EML4550 Fall 2007 1

EML4550 - Engineering Design Methods

Concept development and SpecificationsFrom needs to requirements to specifications, Product Specification

vs. Specs, Refining Specifications

Hyman: Chapter 2Dym and Little: Chapter 5

Ulrich and Eppinger: Chapter 4

EML4550 Fall 2007

Phases of the design process

Concept development Identify customer needs, gather information on competition or

possible alternatives, generate and evaluate alternate concepts, select concept, define form and function of the artifact. [feasibility study]

System-level design Determine system architecture (configuration) as well as all

sub-systems and respective interfaces, produce system layout and specifications for the system and each sub-system

Detailed design Complete and final specification of the system, including

geometry, materials, tolerances, etc. (drawings), complete and final manufacturing process specification.

Testing and refinement Review design, build prototype (if appropriate), alpha and beta

prototype.

Production ramp-up and delivery Production line checked and refined, product ‘launch’

EML4550 Fall 2007

Concept Development Diagram

IdentifyCustomerNeeds

EstablishTargetSpecs

GenerateProductConcepts

SelectProductConcept

RefineSpecs

AnalyzeCompetitiveProducts

PerformEconomicAnalysis

PlanDesign/DevelopmentProject

MissionStatement

ActionPlan

EML4550 Fall 2007

Customer Needs vs. Product Specifications

Subtle but important differences between customer needs and product specifications Customer views a product without any technology bias Customer uses product without knowledge of design

process or internal sub-systems It is important to always distinguish when a specific

need is driving a feature, or when a designer is introducing an assumption in order to arrive at a specification

The specification is not “customer-driven” necessarily, it is “design-centric”, it is how the design engineer sees the product as a “black box” PRIOR to a full design definition. It is usually a QUANTIFICATION of functionality and perfromance

EML4550 Fall 2007

From Customer Needs to Specifications

Turn subjective customer needs into precise engineering terms to guide the design effort

Force agreement among corporate stakeholders (management, marketing, engineering) on what will be designed

Serve as focal point for design trade-offs

Develop confidence that the product will have a successful market introduction

EML4550 Fall 2007

What is a Specification?

Product specification is a precise description of what the product has to do

“Product Specifications” vs. “Specification”

Specification (“spec”): Metric Value (or range of values)

Product Specification as a collection of “specs” (requirements) plus other relevant information (including modes of use, design preferences: design criteria, etc.)

EML4550 Fall 2007

Specifications

When: Ideally specifications should be set early in the project. However, specifications can only be done as an iterative

process Many specifications need to be changed after a concept is

selected, or as a result of engineering-economic trade-offs

Establish target specs: Step 1: Prepare List of Metrics Step 2: Collect Competitive Benchmarking Information Step 3: Set Ideal and Marginally Acceptable Targets for Each

Metric Step 4: Reflect on the Results of the Process

EML4550 Fall 2007

Step 2: Collect Information on Competition (Example)

Need No. Need Imp. Product 1 Product 2 Product 31 Is versatile to use on many types 1 *** *** **2 Completes the job fast 2 *** ** ***3 Batteries are easy to change 2 * *** **4 Very reliable 1 * *** ***5 Looks nice 3 * * ***

Prepare a subjective list of perceived attributes that fulfill the needs

EML4550 Fall 2007

Concept Development Diagram

IdentifyCustomerNeeds

EstablishTargetSpecs

GenerateProductConcepts

SelectProductConcept

RefineSpecs

AnalyzeCompetitiveProducts

PerformEconomicAnalysis

PlanDesign/DevelopmentProject

MissionStatement

ActionPlan

EML4550 Fall 2007

Refine Specification

Final specifications cannot be written in the absence of a design concept

Many trade-offs need to be considered before finalizing the specification document

Not all information for these trade-offs is available at the time the specification is needed

Finalizing a specification document is an iterative, often painful, process

EML4550 Fall 2007

Refine Specification

Step 1: Develop technical model of product Analytical models, computer models, technical

calculations, parametric studies, etc.

Step 2: Develop economic model of product Cost model, materials and manufacturing costs, part

counts, etc.

Step 3: Finalize specification, perform trade-offs Estimate cost associated with each spec, place product

in a ‘competitive’ map

Step 4: Reflect on the results of the process Will the product win? Level of uncertainty? Did we select

the right concept? Do we need better modeling tools?

EML4550 Fall 2007

Final Specification

Tabulate all quantifiable attributes that need to be on the specification

Add any other information that is needed to fully ‘spec’ the product (e.g., graphs, plots for input or desired output, etc.)

Include a narrative to the tabulation or plots to make the document ‘readable’

The “Product Specification” is a ‘stand-alone’ document - it needs to be arranged and prepared as such. Treat it as a CONTRACT!

EML4550 Fall 2007 20

Some useful techniques

From needs to requirements to specification to design

QFD and the House of Quality

EML4550 Fall 2007

Quality Function Deployment (QFD)

How to identify customer requirements and turn them into design/performance parameters (specifications)

Construct a chart the explicitly depicts the relationship between 1) customer requirements, 2) engineering (product) requirements, and 3) competing products (or concepts)

The QFD table has five elements (regions): Customer requirements Engineering requirements Matrix of requirements relations Competitive benchmarks Engineering targets

EML4550 Fall 2007

QFD chart

EML4550 Fall 2007

QFD chart

Customer requirements Unfiltered needs (requirements) using customer’s

own words Exhaustive list compiled through customer

interviews

Engineering requirements Quantifiable aspects of the system that contribute to

fulfillment of customer needs Include as many as you can think of (mixture of

design and performance parameters) Some items may be a repetition of a customer

requirement (sophisticated and technical customer)

EML4550 Fall 2007

QFD chart (cont’d)

Matrix of requirements relations At the center of the QFD “matrix” we correlate on a

one-to-one basis the customer and the engineering requirements

Ask: Are these two requirements (customer vs. engineering) related? If so, mark it with an “x”

Competitive benchmarks Indicate how competitors (external or

internal/concept) match the customer needs

Engineering targets Quantitative target for each of the engineering

requirements This is a first draft of the “specification” Some may be “TBD” in the early stages.

EML4550 Fall 2007

QFD chart - Example (Car Bumper)

EML4550 Fall 2007

Variations on QFD: The House of Quality

• Add triangular region• Keep track of relationships between engineering requirements (specs.)• Indicate positive correlation with a “+”, and negative correlations with a “-”• This is useful to visualize the design trade-offs