The Design Process – Part II
Engineering Design
GE121
The Design Process
(continued – Part II)
Lecture 8A
Unless otherwise stated, all images are from Chapter 2 - Text
GE 121 – Engineering Design - 2008
The Design Process – Part II
Describing/Prescribing
the Engineering Design Process
Description versus Prescription of the
Engineering Design Process
Description: Attempts only to describe the
elements of the Design Process, but does
not attempt to determine how the process is
to proceed
Prescription: Prescribe what should be done
during the design process, or in other words,
indicate how it should be done (procedure).
We will be concerned primarily with a
Prescriptive Model of Engineering Design
GE 121 – Engineering Design - 2008
The Design Process – Part II
Describing the Engineering Design Process
 A simple 3 Stage model
 Between Client Statement (Need) and Final
Design (Fabrication Specification / Document’n)
 Conceptual Design – Develop different concepts
(schemes) that can meet objectives (major
functions and spatial relations – so cost/weight
and basic performance can be estimated)
 Preliminary Design – ‘Embodiment of schemes’
with their most important attributes by selecting
and sizing major subsystems to account for
performance specifications. Extensive use of
rules of thumb and designer experience. Final
Design is chosen from these.
 Detailed Design – Refining the preliminary
design, including selection of all parts and
dimensions. Result is usually a set of documents
and drawing such that another person or
company can complete the fabrication without
further contact with the designer
GE 121 – Engineering Design - 2008
The Design Process – Part II
Describing the Engineering Design Process (cont’d)
 5 Stage model – Adds 2
 Problem Definition – Work done to
focus / refine client’s statement
before Conceptual Design
 Design Communication – Postprocessing that identifies the work
done after the detail design
 Problems
 Both of these Descriptive models do
not bring us any closer to knowing
HOW to do design
 Prescriptive Models will tell us how
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing the Engineering Design Process
 Model shown converts
Descriptive Model into a
Prescriptive Model
 Incorporates the design
tasks specified earlier,
which define what is done
at each stage
 Each stage has Inputs, has
Design Steps that have to
be performed, and
produces Outputs or
product
 Outputs from one stage
become the Inputs for the
next
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
GE 121 – Engineering Design - 2008
The Design Process – Part II
Prescribing Engineering Design Process (cont’d)
We now have a ‘checklist’ against which we
can ensure we have done the required steps
Similar lists often used by design organizations
to specify and propagate approaches to design
within the company
We must still rely on our ability to model the
tasks done within each phase of the design
process
GE 121 – Engineering Design - 2008
The Design Process – Part II
Feedback and Iteration in the Design Process
Previous expressions of
models have all been
linear
In reality feedback and
iteration often occur in
the design process
GE 121 – Engineering Design - 2008
The Design Process – Part II
Feedback and Iteration in the Design Process (cont’d)
Feedback feeds information from the output of
a process back into the process to improve
results
Internal feedback (verification) comes from internal
testing, or from internal departments
External feedback (validation) comes from
customers and users who use the product
Iteration repeats tasks, usually at different
levels of abstraction – as the design is refined,
we have to look back to make sure it is still
consistent with the original objectives and
specifications
GE 121 – Engineering Design - 2008
The Design Process – Part II
Opportunities/Limits in the Design Process
 Text and GE121 focus on conceptual design
 Will be dealing with broad themes and approaches
 They are logical, but not nearly as neat and tidy as a set
of equations or algorithm
 This lack of rigidity actually makes these processes
applicable to problem solving in general (that’s why
engineers often move into higher levels of responsibility)
 Interesting tidbit – the Chinese Politbureau (inner cabinet of the
government) was at one recent point composed entirely of
engineers!
 We are limiting our detailed discussion of design
methods to the earliest phases of design. There are
many other processes and activities that must be
undertaken before a product makes it to market
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study
 Familiarization with Case Study (cont’d) (Text 2.5)
Have previously determined metrics for indicating the
degree to which potential designs, or schemes, satisfy
the objectives
Other Tools allow us to determine the relative
importance of objectives, and required functions, which
are also important when it’s time to select the Final
Design
These tools will be discussed in detail later, but we will
examine them quickly now because they may be
helpful in determining the relative importance of
objectives, as well as any necessary functions for your
Project Proposal
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Pairwise Comparison Charts
Allows Rank Ordering of Objectives in terms of
their relative importance
Result from one design group is shown
Shows most important is reduction in surgeon’s
tremor, while least important is cost
Helps focus design attention and also, to see if
our intuition is correct
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Functions and Performance Specifications
Required functions can be determined by
using one or more of the related tools, which
are discussed in more detail in section 4.1,
some of which include:
Black Box (looks at functional Inputs/Outputs, but
obscures process details)
Glass Box (similar to black box, but also allows
observation of some relevant internal details)
Functions-Means Tree
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
Functions and Performance Specifications
One sample list of functions for the
microlaryngeal stabilizer states that it must:
Stabilize the instrument
Move the instrument
Stabilize the distal end of the instrument
Reduce muscle tension of the surgeon during
surgery (or reduce shaking tremors)
Stabilize itself
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
Functions and Performance Specifications
Performance Specifications (written for the
first function)
Function: Stabilize the Instrument
Performance Specification: This function is not
achieved if the design cannot reduce the
amplitude of a trembling hand to less than 0.5
mm. It is optimally achieved if it controls the
amplitude of a trembling hand to make it less
than 0.05mm. It is overly restrictive if it inhibits
or disallows any instrument or hand use.
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Functions / Means – Morphological Chart
A Morphological Chart requires you to select
one item from each row to produce a potential
instance, allowing a methodical search through
all of the possible changes/options (morphs)
Chart shown has Functions on one axis, and
the Means to achieve that Function on the
other axis
Effectively tells us how large a design space
we are working in
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Functions / Means – Morphological Chart
GE 121 – Engineering Design - 2008
Case Study (cont’d)
The Design Process – Part II
 Functions / Means – Morphological Chart
One candidate design has the instrument held in
and moved by the surgeon’s hands, with distal
support provided by crosswires, which are attached
directly to the laryngoscope. The surgeon uses a
forearm rest to reduce tremor-inducing muscle
tension
A second design alternative has the instrument
supported on a stand, moved by a system of pulleys,
and supported by the stand itself. The instrument
stand removes the need for the surgeon to operate
in a fixed position, which thus reduces tremorinducing muscle tension
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Decision (Selection) Matrix
These 3 potential designs satisfy all constraints
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Decision or Selection Matrix – Find BEST !
GE 121 – Engineering Design - 2008
The Design Process – Part II
Case Study (cont’d)
 Testing of Design Concepts – an input to
the Decision Matrix
Tracing a square with and without an
instrument stand
GE 121 – Engineering Design - 2008
The Design Process – Part II
Activity
Continue working on your Midterm
Report
GE 121 – Engineering Design - 2008