| 1. |
- Axmon, Joakim, et al.
(författare)
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A signal model adapted ESPRIT algorithm for joint estimation of spatial and temporal parameters in vibrational analysis of cylinders
- 2002
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Ingår i: 2002 IEEE Sensor Array and Multichannel Signal Processing Workshop Proceedings (Cat. No.02EX593). - 0780375513 ; , s. 360-364
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Konferensbidrag (refereegranskat)abstract
- This paper deals with the joint estimation of spatial mode shapes and temporal frequencies from transient vibrations measured by a uniform circular array of sensors encircling a cross-section of a cylinder. The geometry allows each 2D mode to be interpreted as two real planar waves impinging from mirrored directions with respect to the broadside of a uniform linear array. Algorithms for joint estimation tend, in the presence of noise, to produce signal roots that express planar waves impinging from only approximately mirrored directions. A recently published 2-D ESPRIT algorithm is modified to take advantage of the geometry, and to force the estimated planar waves to impinge from exactly mirrored directions. This reduces the estimation errors; the main advantage is more easily interpreted results. Memory requirements and complexity are significantly lower for the modified algorithm, since the decomposition of a large data matrix may be broken down into several small ones
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| 2. |
- Axmon, Joakim, et al.
(författare)
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Estimation of circumferential mode parameters of living trees
- 1999
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Ingår i: Signal and image processing : proceedings of the IASTED international conferences : October 18 - 21, 1999, Nassau, The Bahamas / SIP '99 - October 18 - 21, 1999, Nassau, The Bahamas / SIP '99. - 0889862672
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Konferensbidrag (refereegranskat)
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| 3. |
- Axmon, Joakim, et al.
(författare)
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Experimental study on the possibility of detecting internal decay in standing Picea abies by blind impact response analysis
- 2004
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Ingår i: Forestry. - : Oxford University Press (OUP). - 1464-3626 .- 0015-752X. ; 77:3, s. 179-192
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers detection of internal decay in standing trees of species Picea abies (L.) Karst. The novel approach is based on two-dimensional spatiotemporal modal analysis of a cross-section which is excited by the hand-made impact of a hammer. An array of accelerometers is distributed around the cross-section, and the resulting impact response is analysed. The temporal frequency for a special spatial mode-shape is used for comparisons on a tree-to-tree basis. The mechanical properties of wood are inherently variable as they are for most materials of biological origin. This leads to a scatter of the analysed parameters that hinders detection of decay based on the temporal frequencies alone. Using regression analysis, we show that by incorporating the additional information on a surface wave propagation velocity, the scatter of sound trees is significantly reduced. The performance of a detector rule which incorporates the frequency and the surface wave propagation velocity is investigated and found to be better than performance reported for visual tree examination. The analyses are based on the impact responses from 94 standing trees, with 66 sound and 28 in various stages of decay. The proposed technique is yet to be considered an experimental tool. Further research, e.g. on how the mechanical properties are influenced by various environmental factors, is needed before the technique can be applied operationally.
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| 4. |
- Axmon, Joakim, et al.
(författare)
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Partial Forward-Backward Averaging for Enhanced Frequency Estimation of Real X-texture Modes
- 2005
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Ingår i: IEEE Transactions on Signal Processing. - 1053-587X. ; 53:7, s. 2550-2562
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, enhancement of the signal root estimation of a particular kind of real-valued two-dimensional (2-D) sinusoidal modes is considered. To its constitution, each mode corresponds to the superposition of two real-valued plane waves in a particular symmetry. The concept of partial forward-backward averaging, which is applicable for modes that are undamped in at least one dimension, is introduced as a means for improving the signal subspace estimate from which the signal roots are estimated. The consequences of real-valued signals for the signal root estimates are discussed in detail, and it is shown that by applying partial forward-backward averaging, the mean square errors of the estimates, and the breakdown threshold signal-to-noise ratio (SNR), are significantly reduced, compared with forward-only or conventional forward-backward (when applicable) usage of the sampled signals. The practical implication is highlighted by applying the proposed technique to modal analysis of multichannel impact responses from a tree trunk.
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| 5. |
- Axmon, Joakim, et al.
(författare)
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Partial modal analysis for health assessment of living trees
- 2001
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Ingår i: Asia-Pacific conference on non-destructive testing (APCNDT).
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Konferensbidrag (refereegranskat)abstract
- Rot in living trees cause substantial losses for the forestry industry. The common practice when evaluating forest stands for, e.g., purchase, is assessment based on visual signs. In this paper a new non-destructive assessment method based on the impact excitation method is proposed. The trunk of a living tree is excited by the impact of a hammer, and the vibrations are measured by accelerometers. Resonance frequencies, circumferential mode shapes and propagation velocity of a surface wave are analysed. A function describing the expected frequency for a sound tree is derived, and used in a detector whose performance is evaluated for 93 trees of species Norway spruce. The partial mode shape is used to ensure that the corresponding resonance frequencies are compared to each other. It is found that the detector is successful and outperforms assessments by skilled experts in forestry.
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| 6. |
- Axmon, Joakim, et al.
(författare)
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Transients in circular symmetry; estimation of transverse wave propagation velocity
- 2000
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Ingår i: Proceedings of the 2000 IEEE Sensor Array and Multichannel Signal Processing Workshop. SAM 2000. - 0780363396 ; , s. 212-216
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Konferensbidrag (refereegranskat)abstract
- The response of a tree trunk to a hammer blow, measured by sensors positioned around the excited cross section, consists of two kinds of transient mechanical waves: longitudinal P- and transverse S-waves. The aim of this paper is to extract the velocity of the S-waves, using a filtering structure that emphasizes the waves heading in one of the directions along the circumference. By this approach, a more robust estimate of the delay between the channels, compared to, e.g., using the pure discrete Radon transform, is obtained. The method is applied to recordings of a large number of trees, and estimated velocities are reported.
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| 7. |
- Axmon, Joakim
(författare)
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Two-Dimensional Signal Root Estimation in Blind Multichannel Impact Response Analysis
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This doctoral dissertation concerns two-dimensional frequency estimation for modal analysis of single-input multiple-output impact responses from circularly cylindrical objects. The application is within detection of internal defects in trunks of spruce. The material consists of impact responses from 94 trees, with 66 sound and 28 in various stages of decay. Paper I proposes that modal analysis of trunks of spruce may unveil the presence of internal defects, which are hypothesised to result in a reduction of the modal frequencies. The responses, collected using a uniform linear array of accelerometers encircling a cross-section, contain two-dimensional spatiotemporal modes which are sinusoidal and temporally damped. The spatial mode-shapes are used for ensuring that comparisons of modal frequencies on a tree-to-tree basis are performed on corresponding modes. The frequency analysis is based on cascading of a polynomial-based estimator for the temporal dimension and DFTs for the spatial dimension. The results show that the modal frequencies often are lower for trees in decay than for sound trees, but also, that the groups partially overlap, and that the results for sound trees with matching dimensions are scattered. Paper II is based on the results in Paper I, and addresses a method for reduction of scatter within the group of sound trees, using one additional parameter that can be estimated from the impact responses. Furthermore, a detector is proposed, and its performance is found to be better than performance reported elsewhere for visual tree examination. Paper III presents a novel technique, called partial forward-backward averaging, that enhances two-dimensional state-space/subspace rotation-based frequency estimation for the kind of modes encountered in the current application. Conventional estimators do not account for all symmetries inherent in this particular kind of modes, and consequently, the quality of the resulting estimates is poor. The novel technique, model-based enhancement of signal subspace estimation, results in estimates with better properties and lower bias and variance. Paper IV contains a full re-analysis of, and a comparison with, the material reported on in Paper I. A fast implementation of a state-space/subspace rotation-based algorithm, detailed in Paper III, is derived and used for the analysis. It has been hypothesised that the modal frequencies change when a tree is subjected to impact excitation repeatedly, since the impacts are applied onto a screw which inevitably is forced deeper and deeper into the trunk. No such effect is found, so this hypothesis is rejected.
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| 8. |
- Dierschke, Katrin, et al.
(författare)
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Acute respiratory effects and biomarkers of inflammation due to welding-derived nanoparticle aggregates
- 2017
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Ingår i: International Archives of Occupational and Environmental Health. - : Springer Science and Business Media LLC. - 0340-0131 .- 1432-1246. ; 90:5, s. 451-463
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Tidskriftsartikel (refereegranskat)abstract
- Welders are exposed to airborne particles from the welding environment and often develop symptoms work-related from the airways. A large fraction of the particles from welding are in the nano-size range. In this study we investigate if the welders' airways are affected by exposure to particles derived from gas metal arc welding in mild steel in levels corresponding to a normal welding day. In an exposure chamber, 11 welders with and 10 welders without work-related symptoms from the lower airways and 11 non-welders without symptoms, were exposed to welding fumes (1 mg/m(3)) and to filtered air, respectively, in a double-blind manner. Symptoms from eyes and upper and lower airways and lung function were registered. Blood and nasal lavage (NL) were sampled before, immediately after and the morning after exposure for analysis of markers of oxidative stress. Exhaled breath condensate (EBC) for analysis of leukotriene B4 (LT-B4) was sampled before, during and immediately after exposure. No adverse effects of welding exposure were found regarding symptoms and lung function. However, EBC LT-B4 decreased significantly in all participants after welding exposure compared to filtered air. NL IL-6 increased immediately after exposure in the two non-symptomatic groups and blood neutrophils tended to increase in the symptomatic welder group. The morning after, neutrophils and serum IL-8 had decreased in all three groups after welding exposure. Remarkably, the symptomatic welder group had a tenfold higher level of EBC LT-B4 compared to the two groups without symptoms. Despite no clinical adverse effects at welding, changes in inflammatory markers may indicate subclinical effects even at exposure below the present Swedish threshold limit (8 h TWA respirable dust).
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