Engineering Design GE121 Functions

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Engineering DesignGE121Functions
Specifications

• Lecture 6B

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Functions and Specifications

• Moving 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 Specifications
• Tell us how well the designed object must do
  something
• Will 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 doesnt do
  anything it just sits there
• Engineer would see
• Resists weight of books
• Dividers may be used to organize
• May 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 design
• Forensic Engineering (study of failure) has many
  examples!

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What are Functions?

• Engineering Functions
• Like mathematical or business functions
• Specify how some set of inputs is transformed
  into a set of outputs
• Usually a verb (action word) noun pair
• Lift a weight
• Light 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 Projector
• Basic 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 functions
• Enumeration
• Simply making a list of the functions
• Works well if were already familiar with
  artifact/problem
• Some tricks to help extend an enumerated list
  (see next slide)

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How Do We Identify/Specify Functions? (contd)

• 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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How Do We Identify/Specify Functions? (contd)

• Beverage container design example (Functions)
• Contain liquid
• Get 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 extremes
• Resist forces induced by handling in transit
• Identify 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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How Do We Identify/Specify Functions? (contd)

• Black Boxes/Transparent Boxes
• Two methods of associating inputs and outputs
• Black 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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How Do We Identify/Specify Functions? (contd)

• Transparent Boxes
• Cover of the box is then removed, and the
  designer can focus attention on how the various
  inputs are transformed
• Process 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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How Do We Identify/Specify Functions? (contd)

• Must be careful with setting boundaries or limits
  with both black boxes and transparent boxes
• Requires tradeoffs
• Too 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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How Do We Identify/Specify Functions? (contd)

• Functions, Means and your Project
• Means are methods of achieving functions, and
  they imply solutions
• Dont propose solutions to your Project at this
  point in the process
• Only 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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How Do We Identify/Specify Functions? (contd)

• Function-Means Trees
• A graphical representation of a designs basic
  and secondary functions
• Top 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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How Do We Identify/Specify Functions? (contd)

• Function-Means Tree for cigarette lighter

Fig. 4.2 p85
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How Do We Identify/Specify Functions? (contd)

• Some cautions Function-Means trees
• NOT a substitute for either formulating the
  problem or for generating solutions
• Should not be used without using some of the
  other tools listed earlier. Dont adopt a tool
  because it fits with your preconceived idea of a
  solution

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How Do We Identify/Specify Functions? (contd)

• Dissection and Reverse Engineering
• Designer 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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How Do We Identify/Specify Functions? (contd)

• May not be able to use the design
• May not do all things that we require
• May not do them well
• May be too expensive
• May be patented
• May be our competitors 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 function
• May press buttons, or test, and try to determine
  as many functions as possible

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How Do We Identify/Specify Functions? (contd)

• Cautions for Reverse Engineering
• Client / Users for dissected artifact may be
  quite different from yours
• Temptation to limit new means once existing means
  have been identified
• Reverse 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 DO
• Objectives tend to characterize what artifacts
  ARE

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Activity

• Xela-Aid chicken coop
• For 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