Networked Control Systems: Definition and Analysis of a Hybrid Priority Scheme for the Message Scheduling

The message scheduling is an essential mechanism in the context of networked control systems (NCS) which are systems based on communication networks and which allow, in particular, the implementation of closed loop distributed control applications. The scheduling of the messages of the flows of these closed loop distributed applications strongly influences the stability of these applications. The message scheduling is, in several networks, mainly based on fixed (static) priorities associated to the messages (for example the network CAN). In previous works, we have shown that the static priority scheme has intrinsic limitations and so, we propose in this paper, in order to overcome these limitations, the concept of hybrid (hierarchical) priority. Different hybrid priority schemes are presented and evaluated with respect to the stability performances of the closed loop distributed process control applications. We show, in particular, the advantage of a scheduling based on a scheme "hybrid priority + time strategy".