Stability of networked control systems - IEEE Control Systems Magazine

IEEE Control Systems Magazine February 2001 Feedback control systems wherein the control loops are closed through a real-time network are called networked control systems (NCSs) [1]-[4]. The defining feature of an NCS is that information (reference input, plant output, control input, etc.) is exchanged using a network among control system components (sensors, controller, actuators, etc.). Fig. 1 illustrates a typical setup and the information flows of an NCS. The primary advantages of an NCS are reduced system wiring, ease of system diagnosis and maintenance, and increased system agility. The insertion of the communication network in the feedback control loop makes the analysis and design of an NCS complex. Conventional control theories with many ideal assumptions, such as synchronized control and nondelayed sensing and actuation, must be reevaluated before they can be applied to NCSs. Specifically, the following issues need to be addressed. The first issue is the network-induced delay (sensor-to-controller delay and controller-to-actuator delay) that occurs while exchanging data among devices connected to the shared medium. This delay, either constant (up to jitter) or time varying, can degrade the performance of control systems designed without considering the delay and can even destabilize the system. Next, the network can be viewed as a web of unreliable transmission paths. Some packets not only suffer transmission delay but, even worse, can be lost during transmission. Thus, how such packet Stability of Networked Control Systems By Wei Zhang, Michael S. Branicky, and Stephen M. Phillips