Intelligent Control and Automation
Vol.06 No.02(2015), Article ID:55169,13 pages
10.4236/ica.2015.62011
Constrained Feedback Stabilization for Bilinear Parabolic Systems
Azzeddine Tsouli, Ali Boutoulout
TSI Team, MACS Laboratory, Faculty of Sciences, Moulay Ismail University, Meknes, Morocco
Email: azzeddine.tsouli@gmail.com, boutouloutali@yahoo.fr, elalamiabdessamad@gmail.com
Copyright © 2015 by authors and Scientific Research Publishing Inc.
This work is licensed under the Creative Commons Attribution International License (CC BY).
http://creativecommons.org/licenses/by/4.0/
Received 2 January 2015; accepted 27 March 2015; published 30 March 2015
ABSTRACT
In this paper, we shall study the stabilization and the robustness of a constrained feedback control for bilinear parabolic systems defined on a Hilbert state space. Then, we shall show that stabilizing such a system reduces stabilization only in its projection on a suitable subspace. For this purpose, a new constrained stabilizing feedback control that allows a polynomial decay estimate of the stabilized state is given. Also, the robustness of the considered control is discussed. An illustrating example and simulations are presented.
Keywords:
Bilinear Parabolic Systems, Constrained Feedback Stabilisation, Decay Estimate, Robustness
1. Introduction
Bilinear systems represent a small, but important subset of nonlinear systems within which linear systems coexist as a special subclass. Adopting a bilinear model retains a well structured framework, which contains the well- known notional concepts such as time constants and steady-state behaviour. When adopting a bilinear approach, these concepts become operation-dependent quantities which can be appropriately modelled. Bilinear system models represent an important class of nonlinear models that are defined to be linear in both state and control when considered independently, with the nonlinearity (or bilinearity) arising from coupled terms involving products of system state and control (see [1] [2] ). By formulating the model appropriately, the bilinear term could also be represented by products of system output and control input, i.e. the output is defined as a system state. There are numerous combinations of product terms that could be considered, thus potentially increasing the model complexity. However, it has been found in practice that a minimal number of product terms can provide an adequate model for the purpose of control. Bilinear model structures are able to represent nonlinear phenomena more accurately than linear models, and thereby extend the range of satisfactory performance. In this paper, we are concerned with the question of the stabilization by a constrained feedback control for bilinear parabolic systems that can be described in the following form:
(1)
on a real Hilbert space with inner product ; and corresponding norm , where the linear operator generates a contraction semigroup on and . While the real valued function represents a control. A function , , is a mild solution of the system (1) if and only if the solution of the system (1) satisfies the variation of parameters formula:
(2)
(see [3] ). By choosing an adequate feedback control in such a way, the corresponding solution of the system (1) converges to zero when , for all in . For finite-dimensional bilinear systems associated to a skew-adjoint matrix , the question of stabilization has been treated in [4] , under the condition:
(3)
where is defined recursively as , and , .
Using the following assumptions:
(4)
the problem of stabilization has been studied in [5] . In [3] , when the linear operator is compact and is a contraction semigroup, then using the quadratic feedback control
(5)
a weak stabilization result is obtained under the weak observability condition:
(6)
In the case where is sequentially continuous from ( endowed with the weak topology) to , the quadratic feedback control (2) weakly stabilizes the system (1), provided that the following weak observability assumption (4) holds (see [3] ). Under the exact observability assumption
(7)
The strong stabilization result with the following decay estimate
(8)
i.e. , for large enough, has been obtained using the quadratic feedback control (5) (see [6] ). However, in this way the convergence of the resulting closed loop state is not better than (8). In [7] the rational decay rates are established i.e. using the following feedback control:
It has been shown in [8] that, where the resolvent of the operator is compact, and is abounded linear, self-adjoint and monotone, the constrained feedback control law
(9)
strongly stabilizes the system (1), provided that the assumption (6) holds. It has been established in [7] that, if
the linear operator generates a contraction semigroup in , then the system (1) is strongly
stable with the explicit decay estimate (8), using the control (9), provided that the estimate (7) holds. Here, we will establish an explicit decay estimate of the stabilized state and the robustness of the control (9) for a large class of bilinear systems as considered in [3] [8] [9] . The method used in this paper is based on decomposing the system (1) into two suitable subsystems: the stable part and the unstable one. Then, we will show that one can concentrate on the determination of a stabilizing control for the so-called unstable part which maintains the exponential stability of the stable part. The rest of this article is as follows: in Section 2, we will give the main hypotheses that allow the decomposition of the system (1) into two subsystems. Then, under the compactness hypothesis of the operator , we will give a weaker variant of the condition (6) which achieves strong stabilization of the system (1). In Section 3, we will show that under a weaker version of (6), we obtain the stabilization with the decay estimate (8). Section 4 concerns the robustness of the stabilizing controls. The last section is devoted to an illustrating example and simulations.
2. Stabilization Results
Let us now recall the following definition concerning the asymptotic behavior of the system (1).
2.1. Definition
The system (1) is weakly (resp. strongly) stabilizable if there exists a feedback control , , such that the corresponding mild solution of the system (1) satisfies the properties:
1. For each initial state of the system (1) there exists a unique mild solution defined for all of the system (1),
2. is an equilibrium state of the system (1),
3. , weakly (resp. strongly), as , for all .
In the sequel of this section, we will present an appropriate decomposition of the state space and the system (1) via the spectral properties of the operator , and we apply this approach to study the stabilization pro- blem of the system (1). In [10] - [12] , it has been shown that if the spectrum of can be decomposed
into and , then the state space can be decomposed according to
(10)
where , , is given by
(11)
where is a curve surrounding , and for all is the eigenvector associated to
the eigenvalue . The projection operators and commute with , and we have with and . Also, for all , we set and . For linear systems, it
has been shown that the initial system can be decomposed into two subsystems on and . If satisfies the spectrum growth assumption:
(12)
Which is equivalent to:
(13)
where denotes the semigroup generated by in , then stabilizing the whole system turns out to stabilizing its projection on (see [13] ). In the sequel, we suppose that the operator satisfies
(14)
It is easily verified that the condition (14) is equivalent to the fact that the linear operator commutes with . We note that the condition (14) also holds in the special case: . Let us consider that the system (1) can be decomposed in the following two subsystems:
(15)
(16)
in the state spaces and respectively, and . It has been proved that stabilizing a linear system turns out to stabilizing its unstable part (see [13] ).
2.2. Remark
For finite-dimensional systems, the conditions (6) and (7) are equivalent (see [5] [10] ). However, in infinite- dimensional case, and if is compact, then the condition (7) is impossible. Indeed, if is an orthonormal basis of , then applying (7) for and using the fact that , weakly as , we obtain the contradiction:
The following result concerns the strong stabilization of the system (1).
2.3. Theorem
Let
1. generates a linear -contraction semigroup on ,
2. allows the decomposition (10) of with such that (13) holds,
3. be compact such that
(17)
Then, the constrained feedback control law:
(18)
strongly stabilizes the system (1).
Proof
The system (1) controlled by (18) possesses a unique mild solution defined on a maximal interval and given by the variation of constants formula
(19)
corresponds to (18) (see [9] ).
Since is a contraction semigroup, we get:
(20)
It follows from (20) that
(21)
From (19) and using the fact that is a contraction semigroup and the Gronwall inequality, we deduce that the map is continuous from to . Then (21) holds for all by density argument, and hence (see [14] ). Now, let us show that , weakly as . Let
such that weakly converges in and let such that , weakly as . (The ex- istence of the sequence and are ensured by (21) and by the fact that space is reflexive.) Taking as initial state in (19) and using superposition property of the solution, and via the dominated conver-
gence theorem, we obtain , . It follows from (17) that . Hence , weakly as , and since , we have , as . For the component of we have
(22)
Then for all , we have:
(23)
It follows from (13) that
(24)
From Gronwall inequality, we obtain:
Taking , we deduce that , as .
Hence , as .
3. A Decay Rate Estimate of the Stabilized State
In what follows, we will study the strong stabilizability of the system (1) with the decay estimate (8).
Before we state our main result, the following lemmas will be needed (see [15] ).
3.1. Lemma
Let be a sequence of positive real numbers satisfying
(24)
where and are constants. Then there exists a positive constant (depending on and ) such that
(25)
Let us now recall the following existing result (see [9] ).
3.2. Lemma
Let generate a contraction semigroup on and let be linear operator from into itself. Then the system (1), controlled by (18) possesses a unique mild solution for each which satisfies
(26)
For almost all .
Our main result in this section is stated as follows:
3.3. Theorem
Let
1. generates a linear -semigroup such that is a semigroup of isometries and (13) holds,
2. allows the decomposition (10) of with ,
3. such that for all , we have
(27)
Then the constrained feedback control law:
(28)
strongly stabilizes the system (1) with the explicit decay estimate (8).
Proof
Let us consider the system:
(29)
Multiplying the system (29) by , integrating over and using the fact that is a semigroup of isometries, we obtain:
(30)
which proves that the real function is decreasing on , and we have
(31)
Hence, the system (29) admits a unique mild solution defined for almost all (see [9] ).
Integrating now the inequality (30) over the interval , for and , we get:
using now the estimate (26), we deduce that
(32)
for some . Using now the fact that , then the assumption (27) is equivalent to
(33)
From (32) and (33) we have
using the fact that the map is decreasing on , we obtain:
which implies that
Letting , the last inequality can be written as
From Lemma.3.1 we have
For ( designed the integer part of ), then we obtain , , which gives
Hence
(34)
For the component , we shall show that is defined for all and exponentially converges to 0, as . The system (1) excited by the constrained feedback control (28) admits a unique mild solution defined for almost all in a maximal interval defined by
Thus
(35)
It follows from (13) that
For almost all . The Gronwall inequality then yields:
(36)
Taking , it follows from (36) that is bounded on so , and therefore
(36) holds for all . Hence, from (34) and (36), the solution of (1) satisfies the estimate (8). This completes the proof of Theorem 3.3.
3.4. Remark
1. Since the function decreasing in , we have
In this case, we have
Hence, using (13) the system (1) is exponentially stable.
2. The constrained feedback control (28) depends only on the unstable part and we have
3. The constrained feedback control (28) satisfies
4. We note that (27) is weaker than (6). The converse is not true as we can see taking an orthonormal basis of , and .
5. In the case and is nonlinear and locally Lipschitz, such that , then using the same techniques as in [9] , we can obtain the result of Theorem 3.3, if the estimate (7) is changed to (33).
4. Robustness
In this section, we study the robustness of the controls (18) and (28), under a class of perturbations of the system (1).
4.1. Strong Robustness
In this part, we consider the strong robustness of the feedback (18). Then, we will show that the stability property of the system (1) remains invariant under a certain class of bounded perturbations.
Let us consider the following perturbed system
(37)
where the linear bounded operator is such that the system (37) is decomposed into two following subsystems:
(38)
(39)
The following main result concerns the strong stability of the system (37).
4.2. Proposition
Let
1. generates a linear -contraction semigroup on such that (13) holds,
2. The operator is compact such that (6) holds,
3. The linear operator is compact and satisfying and , .
Then the system (37) is strongly stabilizable.
Proof
First, let us note that 0 remains an equilibrium state of the perturbed system (37), which can be written in the form:
(40)
where and
Since and are locally Lipschitz, so is . Also is dissipative: , .
The assumption , , together with (6) guarantees the following implication
Then the weak stability of the perturbed system (37) follows from Theorem 2.4 of Ball [3] , and since , we have , as . For the component of the solution of the system (37), and for all , we have:
(41)
It follows from (13) that
(42)
From Gronwall inequality, we obtain:
Taking , we obtain , as . Hence, the solution of the system (37) strongly converges to as .
4.3. A Polynomial Decay Estimate for the Perturbed System
Our second main result in this section is stated as follows:
4.4. Proposition
Let
1. generate a linear -semigroup such that is a semigroup of isometries and (13) holds,
2. allows the decomposition (10) of with ,
3. satisfies (27),
4. and , for all
Then the constrained feedback control (28) strongly stabilizes the system (37) with the explicit decay estimate (8).
Proof
Let us consider the system:
(43)
Multiplying the system (43) by and integrating over and using the fact that is a semigroup of isometries and the hypothesis: , , we obtain:
(44)
which gives , . Then, the system (43) admits a unique global mild solution de-
fined for almost all . By the same argument as in the proof of Theorem 3.3, the solution of the system (37) satisfies:
which completes the proof of Proposition 4.2.
5. Application and Simulations
5.1. An Application
In this part, we will give an illustrating example of the established results.
Example
Let us consider the following 1-d bilinear heat equation:
(45)
where is the temperature profile at time . We suppose that the system is controlled via the flow of a liquid in an adequate metallic pipeline. Here we take the state space and the operator is defined by , with . The domain of gives the ho-
mogeneous Neumann boundary condition imposed at the ends of the bar which require specifying how the heat flows out of the bar and means that both ends are insulated. The spectrum of is given by the simple eigen-
values , and eigenfunctions and for all . Then the subspace is the one-dimensional space spanned by the eigenfunction , and we have so (the identity) and hence , is a semigroup of isometries. The operator of control , is defined by: , , , such that (see [16] ). From the relation: , we can see that (27) holds if . To examine the estimate (8), remarking for the scalar functions , we have
(46)
which implies that
(47)
Letting , we obtain
Integrating now the last equality from 0 to , we get
from (47), we deduce that the nonnegative scalar map is decreasing for all , and we have
which means that
Then
Furthermore, the control in this case is defined by
(48)
For , the functions are characterized by , and satisfy
which implies that
Then
Hence, the system (45) is strongly stable with the decay rate estimate (8).
Let us reconsider the above example with the perturbation defined by:
It is clear that the function satisfies the conditions of Proposition 4.2.1. Then the perturbed closed-loop system remains stable, i.e., the control (48) still stabilizes the perturbed system i.e., the control (48) is strongly robust.
5.2. Simulations
In this part, taking in the system (45), the operator and .
Then we obtain the results shown in Figures 1-5.
Figure 1. The norm of the free state.
Figure 2. The free state.
Figure 3. The norm of the stabilized state.
Figure 4. The stabilized state.
Figure 5. The stabilizing control.
6. Conclusion
In this work, we have considered the problem of strong stabilization with polynomial decay rate of the stabilized state for bilinear parabolic systems that can be decomposed in the stable and unstable parts (15) and (16) under a weaker condition (27). We have also considered the problem of using a stabilizing feedback control for the unstable part (15) only that can make the whole system (1) stable. Various questions remain open. This is the case
of stabilization for nonlinear systems. Finally, we have studied the robustness problem of the stabilizing controls with respect to a class of perturbations, but a confrontation to more realistic situations remain done. This leads us to consider the stabilization problem for stochastic bilinear system.
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