206 Yoshinobu Kitamura and Riichiro Mizoguchi
that a behavior has to perform a certain function is anti-rigid, since a behavior can perform
different functions according to contexts without loss of its identity. Third, a function can
be performed (realized) by different behaviors. A behavior can perform multiple functions
simultaneously. Thus a function is dynamic and multiple.
We say that “a behavior plays a function role.” If a device performs a behavior
and the behavior plays a function role in a context, then the device plays a function-per-
former role in the context. For example, the heat-exchange behavior plays the heat-remov-
ing function role, and a heat exchanger plays the function-performer role of removing heat
as a radiator. The heat-exchange behavior implies changes in the temperatures of both
fl uids. When its function is recognized, one of the fl uid fl ows becomes of further
interest.
Note that the difference between function and behavior depends on their context. For
example, “to change (increase) temperature” is a function of an electric heater, though the
temperature-changing function is similar to the heat-exchange behavior (i.e., changes in
the temperatures of both fl uid fl ows) of the heat exchanger. In this case the behavioral
model of the heater includes changes both in temperature and electricity. Thus another
possible function of the electric heater is “to consume electricity.” The temperature-chang-
ing function of the heater is dependent on the goal, whereas the heat-exchange behavior
of the heat exchanger is independent of any goal.
Furthermore, note that a device’s function here refers to local behaviors in the device
(which we call a “local function”), though it is dependent on context. Thus its variety is
restricted by the device’s behavior. On the other hand, a conjunct function refers to behav-
iors of other components, the embedded system, or users. For example, the heat exchanger
mentioned earlier can perform conjunct functions such as controlling the temperature of
a room or preventing an engine from overheating. The conjunct functions cannot be
exhaustively enumerated in nature. We use local functions as the base-functions here in
order to realize composability of device models.
12.2.3 Functional Context
The context of use represents teleological goals to be achieved by the function (which we
call the “functional context”). If the device is a whole product, its functional context (and
its goal) is determined by how it is used by users externally (which we call the “external
function context”). A function in an external function context (which we call the “external
function”) is one that is intended by a user. Some external functions are also intended by
the designer, as discussed in section 12.2.5.
On the other hand, a function of a component embedded in a system contributes to
achieving the system’s function. Thus its functional context (which we call the “system
function context”) is determined by a functional structure in which the system’s function
is achieved by a sequence of (relatively fi ner-grained) component functions. We call this