Definitions
In this section we define and outline terms that are often used in product development; that is requirement, function, constructive layout and technology. Our effort is to facilitate and to systematize the determination of abstraction levels in a functional model and to avoid obscurities when distinguishing e.g. between constructive layout and technology.
Lindemann [6] defines requirement as a design objective, which has to be achieved and as prerequisite, which has to be fulfilled. Requirements can be derived from design constraints, customer requests and boundary conditions such as specification of the maximum allowable weight, which determine the values of the design technical characteristics. In the context of this contribution the source of the requirements will not be further examined.
In order to meet the defined requirements, functions have to be set that satisfy them. As Ehrlenspiel [7] points out, a function is the solution independent description of a system activity. Furthermore, Pahl and Beitz [1] define function as the relation between input and output of a system with the goal to complete an activity or task and thus a function can be used in order to describe a system’s activity on an abstract and solution independent level.
Nevertheless, in the next section it will become apparent that not all the functions of a product feature the same solution independency. The extent of the solution independency of a function depends on which hierarchical level the function is arranged. Accordingly, the extent of the solution independency of a function constitutes an accurate criterion in order to define hierarchical levels in a function model.
By constructive layout we mean the form and the design solution that has been chosen in order to realize a function. A constructive layout mostly consists of a set of interacting physical components that are composed in an adequate manner. In order to compose the components adequately and to assure their interactions technology knowhow is required. Thus technologies determine the kind of the components composition and beyond that the functional feasibility of the constructive layout.
The figure below outlines the relations between these terms. Beginning from the defined product requirements functions are set in order to fulfil the requirements. As pointed out, these functions are abstract solution definitions but do not give any technical and technological information about how to realize a solution. They are used in order to structure and subdivide the problem in smaller and easier to solve sub-problems. Furthermore, they feature a hierarchical structure and can be analyzed and broken down to elemental functions. These elemental functions can then be linked to physical components that realize them. The components’ composition and interactions are defined in constructive layouts, which in turn require technology know-how in order to be carried into effect.
In figure 2 the defined requirement is that a leak between the valve piston and piston guidance has to be prevented. In order to satisfy this requirement the function “seal guidance” is set, which is realized by a “sealing ring”. The constructive layout specifies further the solution through the definition of the “sealing ring” and the components that interact with the “sealing ring”. In order to implement this solution and to assure the required component interactions, technology know-how is needed about the ring material, the tribological behaviour of this material with the guidance material and beyond that know-how is required about surface treatments in order to achieve the required tribological conditions between ring and guidance material [8].