Proceedings ArticleDOI

# Adaptive control of single-input, single-output linear systems

01 Jan 1977-Vol. 16, pp 1030-1035

AboutThis article is published in Conference on Decision and Control.The article was published on 1977-01-01 and is currently open access. It has received 3 citation(s) till now. The article focuses on the topic(s): Adaptive control & Linear system.

Topics: Adaptive control (66%), Linear system (63%)

## Summary (2 min read)

### HERE ARE many examples of physical processes

• Twhich require feedback control syestems capable of functioning at a number of different process operating points.
• Nevertheless, it is fair to say that the basic principles governing the design and operation of such systems are only now just beginning to be understood.
• Only under very restrictive assumptions have any actually been shown to result in stable closed-loop systems.
• The only assumptions made about the process are that it admits a transfer function model with left-half plane zeros, and that: 1) an .

### Notation

• In spite of its intuitive appeal, this particular configuration is dfficult to analyze and at present is not well understood.
• There are at least three reasons for this.
• 1) Known results [9]-[ll] characterizing the behavior of dynamic identifiers (e.g., adaptive observers) are not directly applicable, since such results usually require all identifier inputs to be bounded; in the present situation, neither the process input u nor output y can be assumed bounded a priori, since the identifier is in feedback with the process.
• 2) The relationship between process model parameters p and desired feedback gains f is typically a complicated nonlinear functionf(p).

### Process Model Assumptions

• The reference model is a stable, canonical linear system with strictly proper transfer function T,(s) and bounded, piecewise-continuous, reference input r( t).
• Clearly, any reference model not respecting this constraint must involve some form of differentiation.
• Since the authors have stipulated that their adaptive controller be differentiator-free, they must require that (T,(s))O >n*.
• The process model assumptions imply that the process can be represented in an especially useful way.

### p(i?,r(t))-p(Xo(t),r(t)) is globally, uniformly stable, jSo(t)

• On the other hand, it cannot be concluded from Theorem 2 (or Theorem 1) that the parameter errors F(t) and g(t) tend to 'Note that C; is nonsingular because (c,A) is observable.
• The first describes certain algebraic relationships which exist between a positive-definite matrix P and matrices Mi defined by (43) .
• This together with (71) allows us to write.

### REMARKS

• In this paper the authors have outlined a procedure for designing a parameter-adaptive controller capable of causing the output of a process to approach and track the output of a prespecified reference model with zero steady-state error.
• The principal result of this paper-that it is actually possible to adaptively control a linear system with zero steady-state tracking error-is contrary to their earlier expectations [lo].
• What this paper shows is that this problem can be avoided by applying to the process a control signal which, in effect, is an estimate of the product of desired feedback gains and process state rather than the product of distinct estimates of desired feedback gains and process state.
• Recent simulation experiments have shown, that for such models, the overall set of differential equations describing the adaptive system can be quite sensitive to computer roundoff errors unless the initial parameter errors are small.
• Systematic techniques for selecting these gains to reduce sensitivity or to increase convergence rates are obviously needed, but so far have not been developed.

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IEEE
TRANSACTIONS
ON
AUTOMATIC
como~,
VOL.
AC-23,
NO.
4,
AUGUST
1978
557
Adaptive Control
of
Single-Input,
Single-Output Linear Systems
Absrruc-A
procedure
is
presented for
designing
parameter-adaptive
control
for
a single-input, single-output process admitting
an
essentiauy
unknown
but
fiied
linear
model,
so
ttmt the
resulting
closed-loop syshn
is
globally stable
with
zero
steady-state
tracking
error
been
the
output
of
the
process
and
the
output
of a
prespecified
hear
reference
model.
The
adaptive controller
is
a differentiator-free
dynamical
system forced
only
by
the
process
input
and
output,
as
well
as
by
a
reference
input.
INTRODKCTIOW
HERE ARE many examples of physical processes
Twhich require feedback control syestems capable of
functioning at a number of different process operating
points. In some instances, the parameters of the linearized
process model upon which closed-loop control is based
assume such a wide range of values during process opera-
tion that a single, fixed-parameter control system proves
inadequate to regulate the process. In such cases paramet-
ric changes are often dealt with by using several fixed-
parameter controls and an appropriate switching logic;
alternatively, in some cases model parameter values are
precomputed and stored as functions of operating point,
and a single control system with gains functionally depen-
dent on stored model parameters is used. If the number of
process model parameters is large, if the parameters
cannot be computed with sufficient accuracy or if “tight”
control is required to meet rigid specifications, neither
type
of
control system may be capable of providing ade-
quate regulation.
A
promising alternative, potentially applicable in situa-
tions such as these, is
a
parameter-adaptive control sys-
tem. Roughly speaking, a parameter-adaptive system
is
an
adaptive control system with the capability of adjusting its
own parameters to compensate for the slow but significant
changes in process characteristics resulting from process
transfer from one operating point to another. The idea of
a parameter-adaptive system is, of course, not new
[
11-[3].
Nevertheless, it is fair to say that the basic principles
governing the design and operation of such systems are
only now just beginning to be understood.
March 8, 1978. Paper recommended
by
R.
V.
Monopoli, Past
Chairman
Manuscript received April
25,
1977;
revised September
19,
1977
and
of
the
Adaptive,
Learning
Systems, Pattern Recognition Committee.
This
work
was
supported
by
the United States
Air
Force
Office
of
Scientific Research under Grant
77-3176.
The authors are with the Department
of
Engineering
and
Applied
Science, Yale University, New Haven,
CT
06520.
Fig.
1.
Parameter-adaptive sytstem.
In this paper we consider the problem of designing a
parameter-adaptive control system for a single-input,
sin-
gle-output process admitting an essentially
unknown
but
fixed linear model,
so
that for any reference input
r(t),
the
tracking error
e(t)
between the output
y(t)
of the resulting
controlled system
(Fig.
1) and the output
y,(t)
of
a
pre-
specified linear reference model
is
regulated
to
zero
asymptotically. We assume that only the process input
u(t)
and output
y(t)
can be measured (but
not
the process
model state) and we require the controller to be a dif-
ferentiator-free dynamical system realizable with conven-
tional analog components.
Although
a
great many parameter-adaptive controllers
have been proposed in the literature, only under very
restrictive assumptions have any actually been shown to
result
in
stable closed-loop systems. For example, in
[3]
Parks puts forth the idea of Lyapunov redesign to achieve
stable adaptive operation, but the stability analysis given
there is incomplete and the overall approach is limited to
process transfer functions of relative degree one. In
[4],
Astrom and Wittenmark propose a parameter-adaptive
system consisting of an on-line recursive parameter
identifier and
a
minimum-variance control law generator;
the asymptotic properties
of
such “self-tuning regulators”
have recently been examined in
[5],
[6],
but global stability
of these systems has not yet been established. In
[7],
Monopoli suggests an alternative configuration (a simpli-
fication
of
which is used in this paper) based on the
important observation that under certain conditions it is
not necessary to separately identify process model param-
eters and control feedback gains; but the arguments
in
[7]
concerning stability contain errors and do not justify the
paper’s main claims
[8].
In addition to these references
there are numerous others dealing with parameter-adap-
tive control, but for one reason or another none apparen-
0018-9286/78/0800-0557$00.75 01978 IEEE 558 IEEE TRANSA~ONS ON AUTOMATIC CONTROL, VOL. AC-23, NO. 4, AUGUST 1978 tly give an adequate answer to the following fundamental I. SYSTEM STRUCTURE question: Do parameter-adaptive controllers which yield globally stable closed-loop systems actually exist for sin- gle-input, single-output linear processes? The purpose of this paper is to provide an affirmative answer to this question by presenting what we believe is the first parameter-adaptive control configuration, appli- cable to a reasonably large class of linear process models, which is known to result in a globally stable closed-loop system in which all signals and gains are guaranteed to remain bounded. The proposed controller requires no differentiators and can be realized with integrators, summers, gains, and multipliers. The only assumptions made about the process are that it admits a transfer function model with left-half plane zeros, and that: 1) an . upper bound for the transfer function’s “dimension,” 2) the “relative degree” of the transfer function, and 3) the sign of the transfer function’s “gain” are known. In Section I the general structure of the controller is discussed as are the crucial process model assumptions upon which it is based; an interpretation of this structure in state-space terms reveals that the function of the con- troller is, in essence, to adaptively shift one subset of process model poles to prescribed locations, while adap- tively cancelling process transfer function zeros with the others. Detailed descriptions of the control parameter adjustment law and the auxiliary control signal needed to guarantee system stability are given in Section 11. Finally in Section I11 it is shown that application of the controller to any process satisfying the assumptions of Section I results in a globally stable closed-loop system which follows a prespecified linear reference model with zero steady-state output tracking error. Notation It is perhaps most natural to think of a parameter-adap- tive controller as a system consisting of two distinct sub- systems-one which dynamically generates asymptotic estimates act) and 2(t) of process model parameters p and state x(t) respectively, the other which generates an esti- mate At) of desired feedback gains f as a function of estimated model parameters d(t). However, in spite of its intuitive appeal, this particular configuration is dfficult to analyze and at present is not well understood. There are at least three reasons for this. 1) Known results [9]-[ll] characterizing the behavior of dynamic identifiers (e.g., adaptive observers) are not directly applicable, since such results usually require all identifier inputs to be bounded; in the present situation, neither the process input u nor output y can be assumed bounded a priori, since the identifier is in feedback with the process. 2) The relationship between process model parameters p and desired feedback gains f is typically a complicated nonlinear functionf(p). Roughly speaking, this is because the type of parameterized model which is amenable to on-line identification and state estimation is a linear sys- tem which is observable for all values of its free parame- ters, whereas the type of parameterized model which might conceivably yield a linear relationship between its parameters and desired feedback gains is one which must remain controllable for all values of its free parameters. 3) f(p) is usually not well-defined at those points in parameter space at which the parameterized model used for identification is not controllable; thus, if the control generator shown in Fig. 2 is a memoryless realization of this function, then the possibility of an unbounded gain vectorjcannot be ruled out [lo]. By adopting a somewhat different concept of a parame- are two x matrices of time functions, we write M= ble to avoid the preceding problems. The key idea is to (E) if each element of the matrix M- is a linear combi- generate a feedback control without utilizing distinct nation of decaying exponentials. If a(s) is a polyno~al, estimates of desired feedback gains f and process model function a(s)/p(s), written (a(s)/P(s))O, is the integer adaptive system be described- ear system with inputf(t) and transfer function a(s)/p(s) the process input and Output y can be by a is sometimes written as (a/Plf(t). A dynamical system of canonical (i.e., controllable and observable) linear system the form f = g(x, r) with piecewise-continuous input r(t) is smooth if g( .) is a continuous function of its arguments; such a system is global& stuble if for each initial time At>= c,x,(t> In the sequel, prime denotes transpose. If M(t) and N(t) ter-adaptive system than that in Fig. 2, it is POssi- (a(s))O denotes its &gee. The relative degree of a rational state x(t)* In the the general structure Of such an (p(s))o - (a(s))o. me zero-state output response of a fin- Our basic assumption is that the relationship between ip(t)=ApXp(t)+ b,u(t) to> 0 and state x,,, the state response x(x,,t,t) exists for t >to and the Euclidean norm IIx(xo, to, tll is bounded by a with strictly proper transfer function finite constant not depending on to. In the special case of a linear system, stable means asymptotically stable (i.e., all T,(s)=g,pPo system eigenvalues are in the open left-half plane.) Simi- larly, a polynomial transfer matrix or square matrix is where gp is a constant gain and$(s)
and
p,(s)
are
stable if its zeros, poles, or eigenvalues, respectively, lie in coprime monic polynomials. For reasons to be made clear
the open left-half plane.
in
the sequel, we make the following
4
(s)

r------------------
1
6,.(s)
such that
J-
I
I
I
y(t)=gp*(
b(s>
u(t)-
-u(f)-syoy(~))
8
Y
(3)
(s)
Y
(4
(E)(4)
d
T
DY?;>MC
1
IDEX'IIII'IEII
where C?u(s)/y(s) and .li,(s)/y(s) are strictly proper and
I
I'
I
I
I
I
L-------------------l
I
I
proper transfer functions, respectively.
To
understand the
I
I
role of this representation, which has its origin in
[7],
first
e(t)
=y(t)
-Y,(f)
(5)
GEXETUTCI
+
CONTROLLER
observe that
(4)
implies that the
output
tracking
error
Fig.
2.
A
parameter-adaptive system with separate identification and
control.
can be written as
Process Model Assumptions
1)
o$(s) is a stable polynomial. 2) The following data are known: a) The sign of g,; b) An integer n > (P,(s))O; c) The relative degree n*=(o$(s)/pp(s))O.
The principal function of the adaptive controller shown
in Fig.
1
is to force the process output
y
to approach and
track the output
y,
of a prespecified linear reference
model. The reference model is a stable, canonical linear
system
with strictly proper transfer function
T,(s)
and bounded,
piecewise-continuous, reference input
r(
t).
To
determine
what else
is
required of
Z,
for zero tracking error to be
possible, let
us
note that if the adaptive controller shown
in Fig.
1
were replaced by any linear dynamical (i.e.,
differentiator-free) compensator
Z
with reference input
r,
measured input
y
and output
u,
and
if
Tz
were the
resulting closed-loop transfer function from
r
to
y,
then
the relative degree
of
T2
would not be less than
n*.
Indeed, this fundamental constraint on
Tz
can be relaxed
only by incorporating differentiators in
2.
Clearly, any
reference model not respecting
this
constraint must in-
volve some form of differentiation. Since we have stipu-
lated that our adaptive controller be differentiator-free,
we must require that
(T,(s))O
>n*.
The process model assumptions imply that the process
can be represented in an especially useful way.
To
de-
scribe this representation, let
a(s),
b(~)
and y(s) be any
three polynomials which have been selected with knowl-
edge
of
n
and
n*
so
that
a)
a(s),p(s)
and y(s) are monic and stable.
b)
a(s)
and
p(s)
are coprime and
(a(s)//?(~))~
=
n*.
c)
a(s)
divides
y(s)
and
(y(s))"
>n
-
1.
i
(3)
where T(s)r
(b(s)/a(s))T,(s);
the assumptions on
T,(s)
(i.e., stability and
(T,(s))O
>n*),
together with the con-
straints on
a(s)
and
p(s)
dictated by (3a) and (3b)
guarantee that
T(s)
is a stable, proper transfer function.
Next, observe that since S,(s)/y(s) and G,(s)/y(s) are
strictly proper and proper transfer functions, respectively,
it
is
possible to write
and
where the ki are constants,
ny
=
(~(s))",
and
{y,(s); ,y,(s)} is any preselected basis for the vector
space
of
polynomials
of
degree less than
nr
(e.g.,
yi(s)=
si-').'
The implication of these expressions is that it is
now possible to rewrite
(6)
as
where
k
[
k,,
k,,
.
-
-
,
k,,,
+
,,
1
/g,]'
is a vector of unknown
constants;
t9(t)r[Ol(t),02(t),.
.
,02%+,(t)]'
is
a
vector
of
known sensitivity functions obtained by passing
u,
y,
and
r
through the stable, canonical, linear system
x,:
{
}
(8)
~e(t)=ABxe(t)+b,u(t)+b,y(t)+b,r(t)
B(t)
=
Cexe
(t)
+
dYy(t)
+
drr(t)
with transfer matrix To=block diag
{
T,,T,,,T},
where
The expression for
e(t)
in
(7)
shows that
If
k were
known,
then zero-output tracking error could be achieved
Tu=[Yl/Y,-
-.
,Y,/Yl'
and
Ty
=[YI/Y,'.
-
,Y,,/YI
1Y.
It
be
shown
in
the
that
if
p('>
and Y(')
IIf
(?(s))">n-
1,
hen
Gy(s)/y(s)
turns out to be strictly proper [cf.
are
SO
defined, then there must exist polynomials
6Js)
and
Proposition
11;
thus,
in
this case
k%+,
=o.

560
EEE
TRANSACTIONS
ON
AUTOMATIC
CO~OL,
VOL.
AC-23,
NO.
4,
AUGUST
1978
(asymptotically) by setting
u(t)
=
O'(t)k.
Since
k
is not to select polynomials
a,
P,
and
y
for which
(4)
is guaran-
known, what we shall do instead is to set teed to hold, it is not only sufficient to know
n*
and
n,
but
necessary as well. Thus, even though process model
u(t)=B'(t)L(t)+a(t)
(9)
assumptions 2b) and 2c) may appear somewhat severe,
where
i(t)
is a suitable defined estimate of
k
and
g(t)
is
they
are unavoidable consequences of any approach
an
auxiliary whose sole function is
to
guarantee
based
on
system representation
(4).
Whether or not one
system stability. In Section
11
we explain how to select
can design an adaptive system under weaker hWotheses
i(t)
and
a(
5)
SO
that for any reference input
r(t),
all
(e*& bY only assuming bounds for
n*
are known, rather
system signals are bounded and
e(t)+O
as
t-w.
than
n*
itself) is very much an open matter.
The structure of the control law
(9)
is motivated prim-
Remark
1:
It is of Some interest to know whether Or
kly
by the expression for
e
in
(7)
which, in turn, is a
not a particular selection of
a,
P,
and
Y
proides a
consequence of (4).
T~
establish the validity
of
(4),
first
''minimal parameterization," i.e., a parameterization
in
note that since
y(s)
is a stable polynomial,
y(t)
and
u(t)
which
ap
and consequently
k
are uniquely determined
will satisfy (4) just in case
y(t)
is a solution to the
by
gp.
$9 and Pp. It can be shown that if & and Y are differential equation chosen to satisfy (14), (15), and the hypotheses of Proposi- tion 1, then for any pair of coprime, monic polynomials o$
fi(s)y(s)~(t)=gpa(s)((y(s)-GU(s))u(t)-8,(s)y(t))
(E).
and
Pp
satisfying (12) there exist unique polynomials
6,
and
4
satisfying (13) provided: 1)
a
and
5
are coprime,
(lo) and 2) either
y"=(P,)"-
1
or
yo
=(/?')"
and (13b) is
Since process model assumption 1) asserts that
%(s)
is required to hold with inequality. While coprimeness of
a
stable, (1) will imply (10) if and only if and
L$can be guaranteed by simply choosing a = 1, it is clearly not possible to choose y to satisfy 2) above, unless, -- 4s) 4S)(Y(S) - aU(s)) (11) of course, if (j3')" is known exactly. PPW - Y(S)P(S)+8p(s)ay(s) . Justification of (4) thus amounts to showing that if a(s), P(s) and y(s) are polynomials satisfying (3), then there Before concluding this section we briefly outline an must exist polynomials 8Js) and 6Js) such that (1 1) holds alternative approach leading to (9), based on state-space where du(s)/y(s) and G,(s)/y(s) are strictly proper and considerations. This approach, which was what originally proper transfer functions respectively. The following pro- led us to (9), provides further insight by characterizing (9) position implies this and more. as an estimate of a desired state-feedback control law. For simplicity, we outline the approach under the assumption that the polynomials a(s), P(s), and y(s) satisfying (3) (y(s))" = n; we further assume that T,(s)=g,/P(s), where g, is a constant. The approach is based on two easily proved facts: First, acteristic polynomial of A, then there exist n-vectors hp and bp such that State-Space Interpretation Proposition I: Let a,& y be fixed monic polynomials fixed positive integers with n > n*. For each nonzero con- stant gp and each pair of monic and coprime polynomials 05 and Pp satisfving with a and P ''Prime and ./P Proper; and let and n* be have been selected so that a(s)= 1, (P(s))O = n* and ($lPp)"
=
n*
(12a) if
(clxn,Anxn)
is any observable pair, with
y(s)
the char-
and
(P,)"
Qn
(12b)
o$(4 there exist polynomials 8, and 8, satisfving c(sl-A-h,c)-'b,= - Bp(4 * (16) (44/Y)O>O (13a) Second, for at least one pair (hp,bp) satisfying (16), there @,/Y)" 2 0 (13b) exists a row vector f, such that A + hpc + bpgJp is stable and (1 l), if and oniy if a divides y, and (14) (a/P)"=n*, c(sI-A-h,c-bpgJp)-'bp=-. P(s) (17) 1 and Equation (1 6) implies that (y)" >n- 1 (15) ip(t)=(A +h&,(t)+b,g,u(t) If a, fi and y have the required properties and if the y(t)=cxp(t) inequality in (15) is strict, then there exist 8, and 6, satisfv- ing (11) and (13) for which the inequality in (136) is strict. is a process model. sine ip cafl also be written as A proof of this proposition appears in the Appendix. The proposition clearly implies that in order to be able ip (t) = (A + hpC+ bpg&)Xp (t) bpgp (u(t) -f,Xp(t>) FEUER AND MORSE: ADAF’TNE CONTROL OF LINEAR 561 and since A + h,c + bpgJp is stable, we can use (17) to obtain By assumption, y,(t) =(g,/P(s))r(t), so the output track- ing error can now be written as e(t> = BP-(U(t) -f,x,(t) - (g,/gp)r(t)) (4 (19) P(s) This suggests that desired system behavior might be achieved by setting u(t)=f(t)f(t)+g(t)r(t) wheref(t),I(t) and g(t) are suitably defined estimates of f,, x,(t) and g,/g,, respectively-but as mentioned at the beginning of this section, this approach leads to difficult problems which so far have not been overcome. The alternative approach pursued here is to use an estimate of the product f,x,(t) rather than the product of estimates off, and x,(t), respectively. The structure of the proposed estimate is motivated by the fact that the state xp( t) of system (1 8) can be written as .,(t)= Eu(t)bpgp + Ey(t)h, + eA‘q (20) where E,([) and E,(t) are any pair of solutions to the matrix differential equations &(t)=AE,(t)+lu(t) ky(t)=AEy(t)+Zy(t) (21) root set of %(s)P(s). The more general situation, when a, /I, y, and T,(s) are not constrained by these special assumptions, admits a similar state-space interpretation, provided system (18) is replaced with a more general linear model, representing the process together with a dynamic compensator. 11. CONTROL EQUATIONS To complete our description of the proposed adaptive system, we first select a($)
and
P(s)
so that
(3)
holds, and
in addition
so
that
P(s)=
P*(s)S(s),
where
P*(s)
is monic
and
a(s)/P*(s)
is a strictly proper, strictly positive real
transfer function. It is easy to check that these constraints
imply that
S(s)
must be a monic, stable polynomial of
degree
n*
-
1.
It turns out in what follows that nothing essential is lost
if
a(s)/P*(s)
is assumed to be the transfer function of a
one-dimensional system, i.e.,
a(s)/P*(s)=
l/(s
+xo>
for
some
b>O.
For the sake
of
simplicity we henceforth
make this assumption; the more general situation
in
which
a(s)/P*(s)
is the positive real transfer function of a higher
dimensional system can be treated along similar lines
using the Kalman-Yakubovich lemma.
These assumptions together with
(5)
and the control law
defined by (9) imply that the output tracking error
e(t)
can
be
written
as
I
is the n
X
n identity and
q
is
a constant vector depending where
is
the
parameter
error
on the initial values of
x,,
E,,
and
E,
[lo].
Since
A
is a
stable matrix by hypotheiis,
it
followifrom (20) that
Z(t)=Jc(t)-k.
f,xp(t)=f,E,(t)b,gp+f,E,(t)h,
(€1-
(22) We wish to develop a parameter adjustment law for
k(t)
and an expression for
a(t).
Since these equations are
for the case
n*
=
1,
these cases will be treated separately.
Using
just
(21), (22),
and
the
Of
A,
it
is
not considerably more compficated
for
the
n*
>
1
than
difficult to verify that
f,x,(t)
can also be written
as
will yield an estimate off,x,(t)+(g,/g,)r(t) provided
&(t)
is an estimate of
ki
for iE{1,2,...,2n} and
k(2n+I)(t)
is
an
estimate of l/g,.
The preceding development shows, at least under the
special assumptions on
a,
P,
y,
and
T,(s),
that the term
eyt)L(t)
appearing in (9) is really an estimate off,x,(r)+
(g,/g,)r(t) where
f,x,(t)
is a state feedback law, whch
if
applied to process model (18), would have the effect of
shifting the model’s poles from the root set of
P,(s)
to the
Z,-:i(t)=
-(sign(g,))@(t)e(t), (26)
where
Q
is any prespecified, positive definite, constant
gain matrix, then for any reference input r(t), bounded for
t
>
0,
and for any initial time
to
>
0
and state
xz(to),
the
state response
xx(t)
=
(x,(
t),
x,(
t),
xs(t),
k(
t))
of the result-
ing closed-loop adaptive system
Z
described by
(I),
(2),
(5),
(8),
(9),
and (26) exists for
t
>to
and is bounded
uniformly
in
to.
To
understand why this is
so,
first observe
that we can now use (24)-(26)
to
write

##### Citations
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Journal ArticleDOI
Abstract: The stability properties of a fairly general diserete-time adaptive control scheme are analyzed. Sufficient conditions for L^{\infty} stability in the presence of disturbances are given. The stability results are used to prove convergence of the process outputs in the disturbance-free case, without requiring any a priori stability assumption.

72 citations

Proceedings ArticleDOI
01 Jan 1978
Abstract: A new parameter adaptive control scheme is outlined which essentially allows one to arbitrarily position all of the poles of an unknown, linear, scalar system of known dynamical order. The technique is based on identifying the parameters which characterize the dynamical behavior of the system in differential operator form and simultaneously implementing an adaptive observer which generates a feedback control signal which converges to the equivalent of an appropriate pole placing linear state variable control law. It is shown that the overall control configuration, consisting of a system identifier and a complementary adaptive observer, is globally stable provided only that the control input is "sufficiently rich" in frequency content. A study employing an unstable, nonminimum phase plant is presented to illustrate the technique.

66 citations

Journal ArticleDOI
Abstract: Model reference adaptive regulators based on input-output descriptions are examined. By reinterpreting the concept of "augmented error," it is shown that there are no essential differences between the model reference adaptive algorithms and the self-tuning regulators. Both types of schemes can be thought of as composed of a parameter estimator and a control law, based on the parameter estimates. It is shown that many schemes proposed are special cases of a general algorithm. The positive real condition for model reference adaptive systems is also examined. It is shown that this condition is a consequence of the choice of the estimator and that it is not crucial for stability.

53 citations