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Chapter 4Inverse Optimal Redesign

In Chapter 2 we studied the system

X(t) = AX(t)+ BU(tD) (4.1)

with the controller

U(t) = K[

ADX(t)+ t

tDeA(t)BU( )d

], (4.2)

and, using the backstepping method for PDEs, we constructed a LyapunovKrasovskii functional for the closed-loop system (4.1), (4.2). This LyapunovKrasovskii functional is given by

V (t) = X(t)T PX(t)+a

2

ttD

(1 + + D t)W ( )2d , (4.3)

where P is the solution to the Lyapunov equation

P(A + BK)+ (A + BK)TP =Q , (4.4)

P and Q are positive definite and symmetric, the constant a > 0 is sufficiently large,and W ( ) is defined as

W ( ) = U( )K[

tDeA()BU()d + eA(+Dt)X(t)

], (4.5)

with D tD t.The main purpose of a Lyapunov function is the establishment of Lyapunov

stability. But what else might a Lyapunov function be useful for? We explore thisquestion in the present chapter and in Chapter 5.

As we shall see, the utility of a Lyapunov function is in quantitative studies ofrobustness, to additive disturbance and to parameters, as well as in achieving inverseoptimality in addition to stabilization.

M. Krstic, Delay Compensation for Nonlinear, Adaptive, and PDE Systems,

Birkhuser Boston, a part of Springer Science + Business Media, LLC 2009Systems & Control: Foundations & Applications, DOI 10.1007/978-0-8176-4877-0_4,

53

54 4 Inverse Optimal Redesign

In this chapter we highlight inverse optimality and disturbance attenuation, whichare achieved with the help of the transformation (4.5) and the Lyapunov function(4.3).

We first derive an inverse-optimal controller, which incorporates a penalty notonly on the ODE state X(t) and the input U(t) but also on the delay state. The inverseoptimal feedback that we design is of the form (where, for brevity and conceptualclarity, we mix the frequency and time domains, i.e., the lag transfer function on theright should be understood as an operator)

U(t) =c

s+ c

{K

[eADX(t)+

ttD

eA(t)BU( )d]}

, (4.6)

where c > 0 is sufficiently large; i.e., the inverse-optimal feedback is of the form ofa low-pass filtered version of (4.2).

In Section 4.1 we establish the inverse optimality of the feedback law (4.6) andits stabilization property for sufficiently large c. In Section 4.3 we consider the plant(4.1) in the presence of an additive disturbance and establish the inverse optimalityof the feedback (4.6) in the sense of solving a meaningful differential game problem,and we quantify its L disturbance attenuation property.

4.1 Inverse Optimal Redesign

In the formulation of the inverse optimality problem we will consider U(t) as theinput to the system, whereas U(t) is still the actuated variable. Hence, our inverseoptimal design will be implementable after integration in time, i.e., as dynamic feed-back. Treating U(t) as an input is the same as adding an integrator, which has beenobserved as being beneficial in the control design for delay systems in [69].

Theorem 4.1. There exists c such that the feedback system (4.1), (4.6) is exponen-tially stable in the sense of the norm

N(t) =(|X(t)|2 +

ttD

U( )2d +U(t)2)1/2

(4.7)

for all c > c. Furthermore, there exists c > c such that for any c c, thefeedback (4.6) minimizes the cost functional

J =

0

(L (t)+ U(t)2

)dt , (4.8)

where L is a functional of (X(t),U( )), [tD,t], and such thatL (t) N(t)2 (4.9)

4.1 Inverse Optimal Redesign 55

Fig. 4.1 Linear system X(t) = AX(t)+BU(t D) with actuator delay D.

for some (c) > 0 with a property that(c) as c . (4.10)

Proof. We start by writing (4.1) as the ODE-PDE systemX(t) = AX(t)+ Bu(0,t) , (4.11)

ut(x,t) = ux(x,t) , (4.12)u(D,t) = U(t) , (4.13)

whereu(x,t) = U(t + xD) , (4.14)

and therefore the outputu(0,t) = U(tD) (4.15)

gives the delayed input (see Fig. 4.1).Consider the infinite-dimensional backstepping transformation of the delay state

(Chapter 2)

w(x,t) = u(x,t)[ x

0KeA(xy)Bu(y,t)dy+ KeAxX(t)

], (4.16)

which satisfies

X(t) = (A + BK)X(t)+ Bw(0,t) , (4.17)wt(x,t) = wx(x,t) . (4.18)

Let us now consider w(D,t). It is easily seen that

wt (D,t) = ut(D,t)K[

Bu(D,t)+ D

0eA(Dy)Bu(y,t)dy+ AeADX(t)

]. (4.19)

Note thatut(D,t) = U(t) , (4.20)

56 4 Inverse Optimal Redesign

which is designated as the control input penalized in (4.8). The inverse of (4.16) isgiven by1

u(x,t) = w(x,t)+ x

0Ke(A+BK)(xy)Bw(y,t)dy

+ Ke(A+BK)xX(t) . (4.21)

Plugging (4.21) into (4.19), after a lengthy calculation that involves a change of theorder of integration in a double integral, we get

wt(D,t) = ut(D,t)KBw(D,t)

K(A + BK)[ D

0M(y)Bw(y,t)dy+ M(0)X(t)

], (4.22)

where

M(y) = D

yeA(D)BKe(A+BK)(y)d + eA(Dy)

= e(A+BK)(Dy) (4.23)

is a matrix-valued function defined for y [0,D]. Note that N : [0,D] Rnn is inboth L[0,D] and L2[0,D].

Now consider a Lyapunov function

V (t) = X(t)T PX(t)+a

2

D0

(1 + x)w(x,t)2 dx+ 12 w(D,t)2 , (4.24)

where P > 0 is defined in (4.4) and the parameter a > 0 is to be chosen later. We haveV (t) = XT (t)((A + BK)T P+ P(A + BK))X(t)

+ 2XT (t)PBw(0,t)+ a2

D0

(1 + x)w(x,t)wx(x,t)dx

+ w(D,t)wt(D,t) . (4.25)

After the substitution of the Lyapunov equation, we obtain

V (t) =XT (t)QX(t)+ 2XT(t)PBw(0,t)

+a

2(1 + D)w(D,t)2 a

2w(0,t)2 a

2

D0

w(x,t)2 dx

+ w(D,t)wt(D,t) , (4.26)1 The fact that (4.21) is the inverse of (4.16) can be seen in various ways, including a direct substi-tution and manipulation of integrals, as well as by using a Laplace transform in x and employingthe identity ( IABK)1 (IBK( IA)1) = ( IA)1, where is the argument of theLaplace transform in x.

4.1 Inverse Optimal Redesign 57

which gives

V (t)XT (t)QX(t)+ 2a|XT PB|2 a

2

D0

w(x,t)2 dx

+ w(D,t)(

wt(D,t)+a(1 + D)

2w(D,t)

), (4.27)

and finally,

V (t)12

XT (t)QX(t) a2

D0

w(x,t)2 dx

+ w(D,t)(

wt (D,t)+a(1 + D)

2w(D,t)

), (4.28)

where we have chosena = 4max(PBB

T P)min(Q) , (4.29)

where min and max are the minimum and maximum eigenvalues of the corre-sponding matrices. Now we consider (4.28) along with (4.22). With a completion ofsquares, we obtain

V (t)14

XT (t)QX(t) a4

D0

w(x,t)2 dx

+|K(A + BK)M(0)|2

min(Q) w(D,t)2

+K(A + BK)MB2

aw(D,t)2

+(

a(1 + D)2

KB)

w(D,t)2

+ w(D,t)ut(D,t) . (4.30)

[We suppress the details of this step in the calculation but provide the details on thepart that may be the hardest to see:

w(D,t)K(A + BK)MB,w(t) |w(D,t)|K(A + BK)MBw(t)

a4w(t)2 + K(A + BK)MB

2

aw(D,t)2 , (4.31)

where the first inequality is the CauchySchwartz and the second is Youngs, thenotation , denotes the inner product in the spatial variable y, on which both M(y)and w(y,t) depend, and denotes the L2 norm in y.]

58 4 Inverse Optimal Redesign

Then, choosingut(D,t) =cw(D,t) , (4.32)

we arrive at

V (t)14

X(t)T QX(t) a4

D0

w(x,t)2 dx (c c)w(D,t)2 , (4.33)

where

c =a(1 + D)

2KB + |K(A + BK)M(0)|

2

min(Q) +K(A + BK)MB2

a. (4.34)

Using (4.16) for x = D and the fact that u(D,t) = U(t), from (4.32) we get (4.6).Hence, from (4.33), the first statement of the theorem is proved if we can show thatthere exist positive numbers 1 and 2 such that

1N2(t)V (t) 2N2(t) , (4.35)

whereN(t)2 = |X(t)|2 +

D0

u(x,t)2 dx+ u(D,t)2 . (4.36)

This is straightforward to establish by using (4.16), (4.21), and (4.24), and employ-ing the CauchySchwartz inequality and other calculations, following a pattern of asimilar computation in [202]. Thus, the first part of the theorem is proved.

The second part of the theorem is established in a manner very similar to thelengthy proof of Theorem 6 in [202], which is based on the idea of the proof ofTheorem 2.8 in [109]. We choose

c = 4c (4.37)

and

L (t) = 2c V (t)(4.26) with (4.22), (4.32), and c = 2c+ c(c 4c)w(D,t)2

c(

12

XT (t)QX(t)+ a2

D0

w(x,t)2 dx+(c2c)w(D,t)2)

. (4.38)

We haveL (t) N(t)2 (4.39)

for the same reason that (4.35) holds. This completes the proof of inverse optimality.unionsq

Remark 4.1. We have established the stability robustness to varying the parameter cfrom some large value c to , recovering in the limit the basic, unfiltered predictor-based feedback (4.2). This robustness property might be intuitively expected froma singular perturbation idea, though an off-the-shelf theorem for establishing this

4.2 Is Direct Optimality Possible Without Solving Operator Riccati Equations? 59

property would be highly unlikely to be found in the literature due to the infinitedimensionality and the special hybrid (ODE-PDE-ODE) structure of the system athand.

Remark 4.2. The feedback (4.2) is not inverse optimal, but the feedback (4.6) is,for any c [c,). Its optimality holds for a relevant cost functional, which isunderbounded by the temporal L2[0,) norm of the ODE state X(t), the norm of thecontrol U(t), as well as the norm of its derivative U(t) [in addition to 0D U( )2d ,which is fixed because feedback has no influence on it]. The controller (4.6) isstabilizing for c = , namely, in its nominal form (4.2); however, since () = ,it is not optimal with respect to a cost functional that includes a penalty on U(t).

Remark 4.3. Having obtained in