Rosemount OCX 8800, Instruction Manual

Page: 148 / 266

Instruction Manual
IM-106-880, Rev 2.1
April 2017
OCX 8800
7-48
AI
Function
Block
OUT IN
PID
Function
Block
OUT
TT 101 TC 101
BKCAL_IN BKCAL_OUT
Steam
Flow
Input
AI
Function
Block
CAS_IN
PID
Function
Block
OUT
IN
OUT IN
AO
Module
Block
Solution
If the flow is controlled, steam pressure variations will be compensated
before they significantly affect the heat exchanger temperature. The output
from the master temperature loop is used as the setpoint for the slave
steam flow loop. The BKCAL_IN and BKCAL_OUT connections on the
PID blocks are used to prevent controller windup on the master loop when
the slave loop is in Manual or Automatic mode, or it has reached an output
constraint. Figure 7-20 illustrates the correct function block configuration.
Figure 7-20. PID Function Block Diagram for Cascade Control Example
Outlet
Temperature
Input
BKCAL_IN BKCAL_OUT
FT 101 FC 101 TCV 101
Cascade Control with Override
You can use the PID function block with other function blocks for complex
control strategies. Figure 7-21 illustrates the function block diagram for
cascade control with override.
When configured for cascade control with override, if one of the PID function
blocks connected to the selector inputs is deselected, that PID block filters the
integral value to the selected value (the value at its BKCAL_IN). The selected
PID block behaves normally and the deselected controller never winds up. At
steady state, the deselected PID block offsets its OUT value from the selected
value by the proportional term. When the selected block becomes
output-limited, it prevents the integral term from winding further into the limited
region.
When the cascade between the slave PID block and the Control Selector
block is open, the open cascade status is passed to the Control Selector block
and through to the PID blocks supplying input to it. The Control Selector block
and the upstream (master) PID blocks have an actual mode of
IMan .
3
8
7
4
0
1
0
0