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This paper presents an adaptive neural control design for nonlinear pure-feedback systems with an input time-delay. Novel state variables and the corresponding transform are introduced, such that the state-feedback control of a pure-feedback system can be viewed as the output-feedback control of a canonical system. An adaptive predictor incorporated with a high-order neural network (HONN) observer is proposed to obtain the future system states predictions, which are used in the control design to
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Journal
of
Process
Control
22 (2012) 194–
206
Contents
lists
available
at
SciVerse
ScienceDirect
Journal
of
Process
Control
j
ourna
l
ho
me
pag
e:
www.elsevier.com/locate/jprocont
Adaptive
neural
network
predictive
control
for
nonlinear
pure
feedback
systems
with
input
delay
夽
Jing
Na
a,∗
,
Xuemei
Ren
b
,
Cong
Shang
b
,
Yu
Guo
a
a
Faculty
of
Mechanical
&
Electrical
Engineering,
Kunming
University
of
Science
and
Technology,
Kunming
650093,
China
b
School
of
Automation,
Beijing
Institute
of
Technology,
Beijing
100081,
China
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
27
November
2010
Received
in
revised
form
7
September
2011
Accepted
16
September
2011
Available online 15 October 2011
Keywords:
Process
control
Nonlinear
predictor
Pure-feedback
systems
Time-delay
Neural
networks
a
b
s
t
r
a
c
t
This
paper
presents
an
adaptive
neural
control
design
for
nonlinear
pure-feedback
systems
with
an
input
time-delay.
Novel
state
variables
and
the
corresponding
transform
are
introduced,
such
that
the
state-
feedback
control
of
a
pure-feedback
system
can
be
viewed
as
the
output-feedback
control
of
a
canonical
system.
An
adaptive
predictor
incorporated
with
a
high-order
neural
network
(HONN)
observer
is
pro-
posed
to
obtain
the
future
system
states
predictions,
which
are
used
in
the
control
design
to
circumvent
the
input
delay
and
nonlinearities.
The
proposed
predictor,
observer
and
controller
are
all
online
imple-
mented
without
iterative
predictive
calculations,
and
the
closed-loop
system
stability
is
guaranteed.
The
conventional
backstepping
design
and
analysis
for
pure-feedback
systems
are
avoided,
which
renders
the
developed
scheme
simpler
in
its
synthesis
and
application.
Practical
guidelines
on
the
control
imple-
mentation
and
the
parameter
design
are
provided.
Simulation
on
a
continuous
stirred
tank
reactor
(CSTR)
and
practical
experiments
on
a
three-tank
liquid
level
process
control
system
are
included
to
verify
the
reliability
and
effectiveness.
© 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
The
identification
and
control
for
time-delay
systems
have
been
an
active
topic
in
both
academic
and
engineering
fields
since
the
presence
of
delays
in
the
control
systems,
e.g.
process
control,
net-
work
systems,
may
deteriorate
the
performance
or
even
trigger
instability
[1].
During
the
past
decades,
the
stability
analysis
and
stabilization
control
of
system
with
delays
in
its
state
variables
have
been
significantly
advanced
[2,3].
On
the
other
hand,
the
track-
ing
control
of
system
with
delays
in
its
input
is
more
practically
useful
but
technically
challenge
[1].
In
this
issue,
Smith
[4]
pro-
posed
a
predictor-based
structure
to
deprive
the
time-delay
from
the
closed-loop
characteristic
equation,
which
successfully
reme-
dies
the
malicious
effects
of
the
input
delay.
By
taking
the
merit
of
Smith
predictor
(SP),
many
improved
schemes
[5–9]
have
been
developed
to
enhance
the
robustness
and
the
disturbance
rejec-
tion.
To
address
unknown
parameters,
Rad
et
al.
[10]
established
an
adaptive
time-delay
controller
(ATDC)
with
an
online
parame-
ter
estimation.
Recently,
Normey-Rico
et
al.
[11]
proposed
a
novel
夽
This
work
was
supported
by
National
Natural
Science
Foundation
of
China
(60974046
and
61011130163).
This
work
was
conducted
when
Jing
Na
was
with
School
of
Automation,
Beijing
Institute
of
Technology,
Beijing
100081,
China.
∗
Corresponding
author.
E-mail
addresses:
najing25@163.com
(J.
Na),
xmren@bit.edu.cn
(X.
Ren).
kind
of
dead-time
compensators
(DTC)
where
the
analytical
param-
eter
design
results
are
derived
to
retain
the
robustness
and
tracking
performance.
For
the
system
with
communication
delays,
a
con-
troller
based
on
the
network
disturbance
(ND)
and
communication
disturbance
observer
(CDOB)
was
studied
in
[12]
to
achieve
the
same
effectiveness
as
SP.
In
[13],
predictive
control
was
applied
to
networked
control
systems
(NCS)
with
random
delays.
To
elimi-
nate
periodic
disturbances,
repetitive
control
methodology
has
also
been
extended
to
time-delay
systems
[14–16].
Different
to
other
controllers,
the
intrinsic
plant
delay
is
considered
as
a
useful
ele-
ment
in
these
repetitive
control
designs.
Despite
their
successful
applications,
it
is
noted
that
all
of
aforementioned
schemes
primar-
ily
focus
on
linear
time-delay
systems
with
precisely
known
mod-
els,
and
thus
cannot
be
directly
applied
for
nonlinear
time-delay
systems.
To
accommodate
the
nonlinearities
and
input-delay,
the
princi-
ple
of
SP
was
extended
to
nonlinear
time-delay
systems
in
[17,18].
Henson
and
Seborg
[19]
presented
a
time-delay
compensator
for
SISO
nonlinear
processes
by
using
a
nonlinear
state
predictor
and
an
input–output
linearization
controller.
An
alternative
compensa-
tion
strategy
incorporating
the
input–output
linearization
with
the
internal
model
control
(IMC)
and
a
feedback
compensation
term
was
developed
by
Hu
and
Rangaiah
[20],
while
an
input–output
feedback
linearization
with
only
output
measurement
was
further
exploited
in
[21].
In
[22],
an
adaptive
generic
model
control
was
investigated
based
on
the
nonlinear
state
predictor
(NSP)
and
the
modified
strong
tracking
filter
(MSTF).
Nevertheless,
most
of
these
0959-1524/$
–
see
front
matter ©
2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jprocont.2011.09.003
J.
Na
et
al.
/
Journal
of
Process
Control
22 (2012) 194–
206 195
methods
are
derived
with
the
assumption
that
the
plant
model
should
be
known
and
open-loop
stable.
To
facilitate
the
control
design
with
uncertainties
and
input
delay,
several
methods
have
recently
been
reported
in
terms
of
the
approximation
capability
of
neural
networks
(NN).
In
[23],
a
NN-based
linearization
method
was
studied
for
uncertain
nonlin-
ear
systems
with
communication
delays.
Inspired
by
this
idea,
a
time-delay
positive
controller
with
a
NN
compensation
[24]
was
studied
for
more
general
systems.
By
using
a
predictive
scheme,
Lu
et
al.
[25]
investigated
an
adaptive
controller
based
on
a
neural
predictor
for
discrete
time-delay
systems,
which
requires
complex
iterative
predictive
calculations
within
every
sampling
interval.
For
continuous-time
systems,
adaptive
NN
predictors
were
established
in
[26,27]
to
obtain
the
future
system
states
prediction,
for
which
the
iterative
prediction
are
avoided
and
thus
the
computational
cost
is
significantly
reduced.
In
the
latest
work
[28],
an
adaptive
control
was
presented
only
concerning
the
stabilization.
In
viewing
these
results,
only
some
specific
and
simple
systems,
e.g.
Brunovsky
systems
or
strict-feedback
systems,
have
been
studied.
The
pure-feedback
system,
on
the
other
hand,
can
represent
more
general
systems,
such
as
mechanical
systems
[29],
aircraft
systems
[30],
and
biochemical
processes
[31].
Although
many
controllers
have
been
investigated
for
nonlinear
strict-feedback
systems
using
the
backstepping
technique,
only
a
few
of
them
are
applicable
for
pure-feedback
systems
since
the
cascade
and
non-affine
properties
make
it
difficult
to
find
the
explicit
virtual
and
actual
controllers.
In
[32],
Kanellakopoulos
et
al.
advanced
an
effort
to
extend
the
adaptive
backstepping
control
to
parametric
pure-feedback
systems
and
obtained
regionally
stable
results.
The
assumption
of
“linearity
in
the
parameters
(LIP)”
and
the
lengthy
analysis
in
determining
the
“regression
matrices”
were
further
cir-
cumvented
with
the
help
of
NNs
and
fuzzy
systems
(FZ)
[33–35].
In
[33],
a
special
kind
of
affine-in-control
pure-feedback
systems
was
studied
in
terms
of
NN-based
backstepping.
By
introducing
an
implicit
function
theorem
to
assert
the
existence
of
continuous
desired
feedback
controllers,
and
employing
NNs
to
approximate
the
nonlinearities
in
the
ideal
feedback
controllers,
a
novel
adap-
tive
control
was
successfully
derived
in
[36],
where
the
nonlinear
functions
except
the
bounds
of
input
gain
functions
allows
to
be
unknown.
Following
this
idea,
more
general
non-affine
pure-
feedback
systems
with
less
restrictive
assumptions
were
further
studied
[35].
Here,
the
employment
of
ISS-modular
approach
and
the
small
gain
theorem
avoids
the
construction
of
an
overall
Lya-
punov
function
for
the
entire
system
and
provides
a
simple
and
effective
control
synthesis.
It
is
known
that
there
may
be
a
‘cir-
cular
issue’
[37]
when
neural
networks
are
used
to
approximate
the
virtual
and
desired
controllers
in
these
backstepping
designs,
which
is
due
to
the
fact
that
the
control
variable
may
be
adopted
as
one
part
of
NN
approximation
used
in
the
control
itself.
This
fact
motivates
the
latest
work
[37],
in
which
Butterworth
filters
are
used
to
alleviate
the
algebraic
loop
problem.
However,
to
the
best
of
our
knowledge,
most
of
available
control
methods
[33–37]
for
pure-feedback
systems
are
all
derived
based
on
the
backstep-
ping
technique,
which
allows
complicated
design
and
synthesis
procedures.
This
paper
concerns
the
control
design
for
nonlinear
pure-
feedback
systems
with
an
input
delay
and
proposes
an
alternative
neural
network
predictive
control
(NNPC)
without
using
the
back-
stepping.
First,
a
set
of
novel
system
state
variables
are
defined
such
that
a
pure-feedback
system
can
be
represented
in
a
Brunovsky
form
while
the
tracking
objective
is
retained
by
controlling
the
newly
defined
system
output.
In
this
sense,
the
state
feedback
control
of
pure-feedback
systems
can
be
viewed
as
the
output
feed-
back
control
of
Brunovsky
systems,
which
deserves
simpler
control
design
and
stability
analysis.
To
compensate
for
the
input
delay,
a
nonlinear
full-states
predictor
with
an
adaptive
NN
observer
is
established
for
the
transformed
system
where
two
high-order
neural
networks
(HONN)
are
utilized
to
approximate
unknown
nonlinearities.
Finally,
a
feedback
control
is
constructed
to
achieve
output
tracking
by
using
the
predicted
system
states
and
dynamics.
The
closed-loop
stability
and
the
error
convergence
are
guaran-
teed.
In
the
real-time
implementation,
the
past
system
information,
the
off-line
learning
phase
and
iterative
predicative
calculations
are
not
required,
and
thus
the
computational
cost
is
significantly
reduced
in
comparison
to
other
predictive
controllers
for
time-
delay
systems.
The
idea
of
transforming
pure-feedback
systems
into
canonical
systems
can
also
readily
be
extended
to
other
nonlin-
ear
systems,
e.g.
strict-feedback
systems,
to
avoid
the
backstepping
synthesis
[43].
The
theoretical
results
are
validated
by
simulations
in
a
continuous
stirred
tank
reactor
(CSTR)
and
by
practical
exper-
iments
in
a
three-tank
liquid
level
process
control
system.
The
rest
of
the
paper
is
organized
as
follows.
Problem
statement
is
provided
in
Section
2.
Section
3
introduces
the
system
state
trans-
form
and
the
NN-based
observer
design.
The
adaptive
nonlinear
state
predictor
and
the
corresponding
control
are
derived
in
Section
4.
Section
5
is
devoted
to
validate
the
proposed
schemes
by
simu-
lations
and
experiments.
Some
conclusions
are
given
in
Section
6.
2.
Problem
formulation
Consider
a
class
of
pure-feedback
nonlinear
systems
with
an
input
time-delay
as
⎧
⎪
⎨
⎪
⎩
˙
x
1
=
f
1
(x
1
,
x
2
)
˙
x
i
=
f
i
(x
1
,
x
2
,
·
·
·x
i+1
),i
=
2,
·
·
·,
n
−
1
˙
x
n
=
f
n
(x
1
,
x
2
,
·
·
·x
n
,
u(t
−
))
y
=
x
1
(1)
where
¯
x
i
=
[x
1
,
x
2
·
·
·x
i
]
T
∈
R
i
,
i
=
1,
·
·
·,
n,
y(t)
∈
R
and
u(t)
∈
R
are
the
system
states,
the
output
and
input,
respectively,
f
i
(
•
),
i
=
1,
·
·
·,
n
are
unknown
but
smooth
nonlinear
functions,
and
is
a
constant
delay.
The
objective
of
this
paper
is
to
find
a
control
u(t)
such
that
the
output
y(t)
tracks
a
given
reference
y
d
(t)
while
all
signals
involved
in
the
closed-loop
are
bounded.
Assumption
1.
The
functions
f
i
(
•
),
i
=
1,
·
·
·,
n
are
continuously
differentiable
to
n-order
with
respect
to
the
state
variables
¯
x
i
and
the
input
u.
Remark
1.
Many
industrial
processes
can
be
modeled
by
system
(1),
such
as
continuous
stirred
tank
reactor
[20],
cold
rolling
mills
and
multi-tank
liquid
levels
control
[22].
Though
many
control
designs
for
some
time-delay
systems
(e.g.
Brunovsky
systems
[26]
and
linear
systems
[4,10,38])
have
been
developed,
they
may
not
be
directly
applicable
for
system
(1)
due
to
its
cascade
and
non-affine
properties.
Hence,
new
techniques
need
to
be
exploited
taking
into
account
the
specific
properties
of
system
(1).
Remark
2.
System
(1)
is
in
the
so-called
pure-feedback
form
that
covers
most
of
nonlinear
systems,
e.g.
Brunovsky
systems
[26],
strict-feedback
systems
[33].
For
pure-feedback
systems
without
input
delay,
backstepping
control
has
been
well-recognized
and
widely
employed.
However,
it
is
usually
difficult
to
find
explicit
virtual
controls
and
the
actual
control
[37]
for
system
(1)
in
the
presence
of
delay.
Moreover,
compared
to
[33,36,37],
the
studied
pure-feedback
system
(1)
has
no
affine
appearance
of
variables
to
be
used
as
the
virtual
controllers
and
the
actual
control
u
itself,
and
only
an
assumption
that
f
i
(
•
),
i
=
1,
·
·
·,
n
is
continuously
differen-
tiable
is
required.
In
this
paper,
the
following
linear
parameter
neural
network
(LPNN)
[23]
is
employed
to
approximate
nonlinear
functions
over
a
compact
set
˝
as
Q
i
(Z)
=
W
∗T
i
˚
i
(Z)
+
ε
i
,
i
=
1,
·
·
·,
n,
∀Z
∈
˝
⊂
R
n
(2)
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