Control Scheme

7/28/2019 Control Scheme

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Chapter 12

Cascade, Ratio, and

Feedforward Control


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Overall Course Objectives Develop the skills necessary to function as

an industrial process control engineer. Skills

Tuning loops

Control loop design Control loop troubleshooting

Command of the terminology

Fundamental understanding Process dynamics

Feedback control


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Cascade, Ratio, and FeedforwardControl

Each of these techniques offers advantageswith respect to disturbance rejection:

Cascade reduces the effect of specific types of

disturbances.

Ratio reduces the effect of feed flow rates

changes

Feedforward control is a general methodology

for compensating for measured disturbances.


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Compensating for Disturbances

Reduces Deviations from

Setpoint and Settling Time

-6

-3

0

3

6

0 10 20 30 40 50

Time (seconds)

T'(K)

FB-only

Compensating

for disturbances


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Level Controller on a Tank With

and Without Cascade Control

Fin

FC

LT

RSP

FT

Fout

Lsp

LC

Fin

LT

Fout

Lsp

LC


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Analysis of Cascade Example

Without a cascade level controller, changes

in downstream pressure will disturb the tank

level.

With cascade level controller, changes indownstream pressure will be absorbed by the

flow controller before they can significantly

affect tank level because the flow controllerresponds faster to this disturbance than the

tank level process.


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Key Features for CascadeControl to be Successful

Secondary loop should reduce the effect of

one or more disturbances.

Secondary loop must be at least 3 times faster

than master loop.

The CV for the secondary loop should have adirect effect on the CV for the primary loop.

The secondary loop should be tuned tightly.


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Cascade Reactor Temperature

Control

Feed

ProductTT

Cooling

water

TC

Feed

Product

TT

Cooling

water

TCTT

TC

RSP


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Analysis of Example

Without cascade, changes in the coolingwater temperature will create a significant

upset for the reactor temperature.

With cascade, changes in the cooling water

temperature will be absorbed by the slave

loop before they can significantly affect thereactor temperature.


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Multiple Cascade Example

This approach works because the flow control

loop is much faster than the temperature control

loop which is much faster than the composition

control loop.

FT

AC

AT

TCTT

FC

RSP

RSP


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Example

TT

PT

Condensate

Steam

Feed

Draw schematic: A temperature controller on theoutlet stream is cascaded to a pressure controller

on the steam which is cascaded to a control valve

on the condensate.


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Solution

TT

PTPCTC

Condensate

Steam

RSP

Feed


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Ratio Control

Useful when the manipulated variable scalesdirectly with the feed rate to the process.

Dynamic compensation is required when the

controlled variable responds dynamically

different to feed rate changes than it does to

a changes in the manipulated variable.


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Typical Performance

Improvements using Ratio Control

Time

ImpurityConcentration

w/ ratio control

w/o ratio control


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Ratio Control for Wastewater

Neutralization

NaOH

SolutionAcid

Wastewater

Effluent

FTFT

FC

pHTpHC

RSP


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Analysis of Ratio ControlExample

The flow rate of base scales directly withthe flow rate of the acidic wastewater.

The output of the pH controller is the ratio

of NaOH flow rate to acid wastewater flow

rate; therefore, the product of the controller

output and the measured acid wastewater

flow rate become the setpoint for the flow

controller on the NaOH addition.


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Ratio Control Applied for VentComposition Control

Steam

Feed

Product

TT

FT

FCFT AT

AC

Vent


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Ratio Control Requiring

Dynamic Compensation

FT

AC

AT

FC

RSP

FT

Feed

DC


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Example

FT

FT TT

FlueGas

Process

FluidFuel

Draw schematic: For a control system that adjuststhe ratio of fuel flow to the flow rate of the process

fluid to control the outlet temperature of the process

fluid. Use a flow controller on the fuel.


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Solution

FT

FC

FT TT

TC

RSP FlueGas

Process

FluidFuel

Ratio


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Feedforward and Feedback Level

Control

Make-up

Water

To Steam

Users

LT

LC

Make-up

Water

To Steam Users

LT

FT

FF

To Steam

Users

LT

FT FF

LC +

Make-up Water


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Analysis of Feedforward and

Feedback Level Control

Feedback-only must absorb the variations insteam usage by feedback action only.

Feedforward-only handle variation in steam

usage but small errors in metering will

eventually empty or fill the tank.

Combined feedforward and feedback hasbest features of both controllers.


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Derivation of FF Controller

Cff(s)

Y(s)

Gd(s)

D(s)

+

+Ga(s)

Gff(s)

Gp(s)

Gds

(s)

)()()(

)()(

)(forSolving

0)()()()()()()()(

sGsGsG

sGsG

sG

sGsDsGsGsGsGsDsY

pads

dff

ff

dpaffds

=

=+=


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Lead/Lag Element for

Implementing FF Control

ffldff

s

ldff

s

dp

s

pd

ff

d

s

dd

p

s

ppads

K

s

esK

esK

esK

sG

s

eKsG

seKsGsGsG

ff

p

d

d

p

,,,:parametersLead/Lag

)1(

)1(

)1(

)1(

)(

1)(

1)()()(

lg

lg +

+

=+

+

=

+=

+=


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Effect of Lead/Lag Ratio

Time

cff

ld/ lg =

ld/ lg =1

ld/ lg = 2


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Static Feedforward Controller

ffff KsG =)(

A static feedforward controller make a correctionthat is directly proportional to the disturbancechange.

A static feedforward controller is used when theprocess responds in a similar fashion to a changein the disturbance and the manipulated variable.


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Feedforward When p

d

FF

FeedFT

Coolant

Outlet

Coolant

Inlet

TT

FT

FC

1)(

ld

s

+

=

s

eKsG

ff

ff

ff


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Example of Feedforward Control

for d


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Static Feedforward Results

-4

0

4

8

12

0 2 4 6 8 10Time

T'(C)

6.5 C

When the inlet temperature drops by 20C, Q isimmediately increased by 20 kW.

Deviations from setpoint result from dynamic

mismatch


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Perfect Feedforward Control

-30

-15

0

15

30

0 2 4 6 8 10Time (minutes)

T'(C)

Ti effect

Net result

FF Effect

FF correction is mirror image of disturbance effect.

Net effect is no change in controlled variable.


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Required Dynamic Compensation

0

10

20

0 2 4 6 8 10Time (minutes)

T(C)

Q=20kW

Perfect FF

Since the Q affects the process slower than Ti ,

initially overcompensation in Q is required

followed by cutting back on Q to 20 kW.


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Results with Dynamic

Compensation

-4

0

4

8

12

0 2 4 6 8 10

Time (minutes)

T'(C)

w/o DC

w/ DC


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Feedforward Control Action

0 5 10 15 20

Time (minutes)

Q

w/o DC

w/ DC


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Effect of Lead/Lag Ratio

Time

cff

ld/

lg =

ld/ lg = 1

ld/ lg = 2


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Tuning a FF Controller Make initial estimates of lead/lag parameters based

on process knowledge.

Under open loop conditions, adjust Kffuntil steady-

state deviation from setpoint is minimized.

Time

y


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Tuning a FF Controller

Analyzing the dynamic mismatch, adjust

ff.

Time

y


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Finally, adjust (

ld -

lg) until approximately equalareas above and below the setpoint result.

Time

y


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Demonstration: Visual BasicSimulator



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Feedback Control

Can effectively eliminate disturbances forfast responding processes.

But it waits until the disturbance upsets the

process before taking corrective action.

Can become unstable due to nonlinearity

and disturbance upsets.


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Feedforward Control

Compensates for ds before process is affected

Most effective for slow processes and for

processes with significant deadtime.

Can improve reliability of the feedbackcontroller by reducing the deviation from

setpoint.

Since it is a linear controller, its performance

will deteriorate with nonlinearity.


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Combined FF and FB Control

Cfb

(s)Ysp(s)

Gp(s)

Y(s)++

++

+-G

c(s)

D(s)

Gd(s)

Gff(s)

Cff(s)


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Combined FF and FB for the

CSTR

Steam

Feed

Product

TT

FT

FC

+

TC

TT

FF

RSP


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Results for CSTR

-6

-3

0

3

6

0 10 20 30 40 50Time (seconds)

T'(K)

FB-only

FF+FBFF-only


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Analysis of Results for CSTR

FB-only returns to setpoint quickly but haslarge deviation from setpoint.

FF-only reduces the deviation from setpointbut is slow to return to setpoint.

FF+FB reduces deviation from setpoint and

provides fast return to setpoint.


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Example

TTPT

Condensate

Steam

Feed

TT

Draw schematic: For a combined feedforward and

feedback controller in which the inlet feed temperatureis the feedforward variable and the outlet temperature

is the feedback variable. The combined controller

output is the setpoint for a steam pressure controller.


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Solution

TT PT

PCTC

Condensate

Steam

RSP

Feed

TT

FF

+


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Overview Cascade can effectively remove certain

disturbances if the slave loop is at least 3times faster than the master loop.

Ratio control is effective for processes that

scale with the feed rate. Feedforward can be effective for measured

disturbances for slow responding processes

as long as the process nonlinearity is not too

great.

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