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station by exchanging commands and feedback data. A graphical user
interface represents the PLC's local console.
The supervisory station has a graphical user interface that allows the
technician to control the car painting process.
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The second requirement deals with system distribution. Distribution
spans two dimensions: the work cell equipment and the control system.
The real car painting work cell is made up of physically distributed colour
tanks connected to each other through pipes and pumps, which control
the paint flow from one tank to another. The simulator behaviour must
enforce the physical constraints that characterize the work cell's equip-
ment, that is, the paint level of two connected tanks should decrease and
increase consistently without dissipation.
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The real work cell area may be very wide, up to thousands of square
metres. The supervisory station might reside in a control cabin that
jointly supervises several work cells. The SCADA system consists in a
local area network or even in a wide area network of workstations.
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Finally, it is important to guarantee a seamless transition of the SCADA
system from the computer simulation to its deployment in a real work cell.
For this purpose we need to evaluate how many control workstations are
needed, how they are interconnected with the work cell simulator and the
supervisory station, and which communication mechanisms are best
suited.
13.2.1
Domain models
Traditionally, SCADA systems use dedicated high-bandwidth networks to
interconnect physical devices such as sensors and actuators, a central data-
base of process data, and computational resources such as the supervisory
station.
A new generation of virtual SCADA systems builds on internet tech-
nology. The current trend, “embedding the Internet” (Estrin
et al
. 2000),
pursues the interconnection of any kind of physical and virtual device
through the internet. Strictly speaking, from the point of view of automation,
the most challenging opportunity offered by the new web technology is to
achieve tight dynamical interaction over the internet (i.e. real time moni-
toring and control).
Figure 13.2 represents the distributed architecture of a virtual SCADA
system. The rectangular boxes indicate the typical control modules of a
SCADA system. The communication infrastructure is logically structured as
an information bus (Brugali and Menga 2002), i.e. a flat interconnection of
autonomous and decentralized control modules that manage local
resources, support decision making by formulating problem-solving plans,
carry out these plans through querying and exchanging information with
other control modules, and delegate to them the execution of specific
services.
