Image Processing Reference
In-Depth Information
Fieldbus technology does not influence the way calibration is handled, although information that
is required for calibration is stored as part of the properties that describe a device. Such informa-
tion could be, e.g., the minimum calibration span limit. his is the minimum distance between two
calibration points within the supported operation range of a device. Additionally, calibration-related
information, i.e., the individual calibration history can be stored in the devices themselves. his infor-
mation is then remotely available for management tools in order to check the calibration status of
devices. Together with the self-diagnosis capabilities of the field devices this allows performing a
focused and proactive management strategy.
22.9 Maintenance in Fieldbus Systems
Fieldbus maintenance is the activity of keeping the system in good working order. The extensive
management functions provided by fieldbus systems, such as diagnosis, and monitoring greatly help
in maintaining systems. here are several diferent maintenance schemes that influence the way these
steps are executed in detail. Choice of a particular maintenance scheme is usually motivated by the
application requirements []:
•
Reactive maintenance
is a scheme, in which a device is only fixed after it has been
found to be broken. his case should be avoided in environments where downtimes are
costly (such as in factory applications). hus, designers of such applications will usually
choose more active maintenance strategies. Nonetheless, fieldbus systems also provide
advantages for this scheme, since they support the fast detection of faulty devices.
•
Preventive maintenance
is a scheme, in which devices are serviced at regular intervals
even if they are working correctly. This strategy prevents unexpected downtime, thus
improving availability. Due to the associated costs, this approach will only be taken
in safety-related applications such as in aviation, train control, or where unexpected
downtimes would lead to very high costs.
•
Predictive maintenance
is similar to preventive maintenance, differing in a dynamic
service interval that is optimized by using long time statistics on devices.
•
Proactive maintenance
focuses on devices that are expected to require maintenance.
Basically, maintenance involves the following steps:
•
Recognizing a defective device
•
Repairing (replacing) the defective device
•
Reintegrating the serviced device
In fieldbus systems, faulty devices will usually be recognized via the network. This is achieved by
monitoring the fieldbus nodes and the application or with devices that are capable of sending alerts
(also refer to Section .).
After the source of a problem has been found, the responsible node must be serviced. his often
means to disconnect the node from the network. hus, we require strategies how the system should
deal with
disconnecting
the node, as well as reconnecting and
reintegrating
the replacement node.
Incasethewholesystemmustbepowereddownformaintenance,thefaultynodecansimplybe
replaced and the integration of the new node occurs as part of the normal initial startup process. If
powering down the whole system is undesirable or even impossible (in the sense of leading to severe
consequences, as in case of safety-critical applications), this process becomes more complicated. In
this case, we have several options:
•
Implementation of redundancy
: his approach must be taken for safety or mission-critical
devices, where operation must be continued after a device becomes defective, respectively
