A hybrid scheduling protocol to improve quality of service in networked control systems

In many networked control systems (NCSs) only one node is allowed to use the shared medium at any given time. This network constraint can adversely affect the performance of the system and its stability. There are two types of network schedulers, static and dynamic. Static schedulers, such as the token ring protocol, have problems handling large-scale systems. On the other hand, dynamic schedulers, such as try-once-discard (TOD) cannot guarantee good quality of service for each node, especially the low-priority ones. In this paper, a hybrid scheduler, which is a combination of dynamic and static protocols, is proposed. This scheduler improves the medium access strategy in large-scale control systems. We refer to this scheduler as the traffic-division arbitration (TDA) protocol. The network-induced delay error bound and the system stability of the NCS using the proposed scheduler are investigated. Simulations illustrate the performance of the proposed scheduler and difference from TOD are shown. We use two different decision functions to prioritize the scheduling criteria of the protocol.