Proportional-Integral Synchronisation for Non-identical Wireless Packet-Coupled Oscillators with Delays

Precise timing among wireless sensor nodes is a key enabling technology for time-sensitive industrial Wireless Sensor Networks (WSNs). However, the accuracy of timing is degraded by manufacturing tolerance, ageing of crystal oscillators, and communication delays. This paper develops a framework of Packet-Coupled Oscillator (PkCOs) to characterise the dynamics of communication and time synchronisation of clocks in WSNs. A non-identical clock is derived to describe the embedded clock's behaviour accurately. The Proportional-Integral (PI) packet coupling scheme is proposed for synchronising networked embedded clocks, meanwhile, scheduling wireless Sync packets to different slots for transmission. It also possesses the feature of automatically eliminating the effects of unknown processing delay, which further improves synchronisation performance. The rigorous theoretical analysis of PI-based PkCOs is presented via studying a closed-loop time synchronisation system. The performance of PI-based PkCOs is evaluated on a hardware testbed of IEEE 802.15.4 WSN. The experimental results show that the precision of the proportional-integral PkCOs protocol is as high as 60us (i.e., 2 ticks) for 32.768kHz crystal oscillator-based clocks

Modelling and Synchronisation of Delayed Packet-Coupled Oscillators in Industrial Wireless Sensor Networks

In this paper, a Packet-Coupled Oscillators (PkCOs) synchronisation protocol is proposed for time-sensitive Wireless Sensor Networks (WSNs) based on Pulse-Coupled Oscillators (PCO) in mathematical biology. The effects of delays on synchronisation performance are studied through mathematical modelling and analysis of packet exchange and processing delays. The delay compensation strategy (i.e., feedforward control) is utilised to cancel delays effectively. A simple scheduling function is provided with PkCOs to allocate the packet transmission event to a specified time slot, by configuring reference input of the system to a non-zero value, in order to minimise the possibility of packet collision in synchronised wireless networks. The rigorous theoretical proofs are provided to validate the convergence and stability of the proposed synchronisation scheme. Finally, the simulations and experiments examine the effectiveness of PkCOs with delay compensation and scheduling strategies. The experimental results also show that the proposed PkCOs algorithm can achieve synchronisation with the precision of $26.3\mu s$ ($1$ tick)

Robust time synchronisation for industrial internet of things by H∞ output feedback control

Precise timing over timestamped packet exchange communication is an enabling technology in the mission-critical industrial Internet of Things, particularly when satellite-based timing is unavailable. The main challenge is to ensure timing accuracy when the clock synchronisation 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 synchronisation between an arbitrary pair of clocks is modelled 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 modelled as disturbances. A static output controller is adopted to adjust the drifting clock. The H∞ robust control design solution is proposed to guarantee that the ratio between the modulus of synchronisation precision and the magnitude of the disturbances is always less than a given value. Therefore, the proposed time synchronisation protocol is robust against the disturbances, which means that the impacts of drifting frequency and delays on the synchronisation accuracy are limited. The one-hour experimental results demonstrate that the proposed R-PkCOs protocol can realise time synchronisation with the precision of six microseconds 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

Modelling and Synchronisation of Delayed Packet-Coupled Oscillators in Industrial Wireless Sensor Networks

In this paper, a Packet-Coupled Oscillators (PkCOs) synchronisation protocol is proposed for time-sensitive Wireless Sensor Networks (WSNs) based on Pulse-Coupled Oscillators (PCO) in mathematical biology. The effects of delays on synchronisation performance are studied through mathematical modelling and analysis of packet exchange and processing delays. The delay compensation strategy (i.e., feedforward control) is utilised to cancel delays effectively. A simple scheduling function is provided with PkCOs to allocate the packet transmission event to a specified time slot, by configuring reference input of the system to a non-zero value, in order to minimise the possibility of packet collision in synchronised wireless networks. The rigorous theoretical proofs are provided to validate the convergence and stability of the proposed synchronisation scheme. Finally, the simulations and experiments examine the effectiveness of PkCOs with delay compensation and scheduling strategies. The experimental results also show that the proposed PkCOs algorithm can achieve synchronisation with the precision of 26.3µs (1 tick)

APT Weighted MRI as an Effective Imaging Protocol to Predict Clinical Outcome After Acute Ischemic Stroke

To explore the capability of the amide-proton-transfer weighted (APTW) magnetic resonance imaging (MRI) in the evaluation of clinical neurological deficit at the time of hospitalization and assessment of long-term daily functional outcome for patients with acute ischemic stroke (AIS). We recruited 55 AIS patients with brain MRI acquired within 24–48 h of symptom onset and followed up with their 90-day modified Rankin Scale (mRS) score. APT weighted MRI was performed for all the study subjects to measure APTW signal quantitatively in the acute ischemic area (APTWipsi) and the contralateral side (APTWcont). Change of the APT signal between the acute ischemic region and the contralateral side (ΔAPTW) was calculated. Maximum APTW signal (APTWmax) and minimal APTW signal (APTWmin) were also acquired to demonstrate APTW signals heterogeneity (APTWmax−min). In addition, all the patients were divided into 2 groups according to their 90-day mRS score (good prognosis group with mRS score <2 and poor prognosis group with mRS score ≥2). In the meantime, ΔAPTW of these groups was compared. We found that ΔAPTW was in good correlation with National Institutes of Health Stroke Scale (NIHSS) score (R2 = 0.578, p < 0.001) and 90-day mRS score (R2 = 0.55, p < 0.001). There was significant difference of ΔAPTW between patients with good prognosis and patients with poor prognosis. Plus, APTWmax−min was significantly different between two groups. These results suggested that APT weighted MRI could be used as an effective tool to assess the stroke severity and prognosis for patients with AIS, with APTW signal heterogeneity as a possible biomarker

Common characteristics of feedstock stage in life cycle assessments of agricultural residue-based biofuels

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordIn this study, we conducted life cycle assessments (LCAs) for fuels based on different types of agricultural residues and determined the characteristics common to all LCAs. Each fuel type required specific conversion technology during the feedstock stage, particularly during the production and collection processes. We divided the field-to-fuel life cycle into five high-level and relatively independent sub-stages: production of agricultural residues, collection of agricultural residues, conversion of agricultural residues to biofuels, biofuel distribution, and biofuel utilization. We then illustrated the common characteristics during the feedstock stage for the first two field-to-fuel life cycle sub-stages: production and collection of agricultural residues. Agricultural residues-to-grain weight and price ratios and multifactorial LCA allocations were summarized for the production stage. In addition, the energy use availability coefficient, collection radius, and emissions were determined for each fuel type during the collection stage. System boundaries and benefits of direct emissions reduction during the feedstock stage were also discussed. Our results provide guidance for future LCA studies on agricultural residue-based biofuels.National Natural Science Foundation of ChinaChinese Academy of EngineeringHenan Province Talent ProjectHenan Academy of Sciences Research Project

A software simulator of discrete pulse-coupled oscillators (PCO) time synchronization in wireless sensor networks

Time synchronization, aiming to provide a common timescale among distributed sensor nodes, is a key enabling technology for many applications, such as collaborative condition monitoring and localization detection. Due to the complexity of time synchronization in wireless sensor networks, the Discrete Event Simulator is recommended to adopt resulting from the feature that the behavior of a complex is represented as an order sequence of well-defined event in time. In this paper, the PCO clock is firstly implemented into the open-source software simulator OMNeT++ by using desynchronization mechanism under a realistic scenario, and it can also be simulated at an adjustable and higher resolution. The developed relay node, can either be a Full Function Device or a Reduced Function Device, enables the high scalability for multi-node and multi-hop simulation. In addition, the shared code of this project on the GitHub directly benefits the researchers and engineers in communication

Implementation of Timestamped Pulse-Coupled Oscillators in IEEE 802.15.4 Networks

Time Synchronization (TS) is a key enabling technology of mission-critical Wireless Sensor Networks (WSNs) to provide a common timescale for distributed sensor nodes. Inspired by synchronous flashing of fireflies, a bio-inspired model, Pulse-Coupled Oscillators (PCO), has been intensively studied. The most studies on PCOs are theoretical work, and the assumption is given that oscillators broadcast and receive the Pulses simultaneously when synchronization of a network is achieved. This is not true when it comes to any real-world environments. From the viewpoint of WSNs, the clock of a sensor node driven by crystal oscillators can be modelled as an oscillator, and Pulse firing can be implemented by transmitting a packet. However, the concurrent transmission of Pulse packets is impossible due to the packet collision in the single wireless channel. To avoid this issue in WSNs, this paper adopts a desynchronization mechanism, in which the Pulse packets are transmitted to the wireless channel in a uniformly distributed fashion and in accordance with standard IEEE 802.15.4. A hardware testbed is developed to implement the desynchronized pulse-coupled oscillators, and it can also be extended to the large-scale wireless sensor networks

Modelling and Synchronization of Pulse-Coupled Non-identical Oscillators for Wireless Sensor Networks

Time synchronization in wireless sensor networks, aiming to provide a common sense of timing among distributed sensor nodes, is a key enabling technology for many applications, such as collaborative condition monitoring, time-of-flight localization and underwater navigation and tactical surveillance. In order to solve the challenges of the manufacturing tolerance and working condition variations in any real-world environments, a novel state-space model for pulse-coupled non-identical oscillators is proposed to model a realistic clock oscillator with nonidentical and time-varying frequency. A state feedback correction, referred to as hybrid coupling mechanism, is also proposed to ensure the system move into steady state, thus achieving time synchronization in wireless sensor networks. Furthermore, the intensive simulations of single-hop wireless sensor networks have been carried out to evaluate the performance of proposed pulsecoupled non-identical oscillators. It is shown that a partially connected wireless network consisting of 50 non-identical pulsecoupled oscillators can achieve the synchronization with the precision of 40us

Simulation and evaluation of pulse-coupled oscillators in wireless sensor networks

The clock in embedded systems usually is driven by a crystal oscillator and implemented via a counter register, such a crystal clock is non-identical and drifting due to the manufacturing tolerance and variation of working conditions. Thus, a common time among distributed wireless sensor nodes, also referred to as Time Synchronization, is required for many time-sensitive wireless applications, such as collaborative condition monitoring, coordinated control and localization. Inspired by fireflies’ behaviour, the Pulse-Coupled Oscillators (PCO) has been proposed for synchronization in complex networks. Since the concurrent transmission of PCO’s Pulses is impossible in Wireless Sensor Networks (WSNs), the desynchronization mechanism is adopted to ensure the implementation of PCO in WSNs. Moreover, due to the uncertainties in radio channels and the complexities of communication protocols and packet-exchange behaviours in wireless networks, it is challenging to have a closed-form solution to the performance of PCO synchronization in WSNs. The realistic software simulation, in particular, the discrete event simulator has been a powerful tool to exam the performance of communication protocols in various scenarios, since an order sequence of well-defined event in time is to represent the behaviour of a complex system. This paper presents the development of a pulse-coupled oscillators time synchronization simulator on the OMNeT++ platform for simulating and studying its behaviour and performance in sensor networks. A clock module with configurable phase and frequency noises, and adjustable and higher resolution is developed to mimic various crystal oscillators in embedded systems, for example, the real-time clock. The developed simulator also supports the full functions devices defined by ZigBee protocol, which allows realistic simulation of multi-hop IEEE 802.15.4 wireless networks. Finally, the intensive simulations of classical PCO with the refractory period in IEEE 802.15.4-based WSNs have been carried out to demonstrate the features and benefits of the developed simulator. It is shown that for the non-identical and time-varying PCO clocks in the WSNs, the achieved synchronization will lose gradually, and the time that maintained synchronization depends on the length of refractory period