unit5notes - Unit 5 Objectives in conflict: trade-off...

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Unit 5 Objectives in conflict: trade-off methods and value functions
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2 CES EDUPACK Notes for Instructors Frame 5.1 Outline Description The main points to be covered by the Unit. Further Information The Text, Chapters 9 and 10, and “Multi-objective optimisation in material design and selection” by M.F. Ashby, Acta Mater. (2000), vol 48, p. 359 – 369.
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Unit 5: Objectives in conflict 3 Frame 5.2 The problem of conflicting objectives Description Real-life decision-making frequently requires that a compromise be reached between conflicting objectives. The compromises required to strike a balance between wealth and quality of life, between the performance and the cost of a car, or between health and the pleasure of eating rich foods, are familiar ones. Conflict arises because the choice that optimises one objective will not, in general, do the same for the others; then the best choice is a compromise, optimising none but pushing all as close to their optima as their interdependence allows.
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4 CES EDUPACK Notes for Instructors Frame 5.3 Conflicting objectives in design Description An objective, it will be remembered from Unit 2, defines a performance metric. If the objective is to minimise mass , then the mass becomes the metric of “goodness” of a given choice: the lightest solution that meets all the constraints of the problem is the best choice. If the objective is to minimise cost , then the cheapest solution that meets all constraints is the best choice. The metric allows solutions to be ranked. This frame lists common design objectives; there are, of course, many more. It is rare that a design has only one objective. And when there are two a conflict arises: the choice that minimises one metric – mass say – does not generally minimise the other – cost, for example. Then a compromise must be sought. To reach it we need some simple ideas drawn from the field of multi-objective optimisation .
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Unit 5: Objectives in conflict 5 Frame 5.4 Multi-objective optimisation: the terminology Description Multi-objective optimisation is a technique for reaching a compromise between conflicting objectives. It lends itself to visual presentation in a way that fits well with methods developed here thus far. This frame explains the words. They are illustrated by the diagram on the right in which we have specialised a problem to a trade-off between the mass of a component and its cost. The first bullet point on the frame defines a solution : a choice of material to make a component that meets all the necessary constraints and is thus a candidate for the design, although not perhaps the best one. The little circles each represent a solution; each describes the mass and cost of the component if made from a given material. The next two bullet points distinguish between a dominated solution (meaning that other solutions exist that are both lighter and cheaper) and a non-dominated solution (one that is lighter than all others that cost more and cheaper than all others that are heavier – thus there is no
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This note was uploaded on 02/01/2012 for the course MSEG 302 taught by Professor Snively during the Spring '08 term at University of Delaware.

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unit5notes - Unit 5 Objectives in conflict: trade-off...

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