Contents 1. Introduction
Recently there has been much interest in networked control systems (NCS), that is, control systems with a feed-back loop closed through a communications network or a field bus. Traditional point-to-point controller architectures are being replaced with those based on a serial communication channel because with the drop in price of microcontrollers such an architecture is less costly, more reliable, easier to maintain, and more flexible. The presence of the communication network, however, complicates the application of standard results and algorithms of control theory. The problem lies with the communication channel restricting access to the data held by the sensors; decisions must be made on old or on partial data and the intervals between updates are not regular as the network is used for many purposes besides the routing of feedback data. So there will inevitably be time delays in the communication net. As long as the sampling periods are long compared with these delays there is no need to consider the influence of the delays. As the demand on the control system increases it will be more and more important to take the delays into account in the analysis and the design of the control system. While inaccuracies, disturbances, etc., have been extensively studied in the control literature the network time delay in real-time systems have just recently attracted attention, and in the communication literature the feedback control aspect has not been treated to any larger extent. This is thus an area where much can be gained by combining ideas from the fields of control, real-time systems, and communication networks.
