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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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