Stability Analysis of Networked Control Systems

Gregory C. Walsh
University of Maryland at College Park

In many complicated control systems, such as manufacturing plants, vehicles, aircraft and spacecraft, serial communication networks are employed to exchange information and control signals between spatially distributed system components, like supervisory computers, controllers and intelligent I/O devices (e.g. smart sensors and actuators). Each of the system components connected to the network directly is denoted as a node. When a control loop is closed via the serial communication channel, we label it a Networked Control System (NCS). The serial communication channel, which multiplexes signals from the sensors to the controller and/or from the controller to the actuators, serves many other uses besides control.

In contrast to widely used computer networks, a NCS is concerned primarily with the quality of real-time reliable service. Queues are not desirable in a NCS because in real-time control system, the newest data is the best data. If new sampling data is available, the old data that has not been transmitted can be discarded. This observation forms the basis for our Try-Once-Discard (TOD) protocol. Modeling and analysis of a NCS is quite different from that of computer network because of the lack of queues. Our primary objective of NCS design is to efficiently use the finite bus capacity while maintaining good closed loop control system performance.

In this talk we introduce a novel control network protocol, TOD, for Networked Control Systems (NCS), and provide, for the first time, an analytic proof of stability for both the new protocol and the more commonly used (statically scheduled) access methods. The performance of the new network protocol and the statically scheduled protocols are compared in simulations.