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A Formal Methodology for Smart Assembly Design
A Presentation by
Kris Downey – Graduate Student
Alan Parkinson – Faculty Member
15 June 2000
Acknowledgements to NSF Grant 0084880
Presentation Outline
Introduction Research ObjectivesCurrent design techniques and analysis
methodsCase studyCurrent status of researchConclusions
Robust Design
Design that works properly when subjected to variation
Current robust design methodsFocus on key characteristicsFMEADOE by TaguchiSix sigma analysisOptimization techniques
Smart Assembly
A smart assembly has features, not otherwise required by the function of the design, which allow the design to absorb or cancel out the effects of variation [Parkinson, 2000]
Smart Assembly
Examples of smart assembly featuresSlotted holesSprings used for positioningScrew locatorsSliding locatorsShims
Smart AssemblyTypes of smart assemblies
Shim
Screw
Inclined Plane
Passive
Smart AssemblyTypes of smart assemblies
Passive
Can Opener Support
Smart AssemblyTypes of smart assemblies
Passive
Car frame
Smart AssemblyTypes of smart assemblies
Active
Scissors Gear
Smart AssemblyTypes of smart assemblies
Active
Garage Door Roller Bearing
Research Objectives
Create a smart feature implementation methodologyPrinciples developed for methodologyProven analysis methods implemented into
methodology
Apply methodology to case studies
Current Design Techniques and Analysis Methods
Exact constraint designVery close relationship to smart
assembly designScrew theory
Constraint information inferred from analysis
Tolerance analysisCritical dimension information provided
Exact Constraint Design
Degrees of freedom
6 in 3D space3 in 2D space
Exact Constraint Design
ConstraintA mechanical connection between objects that
reduces the degrees of freedom of each object
2D constraints 3D constraints
Exact Constraint DesignExactly one constraint for each degree
of freedom
No two constraints collinear No four constraints in a single plane No three constraints parallel No three constraints intersect at a point
No four constraints are parallel No four constraints intersect at a point No four constraints in the same plane
Rules for exact constraint design:
2D space 3D space
More mathematically-based theory is needed in this area
Taken from [Blanding, 1999] and [Skakoon, 2000]
Exact Constraint DesignExactly one constraint for each degree
of freedom
Exactly constrained
Overconstrained
Underconstrained
Exact Constraint Design
Conclusions:Exact constraint design very closely related to
smart assemblySmart assembly provides solution to
overconstrained designs
Screw Theory
Allows determination of over- or underconstrained designs
Motion analysisPerformed on individual part of assemblyAssumes parts do not break contactEach joint type has distinct screwmatrices
Taken from [Ball, 1900], [Roth 1966], [Konkar, 1993], and [Adams, 1998]
Screw Theory
Steps to constraint analysisTranslate joints into twistsFind reciprocal wrench of each twistmatrixUnion of wrenchesFind reciprocal twist of wrenchResultant twistmatrix
Screw Theory Interpretation of resultant twistmatrix
and wrenchmatrixTwist rows = degrees of freedomWrench rows = overconstraints
Motions
ConstraintsY-Translation
Z-Translation
X-Rotation
Y-Rotation
Z-Rotation
Screw Theory
Conclusions:Screw theory can determine if a design is over-
or underconstrainedLocation of idle degrees of freedom identified Information useful to smart assembly design
Tolerance Analysis
Vector loop analysis (DLM)Sensitivity matrix
Results provide useful information regarding critical part dimensions
80% of variation in angle is attributed to dimension a
Conclusion: Sensitivity matrix determines preferred location of smart features
Case Study
Baffle designOverconstrained in z-direction
Taken from [Kriegel, 1994]
Taken from Kodak design problem
Case Study
Results:Less deflectionDoes not address
constraint problemTabs tear from baffle
Taken from [Kriegel, 1994]
Solution #1Reinforce baffle
and frame
Case Study
Solution #2Double screw Smart feature
Results:Variation absorbedNo deflectionExpensive parts Increased assembly
costs
Taken from [Kriegel, 1994]
Case Study
Solution #3Double-slotted tabs Smart feature
Results:Variation absorbedNo deflection “No cost” solutionMinimal assembly
costs
Taken from [Kriegel, 1994]
Case Study
Conclusions:Worst case tolerance analysis contributed to
need of smart featuresOverconstrained design was not initially
recognizedSmart features allowed variation absorptionSome smart features are more expensive than
others
Current Status of Research
Principles of smart assembly being considered and explored Implementation as nesting forcesAbsorption of tolerancesElimination of overconstraintUse in designs where redundant constraints
are necessary
Smart Assembly PrinciplesSmart assembly features as nesting forces
Eliminate mechanical play and assembly stresses
Rigid constraint
Smart feature
Smart Assembly PrinciplesTolerance absorption
When other methods are not sufficient
Part A
Part B
Part A
Part B
Total gap No gap
Spring
Smart Assembly PrinciplesRedundant constraints become smart features
Eliminate assembly stresses
ConclusionsResearch Objectives:
Create a smart feature implementation methodology
Principles inferred from existing designs: Smart features replace overconstraints Smart features absorb tolerances in exactly
constrained designs Smart features implemented as nesting forces
Proven analysis methods adapted for methodology Exact constraint design, screw theory and
tolerance analysis
Apply methodology to case studies Indirect absorption of variation
