Information Technology Reference
In-Depth Information
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Two input blocks (analogue and digital).
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Two output blocks (analogue and digital).
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Six control blocks, such as PID (proportional-integral-derivative).
Other, less well used, control functions include pulse input, arithmetic, dead time, splitter
and signal characterisation.
Most current control systems use a DCS (distributed control system) to control and the
transmitter simply determines the process variable. Smart transmitters will change this as
basic regulatory control can be moved to the transmitter. This reduces the loading on the
controller, and as the functions become more complex may eliminate the controller all to-
gether.
The control with Fieldbus is relatively easy if the devices are located on the same bus,
and are located in near proximity. Control in the field devices works well if the elements of
control are located relatively close to each other. This allows function blocks to be linked
without having to span different bus segments, and thus reduces delays.
Messages on the bus are divided into two classes:
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Cyclic - these messages involve process data which is transferred between linked func-
tion blocks and can be made part of a network. The 31.25 kbps Fieldbus supports ap-
proximately 30 messages per second
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Acyclic - these are single transfers of data. The scheduling of these is determined by the
control equipment and is flexible in its approach, thus allowing the bandwidth to be used
effectively.
19.3.9 Diagnostic information
Maintenance methods differ from plant to plant. These are:
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Preventative maintenance (PM) - this is where plant is inspected and, if necessary, re-
placed, before faults occur. In some cases, PM can cause more problems than it is worth,
because when a piece of plant is disturbed it can often lead to faults that would not have
happened.
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Unfortunately, to operate PM properly, it requires a great deal of information about the
operation of the plant for its previous history.
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Deferred maintenance (DM) - this is where maintenance is deferred to save costs. Unfor-
tunately, deferred maintenance can often lead to long-term costs, typically causing plant
shutdowns, complete rebuilds for expensive equipment or leading to an unsafe plant.
In the past manufacturers have built in diagnostic information to microprocessor-based de-
vices. Unfortunately, the method of implementation has been non-standard. Typically, each
diagnostic signal required an additional 4-20mA signal to be sent to a host or DCS. In some
cases, a proprietary digital protocol allowed the transmission of multiple diagnostic signals
over the same pair of wires. This all required extra control system programming and alarm
handling.
The Fieldbus overcomes this with a standardised comprehensive alarm-reporting mecha-
nism and the DDL. A host or DCS supporting Fieldbus does not require any special configu-
ration or programming to accept the manufacturer-specific predictive diagnostic information.
The diagnostic information can be used to determine when a device needs to be main-
