CIRCLE CRITERION AND BOUNDARY CONTROL SYSTEMS IN FACTOR FORM:
INPUT–OUTPUT APPROACH
Piotr GRABOWSKI ∗ , Frank M. CALLIER ∗∗
A circle criterion is obtained for a SISO Lur’e feedback control system consist- ing of a nonlinear static sector-type controller and a linear boundary control system in factor form on an infinite-dimensional Hilbert state space H previ- ously introduced by the authors (Grabowski and Callier, 1999). It is assumed for the latter that (a) the observation functional is infinite-time admissible, (b) the factor control vector satisfies a compatibility condition, and (c) the trans- fer function belongs to H ∞ (Π + ) and satisfies a frequency-domain inequality of the circle criterion type. We also require that the closed-loop system be well- posed, i.e. for any initial state x 0 ∈ H the truncated input and output sig- nals u T , y T belong to L 2 (0, T ) for any T > 0. The technique of the proof adapts Desoer-Vidyasagar’s circle criterion method (Desoer and Vidyasagar, 1975, Ch. 3, Secs. 1 and 2, pp. 37–43, Ch. 5, Sec. 2, pp. 139–142 and Ch. 6, Secs. 3 and 4, pp. 172–174), and uses the input-output map developed by the authors (Grabowski and Callier, 2001). The results are illustrated by two trans- mission line examples: (a) that of the loaded distortionless RLCG type, and (b) that of the unloaded RC type. The conclusion contains a discussion on improving the results by the loop-transformation technique.
Keywords: infinite-dimensional control systems, semigroups, input-output relations
1. Introduction
In a Hilbert space H with a scalar product h·, ·i H consider the SISO model of bound- ary control in factor form (Grabowski and Callier, 1999),
( ˙x(t) = A
x(t) + u(t)d ,
y = c # x. (1)
We assume that A : (D(A) ⊂ H) −→ H generates a linear exponentially sta- ble (EXS), C 0 -semigroup {S(t)} t≥0 on H, d ∈ H is a factor control vector,
∗ Institute of Automatics, Academy of Mining and Metallurgy, al. Mickiewicza 30/B1, 30–059 Kraków, Poland, e-mail: pgrab@ia.agh.edu.pl
∗∗ University of Namur (FUNDP), Department of Mathematics, Rempart de la Vierge 8,
B–5000 Namur, Belgium, e-mail: frank.callier@fundp.ac.be
u ∈ L 2 (0, ∞) is a scalar control function, y is a scalar output defined by an A- bounded linear observation functional c # . The restriction of c # to D(A) is repre- sentable as c # | D(A) = h ∗ A for some h ∈ H (for q ∈ H, q ∗ denotes the bounded linear functional q ∗ x := hx, qi H , x ∈ H).
Define two operators:
V ∈ L H, L 2 (0, ∞)
, (V x)(t) := h ∗ S(t)x W ∈ L(L 2 (0, ∞), H), W u :=
Z ∞ 0
S(t) du(t) dt.
Recall that L and R = L ∗ ,
Lf = f 0 , D(L) = W 1,2 (0, ∞), Rf = −f 0 , D(R) =
f ∈ W 1,2 (0, ∞) : f(0) = 0 ,
are the generators of the semigoups of left- and right-shifts on L 2 (0, ∞), respectively.
Definition 1. The observation functional c # is called admissible if the observability operator
P = V A, D(P ) = D(A)
is bounded. The factor control vector d ∈ H is called admissible if Range(W ) ⊂ D(A).
From (Grabowski and Callier, 1999, Thm. 4.1, p. 100; 2001, eqn. (1.8)) the fol- lowing result may be concluded:
Lemma 1. If c # is admissible, then P , the closure of P , has the form Range(V ) ⊂ D(L), P = LV,
while if d is admissible, then the r e a c h a b i l i t y o p e r a t o r Q = AW is in L (L 2 (0, ∞), H).
Furthermore, from (Grabowski and Callier, 2001, Sec. 3) it follows that if the compatibility condition
d ∈ D(c # ) (2)
holds, then the function ˆ
g(s) := sc # (sI − A) −1 d − c # d = sh ∗ A(sI − A) −1 d − c # d (3)
is well-defined and analytic on the complex right half-plane Π + = {s ∈ : Re s > 0}.
If, apart from (2), c # is admissible, then:
(i) ˆ g(s) = s c P d
(s) − c # d with c P d ∈ H ∞ (Π + ) ∩ H 2 (Π + ), where H ∞ (Π + ) denotes the Banach space of analytic functions f on Π + , equipped with the norm kfk H ∞ (Π + ) = sup s∈Π + |f(s)|, and H 2 (Π + ) is the Hardy space of functions f analytic on Π + such that sup σ>0 R ∞
−∞ |f(σ + jω)| 2 dω < ∞, where f(jω) :=
lim σ→0+ f (σ + jω) exists for almost all ω ∈ . The space H 2 (Π + ) is unitarily isomorphic with L 2 (0, ∞) through the normalized Laplace transform. To be more precise,
hf, gi L 2 (0,∞) = 1 2π
Z ∞
−∞
f(jω)ˆ ˆ g(jω) dω,
where ˆ f and ˆ g are the Laplace transforms of f and g, respectively. The latter facts are fundamental ingredients of the Paley-Wiener theory (Duren, 1970, Ch. 11).
(ii) The convolution operator K with kernel P d, i.e. Ku := P d ? u, belongs to L (L 2 (0, ∞)), and it maps the domain of R into itself.
Finally, by (Grabowski and Callier, 2001, Thm. 4.1) the following result holds:
Lemma 2. If (2) is satisfied, c # is admissible and
ˆ g ∈ H ∞ (Π + ), (4)
then the i n p u t - o u t p u t operator F ,
F = −KR − c # dI, D(F ) = D(R), is bounded and its closure F is given by
Range(K) ⊂ D(R), F = −RK − c # dI.
Moreover, ˆ g is then the t r a n s f e r f u n c t i o n of the system (1).
2. Additional Properties of the Input-Output Map
Definition 2. The operator H ∈ L(L 2 (0, ∞)) is called causal or nonanticipative if (Hu T ) T = (Hu) T , ∀u ∈ L 2 (0, ∞),
where u T denotes the truncation of u at time T > 0,
u T (t) =
( u(t) if t < T,
0 otherwise.
Lemma 3. The closure F of the input-output map F has the following properties:
(i) F is causal.
(ii) If k 1 < 0 and k 2 > 0 are such that
ω∈ inf
h k 1 k 2 |ˆg(jω)| 2 − (k 1 + k 2 ) Re ˆ g(jω) + 1 i
≥ δ > 0, (5)
then for any u ∈ L 2 (0, ∞) we have (I − k 1 F )u, (I − k 2 F )u
L 2 (0,∞) ≥ δ kuk 2 L 2 (0,∞) , (6) and the operator (I − k 1 F ∗ )(I − k 2 F ) is s t r i c t l y p a s s i v e (Desoer and Vidyasagar, 1975, p. 173), i.e.
(I − k 1 F )u T
T ,
(I − k 2 F )u T
T
L 2 (0,T ) ≥ δ ku T k 2 L 2 (0,T ) , ∀T > 0. (7) Proof. As for (i), observe that
F u
T =
d dt
Z t
0 P d(t − τ)u(τ) dτ, t < T
0, t > T
− c # du T ,
F u T = d dt
Z t
0 P d(t − τ)
( u(τ ), τ < T 0, τ > T
)
dτ − c # du T
=
d dt
Z t
0 P d(t − τ)u(τ) dτ, t < T d
dt Z T
0 P d(t − τ)u(τ) dτ, t > T
− c # du T ,
so that F u T
T = F u
T and F is nonanticipative.
As for (ii), applying the Paley-Wiener theory, for any u ∈ L 2 (0, ∞) we get (k 1 F − I)u, (k 2 F − I)u
L 2 (0,∞)
= 1 2π
Z ∞
−∞
k 1 g(jω) − 1 ˆ ˆ
u(jω)[k 2 g(jω) − 1] ˆ ˆ u(jω) dω
= 1 2π
Z ∞
−∞
h k 1 k 2 |ˆg(jω)| 2 − (k 1 + k 2 ) Re ˆ g(jω) + 1 i
|ˆ u(jω)| 2 dω
≥ inf
ω∈
h k 1 k 2 |ˆg(jω)| 2 − (k 1 + k 2 ) Re ˆ g(jω) + 1 i 1 2π
Z ∞
−∞ |ˆu(jω)| 2 dω
≥ δ kuk 2 L 2 (0,∞) .
By (6) with u = u T we have (I−k 1 F )u T , (I−k 2 F )u T
L 2 (0,∞) ≥ δ ku T k 2 L 2 (0,∞) = δ ku T k 2 L 2 (0,T ) , ∀T > 0, (8)
but
(I − k 1 F )u T , (I − k 2 F )u T
L 2 (0,∞)
= ku T k 2 L 2 (0,∞) − k 1
F u T , u T
L 2 (0,∞)
− k 2
u T , F u T
L 2 (0,∞) + k 1 k 2 F u T
2 L 2 (0,∞)
= ku T k 2 L 2 (0,T ) − k 1 F u T
T , u T
L 2 (0,T )
− k 2
u T , F u T
T
L 2 (0,T ) + k 1 k 2 F u T
2 L 2 (0,∞) . Since
F u T
2
L 2 (0,∞) ≥ F u T
T
2
L 2 (0,T )
and k 1 k 2 < 0, we obtain the estimate k 1 k 2
F u T
2 L 2 (0,∞) ≤ k 1 k 2
F u T
T
2
L 2 (0,T ) . Finally,
(I − k 1 F )u T , (I − k 2 F )u T
L 2 (0,∞) ≤
(I − k 1 F )u T
T ,
(I − k 2 F )u T
T
L 2 (0,T ) , and (7) follows from (8).
Remark 1. A similar result is found in (Desoer and Vidyasagar, 1975, Ex. 1, p. 174).
Since F is causal, its adjoint operator F ∗ is anticausal (Desoer and Vidyasagar, 1975, Lemma 9.1.8, p. 201).
Remark 2. The frequency-domain inequality (5) means geometrically that the plot of the transfer function ˆ g(jω) is located strictly inside the circle with centre at
k 1 −1 + k −1 2
/2 and radius k 2 −1 − k −1 1
/2.
3. The Circle Criterion
For the feedback system given in Fig. 1, we assume the following:
(A1) The linear part of the feedback system from u to y is our boundary control system in factor form, where A generates a linear EXS semigroup {S(t)} t≥0
on H, c # is admissible, d ∈ D(c # ) and ˆ g ∈ H ∞ (Π + ). Hence, for any x 0 ∈ H, its input-output equation in L 2 (0, T ) for any T > 0 is given by
y T = P x 0
T + F u
T = P x 0
T + F u T
T . (9)
The last equality is due to the causality of F .
y(t) CONTROLLER
u(t)
PLANT
f (y)
˙x(t) = A
x(t) + du(t) x(0) = x 0
y(t) = c # x(t)