Feedforward Control ( 前馈控制 )
Lei Xie
Institute of Industrial Control, Zhejiang University, Hangzhou, P. R.
China
2013
Contents Feedforward Concept Design of Linear Feedforward Controllers Design of Nonlinear Feedforward
Controllers Design Examples of Feedforward Control Feedforward-Feedback Control Simulation Results Summary
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 of PV and RF varies frequently.
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
Process Fluid
Condensate
Tsp
Feedback Control Scheme Feedforward Control Scheme
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.
Feedforward / Feedback Control
Process
y(t)
u(t)
D(t)
ysp Feedforward control system
Feedforward control system
ysp
∑+ +
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 PathGYC (s)
Disturbance PathGYD (s)
Extended Controlled Process
Design Objective:
( )( ) ( ) ( ) ( )
( )
0
mYD YC FF DM
Y sG s G s G s G s
D s
Design of Linear Feedforward Controllers
(cont.)
Design formula for the feedforward controller:
( )( )
( ) ( )YD
FFYC DM
G sG s
G s G s
++
D (t)
ym (t)
Disturbance Measurement
GDM (s)
Feedforward Controller
GFF (s)
Dm (t) u(t) Control PathGYC (s)
Disturbance PathGYD (s)
Extended Controlled Process
Design Objective:
( )( ) ( ) ( ) ( )
( )
0
mYD YC FF DM
Y sG s G s G s G s
D s
Design of Linear Feedforward Controllers
(cont.)Design formula for the feedforward controller:
( )( )
( ) ( )YD
FFYC DM
G sG s
G s G s
ssT
KsG D
YD
YDYD
exp
1)( s
sT
KsG C
YC
YCYC
exp
1)(
DMDM KsG )(
ssT
sTKsG FF
YD
YCFFFF
exp1
1)(
DMYC
YDFF KK
KK CDFF ,0max ( Why ? )
Types of Feedforward Controllers
Steady-State or Dynamic Feedforward Controller ( 静态前馈控制与动态前馈控制 )
Linear or nonlinear Feedforward Controller ( 线性前馈控制与非线性前馈控制 )
Discuss the common-used feedforward controller
ssT
sTKsG FF
YD
YCFFFF
exp1
1)(
DMYC
YDFF KK
KK
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)
Nonlinear Steady-state Feedforward Control
Steady-state Equation
VVFp RHTTRc )( 12
pVv
spF
v
spV
cHK
TTRKR
/
,)(112
Discuss the realization of nonlinear feedforward controller and the effect of relevant measurements
T2
RVm
RF
FC
T2sp
RVsp Steam
Process Fluid
Condensate
cp, RF , T1
T1
HV, RV
NonlinearFFC
Nonlinear Steady-State Feedforward Control
Simulation
(Please see …/FFControl/ExHeaterStaticFFC.mdl)
Comparison of Feedforward and Feedback Control
Feedforward Control Feedback Control
Disturbances are measurable CV is measurable
Control MV based on disturbances
Control MV based on control ERROR
Open-loop, No Stability Problem
Closed-loop, Stability is the most important
Only some disturbances are detected
All disturbances are detected
Accurate model needed for both of Control and Disturbance Paths
No accurate model needed
Not adaptable to nonlinear or time-varied systems
Adaptable to nonlinear or time-varied systems
FFC+FBC Scheme #1 for Heat Exchanger
T2
RV
RF
FC
T2sp
RVsp
TC
T1
∑∑ ×+
-k1
FFCSteam
Process Fluid
Condensate
Steady-state output and smooth switch problem ?
FFC+FBC Scheme #2 for Heat Exchanger
T2
RV
RF
FC
T2sp
RVsp
TC
T1
∑ ×+
-k1
FFC
Steam
Process Fluid
Condensate
Summary Cases which may introduce feedforward
control(1) Primary Controlled Variable is not measurable(2) Primary CV is measurable, but some disturbances are too strong for feedback control systems
Conditions to apply feedforward control(1) Main disturbances are measurable(2) Response speed of disturbance path is slower than that of control path(3) Characteristics of disturbance and control path are almost invariable
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