Abstract
Precise timing over timestamped packet-exchange communication is an enabling technology in the mission-critical industrial Internet of Things (IIoT), particularly when satellite-based timing is unavailable. The main challenge is to ensure timing accuracy when the clock synchronization system is subject to disturbances caused by the drifting frequency, time-varying delay, jitter, and timestamping uncertainty. In this work, a robust packet-coupled oscillators (R-PkCOs) protocol is proposed to reduce the effects of perturbations manifested in the drifting clock, timestamping uncertainty, and delays. First, in the spanning-tree clock topology, time synchronization between an arbitrary pair of clocks is modeled as a state-space model, where clock states are coupled with each other by one-way timestamped packet exchange (referred to as packet coupling), and the impacts of both drifting frequency and delays are modeled as disturbances. A static output controller is adopted to adjust the drifting clock. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{\infty }$ </tex-math></inline-formula> robust control design solution is proposed to guarantee that the ratio between the modulus of synchronization precision and the magnitude of the disturbances are always less than a given value. Therefore, the proposed time synchronization protocol is robust against the disturbances, which means that the impacts of drifting frequency and delays on the synchronization accuracy are limited. The one-hour experimental results demonstrate that the proposed R-PkCO’s protocol can realize time synchronization with the precision of 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> in a 21-node IEEE 802.15.4 network. This work has widespread impacts in the process automation of automotive, mining, oil, and gas industries.
Highlights
O VER the last decade, the rapid proliferation of Internet of Things (IoT) has been instrumental in the digital manufacturing revolution, and a new era of the Industrial Internet of Things (IIoT) has emerged with different requirements to traditional IoT systems
Precise timing is one of the most sought after IIoT attributes in mission-critical industrial applications, especially those that have control loops commonly found in chemical engineering and precision manufacturing
In many mission-critical industrial applications, the slotbased contention-free packet transmission mechanism is used to guarantee that the packet exchange delay is almost deterministic, and to insure a high Quality of Service (QoS) [11], [17]
Summary
O VER the last decade, the rapid proliferation of Internet of Things (IoT) has been instrumental in the digital manufacturing revolution (fourth industrial revolution), and a new era of the Industrial Internet of Things (IIoT) has emerged with different requirements to traditional IoT systems. Precise timing is one of the most sought after IIoT attributes in mission-critical industrial applications, especially those that have control loops commonly found in chemical engineering and precision manufacturing. The assumptions of PCO [e.g. failure of producing the physical Pulse signal, and no delays exist during the firing information (i.e. Pulse) exchange among oscillators] limit its application in Offthe-Shelf wireless networks. It needs to be improved for employment in industrial applications.
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