Solution
The PID loop uses TT101 as an input and provides a signal to the analog output TCV101. The
BKCAL_OUT of the AO block and the BKCAL_IN of the PID block communicate the status and
quality of information being passed between the blocks. The status indication shows that
communications is functioning and the I/O is working properly. Figure 7-19 illustrates the
correct function block configuration.
Figure 7-19 PID Function Block Diagram for Steam Heater Control Example
Feedforward Control
Situation
In the previous example, control problems can arise because of a time delay caused by thermal
inertia between the two flow streams (TT100 and TT101). Variations in the inlet temperature
(TT100) take an excessive amount of time to be sensed in the outlet (TT101). This delay causes
the product to be out of the desired temperature range.
Solution
Feedforward control is added to improve the response time of the basic PID control. The
temperature of the inlet process fluid (TT100) is input to an AI function block and is connected
to the FF_VAL connector on the PID block. Feedforward control is then enabled (FF_ENABLE),
the feedforward value is scaled (FF_SCALE), and a gain (FF_GAIN) is determined. Figure 7-20
illustrates the process instrumentation diagram, and Figure 7-21 illustrates the correct function
block configuration.
Figure 7-20 PID Function Block Feedforward Control Example
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