1、華中科技大學文華學院畢業設計（論文）外文文獻翻譯（2012年12月15日 至2013 年 5月20日） 畢業設計（論文）題目：十字頭的加工工藝及夾具設計學部（系）：機械與電氣工程學部 專業班級：機械設計制造及其自動化09學生姓名： 黃柏程 指 導 教 師： 趙鎮鋒 外文文獻翻譯基于事例推理的夾具設計研究與應用摘要:根據基于事例的設計方法,提出采用工序件的特征信息和夾具的結構特征信息來描述夾具的相似性,并建立了包括這2方面主要特征信息為基礎的事例索引碼,設計了事例庫的結構形式,創建了層次化的事例組織方式;同時,提出了基于知識引導的夾具事例檢索算法,以及事例的修改和采用同族事例碼進行相似事例的存貯

2、,形成了基于事例推理的夾具設計.所開發的原型系統在型號工程夾具設計等項目的設計過程中得到了應用,并取得了令人滿意的使用效果.關鍵詞: 基于事例的推理 夾具設計 CAD 夾具是以確定工件安全定位準確為目的的裝置，并在加工過程中保持工件與刀具或機床的位置一致不變。因為夾具的結構依賴于產品的特點和在企業規劃中加工工序的地位,所以它的設計是制造過程中的瓶頸,制約著效率的提高. 夾具設計是一個復雜的過程,需要有從大量的設計論文中了解質量知識的經驗，這些設計論文包括工件的結構設計、涉及加工工藝，和加工環境。當用這些擅長繪制詳細設計圖的傳統的CAD工具（如Unigraphics、CATIA、Pro/E)時,

3、這仍然是一項非常耗時的工作，但是利用以往的設計經驗和資源也不能提供一些益處,而這正是提高效率的關鍵因素. 基于事例推理 (CBR) 的方法適應以往個案解決的辦法,建立一個新問題的方法,主要有以下四步驟:檢索、利用、修改,并保留.這是一個比用專業系統模仿人類思維有用的使用方法，因為提出一個類似的情況,和采用一些修改,似乎不言自明,而且比人類更直觀.所以支持不同事例的設計工具已經在諸多領域中發展起來，如在注射成型及設計、建筑設計、模具設計投死, 規劃過程中,還有夾具設計. 孫用六個數字組成代碼參數,包括工件的形狀、機械部分、軸襯,第一定位裝置,第二定位裝置和夾緊裝置. 但這個系統不能用于除鉆床夾具

4、外的其他夾具類型,不能解決儲存需要保留的同一參數代碼的問題,這在CBR中是非常重要的.1事例參數和事例圖書館的建立1.1事例參數事例參數應該由工件的所有的特征組成，來區別不同的夾具. 使用他們能夠使操作方便. 因為零件的形狀是多種多樣的, 在生產企業中制造的技術要求也不斷發展,許多特征作被用做事例參數將會使搜索速度降低,其主要特征是不重要的,因為分配給每個特征的比重必須減少. 另一方面,事例參數包含所有的特征是困難的。 因此,考慮到實際和快速設計的需求,事例參數要包含工件的主要特征和夾具的結構。事例參數代碼由16位數組成：13位數是事例特征 3位數是事例識別數字。 前13位數代表13個特征。

5、每個數字與特征的一個屬性相一致,這可能是"*"、"？"、"1"、"2"，"A"、"B",，"Z"，等其中的一個。其中，"*"是指任何一個,"?"代表不確定,"0"代表沒有。 系統規定:夾具的類型，工件的形狀，位置模式不能是"*"和"?"。在設計系統時,三個項目的屬性信息沒有這些選擇,這就意味著必須選擇確定的屬性。最后三位數是事例識別號碼,如果事例特征的13位

6、數是一樣的,這三個數字就用來區別他們。該系統還規定:"000"是用于修正的一個典型事例,其他事例"001"、"002"、,這些是用于設計師查找參考事例的. 如果其中一個偶爾需要改變成典型事例,首先它必須要求改成"000",前面的自動變成參考事例. 事例索引碼的結構如圖1所示。 1夾具類型； 6工件重量； 11夾緊模型； 2工件形狀； 7工件剛度； 12夾具體； 3工件材料； 8加工內容； 13其他； 4批 量； 9過程所有物； 14到16事例識別碼； 5工件比例； 10定位模型； 圖1 事例索引碼的結構1.2事例庫

7、事例庫由許多預定義的事例組成。事例的描述是基于事例推理的最重要的問題之一。所以由索引碼復合。1.3 事例的層次化夾具的結構相似被認為是整個夾具，成分和內容相似。所以，整個夾具事例庫，成分事例庫，夾具的成分事例庫形成相同。整個夾具的設計資料通常是由工件資料和工件加工資料組成,這就意味著夾具的設計應滿足特別功能的需求.全部夾具事例是由功能成分組成，它是用功能成分的名字和數字來進行描述的。成分事例代表成員（成分功能和其他結構成分，主要驅動參數，數字，和它們的約束關系）。成分事例（夾具的最低層）是功能成分和和其他成分的結構。在現代夾具設計中有很多參數化準件和普通非標準件。所以成分事例圖書館應記錄特殊參

8、數和保持它們的方法。2事例修改的策略在基于事例的夾具設計中，最重要的是相似點的修改，這樣能有助于獲得最相似的事例，以及縮短適應時間。根據夾具設計的需求，事例修改的策略使最接近的事例方法和知識指導結合起來。首先在深度上查找，然后在寬度上；知識指導策略意味著在來自客觀事物根源的知識規則上查找，這就要首先查找固定類型，然后查找工件的形狀，第三查找定位方法。例如，如果事例索引碼包括夾具類型的磨削夾具，就只查找所有的磨削夾具，然后查找工件形狀的盒子，第三查找一個平面兩個銷的定位方法。如果沒有合適的，就查找深度標點，然后回到最上層，然后再找所有與寬度相關的事例。 修改方法：(1)根據夾具事例庫的事例索引信

9、息，查找有關事例庫。(2)將事例索引碼與事例庫的每個事例碼匹配，然后計算相似尺寸的價值。(3)整理相似尺寸的次序，最大的架子是最類似的事例。兩個事例之間的相似點是基于兩個事例特征之間的相似點。相似點尺寸的計算依靠特征的類型。相似點的價值可以通過數字化的價值來計算，例如比較重量分別是50kg 和 20kg的工件。非數字化的價值也能計算，例如，現在前13位索引碼都是非數字化的價值。一個夾具的相似尺寸的計算公式如下：其中S表示通用夾具的相似尺寸，n表示索引特性數，表示每個特性的重量，表示事例庫中特性和相關夾具的特性的相似尺寸。同時， ，數值計算如下： 其中表示第i個特征的索引特性值，表示事例庫中第j

10、個事例的第i個特征的特性值。 所以有兩種方法選擇相似夾具。一個方法是建立數值。如果通用事例的相似尺寸值比給定的數值小，這些事例就不能選來作相似事例。事例庫最初建立的時候，只有一些事例，數值可以建小一點。如果有大量的相似事例，數值就應該建的大一些。另外一個方法是只建立相似事例的數字（例如10），這是類型單里相似尺寸的最大值。3 事例的修改和存儲3.1事例的修改 夾具設計中相似事例的修改包括以下三個階段：(1)成分的替代(2)保持形式不變，調整成分的特性(3)模型重新設計如果夾具的成分是普通的物品，它們能通過使用工具被修改，代替以及刪除，這些已經被設計好了。3.2事例的存儲 在將一個新的事例保存到

11、事例庫之前，設計者必須考慮保存是否有價值。如果這個事例不能增加系統的知識，就沒有必要把它保存到事例庫里。如果它有價值的話，設計者在保存之前必須分析一下，看看這個事例是否作為標準事例或參考事例被存儲了。一個標準事例是一個描述同族事例主要特征的標準。一個同族事例是有事例庫中索引碼前13位相同而最后三位不同的那些事例組成的。一個標準事例的最后三位通常是“000”。一個參考事例屬于同族標準事例，最后三位用不同數字區分。從被解釋的概念中，可采用以下方法：(1)如果一個新的事例和任何一個存在的事例族一致，和一個存在的標準事例的前13位數相同，那么這個事例就不能存儲因為已經這種標準事例了。或者只能作為一個參

12、考事例保存（最后三位不是“000”，而且和其它的不一樣）在事例庫中。 (2)如果一個新的事例和任何一個存在的事例族一致，并且被認為代替這個事例族要比以前的標準事例好，那么這個標準事例就被這個新的事例代替，以前的標準事例作為一個參考事例保存。 (3)如果一個新的事例和任何一個存在的事例族不一致，一個新的事例族將會自動產生，并作為標準事例保存到事例庫中。4夾具設計中基于事例推理的過程根據夾具設計的特性，夾具設計的基本信息，例如夾具的名字，零件，生產和設計者等等，必須先輸入。然后，輸入或設計工件的模型。輸入有關工件的細節信息，建立事例索引碼，然后CBR開始依靠相似尺寸查找相似事例，選出最相似的事例。

13、如果需要的話，事例要滿足通用性設計，再存儲到事例庫中。程序流程圖如圖2所示 圖2 基于事例推理的夾具設計流程圖5基于事例推理的夾具設計說明這是一個工件如圖3所示。材料是45鋼，底座，形狀為塊狀，生產批量為中批等。需要設計成一個用來旋轉孔的旋轉夾具。圖3 需要設計夾具的一個工件（最大尺寸80mmx49mmx22mm）工件的特征值，屬性值，事例索引碼和重量在表1中列出。 表1 工件的事例索引碼和重量 特征名稱 特性值 索引碼 重量 夾具類型 車床夾具 1 100 工件形狀 塊狀 9 90 工件材料 中碳鋼 3 70 批量 中批 2 60 工件比例 小 5 60 工件重量 輕 5 60 工件剛度 硬

14、度強 1 60 加工內容 孔 3 80 程序要求 完成加工 3 70 定位方法 三個平面 1 100 夾緊方法 不確定 ？ 90 夾具體 復合 4 80 其他 沒有 0 60 表2 最相似事例的事例索引碼特征名稱 特性值 索引碼夾具類型 車床夾具 1 工件形狀 塊狀 9 工件材料 中碳鋼 3 批量 中批 2 工件比例 小 5 工件重量 輕 5 工件剛度 硬度強 1 加工內容 孔 3 程序要求 完成加工 3 定位方法 三個平面 1 夾緊方法 不確定 ？ 夾具體 復合 4 其他 沒有 0 相似點的計算如下： 所以夾具的相似尺寸值是0.806，這是在事例庫中用于設計的最相似的事例，最相似的事例的結構

15、如圖4所示圖4 最相似的夾具當成分替代，修改定位模型和夾緊模型，以及調節相關尺寸之后，新的夾具被設計出來，圖形如圖6所示 圖5 需要設計的新夾具6 結論 基于事例推理，作為一個問題解決的方法，是一個比模仿人類思想的專業系統更有效的方法，已經在很多難獲取知識的領域里得到發展。基于事例推理的優點如下：它和人類的思想很相似；一個事例庫通過保存新事例獲得自學能力，它比有慣例庫更快更容易，它可以更好的傳遞和解釋新的知識，這和慣例庫有很大的不同。基于事例推理中提出的一個夾具設計的框架已經被實行了，使用的是支持基礎數據的VC+，UG電腦繪圖軟件。這個框架也已經和普通成分庫和典型夾具庫結合起來。這個發展的標準

16、系統，用于航空項目，幫助夾具設計者提高設計效率和重新使用先前的設計資源。Application and developmentOf case based reasoning in fixture design Abstract: Based on the case based designing (CBD) methodology, the fixture similarity is in two respects: the function and the structure information. Then, the computer aided fixture design syste

17、m is created on case based reasoning (CBR),in which the attributes of the main features of work piece and structure of fixture as case index code are designed for the retrieve of the similar cases, and the structure and hierarchical relation of case library are set up for store. Meanwhile, the algor

18、ithm based on the knowledge guided in the retrieve of the similar cases, the strategy of case adapt at ion and case storage in which the case indent if cat ion number is used to distinguish from similar cases are presented. The application of the system in some projects improves the design efficienc

19、y and gets a good result .Keywords: case based reasoning ;fixture design; computer aided design(CAD) Fixtures are devices that serve as the purpose of holding the work piece securely and accurately, and maintaining a consistent relationship with respect to the tools while machining. Because the fixt

20、ure structure depends on the feature of the product and the status of the process planning in the enterprise, its design is the bottleneck during manufacturing, which restrains to improve the efficiency and lead-time. And fixture design is a complicated process, based on experience that needs compre

21、hensive qualitative knowledge about a number of design issues including work piece configuration, manufacturing processes involved, and machining environment. This is also a very time consuming work when using traditional CAD tools (such as Epigraphic, CATIA or Pro/E), which are good at performing d

22、etailed design tasks, but provide few benefits for taking advantage of the previous design experience and resources, which are precisely the key factors in improving the efficiency. The methodology of case based reasoning (CBR) adapts the solution of a previously solved case to build a solution for

23、a new problem with the following four steps: retrieve, reuse, revise, and retain 1. This is a more useful method than the use of an expert system to simulate human thought because proposing a similar case and applying a few modifications seems to be self explanatory and more intuitive to humans .So

24、various case based design support tools have been developed for numerous areas2-4, such as in injection molding and design, architectural design, die casting die design, process planning, and also in fixture design. Sun used six digitals to compose the index code that included work piece shape, mach

25、ine portion, bushing, the 1st locating device, the 2nd locating device and clamping device5. But the system cannot be used for other fixture types except for drill fixtures, and cannot solve the problem of storage of the same index code that needs to be retained, which is very important in CBR6.1 Co

26、nstruction of a Case Index and Case Library1.1 Case index The case index should be composed of all features of the work piece, which are distinguished from different fixtures. Using all of them would make the operation in convenient. Because the forms of the parts are diverse, and the technology req

27、uirements of manufacture in the enterprise also develop continuously, lots of features used as the case index will make the search rate slow, and the main feature unimportant, for the reason that the relative weight which is allotted to every feature must diminish. And on the other hand, it is hard

28、to include all the features in the case index.Therefore, considering the practicality and the demand of rapid design, the case index includes both the major feature of the work piece and the structure of fixture. The case index code is made up of 16 digits: 13 digits for case features and 3 digits f

29、or case identification number.The first 13 digits represent 13 features. Each digit is corresponding to an attribute of the feature, which may be one of“*”, “？”, “1”, “2”,“A”,“B”, “Z”, etc. In which, “*” means anyone, “?” uncertain, “0” nothing.The system rules: fixture type, work piece shape, locat

30、ing model cannot be “*”or“?”. When the system is designed, the attribute information of the three items does not have these options, which means the certain attribute must be selected. The last three digits are the case identification number, which means the 13 digits of the case feature are the sam

31、e, and the number of these three digits is used for distinguishing them.The system also rules: “000” is a prototype case, which is used for retrieval, and other cases are “001”,“002”, which are used for reference cases to be searched by designers. If occasionally one of them needs to be changed as t

32、he prototype case, first it must be required to apply to change the one to “000”, and the former is changed to referential case automatically.The construction of the case index code is shown in Fig.1.1.2 Case libraryThe case library consists of lots of predefined cases. Case representation is one of

33、 the most important issues in case based reasoning. So compounding with the index code,.1.3 Hierarchical form of CaseThe structure similarity of the fixture is represented as the whole fixture similarity, components similarity and component similarity. So the whole fixture case library, components c

34、ase library, component case library of fixture are formed correspondingly. Usually design information of the whole fixture is composed of work piece information and work piece procedure information, which represent the fixture satisfying the specifically designing function demand. The whole fixture

35、case is made up of function components, which are described by the function components names and numbers. The components case represents the members. (function component and other structure components， main driven parameter, the number, and their constrain relations.) The component case (the lowest

36、layer of the fixture) is the structure of function component and other components. In the modern fixture design there are lots of parametric standard parts and common non standard parts. So the component case library should record the specification parameter and the way in which it keeps them. 2 Str

37、ategy of Case RetrievalIn the case based design of fixtures ,the most important thing is the retrieval of the similarity, which can help to obtain the most similar case, and to cut down the time of adaptation. According to the requirement of fixture design, the strategy of case retrieval combines th

38、e way of the nearest neighbor and knowledge guided. That is, first search on depth, then on breadth; the knowledge guided strategy means to search on the knowledge rule from root to the object, which is firstly searched by the fixture type, then by the shape of the work piece, thirdly by the locatin

39、g method. For example, if the case index code includes the milling fixture of fixture type, the search is just for all milling fixtures, then for box of work piece shape, the third for 1plane+ 2pine of locating method. If there is no match of it, then the search stops on depth, and returns to the up

40、per layer, and retrieves all the relative cases on breadth.Retrieval algorithms:1)According to the case index information of fixture case library, search the relevant case library;2)Match the case index code with the code of each case of the case library, and calculate the value of the similarity me

41、asure;3)Sort the order of similarity measure, the biggest value, which is the most analogical case.Similarity between two cases is based on the similarity between the two cases. features. The calculation of similarity measure depends on the type of the feature. The value of similarity can be calcula

42、ted for numerical values, for example, compareWork piece with the weight of 50kg and 20kg. The value can also be calculated between non numerical values, for example, now the first 13 digits index code is all non numerical values. The similarity measure of a fixture is calculated as follows: where S

43、 is the similarity measure of current fixture, n is the number of the index feature, is the weight of each feature, is the similarity measure of the attribute of the i2th feature with the attributeof relative feature of the j-th case in the case library. At the same time, , the value counts as follo

44、ws:.Where is the value of the index attribute of the i-th feature, and is the value of attribute of the relative i-th feature of the j-th case in case library.So there are two methods to select the analogical fixture. One is to set the value. If the values of similarity measure of current cases were

45、 less than a given value, those cases would not be selected as analogical cases. When the case library is initially set up, and there are only a few cases, the value can be set smaller. If there are lots of analogical cases, the value should get larger. The other is just to set the number of the ana

46、logical cases (such as10), which is the largest value of similarity measure from the sorted order.3 Case adaptation and Case Storage3.1 Case adaptation The modification of the analogical case in the fixture design includes the following three cases:1) The substitution of components and the component

47、;2) Adjusting the dimension of components and the component while the form remains; 3) The redesign of the model.If the components and component of the fixture are common objects, they can be edited, substituted and deleted with tools, which have been designed.3.2 Case storageBefore saving a new fix

48、ture case in the case library, the designer must consider whether the saving is valuable. If the case does not increase the knowledge of the system, it is not necessary to store it in the case library. If it is valuable, then the designer must analyze it before saving it to see whether the case is s

49、tored as a prototype case or as reference case. A prototype case is a representation that can describe the main features of a case family. A case family consists of those cases whose index codes have the same first 13 digits and different last three digits in the case library. The last three digits

50、of a prototype case are always “000”. A reference case belongs to the same family as the prototype case and is distinguished by the different last three digits.From the concept that has been explained, the following strategies are adopted:1) If a new case matches any existing case family, it has the

51、 same first 13 digits as an existing prototype case, so the case is not saved because it is represented well by the prototype case. Or is just saved as a reference case (the last 3 digits are not “000”, and not the same with others) in the case library.2) If a new case matches any existing case fami

52、ly and is thought to be better at representing this case family than the previous prototype case, then the prototype case is substituted by this new case, and the previous prototype case is saved as a reference case.3) If a new case does not match any existing case family, a new case family will be

53、generated automatically and the case is stored as the prototype case in the case library.4 Process of CBR in Fixture DesignAccording to the characteristics of fixture design, the basic information of the fixture design such as the name of fixture, part, product and the designer, etc. must be input f

54、irst. Then the fixture file is set up automatically, in which all components of the fixture are put together. Then the model of the work piece is input or designed. The detailed information about the work piece is input, the case index code is set up, and then the CBR begins to search the analogical

55、 cases, relying on the similarity measure, and the most analogical case is selected out. If needed, the case is adapted to satisfy the current design, and restored into the case library. The flowchart of the process is shown in Fig.3.5 Illustrating for Fixture Design by CBRThis is a work piece (seeFig.4). Its material is 45# steel. Its name is seat. Its shape is block, and the product batch size is middle, etc. A fixture is turning fixture that serves to tu