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畢業(yè)設(shè)計(論文)譯文題目 平面機構(gòu)綜合分析 Chapter 3 Graphical Linkage Synthesis3.0 INTRODUCTIONMost engineering design practice involves a combination of synthesis and analysis. Most engineering courses deal primarily with analysis techniques for various situations. However, one cannot analyze anything until it has been synthesized into existence. Many machine design problems require the creation of a device with particular motion characteristics. Perhaps you need to move a tool from position A to position B in a particular time interval. Perhaps you need to trace out a particular path in space to insert a part into an assembly. The possibilities are endless, but a common denominator is often the need for a linkage to generate the desired motions. So, we will now explore some simple synthesis techniques to enable you to create potential linkage design solutions for some typical kinematic applications.3.1 SYNTHESISQUALITATIVE SYNTHESIS means the creation of potential solutions in the absence of a well-defined algorithm which configures or predicts the solution. Since most real design problems will have many more unknown variables than you will have equations to describe the systems behavior, you cannot simply solve the equations to get a solution. Nevertheless you must work in this fuzzy context to create a potential solution and to also judge its quality. You can then analyze the proposed solution to determine its viability, and iterate between synthesis and analysis, as outlined in the design process, until you are satisfied with the result. Several tools and techniques exist to assist you in this process. The traditional tool is the drafting board, on which you layout, to scale, multiple orthographic views of the design, and investigate its motions by drawing arcs, showing multiple positions, and using transparent, movable overlays. Computer-aided drafting (CAD) systems can speed this process to some degree, but you will probably find that the quickest way to get a sense of the quality of your linkage design is to model it, to scale, in cardboard or drafting Mylar? and see the motions directly. Other tools are available in the form of computer programs such as FOURBAR,FIVEBAR, SIXBAR,SLIDER,DYNACAM,ENGINE, and MATRIX(all included with this text), some of which do synthesis, but these are mainly analysis tools. They can analyze a trial mechanism solution so rapidly that their dynamic graphical output gives almost instantaneous visual feedback on the quality of the design. Commercially available programs such as Working Model* also allow rapid analysis of a proposed mechanical design. The process then becomes one of qualitative design by successive analysis which is really an iteration between synthesis and analysis. Very many trial solutions can be examined in a short time using these Computer-aided engineering (CAE) tools. We will develop the mathematical solutions used in these programs in subsequent chapters in order to provide the proper foundation for understanding their operation. But, if you want to try these programs to reinforce some of the concepts in these early chapters, you may do so. Appendix A is a manual for the use of these programs, and it can be read at any time. Reference will be made to program features which are germane to topics in each chapter, as they are introduced. Data files for input to these computer programs are also provided on disk for example problems and figures in these chapters. The data file names are noted near the figure or example. The student is encouraged to input these sample files to the programs in order to observe more dynamic examples than the printed page can provide. These examples can be run by merely accepting the defaults provided for all inputs. TYPE SYNTHESIS refers to the definition of the proper type of mechanism best suited to the problem and is a form of qualitative synthesis.t This is perhaps the most difficult task for the student as it requires some experience and knowledge of the various types of mechanisms which exist and which also may be feasible from a performance and manufacturing standpoint. As an example, assume that the task is to design a device to track the straight-line motion of a part on a conveyor belt and spray it with a chemical coating as it passes by. This has to be done at high, constant speed, with good accuracy and repeatability, and it must be reliable. Moreover, the solution must be inexpensive. Unless you have had the opportunity to see a wide variety of mechanical equipment, you might not be aware that this task could conceivably be accomplished by any of the following devices:- A straight-line linkage- A carn and follower- An air cylinder- A hydraulic cylinder- A robot- A solenoidEach of these solutions, while possible, may not be optimal or even practical. More detail needs to be known about the problem to make that judgment, and that detail will come from the research phase of the design process. The straight-line linkage may prove to be too large and to have undesirable accelerations; the cam and follower will be expensive, though accurate and repeatable. The air cylinder itself is inexpensive but is noisy and unreliable. The hydraulic cylinder is more expensive, as is the robot. The so- lenoid, while cheap, has high impact loads and high impact velocity. So, you can see that the choice of device type can have a large effect on the quality of the design. A poor choice at the type synthesis stage can create insoluble problems later on. The design might have to be scrapped after completion, at great expense. Design is essentially an exercise in trade-offs. Each proposed type of solution in this example has good and bad points. Seldom will there be a clear-cut, obvious solution to a real engineering design problem. It will be your job as a design engineer to balance these conflicting features and find a solution which gives the best trade-off of functionality against cost, reliability, and all other factors of interest. Remember, an engineer can do, with one dollar, what any fool can do for ten dollars. Cost is always an important constraint in engineering design.QUANTITATIVESYNTHESIS,OR ANALYTICALSYNTHESIS means the generation of one or more solutions of a particular type which you know to be suitable to the problem, and more importantly, one for which there is a synthesis algorithm defined. As the name suggests, this type of solution can be quantified, as some set of equations exists which will give a numerical answer. Whether that answer is a good or suitable one is still a matter for the judgment of the designer and requires analysis and iteration to optimize the design. Often the available equations are fewer than the number of potential variables, in which case you must assume some reasonable values for enough unknowns to reduce the remaining set to the number of available equations. Thus some qualitative judgment enters into the synthesis in this case as well. Except for very simple cases, a CAE tool is needed to do quantitative synthesis. One example of such a tool is the program LlNCAGES,* by A. Erdman et aI., of the University of Minnesota 1 which solves the three-position and four-position linkage synthesis problems. The computer programs provided with this text also allow you to do three-position analytical synthesis and general linkage design by successive analysis. The fast computation of these programs allows one to analyze the performance of many trial mechanism designs in a short time and promotes rapid iteration to a better solution. DIMENSIONALSYNTHESIS of a linkage is the determination of the proportions (lengths) of the links necessary to accomplish the desired motions and can be a form of quantitative synthesis if an algorithm is defined for the particular problem, but can also be a form of qualitative synthesis if there are more variables than equations. The latter situation is more common for linkages. (Dimensional synthesis of cams is quantitative.) Dimensional synthesis assumes that, through type synthesis, you have already determined that a linkage (or a cam) is the most appropriate solution to the problem. This chapter discusses graphical dimensional synthesis of linkages in detail. Chapter 5 presents methods of analytical linkage synthesis, and Chapter 8 presents cam synthesis.3.2 FUNCTION, PATH, AND MOTION GENERATIONFUNCTION GENERATION is defined as the correlation of an input motion with an output motion in a mechanism. A function generator is conceptually a black box which delivers some predictable output in response to a known input. Historically, before the advent of electronic computers, mechanical function generators found wide application in artillery rangefinders and shipboard gun aiming systems, and many other tasks. They are, in fact, mechanical analog computers. The development of inexpensive digital electronic microcomputers for control systems coupled with the availability of compact servo and stepper motors has reduced the demand for these mechanical function generator linkage devices. Many such applications can now be served more economically and efficiently with electromechanical devices. * Moreover, the computer-controlled electromechanical function generator is programmable, allowing rapid modification of the function generated as demands change. For this reason, while presenting some simple examples in this chapter and a general, analytical design method in Chapter 5, we will not emphasize mechanical linkage function generators in this text. Note however that the cam-follower system, discussed extensively in Chapter 8, is in fact a form of mechanical function generator, and it is typically capable of higher force and power levels per dollar than electromechanical systems.PATH GENERATION is defined as the control of a point in the plane such that it follows some prescribed path. This is typically accomplished with at least four bars, wherein a point on the coupler traces the desired path. Specific examples are presented in the section on coupler curves below. Note that no attempt is made in path generation to control the orientation of the link which contains the point of interest. However, it is common for the timing of the arrival of the point at particular locations along the path to be defined. This case is called path generation with prescribed timing and is analogous to function generation in that a particular output function is specified. Analytical path and function generation will be dealt with in Chapter 5.MOTION GENERATION is defined as the control of a line in the plane such that it assumes some prescribed set of sequential positions. Here orientation of the link containing the line is important. This is a more general problem than path generation, and in fact, path generation is a subset of motion generation. An example of a motion generation problem is the control of the bucket on a bulldozer. The bucket must assume a set of positions to dig, pick up, and dump the excavated earth. Conceptually, the motion of a line, painted on the side of the bucket, must be made to assume the desired positions. A linkage is the usual solution.PLANAR MECHANISMS VERSUS SPATIAL MECHANISMS The above discussion of controlled movement has assumed that the motions desired are planar (2-D). We live in a three-dimensional world, however, and our mechanisms must function in that world. Spatial mechanisms are 3-D devices. Their design and analysis is much more complex than that of planar mechanisms, which are 2-D devices. The study of spatial mechanisms is beyond the scope of this introductory text. Some references for further study are in the bibliography to this chapter. However, the study of planar mechanisms is not as practically limiting as it might first appear since many devices in three dimensions are constructed of multiple sets of 2-D devices coupled together. An example is any folding chair. It will have some sort of linkage in the left side plane which allows folding. There will be an identical linkage on the right side of the chair. These two XY planar linkages will be connected by some structure along the Z direction, which ties the two planar linkages into a 3-D assembly. Many real mechanisms are arranged in this way, as duplicate planar linkages, displaced in the Z direction in parallel planes and rigidly connected. When you open the hood of a car, take note of the hood hinge mechanism. It will be duplicated on each side of the car. The hood and the car body tie the two planar linkages together into a 3-D assembly. Look and you will see many other such examples of assemblies of planar linkages into 3-D configurations. So, the 2-D techniques of synthesis and analysis presented here will prove to be of practical value in designing in 3-D as well. 平面機構(gòu)綜合分析 機械原理Norton, Robert L.第三章 平面連桿機構(gòu)的理論分析3.0引言大多數(shù)工程設(shè)計實踐都會將綜合的理論推理與分析相結(jié)合。大多數(shù)的工程課程主要對各種案例進行技術(shù)分析。然而,只有將知識都綜合起來才能進行分析。解決許多機器的設(shè)計問題都需要創(chuàng)造一個具有特定的運動特性的設(shè)備。也許你需要在一個特定的時間不斷地將刀具從位置A移動到位置B。也許你需要在一個空間內(nèi)沿著一個特定的路徑將一個零件裝配到機體上??赡苄允菬o窮無盡的,但共同點往往是由某個機構(gòu)來完成所需的運動。所以,現(xiàn)在我們將探討一些簡單的綜合技術(shù),使你能夠做出的好的機構(gòu)的設(shè)計方案并在一些典型場合進行靈活應(yīng)用。3.1理論分析理論分析的本質(zhì)是在缺少明確的能定義或預(yù)測結(jié)果的運行規(guī)則的情況下創(chuàng)造潛在的解決方案。由于大多數(shù)實際設(shè)計問題的未知變量比描述系統(tǒng)行為的方程多,所以你不能簡單的通過求解方程來解決問題。不管怎樣你必須在這種模糊的環(huán)境中創(chuàng)造一個潛在的解決方案,并理解其本質(zhì)。然后你才能分析所提出的解決方案以確定其可行性,并在設(shè)計過程不斷地的推理和分析,直到你對你的結(jié)果滿意。在這個過程中有幾種工具和技術(shù)來幫助你。傳統(tǒng)的工具是繪圖板,你可以在上面布局,縮放,設(shè)計的多個正交視圖,并通過弧線的繪制來預(yù)測運動軌跡,顯示多個位置,還可以使用透視和消隱。計算機輔助繪圖(CAD)系統(tǒng)可以在一定程度上加快這一進程,但你可能會發(fā)現(xiàn)要了解你的機構(gòu)的設(shè)計質(zhì)量,最快的方法是在紙板或繪圖板上做模型和縮放以便于直接觀察運動情況。其他可以利用工具軟件,有:四,六,五桿機構(gòu),滑塊,凸輪機構(gòu),驅(qū)動引擎和矩陣法(這本書都有所提及),雖然其中一些是做綜合推理的,但這些也是主要的分析工具。他們可以非常迅速分析一個實驗機構(gòu),以至于設(shè)計質(zhì)量的動態(tài)圖形輸出幾乎與視覺反饋同步。你能買到一款叫做Working Model的軟件可以對所提出的機械設(shè)計進行快速分析。通過不斷地分析和推理,這就是定性設(shè)計的過程。通過使用這些計算機輔助工程(CAE)工具可以在短時間內(nèi)檢測很多試驗方案。我們在之后的章節(jié)中將用這些程序建立數(shù)學(xué)模型來為你們打好基礎(chǔ)以便理解它們是如何運作的。但是,如果你想通過使用這些軟件來加深對前面章節(jié)的一些概念的理解,你完全可以這樣做。附錄A是這些程序的使用手冊,你可以在任何時候去看。在每一章中,我們會根據(jù)主題來對這些軟件的相關(guān)功能做一些說明。對這幾章的一些典型問題和圖表,我們將在光盤中提供導(dǎo)入到這些計算機程序的數(shù)據(jù)文件。這些數(shù)據(jù)文件名都標注在圖或例子的附近。我們鼓勵學(xué)生導(dǎo)入這些示例文件到程序中來觀察比課文中更多的動態(tài)的實例。這些例子僅用初始輸入的數(shù)值就可以運行。典型理論分析是指對每個問題我們都會拿出最適合的典型機構(gòu)來加以分析和說明,這也是定性推理的一種形式。對某些學(xué)生來說,這也許是最困難的任務(wù),因為這需要一定的經(jīng)驗和關(guān)于機械的各種類型的知識,這些機器可能是現(xiàn)實存在的,也可能僅是性能和制造的理論上具體有可行性。舉個例子,假設(shè)有個任務(wù)是設(shè)計一個裝置,當(dāng)輸送帶上有做直線運動的零件經(jīng)過時,對其噴一種化學(xué)涂層。這必須在連續(xù)高速運轉(zhuǎn)的情況下具備良好的準確性和可重復(fù)性。此外,該解決方案必須是經(jīng)濟的。除非你有機會接觸到各種各樣的機械設(shè)備,要不然你不可能想象到這個任務(wù)可以由下列設(shè)備完成:- 直動連桿機構(gòu)- 凸輪機構(gòu)- 氣缸- 液壓缸- 中控臺- 繼電器這些方法,有可能不是最好的或最實用的。如何抉擇需要知道更多關(guān)于這個問題的細節(jié),而這些細節(jié)來自于設(shè)計過程的研究階段。直動連桿機構(gòu)可能過于龐大,并且加速性能也不理想。凸輪機構(gòu)可能過于昂貴,雖然其具有良好的準確性和可重復(fù)性。氣缸本身便宜但是有噪音并且不太可靠的。液壓缸比較貴的但有利于實現(xiàn)直動化。繼電器雖然便宜,但是受高負載和過高的速度影響。所以你看設(shè)備類型的選擇會對設(shè)計的質(zhì)量有很大的影響。在理論分析階段,一個錯誤的選擇會在后面造成一堆無法解決的問題。這個設(shè)計完成后可能會失敗并造成巨大的損失。設(shè)計的本質(zhì)就是練習(xí)取舍。在這個例子中,每種解決方案都有優(yōu)缺點。在實際工程設(shè)計中,很少會有一個明確直接的解決方案。作為一名設(shè)計師,你的工作就是平衡這些矛盾,在性能和成本,可靠性和其他可能的因素間做出最好的取舍并且找到合理的解決方案。記住,一個工程師花一美元要做一個傻瓜花十美元才能做得事。在工程設(shè)計中,成本永遠是一個重要的約束。數(shù)值的綜合推理和理論分析意味著,應(yīng)對典型問題你可以想出不止一個的解決方法,更重要的是,這里面有一套明確的推理規(guī)律。顧名思義,這種類型的解決方案是一定量的,就像一些方程組的的根就那么幾個。不管這個結(jié)果是好或僅僅只是合適,這都是一個設(shè)計師判斷的依據(jù)并且他需要實際分析和不斷地優(yōu)化設(shè)計。通??梢粤谐龅姆匠虝儆谖粗兞康臄?shù)目,在這種情況下你必須為這些未知量設(shè)一些合理的值來減少方程的數(shù)目。因此在做理論分析的時候就應(yīng)該做一些定性的判斷。除了非常簡單的情況下,還是有必要用CAE工具來做數(shù)值的理論分析。比如明尼蘇達大學(xué)厄爾德曼的工具軟件LINCAGES可以解決平面四桿機構(gòu)和平面五桿機構(gòu)的綜合分析問題。本書所提供的軟件還可以讓你做三位機構(gòu)理論分析,還有普通連桿的連續(xù)性分析。這些快速計算的程序可以讓你在很短的時間內(nèi)分析試驗機構(gòu)的設(shè)計的運行效果并且有利于迅速的找到更好的解決方案。對于那些需要完成目的動作的連桿機構(gòu),尺寸分析決定了其各部分的比例(長度),并且對于特定的問題,如果定義了運動軌跡或者變量多余方程數(shù),尺寸分析也是數(shù)值分析的一種形式。對于連桿機構(gòu)來說,后面的情況更為常見。(如凸輪的尺寸數(shù)值分析。)尺寸分析可以基本判定,通過典型的理論分析,你已經(jīng)可以確定一個連桿機構(gòu)(或一個凸輪機構(gòu))是最合適的解決方案對于某些問題來說。這一章詳細討論了平面連桿機構(gòu)的尺寸分析。第五章提供了一些理論分析的方法,第八章講了凸輪機構(gòu)的理論分析。3.2功能,軌跡,和運動的產(chǎn)生功能的產(chǎn)生被定義為在一個機構(gòu)中的輸出運動輸入運動之間的關(guān)系。功能的產(chǎn)生者在概念上是對已知輸入傳遞一些可預(yù)測的輸出的“黑盒子”歷史上,在電子計算機出現(xiàn)以前,機械功能產(chǎn)生器在火炮測距儀,艦載機關(guān)槍的瞄準系統(tǒng)等領(lǐng)域得到了廣泛的應(yīng)用。事實上他們是機械模擬計算機。由廉價的數(shù)字電子微型電腦組成的控制系統(tǒng)結(jié)合實用緊湊的伺服步進電機減少了對這些機械功能發(fā)生器連桿機構(gòu)的需求?,F(xiàn)在許多帶機電設(shè)備的應(yīng)用程序運行起來更經(jīng)濟且高效。并且電腦控制的機電功能發(fā)生器還是可編程的。因此,雖然在這一章我們會提供一些簡單的例子,并在第五章會有一些通用的分析性設(shè)計的方法,但我們不會在這本書中強調(diào)機械連桿機構(gòu)功能發(fā)生器。但是注意,凸輪機構(gòu),將在8章有大量的介紹,其實這就是一種機械功能發(fā)生器,它比機電系統(tǒng)有更高的載荷和功率水平。軌跡的產(chǎn)生被定義為在一個平面內(nèi),控制某個點按照規(guī)定的路徑運行。通常這至少需要四根桿來完成,由其中某個連桿的某個點來按照預(yù)定軌跡運行。具體的例子將在下一節(jié)連桿曲線里介紹。請注意,不要嘗試通過生成一個軌跡來控制連桿上某個點的方向。然而,通常某個點會沿著預(yù)定的軌跡在某一時刻到達特定的位置。這種情況被稱為規(guī)定時序生成軌跡,這類似于在輸出某個指定的特殊功能時生成一個功能。軌跡的分析和功能的生成將在第5章介紹。運動的產(chǎn)生被定義為在某個平面上,控制一條線沿著一些預(yù)設(shè)的連續(xù)軌跡運行。這個包含這條線的連桿的方向是非常重要的。這個問題比軌跡生成更常見,而事實上,軌跡的生成只是運動生成的一個分支。有個運動生成問題的一個例子如“推土機鏟斗的控制”。鏟斗必須完成一系列的動作如挖,鏟和倒掉鏟起來的土。從理論上講,鏟斗上某條線必須按照預(yù)期的位置運動,連桿機構(gòu)是最常見的解決方案。平面機構(gòu)和空間機構(gòu) 上面討論的運動的控制都是在平面(二維)內(nèi)完成預(yù)期動作。然而我們生活在三維世界并且我們的機械也必須在三維世界里運行。空間機構(gòu)就是三維的設(shè)備。它們的設(shè)計和分析都要比二維平面機械更復(fù)雜。研究空間機構(gòu)超出了我們這僅僅是介紹性質(zhì)課程的范疇。在本章參考文獻里我們會列出一些在今后學(xué)習(xí)過程中可能會用到的參考書目。然而,研究平面機構(gòu)沒有那么多的實際限制,像之前那些三維設(shè)備都是由二維設(shè)備組成的。比如說所有的可折疊椅,它的左側(cè)平面會有某種連桿機構(gòu)來完成折疊功能。椅子的右側(cè)平面會有個同樣的連桿機構(gòu)。這些有兩個XY平面的連桿機構(gòu)會由一些Z方向的結(jié)構(gòu)連接起來,這就可以將兩個平面機構(gòu)組裝成一個三維空間機構(gòu)。許多真實的機器就是這樣設(shè)計的,如重復(fù)的平面機構(gòu),在平行平面替換Z方向的緊密連接的結(jié)構(gòu)。當(dāng)你打開一個汽車的發(fā)動機罩時,留意一下發(fā)動機罩的鉸鏈機構(gòu)它在車的兩邊是一模一樣的。發(fā)動機罩和車身將兩個平面機構(gòu)組裝成一個三維裝置。多觀察你會發(fā)現(xiàn)更多這樣的例子。所以這里做的二維平面機構(gòu)的綜合推理和理論分析對三維設(shè)計也有一定的實際價值。本文譯自:Robert L. Norton,“Design of Machinery: An Introduction to the Synthesis and Analysis of Mechanisms and Machines”,Third Edition, McGraw-Hill Companies Inc.,2007, 76-79.
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