Rosemount Oxymitter 5000, Instruction Manual

Page: 204 / 214

Hazardous Area Oxymitter 5000
E-12
Instruction Manual
IM-106-350C, Rev 2.2
July 2008
Figure E-4. 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 E-5
illustrates the process instrumentation diagram, and Figure E-6 illustrates the
correct function block configuration.
Figure E-5. PID Function Block
Feedforward Control Example
Outlet
Temperature
Input
AI
Function
Block
PID
Function
Block
AO
Function
Block
TT101 TC101 TCV101
BKCAL_IN BKCAL_OUT
OUT IN
CAS_IN OUT
OUT
38740096
38740097
Steam Supply
TCV
101
Steam Heater
Condensate
TC
101
TT
101
TT
100
FF