Rosemount OCX 8800, Instruction Manual

Page: 146 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-46
Figure 7-16. PID Function Block Diagram for Steam Heater Control Example
Outlet
Temperature
Input
BKCAL_IN BKCAL_OUT
TT101 TC101 TCV101
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 7-17 illustrates the process instrumentation diagram,
and Figure 7-18 illustrates the correct function block configuration.
Figure 7-17. PID Function Block
Feedforward Control Example
AI
Function
Block
PID
Function
Block
OUT CAS_IN
AO
Function
Block
OUT
OUT IN
TCV
101
FF TC
101
Steam Supply
TT
100
TT
101
Steam Heater
Condensate
3
8
7
4
0
0
9
6
3
8
7
4
0
0
9
7