Estimating the safety function response time for wireless control systems

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University of New Brunswick


Safety function response time (SFRT) is a metric for safety-critical automation systems defined in the IEC 61784-3-3 standard for single input and single output systems communicating over wired technologies. This thesis proposes a model to estimate the SFRT for multiple input and multiple output feedback control systems communicating over the IEEE 802.15.4e wireless medium access control standard designed for process automation. The wireless SFRT model provides equations for the worst case delay time and watchdog timer of participating network entities, including wireless communication channels. Thirty-nine on board, wired and wireless control experiments using real devices were carried out to evaluate control performance, and the applicability of the wireless SFRT model. The estimated SFRT for the wired implementation is 38.2 ms. For the wireless experiments, the best SFRT obtained was 655.4 ms with no acceptable packet loss. The wireless implementation failed to provide successful control on 15 of the 21 experiments.