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  • 1.
    Bagge, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Nilimaa, Jonny
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Enochsson, Ola
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Emborg, Mats
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Lundmark, Tore
    Ramböll Sverige AB, Luleå.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Protecting a five span prestressed bridge against ground deformations2015Inngår i: IABSE Conference Geneva 2015: Structural Engineering: Providing Solutions to Global Challenges, Geneva: International Association for Bridge and Structural Engineering, 2015, s. 255-262Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    A 55 year-old, 121.5 m long, five span prestressed bridge was situated in the deformation zone close to a mine in Kiruna in northern Sweden. There was a risk for uneven ground deformations so the bridge was analyzed and monitored. Results and measures taken to ascertain the robustness of the bridge are presented.The analysis resulted in an estimate that the bridge could sustain 24 mm in uneven horizontal and 83 mm in uneven vertical displacement of the two supports of a span. To be able to sustain larger deformations, the columns of the bridge were provided with joints, where shims could be inserted to counteract the settlements. To accomplish this, each one of the 18 columns of the bridge was unloaded by help of provisional steel supports. The column was then cut and a new foot was mounted to it. This made it possible to lift each individual column with two jacks, when needed, and to adjust its height by inserting or taking away shim plates.The deformations of the bridge and the surrounding ground were monitored. The eigenmodes of the bridge were studied with accelerometers and by analysis with finite elements (FE) models. Comparison indicated good agreement between the model and the actual bridge, with calculated eigenfrequencies of 2.17, 4.15 and 4.67 Hz, for the first transversal, vertical and torsional modes, respectively. Measurements during winter resulted in higher values due to increased stiffness caused by frozen materials.

  • 2. Bengtsson, Daniel
    et al.
    Deutgren, Petra
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Isaksson, Mikael
    Olsson, Lennart
    Luleå tekniska universitet.
    Sjöberg, Frank
    Luleå tekniska universitet.
    Öhman, Hans
    Zipper performance when mixing ADSL and VDSL in terms of reach and capacity1997Rapport (Annet vitenskapelig)
    Abstract [en]

    In this contribution we have examined how the maximum reach vary at different bit rates on a VDSL system, when different numbers of ADSL and VDSL systems share the same binder. In this context it is concluded that when using the Zipper duplex scheme, VDSL can coexist with ADSL in the same binder without a significant degradation in reach. Further, it is shown that the Zipper duplex scheme secure that ADSL is not disturbed by NEXT from VDSL The Zipper performance has also been compared with both a TDD proposal and a FDD proposal. The results from this comparison shows that Zipper outperforms FDD and TDD in terms of extended reach at all studied mixes of ADSL and VDSL sharing the same binder. For example with a moderate number (5) of ADSL disturbers, the Zipper reach is 150 meters longer than for TDD, and is 350 meters longer than for FDD when studying the medium range asymmetrical bit rate (26:3.2 Mbps).

  • 3.
    Bennitz, Anders
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Schmidt, Jacob W.
    Technical University of Denmark.
    Thick-walled cylinder theory applied on a conical wedge anchorage2011Inngår i: Meccanica (Milano. Print), ISSN 0025-6455, E-ISSN 1572-9648, Vol. 46, nr 5, s. 959-977Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, we derive and examine an analytical model for the internal stresses and strains within the anchorage for a prescribed presetting distance. This model is derived from the theory of thick walled cylinders under the assumptions regarding plane stress and axial symmetry. We simplify the resulting system of ten nonlinear equations and derive a method for solving them numerically. A comparison of plotted results for three different angles on the wedge's outer surface and six different presetting distances follows. These results are also compared to both axi-symmetric and 3D FE (Finite Element) models. Analytical and FE axi-symmetric models show good correspondence, though compared to the 3D FE model, they show a clear difference in the predicted radial stress distribution on the FRP. Thus, the derived analytical model can be a useful and faster alternative to FE modeling of axi-symmetric anchorages. However, the model is of more restricted value and should be complemented by, for example, 3D FE models for other designs.

  • 4.
    Bennitz, Anders
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Schmidt, Jacob W.
    Department of Civil Engineering, Technical University of Denmark.
    Thick-walled cylinder theory applied on a conical wedge anchorage: Research report with Matlab implementation documentation2010Rapport (Annet vitenskapelig)
    Abstract [en]

    Conical wedge anchorages are frequently used to anchor steel tendons in prestressing applications within the construction industry. To replace the steel tendons with non-corrosive and low weight FRPs (Fiber Reinforced Polymers), the different mechanical interactions between the steel and FRPs call for further development of the anchorage.In this paper, we derive and examine an analytical model for the internal stresses and strains within the anchorage for a prescribed presetting distance. This model is derived from the theory of thick walled cylinders under the assumptions regarding plane stress and axial symmetry. We simplify the resulting system of ten nonlinear equations and derive a method for solving them numerically. A comparison of plotted results for three different angles on the wedge's outer surface and six different presetting distances follows.These results are also compared to both axi-symmetric and 3D FE (Finite Element) models. Analytical and FE axi-symmetric models show good correspondence, though compared to the 3D FE model, they show a clear difference in the predicted radial stress distribution on the FRP. Thus, the derived analytical model can be a useful and fasteralternative to FE modeling of axi-symmetric anchorages. However, the model is of more restricted value and should be complemented by, for example, 3D FE models for other designs.

  • 5.
    Blanksvärd, Thomas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Häggström, Jens
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Nilimaa, Jonny
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Täljsten, Björn
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Carolin, Anders
    Paulsson, Björn
    UIC, Trafikverket.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Test to failure of a steel truss bridge: Calibration of assessment methods2014Inngår i: Bridge Maintenance, Safety, Management and Life Extension: proceedings of the Seventh International Conference of Bridge Maintenance, Safety and Management, 7-11 July 2014, Shanghai, China / [ed] Airong Chen; Dan M. Frangopol; Xin Ruan, London: CRC Press, Taylor & Francis Group , 2014, s. 1076-1081Konferansepaper (Fagfellevurdert)
    Abstract [en]

    The steel truss railway bridge at Åby River was built in 1957 with a span of 32 m (105 feet). In 2012 it was replaced by a new steel beam bridge and the old bridge was placed beside the river. It was tested to failure to study its remaining load-carrying capacity in September 2013. The test was carried out by Luleå University of Technology by commission from Trafikverket as a part of the European Research Project MAINLINE (www.mainline-project.eu). In this paper some preliminary results are given. Two hydraulic jacks, anchored by cables to the bedrock, pulled the bridge downwards. The bridge remained elastic up to about three times the original design load and the load could then be almost doubled with substantial yielding deformations before a buckling failure appeared in the top girders for a load of ca. 11 MN (1000 short tons) for a midpoint deflection of ca. 0, 2 m (8 inches). No brittle or fatigue failure in any of the joints appeared and the bridge proved to behave in a ductile way with a substantial hidden capacity.

  • 6. Börjesson, Per Ola
    et al.
    Feichtinger, Hans G
    NUHAG (Numerical Harmonic Analysis Group), Dept. of Mathematics, University of Vienna.
    Grip, Niklas
    Isaksson, Mikael
    Telia Research AB.
    Kaiblinger, Norbert
    NUHAG (Numerical Harmonic Analysis Group), Dept. of Mathematics, University of Vienna.
    Ödling, Per
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    A low-complexity PAR-reduction method for DMT-VDSL1999Inngår i: Proceedings of the 5th International Symposium on Digital Signal Processing for Communication Systems: DSPCS'99, Perth, Australia, Feb. 1999, 1999, s. 164-169Konferansepaper (Fagfellevurdert)
    Abstract [en]

    DMT-VDSL signals have a high peak-to-average power ratio (PAR). In the transmitters, the PAR governs the necessary resolution of the digital-to-analog converter (DAC) and is an important factor for the power consumption of the line-driver. Aiming at implementation in a specific system, we propose a low complexity PAR-reduction method based on the iterative algorithm derived in [5, 20, 24, 25]. We maintain good performance while stressing a straigthforward and low-complex implementation. Key elements of the method are: low latency; no loss in data rate; precalculated and stored peak-cancellation waveform; and bit-shifts (multiplication with powers of two) replacing the scaling of the waveform. Computer simulations show that, for a DMT frame length of 4096 samples and a frame clip rate of 10-4, the PAR can be reduced about 1.5 to 2.0 dB, depending on the number of peaks cancelled. When multiplication is replaced by bit-shifts, the reduction is still 1.5-1.7 dB.

  • 7.
    Dechevsky, Lubomir
    et al.
    Narvik University College, 8505 Narvik, Norway.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Gundersen, Joakim
    Narvik University College, 8505 Narvik, Norway.
    A new generation of wavelet shrinkage: adaptive strategies based on composition of Lorentz-type thresholding and Besov-type non-threshold shrinkage2007Inngår i: Wavelet Applications in Industrial Processing V: 11 - 12 September 2007, Boston, Massachusetts, USA / [ed] Frédéric Truchetet; Olivier Laligant, Bellingham, Wash.: SPIE - International Society for Optical Engineering, 2007, s. 676304-Konferansepaper (Fagfellevurdert)
    Abstract [en]

    This article is a systematic overview of compression, smoothing and denoising techniques based on shrinkage of wavelet coefficients, and proposes (in Sections 5 and 6) an advanced technique for generating enhanced composite wavelet shrinkage strategies.

  • 8.
    Dechevsky, Lubomir T.
    et al.
    Narvik University College, 8505 Narvik, Norway.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Relations between functional norms of a non-negative function and its square root on the positive cone of Besov and Triebel-Lizorkin spaces2009Inngår i: Applications of Mathematics in Engineering and Economics: proceedings of the 35th International Conference, Sozopol, Bulgaria, 7 - 12 June 2009 / [ed] George Venkov; Ralitza Kovacheva; Vesela Pasheva, Melville, NY: American Institute of Physics (AIP), 2009, s. 3-15Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this communication we study in detail the relations between the smoothness of f and √f in the case when the smoothness of the univariate non-negative functions f is measured via Besov and Triebel-Lizorkin space scales. The results obtained can be considered also as embedding theorems for usual Besov and Triebel-Lizorkin spaces and their analogues in Hellinger metric. These results can be used in constrained approximation using wavelets, with applications to probability density estimation in speech recognition, non-negative non-parametric regression-function estimation in positron-emission tomography (PET) imaging, shape/order-preserving and/or one-sided approximation and many others.

  • 9.
    Dechevsky, Lubomir T.
    et al.
    Narvik University College, 8505 Narvik, Norway.
    Grip, Niklas
    Persson, Lars-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sharp error estimates for approximation by wavelet frames in Lebesgue spaces2003Inngår i: Journal of Analysis and Applications, ISSN 0972-5954, Vol. 1, nr 1, s. 11-31Artikkel i tidsskrift (Fagfellevurdert)
  • 10.
    Dechevsky, Lubomir T.
    et al.
    Narvik University College, 8505 Narvik, Norway.
    Gundersen, Joakim
    Narvik University College, 8505 Narvik, Norway.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wavelet compression, data fitting and approximation based on adaptive composition of Lorentz-type thresholding and Besov-type non-threshold shrinkage2010Inngår i: Large-Scale Scientific Computing: 7th International Conference, LSSC 2009, Sozopol, Bulgaria, June 4-8, 2009 / [ed] Ivan Lirkov; Svetozar Margenov; Jerzy Wasniewski, Berlin: Springer Science+Business Media B.V., 2010, s. 738-746Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this study we initiate the investigation of a new advanced technique, proposed in Section 6 of [3], for generating adaptive Besov-Lorentz composite wavelet shrinkage strategies. We discuss some advantages of the Besov-Lorentz approach compared to firm thresholding.

  • 11.
    Duvnjak, Ivan
    et al.
    University of Zagreb, Croatia.
    Damjanović, Domagoj
    University of Zagreb, Croatia.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ohlsson, Ulf
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yongming
    School of Civil Engineering, Southeast University, Nanjing, China.
    Damage Detection in Structures – Examples2019Inngår i: IABSE Symposium 2019: Towards a Resilent Built Environment - Risk and Asset Management, 2019Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Damage assessment of structures includes estimation of location and severity of damage. Quite often it is done by using changes of dynamic properties, such as natural frequencies, mode shapes and damping ratios, determined on undamaged and damaged structures. The basic principle is to use dynamic properties of a structure as indicators of any change of its stiffness and/or mass. In this paper, two new methods for damage detection are presented and compared. The first method is based on comparison of normalised modal shape vectors determined before and after damage. The second method uses so-called 𝑙1-norm regularized finite element model updating. Some important properties of these methods are demonstrated using simulations on a Kirchhoff plate. The pros and cons of the two methods are discussed. Unique aspects of the methods are highlighted.

  • 12.
    Edrees, Tarek
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Nikolakopoulos, George
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jonasson, Jan-Erik
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Investigation of changes in modal characteristics before and after damage of a railway bridge: A case study2015Inngår i: The IES Journal Part A: Civil & Structural Engineering, ISSN 1937-3260, E-ISSN 1937-3279, Vol. 8, nr 2, s. 131-144Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The linear, time-invariant transfer function Txy has been utilized for the construction of FRF, based on the ambient vibration measurements. The results presented here indicated the possibility to identify and localize damages in steel railway bridges from the variations in the modal characteristics of the structure. The comparison between the modal characteristics for the healthy and collapsed bridge confirmed that damage had been existed. The abnormal percentage of change in modal damping, between the healthy and any other condition for a structure, can be regarded as a serious indicator for early stages of damage, while the high percentage of change in modal damping can clearly indicate the existence of damage in that structure. The average ratio of change in the damping ratio from the healthy to the collapsed bridge was about 206 % and this ration could be regarded as an index for the existence of a serious damage in steel bridges, which needs further evaluation in other test cases.

  • 13.
    Elfgren, Lennart
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ohlsson, Ulf
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Signal processing in modal analysis of bridges2009Konferansepaper (Annet vitenskapelig)
  • 14. Ericsson, Stefan
    et al.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    An analysis method for sampling in shift-invariant spaces2005Inngår i: International Journal of Wavelets, Multiresolution and Information Processing, ISSN 0219-6913, E-ISSN 1793-690X, Vol. 3, nr 3, s. 301-319Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A subspace V of L2(ℝ) is called shift-invariant if it is the closed linear span of integer-shifted copies of a single function.As a complement to classical analysis techniques for sampling in such spaces, we propose a method which is based on a simple interpolation estimate of a certain coefficient mapping. Then we use this method to derive both new results and relatively simple proofs of some previously known results. Among these are some results of rather general nature and some more specialized results for B-spline wavelets. The main problem under study is to find a shift x0 and an upper bound δ such that any function f ∈ V can be reconstructed from a sequence of sample values (f(x0 + k + δk))k∈ℤ, either when all δk = 0 or in the irregular sampling case with an upper bound supk|δk| < δ.

  • 15. Ericsson, Stefan
    et al.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Efficient wavelet prefilters with optimal time-shifts2005Inngår i: IEEE Transactions on Signal Processing, ISSN 1053-587X, E-ISSN 1941-0476, Vol. 53, nr 7, s. 2451-2461Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A wavelet prefilter maps sample values of an analyzed signal to the scaling function coefficient input of standard discrete wavelet transform (DWT) algorithms. The prefilter is the inverse of a certain postfilter convolution matrix consisting of integer sample values of a noninteger-shifted wavelet scaling function. For the prefilter and the DWT algorithms to have similar computational complexity, it is often necessary to use a "short enough" approximation of the prefilter. In addition to well-known quadrature formula and identity matrix prefilter approximations, we propose a Neumann series approximation, which is a band matrix truncation of the optimal prefilter, and derive simple formulas for the operator norm approximation error. This error shows a dramatic dependence on how the postfilter noninteger shift is chosen. We explain the meaning of this shift in practical applications, describe how to choose it, and plot optimally shifted prefilter approximation errors for 95 different Daubechies, Symlet, and B-spline wavelets. Whereas the truncated inverse is overall superior, the Neumann filters are by far the easiest ones to compute, and for some short support wavelets, they also give the smallest approximation error. For example, for Daubechies 1-5 wavelets, the simplest Neumann prefilter provide an approximation error reduction corresponding to 100-10 000 times oversampling in a nonprefiltered system.

  • 16.
    Ericsson, Stefan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Grip, Niklas
    Using a natural deconvolution for analysis of perturbed integer sampling in shift-invariant spaces2011Inngår i: Journal of Mathematical Analysis and Applications, ISSN 0022-247X, E-ISSN 1096-0813, Vol. 373, nr 1, s. 271-286Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An important cornerstone of both wavelet and sampling theory is shift-invariant spaces, that is, spaces V spanned by a Riesz basis of integer-translates of a single function. Under some mild differentiability and decay assumptions on the Fourier transform of this function, we show that V also is generated by a function ϕ with Fourier transform equal to the convolution of g with the characteristic function living on the interval [-pi,pi]. We explain why analysis of this particular generating function can be more likely to provide large jitter bounds ε such that any f ∈ V can be reconstructed from perturbed integer samples f(k + ε_k) whenever the supremum of |ε_k| is smaller than ε. We use this natural deconvolution to further develop analysis techniques from a previous paper. Then we demonstrate the resulting analysis method on the class of spaces for which g has compact support and bounded variation (including all spaces generated by Meyer wavelet scaling functions), on some particular choices of ϕ for which we know of no previously published bounds and finally, we use it to improve some previously known bounds for B-spline shift-invariant spaces.

  • 17. Ericsson, Stefan
    et al.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Johansson, Elin
    Luleå tekniska universitet.
    Persson, Lars-Erik
    Sjöberg, Ronny
    Nåiden Teknik AB.
    Strömberg, Jan-Olov
    Department of Mathematics/NADA, Royal Institute of Technology.
    Towards automatic detection of local bearing defects in rotating machines2005Inngår i: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 19, nr 3, s. 509-535Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we derive and compare several different vibration analysis techniques for automatic detection of local defects in bearings. Based on a signal model and a discussion on to what extent a good bearing monitoring method should trust it, we present several analysis tools for bearing condition monitoring and conclude that wavelets are especially well suited for this task. Then we describe a large-scale evaluation of several different automatic bearing monitoring methods using 103 laboratory and industrial environment test signals for which the true condition of the bearing is known from visual inspection. We describe the four best performing methods in detail (two wavelet-based, and two based on envelope and periodisation techniques). In our basic implementation, without using historical data or adapting the methods to (roughly) known machine or signal parameters, the four best methods had 9–13% error rate and are all good candidates for further fine-tuning and optimisation. Especially for the wavelet-based methods, there are several potentially performance improving additions, which we finally summarise into a guiding list of suggestion.

  • 18.
    Forsberg, Thomas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Non-iterative calibration for accelerometers with three non-orthogonal axes and cross-axis interference2012Rapport (Annet vitenskapelig)
  • 19.
    Forsberg, Thomas
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Non-iterative calibration for accelerometers with three non-orthogonal axes, reliable measurement setups and simple supplementary equipment2013Inngår i: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 24, nr 3Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The input-output relationship of an accelerometer depends on parameters that are sensitive to temperature and air humidity. High accuracy field measurements therefore require simple in-field estimation of these parameters.We present an extension of a simple non-iterative six-parameter calibration method for triaxial accelerometers with orthogonal input axes to a nine-parameter method that also handles non-orthogonal axes and cross-axis interference.We derive necessary and sufficient conditions on the accelerometer output that guarantee that the nine parameters can be uniquely determined from the calibration measurements in an idealized scenario with no noise or quantization errors. The method is based on measurements of the Earth gravity with the accelerometer placed at rest in at least nine different orientations.The choice of orientations is important for measurement accuracy. We compare two different setups, one called A090-45, which is based on 90 and 45 degree rotations of the accelerometer and one called A0max sep that has maximized smallest angle between any two of the orientations. For the A090-45 setup we have constructed a simple test equipment for quick positionings of the accelerometer. For the A0max sep setup, a similar equipment is more complicated to construct, but equally simple to use.We have done Monte Carlo simulations with accelerometer orientations deviating at most D degrees from the desired A090-45 or A0max sep and with D ranging from 1◦ to 30◦. For real-world noisy environments and D up to 18◦, our simulations showed slightly smaller errors for the A0max sep than for the A090-45 setup. For noise standard deviation typical for our field measurements, the measurement errors after nine-parameter calibration were about 100 times smaller than those for six-parameter calibration both for the A0max sep setup and, as long as D ≤ 13◦ for the A090-45 setup. For the A090-45 setup, however, we found that combinations of large noise levels and/or large D can makesix-parameter calibration the better choice.

  • 20.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hilbert space frames and bases: a comparison of Gabor and wavelet frames and applications to multicarrier digital communications2000Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    Several signal processing applications today are based on the use of different transforms. The signals under consideration are written as a linear combination (or series) of some predefined set of functions. Traditionally, orthogonal bases have been used for this purpose, for example, in the discrete Fourier transform. The theory for orthogonal bases for Hilbert spaces can, however, be generalized to other sequences of functions, called frames. The first part of this thesis begins with an application-oriented introduction to the theory of frames and bases for separable Hilbert spaces. We explain similarities with and differences from the theory of orthogonal bases. Special attention is given to the relatively new theory of Gabor and Wavelet frames. We explain how they can be used for so-called time-frequency analysis. The main emphasis is on explaining fundamental similarities and differences between Gabor and wavelet frames. We also give an example of an application (OFDM) related to the second part of the thesis, for which nonorthogonal Gabor frames are superior to any orthogonal basis. The second part of this thesis concerns the current development of a standard for very high speed digital communication in ordinary telephone copper wires. It is the result of a cooperation with the Division of Signal Processing and Telia Research. We present a novel duplex method for Very high bitrate Digital Subscriber Lines (VDSL), called Zipper. It is intended to provide bit rates up to 52 Mbit per second, about 1000 times faster than the most common modems today. Zipper is based on Discrete Multi Tone (DMT) modulation. It uses an orthogonal basis of Gabor type for the signal transmission. Certain cyclical extensions are used to ensure the orthogonality between the basis functions. Zipper is proposed as a standard for VDSL to both the American National Standards Institute (ANSI) T1E1.4 group and in the European Telecommunication Standards Institute (ETSI) TM6 group. It will also be presented for the International Telecommunication Union (ITU). Telia Research is currently building a prototype together with ST Microelectronics (former SGS-Thomson), France. The first Zipper-VDSL modems are expected to be available on the mass market in the year 2001. The second part of this thext consists of a brief introduction to Zipper, an ANSI standard contribution and three conference papers. The standard contribution compares Zipper performance with competing standard proposals at that time: TDD and FDD. In the first conference paper we present a new and patented method for reducing the interference that the unshielded copper wires experience from radio transmissions. The two last conference papers present a low complexity method for reducing the so-called Peak to Average power Ratio (PAR) of the transmitted signal. PAR is a measure for the amount of rare but very high peaks in the signal. A reduced PAR allows for using a cheaper digital-to-analog converter and amplifier in the transmitter.

  • 21.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Inte bara broar: vibrationsanalys för tillståndsbedömning2013Rapport (Annet vitenskapelig)
    Abstract [sv]

    Behovet av tillståndsbedömning av byggnader och andra konstruktioner har ökat under senare år. Metoder för tillståndsbedömning kan användas för att avgöra om en konstruktion kan fortsätta att bära den last den dimensionerats för eller om den rentav kan bära en ökad last. Alternativt kan den klargöra vilka förstärkningar som behövs för att förlänga byggnadens livslängd till nytta för både miljö och samhällsekonomi.En resurssnål metod för tillståndsbedömning av en konstruktion bygger på numerisk analys av så kallade fria vibrationer orsakade av naturligt förekommande excitering, som till exempel vågor, vind, människor och trafik. Ett mer resurskrävande alternativ bygger på analys av konstruktionens vibrationssvar på en väl känd konstgjord excitering. Den exciterande kraften måste då mätas, och konstruktionen måste vara isolerad från andra större exciteringskrafter.I båda fallen finns mät- och analystekniker för att dela upp konstruktionens uppmätta vibrationer i en summa av så kallade vibrationsmoder. Sedan kan numeriska optimiseringsmetoder användas för att justera utvalda nyckelparametrar i en finit elementmodell av konstruktionen så att skillnaden minimeras mellan uppmätta svängningsmoder och de som förutspås av finita elementmodellen. Nyckelparametrarna kan till exempel vara randvillkor och/eller elasticitetsmodul i olika delar av konstruktionen. Lokala variationer av dessa kan då indikera en lokal svaghet i konstruktionen.Med huvudfinansiering från SBUF har nya vibrationsmätningar utförts på en betongplatta i laboratoriemiljö och på två broar i fält. I båda fallen har vi nyttjat både friavibrationer och konstgjord excitering. Vibrationsmoder har räknats ut med några olika tekniker och detaljerade finita elementmodeller har gjorts för de olika konstruktionerna. Vi analyserar även vibrationsmoder och jämför mot en finita elementmodell för ett flervåningshus, samt beskriver några faktorer som är speciella för mätning och analys på sådana byggnader.För betongplattan och en av broarna har mätningarna upprepats efter att ha tillfört olika grader av skada. Analystekniker för tillståndsbedömning har implementerats i Matlab. I en första utvärdering på betongplattan så är grunda sprickor svåra att detektera medan ökande djup på sprickorna gör att de kan detekteras på rätt halva av plattan eller mer noggrant. Matlab-koden struktureras för närvarande om för att publiceras i en form lämplig för andra att anpassa till egna liknande tillämpningar. Matlab-koden skall i nästa skede anpassas för att utvärdera tillståndsbedömningen på de omfattande mätningar som genomförts i samband med fullskaleförsök på en 33 m lång stålfackverksbro. Tillståndsbedömningen innefattar mätningar från både intakt bro, bro med delar skadade samt efter att bron belastats till brott.Dessutom har enkel utrustning och enkla analysmetoder utvecklats för att säkerställa snabb och säker kalibrering av accelerometrar i fält. Detta behövs för att få jämförbara mätsignaler från de olika accelerometrarna oavsett ändringar i temperatur och väderförhållanden.

  • 22.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Some Essentials of Data Analysis with Wavelets: Lecture notes in the wavelet part of the course in data analysis at The Swedish National Graduate School of Space Technology, Kiruna and Luleå, November–December 20092009Annet (Annet (populærvitenskap, debatt, mm))
  • 23.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Wavelet and gabor frames and bases: approximation, sampling and applications2002Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis is devoted to both theoretical and practical aspects of applied mathematics. It consists of three main parts: Part I consists of an application-oriented introduction to the theory of frames and bases for separable Hilbert spaces, as well as an introduction to the main tools used in the remaining Chapters: Time-frequency analysis, Gabor frames and wavelet frames. Part II contain five publications in the fields of approximation theory, sampling and perturbation stability. One paper and one research report consider different estimates of the error (measured in L^p, Besov or Triebel-Lizorkin norm) when a function is projected on certain so-called shift-invariant spaces. This is closely connected to a certain class of wavelet subspace sampling problems (containing the classical Shannon sampling theorem as a special case), prefiltering of the discrete wavelet transform and a certain perturbation stability theorem called the Kadec 1/4-theorem. These are the topics of the three remaining papers in Part II. Chapter 3 contains estimates of the L^p-norm error in certain projections of a function f onto a shift-invariant space V_j, spanned by translated copies of some given function. In Chapter 4, we propose a method which is well-suited for studying irregular sampling problems in such spaces. The main advantage of our method is that it provides an intuitive understanding and relatively simple proofs of some studied problems. The method builds on a study of an equivalent coefficient mapping, which we also use in Chapter 5. There we propose an improved low-complexity approximation of an often neglected prefilter which is needed when the discrete wavelet transform is used for analysis of real-world (i.e., non-discrete) signals. Chapter 6 consists of further results and remarks. First we prove some new results related to Chapter 3, but now the error estimates are measured in other function space norms. Then we conclude Part II with a collection of remarks on some recent proofs of some perturbation theory theorems, including the Kadec 1/4-theorem, which is equivalent to the classical Shannon sampling problem and a special case of the problems studied in Chapter 4. Part III contains one selected publication from each the two practical applications which I have been working with during my years as Ph.D. student: VDSL signal transmission and bearing condition monitoring. The first paper is one of four papers which were included and defended in my licentiate thesis. They describe the solutions to some important problems during the development of a duplex method (called Zipper) for very high speed digital communication in ordinary unshielded telephone copper wires. This was done together with Telia Research and the Division of Signal processing and the end-product (VDSL modems for up to 52 Mbit/s) is currently under development by the French-Italian company ST Microelectronics (former SGS-Thomson). The paper included here describes a patented method for reducing the interference that the unshielded copper wires experience from high-power narrowband radio transmissions, such as radio amateurs. The last paper, finally, is an overview article about bearing condition monitoring. Here the main problem is to find a method for predicting bearing failures by analysis of vibration measurements from rotating machines. The paper contains the main results of a co-operation with (among others) Nåiden Teknik and three forestry combines. We argue that time-frequency analysis based methods are well suited for this task and compare such methods with different old and new methods, using a large number of (mainly industrial environment) test signals.

  • 24.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Brämming, Mats
    SSAB Tunnplåt AB.
    Matlab-program för tid-frekvensanalys av vibrations-och ljudmätningar2008Rapport (Annet vitenskapelig)
    Abstract [sv]

    Detta är dokumentation för en Matlab-implementation av ett program som läser in vibrations- och/eller ljudmätningar från exempelvis en LD-ugn samt räknar ut och plottar RMS-data som beskriver hur frekvensinnehållet i mätdata varierar med tiden.

  • 25.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ericsson, Stefan
    Deconvolution based analysis of perturbed integer sampling in shift-invariant spaces2007Konferansepaper (Annet vitenskapelig)
  • 26.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Grip, Carl-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Nilsson, Leif
    SSAB EMEA, Luleå.
    Wavelet study of dynamic variations in steel and ironmaking rest gases: Potential effect on external use2013Inngår i: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 112, s. 1032-1040Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The surplus energy in rest gases is an important by-product in the SSAB EMEA steel plant in Luleå. The surplus is used in a local heat and power plant. Studies have been and are being carried out to find alternative use for the high calorific gases, especially the coke oven gas. The different gas flows typically are mixtures of irregularly distributed transients and non-periodic contents of different duration. Slowly varying fluctuations can probably be parried by good production planning. For shorter wavelengths some kind of safety margin could be appropriate, perhaps in the range 0.75–1.5 kN m3/h. However, the existing buffer capacity in gasholders, etc. is too small to cover the fluctuations. Good production planning can partly compensate low buffer capacity. An example from a period without gas holder is discussed.We describe analysis of the gas flows that can be used both for better production planning and for efficient external use, e.g., for fuel production. Rather than traditional Fourier analysis, we use wavelet analysis, which is better suited for analyzing the irregular and nonperiodic characteristics of the gas flows. We demonstrate such analysis on collected gas flow data from the SSAB EMEA steel plant and give some suggestions for how to make a good choice of wavelet basis for the analyzed signals.

  • 27.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    Jacobs University, Bremen.
    A discrete model for efficient analysis of time-varying narrowband communication channels2008Inngår i: Multidimensional systems and signal processing, ISSN 0923-6082, E-ISSN 1573-0824, Vol. 19, nr 1, s. 3-40Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We derive an efficient numerical algorithm for the analysis of certain classes of Hilbert-Schmidt operators that naturally occur in models of wireless radio and sonar communications channels. We show that many narrowband finite lifelength systems such as wireless radio communications can be well modelled by smooth and compactly supported spreading functions. Further, we exploit this fact to derive a fast algorithm for computing the matrix representation of such operators with respect to well time-frequency localized Gabor bases (such as pulseshaped OFDM bases). Hereby we use a minimum of approximations, simplifications, and assumptions on the channel. Moreover, we use a multivariate setting to allow for applications to, for example, antenna arrays. The derived algorithm and software can be used, for example, for comparing how different system settings and pulse shapes affect the diagonalization properties of an OFDM system acting on a given channel.

  • 28.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    Jacobs University, Bremen.
    Efficient analysis of OFDM channels2011Inngår i: OFDM: Concepts for Future Communication Systems, Heidelberg: Encyclopedia of Global Archaeology/Springer Verlag, 2011, s. 107-112Kapittel i bok, del av antologi (Fagfellevurdert)
  • 29.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    Jacobs University, Bremen.
    Time varying narrowband communications channels: analysis and implementation2007Rapport (Annet vitenskapelig)
    Abstract [en]

    An efficient numerical algorithm for the analysis of wireless radio and sonar communications channels in the framework of Hilbert-Schmidt operators is derived. The underlying model is based on the description of the short-time behavior of these channels as a weighted superposition of time and frequency shift operators, where the weight function is called the spreading function of the channel operator. A description of an implementation of the developed algorithm in MatLab completes this technical report.

  • 30.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    School of Engineering and Science, Jacobs University Bremen.
    Rashkov, Peter
    Fachbereich Mathematik und Informatik, Philipps-Universität Marburg.
    A time-frequency density criterion for operator identification2013Inngår i: Sampling Theory in Signal and Image Processing, ISSN 1530-6429, Vol. 12, nr 1Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We discuss necessary and sufficient density conditions for the identifiability of time–frequency localized operators. In particular, we give a necessary density condition for identifiability based on the density criterion for Gabor frames and Riesz bases in the space of square integrable functions.

  • 31.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    Jacobs University, Bremen.
    Rashkov, Peter
    Jacobs University, Bremen.
    Identification of time-frequency localized operators2010Rapport (Annet vitenskapelig)
    Abstract [en]

    We consider identi cation of operator families de ned via a time-frequency series expansion of the operator spreading function. The identi cation problem is transformed into an in nite-dimensional linear algebra problem. Our aim is to establish a connection between the identi ability of the operator family and a density measure of the time-frequency index set. In this way, the identi cation problem can be compared to the classical density condition for existence of Gabor frames. The conclusion is that the relationship between identi ability of such operator families and the \critical" density is highly intricate because of the presence of additional conditions. Criteria for identi ability are developed for families of time-frequency localized operators de ned via time-frequency series expansions of the spreading function based on the Gaussian function

  • 32.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Pfander, Götz E.
    Jacobs University, Bremen.
    Rashkov, Peter
    Philipps-Universität Marburg.
    Time frequency analysis of operators and operator identification2011Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Analogous to the density criterion for Gabor frames and Riesz bases in the space of square integrable functions, we develop a necessary density condition for time–frequency localized operators in the realm of operator identification. The developed framework can be seen as a generalization of both, the density result for Gabor frames and Riesz bases, and the identifiability theorem for so-called operator Paley–Wiener spaces.

  • 33.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Simple non-iterative calibration for triaxial accelerometers2011Rapport (Annet vitenskapelig)
  • 34.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Simple non-iterative calibration for triaxial accelerometers2011Inngår i: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 22, nr 12Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For high precision measurements, accelerometers need recalibration between different measurement occasions. In this paper we derive a simple calibration method for triaxial accelerometers with orthogonal axes. Just like previously proposed iterative methods, we compute the calibration parameters (biases and gains) from measurements of the Earth gravity for six different unknown orientations of the accelerometer. However, our method is non-iterative, so there are no complicated convergence issues depending on input parameters, round-off errors etc.The main advantages of our method are that only from the accelerometer output voltages it gives a complete knowledge of whether it is possible, with any method, to recover the accelerometer biases and gains from the output voltages, and when this is possible, we have a simple explicit formula for computing them with a smaller number of arithmetic operations than previous iterative approaches. Moreover, we show that such successful recovery is guaranteed if the six calibration measurements deviate with angles smaller than some upper bound from a natural setup with two horizontal axes. We provide an estimate from below of this upper bound that, for instance, allows 5 degree deviations in arbitrary directions for the Colibrys SF3000L accelerometers in our lab.

  • 35.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yongming
    School of Civil Engineering, Southeast University, Nanjing.
    Sensitivity-based model updating for structural damage identification using total variation regularization2017Inngår i: Mechanical systems and signal processing, ISSN 0888-3270, E-ISSN 1096-1216, Vol. 84, nr A, s. 365-383Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Sensitivity-based Finite Element Model Updating (FEMU) is one of the widely accepted techniques used for damage identification in structures. FEMU can be formulated as a numerical optimization problem and solved iteratively making automatic updating of the unknown model parameters by minimizing the difference between measured and analytical structural properties. However, in the presence of noise in the measurements, the updating results are usually prone to errors. This is mathematically described as instability of the damage identification as an inverse problem. One way to resolve this problem is by using regularization. In this paper, we compare a well established interpolation-based regularization method against methods based on the minimization of the total variation of the unknown model parameters. These are new regularization methods for structural damage identification. We investigate how using Huber and pseudo Huber functions in the definition of total variation affects important properties of the methods. For instance, for well-localized damages the results show a clear advantage of the total variation based regularization in terms of the identified location and severity of damage compared with the interpolation-based solution.For a practical test of the proposed method we use a reinforced concrete plate. Measurements and analysis were performed first on an undamaged plate, and then repeated after applying four different degrees of damage.

  • 36.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand. Southeast University, Nanjing.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Vibrationsanalys för tillståndsbedömning av byggkonstruktioner: Tillämpningsexempel : (Main results and summary in Swedish. Detailed results in English Appendices.)2017Rapport (Annet vitenskapelig)
    Abstract [sv]

    I projektet beskrivs och tillämpasnågra metoder för tillståndsbedömningav olika typer av konstruktioner med och utan skador.Tillståndsbedömningen skermed hjälp av vibrationsanalys.Detektering av skador av olika typ och storlekdiskuteras. Randvillkorens betydelse undersöks för att få överensstämmelse mellan förutspådda och uppmätta egenmoder och egenfrekvenser.Med den programvara som utvecklats är det möjligt att detektera sprickor i enkla betongplattor med hjälp av vibrationsmätningar. För mer avancerade konstruktioner finns en potential att vidareutveckla programvaran.Mätningar och analyser presenterasför femkonstruktioner: (1) En betongplatta –Sprickor har kunnat identifieras utifrån vibrationsmätningar och en successivt uppdaterad finit element modell (FEM).(2) En bågbro av betong över Långforsen i Kalix älv–Okända randvillkor har kunnat bestämmasför t ex undergrundens styvhetmedhjälp avuppmätta vibrationer. Detekteringav specifika skadorhar ännu inte kunnat genomföras.(3) En fackverksbro av stålöver Åby älv–Enviss skadedetektering har kunnat genomföras med vibrationsmätningar. Brons sårbarhet för korrosion och lokala mekaniska skador har studerats.(4) En förspänd betongbrotill gruvan i Kiruna–Modeller av bron har arbetats framför obelastatoch brottbelastat tillståndoch jämförts med uppmätta vibrationer med gott resultat.(5) Ett betonghus med nio våningar i Luleå –En FEM-modell av huseti bruksstadiethar tagits fram och resultat från den har jämförts med uppmätta vibrationer med gott resultat.Metoder och resultatpresenteras översiktligti rapportens huvudtext. Detaljerade beskrivningar gesi appendix.I appendix presenterasäven en programvaramed färdiga moduler för de undersökta konstruktionerna. Programmet kan laddas ner och användasfritt

  • 37.
    Grip, Niklas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sun, Wenchang
    Department of Mathematics, Nankai University.
    Remarks on the article "On the stability of wavelet and Gabor frames (Riesz Bases)" by J. Zhang2003Inngår i: Journal of Fourier Analysis and Applications, ISSN 1069-5869, E-ISSN 1531-5851, Vol. 9, nr 1, s. 97-100Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    This article consists of some remarks on stability results for Gabor and wavelet frames from Zhang Jing (J. Fourier Anal. Appl. 5, (1999), 105-125), as well as a correction of a proof of a multivariate version of the Kadec's 1/4-theorem in the same article.

  • 38.
    Huang, Zheng
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Bagge, Niklas
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yong-Ming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Modelling of Damage and its Use in Assessment of a Prestressed Bridge2016Rapport (Annet vitenskapelig)
    Abstract [en]

    A 55 years old and 121.5 m long ve-span prestressed bridge has been subjected toshear failure test in Kiruna, Sweden. This in-situ test is a desirable test to validate andcalibrate the existing nonlinear nite element program for predicting the shear behaviorof reinforced and prestressed concrete structures.Two 3D nite element (FE) models of the Kiruna Bridge are built in commercial soft-ware Abaqus, one using shell-elements and one using a combination of shell and beam ele-ments. Predictions obtained from these two models are well consistent with mode shapesand eigenfrequencies computed from acceleration measurements on the bridge before andafter loading it to failure. Shear-failure test of this bridge performed by Lulea Universityof Technology (LTU) is also simulated using the built-in concrete damage plasticity (CDP)model in Abaqus. The predicted load-displacement curve is in good agreement with themeasurement. Verication of the CDP model is conducted at element and member levelwith two dierent damage parameter evolutions. According to the verication, it indi-cates the damage parameter will aect the predicted shear behavior of reinforced concretestructures and it is not reliable to adopt the CDP model to simulate the shear behaviorof reinforced concrete structures based on the present research.A long term goal is to use use the measured mode shapes, eigenfrequencies and FEmodels for evaluating methods for damage identication. Such methods are important formaintenance of dierent structures, for extending their life span and for better knowledgeof their load carrying capacity. We describe how so-called sparse regularization niteelement method updating (FEMU) methods can be used. We then demonstrate someimportant properties of such methods with simulations on a Kirchho plate. For instance,the simulations suggest that both eigenfrequencies and mode shapes should be used forprecise localization of the damage.

  • 39.
    Huang, Zheng
    et al.
    Southeast University, Nanjing, China.
    Tu, Yong-Ming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Sabourova, Natalia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Bagge, Niklas
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Blanksvärd, Thomas
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Ohlsson, Ulf
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Modelling of Damage and its use in Assessment of a PrestressedConcrete Bridge2016Inngår i: IABSE CONGRESS, STOCKHOLM, 2016: Challenges in Design and Construction of an Innovativeand Sustainable Built Environment / [ed] Lennart Elfgren, Johan Jonsson, Mats Karlsson, Lahja Rydberg-Forssbeck and Britt Sigfrid, CH - 8093 Zürich, Switzerland, 2016, s. 2093-2108Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A five-span prestressed concrete bridge has been subjected to a loading test up to failure in Kiruna, Sweden. The bridge was 55 years old and had a length of 121.5 m. The test has been used to validate and calibrate existing nonlinear finite element programs for predicting the shear behavior of reinforced and prestressed concrete structures. Two 3D finite element (FE) models of the Kiruna Bridge are built in commercial software Abaqus, one using shell-elements and one using a combination of shell and beam elements. Predictions obtained from these two models are well consistent with mode shapes and eigenfrequencies computed from acceleration measurements on the bridge before and after loading it to failure.The shear failure of the bridge is also simulated using the built-in concrete damage plasticity (CDP)model in Abaqus. The predicted load-displacement curve is in good agreement with the measurements. Verification of the CDP model is conducted at element and member level with two different damage parameter evolutions. The verification indicates that the damage parameter will affect the predicted shear behavior. It does not seem to be reliable to adopt the CDP model to simulate the shear behavior in the present research. A long term goal is to use use the measured mode shapes, eigenfrequencies and FE models for evaluating methods for damage identification. Such methods are important for maintenance of different structures, for extending their life span and for better knowledge of their load carrying capacity. The use is described of so-called sparse regularized finite element method updating (FEMU) methods. Some important properties of such methods are demonstrated using simulations on a Kirchhoff plate. For instance, the simulations suggest that both eigenfrequencies and mode shapes should be used for precise localization of the damage.

  • 40.
    Lintzén, Nina
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geoteknologi.
    Almqvist, Andreas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Maskinelement.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Span curve temperature dependence of classic style cross country skis2016Inngår i: Sports Technology, ISSN 1934-6182, E-ISSN 1934-6190Artikkel i tidsskrift (Fagfellevurdert)
  • 41.
    Lundgren, Joakim
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Grip, Carl-Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.
    Ekbom, Tomas
    Grontmij AB, Energy and Power, Stockholm.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hulteberg, Christian
    Nordlight AB, Limhamn.
    Larsson, Mikael
    Nilsson, Leif
    SSAB EMEA, Luleå.
    Increased energy efficiency and carbon dioxide - reduction in steel mills: methanol from steel work off-gases2012Rapport (Annet vitenskapelig)
  • 42. Moguchaya, T. N.
    et al.
    Gundersen, J.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Dechevsky, L.T.
    Bang, B.
    Lakså, A.
    Quak, Ewald
    Tong, B.
    Data fitting by wavelet shrinkage using GM-waves2005Inngår i: Mathematical Methods for Curves and Surfaces: Tromsø 2004, Brentwood, Tenn.: Nashboro Press, Ltd., 2005, s. 263-274Kapittel i bok, del av antologi (Annet vitenskapelig)
  • 43.
    Sabourova, Natalia
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ohlsson, Ulf
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och brand.
    Tu, Yongming
    School of Civil Engineering, Southeast University, Nanjing, China.
    Duvnjak, Ivan
    University of Zagreb, Croatia.
    Damjanović, Domagoj
    University of Zagreb, Croatia.
    Detection of Sparse Damages in Structures2019Inngår i: IABSE Symposium 2019: Towards a Resilent Built Environment - Risk and Asset Management, 2019Konferansepaper (Fagfellevurdert)
    Abstract [en]

    Structural damage is often a spatially sparse phenomenon, i.e. it occurs only in a small part of the structure. This property of damage has not been utilized in the field of structural damage identification until quite recently, when the sparsity-based regularization developed in the compressed sensing found its application in this field.

    In this paper we consider classical sensitivity-based finite element model updating combined with a regularization technique appropriate for the expected type of sparse damage. Traditionally (1) 𝑙2-norm regularization was used to solve the ill-posed inverse problems, such as damage identification. However, using (2) already well established 𝑙1-norm regularization or (3) our proposed 𝑙1-norm total variation regularization and (4) general dictionary-based regularization allows us to find damages with special spatial properties quite precisely using much fewer measurement locations than the number of possibly damaged elements of the structure. The validity of the proposed methods is demonstrated using simulations on a Kirchhoff plate model. The pros and cons of these methods are discussed.

  • 44.
    Sabourova, Natalia
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Puurula, Arto
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Enochsson, Ola
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Tu, Yongming
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Ohlsson, Ulf
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Nilsson, Martin
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Elfgren, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Byggkonstruktion och -produktion.
    Carolin, Anders
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser.
    Thun, Håkan
    The railway concrete arch bridge over Kalix river: dynamic properties and load carrying capacity2012Inngår i: Concrete Structures for Sustainable Community: proceedings of the International FIB Symposium 2012, Stockholm, Sweden, 11 - 14 June 2012 / [ed] Dirch H Bager; Johan Silfwerbrand, Stockholm: Swedish Concrete Association , 2012, s. 609-612Konferansepaper (Fagfellevurdert)
  • 45.
    Sjöberg, Frank
    et al.
    Luleå tekniska universitet.
    Nilsson, Rickard
    Luleå tekniska universitet, Institutionen för system- och rymdteknik, Signaler och system.
    Grip, Niklas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Börjesson, Per Ola
    Luleå tekniska universitet.
    Wilson, Sarah Kate
    Luleå tekniska universitet.
    Ödling, Per
    Luleå tekniska universitet.
    Digital RFI suppression in DMT-based VDSL systems1998Inngår i: ICT '98: International Conference on Telecommunications : bridging east & west through telecommunications : 21-25 June 1998, Porto Carras, Greece / [ed] F-N. Pavlidou, 1998, Vol. 2, s. 189-193Konferansepaper (Fagfellevurdert)
    Abstract [en]

    In this paper we describe a method for supressing Radio Frequency Interference (RFI) in Discrete Multitone (DMT) based Very High bit rate Digital Subscriber Line systems. The method operates in the frequency domain of a DMT-system. It is based on a Taylor-expanded transfer function which maps the disturbance from the RF-signal onto the DMT-carriers. By measuring the RF-disturbance on a few unmodulated DMT-carriers we are able to subtract estimates of the disturbance from every modulated DMT-carrier. The RF-signal is assumed to be an unknown narrowband signal. Simulation results show that this method, applied to an RFI-signal with the same average power as the VDSL-signal, reduces the SNR-loss from about 20 dB to less than 1 dB.

1 - 45 of 45
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