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