A Simple Multi-objective Optimization Problem

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    A simple multi-objective

    optimization problem

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    Introduction

    Lets introduce a geometrical optimization problem, named cones problem, with thefollowing characteristics:

    multi-objective problem (two objective functions): the solution is not a single optimum

    design, but instead it is represented by the set of designs belonging to the Pareto frontie

    simple mathematical formulation: easy and quick implementation from scratch of the

    relevant modeFRONTIER project

    constrained problem: objectives space and designs space present feasible and

    unfeasible regions

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    Problem definition

    Right circular cone:

    r= base radius

    h = height

    s= slant height

    V= volume

    B = base area

    S= lateral surface area

    T= total area

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    Cones problem

    two input variables: r, h

    two objectives:

    one constraint:

    The cone shape (i.e. the

    design) is defined univocallywhen both rand h are given.

    We want to minimize both

    the lateral surface area andthe total surface area

    A constraint for the cone volume is

    given, in order to guarantee aminimum volume.

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    Project building

    Lets build from scratch the pertinent modeFRONTIERproject:

    1. Work Flow setup: fill the work canvas with the projects

    building blocks

    2. Script Node setup: use your favourite math tool

    Jython script

    Matlab node

    Excel Workbook node OpenOffice Spreadsheet node

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    Work Flow setup

    two input variables

    three output variablestwo objectivesone constraintscript node

    logic flow

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    Work Flow setup

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    Script node: Jython

    Jython (Python) script case:

    Load math module

    Note the syntax of mathematical functions and constants

    Write down the formulae

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    Script node: MatlabMatlab case:

    Write down the formulae

    Check Matlab version

    Load the matlab file

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    Script node: ExcelExcel Workbook case:

    Insert the formulae

    Build the spreadsheet

    Load the xls file

    Link variables to cells

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    Script node: OpenOfficeOpenOffice Spreadsheet case:

    Insert the formulae

    Build the spreadsheet

    Link variables to cells

    Load the sxc file

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    Runs examples

    Lets see some examples of runs with different DOEsand/or schedulers:

    Full Factorial DOE

    random samplings: Random Sequence and Sobol

    DOEs

    genetic algorithms: MOGA-II, NSGA-II

    MOSA

    NBI-NLPQLP

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    Full Factorial

    Full Factorial DOE

    10 levels per variable

    100 eval. designs

    feasible designsunfeasible designs

    Pareto designs

    objectives space

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    Random Sequence

    Random Sequence DOE

    1000 eval. designs

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    Sobol

    Sobol DOE

    1000 eval. designs

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    MOGA-II

    MOGA-II

    20 individuals (Sobol)

    50 generations

    1000 eval. designs

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    MOGA-II

    MOGA-II

    20 individuals (Sobol)

    50 generations

    1000 eval. designs

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    NSGA-II

    NSGA-II

    20 individuals (Sobol)

    50 generations

    1000 eval. designs

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    MOSA

    MOSA

    10 points (Sobol)

    100 iterations

    1000 eval. designs

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    NBI-NLPQLP

    NBI-NLPQLP

    (DOE: 10 Sobol)

    20 NBI-subproblems

    346 eval. designs

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    Final considerations

    Lets consider the difference between single-objective problem solutions: two different minima

    multi-objective problem solutions: the Pareto frontier

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    Single-objectives minimaEach design represents

    the optimum solution for

    its corresponding single-objective problem.

    ...but what about the in

    between designs?

    ?

    ...we would like to get a

    compromise solution. A

    trade-off of the twoobjectives...

    What we want is the Pareto frontier!

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    The Pareto frontier

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