Evolutionary design techniques

In any design problem there is a multi-dimensional spaee of possible solutions. Some of these solutions may be aeeeptable, but not the best (loeal optima) and there may exist a single best solution (global optimum). 

It has been shown in Chapter 9 that it is possible to obtain an optimal mathemat-ieal solution for a eontrol system with linear plant dynamies. An alternative approaeh is to use heuristies , or knowledge aequired through experienee, to seareh for optimal solutions. One sueh teehnique is to employ a Genetie Algorithm (GA). 

This is a seareh algorithm based upon the evolutional proeess of natural seleetion of the fittest members of a given population to breed the next generation. 

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In addition to evolutionary search strategies such as GAs, there are a number of other search techniques that are employed for design optimization. 

AI methods may be used in various ways. The models may be used as a standalone application, e.g., in recent work on the design of microwave absorbers using particle swarm optimization (PSO).6 Alternatively, a computational tool, such as a finite element analysis or a quantum mechanical calculation, may be combined with an AI technique, such as an evolutionary algorithm. 

The methods of simulated annealing (26), genetic algorithms (27), and taboo search (29) are three of the most popular stochastic optimization techniques, inspired by ideas from statistical mechanics, theory of evolutionary biology, and operations research, respectively. They are applicable to our current problem and have been used by researchers for computational library design. Because SA is employed in this chapter, a more-detailed description of the (generalized) SA is given below. 

Three demonstration models were made for the Office of Saline Water. Figures 3, 4, and 5 give the basic elements of these models. The most obvious design change was the use of cellophane sheets for the capillary surface instead of the porous ceramic plates. The use of cellophane with its high entry pressure permitted tests on sea water as well as dilute sodium chloride solutions. Other evolutionary changes had to do with better techniques of gap fabrication and pressure control. 

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