Image Processing Reference
In-Depth Information
Application Class
RT
GD
BE
Monitoring
+
+
±
Open-loop control
±
++
+
Closed-loop control
+
-
+
File transfer
±
+
-
FIGURE .
Application classes and possible requirements (RT, hard real-time; GD, guaranteed delivery; BE, best-
effort) imposed on the network. (From Sauter, T., in
he Industrial Communication Handbook
,CRCPress,BocaRaton,
F L , , . - . . Wi t h p e r m i s s i o n . )
Distributed applications implemented on automation networks typically cope with process data
acquisition, processing, and signaling. They can be categorized into several “application classes,”
which can be mapped onto the performance classes introduced before (Figure .).
Monitoring/Logging
This application class allows to view or record data within a control application. Typically, selected
process parameters are sent to a monitoring station, where they can be tracked or logged. he QoS
requirements can range from hard real-time to no requirements at all (meaning an entirely unreliable
network is acceptable as well). Examples are remote power meters, weather sensors, or occupancy
detectors.
Open-Loop Control
Open-loop control appears where the process characteristics and disturbances are well-known so
that feedback paths are not required. he network should provide at least reliable data transfer, but
the timing requirements can range from non-real-time to hard real-time. Multidimensional open-
loop controls can pose additional timing requirements originating from the time dependencies of the
output values. Control data are typically sent periodically, in which case the overhead for providing
guaranteed delivery, e.g., by packet retransmission, can be avoided. Examples are light switches,
intelligent power outlets, or dot matrix displays.
Closed-Loop Control
Closed-loop control applications are commonly found in automation tasks, where control quality
and error compensation are required. The controller does not need to be part of the application;
it can be an external system such as a human operator. Timing variations influence stability and
performance of a closed-loop control, so this application class has at least a demand for soft real-
time behavior. Again, multidimensional control is possible and can enforce more strict timing
requirements. Examples are heating, industrial control, and power management.
File Transfer
All distributed automation systems need in one form or the other the transmission of files or at
least larger data blocks instead of process data. The typical application is to configure or parame-
terize field devices either at system start-up or later during runtime. Sometimes, even executable
code is transferred, e.g., to update programs running on controllers. he minimum requirement for
the data transmission is reliable and guaranteed data transfer. In rare cases, when the upload of a
device configuration is time-critical, also hard real-time requirements may apply.
