Mapping of the process ontology to EngMath is depicted in the right-hand
side of Figure 9. Informally, the mapping states that
for each energy flow there are two time-dependent physical quantities,
one for the effort and one for the flow. The domain of the energy flow
determines the dimension of the quantities. For instance, an electrical
effort quantity has the dimension `energy/electrical-current`
(voltage) and the flow quantity the `electrical-current`
dimension. For each mechanism there is a mathematical relation that
relates the values of the physical quantities of the energy flows
connecting the mechanism to each other. The mapping also imposes
constraints on the relation. These constraints only depend on the
mechanism type and they are independent of the domains of the energy
flows. The mathematical relation in Figure 9 belongs
to a dissipator mechanism. The constraints on such a relation are that
it is a continuous function that lies in the first and
third quadrant and that *r*(0) = 0. For an electrical energy flow, this can
be an instantiation of Ohm's law whereas in the
mechanical domain it can model some kind of friction with .

**Figure 11:**
Excerpt from the second part of the PHYSSYS ontology where physical
processes are projected onto mathematical relations. This is an
example of a domain ontology that only contains formalizations of
the interdependencies between the viewpoints it includes.

Figure 11 shows an excerpt of the second part of
PHYSSYS, the part that performs the process to mathematics projection
described above. Only a part of the definition of the relation
`mech.mathrel`, the relation that relates a mathematical relation
to a process, is shown. Axiom 7a states that for every dissipator a
relation between the effort and flow quantities of the energy flow to
the dissipator must exist. In the axiom the relations
`ef.effortq` and `ef.flowq` are used. These relations link
each energy flow to physical quantities for the effort
and the flow. Axioms not incorporated in Figure 11
ensure that these quantities have the proper physical dimension. The
constraints on the relation for dissipators have been formalized by
stating that the effort is zero if and only if the flow is zero and
that the product of effort and flow, i.e. the energy flow, must be
positive. In other words, the dissipator must dissipate energy.
Furthermore, it is probably needless to say that PHYSSYS contains
axioms like axiom 7a for each type of mechanism defined in the process
view.

Fri Sep 27 13:28:43 MET DST 1996