AE 207
Introduction to Engineering Design
Concept Evaluation/Selection
Dept. of Aerospace Engineering
IIT Bombay
Journey So Far
• Need/Mission statement/Design Brief
• Stakeholder Identification – Active, Passive
• Requirements capture, grouping & prioritization –
Stakeholder needs
• Functional Decomposition
• Concept Generation – Number of alternate solution
concepts attempting to satisfy the need
Next Step
• Decide which conceptual solutions are good
potential candidates for further investigation/
product development
– ALL ?
– Some ?
– One ?
Resources, Time, Risk, Policy,
Culture
• Concept Evaluation
– Evaluation criteria
– Compare generated concepts: strengths &
weaknesses wrt criteria
• Concept Selection
– Decision making: Eliminate, Retain, Combine, Modify
– One Shot / Stages/ Iterative (?)
– Final Selection
Difficulties/Issues
• Limited knowledge/data
• Conceptual solutions - Rough/abstract
ideas
• Concensus among team members
• Other ??
Some methods
• External Decision – company chairman?, customer
• Product Champion – personal preference
• Voting – team concensus (subjective)
• Intuition – feel good factor
• Prototype & test – Expensive & time consuming
• Decision Matrices – relatively more objective
Process overview
• Initial filtering – qualitative, absolute comparison
• Quantitative screening based on decision matrix
– Pugh Matrix
–
–
–
–
–
Prepare matrix
Rate concepts
Rank concepts
Combine/improve/retain promising concepts
Select one or more concepts
• Concept Scoring – similar steps, weighted
scores, more refinement and advancement over
Pugh matrix
Evaluation Criteria
• Extracted from stakeholder requirements
• Identify a few key requirements through
prioritization
– Primary, secondary, tertiary
• Level of abstraction/refinement of requirements
must be consistent with concept abstraction/
refinement
There is No Sense in Being Exact About
Something if You Don’t Even Know What You
are Talking About
(John von Neuman, 1950)
Initial Filtering (absolute)
(In case of large number of concepts)
• “Gut feel” feasibility judgment
– Comparison made with prior experience
(“design knowledge”)
• Concept will not work
– Too radical
– Not original
• Conditional feasibility – something else required
• Feasible - Worth considering
• Go - No go screening
• Customer requirements
• Technology readiness/maturity/availability
Pugh Matrix
• Select one concept as a reference or datum
–
–
–
–
Familiar, straightforward concept
Existing similar benchmark product in market
Existing product in case of re-design/ modification problem
Arbitrarily picked from the generated concepts
• Pair-wise relative rating of each concept vs datum for
each criteria
– Better than datum: +
– Same as datum:
0
– Worse than datum: -
• Count number of pluses and minuses for each concept.
Overall rating is difference between no. of pluses and no.
of minuses.
• Rank the concepts based on overall rating
Pugh Matrix Structure
Concepts
Selection Criteria
A
B
C
D
E
F
G
Handling ease
0
0
-
0
0
-
-
Ease of use
0
-
-
0
0
+
0
Readibility of settings
0
0
+
0
+
0
+
Metering accuracy
0
0
0
0
-
0
0
Durability
0
0
0
0
0
+
0
Ease of Manufacture
+
-
-
0
0
-
0
Portability
+
+
0
0
+
0
0
Sum of +’s
2
1
1
0
2
2
1
Sum of –’s
0
2
3
0
1
2
1
Net rating
2
-1
-2
0
1
2
1
Rank
1
6
7
3
2
3
3
Continue?
Yes
No
No
Comb
Yes
Comb
Rev
Concept Scoring Matrix
• Select one concept as a reference or datum
– Best in Pugh Matrix
• Select weights for each criteria
– Importance rating of each criterion: scale of 1 to 5 or %age with total =
100% etc.
• Relative rating of each concept vs datum for each criteria. Datum
maybe different for each criterion
–
–
–
–
–
–
•
•
•
•
Much better than datum: 5
Better than datum:
4
Same as datum:
3
Worse than datum:
2
Much worse than datum: 1
Finer scales (1 to 9) etc. may also be used.
Weighted score for each concept – criterion pair
Rank the concepts based on total score (sum of weighted scores)
Combine/ Improve/ Retain concepts
Select one or more concepts
Concept Scoring Matrix Structure
Concepts
Selection Criteria
A
DF
E
G+
Weights
R
Scor
R
Scor
R
Scor
R
Scor
Handling ease
3
0.15
3
0.15
4
0.2
4
0.2
5%
Ease of use
3
0.45
4
0.6
4
0.6
3
0.45
15%
Readibility of settings
2
0.2
3
0.3
5
0.5
5
0.5
10%
Metering accuracy
3
0.75
3
0.75
2
0.5
3
0.75
25%
Durability
2
0.3
5
0.75
4
0.6
3
0.45
15%
Ease of Manufacture
3
0.6
3
0.6
2
0.4
2
0.4
20%
Portability
3
0.3
3
0.3
3
0.3
3
0.3
10%
Total Score
2.75
3.45
3.10
3.05
Rank
4
1
2
3
Continue?
No
Develop
No
No
Selection process
Generation
Screening
Scoring
Selected
Additional Reading
• Product Design & Development, Ullrich &
Eppinger – Chapter 7
• The Mechanical Design Process, Ullman –
Chapter 8