70 research outputs found
A Circular Approach To Multi-Class Change Detection In Multitemporal Sentinel-1 Sar Image Time Series
No full textThis paper presents a multitemporal technique for multi-class Change Detection (CD) between pairs of images of a satellite image time series. Changes between different pair of images within a time series must be consistent with each other since images acquired over the same scene are causally related with one another. The temporal consistency of the pixel status can be used to formulate a principle that constrains the CD results within the series to be mutually consistent. This principle coincides with the conservative property of the change variable and it allows the unsupervised validation of changes detected between arbitrary image pairs. Thus, all images in the series, rather than a single couple, are used in the pair-wise CD. The proposed technique was applied to a dataset of dual-polarized terrain-corrected SAR images acquired by Sentinel-1. Experimental results show the validity of the proposed multitemporal approach in improving the CD results
Coupled measurement-simulation procedure for very high power fast recovery – Soft behavior diode design and testing
No full textReliability requirements for very high power devices are growing for their importance in industrial drives and renewable energy; testing those devices in operating condition is more and more difficult.
A coupled measurement-simulation based design procedure is presented and applied to high power PiN diodes for application with fast IGBTs or IGCTs, in which high di/dt’s can result in too high current or voltage or energy peaks during turn-off, limiting the reliability of the circuit.
Appropriately tuned simulation of the semiconductor device embedded in the test circuit allows to overcome measurement capability limits and to properly design the diode itself and a specific test circuit
The virtual element method on polygonal pixel–based tessellations
No full textWe analyze and validate the virtual element method combined with a boundary correction similar to the one in [1,2], to solve problems on two dimensional domains with curved boundaries approximated by polygonal domains. We focus on the case of approximating domains obtained
as the union of squared elements out of a uniform structured mesh, such as the one that naturally arises when the domain is issued from an image. We show, both theoretically and numerically, that resorting to polygonal elements allows the assumptions required for stability to be satisfied for any
polynomial order. This allows us to fully exploit the potential of higher order methods. Efficiency is ensured by a novel static condensation strategy acting on the edges of the decomposition
The virtual element method for a minimal surface problem
No full textIn this paper we consider the Virtual Element discretization of a minimal surface problem, a quasi-linear elliptic partial differential equation modeling the problem of minimizing the area of a surface subject to a prescribed boundary condition. We derive an optimal error estimate and present several numerical tests assessing the validity of the theoretical results
Wavelets and convolution quadrature for the efficient solution of a 2D space-time BIE for the wave equation
No full textWe consider a wave propagation problem in 2D, reformulated in terms of a Boundary Integral Equation (BIE) in the space-time domain. For its solution, we propose a numerical scheme based on a convolution quadrature formula by Lubich for the discretization in
time, and on a Galerkin method in space. It is known that the main advantage of Lubich’s
formulas is the use of the FFT algorithm to retrieve discrete time integral operators with a
computational complexity of order R log R, R being twice the total number of time steps
performed. Since the discretization in space leads in general to a quadratic complexity, the
global computational complexity is of order M2R log R and the working storage required
is M2R/2, where M is the number of grid points on the domain boundary.
To reduce the complexity in space, we consider here approximant functions of wavelet
type. By virtue of the properties of wavelet bases, the discrete integral operators have a
rapid decay to zero with respect to time, and the overwhelming majority of the associated
matrix entries assume negligible values. Based on an a priori estimate of the decaying
behaviour in time of the matrix entries, we devise a time downsampling strategy that allows to compute only those elements which are significant with respect to a prescribed
tolerance. Such an approach allows to retrieve the temporal history of each entry e (corresponding to a fixed couple of wavelet basis functions), via a Fast Fourier Transform, with
computational complexity of order Re log Re. The parameter Re depends on the two basis
functions, and it satisfies Re R for a relevant percentage of matrix entries, percentage
which increases significantly as time and/or space discretization are refined. Globally, the
numerical tests show that the computational complexity and memory storage of the overall procedure are linear in space and time for small velocities of the wave propagation, and
even sub-linear for high velocities
Feedback Control Simulation of PowerElectronic Converters for Renewable Energies
No full textdoi: 10.1109/ICCEP.2009.521202
SiO2 gel-glasses prepared from alkoxide, colloidal and amine-silicate solutions, a comparison
No full text
Raman spectra of SiO2 gel glasses prepared from alkoxide, colloidal and amine silicate solutions
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