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ATTRIBUTE-BASED DESIGN DESCRIPTION SYSTEM IN

DESIGN FOR MANUFACTURABILITY AND ASSEMBLY

Srinivas Paluri1 and John K Gershenson2

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

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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:

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• 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,

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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.

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• 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 attribute-

based 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.

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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

RuleEditor

DesignEvaluator

KnowledgeBases

UserInterface

Attribute-based Design DescriptionEnvironment

MaterialDatabase

Design Worksheet

ComponentCard

ComponentLibrary

ComponentPalette

ConnectorPalette

LinkCard

AssemblyFeatureLibrary

Assembly Rule-based System

AssemblyRule Editor

AssemblyValidator

AssemblyConditions

EndUser

Solid Modeling

3D Solid Model(Assembly Drawing)

CAD Translation(ActiveX Automation)

GeometryValidation

Design ExportInterface

AssemblyValidation

Component/Assembly

Library

CADLibrary

Figure 1. System architecture of modified ADDS.

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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.

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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.

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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.

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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.

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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.

Solid model Feature: (Spur Gear) Keyway = 0.250 x 0.125 (Width x Depth)Web = 0.670 x 5.00 (Depth x Diameter)

Basic Geometry (Spur gear) (Shaft) No of teeth = 24 No of steps =2Diametrical pitch = 20 Step1 Diameter = 1 inchPressure angle = 20

degree Step1 Length = 3 inch

Bore diameter = 0.5 inch Step 2 Diameter = 0.5 inchFace width = 2 .0 inch Step2 Length = 3 inchHub diameter = 1.0625 inchHub projection = 0.4375 inch

Assembly Information Step number on which Gear is assemble = 2X position of Gear on Step2 = 1.5 inch

Gear 1

Design Attributes

Convert

CADtranslator

Shaft 1Link

Figure 5. Conversion of design attributes as solid assembly.

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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.

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Speed Input

Speed Output

Gear

Pinion

Shaft_gear

Shaft_pinion

Bearing_gear1

Bearing_gear2

Bearing_pinion1

Bearing_pinion2

Figure 6. Speed reducer example.

Figure 7. Speed reducer in ADDS.

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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.

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Table 1. Validity sheet for the speed reducer.

Design Name Speed ReducerNo. of Parts 8No. of Unique Parts 3No. of Links 7No. of Unique Links 3

Component/Link Type

Component/LinkName Element Name

Elements inADDS

ElementsCorrectly

Exported toAutoCAD

Gear Gear Hub(2) 4 4Pinion Keyway(3) 6 6

Teeth #(1) 2 2Gear type(2) 4 4Bore(1) 2 2Web(2) 0 0Face width(1) 2 2

Shaft Shaft_gear Length(1) 2 2Shaft_pinion Diameter(1) 2 2

Keyway(3) 6 6

Bearing Bearing_gear1 Inner racediameter(1)

4 4

Bearing_gear2 Outer racediameter(1)

4 4

Bearing_pinion1 Ball diamter(1) 4 4Bearing_pinion2 Ball #(1) 4 4

Bore(1) 4 4Facewidth(1) 4 4Outer diamter(1) 4 4

Shaft_Bearing Link4 Bearing position(1) 4 4Link5 Insertion(1) 4 4Link6Link7

Shaft_Gear Link2 Gear position(1) 2 2Link3 Keyway(1) 2 2

Insertion(1) 2 2

Gear_Gear Link1 Center distance(1) 1 1Inclination(2) 2 2Meshing(1) 1 0Total 76 75Validity 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

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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 ValiditySpeed reducer example 98.68%Transmission for mini baja 95.62%Transmission for contour machines 100.00%Average ADDS Validity 98.10%

Table 3. Validity by design elements.

ExampleDesign

ElementsDesign Elements

Exported CorrectlySpeed reducer example 76 75Transmission for mini baja 134 128Transmission for contour machines 146 146Total 359 352ADDS 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

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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.

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