Controller Enhancements

1
Controller Enhancements

• Cascade Control (Ch. 14, Marlin)
• Feedforward Control (Ch. 15, Marlin)

2
Hot Oil Tank
TI
TIC
(from Chapter 14, Marlin)
FI
3
Cascade Control

• scenario
• existing temperature controller gives
  unsatisfactory control performance
• disturbances in hot oil flow rate are a main
  cause of fluctuations in tank exit temperature
• hot oil flow rate measurement is available
• adjusting the valve causes hot oil flow rate to
  change
• valve influences hot oil flow faster than it
  influences temperature.

4
Hot Oil Tank with Cascade Control
TI
TIC
FIC
5
Cascade Control

• How it works
• Temperature controller (primary loop or master)
  manipulates the setpoint of a flow controller
  (secondary loop or slave) to maintain tank exit
  temperature
• Cascade control can be applied when the following
  criteria are satisfied
• Regular feed back control isnt good enough
• Important disturbances can be detected using a
  measured secondary variable
• Cause/effect relationship exists between
  manipulated variable and secondary variable
• Secondary variable responds faster than primary
  controlled variable.

6
Reactor Example
F2
F1
T2
T3
A2
T1
How could cascade control be used? When wouldnt
it work?
AC1
7
Reactor Example
FC
F1
T2
T3
A2
T1
(from Chapter 14, Marlin)
AC1
8
Tuning Cascade Loops

• Tune inside out - start with secondary
• want fast response
• little or no integral action - some offset
  permissable which can be eliminated by primary
  loop
• tune primary loop after secondary is tuned
• secondary closed-loop dynamics are part of the
  apparent process for primary loop

9
Feed Forward Control - Hot Oil Tank Revisited
TI
TIC
(from Chapter 15, Marlin)
FI
10
Feedforward Control

• Hot Oil Tank Example revisited
• unsatisfactory performance from existing feedback
  temperature controller
• disturbances in feed temperature are main problem
• feed temperature is measured
• Why wont cascade control help us?
• adjusting the valve has no influence on feed
  temperature
• response of reactor temperature to inlet
  temperature is slower than response of
  temperature to valve.

11
Feedforward Control

• solution
• use measured temperature disturbance and process
  model to figure out adjustments to valve that
  will cancel out the effects of the disturbance.
• called feedforward control
• requires two transfer functions
• between inlet temperature and tank temperature
• between valve and tank temperature

12
Criteria for Feedforward Control

• given unsatisfactory existing performance
• secondary measurement indicating disturbance
  available?
• no cause and effect relationship between MV and
  secondary measured variable?
• disturbance dynamics slower than control loop
  dynamics?
• if YES to these criteria, a feedforward control
  loop will work

13
Reactor Example
F2
F1
T2
T3
A2
T1
What disturbances would a feedforward controller
help us to reject?
AC1
14
Designing a Feedforward Controller

• See notes on blackboard
• We will also talk about how feedforward and
  feedback controllers work together.