CDS Lecture Series

Friday, August 15, 2003, 2:00 p.m.

Babak Azimi-Sadjadi
Department of Electrical, Systems and Computer Engineering, Rensselaer Polytechnic Institute
Institute for Systems Research, University of Maryland

Stability of Networked Control Systems in the Presence of Packet Losses

Networked control systems are systems whose sensors, actuators, estimator units, and control units are connected through communication networks. This type of system has the advantage of greater flexibility with respect to traditional control systems. Also, it allows for reduced wiring, as well as a lower installation cost. It also permits greater agility in diagnosis and maintenance procedures.

Unlike stand-alone control systems, in networked control systems the synchronization between different sensors, actuators and control units is not guaranteed. Furthermore, there is no guarantee for zero delay or even constant delay in sending information from sensors to the control units and control signals from the control units to the actuators. When there is congestion in the communication networks, some packets are dropped to either reduce the queue size in the path or to inform the senders to reduce their transmission rates. In real time systems, particularly control systems, delays or dropped packets may be catastrophic and may cause instability in the control system.

In this talk we present a general framework for networked control systems, where all components are assumed to be connected through a communication network. We consider network delay in our analysis only inasmuch as it relates to dropping packets due to an extensive delay, and the effect of this on the stability of the system. We use the uncertainty threshold principle to show that under certain conditions there is a rate for dropped packets for which an undisturbed networked control system is mean square stable.

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