| 21. |
- Lundbäck, Jonas, et al.
(författare)
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A Digital Directional Coupler with Applications to Partial Discharge Measurements
- 2008
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Ingår i: IEEE Transactions on Instrumentation and Measurements. - 0018-9456. ; 57:11, s. 2561-2567
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a digital directional coupler (DDC) that separates forward- and backward-traveling waves on a transmission line. Based on two independent wideband measurements of voltage and current and on frequency-domain digital wave splitting using a fast Fourier transform (FFT), the DDC is a versatile device for direction separation. A practical procedure is described for the calibration of the digital processor with respect to the particular transmission line and the voltage and current sensors that are being used. As an experiment, a DDC was designed and implemented using low-cost wideband sensors and was installed with medium-voltage equipment in a power distribution station. Partial discharge (PD) measurements were conducted on cross-linked polyethylene (XLPE)-insulated power cables to illustrate the directional separation capabilities of the DDC.
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| 22. |
- Lundbäck, Jonas, et al.
(författare)
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A Generic Electromagnetic Model for DOA Estimation using Arbitrary Multiport Antennas
- 2006
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Ingår i: IEEE International Symposium on Antennas and Propagation. ; , s. 2549-2552
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Konferensbidrag (refereegranskat)abstract
- A mathematical framework is introduced for optimization of antenna near-field imaging problems, based on the multipole expansion of the electromagnetic field, the Fisher information to quantify the quality of data and use of modern interior point convex optimization techniques. We consider the general problem of optimizing the measurement sensor allocation for parameter estimation in distributed systems, and in particular the problem of optimizing the measurement set-up for antenna near-field estimation. As an application example for antenna near-field imaging, we consider a relevant measurement set-up using cylindrical probing coordinates. The convex optimization problem is examined using duality theory, and it is shown that several structural properties of the optimal measurement problem can be exploited in developing an efficient interior point optimization method. In particular, we show that the cylindrical measurement set-up yields a Fisher information matrix with block diagonal structure, a feature which can be directly exploited in the optimization algorithm by reducing the number of dual decision variables.
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| 23. |
- Lundbäck, Jonas, et al.
(författare)
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A Generic Electromagnetic Model for DOA Estimation Using Arbitrary Multiport Antennas
- 2006
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Ingår i: IEEE Antennas and Propagation Society International Symposium 2006. - 1424401232 ; , s. 2549-2552
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Konferensbidrag (refereegranskat)abstract
- In this paper, a generic electromagnetic model is utilized for arbitrary multiport antennas e.g., vector sensors applied in DOA estimation, which was exploited for the statistical analysis of DOA and polarization estimation. By utilizing the spherical vector wave expansion of the electromagnetic field to completely characterize the multiport antennas a general framework for modelling an antenna configuration based on measurements or computations including e.g., mutual coupling is obtained. The array manifold can easily be expanded over a dense spatial grid to enable high-resolution DOA estimation without demanding further measurements of the antenna characteristics.
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| 24. |
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| 25. |
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| 26. |
- Nilsson, Börje, et al.
(författare)
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A method for under bark detection of the wood grain angle radial dependence
- 2007
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Ingår i: Wood Material Science and Engineering. - : Taylor & Francis. - 1748-0272. ; 2:3,4, s. 118-129
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Tidskriftsartikel (refereegranskat)abstract
- Twist in wood, being closely related to spiral grain, may cause serious problems in building structures, furniture, and joinery. It is therefore of great interest to sort out, at an early stage in the manufacturing process, trees, logs and boards that have an access of spiral grain. The spiral grain pattern is described by a helical deviation of the fibre direction in relation to the longitudinal direction of a living tree or a log and seems to be an indicator for other defects such as compression wood. Remote microwave sensing of spiral grain has received a large interest during the latest two decades. Its development has been impeded by the large variation with moisture content of the microwave properties of wood and by the complexity in modelling the electromagnetic field in a log with spiral grain. A review is presented of a direct method with no requirement of information on moisture content for boards. This procedure has recently been generalized to cylindrical logs and trees having a constant slope of the grain. A further generalization is presented here to allow for the normal spiral grain pattern with radially changing slope of grain in wood under bark. Based on this theory, a measurement procedure is proposed for the detection of wood grain angle with radial dependence, requiring no information on moisture content in the sapwood, also applicable for completely or partially frozen wood. A suitable application would be an instrument to use in the forest for measurements on living trees or logs.
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| 27. |
- Nilsson, Börje, et al.
(författare)
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Estimation of Twist in Uniaxial Cylinders WithInverse Electromagnetic Scattering
- 2009
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Ingår i: IEEE Transactions on Antennas and Propagation. - 0018-926X .- 1558-2221. ; 57:10, s. 3264-3273
-
Tidskriftsartikel (refereegranskat)abstract
- For the purpose of determining the twist of a homogeneous, locally reacting, uniaxial cylinder, an inverse microwave scattering theory is presented. Remote measurements of the spiral grain of trees and logs are the prime application. Based on practical considerations, it is assumed that the transmitting and receiving antennas are collocated, requiring a three-dimensional modelling. A general theory is first developed, followed by an asymptotic analysis assuming that the distance from the antennas to the cylinder is many wavelengths and many cylinder radii. In this way, a substantial reduction of the numerical complexity, to the level of the two-dimensional case, is achieved. The error of the determined twist angle as function of inherent parameters of the problem using a CramEacuter-Rao analysis is given. The results from numerical simulations show that this error is low enough for determining the grain angle. Presented parameter studies of the error can be used for minimizing the errors in a measurement set up, of particular interest for non-sophisticated instruments and non-ideal laboratory conditions, by selecting optimum parameters such as frequency and antenna gain. Finally, it is stated that the model has a great potential for developing efficient algorithms for measuring the twist angle.
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| 28. |
- Nilsson, Börje, et al.
(författare)
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Inverse scattering for determination of twist in uniaxial cylinders
- 2007
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Ingår i: EMB 07 — Proceedings. - : Lunds universitet. ; , s. 177-184
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- An inverse microwave scattering theory is presented for the determination of twist of a homogeneous locally reacting uniaxial cylinder. The prime application is remote measurements of spiral grain of trees and logs. From practical considerations, the transmitting and receiving antennas are near each other requiring a three-dimensional modelling. First a general theory is developed. Then asymptotic formulae are derived assuming that the distance from the antennas to the cylinder is many wavelengths and many cylinder radii granting a substantial reduction of the numerical complexity compared to the twodimensional case. Finally, the error of the determined twist angle as function of inherent parameters of the problem is given using a Cramer Rao analysis.
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| 29. |
- Nordebo, Sven, 1963-, et al.
(författare)
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A priori modeling for gradient based inverse scattering algorithms
- 2009
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Ingår i: Progress in Electromagnetics Research B. - 1937-6472. ; 16, s. 407-432
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a Fisher information based Bayesian approach to analysis and design of the regularization and preconditioning parameters used with gradient based inverse scattering algorithms. In particular, a one-dimensional inverse problem is considered where the permittivity and conductivity profiles are unknown and the input data consist of the scattered field over a certain bandwidth. A priori parameter modeling is considered with linear, exponential and arctangential parameter scalings and robust preconditioners are obtained by choosing the related scaling parameters based on a Fisher information analysis of the known background. The Bayesian approach and a principal parameter (singular value) analysis of the stochastic Cramer-Rao bound provide a natural interpretation of the regularization that is necessary to achieve stable inversion, as well as an indicator to predict the feasibility of achieving successful reconstruction in a given problem set-up. In particular, the Tikhonov regularization scheme is put into a Bayesian estimation framework. A time-domain least-squares inversion algorithm is employed which is based on a quasi-Newton algorithm together with an FDTD-electromagnetic solver. Numerical examples are included to illustrate and verify the analysis.
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| 30. |
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