Week 06 Assembly Modeling

7/29/2019 Week 06 Assembly Modeling

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Assembly

Modelling


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Assembly Modelling

Assembly modellers which form part of anintegrated product design system.

Additional benefits include the ability to checkfor interference between parts.

Assembly modeling allows the integration of

design and manufacturing to productionplanning and control.


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Assembly Modelling

Bill-of-materials (BOM) can be generated for eachsubassembly and assembly.

BOM is a full definition of how a product is

structedContext of

materials requirements planning (MRP)

manufacturing resource planning (MRP II).


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Assembly Modelling

Relationship data includes

Constraint information

orientation and location of components with

respect to one another variational relationship between features of

different parts


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Assembly Modelling

Relationship data includes ...

Instancing information

multiple occurrences of the same component

Tolerance and fit information

part interference and clearance


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Assembly Model Used for ...

Creation of orthographic assemblydrawings.

Creation of exploded assemblies.Facilitate packaging

Perform interference and clearance

checks.


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Part/Sub-assembly Placement

Examples of constraints applied to assemblecomponents

alignment

surfaces, axes with offset distances

mating surfaces, edges

coincidence points, edges


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Assembly modeling

Parts modeling and representation

Hierarchical relation Mating conditions

8


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Assembly modeling

Dependency among parts

Hierarchical relation and mating

condition Mating condition and spatial relation

Assembly analysis: interference(Mating

tolerance), mass property, kinematics/dynamics

Design change=> auto updates of the

assembly


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Generation of an assembly model


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Representation of Parts model

Individual part

Geometric modeling Attributes


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Simplified Representation

Large assembly models can serious stain theprocessor capabilities of the hardwaresystem being used.

In these cases, many assembly modelers providemeans for simplifying the assembly.

The use of instancing helps reduce complexity.


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Assembly tree


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Another tree

Assembly

Subassemblies

Parts

Position& orientation


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Assembly Hierarchy

The assembly hierarchy definesrelationships of parts to each other

There can be multiple levels of sub-assemblies


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Assembly Hierarchy Example

Pump Assembly

Link assembly

offset link

offset link Piping assembly

well casing

well seal

Handle assembly

...

subassembly

Multipleinstances


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Positioning Parts in an Assembly

Parts can be positioned by translating androtating them into the right locations

This requires careful measurement of relative

locations, knowledge of coordinate systems,and entry of numerical values

If position or dimensions of one part change,this has to be redone


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Outline . . .

Base part: Use jig and fixture Modeling, representation, construction

sequence, analysis of assembly

Hierarchical relationship among parts Design change of a part affects the

assembly

kinematics, dynamics


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Design for assembly


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Assembly Constraints

Constraints can be used to create permanentrelationships between parts

THEY use the same commands as 2D constraints

Typical constraints: two faces meet

axes coincident

two faces parallel at fixed distance

etc.


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Assembly Constraints

Concentric

Coincidence

Parallel

Mating


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Assembly of electric clutch


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Assembly tree ofelectric clutch


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Mating condition

Part coord. MCS (modeling coord)

Base part: Datum

Global CS Local CS

Explicit position and direction vs.Mating conditions

4 x 4 homogeneous transformationmatrix


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Positions ofeach part


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Assembly

viaGCS


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Mating condition . . .

Exploded view Instead of given transformation matrices User friendly: transformation matrices By changing conditions the assembly can be

changed Assemblability

Mating feature Types: against, fits, contact, coplanar fits: center lines are concentric


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against condition


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Mating condition . . .

Mating condition = mating type + twofaces

Normal vector + one point on the face

against: two normal vectors are inagainst directions

fits: between two cylinders: centerlines are concentric

Against and fits allows rotation andtranslation between parts


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Fits condition


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Against and fits

Against and fits do not allow relativemotion between parts: contact and tightfits

Contact condition requires two pointscorrespond: Rotation is allowed around thecontact point

First index = face number, 2nd index = partnumber: between two parts againstconditions among 3 faces, contact conditionbetween Ps


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contact condition


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Interference fit

Fits is clearance fit,

tight fits is interference fit

Coplanar: two normal vectors areparallel

Coplanarcomplements against


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coplanar condition


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Calculating positions frommating conditions

Matrix calculation to find the positionand orientation of each part

Solve the system of equations Under constrained, Over constrained

Over constrained: More eqs than

number of free variables, eliminateredundant eq. or least means square.


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Positions from mating

conditions. . . 3 mating conditions: against, fits,

coplanar

against : 2 normal vectors and 2 points 4 equations

n1 = - n2

n1 . (P1 P2) = 0 : n directional component= 0


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against cond.


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Fits and coplanar Fits: Centerlines are on

the same line 2 eqs are redundant

among 6 eqs

Coplanar: same as againstexcept that normals areopposite direction

If free rotation isallowed the matrixcalculation becomesdiverge:

f = 0 : Constrain the freerotation


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Solving the equation

System of nonlinear eqs

Least square : remove redundancy

Newton-Raphson Jacobian matrix(Fi/xk, 1st partial

derivative, approximating linear mapping)

Linearization


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Pin and blockAssembly


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Assembly

frominstances


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Representation Scheme

Graph Structure Location Graph

Virtual Link


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Representation Scheme

An assembly database stores the geometric models of individual parts spatial positions and orientations of the

parts in assembly the assembly or attachment relationships

between parts

The inherent problem; how to assign assembly data

interactively to build or develop theassembly


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Representation Scheme . . .

The way the user provides the assemblydata the locations and orientations of the various

parts. their hierarchical relationship.

2 type schemes ; utilize the WCS method : Graph structure Location graph utilize the mating conditions method ->

Virtual link


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Graph Structure

Use WCS (working coord. sys.) method.

Each node represents an individual parts/ subassembly.

Arc (branch) represents relationshipamong parts


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Graph Structure . . .

P : part-of relationship Logical containment of one object in

another.

A : attachment relationship C : constraint relationship

physical constraints

SA : sub-assembly relationship


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A : attachment relationship

rigid attachment: no relative motion non-rigid attachment

relative motion parts cannot be separated by an arbitrarily

large distance useful in mechanisms analysis (joints)

conditional attachment related to parts supported by gravity, but

not strictly attached


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Graph structure ofelectric clutch


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Location Graph

WCS method

Location of a part is a relative property.

A chain of locations can be defined suchthat each location is defined in terms of[T] another part's coordinate system:Relativeposition.

A set of these chains results in alocation graph.


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Location graph ofelectric clutch

G h


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Location Graph . . .

The 'frame' CS (coord. sys) is ; the root of the location graph

taken as the global CS of the assembly .

Related by the transformation matrix[T]

V l L k


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Virtual Link

Mating conditions. Graph structure + location graph

requires the user to input

transformation matrix. Virtual link requires more basic

information (mating conditions)

used to calculate the transformationmatrices Based on the concept of virtual link.

Vi l Li k


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Virtual Link . . .

Virtual link: defined as the complete setof information required to describe thetype of attachment and the mating

conditions between the mating pair. In the assembly data structure;

any mating pair is connected by a virtuallink

if more than 2 parts are mutually related several virtual links

Assembl raph structure


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Assembly graph structurebased on virtual link

Vi l Li k


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Virtual Link . . .

Top node

The leaf nodes are the parts of theassembly

The geometric data for each one ofthese parts (leaf) are connected tothese leaf nodes via pointers in the datastructures


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Data structure

based onvirtual link

Graph structure of


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Graph structure ofelectric clutch based on virtual link


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Data structure ofclutch

based on

virtual link


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Generation of AssemblingSequences

Precedence Diagram Liaison - Sequence Analysis

Precedence Graph


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Generation of Assembling

Sequences In most assembly, there are multiple

assembly sequence: Not unique=> must decide on the most optimum

sequence

Assembling sequences of cars, ships,missile, Lego blocks

A bl ff t


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Assembly sequence affects

difficulty of assembly steps need for fixture potential for parts damage during

assembly and part mating ability to do in-process testing occurrence of the need for reworking

time of assembly assembly skill level unit cost of assembly

A bli S


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Assembling Sequences . . .

Trial-and error is practically impossiblebecause; large # of valid sequences

minor design changes can drastically modifythe available choice of assembly sequences:design for production

Few techniques: manual vs. algorithmic

P d Di


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Precedence Diagram

Designed to show all the possibleassembly sequences of a product.

Each individual assembly operation isassigned a number.

Diagram is usually organized intocolumns in column I : the base part


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Precedence diagramof electric clutch

P d Di


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Precedence Diagram . . .

Many operations impractical on a single assembling

machine

=> treat as sub-assemblies

easily studied & evaluated

multiple assembling machines can be used.

Li i S A l i


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Liaison - Sequence Analysis

Use precedence relations;in precedence diagram: engineer

generates possible sequence directly

in liaison method: asks a series ofquestions to engineers about matingcondition and precedence.

=> generate sequences: manually oralgorithmically

Li is n S qu nc An l sis


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Liaison - Sequence Analysis . . .

Develops all possible assemblysequences in 2 steps ;

1) characterize the assembly by anetwork (liaison diagram)

2) generate all possible assembly

sequences and represent them inliaison-sequence graph.

Liaison Sequence Analysis


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Liaison - Sequence Analysis . . .

Node: represent partsLine: represent any mating conditions

between parts.

The liaison sequence diagram and list arenot unique.

Determine the precedence relationships

between liaisons by asking andanswering.Generate assembly sequences.


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Liaison-sequence graphfor electric clutch

Precedence Graph


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Precedence Graph

Fully automaticBased on virtual-link data structureRequires the mating conditions as input.

Assembly sequence is generated with theaid of interferencechecking.

1) input mating conditions.

2) generate mating graph


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Mating graph for clutch

Precedence Graph


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Precedence Graph . . .

identify the part that is connected to thelargest # of parts by virtual links => thebase part.

gather all the parts directly connected tothe base part by virtual link. 1st step: a part hierarchy is developed

based on the type of virtual link. 2nd step: develop the precedence graph

with interference checking

Part hierarchy of clutch assembly


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Part hierarchy of clutch assembly

P d h f l h bl


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Precedence graph of clutch assembly


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Assembly Analysis

Analysis tools ;- generation of assembly drawings- exploded views of assemblies

- shaded images of assembly- cross sectional views- mass property calculations- interference checking

- kinematics & dynamic analysis- finite element analysis- animation & simulation

E l d d i f i l j i t


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Exploded view of universal joint

A bl i f i l j i t


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Assembly view of universal joint

A li i f A bl M d l


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Applications of Assembly Models

Interference checkingVisualization

rendered

explodedAnimation

Mechanism analysis

A i i d M i


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Animations and MotionSubmit the mechanism tothe embedded simulationengine, so it can determinehow the mechanism willperform and behave.

The results of thesimulation as an animationshowing the motion of yourmechanism or as numeric

output.

M i T


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Motion Type

ConstantDisplacement

Constant Velocity Constant

Acceleration Motion Expression

M h i D i


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Mechanism Design

Types of joints: rotational sliding spherical cylindrical screw

A mechanism is an assembly that allows relativemotion between parts

The mechanism consists of linksconnected byjoints

Mechanism


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Joints

Joint TranslationalDOF RotationalDOF Total DOFConstrained

Revolute 3 2 5

Translational 2 3 5

Cylindrical 2 2 4Spherical 3 0 3

Universal 3 1 4

Screw .5 .5 1

Planar 1 2 3Fixed 3 3 6

Point-curve 2 0 2

Curve-curve 2 0 2Contact

Constraints:

Revolute J oint Translational J ointMechanism


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J J

Cylindrical J ointSphericalJ oint

Joints


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Mechanism


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Constraints and Joints

Planar J oint

Fixed J oint

Point/Curve Constraint

M h i A l i


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Mechanism Analysis

The mechanism to be drivenby defining an input motion forone of the links

The other links move based onthe kinematics

Reaction forces based ongravity and applied forces

calculated

D i A l i


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Dynamic Analysis

More advanced software can model andanalyse complete dynamics

Applications:

vehicle dynamics suspension analysis

biomechanics

robotics

A Simple Example:


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mp E mpPendulum

Dynamic analysissoftwarewill generate themotionby solving theequations

of motion

Other Examples:


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mp

Dynamic analysis software will generate the

motion by solving the equations of motion

Radial Piston Air Motor Motorcycle

Other Examples:


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p

Dynamic analysis software will generate the

motion by solving the equations of motion

Radial Piston Air Motor Piston rod

Intelligent Assembly Modeling


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Intelligent Assembly Modelingand Simulation

The goal of IAMS is to avoid this expensive and time-consuming process by facilitating assemblability checkingin a virtual, simulated environment.

In addition to part-partinterference checking, theIAMS tool will check for

tool accessibility, stability,and ergonomics.

Intelligent Assembly Modeling


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Intelligent Assembly Modelingand Simulation

Week 06 Assembly Modeling

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