The science (and art) of design

• introduction• the nature of design• the design process• specifications• conceptual design• detailed design• testing• documentation

Hugh GriffithsDept. Electronic & Electrical EngineeringUniversity College LondonTorrington PlaceLONDON WC1E 7JE

tel: +44 (0)20 7679 7310fax: +44 (0)20 7388 9325email: h.griffiths@ee.ucl.ac.ukwww: http://www.ee.ucl.ac.uk/

… to provide some guidance to you on the general subject of design, and also some guidance to you on how to conduct your third-year or fourth-year project.

The nature of design

• the definition in Chambers dictionary gives:

• essentially, the process which takes an abstract idea for something and turns it into a detailed and optimal recipe for making it

• widely applicable !

• as well as being based on sound scientific principles, design will also involve creativity, since in general there is no single correct answer to a given design problem.

design ( ) – v.t. to draw; to form a plan of; to contrive; to intend; to set apart or destine. – n. a drawing or sketch; a plan or scheme formed in the mind; plot; intention.

di zin

Design Exercise 1

Write down two examples taken from everyday life of what you consider to be good design, and two examples of bad design. In each case explain why you consider them to be particularly good or bad.

The design process

• the figures on the next slides show four attempts to depict the way in which the design process works

• Whilst at first sight they all look different, they all show how the process starts with a specification, develops a conceptual design, then a detailed design, produces an example (prototype), and finally measures and compares the performance against the original specification

Flowchart showing stages in a design process (taken from Open University ‘Design Project Guide: Unit 12) – after Ray [1].

The design core (after Pugh [2]).

Design steps according to Hansen [3]:1. From the task via working means to the concept;2. From the concept via working principles to the layout;3. From the layout via form design principles to the production documents.(after Pahl & Beitz [4]).

Phases and steps of the design process according to Roth [5].(after Pahl & Beitz, [4])

Specifications

• a list of functions that whatever you are going to produce should satisfy

• example: an amplifier

• specifications should be things that can actually measured or assessed

• it follows that they should be justifiable in a quantitative manner

• and they should not be more stringent than they need to be

Design Exercise 2

Write down a set of specifications for:

(i) A power amplifier for a television transmitter

(ii) A computer program to simulate the operation of a aircraft- borne radar system to detect small boat targets on the sea surface

In each case, where possible, your specifications should be quantitative

Conceptual design

• In other words, how to meet the specification ….

• may involve looking at a number of alternative approaches, and evaluating the tradeoffs

• if you are designing a system, you would almost certainly want to split it into subsystems by drawing a block diagram, and to develop specifications for the individual subsystems

• the most important tool in this part of the process is experience

• thus an appropriate literature review is invaluable

• on the other hand, there is always scope for innovation, so you should not be inhibited from trying new approaches

Detailed design

• The exact nature of this will depend on exactly what you are designing

• may involve analysis, experiment and/or simulation – ask yourself which are the most appropriate approaches to your own design problem

• there are many powerful simulation programs available, or you can write your own. But certain assumptions will have been made, which may not have been explicitly stated

• so don’t necessarily believe everything your computer tells you !

• The results of any computer simulation should therefore be validated, by comparing the results from the computer simulation with equivalent experimental data or analysis

Testing

• once you have completed whatever it is you have designed, it should be assessed against your specification

• for some parts of the specification this may be trivial; for others it may involve some detailed and complicated measurements

• any measurement involves some uncertainty, and you should therefore identify the sources of error in your measurement and estimate the uncertainty in any results that you quote

• design is almost always an iterative process. If there are parts of the specification that are not met, it may be necessary to refine the design iteratively

Documentation

• it is important that the whole of this process – including the methods and results of testing – is properly documented

• both so that there is a proper record of the design, and so that you (and others) can learn from your successes and failures

• you will, of course, have kept a logbook, and you will almost certainly have to write the process up in a report and to make a presentation on your work.

A horse designed by a committee …..?

Design Exercise 3

There is an island off the north-west coast of Scotland, about 50 km offshore, which currently does not have television. Your company intends to bid for a contract to provide a television service (the standard 5 analogue channels) to the inhabitants.

(i) Write down a specification for a system of this kind

(ii) Identify a number approaches which may be appropriate, and undertake conceptual designs to establish the preferred approach.

You will probably need to make assumptions – in which case identify what they are and justify them