的螺絲，但用這種方法是可以實(shí)現(xiàn)的（即，使用多個(gè)螺絲產(chǎn)生高效率）。要設(shè)計(jì)一個(gè)特殊的絲杠，效率是沒(méi)有極限的。絲杠設(shè)計(jì)II顯示，有一種可行的

32、解決方案可以解決腳腕的問(wèn)題，校正功率質(zhì)量比參數(shù)使其非常接近于人的肌肉。使用一種相似的方法，球形螺絲機(jī)制能有益于它的表現(xiàn)，一般方法是創(chuàng)建一個(gè)驅(qū)動(dòng)的背面，低重量和高效率的螺絲系統(tǒng)可以使基于dc馬達(dá)的動(dòng)作器的便攜機(jī)器人應(yīng)用有一種有力解答。</p><p>　　圖3 原型作動(dòng)器，高效率絲杠</p><p>　　前面提到，一臺(tái)便攜機(jī)器人作動(dòng)器不僅要有好的執(zhí)行能力，而且還要對(duì)它的用戶有一定的安全性。在

33、考慮安全方面時(shí)，（駕駛）是便攜絲杠作動(dòng)器所需要的。（ 駕駛）允許操作者任意安裝沒(méi)有動(dòng)力的螺釘，因而使它的阻礙減到最小。另一方面，在螺絲的末端設(shè)計(jì)一塊閑置的部分以防止馬達(dá)和用戶受到損壞。對(duì)人的損傷可以通過(guò)安置螺絲的末端范圍在用戶的生理安全極限內(nèi)來(lái)避免，即一旦遇到危險(xiǎn)強(qiáng)度，可以得到短期的脫離。所有這些方法都需要得到重點(diǎn)考慮，并且應(yīng)該在設(shè)計(jì)過(guò)程中早期解決。安置機(jī)械部件必須包括特別的防備措施。 防備措施必須超出軟件或控制器范圍; 因此，在機(jī)械設(shè)

34、計(jì)中應(yīng)該包括他們。保證用戶的安全是在設(shè)計(jì)所有協(xié)助機(jī)器人時(shí)應(yīng)該是最優(yōu)先考慮的事，我們的實(shí)驗(yàn)室也調(diào)查了便攜作動(dòng)器的其他類(lèi)型。看圖3。這些技術(shù)幫助我們保持設(shè)備的整體大小和重量打到最低。</p><p><b>　　7 結(jié)論</b></p><p>　　一臺(tái)便攜機(jī)器人作動(dòng)器必須有好的功率勢(shì)能比，好的機(jī)械效率，好的強(qiáng)度質(zhì)量比，并且一定是安全的。對(duì)于一個(gè)具有好的功率的dc馬達(dá)，改

35、進(jìn)它力量的唯一方法是增加傳動(dòng)系統(tǒng)。傳統(tǒng)上，這種方法導(dǎo)致了dc馬達(dá)作動(dòng)器功率質(zhì)量比的增加以至于它的執(zhí)行力筆直下降?？墒?，我們的方法可以用于設(shè)計(jì)絲杠和球形絲杠的力，例如一個(gè)便攜的協(xié)助機(jī)器人。</p><p>　　Design of Lightweight Lead Screw Actuators for Wearable Robotic Applications</p><p>　　Journ

36、al of Mechanical Design</p><p>　　Kevin W. Hollander Thomas G. Sugar</p><p>　　A wearable robot is a controlled and actuated device that is in direct contact with its user. As such, the implied re

37、quirements of this device are that it must be portable, lightweight, and most importantly safe. To achieve these goals, The design of the standard lead screw does not normally perform well in any of these categories. The

38、 typical lead screw has low pitch angles and large radii, thereby yielding low mechanical efficiencies and heavy weight. However, using the design procedure outlin</p><p>　　1 Introduction</p><p>

39、;　　One in five persons in the United States live with some form of disability, with 61% of those suffering from either a sensory or physical disability.As an example, within the elderly population,8% to 19% are affected

40、by gait disorders . Many disabled individuals could benefit from some form of robotic intervention. A wearable robot is a computer controlled and actuated device that is in direct contact with its user. The purpose of su

41、ch a device is the performance/strength enhancement of the wear</p><p>　　1 Low battery power density；</p><p>　　2 motors with low “strength to weight” ratios；</p><p>　　3 weight and s

42、afety of a mechanical transmission system.</p><p>　　The goal of this work is to review the design process of a lead screw actuator; the result of which will demonstrate significant improvements over the limi

43、tations described in item number 3, i.e., the weight and safety of the mechanical transmission system.</p><p>　　2 Background</p><p>　　Interest in the area of wearable robotics has grown over the

44、 last decade. The recent surge of interest can be attributed to advancements in electronic miniaturization, microprocessor capabilities, and wireless technology proliferation. The feasibility of a portable computer contr

45、olled strength enhancing device is closer to reality</p><p>　　However, aside from the availability of portable computation platforms, issues of the physical mechanism must still be addressed. The main issues

46、 in any wearable robot development are power, weight, and safety. How much power is available to do mechanical work? How much additional weight does the robotic device add to the person? And, how can this power be transf

47、erred and still maintain safety? The safe interaction between the wearer and theactuated robot has to be the primary concern in a weara</p><p>　　The purpose of a wearable robotic system is to offset the effo

48、rt or energy of the operator by some amount of energy from a storage device, i.e., battery, fuel cell, and air tank. The sharing of the work load between the operator and the robot is heavily influenced by actuator effic

49、iencies and the overall system weight. The additional weight that the robot adds to the user, in many cases, can increase the total amount of work required to accomplish a given task. This means that the robot not only&l

50、t;/p><p>　　2.1 Actuator Comparisons. </p><p>　　Human skeletal muscle is the “gold” standard by which many robotic actuators are compared. Known for their good “power to weight” ratios and excellent

51、 force production capabilities, skeletal muscle performance is what most actuator designers would like to match. In order to match the performance capabilities of skeletal muscle, it is important to know some of its meas

52、ures. Unfortunately, common throughout biological literature is a wide variation of measured muscle properties. Although reported </p><p>　　Table1：Actuator comparison: Compares various actuator types by mech

53、anical efficiency, power to weight ratio, “corrected”power to weight ratio, and strength to weight ratio Measures</p><p>　　allows the direct comparisons to be made based upon utilization of available energy.

54、 However, both of these parameters need to be examined in the development of a wearable robotic actuator. Consider that if all actuators were to operate at 100% efficiency, then the entire group could be compared directl

55、y by their respective power to weight ratios. However, if only the power stated in the power to weight ratio were supplied to each actuator, then because of their respective efficiency, only a fra</p><p><

56、;b>　　(1)</b></p><p>　　where is the mechanical efficiency and Pwt is the original power to weight ratio. The results of this calculation for various kinds of actuators can be seen in Table 1.</p&

57、gt;<p>　　Values in Table 1 were obtained either by referenced literature or estimations based upon that literature. The values for the dc motor are for the Maxon RE40 motor. The values for the + gearbox combinatio

58、n were also found in the Maxon 2004 catalog. values from an electric Series Elastic Actuator were used to estimate these parameters. However, a similiarly sized lead screw system will likely have a better strength to wei

59、ght ratio, due to its ability to carry higher loads and its nut is of lower </p><p>　　Immediately evident in this comparison is that the corrected power to weight, cP, values of the dc motor, the air muscle

60、and human skeletal muscle are all similarly matched. However, once additional hardware is added to the dc motor, its performance decreases significantly. If one could create a mechanical transmission system that did not

61、significantly alter the weight of the dc motor based actuator, then performances very near that of human skeletal muscle could be achieved.</p><p>　　3 Lead Screw Design。</p><p>　　Seen above, the

62、 performance of a typical lead screw system is limited when compared to other wearable robotic actuator concepts. The primary reason for its low performance is poor mechanical efficiency. The coefficient of friction in a

63、 standard lead screw system is approximately =0.36., metal on metal, better results are possible if lubrication is used.</p><p>　　In contrast, the typical ball screw system has very good mechanical efficien

64、cy. The rolling contact of the ball bearings keeps the frictional effects on this system to an absolute minimum. However, even with its improved efficiencies, the cP value for the ball screw actuator is still well below

65、that of skeletal muscle, due directly to the considerable weight of the ball screw system. To improve the cP performance of a ball screw, a significant</p><p>　　reduction of weight must be achieved.</p>

66、;<p>　　Journal of Mechanical Design</p><p>　　Fig. 1 Lead screw geometry; as drawn, pitch ?p… and lead ?l…</p><p>　　are equivalent in a single helix screw</p><p>　　The basic m

67、athematics surrounding the design of a lead screw can also apply to a ball screw system. The primary difference between these two mechanical transmissions is their coefficient of friction. In the following section, an ex

68、ploration of the design parameters that influence weight and mechanical efficiency of a lead screw will be considered and thus improvements to its ccan be made.</p><p>　　3.1 Lead Screw Geometry.</p>&

69、lt;p>　　Shown in Fig. 1 is the basic geometry of a common lead screw. The key parameter of a lead screw is the lead, l, which is dependent on screw radius, r, and lead angle. The lead, l, is the amount of displacement

70、achieved for each revolution of the screw. A high precision screw has a very short or fine lead. The right triangle in Fig. 1 shows the unwrapped geometry of a single revolution of a screw. The lead angle , represents th

71、e incline or slope of the screw thread. The base of the triangle is th</p><p>　　Also seen on the right triangle are the forces present on a screw that is lifting a load. The force of the load is shown as Fw,

72、 the force resulting from the torque on the screw is F, the normal reaction force on the thread of the screw isN, and the frictional force is N. From this diagram, the following equation for a lifting torque can be deriv

73、ed </p><p><b>　　(2) </b></p><p>　　3.2 Alpha Versus R.</p><p>　　Considering, again, the geometry of a lead screw in Fig. 1, it can be shown that leadl, is described both

74、by screw radiusr, and lead angle. The relationship between these variables is given in</p><p><b>　　(3)</b></p><p><b>　　（4）</b></p><p>　　The meaning of Eq（4）i

75、s that both r, screw radius, and, lead angle, are necessary to create a screw lead, l. This means that there exists a continuous relationship between r and . Although this continuous relationship exists, most screw syste

76、ms are designed with very small lead angles. A review of the preferred ACME screw sizes reveal that although the individual diameters vary, the lead angles are all less than 3°.</p><p>　　From Eq（4）.it

77、is shown that for any screw lead desired, a variety of radii could be used. The significance of this is that as screw radius, r, shrinks, the weight of the screw shrinks by a factor.r2 Thus, to compensate for small screw

78、 radii, a larger value of lead angle , must be considered.</p><p>　　Fig. 2 Mechanical efficiency of lead screw systems: Shaded part of the graph is the typical design region for the majority of lead screws.

79、 is small, radius is large, weight is large, and efficiencies are lower. Designs in the unshaded region of the graph, where is large, implies smaller radii, lower weight, and higher efficiencies. </p><p>　　

80、3.3 Efficiency Versus Alpha. </p><p>　　For a wearable robot design, not only is the weight of a lead screw actuator an important issue, but the efficiency of an actuator is also key. As mentioned before, a d

81、ecrease in screw radius can achieve significant reductions in actuator weight. However, while the screw radius is reduced, the lead angle, must be increased to maintain a constant lead. When looking at Eq(2). it is seen

82、that the torque required to lift a load, Fw, is dependent upon both lead angle, as well as the coefficient of fr</p><p>　　Relating the efficiency of a screw to both lead angle and coefficient of friction, Fi

83、gure 2 shows the impact on both coefficient of friction, and lead angle, on the efficiency of a lead screw system</p><p><b>　　（5）</b></p><p>　　Each line in Fig. 2 is based upon a dif

84、ferent value of the coefficient of friction. Several common engineering materials are given as examples to give the reader a sense of what effect different materials or coatings could have on the efficiency of a lead scr

85、ew system. This figure shows that as the lead angle increases, so does the mechanical efficiency; or at least until a peak value is reached.</p><p>　　Ideally, it would be advantageous to pick the angle, base

86、d upon maximum efficiency. A lead screw system operating at peak efficiency minimizes the input torque requirements to lift the load Fw. The angle at which peak efficiency occurs can be determined by taking the derivativ

87、e of efficiency with respect to angle, the result of which can be seen in</p><p><b>　　(6)</b></p><p>　　Although a high lead angle can lead to a high efficiency, it can also lead to a

88、 system that is “back-drivable”. A back-driveable system is one in which the load, Fw, can cause a rotation of the screw without the assistance of applied torque, thus allowing the load, Fw, to self-lower. A back-driveab

89、le lead screw is a bad idea for a car jack, but is desirable in a wearable robot. For the lead angles in which back-drive will occur</p><p><b>　　(7)</b></p><p>　　Lead angle and coeff

90、icient of friction are all that influence this condition, regardless of how high the load force becomes. Fora very low coefficient of friction system, such as a ball screw,back-drive is an inevitable consequence.</p&g

91、t;<p>　　4 Practical Considerations</p><p>　　Ideally, as shown in the previous text, it would be desirable to reduce our screw radius, r, to an almost microscopic scale. However, this is not a practica

92、l solution, neither from a design nor manufacturing perspective. Although small screw diameters and high lead angles are desired from the perspective of weight and efficiency, they may not allow the designer to meet the

93、strength demands of the physical system. Issues, such as axial yielding,compression buckling, and mechanism bind, need to be</p><p>　　For those designers who would push the limits of the screw radius and thu

94、s lead angle to beyond that of maximum efficiency, the presence of friction limits just how far the angle can be inclined. The physical interpretation of this is that the system willbind or lock. This can be seen by eval

95、uating Eq.（2）. An evaluation of the denominator in Eq.（2）. yields the following relation。</p><p><b>　　(8)</b></p><p>　　In addition to the practical considerations listed here, there

96、exists many other issues that could be detailed. Examples of which may include torsional stiffness/yielding or even heat dissipation. Each of these factors are important and worthy of consideration, however, the purpose

97、of this exercise is to demonstrate an alternative</p><p>　　to the typical approaches of designing or selecting screw systems. The benefits of this alternative approach are directly applicable to the design i

98、ssues of a wearable robotic system.</p><p>　　5 Example Problem</p><p>　　To demonstrate a crude design exercise, consider the peak ankle joint torque during gait of an able-bodied or normal indiv

99、idual that weighs 80 kg and walks at 0.8 Hz stepping frequency. The peak ankle torque during gait is approximately 100 Nm. This peak occurs at roughly 45% of the gait cycle, A gait cycle is defined by the heel strike of

100、a foot to the next heel strike of the same foot. Toe off is the point in which the weight of the individual has transferred to the opposite leg and the initia</p><p>　　As an example, let us consider building

101、 a lead screw actuator for ankle gait assistance. For our problem, let us assume the level</p><p>　　Table 2： Example problem actuator comparison: Compares lead screw designs I and II to human muscle in terms

102、 of mechanical efficiency, power to weight ratio, corrected power to weight ratio and strength to weight ratio, measures</p><p>　　of assistance to be at 30% and that the actuator acts with a 12 cm moment arm

103、 to the ankle joint. These values can be changed but, based upon personal experience, are reasonable in their scale. Using these values and parameters available for a chosen Maxon motor, the RE40, a range of lead lengths