Image Processing Reference
In-Depth Information
Sensor
Analog in
PID
Analog out
Actuator
Sensor
Analog in
PID
FIGURE .
Example for an application model. (From Pitzek, S. and Elmenreich, W., Configuration and manage-
ment of fieldbus systems. In R. Zurawski, ed.,
he Industrial Communication Technology Handbook
.CRCPress,Boca
Raton, . With permission.)
22.6 Application Development
At the center of a fieldbus system is the actual fieldbus application. In the following we examine
several application development approaches, and how they influence system configuration.
A widely used development approach for fieldbus applications is the model-based development
approach. he basic idea behind this approach is to create a model of the application that consists of
components that are connected via links that represent the communication flow between the com-
ponents. Different approaches usually differ in what constitutes a component (e.g., function blocks,
subsystems, services, functional profiles, physical devices) and the detailed semantics of a link. Many
approaches support the recursive definition of components, which allows grouping multiple lower-
level components into one higher-level component. Figure . depicts the model of a typical small
control application consisting of two analog inputs receiving values from two sensors, two PIDs, and
one analog output controlling an actuator.
But the model-based approach is not the only application design approach. Another approach used
by multiple fieldbus configuration tools is the ANSI/ISA-.- procedural control model [].
his modeling approach enforces a strictly modular hierarchical organization of the application (see
Figure .). .).There should be no or hardly any interaction between multiple process cells. Interaction
betweencomponentsinaprocesscellisallowed.Tomakebestuseofthisapproach,thestructureofthe
network site and the application should closely correspond to the hierarchy specified by this model.
he modeling approach conceptually follows the typical hierarchy of process control applications
with multiple locally centralized programmable logic controllers that drive several associated control
devices. This eases transition from predecessor systems and improves overall robustness, since this
approach provides fault containment at the process cell level. As a downside, the coupling between
the physical properties of the system and the application is rather tight. An example for a fieldbus
protocol that supports this modeling approach is the PROFIBUS PA protocol that supports this model
by providing a universal function block parameter for batch identification [].
Another design approach is the two-level design approach [], which originated in the domain of
safety critical systems. In this approach the communication between components must be configured
before configuring the devices. While this requires that many design decisions must be taken very
early in the design process, this approach greatly improves overall composability of the components
in the system.
Abstract application models provide several advantages for application development:
•
Modular design of applications helps to deal with complexity by applying a “divide-and-
conquer” strategy. Furthermore, it supports reuse of application components and physical
separation.
