346 research outputs found
A Model of an Oscillatory Neural Network with Multilevel Neurons for Pattern Recognition and Computing
The current study uses a novel method of multilevel neurons and high order
synchronization effects described by a family of special metrics, for pattern
recognition in an oscillatory neural network (ONN). The output oscillator
(neuron) of the network has multilevel variations in its synchronization value
with the reference oscillator, and allows classification of an input pattern
into a set of classes. The ONN model is implemented on thermally-coupled
vanadium dioxide oscillators. The ONN is trained by the simulated annealing
algorithm for selection of the network parameters. The results demonstrate that
ONN is capable of classifying 512 visual patterns (as a cell array 3 * 3,
distributed by symmetry into 102 classes) into a set of classes with a maximum
number of elements up to fourteen. The classification capability of the network
depends on the interior noise level and synchronization effectiveness
parameter. The model allows for designing multilevel output cascades of neural
networks with high net data throughput. The presented method can be applied in
ONNs with various coupling mechanisms and oscillator topology.Comment: 26 pages, 24 figure
Method of increasing the information capacity of associative memory of oscillator neural networks using high-order synchronization effect
Computational modelling of two- and three-oscillator schemes with thermally
coupled -switches is used to demonstrate a novel method of pattern
storage and recognition in an impulse oscillator neural network (ONN) based on
the high-order synchronization effect. The method ensures high information
capacity of associative memory, i.e. a large number of synchronous states
. Each state in the system is characterized by the synchronization order
determined as the ratio of harmonics number at the common synchronization
frequency. The modelling demonstrates attainment of of several orders
both for a three-oscillator scheme ~650 and for a two-oscillator scheme
~260. A number of regularities are obtained, in particular, an optimal
strength of oscillator coupling is revealed when has a maximum. A general
tendency toward information capacity decrease is shown when the coupling
strength and switch inner noise amplitude increase. An algorithm of pattern
storage and test vector recognition is suggested. It is also shown that the
coordinate number in each vector should be one less than the switch number to
reduce recognition ambiguity. The demonstrated method of associative memory
realization is a general one and it may be applied in ONNs with various
mechanisms and oscillator coupling topology.Comment: 18 pages, 8 figure
Threshold Switching and Self-Oscillation in Niobium Oxide
Volatile threshold switching, or current controlled negative
differential resistance (CC-NDR), has been observed in a range of
transition metal oxides. Threshold switching devices exhibit a
large non-linear change in electrical conductivity, switching
from an insulating to a metallic state under external stimuli.
Compact, scalable and low power threshold switching devices are
of significant interest for use in existing and emerging
technologies, including as a selector element in high-density
memory arrays and as solid-state oscillators for hardware-based
neuromorphic computing.
This thesis explores the threshold switching in amorphous NbOx
and the properties of individual and coupled oscillators based on
this response. The study begins with an investigation of
threshold switching in Pt/NbOx/TiN devices as a function device
area, NbOx film thickness and temperature, which provides
important insight into the structure of the self-assembled
switching region. The devices exhibit combined threshold-memory
behaviour after an initial voltage-controlled forming
process, but exhibit symmetric threshold switching when the RESET
and SET currents are kept below a critical value. In this mode,
the threshold and hold voltages are shown to be independent of
the device area and film thickness, and the threshold power,
while independent of device area, is shown to decrease with
increasing film thickness. These results are shown to be
consistent with a structure in which the threshold switching
volume is confined, both laterally and vertically, to the region
between the residual memory filament and the electrode, and where
the memory filament has a core-shell structure comprising a
metallic core and a semiconducting shell. The veracity of this
structure is demonstrated by comparing experimental results with
the predictions of a resistor network model, and detailed finite
element simulations.
The next study focuses on electrical self-oscillation of an NbOx
threshold switching device incorporated into a Pearson-Anson
circuit configuration. Measurements confirm stable operation of
the oscillator at source voltages as low as 1.06 V, and
demonstrate frequency control in the range from 2.5 to 20.5 MHz
with maximum frequency tuning range of 18 MHz/V. The oscillator
exhibit three distinct oscillation regimes: sporadic spiking,
stable oscillation and damped oscillation. The oscillation
frequency, peak-to-peak amplitude and frequency are shown to be
temperature and voltage dependent with stable oscillation
achieved for temperatures up to ∼380 K. A physics-based
threshold switching model with inclusion of device and circuit
parameters is shown to explain the oscillation waveform and
characteristic.
The final study explores the oscillation dynamics of capacitively
coupled Nb/Nb2O5 relaxation oscillators. The coupled system
exhibits rich collective behaviour, from weak coupling to
synchronisation, depending on the negative differential
resistance response of the individual devices, the operating
voltage and the coupling capacitance. These coupled oscillators
are shown to exhibit stable frequency and phase locking states at
source voltages as low as 2.2 V with MHz frequency tunable range.
The numerical simulation of the coupled system highlights the
role of source voltage, and circuit and device capacitance in
controlling the coupling modes and dynamics
Hardware Implementation of Differential Oscillatory Neural Networks Using VO 2-Based Oscillators and Memristor-Bridge Circuits
Oscillatory Neural Networks (ONNs) are currently arousing interest in the research community for their potential to implement very fast, ultra-low-power computing tasks by exploiting specific emerging technologies. From the architectural point of view, ONNs are based on the synchronization of oscillatory neurons in cognitive processing, as occurs in the human brain. As emerging technologies, VO2 and memristive devices show promising potential for the efficient implementation of ONNs. Abundant literature is now becoming available pertaining to the study and building of ONNs based on VO2 devices and resistive coupling, such as memristors. One drawback of direct resistive coupling is that physical resistances cannot be negative, but from the architectural and computational perspective this would be a powerful advantage when interconnecting weights in ONNs. Here we solve the problem by proposing a hardware implementation technique based on differential oscillatory neurons for ONNs (DONNs) with VO2-based oscillators and memristor-bridge circuits. Each differential oscillatory neuron is made of a pair of VO2 oscillators operating in anti-phase. This way, the neurons provide a pair of differential output signals in opposite phase. The memristor-bridge circuit is used as an adjustable coupling function that is compatible with differential structures and capable of providing both positive and negative weights. By combining differential oscillatory neurons and memristor-bridge circuits, we propose the hardware implementation of a fully connected differential ONN (DONN) and use it as an associative memory. The standard Hebbian rule is used for training, and the weights are then mapped to the memristor-bridge circuit through a proposed mapping rule. The paper also introduces some functional and hardware specifications to evaluate the design. Evaluation is performed by circuit-level electrical simulations and shows that the retrieval accuracy of the proposed design is comparable to that of classic Hopfield Neural Networks.Unión Europea H2020 grant 871501 “NeurONN,”Spanish Ministry of Economy and Competitivity grant PID2019-105556GB-C31 (NANOMIND) (with support from the European Regional Development Fund)
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