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.