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Nonlinear Control Sy.. - Free

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8.5. PASSIVITY OF LINEAR TIME-INVARIANT SYSTEMS<br />

(x, Hx)<br />

(x,h*x)<br />

J<br />

27r f<br />

1 te[H(7w)] lX (7w)12 dw +<br />

27r<br />

x(t)[h(t) * x(t)] dt<br />

00<br />

X (7w)[H(?w)X (7w)] du)<br />

00<br />

1 IX(.7w)I2 H(7w)' dw<br />

27r f00<br />

00<br />

f<br />

00<br />

3<br />

2rrm[H(7w)]<br />

00<br />

and sincem[H(yw)] is an odd function of w, the second integral is zero. It follows that<br />

and noticing that<br />

we have that<br />

(x, Hx) _ 1<br />

27r<br />

00<br />

J Re[H(34 dw<br />

(x, x) I I -`f (7w) I2 dw<br />

27r<br />

(x,Hx) > inf te[H(yw)]<br />

W<br />

from where the sufficiency of conditions (i) and (ii) follows immediately. To prove necessity,<br />

assume that 2e[H(3w)] < 0 at some frequency w = w'. By the continuity of the Fourier<br />

transform as a function of w, it must be true that te[H(tw)] < 0 Vu ;E 1w - w' I 0. We can now construct 9(y,) as follows:<br />

X(pw) > M<br />

OW) < m<br />

VwE 1w-w*I

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