EE/CpE322 Lecture 4. Bruce McNair Based on Engineering Design: A Project-Based Introduction (the 3 rd ed.), by C.L. Dym and P.

Transcription

1 EE/CpE322 Lecture 4 Bruce McNair Based on Engineering Design: A Project-Based Introduction (the 3 rd ed.), by C.L. Dym and P. Little

2 A Model of the Design Process

3 Defining Client s Problem! Identify and clarify what the client wants or needs! Start with some problem statement! It may contain errors, biases, and possible implied solutions.! Talk to the client and ask questions, encourage the client to discuss the problem, not merely re-confirm designer s understanding.! Teams may engage in brainstorming and other activities to elicit the client s ideas on the problem

4 Definitions of Design Concepts! Goals or objectives: expressions of desirable attributes of a design or solution to the problem.! Can often be expressed in terms of more or less of something, such as more durable which is better than a less durable design.! Constraints: limitations on the design which must be met for the design to be acceptable.! These can be thought of as binary in that they are either met or not. Any design that fails to meet a constraint is to be rejected.! Functions: the things the design must do, usually expressed in terms of verb-object combinations, such as resist force or convert power to heat.! Means or Implementations: ways of performing the functions that the design must do.

5 Object Attributes and Attribute List! Example: beverage container

6 Objectives Trees! Objectives trees are hierarchical structures based on attribute list.! They are for the object being designed, not for the design process.! The top levels represent the most important goals, the lower levels branch out to describe these goals in terms of subgoals.! Reading an objectives tree downward reveals how a goal is defined.! Reading an objectives tree upward reveals why a subgoal is important.

7 Objectives Trees! Example: beverage container

8 Objectives Trees! Example: beverage container (including constraints)

9 Objectives Trees! Another example: safe ladder

10 Rank Ordering Objectives! Even designs that operate very efficiently, or very creatively may not appeal to the value set of the client.! It is necessary for the design team to understand the relative importance of the client s objectives.! Pairwise comparison charts are one way to arrange the values that clients or designers attach to design objectives.

11 Pairwise Comparison Charts! Example: beverage container

12 Pairwise Comparison Charts! Example: safe ladder! Aggregated PCC! 12 designers produced following preference ordering set: 1 preferred A B C 4 preferred B C A 4 preferred A C B 3 preferred C B A

13 PCC Comments! Pairwise comparison should be used in a constrained, top-down fashion to insure that objectives are only compared to others emanating from a common node.! While there is a temptation to use them as a basis for numerical evaluations, we need to remember that pairwise comparisons can give us no more than relative rankings. They are not strong measurements.! Just because an objective is ranked last, this does not mean that it is necessarily of no value to the client.

14 Measuring Objective Achievement! Metrics are used to determine how well objectives have been attained.! Steps for developing metrics:! Identify units and scales for the thing to be measured.! Identify means of assessing values of a design in terms of the units and scale selected.! Evaluate whether or not the particular measurement is feasible and appropriate for the designs being considered.

15 Characteristics of Metrics! Should be defined early in the design process! Actually measures the objective! Correct level of accuracy and tolerance! Repeatable! Understandable units of measure! Unambiguous interpretation

16 Objective Assessment! Example: an arm support device for CP students