Using an hybrid traffic scheduling in Networked Control Systems

The scheduling of the exchanged messages is an essential mechanism in the context of Networked Control Systems. The more often, the scheduling is based on static priorities but this scheme has shown limits when process control applications are considered. These limits have yet been demonstrated by the authors by considering a Networked Control System implemented on the Fielbus CAN. The goal of this work is to propose an hybrid traffic scheduling (priorities composed of two parts - a first level part for representing the flows, a second level part for representing the needs, static or dynamic - ) and to show its advantage with respect to a strictly static scheduling. A proposal is done in the context of the network CAN for implementing the needs part as a function of the control signal of process control applications. Several functions are analysed and compared. Some interesting and encouraging results have been obtained. The drawback of the advantage of the scheme (i.e. the reduction of the number of flows which can be scheduled) is also shown.