1. 26 Nov 12 Process Linearity, Integral Windup,
CBE 491 / CBE 433
Linearity, Windup, & PID
26 Nov 12 Process Linearity, Integral Windup,
PID Controllers
30 Mar 07
2 Apr 08
27 Mar 09

2. Process Linearity Test the Heat Exchanger process linearity by:
Starting Loop Pro trainer
Set %CO to 80%
Make steps down (say 10% down) to the %CO
Measure the response
Calculate the process gain

3. K = -1.09
K = -0.69
K = 0.-45
K = -0.33
K = -0.26
K = -0.15
Adaptive Control ?

4. Integral (Reset) Windup
“Windup” can occur if integral action present
Most modern controllers have anti-windup protection
If doesn’t have windup protection, set to manual when reach point of saturation, then switch back to auto, when drops below sat. level
IE: LoopPro Trainer, select Heat Exchanger
Set %CO to 90%; SP to 126; Kc to 1 %/deg C; Tau I to 1.0 min
Set Integral with Anti-Reset Windup ON
Change Set Point to 120 deg. C. (~10 min); then change back to 126 deg. C
Repeat with controller at ON: (Integral with Windup)

5. Integral (Reset) Windup

6. In-Class PID Controller Exercise
Tune the Heat Exchanger for a PID Controller:
Use the built in IMC, and choose Moderately Aggressive
Start Loop Pro trainer
Tune at the initial %CO and exit temperature
Compare PI with PID
Compare PID with PID with filter

7. Advanced control schemes 26 Nov 12 Cascade Control: Ch 9
CBE 491 / CBE 433
Advanced control schemes
26 Nov 12 Cascade Control: Ch 9
30 Mar 07
2 Apr 08
27 Mar 09

8. Improve Feedback Control
Disturbance must be measured before action taken
~ 80% of control strategies are simple FB control
Reacts to disturbances that were not expected
We’ll look at:
Cascade Control (Master – Slave)
Ratio Control
Feed Forward

9. Cascade Control Control w/ multiple loops
Used to better reject specific disturbances + -
Take slow process:
Split into 2 “processes” that can measure intermediate variable? + + - -
Gp2 must be quicker responding than GP1.
Inner (2nd-dary) loop faster than primary loop
Outer loop is primary loop

10. Material Dryer ExampleMT
% moisture MC
steam
Heat Exchger
air
blower T + -

11. Separate Gp into 2 blocksMT
% moisture MC TT TC
steam
Heat Exchger
air
blower T + + - -

12. A cascade is comprised of two normal PID controllers
The secondary loop is nested inside the primary loop.

13. Problem Solving Exercise: Heat ExchangerTC
Single feedback loop.
Suppose known there will be steam pressure fluctuations…
steam TT
Hot water
Heat Exchger T
Design cascade system that measures (uses) the steam pressure in the HX shell. PT
steam TT
Hot water
Heat Exchger T

14. Temperature Control of a Well-Mixed Reactor (CSTR)Ti
Responds quicker to Ti
changes than coolant temperature changes.

15. Temperature Control of a Well-Mixed Reactor (CSTR)
Use Cascade Control to improve control. Ti
If Tout (jacket) changes it is sensed and controlled before “seen” by primary T sensor.
Secondary Loop
Measures Tout (jacket)
Faster loop
SP by output primary loop
Primary Loop:
Measures controlled var.
SP by operator

16. Cascade Control Benefits:
Disturbances in secondary loop corrected by 2ndary loop controller
Flowrate loops are frequently cascaded with another control loop
Improves regulatory control, but doesn’t affect set point tracking
Can address different disturbances, as long as they impact the secondary loop before it significantly impacts the primary (outer loop).
Challenges:
Secondary loop must be faster than primary loop
Bit more complex to tune
Requires additional sensor and controller

17. Cascade Control Examples
Distillation Columns
Cascade Control Examples
Objective:
Regulate temperature (composition) at top and bottom of column

18. Heat Exchanger Furnace TP out Objective: Keep TP out at the set point
T2 out
Objective:
Keep T2 out
at the
set point

19. In-Class Exercise: Cascade Control System Design
What affects flowrate?
Valve position
Height of liquid
P (delta P across valve)
Design a cascade system to control level (note overhead P can’t be controlled)

20. In-Class Exercise: Cascade Control System Design
Does this design reject P changes in the overhead vapor space?

21. Tuning a Cascade System
Both controllers in manual
Secondary controller set as P-only (could be PI, but this might slow sys)
Tune secondary controller for set point tracking
Check secondary loop for satisfactory set point tracking performance
Leave secondary controller in Auto
Tune primary controller for disturbance rejection (PI or PID)
Both controllers in Auto now
Verify acceptable performance

22. In-Class Exercise: Tuning Cascade Controllers
Select Jacketed Reactor
Set T cooling inlet at 46 oC (normal operation temperature; sometimes it drops to 40 oC)
Set output of controller at 50%.
Desired Tout set point is 86 oC (this is steady state temperature)
Tune the single loop PI control
Criteria: IMC aggressive tuning
Use doublet test with +/- 5 %CO
Test your tuning with disturbance from 46 oC to 40 oC

23. In-Class Exercise: Tuning Cascade Controllers
Select Cascade Jacketed Reactor
Set T cooling inlet at 46 oC (again)
Set output of controller (secondary) at 50%.
Desired Tout set point is 86 oC (as before)
Note the secondary outlet temperature (69 oC) is the SP of the secondary controller
Tune the secondary loop; use 5 %CO doublet open loop
Criteria: ITAE for set point tracking (P only)
Use doublet test with +/- 5 %CO
Test your tuning with 3 oC setpoint changes
Tune the primary loop for PI control; make 3 oC set point changes (2nd-dary controller)
Note: MV = sp signal; and PV = T out of reactor
Criteria: IAE for aggressive tuning (PI)
Implement and with both controllers in Auto… change disturbance from 46 to 40 oC.
How does response compare to single PI feedback loop?

24. Advanced control schemes
CBE 491 / CBE 433
Advanced control schemes
26 Nov 12 Ratio Control: Ch 10
30 Mar 07
2 Apr 08
27 Mar 09

25. Ratio Control Special type of feed forward control A B
Blending/Reaction/Flocculation
A and B must be in certain ratio to each other

26. Ratio Control Possible control system: B AFC FY FC FY FT FT A B
What if one stream could not be controlled?
i.e., suppose stream A was “wild”; or it came from an upstream process and couldn’t be controlled.

27. Ratio Control Possible cascade control systems: A B A B “wild” streamFT
Desired Ratio FY FC FT B
“wild” stream A FT
Desired Ratio
This unit multiplies A by the desired ratio; so output = FY FC FT B

28. Ratio Control Uses:
Constant ratio between feed flowrate and steam in reboiler of distillation column
Constant reflux ratio
Ratio of reactants entering reactor
Ratio for blending two streams
Flocculent addition dependent on feed stream
Purge stream ratio
Fuel/air ratio in burner
Neutralization/pH

29. In-Class Exercise: Furnace Air/Fuel Ratio
Furnace Air/Fuel Ratio model
disturbance: liquid flowrate
“wild” stream: air flowrate
ratioed stream: fuel flowrate
Minimum Air/Fuel Ratio 10/1
Fuel-rich undesired (enviro, econ, safety)
If air fails; fuel is shut down
Check TC tuning to disturbance & SP changes. PV
Desired 2 – 5% excess O2
Disturbance var. TC
Dependent MV
TC output
Ratio set point
Independent MV

30. Advanced control schemes 26 Nov 12 Feed Forward Control: Ch 11
CBE 491 / CBE 433
Advanced control schemes
26 Nov 12 Feed Forward Control: Ch 11
30 Mar 07
2 Apr 08
27 Mar 09

31. FF must be done with FB control!
Feed Forward Control
steam
Suppose qi is primary disturbance TC TT
Heat Exchanger
? What is a drawback to this feedback control loop?
? Is there a potentially better way?
steam
Heat Exchanger
What if Ti changes? FF
Draw block diagram for this feedback control loop FT TT
FF must be done with FB control!

32. Feed Forward and Feedback ControlTC
steam FF TY TY FT TT
Heat Exchanger
Block diagram: + + + + + -

33. Feed Forward Control + + + + + - Response to MFF
No change; perfect compensation!

34. Feed Forward Control + + + + + - Examine FFC T.F. + +
For “perfect” FF control: + +

35. Feed Forward Control: FFC Identification
Set by traditional means:
Model fit to FOPDT equation:
FF Gain
Lead/lag unit
Dead time compensator
Often ignored; if set term to 1
Accounts for time differences in 2 legs
{ FFC ss }
steady state FF control
{ FFC dyn }
dynamic FF control

36. Feed Forward Control: FFC Identification
How to determine FOPDT models :
With Gc disconnected:
Step change COFB, say 5%
Fit C(s) response to FOPDT + +
Still in open loop:
Step change Q, say 5 gpm
Fit C(s) response to FOPDT
lead time
lag time

37. Lead/Lag or Dynamic Compensator
Look at effect of these two to step change in input
Output or response
Final Change from:
Magnitude of step change,
Initial response by the lead/lag,
Exponential decay from lag,

38. + - Feed Forward Control
Rule of Thumb: if lead-lag won’t help much; use FFCss
(p 389)
In text: pp , useful comments if implementing FFC + -
1. Compensates for disturbances before they affect the process
1. Requires measurement or estimation of the disturbance
2. Can improve the reliability of the feedback controller by reducing the deviation from set point
2. Does not compensate for unmeasured disturbances
3. Offers advantages for slow processes or processes with large deadtime.
3. Linear based correction; only as good as the models; performance decreases with nonlinear processes.
No improvement using FFC with set point changes.

39. In-Class PS Exercise: Feed Forward Control
What is the Gm, and what is the GD?
Determine FCC
Tune PI controller to aggressive IMC
For disturbance: Tjacket in
50oC – 60oC – 50oC
Test PI Controller
Test PI + FFCss only
Test PI + FFC full

40. In-Class PS Exercise: Feed Forward Control
PI only
PI + FFCss only
PI + full FFC

41. CBE 491 / CBE 433
30 Mar 07
2 Apr 08
27 Mar 09

42. Problem Solving Exercise: Heat ExchangerTC PC PT
steam TT
Hot water
Heat Exchger T
Draw the block diagram: what is the primary and what is the secondary loop? + + - -