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Compositional Models for Complex Systems 247

meas
Therm
set air set on/off air
HVAC Ctrl AC
Heat
on/off
A black-box A system
(A) component (B) decomposition
Fig. 13.1 Graphical representations of HVAC as a black-box component and as a
system. (A) A black-box component; (B) a system decomposition.

Bob or Charlie (e.g., cooling rate to Charlie for the AC design). Other con-
straints will be generated by Alice in order to ensure that her control strategy
meets the global requirements of the HVAC system, usually determined or
justified by one or more models of the system.
This is a recursive process because Bob and Charlie are now in a position
that is structurally similar to Alice’s at the outset. Each is tasked with designing
a certain system subject to a given set of requirements. Now Bob and Charlie
proceed in exactly the same fashion that Alice did: decomposing their subsys-
tems, realizing some components, and outsourcing other designs.
Eventually, this iterated series of decompositions must bottom out with a
concrete implementation for each component. Bob and Charlie come back
to Alice with completed designs for their components, which they have
tested and verified against the requirements she gave them.
Now Alice must integrate the new AC and heater designs with her con-
troller and the off-the-shelf thermometer she selected. Once this is done she
performs her own tests to make sure that the resulting HVAC system meets
the requirements that she was given. If all works as expected, she hands back
the completed HVAC design to her customer or up the chain to the next
level of system abstraction.
With this example in mind, we can give a rational reconstruction of the
1
engineering design process. We will use the language of systems, but noth-
ing we say is specific to systems engineering as it is traditionally understood.
One important thing to keep in mind is that words like “component” and
“system” are context-relative: the whole point is that Alice’s component is
Bob’s system. Something is a component when we treat it as a black box, and

1
For now we will focus only on the constraint-satisfaction aspects of engineering, leaving
aside optimization, though this could be added on top of the proposed framework. For a
more detailed characterization of the design process from a comprehensive AI perspective
see Tong and Sriram (1992).