Wireless control networks with real-time constraints

The trend for today’s industrial control systems is to design them as distributed real-time systems for reliability, cost effectiveness, ability for fault diagnosis, and upgradability. Industrial control systems and networks, traditionally realized through wired communication systems/protocols suffer from maintenance costs due to numerous installations of expensive cables. Utilization of wireless technologies in industrial control systems is an urgent need. However, one cannot simply combine a general-purpose wireless network and a real-time system and expect satisfactory performance, due to the conflicting natures of the following two entities: A real-time system, such as an industrial control system, does not expect the data traveling in the system to be lost or delayed, whereas packet losses and delays are only typical in a wireless network because of its stochastic nature. Since wireless communication channels are inherently prone to data loss and corruption resulting in unpredictable delays, it is necessary to implement methods that will bound the latency of data delivery between the communicating nodes. This is especially important when a feedback loop is closed over the network as in a networked control system (NCS) [3, 2, 13, 7], because unbounded delays directly affect the phase delay, and thus the stability of the control loop.