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This lecture was delivered by Dr. Sakal Japendu for Process Control course at Ambedkar University, Delhi. It includes: Cascade, Control, Design, Rules, Tuning, Procedure, Improve, 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 cascade is agood control enhancement
Design cascade control using the fivedesign rules
Apply the tuning procedure to cascadecontrol
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Outline of the lesson.
A process challenge - improveperformance
Cascade design rules
Good features and applicationguidelines
Several process examples
Analogy to management principle
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TC
2
T
1
F
1
F 2
T 3
L 1
feed
heating stream
Disturbance =
heating pressure
Control
performance
not acceptable!
pressure
0
20
40
60
80
100
120
140
160
180
200
76 75 74 73 72
IAE = 147.9971 ISE = 285.
temperature
minimum
Class exercise: What do
we do?
TC
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TC
2
T
1
F 1
F
2
T
3
L 1
feed
product
heating stream
Let’s think about theprocess behavior.
Causal relationshipfrom P disturbance toT (without control)
What measurableeffect always occurswhen P changes?
v (valve)
(heating oil)
P
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TC
2
T
1
F
1
FC
2
T 3
L 1
feed
product
heating stream
Key variables
for the two
PID controllers.
SP
1
from person
SP
2
= MV
1
CV
2
MV
2
CV
1
A New Control Structure!!
primary
secondary
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Define the
calculations
performed inthe computer.
Class exercise
computer
plant
T
2 F
2
computer
person
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Control Performance Comparison for CST Heater
Single-Loop
Cascade
0
20
40
60
80
100
120
140
160
180
200
76 75 74 73 72
IAE = 147.9971 ISE = 285.
temperature
0
50
100
150
200
76 75 74 73 72
IAE = 11.5025 ISE = 1.
Much better
performance!
WHY?
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Cascade Control Performance for CST Heater
WHY?
Disturbance in
flow is quickly
corrected.
This compensates for
the disturbance!
0
50
100
150
200
76 75 74 73 72
IAE = 11.5025 ISE = 1.
temperature
0
50
100
150
200
18
19
20
IAE = 11.6538 ISE = 11.
Time
heating flow
0
50
100
150
200
58 56 54 52 50
SAM = 5.8711 SSM = 4.
Time
heating valve (% open)
Valve adjustmentis not aggressive!
Disturbance affectsflow sooner
Small deviation,returns to set point
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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 importantdisturbance
Have a causal relationship from valve tosecondary
Have a faster response than the primary
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Large improvement in performance when thesecondary is much faster than primary
Simple technology with PID algorithms
Use of feedback at all levels. Primary has zero offsetfor “step-like” disturbances.
Plant operating personnel find cascades easy tooperate. Open a cascade at one level, and allcontrollers above are inactive.
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feed
product
heating stream
packed bed
reactor
A 1
T 3
T 2 F 2
F 1
T 1
A 2
Notes:1. A1 measures reactant concentration2. “Circle” is shell & tube heat exchanger3. Feed valve is adjusted by upstream process4. Increasing temperature increases reaction rate
Discuss this packed bed
reactor.
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feed
product
heating stream
packed bed
reactor
AC
1
T 3
T 2
F 2
F 1
T 1
A 2
Class exercise: Design a cascade control structure to improve performance.
0
100
200
300
400
500
-0.
0
CV
maximum
Performance
not acceptable
AC
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Class exercise: Design a cascade control structure to improve performance.
Let’s use the
cascade design
rules!
Cascade design criteria
A
F
F
T
T
T
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
N
N
N
N
Y
Y
→→→→
secondary
N
N
Y
N
N
Y
N/A
N/A
N/A
N/A
N/A
Y
T3 satisfies all of the rulesand can be used as asecondary in a cascade.
T2 is the disturbancebut cannot be usedin cascade!
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feed
product
heating stream
packed bed
reactor
AC
1
TC
3
T
2
F 2
F
1
T
1
A
2
SP
1
from
person
SP
2
= MV
1
CV
2
MV
2
CV
1
primary
secondary
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