Post on 06-Apr-2018
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CRITERIA FOR SELECTING CONTROLLER MODES
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Special Control Structures
Override control is used to take control of an output from one loop toallow a more important loop to manipulate the output.
The output from two or more controllers are combined in a high orlow selector. The output from the selector is the highest or lowestindividual controller output.
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LSS: Low Selective Switch Output a lower of two inputsPrevents: 1. Level from going too low, 2. Pressure fromexceeding limit (lower)
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motor
S
C
HSS
P
C
F
C
Gas out
Gas in
Normal
loop
Example: Compressor Surge Control
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High PressureLine
Low PressureLine
PC
PC
HSS
Example: Steam Distribution System
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Length ofreactor
T
emperat
e
T1T2
Hotspot
Temperature profiles in a tubularreactor
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Auctioneering Control Systems
TT
TT
TT
TT
Cooling
flow
TC
Auctioneering
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Split Range Control: More than one manipulated variable is
adjusted by the controller
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FT
Divider
FT
Wild stream
Gc
3. Ratio Control Type of feedforward control
However, one stream in proportion to another. Use if theratio
Disadvantage:Ratio may goTo erratic
A
BFB
Desired
Ratio
FA
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A Wild stream
FT
Desired Ratio
Multiplier
FC
Controlled streamFB
FA
Another implementation of RatioControl
+
-
FT
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Ratio control is used to ensure that two or more flows are kept atthe same ratio even if the flows are changing.
Applications of ratio control:
Blending two or more flows to produce a mixture with specifiedcomposition.
Blending two or more flows to produce a mixture with specifiedphysical properties.
Maintaining correct air and fuel mixture to combustion.
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The controlled flow(FIC-102) is increased and decreased tokeep it at the correct ratio with the wild flow.
The "wild flow" (FI-101) is the flow not controlled by this loop. Itmay be controlled by some other control loop.
The "controlled flow" is controlled by this loop with a setpoint
equal to the measured wild flow multiplied by some value (FF-102). The measured wild flow is multiplied by a value that may be
fixed or may be adjustable by the operator. The result of themultiplication becomes the setpoint of the controlled flowcontroller.
The options, such as tracking, that apply to cascade controlalso apply to ratio control. The controlled flow controller is a
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