The Need Specification
References
Adapted from:
 Design for Electrical and Computer
Engineers, first edition, by Ralph M. Ford
and Chris S. Coulston
 Excerpts from the book “Engineering
Design, a Project Based Introduction”,
second edition by Clive I. Dym and Patrick
Little. John Wiley and Sons, Inc. ISBN 0471-25687-0

What are Specifications?
Definitions

Specification A detailed and exact statement of
particulars, a statement fully describing
something to be built.


Specifications A detailed, exact statement of
particulars, especially a statement prescribing
materials, dimensions, and quality of work for
something to be built, installed, or manufactured.


American Heritage Dictionary
answers.com
Specifications may indicate as well acceptance
criteria (very important).
What to Specify?
What to Specify?

The Objectives and The Problem Statement
resulting from the attributes in the Need Analysis




Short statements that describe the need in the language
of the client (or the user) like:
“The amplifier should have good sound quality” resulting
from:
Interview, survey, brainstorming
Translate the Objectives into Engineering
Language and attach numbers to them
The system should have Low Total Harmonic
Distortion
 Specification: THD < 1%
 Constraining the Objectives

6
Translating the Need into Engineering
Language and Technical Specifications
Performance

Performance specifications identify
performance levels that signify the achieved
desired functional behavior.
The system should detect 90% of all
human faces in an image.
 The amplifier will have a total harmonic
distortion less than 1%.

7
Who defines the
specifications?
Who defines the specifications?

The Client and the Team


After the Need Analysis, requires
One or more additional meetings
Objectives and Problem
Statement Properties
1)
2)
3.
4.
Abstract: What the system will do, not
how it will be implemented
Verifiable: There should be a way to
measure or demonstrate that the
objectives met in the final realization
Unambiguous: Each requirement should
have a single unambiguous meaning and
be stated with short complete sentences.
Traceable: Each Engineering
requirement should be traceable to
client, users or designers
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Objectives and Problem
Statement Properties

Good Example


The robot must have move forward fast, with
the ability to accelerate from standstill to the
average speed in under one second.
Bad Example

The robot uses IR sensors to sense its external
environment and navigate autonomously
controlled by a Raspberry Pi
11
Validation VS.
Verification
Validation
Validation = Are we building the right
product? (Objectives)
Verification = Are we building the product
correctly? (Specifications)
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Technical specification
A complete list of the technical details for
a given system, e.g., accuracy, speed,
dimensions, etc
 List as a table at the end of section

One Example
Car Audio Amp
Marketing Requirements
1.The system should have excellent sound quality.
2.The system should have high output power.
3.The system should be easy to install.
4.The system should have low cost.
Car Audio Amp
THD
Output Power
Marketing Requirements
1.The system should have excellent sound quality.
2.The system should have high output power.
3.The system should be easy to install.
4.The system should have low cost.
Translate Marketing language into Engineering Language and find the
technical specs and justifications
Car Audio Amp
1, 2, 4
1–4
The total harmonic distortion
should be <0.1%.
Based
upon
competitive
benchmarking and existing amplifier
technology. Class A, B, and AB
amplifiers are able to obtain this level
of THD.
Should be able to sustain an
output power that averages 
35 watts with a peak value of
 70 watts.
This power range provides more than
adequate sound throughout the
automobile compartment. It is a
sustainable
output
power
for
projected amplifier complexity.
Marketing Requirements
1.The system should have excellent sound quality.
2.The system should have high output power.
3.The system should be easy to install.
4.The system should have low cost.
Probably most of user will not
distinguish between .1% and
.01% THD
Or Probably 200 W will represent
a hazard for users in a car
Probably nobody will buy a 10% THD
Or one that delivers a max of 5W
How to determine the values:
Trade-Off
THD
Power
Usually the better the spec the
more expensive to produce
How to determine the values:
Trade-Off
Power
Probably improving the THD spec
may require to impair a little the
power spec to keep price
reasonable
THD
THD
Power
Usually the better the spec the
more expensive to produce
How to determine the values:
Competitive Benchmarks
 Cost
 Perception

Competitive Benchmarks
24
Apex Audio
Monster Amps
Our Design
THD
0.05%
0.15%
0.1%
Power
30W
50W
35W
Efficiency
70%
30%
40%
Cost
$250
$120
$100
Car Audio Amp.
Objectives
Engineering Requirements
Justification
1, 2, 4
1. The total harmonic distortion
should be <0.1%.
1–4
1. Should be able to sustain an
output power that averages 
35 watts with a peak value
of  70 watts.
2, 4
1. Should have an efficiency ()
>40 %.
Achievable with several different
classes of power amplifiers.
1. Average installation time for
the power and audio
connections should not
exceed 5 minutes.
Past trials using standard audio and
power jacks demonstrate that this is
a reasonable installation time.
3
Based
upon
competitive
benchmarking
and
existing
amplifier technology. Class A, B,
and AB amplifiers are able to obtain
this level of THD.
This power range provides more
than adequate sound throughout
the automobile compartment. It is a
sustainable output power for
projected amplifier complexity.
25
1–4
1. The dimensions should
not exceed 6” x 8”x 3”.
Fits under a typical car seat.
Prior models and estimates
show that all components
should fit within this package
size.
1–4
1. Production cost should not This is based upon competitive
exceed $100.
market analysis and previous
system designs.
Marketing Requirements
1.The system should have excellent sound quality.
2.The system should have high output power.
3.The system should be easy to install.
4.The system should have low cost.
26
Assumptions and Limitations

Assumption – The result of any project decision, which is
required to complete the project definition, but is not a physical
limit (minimum or maximum) that was imposed by the client, the
technology used, or a physical law. Assumptions are the result of
decisions that can be made by the team and affect the endproduct design and implementation. Examples would include:



The maximum number of simultaneous users of a computer program,
or
The maximum number of books to be stored on the shelves of a
bookcase.
Limitation – The result of any project decision, which is required
to complete the project definition, but is a physical limit
(minimum or maximum) that was imposed by the technology
used, or a physical law. Limitations are the result of things over
which the team has no control, but must consider in its endproduct design and implementation. Examples would include:



The maximum weight or size of user that would fit in the product
without damaging it.
The maximum power consumption, or (Limited by size of PS or
Breakers)
The maximum speed of the end product (limited by the type of gates
or microcontroller)
Assumptions and Limitations

We care about technical Assumptions
and Limitations. Please do not list in
your assumptions things like:



The user has technical background or
The weather will be fairly good
Do not confuse constraints with
limitations
Operating environment
For any end product other than simply a
calculation, it is essential to know the
environment to which the end product will
be exposed or experience.
 For example, will the end product be
exposed to dusty conditions, extreme
temperatures, or rain or other weather
elements?
 Is the end product likely to be dropped or
thrown?
 This information is necessary in order to
design an end product that can withstand
the hazards to which it is expected to be
exposed. This element shall be at least

Intended user(s) and intended use(s).
Knowing the characteristics of the end
users makes it much easier to design an
end product that will be accepted and
used.
 The expected end uses are equally
important. This description should include
what uses are expected as well as what
uses are not to be considered

Specifications











THD < 0.1%.
Prms 35W
Ppeak 70 W
ƞ > 40 %
Average installation time < 5 minutes
Dimension 6” x 8”x 3”
Cost < $100
Input Voltage 12V (Operating Environment)
Temperature in Operation 0 < T < 50C (Operating
Environment)
12 hour operation when car is off (limitation)
Human Interface: Touch Screen (Assumption)
&
Questions
Answers