42 research outputs found
Generalised balance equations for charged particle transport via localised and delocalised states: Mobility, generalised Einstein relations and fractional transport
A generalised phase-space kinetic Boltzmann equation for highly
non-equilibrium charged particle transport via localised and delocalised states
is used to develop continuity, momentum and energy balance equations,
accounting explicitly for scattering, trapping/detrapping and recombination
loss processes. Analytic expressions detail the effect of these microscopic
processes on the mobility and diffusivity. Generalised Einstein relations (GER)
are developed that enable the anisotropic nature of diffusion to be determined
in terms of the measured field-dependence of the mobility. Interesting
phenomena such as negative differential conductivity and recombination
heating/cooling are shown to arise from recombination loss processes and the
localised and delocalised nature of transport. Fractional transport emerges
naturally within this framework through the appropriate choice of divergent
mean waiting time distributions for localised states, and fractional
generalisations of the GER and mobility are presented. Signature impacts on
time-of-flight current transients of recombination loss processes via both
localised and delocalised states are presented.Comment: 21 pages, 4 figure
Efficient numerical solution of the time fractional diffusion equation by mapping from its Brownian counterpart
The solution of a Caputo time fractional diffusion equation of order
is expressed in terms of the solution of a corresponding integer
order diffusion equation. We demonstrate a linear time mapping between these
solutions that allows for accelerated computation of the solution of the
fractional order problem. In the context of an -point finite difference time
discretisation, the mapping allows for an improvement in time computational
complexity from to , given a
precomputation of . The mapping is applied
successfully to the least-squares fitting of a fractional advection diffusion
model for the current in a time-of-flight experiment, resulting in a
computational speed up in the range of one to three orders of magnitude for
realistic problem sizes.Comment: 9 pages, 5 figures; added references for section
Generalized phase-space kinetic and diffusion equations for classical and dispersive transport
We formulate and solve a physically-based, phase space kinetic equation for transport in the presence of trapping. Trapping is incorporated through a waiting time distribution function. From the phase-space analysis, we obtain a generalized diffusion equation in configuration space. We analyse the impact of the waiting time distribution, and give examples that lead to dispersive or non-dispersive transport. With an appropriate choice of the waiting time distribution, our model is related to fractional diffusion in the sense that fractional equations can be obtained in the limit of long times. Finally, we demonstrate the application of this theory to disordered semiconductors
Third-order transport coefficients for localised and delocalised charged-particle transport
We derive third order transport coefficients of skewness for a phase-space
kinetic model that considers the processes of scattering collisions, trapping,
detrapping and recombination losses. The resulting expression for the skewness
tensor provides an extension to Fick's law which is in turn applied to yield a
corresponding generalised advection-diffusion-skewness equation. A physical
interpretation of trap-induced skewness is presented and used to describe an
observed negative skewness due to traps. A relationship between skewness,
diffusion, mobility and temperature is formed by analogy with Einstein's
relation. Fractional transport is explored and its effects on the flux
transport coefficients are also outlined.Comment: 11 pages, 3 figure
A Survey of Asynchronous Programming Using Coroutines in the Internet of Things and Embedded Systems
Many Internet of Things and embedded projects are event-driven, and therefore
require asynchronous and concurrent programming. Current proposals for C++20
suggest that coroutines will have native language support. It is timely to
survey the current use of coroutines in embedded systems development. This
paper investigates existing research which uses or describes coroutines on
resource-constrained platforms. The existing research is analysed with regard
to: software platform, hardware platform and capacity; use cases and intended
benefits; and the application programming interface design used for coroutines.
A systematic mapping study was performed, to select studies published between
2007 and 2018 which contained original research into the application of
coroutines on resource-constrained platforms. An initial set of 566 candidate
papers were reduced to only 35 after filters were applied, revealing the
following taxonomy. The C & C++ programming languages were used by 22 studies
out of 35. As regards hardware, 16 studies used 8- or 16-bit processors while
13 used 32-bit processors. The four most common use cases were concurrency (17
papers), network communication (15), sensor readings (9) and data flow (7). The
leading intended benefits were code style and simplicity (12 papers),
scheduling (9) and efficiency (8). A wide variety of techniques have been used
to implement coroutines, including native macros, additional tool chain steps,
new language features and non-portable assembly language. We conclude that
there is widespread demand for coroutines on resource-constrained devices. Our
findings suggest that there is significant demand for a formalised, stable,
well-supported implementation of coroutines in C++, designed with consideration
of the special needs of resource-constrained devices, and further that such an
implementation would bring benefits specific to such devices.Comment: 22 pages, 8 figures, to be published in ACM Transactions on Embedded
Computing Systems (TECS
Plasticiser leaching from polyvinyl chloride microplastics and the implications for environmental risk assessment
Microplastics in aquatic environments is a growing concern, particularly due to the leaching of chemical additives such as plasticisers. To develop comprehensive environmental risk assessments (ERAs) of high-concern polymers and plasticisers, an understanding of their leachability is required. This work investigated diethylhexyl phthalate (DEHP) and bisphenol A (BPA) leaching from polyvinyl chloride (PVC) microplastics (average diameter = 191 μm) under simulated marine conditions. Leaching behaviours were quantified using gel permeation chromatography (GPC) and thermal gravimetric analysis (TGA), and the polymer's physiochemical properties analysed using differential scanning calorimetry (DSC), Fourier Transform-Infrared Spectroscopy (FT-IR) and optical microscopy. Experimental data were fitted to a diffusion and boundary layer model, which found that BPA leaching was temperature-dependent (diffusion-limited), whereas DEHP leaching was controlled by surface rinsing. Model predictions also highlighted the importance of microplastic size on leaching dynamics. These data contribute towards greater accuracy in ERAs of microplastics, with implications for water quality and waste management, including decommissioning of plastic infrastructure
WeedCLR: Weed Contrastive Learning through Visual Representations with Class-Optimized Loss in Long-Tailed Datasets
Image classification is a crucial task in modern weed management and crop
intervention technologies. However, the limited size, diversity, and balance of
existing weed datasets hinder the development of deep learning models for
generalizable weed identification. In addition, the expensive labelling
requirements of mainstream fully-supervised weed classifiers make them cost-
and time-prohibitive to deploy widely, for new weed species, and in
site-specific weed management. This paper proposes a novel method for Weed
Contrastive Learning through visual Representations (WeedCLR), that uses
class-optimized loss with Von Neumann Entropy of deep representation for weed
classification in long-tailed datasets. WeedCLR leverages self-supervised
learning to learn rich and robust visual features without any labels and
applies a class-optimized loss function to address the class imbalance problem
in long-tailed datasets. WeedCLR is evaluated on two public weed datasets:
CottonWeedID15, containing 15 weed species, and DeepWeeds, containing 8 weed
species. WeedCLR achieves an average accuracy improvement of 4.3\% on
CottonWeedID15 and 5.6\% on DeepWeeds over previous methods. It also
demonstrates better generalization ability and robustness to different
environmental conditions than existing methods without the need for expensive
and time-consuming human annotations. These significant improvements make
WeedCLR an effective tool for weed classification in long-tailed datasets and
allows for more rapid and widespread deployment of site-specific weed
management and crop intervention technologies.Comment: 24 pages, 10 figures, 8 tables. Submitted to the Computers and
Electronics in Agriculture journa
Comparing inertial measurement units and marker-based biomechanical models during dynamic rotation of the torso
Inertial measurement units (IMUs) enable human movements to be captured in the field and are being used increasingly in high performance sport. One key metric that can be derived from IMUs are relative angles of body segments which are important for monitoring form in many sports. The purpose of this study was to a) examine the validity of relative angles derived from IMUs placed on the torso and pelvis; and b) determine optimal positioning for torso mounted sensors such that the IMU relative angles match closely with gold standard torso-pelvis and thorax-pelvis relative angle data derived from an optoelectronic camera system. Seventeen adult participants undertook a variety of motion tasks. Four IMUs were positioned on the torso and one was positioned on the pelvis between the posterior superior iliac spines. Reflective markers were positioned around each IMU and over torso and pelvis landmarks. Results showed that the IMUs are valid with the root mean square errors expressed as a percentage of the angle range (RMSE%) ranging between 1% and 7%. Comparison between the IMU relative angles and the torso-pelvis and thorax-pelvis relative angles showed there were moderate to large differences with RMSE% values ranging between 4% and 57%. IMUs are highly accurate at measuring orientation data; however, further work is needed to optimize positioning and modelling approaches so IMU relative angles align more closely with relative angles derived using traditional motion capture methods
DeepWeeds: a multiclass weed species image dataset for deep learning
Robotic weed control has seen increased research of late with its potential for boosting productivity in agriculture. Majority of works focus on developing robotics for croplands, ignoring the weed management problems facing rangeland stock farmers. perhaps the greatest obstacle to widespread uptake of robotic weed control is the robust classification of weed species in their natural environment. the unparalleled successes of deep learning make it an ideal candidate for recognising various weed species in the complex rangeland environment. This work contributes the first large, public, multiclass image dataset of weed species from the Australian rangelands; allowing for the development of robust classification methods to make robotic weed control viable. The DeepWeeds dataset consists of 17,509 labelled images of eight nationally significant weed species native to eight locations across northern Australia. This paper presents a baseline for classification performance on the dataset using the benchmark deep learning models, Inception-v3 and ResNet-50. These models achieved an average classification accuracy of 95.1% and 95.7%, respectively. We also demonstrate real time performance of the ResNet-50 architecture, with an average inference time of 53.4 ms per image. These strong results bode well for future field implementation of robotic weed control methods in the Australian rangelands
Charge transport in organic solar cells
The process of charge transport is fundamental to the operation of all electronic devices. In organic photovoltaics, high efficiencies can only be achieved if charge transport is able to extract charge carriers from the active layer with minimal recombination losses. This work presents new insights into the measurement of charge transport, the underlying physics, as well as new approaches for modelling. Numeric simulation software using a drift-diffusion-recombination model is developed and applied to organic photovoltaic devices. Specifically, this model is used to design and interpret charge transport experiments that are applicable to operational organic solar cells.\ud
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Charge carrier mobility is studied using photogenerated charge extraction by linearly increasing voltage (photo-CELIV) and the novel technique of resistance-dependent photovoltage (RPV). These experiments demonstrate the absence of "hot carrier" relaxation effects on the timescales of charge transport in several organic photovoltaic polymer:fullerene blends. This is surprising because it has previously been argued that such relaxation is the cause of the deterimental dispersive transport that affects many organic semiconductor devices. It is argued instead that dispersive transport arises from the loss of carriers to trap states. Next, the techniques are extended to recombination measurements, where the recombination coefficient in a benchmark polymer:fullerene system is found to depend upon the polymer's molecular weight.\ud
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Modelling of the steady-state photocurrent produced by a solar cell demonstrates the conditions under which non-geminate recombination may be avoided, and presents a design rule for avoiding non-geminate recombination. Experimental measurements on devices of varying thickness support the conclusion that the space-charge limited current is a fundamental threshold for high-efficiency photocurrent extraction.\ud
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Finally, fractional kinetics and generalised diffusion equations are explored. We show that the Poisson summation theorem permits the analytic solution of a fractional diffusion equation to be collapsed into closed form. Subsequently, these techniques are applied to a new type of kinetic model that is capable of unifying normal and dispersive transport within a single framework