機(jī)械手夾持器設(shè)計(jì)
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中原工學(xué)院畢業(yè)設(shè)計(jì)翻譯
靈巧機(jī)器手的機(jī)械系統(tǒng)和控制系統(tǒng)
文章來源：德克-亨氏卡爾斯魯厄大學(xué)計(jì)算機(jī)科學(xué)系研究所過程控制和機(jī)器人
技術(shù) 恩格勒-班泰環(huán)8 樓40.28 的D - 76131 卡爾斯魯厄
摘要：最近，全球內(nèi)帶有多指夾子或手的機(jī)械人系統(tǒng)已經(jīng)發(fā)展起來了，多種方法應(yīng)用其上，有擬人化和非擬人化的，不僅調(diào)查了這些系統(tǒng)的機(jī)械結(jié)構(gòu)，而且還包括其必要的控制系統(tǒng)，如同人手一樣，這些機(jī)械人系統(tǒng)可以用它們的手去抓不同的物體，而且不用改換夾子，這些機(jī)械手具備特殊的運(yùn)動(dòng)能力比如小質(zhì)量和小慣量，這使被抓物體在機(jī)械手的工作范圍內(nèi)做更復(fù)雜、更精確的操作變得可能。這些復(fù)雜的操作被抓物體繞任意角度和軸旋轉(zhuǎn)。本文概述了這種機(jī)械手的一般設(shè)計(jì)方法，同時(shí)給出此類機(jī)械手的一個(gè)示例，如卡爾斯魯靈巧手Ⅱ。本文末介紹了一些新的構(gòu)想，如利用液體驅(qū)動(dòng)器為類人型機(jī)器人設(shè)計(jì)一個(gè)全新的機(jī)械手。
關(guān)鍵詞：多指機(jī)器手；機(jī)器人手；精操作；機(jī)械系統(tǒng)；控制系統(tǒng)
1、引言
2010 年7 月在德國(guó)卡爾斯魯厄開展的“人形機(jī)器人”特別研究，是為了開發(fā)在正常環(huán)境如廚房或客廳下能夠和人類合作和互動(dòng)的機(jī)器人系統(tǒng)。設(shè)計(jì)這些機(jī)器人系統(tǒng)是為了能夠在非專業(yè)、非工業(yè)的條件下如身處多物之中，幫我們抓取不同大小、形狀和重量的物體。同時(shí)，它們必須能夠精細(xì)的操縱被抓物體。這種極強(qiáng)的靈活性只能通過一個(gè)適應(yīng)性極強(qiáng)的機(jī)械人手抓系統(tǒng)來獲得，即所謂的多指機(jī)械手或機(jī)器人。上文提到的研究項(xiàng)目，就是要制造一個(gè)人形機(jī)器人，此機(jī)器人將裝備這種機(jī)器人手系統(tǒng)。這個(gè)新手將有兩個(gè)機(jī)構(gòu)合作制造，它們是卡爾斯魯厄大學(xué)的IPR（過程控制和機(jī)器人技術(shù)研究院）和IAI 公司（計(jì)算機(jī)應(yīng)用科學(xué)研究院）。這兩個(gè)組織都有制造此種系統(tǒng)的相關(guān)經(jīng)驗(yàn)，但是稍有不同的觀點(diǎn)。
圖1：IPR 制造的卡爾斯魯厄靈巧手Ⅱ
IPR 制造的卡爾斯魯厄靈巧手Ⅱ（如圖1 所示），是一個(gè)四指相互獨(dú)立的手爪，我們將在此文中詳細(xì)介紹。IAI 制造的手（如圖17 所示）是作為殘疾人的假肢。
2、 機(jī)器人手的一般結(jié)構(gòu)一個(gè)機(jī)器人手可分成兩大主要子系統(tǒng)：
·機(jī)械系統(tǒng)
·控制系統(tǒng)
機(jī)械系統(tǒng)我們將在第三部分作進(jìn)一步介紹，能分別為以下三部分：
- 結(jié)構(gòu)設(shè)計(jì)
- 驅(qū)動(dòng)系統(tǒng)
- 傳感系統(tǒng)
在第四部分介紹的控制系統(tǒng)至少包括：
- 控制硬件
- 控制軟件
我們將對(duì)這兩大子系統(tǒng)的問題作一番基本介紹，然后用卡爾斯魯厄靈巧手Ⅱ演示一下。
3、 機(jī)械系統(tǒng)
機(jī)械系統(tǒng)將描述這個(gè)手看起來如何以及有什么元件組成。它決定結(jié)構(gòu)設(shè)計(jì)、手指的數(shù)量及使用的材料。此外，還確定驅(qū)動(dòng)器（如電動(dòng)機(jī)）、傳感器（如位置編碼器）的位置。
3.1結(jié)構(gòu)設(shè)計(jì)
結(jié)構(gòu)設(shè)計(jì)將對(duì)機(jī)械手的靈活度起很大的作用，即它能抓取何種類型的物體以及能對(duì)被抓物體進(jìn)行何種操作。設(shè)計(jì)一個(gè)機(jī)器人手的時(shí)候，必須確定三個(gè)基本要素：
·手指的數(shù)量
·手指的關(guān)節(jié)數(shù)量
·手指的尺寸和安置位置
為了能夠在機(jī)械手的工作范圍內(nèi)安全的抓取和操作物體，至少需要3根手指。為了能夠?qū)Ρ蛔ノ矬w的操作獲得6 個(gè)自由度（3 個(gè)平移和3 個(gè)旋轉(zhuǎn)自由度），每個(gè)手指必須具備3 個(gè)獨(dú)立的關(guān)節(jié)。這種方法在第一代卡爾斯魯厄靈巧手上被采用過。但是，為了能夠重抓一個(gè)物體而無需將它先釋放再拾取的話，至少需要4 根手指。
要確定手指的尺寸和安置位置，可以采用兩種方法：
·擬人化
·非擬人化
然后將取決于被操作的物體以及選擇何種期望的操作類型。擬人化的安置方式很容易從人手到機(jī)器人手的轉(zhuǎn)移抓取意圖。但是每個(gè)手指不同的尺寸和不對(duì)稱的安置位置將增加加工費(fèi)用，并且使控制系統(tǒng)變得更加復(fù)雜因?yàn)槊總€(gè)手指都必須分別加以控制。
對(duì)于相同手指的對(duì)稱布置，常采用非擬人化方法。因?yàn)橹恍杓庸ず蜆?gòu)建單一的“手指模塊”，因此可減少加工費(fèi)用，同時(shí)也可使控制系統(tǒng)簡(jiǎn)化。
3.2驅(qū)動(dòng)系統(tǒng)
指關(guān)節(jié)的驅(qū)動(dòng)器對(duì)手的靈活度也有很大的影響，因?yàn)樗鼪Q定潛在的力量、精度及關(guān)節(jié)運(yùn)動(dòng)的速度。機(jī)械運(yùn)動(dòng)的兩個(gè)方面需加以考慮：
·運(yùn)動(dòng)來源
·運(yùn)動(dòng)方向
在這方面，文獻(xiàn)里描述了有幾種不同的方法，如文獻(xiàn)[3]中說運(yùn)動(dòng)可由液壓缸或氣壓缸產(chǎn)生，或者正如大部分情況一樣使用電動(dòng)馬達(dá)。在大多數(shù)情況下，運(yùn)動(dòng)驅(qū)動(dòng)器（如電動(dòng)機(jī)）太大而不能直接與相應(yīng)的指關(guān)節(jié)結(jié)合在一起，因此，這個(gè)運(yùn)動(dòng)必須由驅(qū)動(dòng)器（一般位于機(jī)器臂最后的連接點(diǎn)處）轉(zhuǎn)移過來。有幾種不同的方法可實(shí)現(xiàn)這種運(yùn)動(dòng)方式，如使用鍵、傳動(dòng)帶以及軟軸。使用這種間接驅(qū)動(dòng)指關(guān)節(jié)的方法，或多或少地降低了整個(gè)系統(tǒng)的強(qiáng)度和精度，同時(shí)也使控制系統(tǒng)復(fù)雜化，因?yàn)槊扛种傅牟煌P(guān)節(jié)常常是機(jī)械地連在一起，但是在控制系統(tǒng)的軟件里卻要將它們分別獨(dú)立控制。由于具有這些缺點(diǎn)，因此小型化的運(yùn)動(dòng)驅(qū)動(dòng)器與指關(guān)節(jié)的直接融合就顯得相當(dāng)必要。
3.3傳感系統(tǒng)
機(jī)器手的傳感系統(tǒng)可將反饋信息從硬件傳給控制軟件。對(duì)手指或被抓物體建立一個(gè)閉環(huán)控制系統(tǒng)是很必要的。在機(jī)器手中使用了3 種類型的傳感器：
·手爪狀態(tài)傳感器確定指關(guān)節(jié)和指尖的位置以及手指上的作用力情況。知道了指尖的精確位置將使精確控制變得可能。另外，知道手指作用在被抓物體上的力，就可以抓取易碎物件而不會(huì)打破它。
·抓取狀態(tài)傳感器提供手指與被抓物體之間的接觸狀態(tài)信息。這種觸覺信息可在抓取過程中及時(shí)確定與物體第一次接觸的位置點(diǎn)，同時(shí)也可避免不正確的抓取，如抓到物體的邊緣和尖端。另外還能觀察覺到已抓物體是否滑落，從而避免物體因跌落而損壞。
·物體狀態(tài)或姿態(tài)傳感器用于確定手指內(nèi)物體的形狀、位置和方向。如果在抓取物體之前并不清楚這些信息的情況下，這種傳感器是非常必要的。如果此傳感器還能作用于已抓物體上的話，它也能控制物體的姿態(tài)（位置和方向），從而監(jiān)測(cè)是否滑落。
根據(jù)不同的驅(qū)動(dòng)系統(tǒng)，有關(guān)指關(guān)節(jié)位置的幾何信息可以在運(yùn)動(dòng)驅(qū)動(dòng)器或直接在關(guān)節(jié)處測(cè)量。例如，如在電動(dòng)機(jī)和指關(guān)節(jié)之間有一剛性聯(lián)軸器，那么就可以用電機(jī)軸上的一個(gè)角度編碼器（在齒輪前或齒輪后）來測(cè)量關(guān)節(jié)的位置。但是如果此聯(lián)軸器剛度不夠或者要獲得很高的精度的話，就不能使用這種方法。
3.4 卡爾斯魯厄靈巧手Ⅱ的機(jī)械
系統(tǒng)為了能夠獲得如重抓等更加復(fù)雜的操作，卡爾斯魯厄靈巧手Ⅱ（KDHⅡ）由4 根手指組成，且每根手指由3 個(gè)相互獨(dú)立的關(guān)節(jié)組成。設(shè)計(jì)該手是為了能夠在工業(yè)環(huán)境中應(yīng)用（圖2 所示）和操縱箱、缸及螺釘螺帽等物體。因此，我們選用四個(gè)相同手指，將它們作對(duì)稱、非擬人化裝置，且每個(gè)手指都能旋轉(zhuǎn)90°（圖3 所示）。
圖2：裝在一個(gè)工業(yè)機(jī)器人上的KDH Ⅱ
圖3： KDH Ⅱ的俯視圖
圖4：KDH Ⅱ的側(cè)視圖
鑒于從第一代卡爾斯魯厄靈巧手設(shè)計(jì)中得到的經(jīng)驗(yàn)，比如因傳動(dòng)帶而導(dǎo)致的機(jī)械問題以及較大摩擦因數(shù)導(dǎo)致的控制問題，卡爾斯魯厄靈巧手Ⅱ采用了一些不同的設(shè)計(jì)方案。每根手指的關(guān)節(jié)2 和關(guān)節(jié)3 之間的直流電機(jī)被整合到手指前部肢體中（圖4 所示）。這種布置可使用很硬的球軸齒輪將運(yùn)動(dòng)傳遞到手指的關(guān)節(jié)處。處在電機(jī)軸上的角度編碼器（在齒輪前）此時(shí)可作為一個(gè)精度很高的位置狀態(tài)傳感器。
為了感知作用在物體上的手指力量，我們發(fā)明了一個(gè)6 維力矩傳感器（圖5 所示）。這個(gè)傳感器可當(dāng)作手指末端肢體使用，且配有一個(gè)球形指尖。它可以抓取較輕的物體，同時(shí)也能抓取3~5kg 相近重量的物體。此傳感器能測(cè)量X、Y 和Z 方向的力及繞相關(guān)軸的力矩。另外，3 個(gè)共線的激光三角測(cè)量傳感器被安置在KDHⅡ的手掌上（圖4 所示）。因?yàn)橛? 個(gè)這樣的傳感器，因此不僅可以測(cè)量3 個(gè)單點(diǎn)之間的距離，如果知道物體的形狀，還能測(cè)出被抓物體表面之間的距離和方向。物體狀態(tài)傳感器的工作頻率為1HZ，它能檢測(cè)和避免物體的滑落。
圖5：用于KDH Ⅱ指尖的6 維力矩傳感器
4、 控制系統(tǒng)
機(jī)器人手的控制系統(tǒng)決定哪些潛在的靈巧技能能夠被實(shí)際利用，這些技能都是由機(jī)械系統(tǒng)所提供的。如前所述，控制系統(tǒng)可分為控制計(jì)算機(jī)即硬件和控制算法即軟件。
控制系統(tǒng)必須滿足以下幾個(gè)的條件：
·必須要有足夠的輸入輸出端口。例如，一具有9 個(gè)自由度的低級(jí)手，其驅(qū)動(dòng)器至少需要9 路模擬輸出端口，且要有9 路從角度編碼器的輸入端口。如再加上每個(gè)手指上的力傳感器、觸覺傳感器及物體狀態(tài)傳感器的話，則端口數(shù)量將增加幾倍。
·需具備對(duì)外部事件快速實(shí)時(shí)反應(yīng)的能力。例如，當(dāng)檢測(cè)到物體滑落時(shí)，能立即采取相應(yīng)的措施。
·需具備較高的計(jì)算能力以應(yīng)對(duì)一些不同的任務(wù)。如可以對(duì)多指及物體并執(zhí)行路徑規(guī)劃、坐標(biāo)轉(zhuǎn)換及閉環(huán)控制系統(tǒng)等任務(wù)。
·控制系統(tǒng)的體積要小，以便能夠?qū)⑵渲苯蛹傻讲僮飨到y(tǒng)當(dāng)中。
·在控制系統(tǒng)與驅(qū)動(dòng)器及傳感器之間必須要電氣短接。特別是對(duì)傳感器來說，若沒有的話，很多的干擾信號(hào)將會(huì)干擾傳感器信號(hào)。
4.1控制硬件
為了應(yīng)對(duì)系統(tǒng)的要求，控制硬件一般分布在幾個(gè)專門的處理器中。如可通過一個(gè)簡(jiǎn)單的微控制器處理很低端的輸入輸出接口（馬達(dá)和傳感器），因此控制器尺寸很小，能輕易地集成到操作系統(tǒng)中。但是較高水平的控制端口則需要較高的計(jì)算能力，且需要一個(gè)靈活實(shí)時(shí)操作系統(tǒng)的支持。這可以通過PC 機(jī)輕易地解決。
因此，控制硬件常有一個(gè)非均勻的分布式計(jì)算機(jī)系統(tǒng)組成，它的一端是微控制器，而另一端則是一個(gè)功能強(qiáng)大的處理器。不同的計(jì)算單元?jiǎng)t通過一個(gè)通信系統(tǒng)連接起來，比如總線系統(tǒng)。
4.2控制軟件
機(jī)器人手的控制軟件是相當(dāng)復(fù)雜的。必須要對(duì)手指進(jìn)行實(shí)時(shí)及平行_控制，同時(shí)還要計(jì)劃手指和物體的新的軌跡。因此，為了減少問題的復(fù)雜性，就有必要將此問題分成幾個(gè)子問題來處理。
另一方面涉及軟件的開發(fā)。機(jī)器人手其實(shí)是一個(gè)研究項(xiàng)目，它的編程環(huán)境如用戶界面，編程工具和調(diào)試設(shè)施都必須十分強(qiáng)大和靈活。這些只能使用一個(gè)標(biāo)準(zhǔn)的操作系統(tǒng)才能得到滿足。在機(jī)械人中普遍使用的分層控制系統(tǒng)方法都經(jīng)過了修剪，以滿足機(jī)械手的特殊控制要求。
圖6：KDH II 的控制硬件架構(gòu)
4.3 卡爾斯魯厄靈巧手Ⅱ的控制系統(tǒng)
如在4.1 節(jié)中所說，對(duì)于卡爾斯魯厄靈巧手Ⅱ的控制硬件，采用了一種分布式方法（圖6 所示）。一個(gè)微控制器分別控制一個(gè)手指的驅(qū)動(dòng)器和傳感器，另外一個(gè)微控制器用于控制物體狀態(tài)傳感器（激光三角傳感器）。這些微控制器（圖6 左側(cè)和右側(cè)的外箱）直接安裝在手上，所以可以保證和驅(qū)動(dòng)器及傳感器之間較短的電器連接。
這些微控制器都是使用串行總線系統(tǒng)CAN 和主控計(jì)算機(jī)連在一起的。這個(gè)主控計(jì)算機(jī)（圖6、圖7 中的灰色方塊）是由六臺(tái)工業(yè)計(jì)算機(jī)組成的一個(gè)并行計(jì)算機(jī)（PC104 標(biāo)準(zhǔn)）。這些電腦都被排列在一個(gè)二維平面。相鄰電腦模塊（一臺(tái)電腦最多有8 個(gè)相鄰模塊）使用雙端口RAM 進(jìn)行快速通信（圖6 中暗灰色方塊所示）。一臺(tái)電腦用于控制一個(gè)手指，另一臺(tái)用于控制物體狀態(tài)傳感器及計(jì)算物體之間的位置。其余的電腦被安裝在前面提到的電腦的周圍。這些電腦用于協(xié)調(diào)整個(gè)控制系統(tǒng)。控制軟件的結(jié)構(gòu)反映了控制硬件的架構(gòu)。如圖8 所示。
圖7：用于控制KDH II 的并行計(jì)算機(jī)
圖8：KDH II 本地控制系統(tǒng)
一個(gè)關(guān)于此手控制系統(tǒng)的三個(gè)最高層的網(wǎng)上計(jì)劃正在規(guī)劃。理想的物體位移命令可由優(yōu)越的機(jī)器人控制系統(tǒng)得到，并可用作物體路徑的精確規(guī)劃。根據(jù)已生產(chǎn)的目標(biāo)路徑就可規(guī)劃可行的抓取行為（手指作用在物體上的可行抓取位置點(diǎn)）。現(xiàn)在知道了物體的運(yùn)動(dòng)計(jì)劃，就可以由手指路徑規(guī)劃得出每個(gè)手指的運(yùn)動(dòng)軌跡，并傳遞給系統(tǒng)的實(shí)時(shí)能力部分。如果一個(gè)物體被抓取了，那么其手指的運(yùn)動(dòng)軌跡就
傳給了物體的狀態(tài)控制器。這個(gè)控制器控制物體的姿態(tài)，它由手指和物體狀態(tài)傳感器所決定，用以獲得所需的物體姿態(tài)。如果一個(gè)手指沒有跟物體接觸，那么它的移動(dòng)路徑將會(huì)直接傳遞給手控制器。這個(gè)手控制器將相關(guān)的預(yù)期手指位置傳遞給所有的手指控制器，以協(xié)調(diào)所有手指的運(yùn)動(dòng)。這些在手指?jìng)鞲衅鞯膸椭掠址催^來。
Mechanical System and Control System of a Dexterous
Robot Hand
Dirk Osswald, Heinz W?rn
University of Karlsruhe
Department of Computer Science
Institute for Process Control and Robotics (IPR)
Engler-Bunte-Ring 8 - Building 40.28
D-76131 Karlsruhe
Abstract: In recent years numerous robot systems with multifingered grippers or hands have been
developed all around the world. Many different approaches have been taken, anthropomorphic and non-anthropomorphic ones. Not only the mechanical structure of such systems was investigated, but also the necessary control system. With the human hand as an exemplar, such robot systems use their hands to grasp diverse objects without the need to change the gripper. The special kinematic abilities of such a robot hand, like small masses and inertia, make even complex manipulations and very fine manipulations of a grasped object within the own workspace of the hand possible. Such complex manipulations are for example regrasping operations needed for the rotation of a grasped object around arbitrary angles and axis without depositing the object and picking it up again. In this paper an overview on the design of such robot hands in general is given, as well as a presentation of an example of such a robot hand, the Karlsruhe Dexterous Hand II. The paper then ends with the presentation of some new ideas which will be used to build an entire new robot hand for a humanoid robot using fluidic actuators.
Keywords: Multifingered gripper, robot hand, fine manipulation, mechanical system, control system
1 Introduction
The special research area 'Humanoid Robots' founded in Karlsruhe, Germany in July 2001 is
aimed at the development of a robot system which cooperates and interacts physically with human
beings in 'normal' environments like kitchen or living rooms. Such a robot system which is
designed to support humans in non-specialized, non-industrial surroundings like these must, among
many other things, be able to grasp objects of different size, shape and weight. And it must also
be able to fine-manipulate a grasped object. Such great flexibility can only be reached with an
adaptable robot gripper system, a so called multifingered gripper or robot hand.
The humanoid robot, which will be built in the above mentioned research project, will be equipped
with such a robot hand system. This new hand will be built by the cooperation of two institutes, the
IPR (Institute for Process Control and Robotics) at the University of Karlsruhe and the IAI (Institute
for Applied Computer Science) at the Karlsruhe Research Center. Both organizations already have
experience in building such kind of systems, but from slightly different points of view.
The 'Karlsruhe Dexterous Hand II' (see figure 1) built at the IPR, which is described here in detail, is
a four fingered autonomous gripper. The hands built at the IAI (see figure 17) are built as prosthesis for handicapped people.
The approach taken so far will be presented and discussed in the following sections, as it founds the
basis for the novel hand of the humanoid robot.
Figure 1: Karlsruhe Dextrous Hand II from IPR
2 General structure of a robot hand
A robot hand can be split up in two major subsystems:
? The mechanical system
? The control system
The mechanical system, further described in section 3, can be subdivided into:
- The mechanical design
- The actuator system
- The sensor system
And the control system described in section 4 consists at least of :
- The control hardware
- The control software
For each of these parts we will describe the considerations for a robot hand in general and then
present the exemplary implementation in the Karlsruhe Dexterous Hand II.
3 Mechanical system
The mechanical system describes how the hand looks like and what kind of components it is made
of. It defines the mechanical design, e.g. the number of fingers and the kind of materials used.
Additionally actuators, e.g. electric motors, and sensors, e.g. position encoders, are settled.
3.1 Mechanical design
The mechanical design determines the fundamental 'dexterousness' of the hand, i.e. what kind of
objects can be grasped and what kind of manipulations can be performed with a grasped object. Three basic aspects have to be settled when designing a robot hand:
? The number of fingers
? The number of joints per finger
? The size and placement of the fingers
To be able to grasp and manipulate an object safely within the workspace of the hand at least 3 fingers are required. To achieve the full 6 degrees of freedom (3 translatory and 3 rotatory DOF) for the
manipulation of a grasped object at least 3 independent joints are needed for each finger. This approach was taken for the first Karlsruhe Dexterous Hand [1,2]. However, to be able to regrasp an object without having to release it and then pick it up again, at least 4 fingers are necessary.
To determine the size and the placement of the fingers two different approaches can be taken:
? Anthropomorphic
? Non-anthropomorphic
It then depends on the objects to manipulate and on the type of manipulations desired which one is
chosen. An anthropomorphic placement allows to easily transfer e.g. grasp strategies from a human
hand to the robot hand. But the different sizes of each finger and their asymmetric placement makes
the construction more expensive and the control system more complicated, because each finger has
to be treated separately. When a non-anthropomorphic approach is taken most often identical fingers are arranged symmetrically. This reduces the costs for the construction and simplifies the control system because there is only one single 'finger module' to be constructed and controlled.
3.2 Actuator system
The actuation of the finger joints also has a great influence in the dexterousness of the hand, because
it determines the potential forces, precision and speed of the joint movements. Two different aspects of the mechanical movement have to be considered:
? Movement generation
? Movement forwarding
Several different approaches for these aspects are described in the literature. E.g. the movement can
be generated by hydraulic or pneumatic cylinders [3] or, as in most cases, by electric motors.
As the movement generators (motors) are in most cases to big to be integrated in the corresponding
finger joint directly, the movement must be forwarded from the generator (most times located in the last link of the robot arm) to the finger joint. Again different methods can be used, like tendons
[4,5,6], drive belts [1,2] or flexible shafts. The use of such more or less indirect actuation of the finger
joint reduces the robustness and the precision of the system and it complicates the control system
because different joints of one finger are often mechanically coupled and must be decoupled in
software by the control system. Due to these drawbacks an integration of miniaturized movement generators directly into the finger joints is desirable.
3.3 Sensor system
The sensor system of a robot hand provides the feedback information from the hardware back to
the control software. This is necessary to perform a closed loop control of the fingers or a grasped
object. Three types of sensors are used in robot hands [7,8]:
? Gripper state sensors determine the position of the finger joints, and hence the finger tip, and
the forces which act upon the finger. Knowing the exact position of the fingertip makes precise
position control possible, which is necessary for dexterous fine manipulations. With the knowledge of the forces applied to a grasped object by the fingers it is possible to grasp a fragile object without breaking it.
? Grasp state sensors provide information about the contact situation between the finger and the object. This tactile information can be used to determine the point in time of the first contact with the object while grasping, and to avoid undesired grasps, like grasping at an edge or a tip of the object. But it can also be used to detect slippage of an already grasped object, which might lead to a loss of the object.
? Object state or pose sensors are used to determine the shape, position and orientation of an object in the workspace of the gripper. This is necessary if these data is not known exactly, prior to grasping the object. If the object state sensors still works on a grasped object it can be used to control the pose (position and orientation) of a grasped object too, e.g. to detect slippage.
Depending on the actuator system the geometrical information about the finger joint position can be
measured at the movement generator or directly at the joint. For example if there is a stiff coupling
between an electric motor and the finger joint then the joint position can be measured by an angle
encoder at the axis of the motor (before or after the gear). This is not possible if the coupling is less
stiff and a high position precision is desired.
Figure 2: KDH II mounted on an industrial robot
圖3： KDH Ⅱ的俯視圖
圖4：KDH Ⅱ的側(cè)視圖
3.4 The mechanical system of the Karlsruhe
Dexterous Hand II
In order to permit more complex manipulations like regrasping the current Karlsruhe Dexterous Hand II(KDH II) was built with 4 fingers and 3 independent joints per finger. It is designated for applications in industrial environments (see figure 2) and for manipulation of objects like boxes, cylinders, screws or nuts. Therefore a symmetric, non-anthropomorphic configuration of four identical fingers, each rotated by 90° was chosen (see figure 3).
Due to the experiences gained with the first Karlsruhe Dexterous Hand, like e.g. mechanical
problems caused by the drive belts or controlling problems caused by large friction factors, some
different design decisions were chosen for the KDH II. The dc-motors for joint 2 and 3 of each
finger are integrated into the previous finger limb (see figure 4). This permits the use of very stiff
ball-spindle-gears for the forwarding of the movement to the finger joint. Angle encoders directly on the motor axis (before the gear) are used as very precise position state sensors.
For sensing the forces applied to an object by a finger a prototype of a 6 dimensional force torque
sensor has been developed (see figure 5). It can be used as the last finger limb and is equipped with a
spherical finger tip. It is able to grasp light objects as well as relatively heavy objects up to 3 to 5 kg.
The sensor is able to measure forces in x- y- and zdirection and torques around these axes. Additionally 3 colinear laser triangulation sensors are mounted in the palm of the KDH II (see figure 4) [11]. Because there are three such sensors not only the distances of 3 single points can be measured, but also the distance and orientation of the surface of a grasped object, if the shape of the object is known. This object pose sensor works with a frequency of 1 kHz which allows the detection and avoidance of a slipping object.
4 Control system
The control system of a robot hand determines which of the potential dexterous skills provided by
the mechanical system can actually be exploited. As mentioned before the control system can be
subdivided in the control computer or hardware and the control algorithms or software.
The control system must meet several conflicting requirements:
Figure 5: 6 DOF force torque sensor with strain gage sensors used as the last finger limb of the KDH II
? Many input/output resources like actor or sensor signals must be attached. For example for a minimum hand with 9 degrees of freedom, at least 9 analog outputs to the motors and 9 inputs from angle encoders must be estimated. With force and tactile sensors for every finger and additional object state sensors the number of inputs quickly increases to several dozens.
? Quick reactions in real-time to external events are required. If for example a slipping of the grasped object is detected immediate counter measures must be taken.
? High computing power for several different tasks must be available. For example path planning, coordinate transformations, closed loop control in software are executed in parallel for multiple fingers as well as for the object.
? Small physical size is needed to be able to integrate the control system into the manipulation system.
? Short electrical connections between the control system and the actuators and sensors should be
used. This is especially relevant for the sensors because otherwise massive interference might disturb the sensor signal.
4.1 Control hardware
To cope with the requirements the control hardware is usually distributed among several specialized
processors. For example the input/output on the lowest level (motors and sensors) can be handled by a simple microcontroller, which is also of small size and thus can be integrated more easily into the
manipulation system. But the higher levels of control need more computing power and the support of a flexible real time capable operating system. This can be achieved most easily with PClike components.
Therefore the control hardware often consist of a non-uniform, distributed computing system with
microcontrollers on the one end and more powerful processors on the other. The different computing
units then have to be connected with a communication system, like for example a bus system.
4.2 Control software
The control software of a robot hand is quite complex. Several fingers must be controlled in realtime
and in parallel while new trajectories for the fingers and the object must be planned at the same time. Therefore it is necessary to reduce the complexity by dividing the problem into sub problems.
Another aspect concerns software development. As a robotic hand is usually a research project for most of it's lifetime, the programming environment, like user interface, programming tools and debugging facilities, should be powerful and flexible. This can only be achieved if a standard operating system is used.
Figure 6: control hardware architecture of the KDH II
The usual hierarchical control system approach used in robotics has to be trimmed to fit the special
needs of the controlling of a robot hand.
4.3 The control system of the Karlsruhe
Dexterous Hand II
As suggested in section 4.1 a distributed approach to the control hardware was taken for the Karlsruhe Dexterous Hand II (KDH II) (see figure 6) [8]. One microcontroller is used to control the actuators and sensors of one finger respectively. An additional microcontroller is used for the object state sensor (laser triangulation sensors). These microcontrollers (the outer boxes to the left and right in figure 6) are mounted directly on the hand, thus short electrical connections to the actuators and sensors are guaranteed. The microcontrollers are connected to the main control computer by serial bus systems (CAN-bus).
The main control computer of the KDH II (the light grey box in figure 6, and figure 7) is implemented as a parallel computer consisting of 6 industrial PCs (PC104 standard). These PCs are arrang
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