Poles of a State Space Filter

In this section, we show that the poles of a state-space model are given by the eigenvalues of the state-transition matrix $ A$.

Beginning again with the transfer function of the general state-space model,

$\displaystyle H(z) = D + C \left(zI - A\right)^{-1}B,
$

we may first observe that the poles of $ H(z)$ are either the same as or some subset of the poles of

$\displaystyle H_p(z) \isdef \left(zI - A\right)^{-1}.
$

(They are the same when all modes are controllable and observable [37].) By Cramer's rule for matrix inversion, the denominator polynomial for $ \left(zI - A\right)^{-1}$ is given by the determinant

$\displaystyle D(z) \isdef \det(zI - A)
$

where $ \det(Q)$ denotes the determinant of the square matrix $ Q$. (The determinant of $ Q$ is also often written $ \left\vert Q\right\vert$.) In linear algebra, the polynomial $ D(z) = \left\vert zI-A\right\vert$ is called the characteristic polynomial for the matrix $ A$. The roots of the characteristic polynomial are called the eigenvalues of $ A$.

Thus, the eigenvalues of the state transition matrix $ A$ are the poles of the corresponding linear time-invariant system. In particular, note that the poles of the system do not depend on the matrices $ B,C,D$, although these matrices, by placing system zeros, can cause pole-zero cancellations (unobservable or uncontrollable modes).


Next Section:
Difference Equations to State Space
Previous Section:
Transposition of a State Space Filter