L.A.L. de Almeida

Modeling and performance of vanadium–oxide transition edge microbolometers

The performance of a VO2 thin-film microbolometer has been investigated. The device is operated within 35°C<T<60°C, in the hysteretic metal-insulator transition region. An algebraic hysteresis model has been used to model the resistance-temperature characteristic of the sensor. It accurately describes the resistance versus temperature characteristics of the material. Employing this model, and in conjunction with established bolometer theory, the responsivity of a VO2 film is calculated and compared with experimental data. Superior performance of the device is achievable under conditions of single pulse incident radiation where the operating point remains on the major hysteresis loop. This results in a pronounced responsivity peak within the center of the metal-insulator transition. Continuous periodic excitation, in contrast, leads to a steadily decreasing and much lower sensitivity at higher temperature, due to the formation of minor hysteresis loops and the loop accommodation process.

Improving the signal data acquisition in condition monitoring of electrical machines

The occurrence of a broken bar in induction machines causes an internal magnetic imbalance, which is reflected in the stator current. This effect may be detected by estimating the spectral signature of the stator current, particularly the sidebands around the fundamental frequency. However, the fundamental 60-Hz amplitude is considerably greater than the sideband amplitude, which usually requires an analog 60-Hz notch filter in the data acquisition system. Analog filters are sensitive to temperature variations, which, in this case, may shift the filter resonance frequency and degrade the desired response. It is proposed in this paper that the analog notch filter may be replaced by a digital/analog cancelling technique based on the recursive discrete-time Fourier transform. The technique is shown to attenuate the fundamental 60-Hz amplitude considerably before A/D conversion, leaving only the sidebands in the current signal.

Thermal dynamics of VO2 films within the metal–insulator transition: Evidence for chaos near percolation threshold

The thermal dynamics of thin vanadium dioxide films at the martensitic metal–insulator phase transition has been evaluated experimentally by thermal excitation spectroscopy. Over the transition region, the device becomes highly nonlinear, and its bolometric performance is affected. At low thermal cycling rates for a temperature around the percolation threshold, the device stochastically switches into an unusual pattern. The originally smooth and monotonic shape of the R(T) curve for minor loops suddenly becomes unstable and unpredictable. By direct observation of at least two strange attractors, the phenomenon clearly has been identified as chaotic. Bolometric performance of VO2 based devices in the transition region may suffer strong degradation for low thermal cycling rates. In this region, sensor responsivity for periodic thermal excitation is significantly reduced. Resistance noise is 1/f-type and self-generated oscillations were observed at frequency <10−2 Hz.

A hysteresis model for a vanadium dioxide transition-edge microbolometer

This paper presents the adaptation of the Preisach model, originally developed for magnetic hysteresis, to describe mathematically the hysteresis in the resistance-temperature characteristics of vanadium-dioxide (VO/sub 2/) thin film radiation sensors. The necessary and sufficient conditions for the applicability of the Preisach model to a VO/sub 2/ film sensor are experimentally verified. Experimentally measured characteristics are compared with those given by the model for minor and major loops.

Limiting loop proximity hysteresis model

This paper introduces a simple algebraic limiting loop proximity ((L/sup 2/P)) model to describe magnetic hysteresis. With only four parameters, it has low computational cost and reduced mathematical complexity, thus permitting a fast numerical implementation and simple parameter estimation procedure. The paper presents simulation results and discusses the model performance in terms of its capacity to represent common nonlinearities of the magnetic hysteresis phenomenon. The model avoids the heavy computational burden that results when magnetic hysteresis is included in computer-aided tools for electromagnetic devices and circuit analysis - for example, when hysteresis is combined with the finite-element method. The model also avoids the need to use repeated numerical approximation steps to adjust traditional models to experimental data.

Stability conditions, nonlinear dynamics, and thermal runaway in microbolometers

The nonlinear dynamic behavior of microbolometers, operating at room temperature (300 K) under conditions of positive electrothermal feedback is investigated. An improved device model, based on the heat balance equation is developed. It takes into account the temperature dependence of the thermophysical parameters, such as thermal coupling coefficient between the sensor and its surroundings, and sensor heat capacity and its thermal resistance coefficient. Operational considerations for thermoresistive microbolometer with positive and negative temperature coefficient of resistance are discussed for both, constant current and constant voltage modes of operation. Analytical expressions are derived for predicting stable and unstable operation. Safety factors L0, establishing the biasing conditions for stable device operation are proposed for the positive temperature coefficient of resistance and negative temperature coefficient of resistance type sensors. Limits for fast catastrophic destruction are provided,...

Covariance management based RLS algorithm for phasor estimation in severely noisy systems

This paper presents a new algorithm, derived from the recursive least squares (RLS), to estimate power system voltage and current harmonics for digital relay purposes. The algorithm is based on the RLS covariance matrix management, by making use of the covariance resetting in a different way. The approach provides a kind of attractor that absorbs all nonmodeled dynamics. In order to demonstrate its effectiveness, the proposed algorithm is applied to simulated signals and its performance is compared with other conventional methods.

Vibration Monitoring of On-Load Tap Changers Using a Genetic Algorithm

On load tap changers (OLTC) are widely used for voltage regulation in electricity networks. Noninvasive vibration methods for condition monitoring of internal electrical contacts have been recently proposed in the literature. The vibration signals emitted during the tap changes are usually recorded and post-processed using spectral analysis and some pattern classifier technique. To reduce the complexity of the classifier, a new technique based on genetic algorithm is proposed in this paper. A description of the data acquisition system and the corresponding collected experimental data are presented. The proposed technique is detailed and preliminary experimental results are discussed

Self-Organized Classification of On-load Tap Changers Acoustic Signatures

On-load Tap Changers (OLTC) are widely used for voltage regulation in electricity networks. A set of electrical contacts (taps) is used to change the transformer input/output ratio as the input voltage varies. Non-invasive acoustic methods for condition monitoring of internal electrical contacts have been recently proposed in the literature. A limitation of these analysis is that there is no information on gradual degradation of the equipment condition. In this work a classifier system using Self-organizing Maps (SOM) is developed in order to identify different levels of electrical contacts wear. The method proposed here was able to separate the available signatures into four distinct operation conditions: post-maintenance (no-wear out), low, intermediate and severe wear out.

A New Approach for Condition Assessment of On-Load Tap-Changers using DiscreteWavelet Transform

On-load tap changers (OLTCs) are electromechanical devices used in the voltage regulation of the electrical networks. A non-invasive monitoring method was developed using discrete wavelet transform (DWT) for condition assessment of on-load tap-changers using vibration analysis signals. The transient vibration signals emitted during a tap change are recorded and processed using spectral analysis and standards recognition techniques. The monitoring systems which detect failure during the OLTC operation are quite complexes. In order to reduce this complexity, in this work we demonstrated a new approach based on DWT analysis. The proposed technique is presented and the experimental results as well as its simulation are discussed