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Design of an Automatic Control Algorithm for Energy-Efficient ...

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6 The optimiser: <strong>an</strong> evolutionary algorithm approach 53<br />

objective functions. Based on this some are selected as parents. With different methods,<br />

new individuals are created through a recombination <strong>of</strong> the chosen parents. Mutation, the<br />

r<strong>an</strong>dom ch<strong>an</strong>ge <strong>of</strong> design variables, is applied <strong>an</strong>d the resulting population is evaluated.<br />

Finally, the replacement operator deletes the worst per<strong>for</strong>ming individuals.<br />

An operation sequence <strong>of</strong> this basic evolutionary algorithm is found in Figure 6.2.<br />

Terms <strong>an</strong>d definitions that will be used in the following which are used <strong>for</strong> evolutionary<br />

algorithms are explained in Table 6.1.<br />

Figure 6.2: General operation sequence <strong>for</strong> <strong>an</strong> evolutionary algorithm. [31]<br />

Based on the first representatives, evolutionary programming (EP) (Fogel, Owens,<br />

& Walsh, 1966), evolution strategies (ES) (Schwefel, 1965; Rechenberg, 1973), genetic<br />

algorithms (GA) (Holl<strong>an</strong>d, 1975), <strong>an</strong>d genetic programming (GP) (Koza, 1992), a broad<br />

variety <strong>of</strong> algorithms with bigger or smaller ch<strong>an</strong>ges <strong>an</strong>d improvements exists. Some c<strong>an</strong><br />

h<strong>an</strong>dle multiple objectives <strong>an</strong>d provide the pareto front - the set <strong>of</strong> the best compromises.<br />

More <strong>an</strong>d more feature elitism, where the best parents are stored <strong>an</strong>d inserted be<strong>for</strong>e the<br />

replacement operation.<br />

It is one characteristic <strong>of</strong> this class <strong>of</strong> methods is that, due to its heuristic nature,<br />

one run is not repeatable. The solution is never exactly the same if per<strong>for</strong>med twice.<br />

They proved to be very robust <strong>an</strong>d are able to h<strong>an</strong>dle discrete variables as well as non-<br />

continuous objective functions because <strong>of</strong> the this property, too. However, there is no<br />

indication that a found solution is optimal.<br />

The fact that these algorithms are working with a group <strong>of</strong> points permits <strong>an</strong><br />

independent evaluation <strong>of</strong> the fitness <strong>an</strong>d makes them perfectly parallelisable, which is<br />

import<strong>an</strong>t <strong>for</strong> multi-core processors or clusters. On the other h<strong>an</strong>d, the number <strong>of</strong> required<br />

function evaluations is quite high. For this reason, special care has to be taken that the<br />

resulting algorithm <strong>an</strong>d the system simulation is not too computationally expensive.

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