Quasi-ISS/ISDS reduced-order observers and quantized output feedback for interconnected systems

In this paper the notion of quasi-input-to-state dynamical stability (quasi-ISDS) for reduced-order observer design is introduced. It combines the main advantage of ISDS over input-to-state stability (ISS), namely the memory fading effect, with reduced-order observers to obtain quantitative information about the state estimate error. As a second topic, interconnections of nonlinear systems are investigated and quasi- ISS reduced-order observers for the subsystems are designed when there exist suitable error Lyapunov functions. As an application of this concept, we prove that quantized output feedback stabilization for each subsystem and the overall system is achievable, when the systems possess a quasi-ISS reduced-order observer and a state feedback law that yields ISS for each subsystem and therefor the overall system with respect to measurement errors.