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This lecture was delivered by Dr. Sakal Japendu for Process Control course at Ambedkar University, Delhi. It includes: Feedforward, Control, Identify, Situations, Enchancement, Design, Rules, Analogy, Management, Principle
Typology: Slides
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When I complete this chapter, I want to be
able to do the following.
Identify situations for which feedforwardis a good control enhancement
Design feedforward control using the fivedesign rules
Apply the feedforward principle to otherchallenges in life
Outline of the lesson.
A process challenge - improveperformance
Feedforward design rules
Good features and applicationguidelines
Several process examples
Analogy to management principle
TC
2
T
1
F
1
F 2
T 3
L 1
feed
heating stream
Disturbance =
feed temperature
Control
performance
not acceptable!
Class exercise: What do
we do?
0
20
40
60
80
100
120
140
160
180
200
76 74 72 70
IAE = 237.6971 ISE = 758.
temperature
minimum
Let’s use
cascade
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Cascade is desired when1.
Single-loop performance unacceptable
A measured variable is available
A secondary variable must3.
Indicate the occurrence of an important disturbance
Have a causal relationship from valve to secondary
Have a faster response than the primary
Cascade not possible. We need another enhancement!
0
20
40
60
80
100
120
140
160
180
200
76 74 72 70 68 66
0
20
40
60
80
100
120
140
160
180
200
Time
0
Time
m
(t) = T
0
CV
B
(t) = compensation effect
We want to
adjust the
valve to
cancel the
effect of the
disturbance.
CV
A
(t) = disturbance effect
CV
A
B
= no deviation
0
20
40
60
80
100
120
140
160
180
200
60 585654 52 50
v
MV(t) = v
We use block diagram algebra to determine the form ofthe calculation [G
ff
(s)] to achieve the desired performance.
d
p
ff
m
(s)
A
(s)
B
(s)
(s)
(s)
Measured disturbance, T
0
Manipulated variable
Controlledvariable, T How do we
measure CV
A
?
Feedforward
controller
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)
(
)
(
)
(
)
(
)
(
s
G
s
G
s
D
s
MV
s
G
p
d
m
ff
−
=
=
s
ld lg
ff
m
ff
ff
θ
−
Dead time
Gain
Lead-lag
Special case of G
p
(s) and
d
(s) being first order
with dead time
Please
verify.
s
ld
ff
ff
ff
e
s
T
s
T
K
s
G
θ
−
=
)
(
lg
1
1
Lead-lag
ld
s+1)/T
lg
s+1)
FF controller gain
ff
d
p
controller dead time
θ θ
θ θ
ff
θθθθ
d
θθθθ
p
Lead time
ld
τ τ
τ τ
p
Lag time
lg
τ τ
τ τ
d
How do we get values for these parameters?
Typical dynamic responses from the lead-lag element inthe feedforward controller. It synchronizes thecompensation and disturbance effects.
Results for severalcases of Tlead/Tlag :a. 0.0b. 0.5c. 1.0d. 1.5e.
TC
2
T 1
F 1
F
2
T 3
L 1
feed
heating stream
TY
1
TY
2
FF
FF high-
lighted
in red
How do we combine
feedback with
feedforward?
MV
ff
MV
fb
0
20
40
60
80
100
120
140
160
180
200
74.8 74.6 74.
75
75.4 75.
IAE = 27.772 ISE = 8.
temperature
0
20
40
60
80
100
120
140
160
180
200
60 58 56 54 52 50
SAM = 11.4394 SSM = 774.
Time
heating valve (% open)
The MV changed before T
deviated from its set point!
Disturbance occurred at this time
Valve adjustment not too aggressive
Why wait after disturbance?
What have we gainedand lost usingfeedforward andfeedback?How does the systemrespond to thefollowing?
A disturbance in feed inlet temperature
A disturbance in heating medium inlet pressure
A disturbance in feed flow rate
A change to the TC set point
TC
2
T
1
F 1
F
2
T 3
L 1
feed
heating stream
TY
1
TY
2
FF
Feedforward
Feedback
Advantages
Compensates for disturbance before CV isaffected
Does not affect the stability of the controlsysytem(if G
ff
(s) stable)
Provides zero steady-
state offset
Effective for all
disturbances
Disadvantages
Cannot eliminate steady- state offset
Requires a sensor and model for eachdisturbance
Does not take control
action until the CVdeviates from its setpoint
Affects the stability of
the control system
Why do we retain the feedback controller?
When would feedforward give zero steady-state offset?
Why does the feedforward controller sometimes delay itscompensation? Don’t we always want fast control?
What is the additional cost for feedforward control?
How can we design a strategy that has two controlersboth adjusting the same valve?
What procedure is used for tuning the feedforwardcontrol?
