ge 121 – engineering design - 2009 engineering design ge121 functions & specifications lecture...

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GE 121 – Engineering Design - 2009

Engineering DesignGE121

Functions & Specifications

Lecture 6B

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Functions and SpecificationsMoving from the language of the client to the

language of the engineer Functions

Tell us what the designed object must do to realize the stated objectives

Have to establish which functions have to be performed before we can specify how well they must be performed

Performance SpecificationsTell us how well the designed object must do

somethingWill look at various tools and techniques for

deriving them

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Identifying Functions to Realize Objectives

Bookcase example A child may say that a bookcase doesn’t ‘do’ anything

– it just sits there Engineer would see:

Resists weight of booksDividers may be used to organizeMay be visually appealing

An engineer looking at designed objects is educated to see that artifacts do things even when they just sit there

There are consequences for engineers that fail to understand / design for ALL functions in a designForensic Engineering (study of failure) has many

examples!

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What are Functions?Engineering Functions

Like mathematical or business functionsSpecify how some set of inputs is transformed into a set of

outputs

Usually a verb (action word) – noun pairLift a weightLight a fireplace

Several important types of functions to consider Basic Functions Secondary Functions

– Required Secondary Functions

– Unwanted Secondary Functions

Avoid tying a function to a particular solution

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What are Functions? (continued)

Example – Overhead ProjectorBasic Functions

Specific work that designed artifact is intended to accomplish Example - Project Images

Secondary Functions Any other function OR Those that result from the basic function Required Secondary Functions

Those secondary functions that are needed for the basic function to be accomplished

Example - Convert energy, generate light, focus image Unwanted Secondary Functions

Functions related to undesired byproducts of either basic or secondary functions

Example - Generate Heat, Generate Noise Unwanted secondary may require new Basic (quiet, dissipate heat)

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How Do We Identify/Specify Functions?Several techniques commonly used to

identify and specify functionsEnumeration

Simply making a list of the functionsWorks well if we’re already familiar with

artifact/problemSome tricks to help extend an enumerated list

(see next slide)

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How Do We Identify/Specify Functions? (cont’d)

Imagine if it suddenly vanished – Example: A Bridge– Cars would fall into river Function = Support loads– If no abutments, deck / superstructure would collapse

Function = Support own weight (1907 Quebec Bridge – 75 fatalities - our iron rings)

– If no on/off ramps – no traffic Function = Connect to roadways

– If opposing traffic in same lane – accidents Function = Separate traffic by direction

Consider how an object might be used and maintained over its lifetime

– May require bridge to be painted Function = Provide maintenance workers access to all part of structure (design ladders, catwalks, elevators etc.)

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Beverage container design example (Functions)Contain liquidGet liquid into container (fill the container)Get liquid out of the container (empty the container)Close the container after opening (if it is to be used more than

once)Resist forces induced by temperature extremesResist forces induced by handling in transitIdentify the product

Getting liquid into and out are distinct functions – may be identified by examining life cycle

Designer needs a verb-noun pair for each function

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Black Boxes/Transparent BoxesTwo methods of associating inputs and outputsBlack Boxes

After indicating the system boundary, all the inputs are shown going into (left) and outputs coming out of (right) the system

Include ALL inputs/outputs including undesirable byproducts from secondary functions

Fig. 4.1(a) p82

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Transparent BoxesCover of the box is then “removed”, and the designer can

focus attention on how the various inputs are transformedProcess can be repeated iteratively (black boxes shown

inside transparent boxes) until designer is satisfied that a suitable means of realizing functions can be developed

Fig. 4.1(b) p83

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Must be careful with setting boundaries or limits with both black boxes and transparent boxesRequires tradeoffsToo wide – may specify functions that are

beyond our control (include AC power?)Too narrow – may limit scope of the design

(speakers included?)

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Functions, Means and your ProjectMeans are methods of achieving functions, and they

imply solutionsDon’t propose solutions to your Project at this point

in the processOnly examine means that have been used in other

designs when looking at current State-of-the-Art or Reverse Engineering, at this point in your project

Means will be examined AFTER the Midterm Report has been submitted


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Function-Means TreesA graphical representation of a design’s basic and

secondary functionsTop level specifies functions to be met

For each function, several means of accomplishing the function are listed

Subsidiary functions that result from that are listed below the means

Continues until the designer is satisfied that all the functions have been developed

Can list functions that are common to most (or all) means, and which functions are related to specific means

Begins the process of associating what we must do with how we might do it

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Function-Means Tree for cigarette lighter

Fig. 4.2 p85

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Some cautions – Function-Means treesNOT a substitute for either formulating the

problem or for generating solutionsShould not be used without using some of the

other tools listed earlier. Don’t adopt a tool because it fits with your preconceived idea of a solution

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Dissection and Reverse EngineeringDesigner can often find an existing artifact that

performs some or all aspects of the basic function

Artifact can then be examined with the question, “what does this do?” being asked of all the components of the artifact

Dissecting, deconstructing or disassembling it to find out how and why it works is called Reverse Engineering

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May not be able to use the designMay not do all things that we requireMay not do them wellMay be too expensiveMay be patentedMay be our competitor’s design

But may allow us insight into our problem by looking at how others have solved the same, or similar, problems

Look at Means, and try to determine functionMay press buttons, or test, and try to determine as

many functions as possible

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Cautions for Reverse EngineeringClient / Users for dissected artifact may be quite

different from yoursTemptation to limit new means once existing means

have been identifiedReverse Engineering goes beyond Dissection, in

that we are trying to determine why and how the artifact was made to do what it does

Need to define functions in the broadest terms – only focus down when it is necessary. To mimic an existing design may violate intellectual property (patent) laws, and you could be sued!

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A Repeated Caution AboutFunctions and Objectives

While young or inexperienced designers confuse the two, a useful distinction is:Functions describe what artifacts DOObjectives tend to characterize what artifacts

ARE

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Activity

Xela-Aid chicken coopFor the list of functions shown (developed by

one student group)Clarify / focus with verb-noun pairs

– Example: Allow for removal of waste Remove Waste

Look for any that imply solutions– Example: Keep Water Fresh implies changing water

frequently. Other options could be to continuously stream water, removing old water, detoxifying water A Better statement might be: Supply chickens with potable water

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Xela-Aid chicken coop

p93

ge 121 – engineering design - 2009 engineering design ge121 functions & specifications lecture...

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