Electronic Journal of Differential Equations, Vol. 2014 (2014), No. 131, pp. 1–10. ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu ftp ejde.math.txstate.edu H∞ CONTROL OF SWITCHED LINEAR PARABOLIC SYSTEMS LEPING BAO, SHUMIN FEI, LIN CHAI Abstract. The H∞ control problem of a class of switched linear parabolic systems is considered. By applying the multiple Lyapunov function method and the average dwell time scheme, sufficient conditions for exponential stability and the H∞ control synthesis are established in terms of LMIs and a family of switching signals. The advantage in this work lies in the fact that sufficient conditions completely depend on the system parameters and the system can be analyzed by using numerical softwares. At the end of the paper, an example is given to illustrate the obtained result. 1. Introduction During the previous decade, the study of switched systems has attracted considerable attention because of its significance in both theoretical research and practical applications [1, 3, 6, 10, 11, 15, 18, 20, 22, 23]. A switched system is a dynamical system described by a family of continuous-time subsystems and a rule that governs the switching among them. In many realistic cases, switched systems can be described by partial differential equations (PDE) or a combination of ordinary differential equations (ODE) and PDE such as in chemical industry process and biomedical engineering. We refer to these switched systems as distributed parameter switched systems (DPSS) or infinite dimensional switched systems [5, 13]. However, there are very few works concerning DPSS (see, eg. [1, 7, 9, 14, 16, 19, 21] and the references cited therein). For example, Sasane generalized the results presented in [15] to infinite dimensional Hilbert spaces [21], and showed when all subsystems are stable and commutative pairwise, the switched linear system is stable under an arbitrary switching via the common Lyapunov function. Michel and Sun provided the stability conditions for switched nonlinear systems on Banach spaces under constrained switching [14]. Hante and Sigalotti gave necessary and sufficient conditions in term of the existence of common Lyapunov functions for DPSS [1, 7]. It seems that the majority of works deal with the stability of DPSS. The H∞ control is an interesting research topic in the field of switched systems. Up to now, most of results in the literature are regarding the H∞ control of switched 2000 Mathematics Subject Classification. 34K20, 35R15, 34D10. Key words and phrases. Switched systems; parabolic systems; H∞ control; linear matrix inequalities; average dwell time. c 2014 Texas State University - San Marcos. Submitted August 16, 2013. Published June 10, 2014. 1 2 L. BAO, S. FEI, L. CHAI EJDE-2014/131 systems which are described by ODEs [6, 12, 23]. For example, the stability, the L2 gain analysis and the H∞ control for switched systems via the multiple Lyapunovlike function methods is considered in [23]. The dynamic output feedback in H∞ control design of switched linear systems are studied in terms of linear matrix inequalities (LMIs) in [6]. To the best of our knowledge, the H∞ control has not been investigated for DPSS. Motivated by the above consideration, we study the H∞ control synthesis for switched linear parabolic systems in this paper. The main contributions of the present paper can be summarized as follows: Firstly, the concept of H∞ control is extended to DPSS. Secondly, sufficient conditions for the exponential stabilization and the H∞ control synthesis of DPSS are developed in terms of LMIs and a class of switching signals. Compared with the work in [7], our sufficient conditions completely depend on the system parameters. n diIn this article, L2 (Ω, Rn ) denotes the Hilbert space of square integrable R mensional vector-valued functions ν(x), x ∈ Ω with the norm kνkL2 = Ω ν T νdx. L2 [t0 , ∞; L2 (Ω, Rn )) is the Hilbert space of square integrable functions ν(t, ·) ∈ L2 [t0 , ∞) with values ν(·, x) ∈ L2 (Ω, Rn ). H 2 (Ω, Rn ) and H02 (Ω, Rn ) denote the ∂2ν n classical Sobolev spaces defined by H 2 (Ω, Rn ) = {ν ∈ L2 (Ω, Rn ) : ∂x 2 ∈ L2 (Ω, R )} ∂ν and H01 (Ω, Rn ) = {ν ∈ L2 (Ω, Rn ) : ∂x ∈ L2 (Ω, Rn ), ν(∂Ω, t) = 0} respectively. γM (P )(γm (P )) denotes the largest (smallest) eigenvalue of P . The symmetric elements of the matrix will be denoted by T . 2. Problem formulation and preliminaries Consider the switched linear parabolic systems ∂ν(x, t) = Dσ(t) ∆ν(x, t) + Aσ(t) ν(x, t) + Bσ(t) u(x, t) + Cσ(t) ω(x, t) ∂t y(x, t) = Eσ(t) ν(x, t) + Fσ(t) ω(x, t) (2.1) ν(t0 ) = ν0 ν(x, t) = 0, (x, t) ∈ ∂Ω × [t0 , +∞) with the static state feedback control u(x, t) = Kσ(t) ν(x, t) n (2.2) where ν(x, t) ∈ L2 [t0 , ∞; L2 (Ω, R )) is a vector-valued function representing the state of the process, u(x, t) ∈ L2 [t0 , ∞; L2 (Ω, Rs )) denotes the manipulated input, q ω(x, t) ∈ L2 [t0 , ∞; L2 (Ω, Rp )) is the disturbance, and y(x, t) ∈ L √2 [t0 , ∞;√L2 (Ω, R2 )) denotes the measured output with (x, t) ∈ Ω×[t0 , +∞). Ω = [0, 2]×[0, 2] ⊂ R is a bounded domain with the smooth boundary. ∆ denotes the Laplace operator; i.e., P2 ∂2 ∆ = k=1 ∂x 2 . Di = diag(di1 , di2 , . . . , din ) represent positive diagonal matrices, k and Ai , Bi , Ci , Ei , Fi (i = 1, 2, . . . , m) represent constant matrices of compatible dimensions. σ(t) : [t0 , ∞) → Θ is the switching signal mapping time to some finite index set Θ = {1, 2, . . . , m}, and the switching signal σ(t) is a piecewise continuous (from the right) function depending on time or state or both. The discontinuities of σ(t) are called switching times or switching instants. The integer m is the number of models (called subsystems) of the switched system. The objective of this article is to establish sufficient conditions of the H∞ control for the system (2.1)–(2.2). That is, we look for controller gain matrices Ki (i ∈ Θ) and a class of switching signals σ(t), such that EJDE-2014/131 CONTROL OF SWITCHED SYSTEMS 3 1. When ω = 0, the system (2.1) is exponentially stabilized by the state feedback control (2.2). 2. The system (2.1) is exponentially stabilized by (2.2) with the H∞ disturbance level γ > 0, i.e., for a prescribed scalar γ > 0, the performance index is Z ∞Z [y T (x, s)y(x, s) − γ 2 ω T (x, s)ω(x, s)] dx ds ≤ 0 J(ω) = t0 Ω for all non-zero ω(x, t) ∈ L2 [t0 , ∞; L2 (Ω, Rp )) under the zero initial condition. The following is the definition of average dwell time (ADT) [10]. Definition 2.1. Given some family of switching signals σ(t) ∈ Θ, for each σ(t) and each t > s ≥ t0 , let Nσ (s, t) denote the number of switching of σ(t) in the open interval (s, t). If Nσ (s, t) ≤ N0 + t−s τa holds for τa > 0 and N0 > 0, then the positive constant τa is called the ADT and N0 is the chatter bound. Lemma 2.2 (Poincare’s inequality [4]). Let the scalar function u ∈ H01 (Ω̄, R) with Ω ⊆ Ω1 , then we have Z Z X Z n ∂u 2 2 2 2 u dx ≤ γ ( ) dx = γ | ∇u |2 dx (2.3) Ω Ω i=1 ∂xi Ω where Ω1 : 0 ≤ xi ≤ δ(i = 1, 2, . . . , n), γ = √δ , n ∂ , ∂ , . . . , ∂x∂n ). and ∇ = ( ∂x 1 ∂x2 3. Stabilization analysis of switched parabolic systems In this section, we consider the exponential stabilization problem of the switched linear parabolic system ∂ν(x, t) = Dσ(t) ∆ν(x, t) + Aσ(t) ν(x, t) + Bσ(t) u(x, t) ∂t (3.1) ν(t0 ) = ν0 ν(x, t) = 0, (x, t) ∈ ∂Ω × [t0 , +∞) with the static state feedback control (2.2). We assume that the state of system does not jump at switching instants; i.e., the state trajectory is continuous and the switching signal σ(t) has the finite switching number in any finite time interval [10]. We start with the well-posedness problem of the closed-loop system (2.2)-(3.1). Define the state function z(t) as z(t) = ν(·, t) on the Hilbert space H = L2 (Ω, Rn ) with the norm k · kL2 , then the equation of closed-loop (2.2) and (3.1) can be rewritten as eσ(t) z(t) + fσ(t) (t), t ≥ t0 ż(t) = A (3.2) ei (A ei x = Di ∆x) have the dense domain in H, where the infinitesimal operators A ei ) = {ν ∈ H 2 (Ω, Rn ) T H 1 (Ω, Rn ) : ν(x) = 0, x ∈ ∂Ω}, fσ(t) (t) = W = D(A 0 [Aσ(t) + Bσ(t) Kσ(t) ]z(t). ei generate analytical semigroups Ti (t) As we know, the infinitesimal operators A [17]. Because the state of system (2.2)-(3.1) does not jump at switching instants, for every initial value ν0 ∈ W , there exists a unique solution for system (2.2)-(3.1). Thus, the initial problem (2.2)-(3.1) turns out to be well-posed on the time interval [t0 , ∞). 4 L. BAO, S. FEI, L. CHAI EJDE-2014/131 Lemma 3.1. For the given scalar λ > 0, if there exist positive constants αi , βi , a constant µ ≥ 1, and continuous functions Vi ∈ C(H × [t0 , +∞), R+ ) such that the functions Vi (t) = Vi (ν, t) are absolutely continuous along the solutions ν of system (2.2)-(3.1) and satisfy αi kν(t)kL2 ≤ Vi (t) ≤ βi kν(t)kL2 (3.3) V̇i (t) + λVi (t) ≤ 0. (3.4) Furthermore, the Lyapunov function of the system satisfies Vi (t) ≤ µVj (t), ∀i, j ∈ Θ. (3.5) Then the closed-loop system (2.2)-(3.1) is exponentially stable for the arbitrary switching signal σ(t) with the ADT τa > lnµ λ . Proof. For t ∈ [tk , tk+1 )(k = 0, 1 . . . ), from (3.4) it follows that Vσ(t) (t) ≤ e−λ(t−tk ) Vσ(tk ) (tk ). This, together with (3.5) gives −λ(t−tk ) Vσ(t) (t) ≤ µVσ(t− ) (t− . k )e k It is easy to show that −λ(t−tk ) Vσ(t) (t) ≤ µVσ(t− ) (t− ≤ µVσ(tk−1 ) (tk−1 )e−λ(t−tk ) e−λ(tk −tk−1 ) k )e k 2 ≤µ −λ(t−tk−1 ) Vσ(t− ) (t− k−1 )e k−1 ≤ . . . · · · ≤ µk Vσ(t0 )(t0 )e−λ(t−t0 ) for all t ≥ t0 and a constant µ ≥ 1. Note that when kτa ≤ t − t0 , we have lnµ Vσ(t) (t) ≤ e−λ(t−t0 ) eklnµ Vσ(t0 )(t0 ) ≤ e−(λ− τa )(t−t0 ) Vσ(t0 )(t0 ). (3.6) Combing (3.3) and (3.6), we obtain kν(t)kL2 ≤ lnµ Vσ(t) (t) 1 ≤ e−(λ− τa )(t−t0 ) Vσ(t0 )(t0 ). α α Thus we have kν(t)kL2 ≤ 1 −(λ− lnµ γ −(λ− lnµ τa )(t−t0 ) · γkν k τa )(t−t0 ) kν k e e 0 L2 ≤ 0 L2 α α where γ = maxi∈Θ {βi } and α = mini∈Θ {αi }. Let h = λ − lnµ τa > 0, and we have lnµ τa > λ . It is obvious that the system (2.2)-(3.1) is exponentially stable for the arbitrary switching signal σ(t) with the ADT τa > lnµ λ . Theorem 3.2. For the given scalar λ > 0, if there exist diagonal matrices Xi > 0, and matrices Yi > 0 such that Πi = −2Di Xi + Ai Xi + XiT ATi + Bi Yi + YiT BiT + λXi < 0. (3.7) Then system (3.1) can be exponentially stabilized by the state feedback control (2.2) with Ki = Yi Xi−1 for the arbitrary switching signal σ(t) with the ADT τa > ln(µ)/λ, where µ is determined by γM (Xk ) µ = max . (3.8) ∀k,l∈Θ γm (Xl ) EJDE-2014/131 CONTROL OF SWITCHED SYSTEMS Proof. Choose the multiple Lyapunov function for the system (2.2)-(3.1) Z V (t) = Vσ(t) (t) = ν T (x, t)Pσ(t) ν(x, t) dx 5 (3.9) Ω with constant diagonal matrices Pi > 0. It is not difficult to see that there exist positive numbers αi , βi and a constant µ ≥ 1 such that inequalities (3.3) and (3.5) (Pk ) } [22]. hold. For inequalities (3.5), we can choose µ = max∀k,l∈Θ { γγM m (Pl ) Differentiating V (t) with respect to t along the trajectory of the closed-loop system (2.2)-(3.1), we have V̇i (t) + λVi (t) Z Z = [∆ν(x, t)]T Di Pi ν(x, t)dx + ν T (x, t)Pi Di ∆ν(x, t)dx Ω Ω Z + ν T (x, t)[Pi Ai + Pi Bi Ki ]ν(x, t)dx Ω Z Z + ν T (x, t)[ATi Pi + KiT BiT Pi ]ν(x, t)dx + ν T (x, t)λPi ν(x, t)dx. Ω (3.10) Ω Because Di and Pi are positive diagonal matrices, we find that Pi Di = Di Pi . Thus we have Z Z [∆ν(x, t)]T Di Pi ν(x, t)dx + ν T (x, t)Pi Di ∆ν(x, t)dx Ω Ω Z T ≤ 2λmax (Pi Di ) ∆ν (x, t)Iν(x, t)dx Ω Z ν1 (x, t) ≤ 2λmax (Pi Di ) [∆ν1 (x, t), . . . , ∆νn (x, t)] . . . dx Ω νn (x, t) Z ≤ −2λmax (Pi Di ) [ν1 (x, t)∆ν1 (x, t) + · · · + νn (x, t)∆νn (x, t)]dx. Ω According to Gauss’s divergence theorem, Poincare’s inequality (2.3) and taking into account the boundary condition in (3.1), we obtain Z Z [∆ν(x, t)]T Di Pi ν(x, t)dx + ν T (x, t)Pi Di ∆ν(x, t)dx Ω Ω Z 2 ≤ −2λmax (Pi Di ) [ν1 (x, t) + · · · + νn (2 x, t)]dx ZΩ (3.11) T ≤ −2λmax (Pi Di ) ν (x, t)Iν(x, t)dx Ω Z ≤ ν T (x, t)(−2Pi Di )ν(x, t)dx < 0. Ω Substituting (3.11) into (3.10) yields Z V̇i (t) + λVi (t) ≤ ν T (x, t)Γi ν(x, t) dx Ω where Γi = −2Pi Di + Pi Ai + ATi Pi + Pi Bi Ki + KiT BiT Pi + λPi . When Γi = −2Pi Di + Pi Ai + ATi Pi + Pi Bi Ki + KiT BiT Pi + λPi < 0, (3.12) 6 L. BAO, S. FEI, L. CHAI EJDE-2014/131 we have V̇i (t) + λVi (t) < 0 (for all ν(x, t) 6= 0). If the switching signal σ(t) satisfies the ADT τa > ln(µ)/λ, all conditions in Lemma 3.1 hold. Hence, the closed-loop system (2.2)-(3.1) is exponentially stable. Left- and right- multiplying (3.12) by Pi−1 and letting Xi = Pi−1 and Yi = Ki Pi−1 , it is not difficult to see that equality (3.12) is equivalent to (3.7) and (Pk ) µ = max∀k,l∈Θ { γγM } leads to (3.8) immediately. Consequently, the proof is m (Pl ) completed. 4. H∞ control synthesis In the section, we consider the H∞ control problem for system (2.1)–(2.2). Lemma 4.1. For given scalars λ > 0 and γ > 0, if there exist diagonal matrices Pi > 0 such that Γi + EiT Ei Pi Ci + EiT Fi < 0, (4.1) CiT Pi + FiT Ei −γ 2 I + FiT Fi then for any t ∈ [tk , tk+1 ), along the trajectory of system (2.1)-(2.2), we have Z tZ Vi (t) ≤ e−λ(t−tk ) Vi (tk ) − e−λ(t−s) Υ(x, s) dx ds (4.2) tk Ω where Γi = −2Pi Di + Pi Ai + ATi Pi + Pi Bi Ki + KiT BiT Pi + λPi , Υ(x, s) = y T (x, s)y(x, s) − γ 2 ω T (x, s)ω(x, s). Proof. Differentiating Vi (t) with respect to t along the trajectory of the closed-loop system (2.1)–(2.2) and using the similar argument described in the previous section, we have Z Γ Pi Ci V̇i (t) + λVi (t) ≤ η T (x, t) T i η(x, t)dx, (4.3) Ci Pi 0 Ω where η T (x, t) = [ν(x, t), ω(x, t)]T . It follows from (4.1) and (4.3) that T Z Z E E EiT Fi V̇i (t) + λVi (t) < − η T (x, t) iT i η(x, t)dx = − Υ(x, s)dx. Fi Ei −γ 2 I + FiT Fi Ω Ω By calculation, we have d λt (e Vi (t)) < −eλt dt Integrating leads to (4.2). Z Υ(x, s)dx. (4.4) Ω Theorem 4.2. For given scalars λ > 0 and γ > 0, if there exist diagonal matrices Pi > 0 such that Γi + EiT Ei Pi Ci + EiT Fi < 0. (4.5) CiT Pi + FiT Ei −γ 2 I + FiT Fi Then, the system (2.1) can be exponentially stabilized by the state feedback control (2.2) with the H∞ disturbance level γ > 0 for the arbitrary switching signal σ(t) γM (Pk ) with the ADT τa > lnµ λ , where µ is determined by µ = max∀k,l∈Θ { γm (Pl ) }. EJDE-2014/131 CONTROL OF SWITCHED SYSTEMS Proof. It follows from (4.5) that Pi Ci + EiT Fi Γi + EiT Ei Γ = i ∗ CiT Pi + FiT Ei −γ 2 I + FiT Fi T Pi Ci E E + iT i −γ 2 I Fi Ei 7 EiT Fi < 0. FiT Fi Since T Ei EiT Fi Ei = T FiT Fi Fi T Ei Ei FiT Ei Fi ≥ 0, we have Γi ∗ Pi Ci < 0. −γ 2 I According to the Schur complement [2], we have Γi < 0. By virtue of the proof of Theorem 3.1, the closed-loop system (2.1)–(2.2) is exponentially stable when ω = 0. On the other hand, combining (3.5) and (4.2), we obtain Z tZ − −λ(t−tk ) Vi (t) ≤ µe Vi (tk ) − e−λ(t−s) Υ(x, s) dx ds. tk Ω Following [22] and repeating the above procedure, we obtain Vi (t) ≤ µk e−λt V (ν0 ) − µk t1 Z t0 − µk−1 t2 Z Z Z e−λ(t−s) Υ(x, s) dx ds Ω e−λ(t−s) Υ(x, s) dx ds − · · · − Ω t1 = µk e−λt V (ν0 ) − Z tZ tk Z tZ t0 e−λ(t−s) Υ(x, s) dx ds (4.6) Ω e−λ(t−s)+Nσ (s,t)lnµ Υ(x, s) dx ds. Ω Because Vi (t) > 0, the zero initial condition implies V (ν0 ) = 0. Using (4.6) yields Z tZ e−λ(t−s)+Nσ (s,t)lnµ Υ(x, s) dx ds ≤ 0. t0 It follows from e Z tZ t0 Ω Nσ (s,t)lnµ ≥ 1(µ ≥ 1, Nσ (s, t) > 0) that e−λ(t−s) [y T (x, s)y(x, s) − γ 2 ω T (x, s)ω(x, s)] dx ds ≤ 0. (4.7) Ω Notice that since Nσ (t0 , s) ≤ N0 + s−t0 τa , N0 > 0, and τa > lnµ λ , we derive that Nσ (t0 , s)lnµ ≤ N0 lnµ + λ(s − t0 ). Multiplying both sides of (4.7) by e−[N0 lnµ+λ(s−t0 )] gives Z tZ e−λ(t−s)−[N0 lnµ+λ(s−t0 )] y T (x, s)y(x, s) dx ds t0 Ω (4.8) Z tZ ≤ e t0 −λ(t−s)−[N0 lnµ+λ(s−t0 )] 2 T γ ω (x, s)ω(x, s) dx ds. Ω Thus we obtain Z tZ Z tZ e−λt y T (x, s)y(x, s) dx ds ≤ e−λt γ 2 ω T (x, s)ω(x, s) dx ds. t0 Ω t0 Ω (4.9) 8 L. BAO, S. FEI, L. CHAI EJDE-2014/131 Integrating both sides from t = t0 to ∞ gives Z ∞Z Z ∞Z γ 2 ω T (x, s)ω(x, s) dx ds y T (x, s)y(x, s) dx ds ≤ t0 Ω t0 (4.10) Ω i.e., J(ω) ≤ 0. This completes the proof. The matrix inequalities (4.5) are not LMIs. Left- and right- multiplying (4.5) by diag{Pi−1 , I}. Let Xi = Pi−1 , Yi = Ki Pi−1 . It follows from the Schur complement that (4.5) is equivalent to the LMIs Xi 0 Πi Ci + Xi EiT Fi ∗ −γ 2 I + F T Fi 0 0 i < 0, (4.11) −1 T ∗ 0 ∗ (−Ei Ei ) ∗ ∗ ∗ −I where Πi = −2Di Xi + Ai Xi + XiT ATi + Bi Yi + YiT BiT + λXi . Next, we show a result which can be obtained using matlab software. Theorem 4.3. For given scalars λ > 0 and γ > 0, if there exist diagonal matrices Xi > 0 and matrices Yi > 0, such that the LMIs (4.11) are feasible. Then the system (2.1) can be exponentially stabilized by the state feedback control (2.2) with Ki = Yi Xi−1 with the H∞ disturbance level γ > 0 for the arbitrary switching signal σ(t) with the ADT τa > lnµ λ , where µ is determined by (3.8). Example 4.4. Consider the switched parabolic equations (2.1) under the state feedback (2.2). Suppose there are two subsystems with parameters D1 = [1 0; 0 2], D2 = [2 0; 0 1], A1 = [1 3; 2 3], A2 = [1 2; 3 1], B1 = [6 7; 5 1], B2 = [5 2; 3 1], C1 = [3 5; 4 2], C2 = [5 2; 3 2], E1 = [1 0; 0 1], E2 = [1 0; 0 1], F1 = [1 2; 2 1], F2 = [2 1; 1 2]. Set λ = 0.6, γ = 0.8, using Theorem 4.3, by resolving LMIs (4.11), we obtain X1 = [489.9209 0; 0 490.7482], The state feedback matrices are −0.1487 −6.4763 K1 = , −4.0296 3.3322 X2 = [195.4787 0; 0 193.5938]. 1.8293 K2 = −5.6515 From (3.8) we obtain that µ = 2.5349 and τa > be exponentially stabilized. lnµ λ −3.8292 9.3622 = 1.5503. So system (2.1) can Conclusion. By using multiple Lyapunov function and ADT method, we establish some new criteria for the exponential stabilization and the H∞ control synthesis of switched linear parabolic systems via the state feedback. All the results are given in terms of LMIs and a class of signals which can be easily tested by matlab software. Acknowledgments. 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Department of Automation, Taiyuan Institute of Technology, Taiyuan, Shangxi 030008, China E-mail address: jsll68@126.com Shumin Fei School of Automation, Southeast University, Nanjing, Jiangsu 210096, China E-mail address: smfei@seu.edu.cn 10 L. BAO, S. FEI, L. CHAI EJDE-2014/131 Lin Chai School of Automation, Southeast University, Nanjing, Jiangsu 210096, China E-mail address: Chailin-1@163.com