探測機器人系統(tǒng)的設(shè)計

探測機器人系統(tǒng)的設(shè)計,探測,機器人,系統(tǒng),設(shè)計 探測機器人系統(tǒng)的設(shè)計 開題報告 班級（學號）：機0405-18 姓名：王亮 指導老師： 王會香 一、綜述 1.課題研究的目的和意義 出于重要的戰(zhàn)略意義，資源領(lǐng)域已成為各科技強國相互競爭的一個焦點，出于安全性等因素的考慮，對探測機器人的研究設(shè)計也成為了開發(fā)資源的重要硬件之一，探測機器人可以幫助人類完成一些不能完成的任務(wù)，其應(yīng)用范圍很廣，有幾下幾方面： 1）行星探測移動機器人 行星探測移動機器人的研究對于發(fā)展行星科學、提高國防能力、提高國家的國際地位等方面均有重要意義，因為:①移動機器人是行星科學研究中著陸探測和取回樣品到實驗室分析的有力工具。②人類在太空中停留數(shù)月之久會嚴重丟失鈣和磷，這似乎意味著人類不可能在重力為零的狀態(tài)下飛行6一9個月或更長一點時間。但機器人不存在這個問題。因此，行星探測移動機器人的研究是對行星進行長期實地考察的需要。③大大節(jié)省探測成本。以月球探測為例，根據(jù)粗略的估計，一次有人駕駛的飛行所花費的錢要比無人駕駛飛行多50一100倍。因此，光就科學上的探索來說，用機器人執(zhí)行無人駕駛飛行任務(wù)是合算的。④有利于提高國家國防自動化的水平和國際地位。因此，行星探測移動機器人的研究受到世界各國的高度重視。[6] 2）海洋探測機器人 海洋探測機器人人已經(jīng)廣泛應(yīng)用于海洋開發(fā)的許多領(lǐng)域，隨著海洋開發(fā)的不斷深入，續(xù)航力大、探測范圍廣、能執(zhí)行多種復(fù)雜任務(wù)的大型機器人需求也越來越大。主要用于海洋石油開發(fā)、海底管道光纜巡查檢修以及其他各種復(fù)雜任務(wù)。為了使機器人能更好的完成指定任務(wù)，水下機器人的運動性能預(yù)報就成為了一個重要的研究課題。[1] 3）油井故障探惻機器人 探測儀器的送進是油田上測井、修井等井下作業(yè)中的一項重要技術(shù)。[3] 4) 履帶式井下探測機器人 中國作為世界產(chǎn)煤大國，也是世界煤礦事故高發(fā)國家，需要非常重視煤礦生產(chǎn)的安全。這種探測機器人可在災(zāi)害發(fā)生前對隱患進行準確及時的檢測與預(yù)防，災(zāi)后進行施救等重要的危險任務(wù)。[5] 關(guān)于探測機器人應(yīng)用范圍比我們想象的要廣泛的多，在軍事方面，已經(jīng)研究出了反坦克雷探測機器人；還有醫(yī)學探測機器人等。 2006年，中國政府制定的《國民經(jīng)濟和社會發(fā)展第十一個五年規(guī)劃綱要》和《國家中長期科學和技術(shù)發(fā)展規(guī)劃綱要（2006－2020年）》，將發(fā)展航天事業(yè)置于重要地位。根據(jù)上述兩個規(guī)劃綱要，中國政府制定了新的航天事業(yè)發(fā)展規(guī)劃，明確了未來五年及稍長一段時期的發(fā)展目標和主要任務(wù)。按照這一發(fā)展規(guī)劃，國家將啟動并繼續(xù)實施載人航天、月球探測、高分辨率對地觀測系統(tǒng)、新一代運載火箭等重大航天科技工程，以及一批重點領(lǐng)域的優(yōu)先項目，加強基礎(chǔ)研究，超前部署和發(fā)展航天領(lǐng)域的若干前沿技術(shù)，加快航天科技的進步和創(chuàng)新。要發(fā)展航空事業(yè)，對月球進行探索，那么研究設(shè)計探測機器人是必不可少的過程。 2.課題的研究現(xiàn)狀及發(fā)展趨勢 1）在行星探測機器人的研制方面，美國和俄羅斯處于世界領(lǐng)先地位。從20世紀60年代開始，美、蘇向月球以及金、火、水、木、土等星球發(fā)射了許多探測器。格林威治時間1997年7月4日17時07分，美國國家航空航天局困ASA)發(fā)射的火星探路者號宇宙飛船成功地在火星表面著陸。探路者登陸器上帶有各種儀器及“索杰納”火星車團。這是上世紀自動化技術(shù)最高成就之一。[6] 日本對機器人的設(shè)計也處于領(lǐng)先地位。日本京都大學科研人員已經(jīng)開發(fā)出一種新型機器人,能在強烈地震發(fā)生后到廢墟中探測被埋人員。還專門進行了實用演示。這種機器人外表象是一條粗大的節(jié)足昆蟲,長1·43 m,由7節(jié)組成,有人的小腿一般粗細,每節(jié)周身都纏滿縱向履帶。它可以在遙控下從瓦礫的夾縫中蜿蜒穿行,裝在頭部的攝像機鏡頭會隨時傳輸觀察到的影像和搜集到的聲音,從而供控制者判斷里面是否有需要救助的存活人員。 未來的空間探測任務(wù)要求機器人系統(tǒng)能夠在預(yù)先未知或非結(jié)構(gòu)化的環(huán)境中執(zhí)行變化的任務(wù)，機器人移動平臺應(yīng)具備良好的幾何通過性、越障性、抗傾覆性、行駛平順性、牽引控制特性和能耗特性。基于不同的原理和性能側(cè)重點，國內(nèi)外提出并試驗了多種類型的空間探測機器人移動機構(gòu)。 2）探測機器人移動系統(tǒng)的發(fā)展趨勢如下: (1)輪腿式，履腿式等復(fù)合型結(jié)構(gòu)的移動機器人是一個研制方向. (2)由于航天器技術(shù)、尺寸、質(zhì)量和費用的限制,微小型行星探測機器人是目前發(fā)展的主流. (3)由于通信時延和微重力作用的緣故,中低速移動機器人是研制的主流. (4)機械結(jié)構(gòu)設(shè)計與控制方案相結(jié)合是研制靈活可靠的行星探測機器人的設(shè)計方向.[6] 3）設(shè)計探測機器人所面臨的問題 盡管國內(nèi)外已經(jīng)研制出了輪式、腿式、輪腿式、履帶式和其它特殊形式的移動機器人,但到目前為止,無論國內(nèi)還是國外,同時具備以下性能的移動機器人還沒有出現(xiàn):(1)能跨越大于輪子直徑的壕溝和高于輪子半徑的臺階;(2機器人陷入軟土壤中時,能自動脫離軟土壤區(qū),恢復(fù)正常的行駛能力;(3)整機的可密封性和可壓縮性良好;(4)克服傾翻對機器人行駛能力的不良影響;(5)行駛的高速高效性;(6)容積可進行擴充，而這些又是行星探測等領(lǐng)域移動機器人運動系統(tǒng)所應(yīng)具備的重要性能,因此,研制出新型的、綜合性能更好的行星探測機器人是行星探測機器人移動系統(tǒng)研究中有待解決的問題之一[6]. 二、研究內(nèi)容 本文以研制履帶便攜式抗摔機器人為目標，采用模塊化設(shè)計，以便根據(jù)要求選擇和定制配置，并在需要的時候方便更換和添加其他模塊，具有良好的機動性，在越障、跨溝、攀爬方面具有明顯優(yōu)勢。該機器人的最大優(yōu)點是具有良好的越障性能、環(huán)境適應(yīng)性能、防摔抗沖擊性能并具備全地形通過能力。 其研究內(nèi)容具體如下： 1、 研究探測機器人系統(tǒng)的設(shè)計原則。 依據(jù)運動學原理，對機器人進行性能指標分析，動態(tài)分析，使機器人能夠自適應(yīng)路面，即具有抗傾覆性、爬坡性能、越障性能、跨溝性能等功能。 2、 確定探測機器人的移動方式，并對整個探測機器人的整體進行規(guī)劃設(shè)計。 1）移動方式的確定 2）總體結(jié)構(gòu)設(shè)計 3）傳動系統(tǒng)設(shè)計 3、對探測機器人系統(tǒng)的硬件設(shè)計，繪制機械圖。 4、給出移動控制系統(tǒng)的設(shè)計方案。 1）選擇傳感器 2）控制系統(tǒng) 3）驅(qū)動器的選擇 目標：掌握探測機器人系統(tǒng)的設(shè)計原則，從實際應(yīng)用環(huán)境出發(fā)確定機器人的移動方式，選用合理的目標監(jiān)測手段，來實現(xiàn)探測目的。 三、實現(xiàn)方法及其預(yù)期目標 1、總體結(jié)構(gòu)設(shè)計 本設(shè)計的探測機器人由四個模塊構(gòu)成，即底盤運動模塊、電源及驅(qū)動模塊、傳感器模塊、控制計算機模塊。大體結(jié)構(gòu)如圖1-1 控制計算機模塊 傳感器模塊 電源驅(qū)動模塊 底盤運動模塊 圖1-1機器人的總體結(jié)構(gòu) 2、 移動機構(gòu)分析 便攜式機器人按移動方式分主要有輪式、履帶式、腿足式三種，另外還有步進移動式、 混合移動式、蛇行移動式等，各種移動方式的機動性能對比如表1-1 表1-1 車輪式、輪、履、腿式移動機構(gòu)性能比較： 移動機構(gòu)方式 輪式 履帶式 腿式 移動速度 快 較快 慢 越障能力 差 一般 好 機構(gòu)復(fù)雜程度 簡單 一般 復(fù)雜 能耗量 小 較小 大 機構(gòu)控制難易程度 易 一般 復(fù)雜 很明顯，履帶式移動機構(gòu)的性能居于輪式和腿式移動機構(gòu)之間，在地面適應(yīng)性能、越障性能方面有良好表現(xiàn)。履帶移動機構(gòu)地面適應(yīng)性能好，在復(fù)雜的野外環(huán)境中能通過各種崎嶇路面以及溝壑等，它的活動范圍廣，性能可靠，使用壽命長，輪式移動機構(gòu)無法與其比擬，適合作為機器人的推進系統(tǒng)。 運動原理： 3、 移動控制系統(tǒng)的設(shè)計 一個在實際工作中的機器人，他的運動由驅(qū)動器系統(tǒng)實現(xiàn)，任務(wù)的具體執(zhí)行有終端機具完成。在執(zhí)行任務(wù)的過程中，感覺系統(tǒng)將內(nèi)感受和外感受的信息反饋給控制系統(tǒng)，有控制系統(tǒng)對整個機器人的活動作決策和付諸實施。系統(tǒng)的控制部分的工作方式要適合于執(zhí)行的任務(wù)。采用某一種工作方式，例如，人進行的干預(yù)很少---自由方式；人的干預(yù)很多---手動方式；斷斷續(xù)續(xù)干預(yù)---監(jiān)督管理方式等，都要按任務(wù)的需求而定。 在控制機器人執(zhí)行任務(wù)過程中涉及三方面： 信息---機器人自身及環(huán)境的信息。他們來自感覺系統(tǒng)即感知的機器人自身狀態(tài)。 決定---產(chǎn)生執(zhí)行任務(wù)的行動方式，任務(wù)的程序設(shè)計。 行動---控制信號的產(chǎn)生和實施。 如圖1-2 所示 人 決定 行動（控制） 信息（感覺） 環(huán)境 操作部分 控制部分 圖1-2 控制系統(tǒng)結(jié)構(gòu)圖 依據(jù)上述要求所需，考慮一下幾個方面： 1） 選擇傳感器 首先，機器人的傳感器就像人體上的感覺器官，它可以探測到周圍的環(huán)境，然后經(jīng)過綜合性的測量、計算，對他的行為進行選擇判斷。其中，視覺是最重要的，因為，即使只有視覺的時候，也能根據(jù)看到的進行前行或是改變行為方式，但如果失去了視覺，即使其他的感覺功能都存在，也很難判斷周圍的環(huán)境，很難對行為方式進行判斷。所以首先應(yīng)選擇視覺傳感器。 其次，視覺傳感器也有他的局限性，實現(xiàn)視覺的功能是需要光的，如果在晚間執(zhí)行任務(wù)時就很難實現(xiàn)其功能，為更好的使機器人完成認為，再加一個超聲波傳感器。 2） 控制系統(tǒng) 控制系統(tǒng)是指機器人的信息處理裝置，在本設(shè)計項目中，選擇數(shù)字電子計算機。這是一個依據(jù)事先和事后的信息而產(chǎn)生對機器人的控制命令的系統(tǒng)。信息主要來自人---機對話和感覺系統(tǒng)。機器人執(zhí)行任務(wù)前。在計算機重要貯存好一個運動模型，一個環(huán)境模型，一些與執(zhí)行任務(wù)有關(guān)的數(shù)據(jù)及一定數(shù)量的執(zhí)行任務(wù)的策略和算法，在執(zhí)行任務(wù)過程中，計算機接受來自傳感器的機器人目前狀態(tài)的信息和涉及到目前包括工作對象在內(nèi)的環(huán)境狀況的信息。依據(jù)上述的所有數(shù)字模型，原始數(shù)據(jù)，感受的信息，利用控制策略和算法，以及過去執(zhí)行任務(wù)的經(jīng)驗等，計算機產(chǎn)生一個對機器人的控制命令。 3）驅(qū)動裝置的選擇 按照能源的不同 ，可分為液動，氣動，電動三大類。電驅(qū)動器由于電能易于獲取，容易傳輸，沒有污垢，易于維修等優(yōu)點而被廣泛采用。在機器人使用的電驅(qū)動器中，步進電機與數(shù)字電子計算機的結(jié)合上，表現(xiàn)出很好的發(fā)展前景。所以驅(qū)動裝置選擇步進電機。 4軟件組成　 　 硬件只是計算機控制系統(tǒng)的軀體；而軟件則是計算機控制系統(tǒng)的大腦和靈魂，是人的思維與系統(tǒng)硬件之間的橋梁。軟件的優(yōu)劣關(guān)系到計算機控制系統(tǒng)正常運行、硬件功能的發(fā)揮以及控制性能的優(yōu)劣等。用以具體實施有關(guān)功能的算法是以軟件包的形式貯存在機器人的計算機系統(tǒng)中。為了控制機器人執(zhí)行任務(wù)，在操作員與機器人之間進行信息交換是必須的，這樣做旨在使機器人按操作員的意圖進行工作，完成任務(wù)。本設(shè)計采用VC語言. 四、對進度的具體安排 1. 第1-3周 實習調(diào)研基本結(jié)束； 2. 第4周 撰寫并提交調(diào)研報告和開題報告； 3. 第5-6周 制定探測移動機器人的設(shè)計原則； 4．第7周 確立機器人的移動方式； 5. 第8-12周 設(shè)計機器人系統(tǒng)的機械結(jié)構(gòu)； 6. 第13-14周 設(shè)計控制方案及監(jiān)測手段； 7．第15-16周 撰寫并提交畢業(yè)論文；審閱、評審并修改畢業(yè)論文； 8．第17周 完成畢業(yè)答辯 五、參考文獻 1、劉曉峰：海洋探測機器人操縱性及仿真研究 哈爾濱工業(yè)大學 2007.1 2、尚建忠 羅自榮 張新訪 范大鵬：基于構(gòu)型組合的空間探測機器人移動機構(gòu)設(shè)計* 3、邵守君：基于虛擬樣機的石抽井故障探惻機器人研究 2007.3 4、賀鑫元 馬書根 李斌 王越超：可重構(gòu)星球探測機器人的機構(gòu)設(shè)計 機械工程報2005、12 5、柴匯：履帶式井下探測機器人底層控制系統(tǒng)研究與設(shè)計 2007、5 6、劉方湖 陳建平 馬培蓀 曹志奎：行星探測機器人的研究現(xiàn)狀和發(fā)展趨勢 2002、5 7、陳芳允：月球探測機器人移動系統(tǒng)的研究現(xiàn)狀和發(fā)展趨勢 井岡山學院學報 2006、8 8、吳芳美：機器人控制基礎(chǔ) 中國鐵道出版社 9、孫樹棟：工業(yè)機器人技術(shù)基礎(chǔ) 北京工業(yè)大學出版社 2006、12 10、為海底探測提供數(shù)據(jù)日本加強深海探測 督導老師： 年 月 日 領(lǐng)導小組審查意見： 年 月 日 審查人簽字： 年 月 日 Design of Lightweight Lead Screw Actuators for Wearable Robotic Applications Journal of Mechanical Design Kevin W. Hollander Thomas G. Sugar A wearable robot is a controlled and actuated device that is in direct contact with its user. As such, the implied requirements 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 typical lead screw has low pitch angles and large radii, thereby yielding low mechanical efficiencies and heavy weight. However, using the design procedure outlined in this text, both efficiency and weight are improved; thus yielding a lead screw system with performances that rival human muscle. The result of an example problem reveals a feasible lead screw design that has a power to weight ratio of 277 W/kg, approaching that of the dc motor driving it, at 312 W/kg, as well as a mechanical efficiency of 0.74, and a maximum strength to weight ratio of 11.3 kN/kg 。 1 Introduction 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 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 such a device is the performance/strength enhancement of the wearer. It can be used in training, in therapy, or simply as a device to assist in functional daily living. The implication of the term “wearable” isthat the robot must be portable, lightweight, and most importantly safe. In contrast, a factory floor robot is none of these things, so the simple adaptation of existing technology is not possible. The standard approach to wearable robot design suffers from three major limitations; 1 Low battery power density； 2 motors with low “strength to weight” ratios； 3 weight and safety of a mechanical transmission system. 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 limitations described in item number 3, i.e., the weight and safety of the mechanical transmission system. 2 Background Interest in the area of wearable robotics has grown over the 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 controlled strength enhancing device is closer to reality However, aside from the availability of portable computation platforms, issues of the physical mechanism must still be addressed. The main issues 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 transferred and still maintain safety? The safe interaction between the wearer and theactuated robot has to be the primary concern in a wearable robot design. The purpose of a wearable robotic system is to offset the effort 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 efficiencies 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 has to augment the operator’s abilities, but must also compensate for its own additional weight. 2.1 Actuator Comparisons. Human skeletal muscle is the “gold” standard by which many robotic actuators are compared. Known for their good “power to weight” ratios and excellent 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 measures. Unfortunately, common throughout biological literature is a wide variation of measured muscle properties. Although reported values have a wide variance, these values can still give a sense of scale in which biological materials behave. Data tabulated and estimated from several sources were used to describe the attributes of human muscle performance, and the result of which can be seen in Table 1. Table1：Actuator comparison: Compares various actuator types by mechanical efficiency, power to weight ratio, “corrected”power to weight ratio, and strength to weight ratio Measures allows the direct comparisons to be made based upon utilization of available energy. 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 directly 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 fraction of that power would be yielded as output. Therefore, to appropriately compare the above described actuators, their corrected power to weight（c） ratios must be computed (1) 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. 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 combination 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 weight ratio, due to its ability to carry higher loads and its nut is of lower weight. For the McKibben style air muscles, a variety of literature was found describing its relevant measures. Immediately evident in this comparison is that the corrected power to weight, cP, values of the dc motor, the air muscle 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 significantly alter the weight of the dc motor based actuator, then performances very near that of human skeletal muscle could be achieved. 3 Lead Screw Design。 Seen above, the 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 standard lead screw system is approximately =0.36., metal on metal, better results are possible if lubrication is used. In contrast, the typical ball screw system has very good mechanical efficiency. 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 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 reduction of weight must be achieved. Journal of Mechanical Design Fig. 1 Lead screw geometry; as drawn, pitch ?p… and lead ?l… are equivalent in a single helix screw The basic mathematics 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 exploration 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. 3.1 Lead Screw Geometry. 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 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 the incline or slope of the screw thread. The base of the triangle is the circumference of the screw shaft, the right leg of the triangle is its lead, and the hypotenuse representsthe path length of the helical thread. 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, 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 derived (2) 3.2 Alpha Versus R. Considering, again, the geometry of a lead screw in Fig. 1, it can be shown that leadl, is described both by screw radiusr, and lead angle. The relationship between these variables is given in (3) （4） The meaning of Eq（4）is 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 systems 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°. From Eq（4）.it 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 radii, a larger value of lead angle , must be considered. Fig. 2 Mechanical efficiency of lead screw systems: Shaded part of the graph is the typical design region for the majority of lead screws. 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. 3.3 Efficiency Versus Alpha. 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 decrease 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 that the torque required to lift a load, Fw, is dependent upon both lead angle, as well as the coefficient of friction。 Relating the efficiency of a screw to both lead angle and coefficient of friction, Figure 2 shows the impact on both coefficient of friction, and lead angle, on the efficiency of a lead screw system （5） Each line in Fig. 2 is based upon a different 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 screw system. This figure shows that as the lead angle increases, so does the mechanical efficiency; or at least until a peak value is reached. Ideally, it would be advantageous to pick the angle, based 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 derivative of efficiency with respect to angle, the result of which can be seen in (6) Although a high lead angle can lead to a high efficiency, it can also lead to a 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-driveable 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 (7) Lead angle and coefficient 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. 4 Practical Considerations 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 practical 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 strength demands of the physical system. Issues, such as axial yielding,compression buckling, and mechanism bind, need to be considered as well. Consider that a single ultrathin screw may be lightweight, although it may not be strong enough to carry the load required by the system. A single or several screws can be used, but must be sized large enough to handle the load placed upon it. As a note,there is no weight advantage to using several small screws to carry a large load, as the computation for both weight and stress are driven by a cross-sectional area of the screw. However, using several small screws to carry the load can allow the continued use of high lead angles and thus operate with high efficiencies, even in the presence of high loads. By pushing the limits of raw material properties of the lead screw, high axial loading can be achieved. This approach works better for a tensional system than it does for a compression bearing system. When considering the compressive loading of a long slender screw, Euler buckling must be addressed . Similar to that of the McKibben actuators or even human muscles, a lead screw actuator could be designed to bear a tensional load only, thus eliminating the consideration of buckling altogether. Creating a tension-only actuation system in a wearable robot does not necessarily mean that an antagonistic pair is required. In fact, for an assistance robot, a disabled person may only have muscle weakness in a single actuated direction and, therefore, a single tensional actuator would be all that is required to aid that person.。 For those designers who would push the limits of the screw radius and thus 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 evaluating Eq.（2）. An evaluation of the denominator in Eq.（2）. yields the following relation。 (8) In addition to the practical considerations listed here, there 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 of this exercise is to demonstrate an alternative to the typical approaches of designing or selecting screw systems. The benefits of this alternative approach are directly applicable to the design issues of a wearable robotic system. 5 Example Problem To demonstrate a crude design exercise, consider the peak ankle joint torque during gait of an able-bodied or normal individual 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 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 initiation of swing begins. The conclusion of the swing phase of gait places the foot back into a heel strike position again and then the next gait cycle can begin. As an example, let us consider building a lead screw actuator for ankle gait assistance. For our problem, let us assume the level Table 2： Example problem actuator comparison: Compares lead screw designs I and II to human muscle in terms of mechanical efficiency, power to weight ratio, corrected power to weight ratio and strength to weight ratio, measures of assistance to be at 30% and that the actuator acts with a 12 cm moment arm 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 for this example solution has been determined; the range of possible screw leads are Example Problem Results. Two lead screw designs were generated to solve this problem. The first design, lead screw I, is a design solved for maximum efficiency. Assuming a lead of 2 mm and a =0.05, yields an efficiency of 0.9 for the screw at =43.5° and a radius of 0.34 mm. With such a small radius, multiple screws are needed to hold the load. Even so, estimates for the actuator power to weight are 280 W/kg. Power to weight has been determined by dividing the peak power required in our example by the weight of the motor and estimated transmission system. From our previous work, the weight of the accessory components was scaled proportionally to the reduced weight of the screw and nut. The second design, lead screw II, uses dimensions available from a commercial vendor. The screw is estimated to have an =13.6° and an efficiency of 0.82. Even with these larger dimensions, the actuator’s power to weight ratio of 277 W/kg =0.74 is expected. The results of this example problem have been tabulated for the purpose of comparison. Table 2 shows the numerical results of both example lead screw designs. These values are compared to the previous values tabulated for a dc motor alone, and the estimated values for human skeletal muscle. The strength to weight properties calculated for these examples is based upon the peak force required by our example. 6 Discussion In the analysis of the maximum efficiency solution, lead screw design I, it was shown that a single small radii screw will not always handle the loads required of it. However, a bundle of screws operating in parallel can perform that task with the same high efficiency. Although a 0.34 mm radius screw would not be easily manufactured using typical techniques, it is possible that this kind of approach i.e., use multiple screws to maintain high efficiency could be useful for a MEMs scaled device. One could imagine a compact “force pack” built up from many high efficiency small diameter screws. Without going to the extremes in efficiency for a particular screw design, it was shown that for lead screw design II, a feasible solution exists for our example problem of ankle gait. Corrected power to weight values were obtained that are very close to those discussed for human muscle. Using a similar approach, a ball screw mechanism could benefit in performance, as well. The general approach to creating back-driveable, low weight, and high efficiency screw system can make a dc motor-based actuator a competitive solution for wearable robot applications. Fig. 3 Prototype actuator, high efficiency lead screw Mentioned earlier, a wearable robot actuator must not only have good p