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Ali KarimpourAssociate Professor
Ferdowsi University of Mashhad
CONTROL IN CONTROL IN INSTRUMENTATIONINSTRUMENTATION
Reference:1- Advanced PID Control by Karl j. Astrom and Tore Hagglund, ISA, 20062- و کنترل صنعتی تالیف فرشاد مریخ بیات مدلسازی3- The Michigan Chemical Process Dynamics and Controls Open Text Book
lecture 9
Ali Karimpour Nov 2013
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Lecture 9
Control ParadigmsControl Paradigms
Topics to be covered Introduction
Cascade Control
Mid-Range and Split-Range Control
Selective and Override Control
Ratio Control
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Cascade Control
Cascade Control: Cascade control uses two or more controllers one driving the other.
Number of measurement signals > Number of controlled variables
Cascade control is particularly useful when there are significant dynamics, e.g, long dead time or long time constants between the control variable and the process variable.
It is also possible to have a cascade control with more nested loop.
Secondary loop
Primary loop
Cascade control Feedback controlAdvantages: One PI can change with two P controller with better performance.
Slave/master control
Inner/Outer control
Secondary/Primary control
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Cascade Control
Example1:Improved load disturbance rejection
11
1 s
P 32 11
s
P
sCp
168.037.0
31)1()(
sCsCsGs
s
Using a conventional PI controller as:
Using a cascade control as:
5sC
sCp
29.055.0
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Ali Karimpour Nov 2013
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Cascade ControlExample1:
Improved load disturbance rejection
Yellow line: Answer to conventional control
Blue line: Answer to cascade control
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Cascade ControlExample1:
Small improvement in set-point response
Yellow line: Answer to conventional control
Blue line: Answer to cascade control
control_in_inst_8_example1.mdlcontrol_in_inst_8_example2.mdl
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Cascade Control
How a cascade controller help?
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Cascade Control
Choice of secondary measured variables
Basic rules for secondary variable selection
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Cascade Control
Choice of secondary measured variables
Basic rules for secondary variable selection
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Cascade Control
Example of cascade systems
Position controlof a motor.
Heat exchanger
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Cascade Control
Design procedure through an example(Example 3)
Consider following system:
)1)(15.0(1
?
ssP
11.01
?
sP
1- In which condition cascade control suggested?
)1)(15.0(1
2
ssP
11.01
1
sP
2- Have a look on feedback controller.
)11(s
KC pp )10)(2(20
21
sssKPPC pp
12formarginStability pK by Ziegler 6pKmethod someby 1pK
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Cascade Control
Design procedure through an example(Example 3)
Consider following system:
)1)(15.0(1
2
ssP
11.01
1
sP
3- First tune the secondary loop.
5sC
11.01
1
sP
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Cascade Control
Design procedure through an example(Example 3)
Consider following system:
)1)(15.0(1
2
ssP
)1)(15.0(1
61.05
1 21
1
sss
PPCPCs
s
4- Tune the primary loop.
)11(s
KC pp )60)(2(
100
sss
KtfLoop p
74formarginStability pK
by Ziegler 37pK
method someby 1pK
11.01
1
sP
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Cascade Control
Design procedure through an example(Example 3)
Consider following system:
Feedback control Cascade control
control_in_inst_8_example3.mdl
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Mid-Range and Split-Range Control
Cascade Control Number of measurement signals > Number of controlled signals
Split-range control
Dual situation for Cascade Control is mid-range and split-range control
Number of control signals > Number of measurement signals
Mid-range control Use two control signal simultaneously
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Ali Karimpour Nov 2013
is in the middle of its operation1u
y
16
Mid-Range Control
Small with high resolution
Large with low resolution
Take care ofcontrol problem
Take care of v1to be in middle spu spy
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Mid-Range Control
Block diagram of mid-range control
C1 is controlled by driving the processoutput y away from the set point.
If this is done slowly, the deviationfrom the set point can be kept small,Otherwise use following:
If the forward compensator is:
Controller C2 will perform the mid-ranging control without any disturbance of the process output y.
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Split-Range Control
Split-range control Number of control signals > Number of measurement signals
Reactor system with split-range control
Output signal andValve coordination
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Split-Range Control
Reactor system with split-range control
Output signal andValve coordination
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Override Control(Selective Control)[The Michigan Chemical Process Dynamics and Controls Open Text Book]
Override Control
Number of measurement signals > Number of controlled variables
This controllers are used in cases where a choice must be made between inputs.
This controller uses some switches (electronic and pneumatic selectors ):
• High Selective Switch (HSS) • Low Selective Switch (LSS)
• Median Value Selector
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Important application of override control are:
1- Equipment Protection 2- Auctioneering
Override Control(Selective Control)[The Michigan Chemical Process Dynamics and Controls Open Text Book]
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Important application of override control are:
3- Instrumentation Redundancy
In the case of failure in a sensor others will help us to have a continuous operation.
Different type of failure:
• Downscale failure
• Upscale failure
Suitable for downscale failure.
Override Control(Selective Control)[The Michigan Chemical Process Dynamics and Controls Open Text Book]
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Important application of override control are:
4- Artificial Measurements
Override Control(Selective Control)[The Michigan Chemical Process Dynamics and Controls Open Text Book]
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Ratio Control
The objective of ratio control is to control the ratio of two variables, at a certain value.
Examples of ratio control are:
• Fuel-to-air supply ratio.
• Blending chemicals.
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Ratio Control
Example 4: Pulp Bleaching Control
Pulp flow
Hydrosulphite flow
151
1 s
P 121
2
sP
sC 05.02.01
sC 073.0078.02
1.010/ 21 soyyLet
1y
210y
Drawback: Delay in the hydrosulphite flow
?????????????
control_in_inst_8_example4.mdl
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Ratio Control
Use a blend stationinstead of
ratio station
Pulp flow
Hydrosulphiteflow )()1()()( 112 tytrtr
1y
210y
1y
210y
0 75.0
control_in_inst_8_example4.mdl
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1- There is a reaction that normally goes from A B under extremely high pressures. However, the pressure in the reactor must not goes below P = 10 atm, and also the temperature in the reactor must not goes below 300 C. What type of a selectors would be used to protect this equipment?
Exercises
2- We have a reactor that carries out the reaction A + B C. However, if there is too much A fed,a highly exothermic side reaction,A + C D occurs and will quickly melt the entire reactor. We need two sets of redundant controlson this reactor to meet safety regulationsand to ensure continuous operation.Which controls do you add? And where?Suppose sensors have downscale failure.