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Research Papers

Process Yield Improvement Through Optimum Design of Fixture Layouts in 3D Multistation Assembly Systems

[+] Author and Article Information
T. Phoomboplab, D. Ceglarek

Warwick Digital Laboratory, WMG University of Warwick, Coventry CV4 7AL, UK; Department of Industrial and Systems Engineering, University of Wisconsin–Madison, Madison, WI 53706

J. Manuf. Sci. Eng 130(6), 061005 (Oct 10, 2008) (17 pages) doi:10.1115/1.2977826 History: Received August 17, 2007; Revised July 18, 2008; Published October 10, 2008

Fixtures control the positions and orientations of parts in an assembly process. Inaccuracies of fixture locators or nonoptimal fixture layouts can result in the deviation of a workpiece from its design nominal and lead to overall product dimensional variability and low process yield. Major challenges involving the design of a set of fixture layouts for multistation assembly system can be enumerated into three categories: (1) high-dimensional design space since a large number of locators are involved in the multistation system, (2) large and complex design space for each locator since the design space represents the area of a particular part or subassembly surfaces on which a locator is placed, (here, the design space varies with a particular part design and is further expanded when parts are assembled into subassemblies), and (3) the nonlinear relations between locator nominal positions and key product characteristics. This paper presents a new approach to improve process yield by determining an optimum set of fixture layouts for a given multistation assembly system, which can satisfy (1) the part and subassembly locating stability in each fixture layout and (2) the fixture system robustness against environmental noises in order to minimize product dimensional variability. The proposed methodology is based on a two-step optimization which involves the integration of genetic algorithm and Hammersley sequence sampling. First, genetic algorithm is used for design space reduction by estimating the areas of optimal fixture locations in initial design spaces. Then, Hammersley sequence sampling uniformly samples the candidate sets of fixture layouts from those predetermined areas for the optimum. The process yield and part instability index are design objectives in evaluating candidate sets of fixture layouts. An industrial case study illustrates and validates the proposed methodology.

Copyright © 2008 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Fixture layout representation for a multistage assembly process

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Figure 2

KPC variations compared with KPC tolerance region: (a) before optimizing fixture layouts (b) after optimizing fixture layouts

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Figure 13

Locator design spaces on floor pan left (FPL, root part) in Station 1

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Figure 14

Locator design spaces on floor pan right (FPR, mating part) in Station 1

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Figure 15

Locator design spaces of the root part (FPL+FPR) in Station 2

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Figure 16

Locator design spaces of bracket left (BrktL, mating part) in Station 2

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Figure 17

Locator design spaces of the root part in Station 3 (the coordinates of the boundaries are the same, as shown in Fig. 1)

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Figure 19

An example of design space discretization

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Figure 20

Locator positions selected by GA in Station 1

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Figure 21

Locator positions selected by GA in Station 2

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Figure 22

Locator positions selected by GA in Station 3

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Figure 23

Locator design spaces of interest

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Figure 24

The optimal locator positions selected by HSS

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Figure 25

The optimal locator positions selected by HSS on (a) bracket left and (b) bracket right

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Figure 8

Example of part-to-part joints used in automotive body assembly

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Figure 9

Illustration of the proposed methodology Steps 1.2–3.1

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Figure 10

Step 2 design space reduction by genetic algorithm

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Figure 11

Sets of fixture layouts generation by HSS

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Figure 12

Floor pan assembly

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Figure 18

Locator design spaces of bracket right (BrktR, mating part) in Station 3

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Figure 3

3-2-1 fixture layout for prismatic workpiece

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Figure 4

External wrench, wd, and twist caused by gravity force, tmg

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Figure 5

The optimization procedure of the proposed methodology

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Figure 6

Mating part degrees of freedom allocation between part-to-part joint and locators

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Figure 7

3-2-1 fixture layout for a single station assembly

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