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Feedforward Control

()

Liankui DAI

Institute of Industrial Control,

Zhejiang University, Hangzhou, P. R. China

2009/04/22

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Contents Feedforward Concept

Design of Linear Feedforward Controllers

Design of Nonlinear Feedforward Controllers

Design Examples of Feedforward Control

Feedforward-Feedback Control

Simulation Results Summary

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Problem DiscussionConsider the heat exchanger shown in

the figure, it is required to control the

outlet temperature of process fluid, T,

by manipulating the steam flow, RV.Please design the relevant control

schemes on the diagram for the

following cases:

Case #1: both of the flow, RF

, and

steam inlet pressure, PV, are stable;

T

RV

RF, Ti

Steam

Condensate

Process

Fluid

PV

Tm

Case #2: RF

is stable, but PV

varies frequently;

Case #3: PV

is stable, but RF

varies frequently;

Case #4: both ofPV

and RF

varies frequently.

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Feedback or Feedforward

Control Examples

TC

cp,RF, T1 T2

HV,RV

Steam

Process

Fluid

Condensate

Tsp

cp,RF, T1 T2

HV,RVFF

RF

Steam

ProcessFluid

Condensate

Tsp

Feedback Control Scheme Feedforward Control Scheme

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Feedforward Concept

Process

y(t)

u(t)

.

.

.

Dn(t)

D1(t)

ysp

Feedforward

control system

D1(t), , Dn(t) represent

some measurable

disturbances to the

controlled variable

Idea: to compensate for some measurable disturbances before

they affect the controlled variable.

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Feedforward / Feedback Control

Process

y(t)

u(t)

D(t)

ysp Feedforward

control system

Feedforward

control system

ysp

+ +

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Design of Linear Feedforward

Controllers

Process

y(t)

u(t)

D(t)

ysp

Feedforward

control system

D (t)

ym(t)

Disturbance

Measurement

GDM(s)

Feedforward

Controller

GFF(s)

Dm(t) u(t) Control Path

GYC(s)

Disturbance Path

GYD(s)

Extended Controlled Process

Design Objective:

( )( ) ( ) ( ) ( )

( )0

mYD YC FF DM

Y sG s G s G s G s

D s

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Design of Linear Feedforward

Controllers (cont.)

Design formula for the feedforward

controller:

( )( )

( ) ( )

YDFF

YC DM

G sG s

G s G s

D (t)

ym(t)

DisturbanceMeasurement

GDM(s)

FeedforwardController

GFF(s)

Dm(t) u(t) Control PathGYC(s)

Disturbance Path

GYD(s)

Extended Controlled Process

Design Objective:

( )( ) ( ) ( ) ( )

( )

0

mYD YC FF DM

Y sG s G s G s G s

D s

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Design of Linear Feedforward

Controllers (cont.)Design formula for the feedforward controller:

( )( )

( ) ( )

YDFF

YC DM

G sG s

G s G s

ssT

KsG D

YD

YDYD

exp1

)( ssT

KsG C

YC

YCYC

exp1

)(

DMDM KsG )(

ssTsTKsG FF

YD

YCFFFF

exp

11)(

DMYC

YDFF

KK

KK CDFF ,0max ( Why ? )

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Types of Feedforward Controllers Steady-Stateor DynamicFeedforward

Controller ()

Linear or nonlinear FeedforwardController ()

Discuss the common-used feedforward controller

ssTsT

KsG FFYD

YC

FFFF

exp1

1

)(DMYC

YD

FF KK

K

K

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Simulation Results of Linear

Feedforward Controllers

40 50 60 70 80 90 100 110 12032

33

34

35

36

37

38

39

40

41

42

Time, min

%

Output of Transmitter

Without Feedforward Control

Steady-State Feedforward Control

Dynamic Feedforward Control

Weak Dynamic Feedforward Control

Strong Dynamic

Feedforward Control

(Please see/FFControl /ExHeaterLinearFFC.mdl)

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Nonlinear Steady-state

Feedforward ControlSteady-state Equation

VVFp RHTTRc )( 12

pVv

sp

Fv

sp

V

cHK

TTRK

R

/

,)(1 12

Discuss the realization of nonlinear feedforward controllerand the effect of relevant measurements

T2

RVm

RF

FC

T2sp

RVsp

Steam

Process

Fluid

Condensate

cp,RF, T1

T1

HV,RV

Nonlinear

FFC

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Nonlinear Steady-StateFeedforward Control Simulation

(Please see /FFControl/ExHeaterStaticFFC.mdl)

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Comparison of Feedforward

and Feedback ControlFeedforward Control Feedback Control

Disturbances are measurable CV is measurable

Control MV based on

disturbances

Control MV based on control

ERROR

Open-loop, No Stability ProblemClosed-loop, Stability is themost important

Only some disturbances aredetected

All disturbances are detected

Accurate model needed for bothof Control and Disturbance Paths

No accurate model needed

Not adaptable to nonlinear ortime-varied systems

Adaptable to nonlinear or time-varied systems

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FFC+FBC Scheme #1 for

Heat Exchanger

T2

RV

RF

FC

T2sp

RVsp

TC

T1

k1

FFCSteam

ProcessFluid

Condensate

Steady-state output and smooth switch problem ?

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FFC+FBC Scheme #2 for

Heat Exchanger

T2

RV

RF

FC

T2sp

RVsp

TC

T1

k1

FFC

Steam

Process

Fluid

Condensate

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Summary Cases which may introduce feedforward control

(1) Primary Controlled Variable is not measurable

(2) Primary CV is measurable, but some disturbancesare too strong for feedback control systems

Conditions to apply feedforward control(1) Main disturbances are measurable

(2) Response speed of disturbance path is slower thanthat of control path

(3) Characteristics of disturbance and control path arealmost invariable

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