Network Protocols for Networked Control Systems

Control systems with networked communication, called networked control systems (NCSs), provide several advantages over point-to-point wired systems such as improvement in reliability through reduced volume of wiring, simpler systems integration, easier troubleshooting and maintenance, and the possibility for distributed processing. There are two types of communication networks. Data networks are characterized by large data packets, relatively infrequent bursty transmission, and high data rates; they generally do not have hard real-time constraints. Control networks, in contrast, must shuttle countless small but frequent packets among a relatively large set of nodes to meet the time-critical requirements. The key element that distinguishes control networks from data networks is the capability to support real-time or time-critical applications [19]. The change of communication architecture from point-to-point to commonbus, however, introduces different forms of time delay uncertainties between sensors, actuators, and controllers. These time delays come from the time sharing of the communication medium as well as the extra time required for physical signal coding and communication processing. The characteristics of time delays may be constant, bounded, or even random, depending on the network protocols adopted and the chosen hardware. This type of time delay could potentially degrade a system’s performance and possibly cause system instability.