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ME 395 Introduction to Mechanical Design
Generation and Evaluation of Design Concepts
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Stages in Design Problem Definition Information Gathering Concept Generation Evaluation of Concepts
Conceptual Design is then followed by Embodiment Design
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Stages in Concept Generation Problem Decomposition
Physical Decomposition Functional Decomposition
Exploration of Solutions: Brainstorming Morphological Box or Chart
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Creativity in Design Particularly important in concept generation Poorly understood Creative ideas emerge slowly often”half-baked” Aided by deliberate process “10% inspiration 90% perspiration” (Edison)
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Enhancing Creativity Develop a creative attitude (self-confidence) Unlock your imagination Be persistent Develop an open mind Suspend your judgment Critical vs Creative thinking (Dieter pp 199)
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Conceptual Decomposition Physical Decomposition
Breaks device into “chunks” Closely related to design segmentation and
to sub-assemblies Functional Decomposition
Separates and analyses functions that product performs
Treats flow of force, energy, material, information etc.
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Physical Decomposition e.g. Carpenter’s hammer
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Steps in Functional Decomposition State the most important function that the
product serves Create descriptions of sub-functions Arrange sub-functions in logical order Refine (decompose) sub-functions as
necessary
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Functional Decompositione.g. carpenter’s hammer Overall function: Drives and pulls nails ? (2 separate functions)
Driving nails - dynamic Pulling nails - static
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F D of hammer (cont.) Functions of parts in driving nail: Head
Stores energy Applies force to nail
Shaft Transmits force Multiplies velocity Absorbs shock Provides guidance
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F D of hammer (cont.) Functions of parts in pulling nail: Head
Transmits torque Applies force to nail Applies reaction to wood
Shaft Accepts hand force Multiplies force Applies torque to head Provides force feedback
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F D of hammer (cont.) Functions of features of head in driving nail: Body
Connects features Stores energy (provides mass)
Striking face Transmits force
Hole Connects to shaft
Claw None?
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F D of hammer (cont.) Functions of features of head in pulling nail: Body
Connects features Transmits force and torque
Striking face none
Hole Connects to shaft
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F D of hammer (cont.) Functions of features of head in pulling (cont.) Claw (groove)
Applies force to nail Claw (curved surface)
Applies force to wood
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Generating Design Concepts Now starts the synthesis process Synthesis based on all analysis done to date Individual consideration of problem often initially
better than team Brainstorming with team very effective Treatment of sub-functions separately then
combination using morphological box or chart often work well
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Morphological box Method of treating combinations of ways
of satisfying sub-functions Discrete design space with one axis
(dimension) per sub-function Can be used directly if small number of
sub-functions or concept for their satisfaction
Concept useful for more complex cases
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Example of Morphological Box
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Morphological Chart Used for more general case with many
dimensions (sub-functions) See Dieter pp.235-236 for disposable
syringe example
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Axiomatic Design Uses to Axioms as basis for design: Axiom 1- Independence Axiom “Maintain the independence of functional requirements” Axiom 2- Information Axiom “Minimize the information content”
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Concept Evaluation Used to pick best conceptual designs for future work Better evaluation at preliminary design
stage Possible (non-exclusive) approaches:
Absolute filters (weeds out losers) Rating process (ranks survivors)
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Evaluation by Absolute Filters Several possible criteria for evaluation: Feasibility of the Design Assessment of Technology Readiness Screening using Customer Requirements
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Filtering based on Feasibility Places designs in three categories: Not feasible (try to fix or abandon) Conditional (resolve condition) Feasible (keep for further development)
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Filtering based on Technology Design is Feasible if: Can be made with existing processes Critical functional parameters identified Sensitivity of parameters known Failure modes identified Existing hardware demonstrating above
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Filtering based on Customer Compares capability of product to meet each customer requirement and places design in one of three categories: Meets all customer requirements (keep for further development) May meet all requirements (keep and try to fix) Clearly does not meet all requirement (abandon)
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Concept Rating (Pugh) Process steps: 1. Choose evaluation criteria 2. Formulate decision matrix 3. Clarify design concepts 4. Choose datum concept 5. Generate the matrix 6. Evaluate the ratings 7. Eliminate and refine concepts 8. Repeat steps 4 to 7 with reduced or modified list of concepts
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Formulating the Decision Matrix Choose evaluation criteria ( extent of
coupling and information content could be two of the evaluation criteria)
Enter criteria as rows Concepts are columns Same level of abstraction for concepts Use sketches if hard to describe
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Clarifying Design Concepts Try to bring all members of team to same
level of understanding of concepts Develop team ownership of all concepts
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Choosing Datum Concept Select a concept as datum for initial
evaluation Concept with which all others are
compared Generally by nomination and voting Should be one of the better concepts For redesign, datum is original design
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Generate the Matrix Each concept and is compared with datum
under each criterion Three level scale:
Better (+ or 1) Same (S or 0 Worse (- or -1)
Usually involves useful discussion
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Evaluate the Ratings Determine number of each rating for each
concept (+’s, S’s and -’s) Eliminate really bad concepts Be generous Consider moving good features of a concept to
other designs Look at strengths of highly rated concepts Try to remove ambiguity
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Reiteration Revise list of design concepts Establish new datum (usually the highest rated concept) Rerun the matrix, evaluate etc Plan further work as necessary