concept selection

Concept Selection

Product Design and DevelopmentChapter 8

Karl T. Ulrich and Steven D. Eppinger

We are spending most of our time on concept development. Concept development is subtask of product development.

Perform Economic Analysis

Benchmark Competitive Products

Build and Test Models and Prototypes

IdentifyCustomer

Needs

EstablishTarget

Specifications

GenerateProduct

Concepts

SelectProduct

Concept(s)

Set Final

Specifications

PlanDownstreamDevelopment

MissionStatement Test

ProductConcept(s)

DevelopmentPlan

PDS complete

Must use goals from PDS

Task for class: List activities that you have performed in each of the boxes for your wind turbine design.

Concept Development Funnel

concept generation

concept screening

concept scoring

concept testing

Concept selection is as much about documenting your process as making the selection.

• Concept Screening is used in the early phases of design when you don’t have a lot of numeric data.– Pugh Concept (your book)– Modified Pugh Concept (this class)

• Concept Scoring is used later in the design process when you have made more decisions about the design.– Weighted decision matrix based on relative ranks– Weighted decision matrix based on scaling – all values must

be numeric or your can’t use this approach.

Your text identifies six steps in the concept selection/screening process.

1. Prepare the Matrix (Select the correct approach)– Criteria– Weightings

2. Rate Concepts– Scale (+ – 0) for concept screening– or (0-4; 0-10) for concept scoring– Compare to Reference Concept or Values

3. Rank Concepts– Sum Weighted Scores

4. Combine and Improve– Remove Bad Features– Combine Good Qualities

5. Select Best Concept– May Be More than One– Beware of Average Concepts

6. Reflect on the Process – does it make sense??

Your text differentiates between concept screening and concept scoring!

• Concept screening is used to narrow the number of concepts quickly and to improve concepts.– Your text uses Pugh Concept Selection. We will use a

modified technique used by IDEO.– This technique is used when you don’t have a lot of

information about your design and you must make decisions.

• Concept scoring is used when increased resolution will better differentiate between concepts.– Your text uses an unusual technique. We will use the

technique introduced in freshmen design (except that you will use it better.)

– Concept scoring should be used when you have more information about your design.

For this class and senior design, you are selecting between concepts that meet the constraints. If a concept does not meet the constraints, it is unacceptable.

This means that you use your goals to make your decision.

The extent to which you can meet your most important goals will guide your design.

There are cases when an item is both a goal and constraint.

The first step is to prepare the matrix, including selecting the correct technique. We are using concept screening.

• Concept screening works best with 20 or fewer criteria.

• Criteria should be based upon the PDS – pay attention to customer needs.

• You may have to pick the most important criteria to obtain the desired limit.

• Criteria

Lightweight

Stable

Pinch Points

Maintenance

Range of Motion (legs)

Comfort

As part of the first step we determine the criteria and the possible options.

• List the concepts• Problem Definition:

– 4 year old, handicapped child cannot support her body weight while learning to creep

• Options– #1 - Catalog Device Modification– #2 - Crane Suspension Device– #3 - Cart Device

Lightweight

Stable

Pinch Points

Maintenance

Range of Motion (legs)

Comfort

1 2 3

Part of the first step is to clarify the concepts. This is about communicating the design beyond the team and for a historical reference.

• Catalog Device Modification: A height adjustable crawler that provides upper body support, but doesn’t keep legs from “kicking out”

• Crane Suspension Device: Provides support of child’s body using a spring suspended from a height adjustable support arm

• Cart Device: Provides a seat that supports child’s upper body. Also has a plate with bungee cords that limits forward and backward motion of the legs.

– Compare each option within each row.

– Indicate +1, -1, or 0

– This is a modification to the Pugh Concept Selection and allows the decision makers to automatically see the best available in each

– A description for the rationale behind each ranking should be included.

Catalog Crane CartLightweight 1 -1 1Stable 0 -1 1Pinch Points 1 -1 0Maintenance 0 1 -1Range of Motion (legs) 0 -1 1Comfort -1 0 1Totals 1 -3 3

Lightweight: Both the catalog design and cart weigh less than 10 lbs. Because the crane is bigger and must be stable, it is much heavier.Stable: The cart is low to the ground and is therefore more stable. The catalog design is higher, but is difficult to turn over. The crane has a greater tendency to turn over because it is higher.

The 2nd step is to rate the concepts. This means discussing each row of the matrix and rating relative to each other.

The third step is to rank the concepts. This is the easiest because it just requires doing the math.


The 4th step is combine and improve. Attack negatives and enhance positives.

• Can you combine portions of one design with another?• Is there a way to eliminate undesirable features?• Remember our functional analysis - is there a way to

change one function and make the design better?

The 5th step is to select the best concept.


The 6th step is to reflect. Does this make sense? Will this be defensible in a court of law in 5 years?

3. Consider a material selection problem for a refrigerated food preparation surface material for use in an ice cream store. The surface will be subsequently coated, but hand mixing of ingredients on the surface is needed. Five materials have been identified as possibilities: 1020 Steel, 304 Stainless Steel, 5052 Aluminum, Copper and Bronze. Use the modified concept screening process discussed in class. To help you in this process a table listing the material properties for each criterion is given below.

Criteria Units DesiredDirection

1020Steel

304SS

5052AL

Cu Bronze

Thickness (in.) less isbetter

0.107 0.107 0.407 0.205 0.205

Conductivity Btu-ft/(hroF-ft2)

higher isbetter

27 9.4 80 200 109

Diffusivity ft2/hr higher isbetter

909 270 3749 6751 3809

Hardness Brinnell higher isbetter

111 95 47 2 1

Yield kips higher isbetter

30 110 13 40 37

Machinability [0-100] less is better

65 90 30 20 20

ThermalMass

Btu/oF lower isbetter

2.93 3.43 7.33 5.45 5.26

Criteria DesiredDirection

1020 Steel

304 SS

5052 Al

Cu Bronze

Thickness less isbetter

Conductivity higher isbetter

Diffusivity higher isbetter

Hardness higher isbetter

Yield higher isbetter

Machinability less is better

ThermalMass

lower isbetter

Total

Use concept screening to complete the matrix below.


1020 Steel

304 SS

5052 Al

Cu Bronze


1 1 -1 0 0


-1 -1 0 1 0


-1 -1 0 1 0


1 1 0 -1 -1


0 1 -1 0 0


0 -1 1 1 1

ThermalMass

lower isbetter

1 1 -1 0 0

Total 1 1 -2 2 0

Next we will use a weighted decision matrix.

• Introduced in Freshman Design.• Specify design criterion• Enumerate alternatives• Establish weightings• Determine performance of each alternative for each

criteria• Score that performance as indicated by the data that you

have.

The data that you have controls the evaluation that you do. Is your data numerical? Is it text? Is it mixed?

• Some numeric and some quantitative – use interval scales

• All numeric = use ratio scales

Let’s return to our matrix for material selection. We have actual numeric data for each item. Let’s see what we would decide if we use ratio scales.


1020 Steel

304 SS

5052 Al

Cu Bronze







ThermalMass

lower isbetter

Total

Complete the Matrix Using Ratio Scales

Answers to the redo of material selection

Evaluation Scheme for Interval Scales

Example of Weighted Decision Matrix Using Interval ScalesTaken from Engineering Design by Dieter and Schmidt

A heavy steel crane hook, for use in supporting ladles filled with molten steel as they are transported through the steel mill, is being designed. Two crane hooks are needed for each ladle. These large, heavy components are usually made to order in the steel mill machine shop when one is damaged and needs to be replaced.

Three concepts have been proposed:1.Built up from flame-cut steel plates, welded together2.Built up from flame-cut steel plates, riveted together3.A monolithic cast-steel hook

The first step is to identify the design criteria by which the concepts will be evaluated. The PDS is used and the design criteria are identified as material cost, manufacturing cost, time to produce a replacement hook if one fails, durability, reliability, and repairability.

Determine Weighting Factor for Each Design Criteria

Crane Hook = 1.0

Cost = 0.6 Quality in Service = 0.4

Material Cost = 0.3

Manufacturing Cost = 0.5

Repairability = 0.2

Durability = 0.6

Reliability = 0.3

Time to produce = 0.1

Weight for Material Cost = 0.3* 0.6 = 0.18

Results for Weighted Decision Matrix using Interval Scales

Remember…

The goal of concept selection is not to• Select the best concept.

The goal of concept selection is to• Develop the best concept.

So remember to combine and refine the concepts to develop better ones!

Caveats

• Beware of the best "average" product.

• Perform concept selection for each different customer group and compare results.

• Check sensitivity of selection to the importance weightings and ratings.

• May want to use all of detailed requirements in final stages of selection.

• Note features which can be applied to other concepts.

Individual QuizName:____________________________ CM:________________

Use the concept screening method presented in class today to evaluate which sport-utility vehicle designs should be pursued. The ratings are given so that you know the relative merits of each vehicle.

Individual QuizName:____________________________ CM:________________

Now use the concept scoring and interval scale method presented in class today to evaluate which sport-utility vehicle designs should be pursued. (Use a scale of 0 to 4.) The ratings are given so that you know the relative merits of each vehicle.

concept selection

Documents