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Click to edit Master subtitle style

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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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