1. Chapter 2
Traditional Advanced Control Approaches – Feedforward, Cascade and Selected Control

2. 2-1 Feed Forward Control (FFC)
Block Diagram
Design of FFC controllers
Examples
Applications

3. Why Feedforward ? Advantages of Feedback Control Disadvantages
Corrective action is independent of sources of disturbances
No knowledge of process (process model) is required
Versatile and robust
Disadvantages
No corrective action until disturbance has affected the output. Perfect control is impossible.
Nothing can be done about known process disturbance
If disturbances occur at a frequency comparable to the settling time of the process. Then process may never settle down.

4. Feedforward Control Feedforward Controller Disturbance Output Process
Manipulated
Variable

5. Feedforward Control Advantages Disadvantages
Corrective action is taken as soon as disturbances arrives.
Controlled variable need not be measured.
Does not affect the stability of the processes
Disadvantages
Load variable must be measured
A process model is required
Errors in modeling can result in poor control

6. EXAMPLES LI FB Boiler Feed control steam FI LI FFC steam FI
Feedback control
Feedforward control LI
steam FI
FFC FB Σ
Combined feedforward-feedback control

7. Design Procedures (Block diagram Method)FF
Controller
GL(s)
Load transfer function
GF(s)
Load
Manipulated Variable
Gp(s)
Process X2 C
Output L M ∑

8. Derivation

9. Examples Example 1 Example 2 Let Gp(s)=Kp/τps+1, GL(s)=KL/τLs+1
Then, GF(s)=-(KL/Kp)(τps+1)/(τLs+1)
Therefore, feedforward controller is a “lead-lag” unit.
Example 2
Let Gp(s)=Kpe-Dps/τps+1, GL(s)=KLe-DLs/τLs+1
Then, GF(s)=-(KL/Kp)(τps+1)/(τLs+1)e(-DL+DP)s
If -DL+DP is positive, then this controller is unrealizable. However, an approximation would be to neglect the delay terms, and readjusting the time constants. In this case, perfect FF compensation is impossible.

10. Tuning feedforward controllers
Let
This has three adjustable constants, K, τ1, τ2
Tuning K, K is selected so that for a persistent disturbance, there is no steady state error in output.
Adjustingτ1, τ2 can be obtained from transfer functions. Fine tune τ1, τ2 such that for a step disturbance, the response is somewhat symmetrical about the set point.

11. Example: A simulated disturbed plant
Disturbed flow rate DV
Waste water treatment
Chemicals MV
BOD
(CV)

12. Simulated Block Disgram
Disturbed flow
rate +
Chemicals

13. Feedforward v.s. Feedback ControlFB FF

14. Example: Distillation Column
Mass Balance: F=D+B
Fz=Dy+Bx
D=F(z-x)/(y-x)
In practice
For example: If light key increase in feed, increase distillate rate.

15. Design of Feedforward Control Using Material and Energy Balances
Consider the hear exchanger
Energy Balance yields Q=WC(T2-T1)=Wsλ
Where λ=hear of vaporization Ws=WC(T2-T1)/λ
This equation tells us the current stream demand based on (1) current flow rate, W, (2) current inlet temperature, T1, (3) desired value of outlet temperature T2. Ws
Steam
w, T1 T2
Condensate

16. Control Law and Design Implementation:
Note no dynamics are incorporated Σ K X
measured
Tset
Gain Ws w T1 - +

17. When to use Feedforward ?
Feedback control is unsatisfactory
Disturbance can be measured and compensated for
Frequency of disturbance variations are comparable to frequency of oscillation of the system
Output variable cannot be measured.
There are large time delays in the system

18. 2-2 Cascade Control
Block Diagram
Design Considerations
Applications

19. Illustrative Example : Steam JacketPC PT TC

20. Illustrative Example: Steam Jacket - Continued
Energy Balance of the Tank:
Energy Balance of Jacket:
Material Balance of the Jacket

21. Illustrative Example: Steam Jacket - Continued
Assume:
Where X=valve position

22. Jacket Pressure Controller
Block Diagram
Feed back Controller
Steam Valve
Stirred Tank
Tset
Steam supply pressure
Valve
position
Jacket
steam pressure
primary
Tank
Temp.
secondary
Primary Controller
Jacket Pressure Controller
Steam Valve
Stirred Tank
Tset
secondary
Jacket
pressure
supply pressure
Tank
Temp.
Secondary loop
Primary loop

23. Principal Advantages and Disadvantages
Disturbances in the secondary loop are corrected by secondary controllers
Response of the secondary loop is improved, thus increasing the speed of response of the primary loop
Gain variations in secondary loop are compensated by secondary loop
Disadvantages
Increased cost of instrumentation
Need to tune two loops instead of one
Secondary variable must be measured

24. Design Considerations
Secondary loop must be fast responding otherwise system will not settle
Time constant in the secondary loop must be smaller than primary loop
Since secondary loop is fast, proportional action alone is sufficient, offset is not a problem in secondary loop
Only disturbances within the secondary loop are compensated by the secondary loop. Hence, cascading improves the response to these disturbances

25. Applications: 1. Valve Position Control
Valve Motor
Desired
position
Secondary loop
Valve position
Air Pressure to Valve Motor
Valve motion is affected by friction and pressure drop in the line. Friction causes dead band. High pressure drop also causes hysteresis in the valve response
Useful in most loops except flow and pressure

26. Application 2. Cascade Flow Loop
Output From
Primary Controller
“ no cascade “
Output From
Primary Controller FC FT
Fset DP
“ cascade “

27. GC2 GC1 GP1 GP2 Σ c m2 e2 mset Secondary loop Primary loop cset GC2
controller
Secondary
process
primary c m2 e2
mset
Secondary loop
Primary loop
cset
GC2
GCL
GP2 Σ
mset m2 c
cset

28. θ Gc 12 Σ + -
Primary
GC2
Secondary
G2(S)
G3(S)
For a cascade system
(open-loop)
Without cascade control θc

29. Illustrative Example: Steam Jacket – Continued – Cascade Case
Wu = 0.53
Mag = 20*log10(AR) = -30 (dB)
 AR = 

30. Illustrative Example: Steam Jacket – Continued – No Cascade Case
Wu = 0.25
Mag = 20*log10(AR) = 0 (dB)
 AR = 1 

31. Illustrative Example: Steam Jacket – Continued – No Cascade Case
Ku = 1;wu = 0.25;Pu = 2*Pi / wu =
 Kc = Ku/1.7 =
 Taui = Pu / 2 =
 Taud = Pu /8 =

32. Illustrative Example: Steam Jacket – Continued – Cascade Case
Ku = 20;wu = 0.53;Pu = 2*Pi / wu = 12
 Kc = Ku/1.7 = 11.8
 Taui = Pu / 2 = 6

33. 2-3 Selective Control Systems
Override Control
Auctioneering Control
Ratio Control
Change from one controlled (CV) or manipulated variables (MV) to another

34. 1. Override Control – Example Boiler ControlLT LC
LSS PC
Normal
loop
water
steam
LSS: Low Selective Switch – Output a lower of two inputs
Prevents: 1. Level from going too low, 2. Pressure from
exceeding limit (lower)

35. Example: Compressor Surge Control
motor SC
HSS PC FC
Gas out
Gas in
Normal loop

36. Example: Steam Distribution System
High Pressure Line
Low Pressure Line PC
HSS

37. 2. Auctioneering Control Systems
Length of reactor
Temperate T1 T2
Hot spot
Temperature profiles in a tubular reactor

38. Auctioneering Control SystemsTT
HSS
Cooling flow TC

39. Temperature Control
Split Range Control: More than one manipulated variable is
adjusted by the controller TC
Bypass
Exchanger T2
Steam

40. Example: Steam Header: Pressure Control
Boiler 2
Steam Header PC

41. 3. Ratio Control – Type of feedforward control
Wild stream FA FT
Disadvantage:
Ratio may go
To erratic
Desired
Ratio ε Gc
Driver FT FB
Controlled Stream B
However, one stream in proportion to another. Use if the ratio
must be measured and displayed

42. Another implementation of Ratio Control
Wild stream FA FT
Multiplier
Desired Ratio + FC ε - FT FB
Controlled stream