Wi-Fi-based hierarchical Wireless Networked Control Systems

Abstract

This paper proposes a novel architecture for a hierarchical Wireless Networked Control System (WNCS). It consists of three cascaded workcells each containing 30 sensors, 30 actuators and one controller. The wireless communication protocol used is IEEE 802.11g with multicasting. The hierarchy of the system is such that the lowest level is that of the sensors and actuators, the intermediate level is the controllers, and the highest level is a supervisory node. This supervisor can be either active or passive. System performance is measured using OPNET simulations and the results are confirmed analytically. The system is shown to tolerate all possible controller failure scenarios. The supervisor can handle the entire control load of all three controllers, should the need arise. The system exhibits zero packet drops and delay constraints are met in all scenarios. The effect of interference is then investigated and the maximum interference that can be tolerated by the system is quantified.

On the Performability of Hierarchical Wireless Networked Control Systems

“…This is the starting state where the system is in the fault-free condition and the three controllers (K) as well as the supervisor (S) are fully operational. It was assumed in [18] that the supervisor is extremely reliable and will fail last. Consequently, if any of the three controllers fail, the system moves to State {2K + S}.…”

“…In [18], hierarchical fault-tolerance was applied to the system in [16] by adding a supervisor above the controller level. The supervisor has two modes of operation: either active or passive.…”

“…In this paper the performability of the active and passive modes in [18] will be compared. Markov reliability models will be built for both systems.…”

“…In [18], a supervisor was added to the three-workcell system in [16] as shown in Figure 1. The supervisor can handle the control of the cells in two ways: passive or active.…”

“…In the active mode, once a controller fails, the supervisor takes over its control tasks. Multicasting was used in [18] to decrease the load on the wireless network. Multicasting allowed each sensor to send only one stream of packets to the AP.…”