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A Device-Oriented Defi nition of Functions of Artifacts and Its Perspectives 211

12.2.7 Relations Among Functions and the Way of Function Achievement

We believe that a clear understanding of relationships among functions contributes to a
clear defi nition of function. We can distinguish “part-of” (called “is-achieved-by”) from
“is-a” relations. As pointed out in section 12.2.3 and further discussed in the appendix, in
a system context a function of a system is (or can be) achieved by a series of fi ner-grained
functions of components. (We call these functions a “goal-function” and “method func-
tions,” respectively). Figure 12.2 shows an example of a functional model of an artifact
using such an “is-achieved-by” relation, a so-called function decomposition tree. In the
literature in engineering, similar relation has been captured as a “degree of complexity”
(Hubka and Eder 1988) and as a function decomposition (Pahl and Beitz 1996). This is-
achieved-by relation shows a part of Cummins’s causal role of the components for capacity
of a containing system (Cummins 1975).

Store

coffee

Heat

water

Functional Concept Ontology

(is-a hierarchy of generic functions)

Example of is-a hierarchy of

generic ways of function

achievement for removing entity

Energy functions

Receive Make energy existent

Convert Shift Change

composition

Take Give Transmit

Remove

Separate

Combine

is-a is-a

Ways for removing entity

Phase way Constituent

way

Weight

way

Size way

Centrifugal

separating way

Filter

way

Extraction

way

Distillation

way

Example of a function

decomposition tree

of a coffee maker

(part-of hierarchy)

is-achieved-by

AND

AND AND

Transform electricity

to heat

Give heat

to water

Move

water

Select ground

coffee

Extract coffee taste

Hot fluid way

Filter way

Remove

ground coffee

Mix ground coffee

and water

Heat transfer

Chemical way Physical way

Figure 12.2
Relations among functions.