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美國國家標準和技術研究所(NIST)對并聯(lián)運動機床過去、現(xiàn)在、未來的研究
摘要
并聯(lián)運動機床在突破了控制器局限性的限制后，其在制造業(yè)方面的應用開始產(chǎn)業(yè)化。并聯(lián)機床由于其投放市場后的商業(yè)價值，在1994年芝加哥國際制造業(yè)展覽會上出現(xiàn)了新發(fā)明的數(shù)控并聯(lián)機床。與傳統(tǒng)機床相比，并聯(lián)驅(qū)動技術在加工制造方面具有許多優(yōu)點，比如：較高的剛度——體積比、較高的速度、較高的精度、安裝次數(shù)少，夾具簡單。在美國，一些機床制造者正在研發(fā)并聯(lián)驅(qū)動技術，同時并聯(lián)驅(qū)動技術的潛在用戶——加工制造商正企劃這種新型的多軸加工技術在操作領域的用途。
NAMT、NIST的研究員及來自各行業(yè)、大學的合作者正在研究這個新型工具的獨特性能。其工作包括對1995年五月搭建的八面體、六驅(qū)動器機床的大范圍的實驗。研究領域包括機床測量、性能測試方法及標準、性能拓展方法、仿真試驗工具和開放式體系結構控制界面。本論文給出了美國國家標準和技術研究所(NIST)對并聯(lián)運動機床的研究現(xiàn)狀及歷史的綜述。并聯(lián)運動的優(yōu)點、所面臨的挑戰(zhàn)、將來的研發(fā)方向也被提到。
1簡介
提高產(chǎn)品質(zhì)量、降低產(chǎn)品成本、縮短產(chǎn)品開發(fā)周期是企業(yè)保持競爭力的迫切需要。這些決定競爭力的關鍵因素要求機床的加工精度、速度、多功能性不斷發(fā)展、提高。這就對機床研究領域提出了挑戰(zhàn)。鞭策機床研發(fā)單位為設計新機床而演化基本的機構。因此，基于并聯(lián)運動機構的機床模型——并聯(lián)運動機床研發(fā)成功。六自由度杰出代表——Stewart平臺是并聯(lián)運動機床的機構模型，最近許多新機床的設計都是基于Stewart平臺機構模型?；赟tewart平臺機構模型的并聯(lián)機床的經(jīng)典結構組成：動平臺與固定的基礎平臺之間由六個同樣的能活動的可伸縮桿聯(lián)接。高的力容積比、高的結構剛性、低動量是Stewart平臺機構的優(yōu)點。從加工應用的角度考慮，Stewart平臺的缺點在于工作空間復雜、運動方向受到限制、對于五自由度工作（磨、鉆、簡單操作）需要六個驅(qū)動器驅(qū)動。一些基于Stewart平臺的機床機構模型通常叫做并聯(lián)機械機構。
這些并聯(lián)機床與傳統(tǒng)機床相比具有許多優(yōu)點。它們還提出了一些重要的問題。這些機床的性能如何描述？計算工作空間的最好方法是什么？機床可以達到的加工精確程度？實際應用時什么是最好的控制算法？在成本方面是否具有競爭力？這些機床最適合于何種加工方式？這些機床的模塊化程序設計如何最優(yōu)化？何種結構在低成本的前提下又具有較多的功能？這些都是從并聯(lián)機床方面提出的重要問題，自從1990年美國國家標準和技術研究所(NIST)在并聯(lián)機床的許多領域進入了研究。這一論文概述了美國國家標準與技術研究對于并聯(lián)運動機床的過去、現(xiàn)在、未來的研究，提出了一些依然具有挑戰(zhàn)性的課題。本論文的討論圍繞著六自由度的Stewart平臺機械機構、并聯(lián)運動機床的機構模型。然而，還有一些并聯(lián)機構的排列方式也許更適合于作為機床的機構模型（可能優(yōu)于Stewart平臺機構本身）。
工業(yè)機器人問世以來, 采用串聯(lián)機構的操作器一直占主導地位，它結構簡單，工作空間大，因而獲得廣泛應用。與之相比,并聯(lián)機器人活動空間小，活動上平臺遠不如串聯(lián)機器人手部靈活。但是，并聯(lián)機器人也有其獨特的優(yōu)點：
并聯(lián)式結構其末端件上平臺同時經(jīng)由6 根桿支撐，與串聯(lián)的懸臂梁相比，剛度大，而且結構穩(wěn)定；由于剛度大，并聯(lián)式較串聯(lián)式在相同的自重或體積下，有高得多的承載能力；串聯(lián)式末端件上的誤差是各個關節(jié)誤差的積累和放大，因而誤差大、精度低，并聯(lián)式則沒有那樣的誤差積累和放大關系，誤差小、精度高；串聯(lián)式機器人的驅(qū)動電機及傳動系統(tǒng)大都放在運動著的大小臂上，增加了系統(tǒng)的慣量，惡化了動力性能，而并聯(lián)式則很容易將電機置于機座上，減小了運動負荷；位置求解上，串聯(lián)機構正解容易, 但反解十分困難, 而并聯(lián)機構正解困難，反解卻非常容易。由于機器人的在線實時計算是要計算反解的，這對串聯(lián)式十分不利,而并聯(lián)式卻容易實現(xiàn)。
并聯(lián)機構的出現(xiàn)，擴大了機器人的應用范圍。隨著對并聯(lián)機器人研究的不斷深入，其應用領域也越來越廣闊。并聯(lián)機器人的應用大體分為六大類：運動模擬器、并聯(lián)機床、工業(yè)機器人、動機構、醫(yī)用機器人和操作器。
運動模擬器 作為運動模擬器,其最廣泛的應用是飛行模擬器。訓練用飛行模擬器具有節(jié)能、經(jīng)濟、全、不受場地和氣象條件限制、訓練周期短、效率高等突出優(yōu)點,目前已成為各類飛行員訓練的必備工具。同時，這種運動模擬器也是研究和開發(fā)各種運載設備的重要工具。通過模擬器可以在早期發(fā)現(xiàn)問題、減少風險、進行綜合系統(tǒng)驗證，解決各系統(tǒng)間的動態(tài)匹配關系、加速系統(tǒng)實驗過程，縮短研制周期，降低開發(fā)費用。
并聯(lián)機床 用作并聯(lián)機床是并聯(lián)機構最具吸引力的應用。并聯(lián)機床結構簡單，傳動鏈短,剛度大、質(zhì)量輕、成本低，容易實現(xiàn)“6軸聯(lián)動”，能加工更加復微雜的三維曲面。還具有環(huán)境適應性強的特點，便于重組和模塊化設計，可構成形式多樣的布局和自由度組合。德國Micromat 公司生產(chǎn)的6X 并聯(lián)機床，這種機床采用伸縮桿的長度變化驅(qū)動主軸部件，實現(xiàn)加工軌跡所需的運動，工件固定在工作臺上不動。
工業(yè)機器人 隨著工業(yè)現(xiàn)代化發(fā)展的高速進程,以及加工業(yè)工藝的不斷完善，技術的不斷進步，工業(yè)機器人的應用被越來越多的企業(yè)認識和接受。工業(yè)機器人既保證了產(chǎn)品質(zhì)量,又減少了特殊環(huán)境工作的危險和實現(xiàn)對人員的勞動強度的降低和人員勞動保護意識的提高。
微動機構 作為微動機構是并聯(lián)機器人的一個重要應用方面。這種微動機構發(fā)揮了并聯(lián)機構的特點，工作空間不很大但精度和分辨率都非常高。
醫(yī)用機器人 醫(yī)療機器人已經(jīng)成為醫(yī)學外科學會和機器人學會共同關注的新興技術領域。近年來，醫(yī)療機器人技術引起美、法、德、意、日等國家學術界的極大關注，研究工作蓬勃興起。醫(yī)療機器人具備選位準確、動作精細、避免病人感染等特點。
操作器 作為操作器, 并聯(lián)機器人可以用作飛船和空間對接器的對接機構，上下平臺中間都有通孔作為對接后的通道，上下平臺作為對接環(huán)，由6 個直線驅(qū)動器驅(qū)動以幫助飛船對正;對接機構還能完成吸收能量和減振，以及主動抓取、對正拉緊、柔性結合、最后鎖住卡緊等工作。對于困難的地下工程，如土方挖掘、煤礦開采，也可以采用這種強力的并聯(lián)機構。Arai 等1991 年提出將并聯(lián)機構裝于履帶式或步行式可移動的小車上，挖掘頭裝于并聯(lián)機構的上平臺，強有力的并聯(lián)機構可以承受巨大的挖掘力。
其他方面應用 此外,Stewart 平臺還在其他方面應用，如力或位移傳感器，造船起重機,主動式振動控制器，體感模擬娛樂機械等。
并聯(lián)機器人具有很多傳統(tǒng)串聯(lián)機器人不具備的優(yōu)點,并聯(lián)機器人還有很多理論問題需要進一步的研究和完善，適用于不同工作要求的新型的并聯(lián)機構有待于進一步開發(fā)。目前,并聯(lián)機器人研究所要解決的問題應包含以下內(nèi)容：
不同自由度的新型并聯(lián)機構的研究。研究新型的并聯(lián)機構，并研究相應的運動學、動力學等理論，必將會進一步豐富并聯(lián)機構領域的研究成果，并進一步擴大并聯(lián)機構的應用范圍。
并聯(lián)機器人運動學正解數(shù)值算法的研究。主要是提高位置正解的計算速度，這項工作是并聯(lián)機器人軌跡規(guī)劃的基礎。并聯(lián)機器人動力學模型研究。建立通用的適用于控制系統(tǒng)設計的并聯(lián)機器人動力學數(shù)學模型，這項工作是計開發(fā)出具有優(yōu)良動力學性能的并聯(lián)機構，對不同類型并聯(lián)機構進行動力學分析的基礎。
并聯(lián)機機器人工作空間研究。研究各種奇異性對工作空間的影響，可以提高我們對并聯(lián)機構運動機理的認識，是進行并聯(lián)機構無奇異路徑規(guī)劃和實現(xiàn)運動的可控性的基礎。
并聯(lián)機器人誤差分析。建立實用的、完整的并聯(lián)機構誤差數(shù)學模型，分析并聯(lián)機構輸入誤差因子對動平臺位資誤差的影響，從而通過控制敏感輸入誤差因子，提高并聯(lián)機器人精度。
少自由度并聯(lián)機構的研究由于少自由度并聯(lián)機構具有結構簡單、造價低廉等特點，有著廣闊的應用前景。但少自由度并聯(lián)機構在某些時候的運動、動力分析反而變得更復雜。
2美國國家標準和技術研究所(NIST)制造工程實驗室對于并聯(lián)運動機床的研究歷史
美國國家標準和技術研究所(NIST)制造工程實驗室對Stewart平臺機械機構的創(chuàng)新應用的研究興趣要追溯到19世紀80年代中期，當時詹姆士和他的從事機器人系統(tǒng)（現(xiàn)在的智能系統(tǒng)）研究的同事研制了一個新項目——繩索機器人吊車，這臺機械采用的是Stewart平臺的機械結構模型。用六個絞車控制的電纜的方式來控制平臺，這臺繩索機器人吊車最初研發(fā)的目的像傳統(tǒng)的吊車那樣是用來穩(wěn)定的起重貨物的。在制造業(yè)和建筑業(yè)領域，為了穩(wěn)地過地搬運貨物、直升機的救援工作、有害廢物的再利用，許多不同型號的這種繩索機器人吊車被研發(fā)出來應用。這種繩索機器人吊車在控制負載的位置和姿態(tài)方面明顯的優(yōu)于傳統(tǒng)的起重機。
美國國家標準和技術研究所(NIST)對Stewart平臺的深入研究生產(chǎn)了這種創(chuàng)新的機械結構繩索機器人吊車。這種八面體結構包括三個用來懸掛工作平臺的上支點，采用三個上支點是為了本身是較輕的結構卻能提供足夠的結構剛度。詹姆士和克萊頓認為這種Stewart平臺結構和八面體空間結構的結合在機床應用方面具有潛在的優(yōu)點?；谶@一思想，他們在1991 年提出了“新型機床”，并且建造了樣機。模型結構表明該機床是由三個可運動的Stewart平臺串連嵌套，第四個Stewart平臺固定的結構組合而成。與傳統(tǒng)的設計相比，這種機構的優(yōu)點在于提高了剛度和精度。
在建立這種新型機床設計的機構模型啟動計劃的過程中，美國國家標準和技術研究所(NIST)的研究員意識到：形成一種商業(yè)性的并聯(lián)機床的過程也就是利用相似原理搭建成一樣機的過程。美國國家標準和技術研究所(NIST)認為商業(yè)性的并聯(lián)機床樣機為將來的研究提供一個好的起點，并且獲得了一個試驗樣機，該機床樣機原型來自 Ingersoll 銑床公司。這種八面體機構，被公司改進后的二代產(chǎn)品，于1995年5月在美國國家標準和技術研究所(NIST)安裝。該機床大約高5米，六個同樣的絞桿，每個絞桿功率11千瓦、絞的轉(zhuǎn)速范圍0轉(zhuǎn)/分鐘~6000轉(zhuǎn)/分鐘，機床上還配備有一個同樣的備用絞桿。
3美國的六條腿機床(Hexapod)用戶群
在美國，隨著對并聯(lián)運動機床研究興趣的上升，桑蒂亞國家實驗室、麻省理工學院、美國國家標準和技術研究所(NIST)等各大組織形成一個強大的六條腿機床(Hexapod)用戶群，這一用戶群包括機床制造商、機床用戶、對并聯(lián)機床的研發(fā)感興趣的研究機構，大約34個企業(yè)，大學和政府組織的代表已經(jīng)參與了第一個六條腿機床(Hexapod)用戶會議，該會議在于1996 年八月在麻省理工學院舉行。第二次會議于1997年3月在美國國家標準和技術研究所(NIST)舉行，后來還有一些非正式會議在召開，美國機械工程師交流(ASME) 暨美國機械工程技術研討和展覽會 (IMECE) 在 1997年十一月、1998年六月、1998年九月舉行, 這些非正式的會議為共享并聯(lián)運動機床的試驗結果、研發(fā)方向的想法和計劃、確定行業(yè)需求提供了寶貴的機會。當然，在六條腿機床(Hexapod)用戶群這一組織成立以前，并聯(lián)機床的研發(fā)還吸引了許多院校及其他組織對并聯(lián)運動機械裝置（大多為機器人應用領域）的研究。
4美國國家標準和技術研究所對并聯(lián)運動機床現(xiàn)在研究領域的研究
美國國家標準和技術研究所(NIST)對并聯(lián)運動機床現(xiàn)狀的研究突出在以下幾點：
對并聯(lián)運動機床特性的更深層次的理解；
評價并聯(lián)運動機床性能的測試方法及檢測手段的標準；
冷卻液走向的排布問題；
與應用、研發(fā)、測試并聯(lián)運動機床相關的建模和仿真問題；
完成同一工作過程中的協(xié)同工作、遠程控制能力的提高；
控制檢測的綜合問題。
因此，并聯(lián)運動機床的研發(fā)問題圍繞著達到以下設計目標而進行： 精確測量、性能拓展、及對并聯(lián)機床協(xié)同研發(fā)、測量學的應用、遠程控制、仿真工具的探討、論證。這一工作的完成要依賴于各個學科的有機綜合及政府、企業(yè)、大學各個合作部門的共同努力。美國國家標準和技術研究所(NIST)的并聯(lián)運動機床研究項目被美國制造工程實驗室引進為先進制造基礎項目（NAMT）。以下部分是美國國家標準和技術研究所(NIST)對并聯(lián)運動機床項目的最新研究成果。
4.1并聯(lián)運動機床的工作性能描述及測量
4.2并聯(lián)運動機床的校準
4.3并聯(lián)運動機床中伸縮桿的測量
4.4并聯(lián)運動機床的機構剛度及測量
4.5并聯(lián)運動機床的建模和仿真
4.6在并聯(lián)運動機床上零件編程和加工的應用
4.7控制器的發(fā)展
5結論、挑戰(zhàn)及并聯(lián)運動機床的研發(fā)方向
致謝
附錄
First European-American Forum on Parallel Kinematic Machines
Theoretical Aspects and Industrial Requirements
31 August – 1 September, 1998
Milan, Italy
Parallel Kinematic Machine Research at NIST:
Past, Present, and Future
Albert J. Wavering
Intelligent Systems Division
National Institute of Standards and Technology (NIST)
Gaithersburg, MD 20899, U.S.A.
Abstract
Unchained from the confines of controller limitations, the industrial application of parallel kinematic machines in manufacturing is beginning to emerge. The launch of commercially available hexapod machine tools at the 1994 International Manufacturing Technology Show in Chicago represented the first radical departure in machine tool design since the introduction of numerical controls. The parallel actuator technology promises to offer manufacturers a number of advantages relative to conventional machine tools, such as a higher stiffness-to-mass ratio, higher speeds, higher accuracy, reduced installation requirements, and mechanical simplicity.Several machine tool makers in the U.S. and around the world are pursuing parallel actuator technology, while their prospective customers—manufacturers—are beginning to contemplate what the novel multi-axis machining technology might mean for their operations.
As part of the Manufacturing Engineering Laboratory's National Advanced Manufacturing Testbed (NAMT), NIST researchers and collaborators from industry and universities are studying the new tools' unique capabilities. The work includes extensive tests on an octahedral hexapod machine tool that was installed at NIST in May 1995. Research areas include machine metrology, performance characterization test methods and standards, performance enhancement methods, simulation and remote experimentation tools, and open architecture controller interfaces. This paper will give an overview of history and current status of research in parallel kinematic machines at NIST. The merits and key challenges of parallel kinematic machine tools will also be addressed, and directions for future research will be identified.
1 Introduction
Improving product quality, reducing product cost, and shortening the product development cycle have always been critical for companies to stay competitive. These competitive drivers result in a continuing need to achieve improvements in accuracy, speed, and versatility in machining operations. These pressing needs pose challenges to the machine tool research community, and have driven several machine tool companies to revisit some of their basic assumptions about machine tool design. As a result, prototypes of a new class of machine tools based on parallel kinematic structures, known as parallel kinematic machines (PKMs), have been introduced.
The six degree-of-freedom Stewart platform [1] is one PKM configuration that has been used recently in a number of new machine tool designs. A Stewart platform machine tool typically consists of a moveable spindle platform connected to a rigid base through six identically jointed and extensible struts. The Stewart platform mechanism is characterized by high force capacity,high structural rigidity, and low moving mass [2]. For machining applications, disadvantages of the Stewart platform include a complex work volume, limited orientation range of motion, and a requirement of six actuators for a five degree-of-freedom task (milling, drilling, and similar operations). Several prototypes of Stewart-platform-based machine tools, commonly called hexapods, have been produced in the U.S. by companies such as Giddings & Lewis1, Hexel Corporation, and the Ingersoll Milling Machine Company.
These machine tools promise some key advantages over conventional machine tools, but they raise important questions as well. How should the performance of these machines be characterized? What is the best way to describe the work volume? What degree of accuracy is achievable? What are the best control algorithms to employ? Can they be cost-competitive? What types of machining applications are best suited to these machines? How can part programs be optimized for them? What configurations provide the most versatility for the least cost? These are just a few of the important issues raised by hexapod machine tools. Since the early 1990’s, NIST has conducted research in a number of these areas. This paper summarizes the past, current, and future work at NIST in parallel kinematic machines, and examines some of the remaining challenges. In this paper, the discussion is focused on the six degree-offreedom Stewart platform mechanism, reflecting the configuration of the prototype machine at NIST. However, there are certainly other parallel kinematic arrangements that may be well suited for use as machine tools (perhaps even more so than the Stewart platform).
Since the development of industrial robots, using tandem with the operation has been dominated by its simple structure, work space, which was widely used. By comparison, parallel robot small space activities, activities on the platform are far less flexible series robot hand. However, the parallel robot has its unique advantages:
Parallel structure of its terminal on the platform at the same time through the six-support, compared with the series of cantilever, stiffness, and structural stability; because of stiffness, a series of parallel with the same dignity or volume, there is high And more carrying capacity; Series at the end of the error is the accumulation of error in all joints and enlarge, thus error, precision low, there is no parallel as the accumulation of errors and to enlarge relations, small error and high precision; Series Robot The drive motor and transmission system mostly on the size of the campaign arm, to increase the system's inertia, the deterioration of the dynamic performance, and the parallel is very easy to put motor-seat, reducing the exercise load; location for , The series of institutions is easy, but the anti-solution is very difficult, and parallel institutions are of the difficulties, the anti-solution is very easy. As the robot on-line real-time calculation is to calculate the anti-solution, which is very unfavorable Series, and the parallel is easy to achieve.
The emergence of parallel institutions, expanded the scope of application of robots. As the parallel robot in-depth research of its increasingly broad applications. The application of parallel robot can be divided into six major categories: Motion Simulator, parallel machine tools, industrial robots, moving bodies, and medical robot for operations.
Motion Simulator as a sports simulator, the most widely used is the flight simulator. Flight simulator training with energy, economy, all from the venue and weather conditions, short training period, the outstanding merits of high efficiency, all kinds of pilot training has become an indispensable tool. At the same time, this movement simulator is also research and develop an important tool for carrying equipment. Through the simulator can be found in the early problems, reduce risk, comprehensive verification system to solve all the dynamic matching system, speed up the process of experimental systems, shorten the development cycle and reduce development costs.
PMT for PMT parallel mechanism is the most attractive application. PMT structure is simple, short transmission chain, stiffness, light weight, low cost, easy to implement "six-axis", to processing more complex three-dimensional surface of the micro-hybrid. Adaptability also has the characteristics of the environment, facilitate restructuring and modular design, can constitute a variety of layout and composition of freedom. Germany Micromat company's 6 X parallel machine tools, such a telescopic machine-driven changes in the length of spindle components, to achieve the required processing track the movement of the workpiece fixed at the stage fixed.
With the modernization of industrial robots industrial development of high-speed process and increase the continuous improvement of industrial technology and technology advances in the application of industrial robots are more and more enterprises understand and accept. Industrial robots not only ensure the quality of products, and reduce the risk of the work of the special environment and realize the strength of the labor and personnel to reduce labor protection awareness raising.
Fretting institutions as a micro-robot body is an important parallel applications. Such fretting agencies played a parallel mechanism to the characteristics of the work space but not great precision and resolution are very high.
Robot has become a medical robot Institute of Medicine and Surgery Robot Institute of common concern emerging technology. In recent years, the medical robot technology from the United States, France, Germany, Italy, Japan and other countries of great concern to the academic, research have mushroomed. Medical robot with the election of accurate, precise movements, to avoid the characteristics of the patient.
Operators as the operation, parallel robot can be used for spacecraft and space docking with the docking mechanism, from top to bottom among all platforms through holes as the channel after docking, and docking ring as a platform from top to bottom, from six straight drive to help drive the spacecraft Is; can complete the docking mechanism of energy absorption and vibration, and take the initiative to crawl, the tension is, flexible integration, the last card locked tight, and so on. The difficulties of underground works, such as the Turkish side mining, coal mining, can also use this powerful parallel institutions. Arai, such as in 1991 proposed a parallel mechanism installed in or walk-tracked to a small mobile vehicles, in parallel with the first excavation of the bodies on the platform, a powerful parallel institutions can withstand the excavation of huge.
Other applications In addition, Stewart also other aspects of application platforms, such as force or displacement sensors, shipbuilding cranes, active vibration control, the sense of mechanical simulation, and other entertainment.
Parallel series robot has many traditional robots do not have the advantage of parallel robot there are many theoretical issues need further study and improve, apply to different job requirements of the new parallel mechanism to be further developed. At present, the Institute for parallel robot to solve the problem should include the following:
Different degrees of freedom parallel mechanism of the new study. New Study on the parallel institutions, and to study the corresponding kinematics, dynamics, and other theories, will be further enriched the field of parallel institutions research results and further expand the application of parallel institutions.
Parallel kinematics robot is the study of the numerical algorithm. Location is the main solution is to increase the computing speed, which is parallel robot trajectory planning base. Parallel robot dynamics model. The establishment of a common control system applicable to the design of the parallel robot dynamics model, the development of this work is to have excellent dynamic performance of parallel institutions, the different types of parallel institutions based on the analysis of the dynamics.
And on-line robot work space research. The study of the various singularity of the impact of the work space, we can improve on the parallel mechanism of awareness campaigns, a parallel mechanism is not singular path planning and realization of the controlled movement of the foundation.
Parallel Robot error analysis. The establishment of practical, complete parallel mechanism error mathematical model of parallel institutions enter error factor on moving platforms in the error-effects, thereby through control of sensitive input error factor, increased parallel robot accuracy.
Parallel institutions less freedom because of fewer degrees of freedom parallel mechanism is simple in structure, low cost, and other characteristics, have broad application prospects. But less freedom parallel mechanism at certain times of movement, dynamic analysis it has become more complex.
2 History of PKM Research at the NIST Manufacturing Engineering Laboratory
Interest within the NIST Manufacturing Engineering Laboratory (MEL) in innovative applications of Stewart platform mechanisms dates back to the mid 1980’s, when James Albus and his colleagues in the Robot Systems (now Intelligent Systems) Division developed a concept for a robot crane (Figure 1). The machine, now called the RoboCrane [3,4], is a Stewart platform configuration where six winch-actuated cables are used to suspend and control the platform. The RoboCrane was developed originally under a Defense Advanced Research Project Agency (DARPA) contract to stabilize loads on conventional cranes. Many different RoboCrane configurations have been developed for applications ranging from large-scale manufacturing and construction, to stabilized cargo handling, helicopter rescue, and hazardous waste remediation. The principal advantage of the RoboCrane over conventional crane systems is improved control of the position and orientation of the load.
NIST research into Stewart platforms also produced an innovative structure from which to
suspend the RoboCrane work platform. An octahedral structure containing the three upper support points necessary to suspend the work platform provides exceptional structural stiffness in a lightweight frame. This combination of a Stewart platform and an octahedral space frame was also recognized by NIST researchers James Albus and Clayton Teague to have potential advantages for machine tool applications. In 1991 they proposed a “New Class of Machine Tools” based on this concept, and built the model shown in Figure 2. The model configuration shown actually can be considered to be three Stewart platforms in series (lower motion platform, “virtual” Stewart platform for laser metrology, and upper fixed platform) nested within a fourth fixed Stewart platform that forms the structural frame. The benefits expected to result from this configuration included greatly improved stiffness and accuracy haracteristics compared with conventional designs.
During the course of pursuing a start-up project to build a prototype of this new machine tool design, NIST researchers became aware that a commercial machine tool builder was in the process of building a prototype machine with a very similar configuration. NIST decided that a commercial prototype would provide a good starting point for investigations, and procured an experimental prototype Octahedral Hexapod machine tool from the Ingersoll
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