Strong stabilization of almost passive linear systems

The plant to be stabilized is a system node Sigma with generating triple (A, B, C) and transfer function G, where A generates a contraction semigroup on the Hilbert space X. The control and observation operators B and C may be unbounded and they are not assumed to be admissible. The crucial assumption is that there exists a bounded operator E such that, if we replace G(s) by G(s) + E, the new system Sigma_E becomes impedance passive. A trivial case is when G is already impedance passive and a special case is when Sigma has colocated sensors and actuators. Such systems include many wave, beam and heat equations with sensors and actuators on the boundary. It has been shown for many particular cases that the feedback u = -ky + v, where u is the input of the plant and k > 0, stabilizes Sigma, strongly or even exponentially. Here, y is the output of Sigma and v is the new input. Our main result is that if for some E isin L(U), Sigma_E is impedance passive, and Sigma is approximately observable or approximately controllable in infinite time, then for sufficiently small k the closed-loop system is weakly stable. If, moreover, sigma(A)capiRopf is countable, then the closed-loop semigroup and its dual are both strongly stable. We illustrate our results with three classes of second order systems, only one of which has colocated actuators and sensors