The Top-Down Design Process

Let us start the development of our functional design methodology by considering the end product i.e. what we want to end up with before making the transition into the physical domain. We would like a description of the functionality of a plant that has the following characteristics  

Such a description is a model of the plant, consisting of real and imaginary functional elements, but it is a very special model. On the one hand, it represents the outcome of design in the functional domain. It is not a model of the stakeholder requirements, but a model of the functionality of a possible solution to the problem of meeting those requirements. The architecture of the primary functional system (which we called the skeleton in Chapter C4) is identical to the functional view of the plant architecture. On the other hand, it provides a measure of the degree to which the plant will satisfy the stakeholder requirements within the context of the project, thereby allowing the design to be optimised. 

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The relationship between the value of n and the level of detail arises only within the top-down development of the system the size of the system (and remember, this is the description of the object) is increased in a step-wise process, from global variables into more and more detailed variables associated with an increasing number of system elements, until all the variables and relations in the original description are accounted for. This process does not (indeed, must not) add anything to the original description it recasts it in a form that will make the design process much more efficient. This is described in Chapter A5 here we shall first look at some of the implications for a set of n elements, no matter where in the top-down design process it is located. 

The point of departure for this step, and therefore the end result of the fust step, is a definition of the plant in terms of what it should do and how well it should do it, the conditions under which it must be able to do it (what is shown as System Specification in Fig. B4.3), and any direct, physical requirements on the object, such as type of materials to be used (or avoided), surface finishes, weight and size restrictions, etc. The process of getting from the start to the finish of this second step has been developed and refined over the last couple of centuries, and we shall look at it in more detail in the next chapter. Here we just note that its main characteristic is a synthesis in terms of known construction elements, and for this reason it is often called the bottom-up process, this also serves to distinguish it from the process in the first step, the top-down design process, with which we shall mainly be concerned in this chapter. 

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