Emerson Oxymitter 5000 Instruction Manual page 155

Application Example: Cascade
Control with Master
and Slave Loops
FIGURE C-7. PID Function Block
Cascade Control Example.
Model 4081FG Oxygen Analyzer with F
Situation
A slave loop is added to a basic PID control configuration to measure
and control steam flow to the steam heater. Variations in the steam
pressure cause the temperature in the heat exchanger to change. The
temperature variation will later be sensed by TT101. The temperature
controller will modify the valve position to compensate for the steam
pressure change. The process is slow and causes variations in the
product temperature. Figure C-7 illustrates the process
instrumentation diagram.
FT
101
Steam
Supply
TT
100
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 C-8 illustrates the correct
function block configuration.
fieldbus Communications
OUNDATION
FC
101
TCV
101
Steam Heater
Condensate
TC
101
TT
101
C-13
C