CAD systems, which require a detailed level of design, prohibit the creative and free

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1 ATTRIBUTE-BASED DESIGN DESCRIPTION SYSTEM IN DESIGN FOR MANUFACTURABILITY AND ASSEMBLY Srinivas Paluri 1 and John K Gershenson 2 1 Volt Computer, Redmond, WA 2 Department of Mechanical and Aerospace Engineering, Utah State University, Logan, UT ABSTRACT Present computer-aided design (CAD) systems, intentionally developed as detail oriented designing tools, do not fully support the activities at the early stage of product development. CAD systems, which require a detailed level of design, prohibit the creative and free expression of a design idea. The solution to the limitations of present CAD systems is to fully utilize the graphical ability of current computer systems to represent a design with an easily understood design description in the conceptual design stage. We have developed a computerized product development tool to support designing activities in the conceptual design phase. The attribute-based design description system (ADDS) is a feature-based system that incorporates life-cycle engineering analysis and solid modeling to form an integrated CAD system. It provides a simple design representation interface and assembly modeling, evaluates the design for life-cycle engineering issues, and exports the design to AutoCAD as a solid model with flexible information input requirements. The research thus provides a starting point to the development of CAD systems that support productivity in the conceptual design stage. ADDS has been validated by describing three 1

2 different design examples of power transmission systems in ADDS and exporting them to AutoCAD. INTRODUCTION A brief background is presented below. Due to the broad base of this topic, we encourage the reader to seek added information. CAD Systems CAD systems have concentrated more on shape information of the product rather than the product information that is useful for life-cycle engineering analysis. Engineering analysis like thermal analysis or finite element analysis requires information regarding product application. This has made CAD systems less attractive to applications where shape information of the product is not prominent or geometric information is expanded with no geometric information (Mantyla, 1989). CAD representations have been difficult to interpret for manufacturing process planning. This drawback can be overcome by implementing feature technology (Ganesan and Devarajan, 1997). Feature-based modeling systems are capable of capturing both geometric and non-geometric information of the product. In the 1990's, commercial CAD developers like Pro-Engineer started adopting feature-based modeling. Although many current CAD systems support assembly modeling, they require additional user interaction to determine the assembly conditions. Assembly features have been developed to alleviate this problem. In summary, drawbacks of current CAD systems include: 2

3 Many CAD tools are applicable in the detail design phase only. Many CAD tools lack concurrent engineering (life-cycle engineering) ability. Many CAD tools have limited parametric support. Most CAD systems restrict innovative and creative design. Most CAD tools do not support assembly modeling. Feature-based Design A product model built by using features is known as design with features or feature-based modeling (Salomons, et al., 1993). In general, the features mean form features. Specifically, a feature is a physical constituent of a part. Features are used in different areas in mechanical engineering such as design, process planning, and assembly planning. Attempts have been made to incorporate feature-based design in commercial CAD systems. Feature-based model representations have been developed for CAD/CAM environments that assist the designer during product definition. Shah and Rogers (1988) developed a generic mapping shell that can be customized to support the extraction, mapping, and reasoning requirements of various engineering applications such as process planning, group technology and manufacturability evaluation. A feature in this testbed is a representation of shape aspects that is mappable to a generic shape with significant functionality. These mappings and the validations allow for a feasibility check and can be used in mapping from feature to form in a standard, application-independent CAD system. Assembly Modeling Assemblies consist of components that can be represented as a hierarchical tree structure. These trees can show multiple levels of abstraction at once, such as component-level, 3

4 feature-level, and geometry-level assembly models (Shah and Mantyla, 1996). Assembly features allow assembly modeling at a higher level by storing the assembly conditions among components. Assembly features allow for design changes that propagate among different parts. Assembly design, using CAD systems, requires considerable and often inconvenient, human interaction. CAD systems do not facilitate dimension changes after parts have been assembled together. Assembly information does not clearly exist. Redesign also is not easy in contemporary CAD systems after final assembly. In summary, the drawbacks in current assembly modeling research include: Most assembly modeling requires cumbersome human interaction. Assembly information is not available at the feature level. Most assembly modelers do not support iterative design. ATTRIBUTE-BASED DESIGN DESCRIPTION SYSTEM A PC-based attribute-based design description system (ADDS) has been developed by creating an integrated CAD system environment, which incorporates creative design in the early stages of the design process and parametric design in the later stages (Ng and Gershenson, 1998). ADDS is a non-commercial test bed for life-cycle engineering analysis. ADDS utilizes an icon-based design environment with the following characteristics that enhance its function: ADDS implements life-cycle engineering concepts in the early stage of product design by detailing and analyzing component interactions in all stages of the product life-cycle. ADDS represents products based not just their geometry but function, properties, and features allowing parametric design and a more comprehensive description of the product. 4

5 ADDS allows for performing an attribute-based analysis based on user-defined rules and allows for the export of this design to a CAD system (AutoCAD) for detailed engineering analysis. ADDS represents the product based upon different characteristics in a attribute-based object that includes geometry, feature, function, property, and value. ADDS establishes a relationship among these attributebased objects through a link that consists of life-cycle connection information. ADDS uses component and attribute libraries to define components. By creating an integrated CAD environment, some problems experienced by current CAD systems can be eliminated. ADDS s integrated CAD environment has the same user interface for both component modeling and assembly modeling. ADDS utilizes graphical symbols to represent components and associated links in a product. This graphical representation of components and links helps the user describe the product easily, thus making ADDS user-friendlier. ADDS allows the description of a new design as well as the ability to retrieve existing similar designs from a database or file. ADDS s allows component and assembly modeling, and a design evaluator to evaluate and benchmark designs using life-cycle engineering knowledge-bases during conceptual design. ADDS allows for assembly validation and life-cycle analysis at any stage of the design. Designed products can be exported to AutoCAD at any stage of the design as a solid model. Default values are assigned to the components and links whenever they are created; thus, the emphasis is on the design rather on the component information. Once design is completed, the designer can change the component information and link information. This allows for creativity while using this design tool and lessens the user interaction in designing the product. 5

6 The architecture of ADDS is shown in Figure 1. ADDS systems consists of four applications: ADDS environment Rule-based system (RBS) Assembly RBS Solid modeling Rule-based System Assembly Rule-based System Knowledge Bases Assembly Conditions Rule Editor Design Evaluator Assembly Rule Editor Assembly Validator End User User Interface Component /Assembly Library CAD Library Solid Modeling Design Export Interface Geometry Validation Assembly Validation CAD Translation (ActiveX Automation) Attribute-based Design Description Environment Material Database Component Card Component Library Link Card Design Worksheet Assembly Feature Library 3D Solid Model (Assembly Drawing) Connector Palette Component Palette Figure 1. System architecture of modified ADDS. 6

7 ADDS Environment A worksheet is the heart of the graphical ADDS environment. Palettes and toolbars are used to access functions and cards are used to store and view information. The design worksheet is the front-end tool for designing, analyzing, and exporting a product. The design worksheet (Figure 2) is used to represent the product, including its assembly using icons that represent the components and connectors. Figure 2. ADDS environment with the main worksheet above the AutoCAD worksheet. 7

8 The ADDS tool box is the user-interface for launching all engineering analysis applications that are available in ADDS. Below is a list of all applications in the toolbox in clockwise order beginning with the open book, each with a brief description. ADDS rule editor builds the life-cycle engineering knowledge bases. ADDS rule analyzer evaluates using the life-cycle engineering knowledge bases. CAD translator retrieves CAD procedures from the library and exports the design. Assembly validator examines the design according to the assembly rules defined in the assembly rule editor. Assembly rule editor defines assembly features between components. The component palette is a collection of icons representing components that are used in power transmission (our example). Similarly, the connector palette is a collection of connective icons that are used to describe the properties of the link. Connective icons represent assembly processes and operations. These connections are used in the assembly feature description and are essential in validating the links before exporting the design. A link is not valid until a valid connector connects it. The connective icons can be extended to other life-cycle engineering processes such as reliability, service, and retirement. The taxonomy of the palettes is shown in Figure 3. Although the component and connector palettes are limited to power transmission elements, other components and connectors can be easily added in the palette. 8

9 Figure 3. Pop-up component card of a gear. ADDS uses component cards to assign component attributes. Default information is assigned to the component upon creation. The component card is the user interface for obtaining component-specific information like features and geometry. Links can be instantiated as soon as components are instantiated. Detailed part design is not required for assembly modeling. The link card is the main user interface for displaying assembly information (Figure 4). Assembly features, defined with default information in the assembly features library, appear on the link card. There is a specific link card for each valid component-component-connector combination. 9

10 Figure 4. Link card with default parameters. The assembly rule editor is a GUI template for editing, defining, and modifying assembly features between two components for storage in the assembly feature library. The assembly validator is the user-interface for validating and correcting the assembly model in ADDS. The assembly validation system checks the link information and component information against the assembly conditions from the assembly database to check the feasibility of the assembly. Once assembly is validated, that means assembly modeling is feasible and the design is ready to export to the CAD system. The assembly validator allows the user to make design changes without exiting the validator. 10

11 Spatial and design evaluations are performed before exporting the design to AutoCAD. Spatial and design errors do not prevent exporting the design into AutoCAD. Feature violations, interference of components, and position violations are the three types errors identified in these evaluations. Additionally, there is the ability to add and edit additional rule bases for similar evaluation. Currently, a design for assembly and a manufacturing process selection database are added. Solid-assembly models are created using ADDS to represent feature information and geometric information. However, ADDS needs a translator to convert the component and link information to AutoCAD (Figure 5). The CAD translator exports the components individually into and generates the relative positions of components with respect to each using component and assembly information available. The CAD translator stores the methods developed to export the individual components in the CAD library. This allows flexibility in creating new parts as one can create new methods and store the methods in the CAD library. Design Attributes Feature: (Spur Gear) Keyway = x (Width x Depth) Web = x 5.00 (Depth x Diameter) Solid model Basic Geometry (Spur gear) (Shaft) No of teeth = 24 No of steps =2 Diametrical pitch = 20 Step1 Diameter = 1 inch Pressure angle = 20degree Step1 Length = 3 inch Bore diameter = 0.5 inch Step 2 Diameter = 0.5 inch Face width = 2.0 inch Step2 Length = 3 inch Hub diameter = inch Hub projection = inch CAD translator Convert Assembly Information Step number on which Gear is assemble = 2 X position of Gear on Step2 = 1.5 inch Gear 1 Link Shaft 1 Figure 5. Conversion of design attributes as solid assembly. 11

12 ADDS VALIDATION The stated objective of this project is that 80% of assembly and geometric information can be correctly exported into AutoCAD. The other 20% of assembly and geometric information can be easily added during embodiment design in AutoCAD. Three validation cases were used a speed reducer, a mini baja transmission, and a contouring machine transmission. The speed reducer validation is detailed and the overall results are explained. Speed Reducer Example The speed reducer example (Figure 6) is a gearbox in a gasoline-engine-powered portable air compressor (Norton, 1996). It is represented in the ADDS main worksheet in Figure 7. No rules were fired by the geometric evaluation and assembly validation modules. The rules fired by the design for assembly (DFA) and manufacturing process selection (MPS) analysis modules did not lead to design changes. The design was exported to AutoCAD (Figure 8). The CAD model was then physically examined to check which components were correctly exported to AutoCAD. 12

13 Bearing_gear1 Gear Shaft_gear Bearing_gear2 Speed Output Speed Input Bearing_pinion1 Shaft_pinion Pinion Bearing_pinion2 Figure 6. Speed reducer example. Figure 7. Speed reducer in ADDS. 13

14 Figure 8. Solid model of speed reducer example. A validation sheet was prepared consisting of five columns (Table 1). The total number of elements in the ADDS and exported designs are calculated and the validity of the design is calculated at the bottom of the chart. Table 1. Validity sheet for the speed reducer. 14

15 Table 1. Validity sheet for the speed reducer. Design Name Speed Reducer No. of Parts 8 No. of Unique Parts 3 No. of Links 7 No. of Unique Links 3 Elements Correctly Component/ Component/Link Elements in Exported to Link Type Name Element Name ADDS AutoCAD Gear Gear Hub(2) 4 4 Pinion Keyway(3) 6 6 Teeth #(1) 2 2 Gear type(2) 4 4 Bore(1) 2 2 Web(2) 0 0 Face width(1) 2 2 Shaft Shaft_gear Length(1) 2 2 Shaft_pinion Diameter(1) 2 2 Keyway(3) 6 6 Bearing Bearing_gear1 Inner race diameter(1) Bearing_gear2 Outer race diameter(1) Bearing_pinion1 Ball diamter(1) 4 4 Bearing_pinion2 Ball #(1) 4 4 Bore(1) 4 4 Facewidth(1) 4 4 Outer diamter(1) 4 4 Shaft_Bearing Link4 Bearing position(1) 4 4 Link5 Insertion(1) 4 4 Link6 Link7 Shaft_Gear Link2 Gear position(1) 2 2 Link3 Keyway(1) 2 2 Insertion(1) 2 2 Gear_Gear Link1 Center distance(1) 1 1 Inclination(2) 2 2 Meshing(1) 1 0 Total Validity 98.68% The speed reducer consists of 76 elements and 75 of these elements were exported to AutoCAD correctly. Therefore, the validity for this design is 98.68%. The meshing of the gear and pinion is the only element not exported to AutoCAD correctly. This is because the meshing rotation of gears in the program does not consider different sized gears. SUMMATION Three different power transmission system examples were represented in ADDS and the validity of ADDS in each case was calculated. Table 2 shows the average ADDS export validity calculated as 98.07%. This table indicates that ADDS can export almost all elements represented in the worksheet, and clearly more than the 80% goal. In addition, the 15

16 total validity was calculated by summing up the three examples as in Table 3. These figures show that ADDS can export all the elements represented in the ADDS worksheet. Table 2. Summary of ADDS validation. Table 2. Summary of ADDS validation Example Validity Speed reducer example 98.68% Transmission for mini baja 95.62% Transmission for contour machines % Average ADDS Validity 98.10% Table 3. Validity by design elements. Example Design Elements Design Elements Exported Correctly Speed reducer example Transmission for mini baja Transmission for contour machines Total ADDS Validity 98.05% Although ADDS can export 98.05% of the elements, there are three possible reasons why an assembly cannot be exported to AutoCAD. Lack of the CAD procedures in the CAD procedure library, to represent the component in AutoCAD. Lack of assembly information about two components from the assembly feature library. Inability to export complex spatial relationships or orientations. CONCLUSIONS Most of the commercial and academic assembly modelers have many drawbacks such as lack of flexible design, lack of a good user interface, and lack of concurrent engineering ability. Using ADDS a designer can perform the conceptual design and then export nearly all elements to AutoCAD for a better view and return to ADDS to implement any modifications if needed. This process can be repeated until the designer is satisfied with the 16

17 design. ADDS provides not only a good user interface, but also flexibility in design by using some degree of intelligence to assign default values when a component or a connector has been created. The rule-based system and the solid modeling system together create an integrated environment for design in ADDS. The solid modeling system can export the design to AutoCAD as an assembly drawing representing all of the relationships among the components or as an individual component drawing without representing the relationships among the components. Recommendations for Future Research Currently, ADDS's scope is limited to mechanical power transmission systems. ADDS can be easily extended to the other mechanical systems by including new components and connectors. ADDS does not support tolerance information. Incorporating tolerance information into ADDS would automate the assembly and solid modeling of components completely. The design representation in ADDS can become cumbersome if the assembly is too big for the screen size. The design representation may have to be reviewed if ADDS will be used to represent very big assemblies. REFERENCES Cantania, G Form-features for mechanical design and manufacturing. Journal of Engineering Design, no. 1: Cunningham, J.J. and J.R. Dixon Designing with features: The origin of features. In Computers in engineering conference: American Society of Mechanical Engineers Proceedings, ASME, San Francisco. 17

18 Dixon, J. R., J. J. Cunningham, and M. M. Simmons Research in designing with features. In Intelligent CAD I: Proceedings IFIP TC 5/WG 5.2 workshop on intelligent CAD, edited by H. Yoshikawa and D. Gossard, New York. Finger, S., and J. R. Dixon A review of research in engineering design, Part 1: descriptive, prescriptive, and computer based models of design processes. Research in Engineering Design 1: Ganesan, R., and V. Devarajan An approach to extracting intersecting features from 2-D CAD. In Concurrent Product Design and Environmentally Conscious Manufacturing: American Society of Mechanical Engineers proceedings, ASME, vol. 5, San Francisco. Gui, J. and M. Mantyla Functional understanding of assembly modeling. Computer- Aided Design 26, no. 6: Lee, K. and D.D. Gossard A hierarchical data structure for representing assemblies: Part1. Computer Aided Design 17, no. 1: Libardi, E.C., J.R. Dixon, and M.K. Simmons Computer environments for the design of mechanical assemblies: A research review. Engineering with Computers 3: Mantyla, M A modeling environment for top-down design of assembled products. IBM RC New York. IBM Corp. Ng, C.L Attribute-based design description system. MS thesis, University of Alabama. Ng, C.L. and J.K. Gershenson Attribute-based design description system. Research Report #LEL The University of Alabama. Norton, R.L Machine design and integrated approach. Upper Saddle River: Prentice-Hall. Salomons, O.W., F.J.A.M. Van Houten, and H.J.J. Kals Review of research in feature based design. Journal of Manufacturing Systems 12, no. 2: Shah, J.J. and M. Mantyla Parametric and feature-based CAD/CAM. New York: John Wiley & Sons, Inc. 18

19 Shah, J.J. and M.T. Rogers Expert form feature modeling shell. Computer-Aided Design 20, no. 9: Shah, J.J. and M.T. Rogers Assembly modeling as an extension of feature based design. Research in Engineering Design 5: