Emerson Rosemount OCX 8800 Reference Manual page 176

Foundation Fieldbus
Figure 7-18:
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 produce 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.
illustrates the correct function block configuration.
Figure 7-19:
166
PID function block for steam heater control example
Figure 7-19 illustrates the process instrumentation diagram, and
PID function block feedforward control example
Figure 7-20
OCX 8800