基于联动机制理论自动化组合夹具规划 毕业论文外文文献翻译.doc

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1、Automated modular fixture planning based onlinkage mechanism theory1. IntroductionModular fixture is a kind of very promising flexible fixture devices in manufacturing. It is also used in assembly and verification processes. The use of modular fixture seems to be a trend in the manufacturing field a

2、s it can meet the desire for greater flexibility (i.e. it is able to deal with the workpieces with irregular shapes by combining modular fixture elements). By using modular fixtures, the flexibility and rapid response capability of manufacturing systems can be improved.However, modular fixture plann

3、ing is a very difficult problem, especially for the modular fixture with the dowel-pin system as the locators of such modular fixtures can only be inserted into the fixed doweled holes. That is why modular fixture planning is still performed by fixture designers in industries up to now, relying on d

4、esigners experiences and trial and error method. The main problem with manual planning of modular fixtures lies in that it is almost impossible for a designer to enumerate all the alternative fixture plans, which makes it extremely hard to find out the optimal fixture plan. Obviously it hinders the

5、use of modular fixtures with dowel-pin system in industries. One way to solve the above problems is to use computer to assist designers in performing modular fixture planning. In recent years, the research on automated modular fixture planning has been paid more and more attention, and several appro

6、aches have been proposed in this area. Asada and By 1 described the basic concept of an adaptable fixturing system and its hardware implementation. The analytic tool of the fixture was developed, including that of accessibility and detachability. The key components of this system are fixture base ta

7、ble with magnetic chucking capability. Such a system is adaptable for flexible assembly and the product with small batch size. Brost and Goldberg 2 and Brost and Peters 3 presented a “complete” algorithm for synthesizing modular fixtures for a polygonal workpiece. The basic assumptions of their meth

8、od are that a workpiece can be represented with simple polygons; locators can be represented by a circle whose radius is less than half of the grid spacing; the fixturing configurations are infinite and all contacts are frictionless. The output of the algorithm includes the positions of three locato

9、rs and the clamp point, as well as the linear displacement and rotational angle of the workpiece relative to the base plate. Their method for finding out the three locators of a given workpiece is based on the enumerative algorithm. Zhuang et al. 4 explored the existence of modular fixture solutions

10、 for a given fixture configuration model and a workpiece, and presented a class of polygons which cannot be fixed by modular fixtures with dowel-pin system. Wallack and Canny 5 developed an automated design algorithm for a fixturing toolkit called the fixture vise, which involves two fixture plates

11、mounted on jaws of a vise and modular fixture elements. They also proposed an output-sensitive algorithm. The algorithm first enumerates all the quartets of the jaw-specified edge segments capable of providing force closure, then for each edge quartet enumerates all the quartets of fixture positions

12、, verifies force closure, and generates all the configurations of a modular fixturing toolkit capable of immobilizing a given prismatic object. Penev and Requicha 6 studied the fixture fool proofing for polygonal workpieces and presented an algorithm for calculating the positions of fool proofing pi

13、ns that make the incorrect loading pose impossible. Overmars et al. 7 introduced a point/edge fixture paradigm wherein objects are immobilizing by a combination of one edge and twopoint contacts. Wagner et al. 8 studied the method of fixturing 3D faceted parts with seven modular struts. Wu et al. 9,

14、10 presented a systematic method for analyzing the geometry, accuracy, clamping and accessibility of a modular fixture. In their method, three locator-workpiece contact situations are determined according to the types of locating faces and the types of contacting faces of locators, and the assembly

15、relationships between locators and the workpiece are described by three triangle function equations. By solving the three equations, the position of the workpiece related to the fixture and the positions of three locators are obtained. Based on the degree-of-freedom analysis of 2-D objects, a step-b

16、y-step algorithm is developed to find out the possible clamp positions by enumerating all the possible clamping edges. Their method is feasible and systematic, but it is complicated and its computation efficiency needs to be further improved.The investigation on optimization algorithms of fixture de

17、sign is also related to this work. In this aspect, King and Hutter 11 presented a method for optimal fixture layout design, which uses the rigid body model of a fixtureworkpiece system but accounts for the contact stiffness. DeMeter 12,13 used a rigid body fixtureworkpiece model and the minmax load

18、criterion to achieve the synthesis of the optimal fixture layout and minimum clamp actuation intensity, and presented a finite element-based method of supporting layout optimization. However, these nonlinear programming methods can neither achieve “global” or “near-global” optimum solutions nor dete

19、rmine the locators and clamp positions explicitly. In addition, the models used are very sensitive to the initial feasible fixture layout. During the last decade, a few researchers utilized genetic algorithm to solve this problem. Wu and Chan 14 used the genetic algorithm to determine the most stati

20、cally stable fixture layout. Ishikawa and Aoyama 15 adopted the genetic algorithm to determine the optimal clamping condition for an elastic workpiece. Krishnakumar and Melkote 16 presented a method of achieving fixture layout optimization using genetic algorithm, in which a finite-element approach

21、is employed to evaluate the generated fixture layouts. Wang and Pelinescu 17 developed an algorithm of fixture layout optimization based on accurate localization, minimal locator contact force and balanced locator contact force. Mervyn et al. 18 presented an approach to automated synthesis of modula

22、r fixture designs using an evolutionary search algorithm. In general, the current achievements on computer-aided modular fixture design still have a certain distance from what industries expect. The main limitations of the current works include: (1) because the assembly relationships between locator

23、s and the workpiece are not described in an analytic way, the current methods of determining location plans are enumerative in nature and thus very time consuming; (2) the current approaches to determination of the side clamping positions are relatively complex; (3) the existing quality metrics abou

24、t fixture design do not consider the geometry structure of the workpiece and the assembly relationship between the workpiece and locators which also affect the quality of location plans. In this paper, we present a new approach to automated planning of modular fixtures with dowel-pin. The approach i

25、dentifies all the alternative location plans of a workpiece comprising both planar and cylindrical faces in a general and efficient way by using four-bar mechanism theory, and performs accessibility and fixturability analysis and generates feasible clamp positions of a fixture plan in an effective m

26、anner based on several new concepts. In the approach, the assembly relationship between locators, side clamping points and workpiece are described in an analytic way. The approach is intended to be more general and more robust.2. Identification of locating plan candidates2.1. Problem statement and m

27、ethod descriptionIn this paper, we assume that the primary locating face set of the workpiece has been determined, which could be a single planar face or consists of two or three parallel planar faces with different heights, and assume that the secondary and tertiary locating faces are perpendicular

28、 to the primary locating face(s), but unlike those in 321 locating scheme, they need not be perpendicular each other. Considering that in most fixture designs, only planar and cylindrical faces are selected as locating faces, especially for modular fixture applications 10, the workpiece in this work

29、 is limited to the part whose side clamping faces consist of planar and cylindrical faces. Without loss of generality, we further assume that the top face of the modular fixtures base plate is the support face of the workpiece and it is coplanar with the primary locating face of the workpiece. Then

30、it indicates that after a workpiece is put on the base plate of a modular fixture, its three DOFs are constrained. In order to make the workpiece completely fixed, the remainder three DOFs of the workpiece need to be constrained as well. For a modular fixture with dowel-pin, three round locators wit

31、h each one inserted into a doweled hole and contacted with a side face of the workpiece are used to completely locate the workpiece. The three locators that can completely locate the workpiece are called a location plan of the workpiece. Obviously, not every three locators inserted into the arbitrar

32、y three doweled holes can form a location plan of the workpiece. Therefore, how one can identify all the alternative location plans becomes the issue that needs to be solved first.Based on the assumption that the secondary and tertiary locating faces of the workpiece are perpendicular to its primary

33、 locating face(s), the problem of identifying location plans of a workpiece can be converted to a 2D problem. Fig. 1 is used to illustrate this, which shows the base plates top face of a modular fixture with dowel-pin, the projection of the workpiece on the base plates top face, and a location plan

34、candidate. In the figure, “o” stands for the locating hole used to fix the round locators, “x” refers to the tapped hole used to connect fixture elements, and the dashed lines refer to the offset of the workpieces profile on the base plate. Here, the offset distance is equal to the radius of the loc

35、ator. With the help of Fig. 1, we can clearly describe the problem of identifying a location plan candidate as follows: to identify a location plan candidate is to find out three doweled holes on the base plate that satisfy following two conditions: first, the workpiece can be put on the base plate

36、with the offset of the workpieces profile projected on the base plate being through all the three doweled holes centers (note that the workpieces profile here just consists of the projections of the planar and cylindrical faces of the workpiece that are perpendicular to the primary locating face); s

37、econd, when three round locators are inserted into the three doweled holes and a clamp force is imposed on the workpiece from a proper position, the workpiece will be completely fixed. As an example, in Fig. 1 three round locators a, b and c, which are inserted into three doweled holes, form a locat

38、ion plan candidate of the workpiece.According to the degree-of-freedom analysis, a 2D object can be located in two ways: (a) two boundary edges of the 2D object are restricted by three fixed points, with one edge restricted by two fixed points, and the other restricted by the third fixed point; (b)

39、three boundary edges of the 2D object are restricted by three fixed points, with each edge restricted by one fixed point. In view that the way (a) is an extension of the traditional 321 location rule 20 and the location plans responding to such location way are easy to be determined, only the second

40、 location way is considered in this paper. Our approach to identifying a location plan consists of two steps: first, determine the first two doweled holes whose centers are on the offset of the workpieces profile; second, identify the third doweled hole whose centers are on the offset of the workpie

41、ces located profile. To effectively identify the third doweled hole, the four-bar mechanism and linkage curve are employed in our approach, which is illustrated by Fig. 2. In Fig. 2,M stands for a fixed plane, a and b are two curves on the plane M, the triangle M1 stands for a 2D region on M, and A,

42、 B are two fixed points of M1. Suppose M1 moves in the M in the way keeping A and B to move along the curve a and b, respectively, then the locus of an arbitrary fixed point C in M1 forms a slide curve. Particularly, when a and b are either linear or circular, the motion of M1 relative to M is a lin

43、kage plane motion, and thus the corresponding locus of the point C in M1 is a linkage curve. Since a and b are the loci of two hinge points of the linkage plane according to the four-bar linkage mechanism theory 21, hereafter, a, b are called hinge point loci, and A, B are called hinge point. In ord

44、er to determine the third doweled hole of a location plan candidate using linkage mechanism theory, we construct a four-bar mechanism and a linkage curve as follows: taking the primary locating face of the workpiece as M, the base plate as M1, the centers of the first two doweled holes on the base p

45、late as two hinge points A and B, and two offset edges that the first two doweled holescenters are, respectively, on as two hinge point loci a and b. Since all the edges of the workpiece considered in this work are either linear or circular, the relative motion of the base plate to the workpiece is

46、exactly a linkage plane motion according to the mechanism theory, and the locus of thecenter of any other doweled hole except the first two doweled holes is a linkage curve. Here the linkage curve is a single parametric curve and the curve parameter is also a position parameter of linkage mechanism.

47、 Therefore, if the linkage curve intersects with any other offset edge of the workpieces profile except a and b, the corresponding doweled hole of linkage curve is the third doweled hole that can form a potential location plan with the first two doweled holes 21. The relative position of the workpie

48、ce with the base plate of fixture can be determined by calculating the parametric value of intersection point, and the relative positions of first two locators to the workpiece can also be determined. It should be pointed out that because four-bar mechanism and linkage curve can effectively represen

49、t the assembly relationship between locators and the workpiece, it is more straightforward and efficient to determine the position of the third locator using the linkage mechanism theory than using other analysis methods such as the screw theory and force closure analysis although the latter is very useful for analyzing the closure of the side locating and side clamping of the workpiece after they are determined.22 Algorithm for determining location plan candidatesThe algorithm for determining location pl