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 0-471-25687-0

What are Specifications?

Specification A detailed and exact statement of particulars, a statement fully describing something to be built. American Heritage Dictionary

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. answers.com

Specifications may indicate as well acceptance criteria (very important).

Definitions

What to Specify?

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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

What to Specify?

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%.

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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 Properties1) Abstract: What the system will do, not how it

will be implemented2) Verifiable: There should be a way to measure

or demonstrate that the objectives met in the final realization

3. Unambiguous: Each requirement should have a single unambiguous meaning and be stated with short complete sentences.

4. 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

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Validation VS. Verification

Validation = Are we building the right product? (Objectives)

Verification = Are we building the product correctly? (Specifications)

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Validation

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

Marketing Requirements1.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 Requirements1.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

Translate Marketing language into Engineering Language and find the technical specs and justifications

1, 2, 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.

1–4 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 Requirements1.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

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

THD

Power

THD

Power

Usually the better the spec the more expensive to produce

Probably improving the THD spec may require to impair a little the power spec to keep price reasonable

How to determine the values: Competitive Benchmarks Cost Perception

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

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Competitive Benchmarks

Objectives Engineering Requirements

Justification

1, 2, 4 1. 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.

1–4 1. 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.

2, 4 1. Should have an efficiency () >40 %.

Achievable with several different classes of power amplifiers.

3 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.

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Car Audio Amp.

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 exceed $100.

This is based upon competitive market analysis and previous system designs.

Marketing Requirements1.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.

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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 end-product 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 end-product 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 one paragraph in length.

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)

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