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| 2. |
- Johansson, Anders, 1982, et al.
(författare)
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Comparison of Several Error Metrics for FE Model Updating
- 2007
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Ingår i: Proceedings of the 25th International Modal Analysis Conference (IMAC-XXV), February 19-22, Orlando, Florida, USA. - 0912053968
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The choice of error metric is one of the most crucial choices of model updating. Many articles have been published advocating the use of certain such metrics, as have comparative studies of various methods based on different error metrics. This article describes the differences between a number of error metrics and their corresponding objective functions using many different evaluation methods, such as plots of objective functions, Cramer-Rao lower bounds and condition numbers of the Hessian. This approach has the advantage that it gives the user a way of assessing error metrics in terms of solvability of the optimization problem and identifiability of parameters of the model updating problem.
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| 3. |
- Johansson, Anders, 1982, et al.
(författare)
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Increased Controllability in Component Testing using Structural Modifications
- 2008
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Ingår i: 23rd International Conference on Noise and Vibration Engineering 2008, ISMA 2008; Leuven; Belgium; 15 September 2008 through 17 September 2008. - 9789073802865 ; , s. 999-1007
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Konferensbidrag (refereegranskat)abstract
- In ordinary component testing for durability, the component's in situ mechanical environment is mapped to what can be achieved in dynamic test rigs under controlled conditions in the testing laboratory. In order for such tests to be meaningful, the response history obtained in the laboratory test needs to closely resemble the targeted response history observed in field. Such responses are more often than not strain and acceleration responses. The reconstruction of a target output time history may prove impossible however, if the controllability in the test system setup is too low. In a typical situation, if it is found that the system is not controllable, the rig's excitation setup may be altered. Such alterations include increasing the number of shakers or repositioning of shakers. Unfortunately, the natural approach to modify the excitation setup is not always readily available for dynamic test rigs, as these typically consist of a number of hydraulic actuators of considerable size. However, it is evident that another possibility to increase the controllability is by changing the system eigenvectors, and thus the system itself. We use a state-space representation of the test system, ẋ = Ax+Bu, y = Cx, in which y is the response and u is the drive signals to the test rig. In mathematical terms test system excitation alterations mean that the B-matrix of the state-space equation is modified, and test system alterations mean that the A-matrix, and thus its eigenvectors, are modified. Using a controllability metric for a certain state φi from a certain input uj, i.e. column vector of B, bj, as proposed by Hamdan & Nayfeh, it is easy to see the relevance of such modifications. Their controllability metric is namely the angles between the invariant subspaces of A (normally the eigenvectors) and the columns of B, calculated as: To be able to study system alterations in an efficient way, a parameterized structural modification of the component is proposed, in which a passive structural component - typically a spring-mass system - is added to the component to be tested. In this article, a two-step method for such modifications is introduced. In the first step we aim at finding a system which is at least marginally controllable, whereafter we maximize the controllability of the resulting system with respect to a given parameter set to obtain a set of optimal structural modifications. These are then elected as starting positions when optimizing the passive component parameters with respect to the minimization of input force and output error of the combined system.
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- Johansson, Anders, 1982, et al.
(författare)
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Jämförande studie av felmått för modelluppdatering
- 2007
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Ingår i: Svenska Mekanikdagar, 13-15 juni, Luleå, 2007 (1 page abstract). - 1402-1528. ; , s. 46-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 5. |
- Johansson, Anders, 1982
(författare)
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Vibrational Response Reconstruction and Model Validation
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, laboratory response reconstruction andcomputational model validation is investigated. The former concernsapplications of control theory as the objective is to reproduce acertain loading for a known component, while the latter concernsoptimization of a parameterized model as the objective is to modifya computational model to mimic a certain component's behavior. Thesesibling problems are also often referred to as drive signalidentification for rig testing and model updating, respectively.They are both inverse problems, as in the case of drive signalidentification the objective is to find a suitable input based onknowledge of the system and its output, while the model updatingobjective is to modify a system based on knowledge of its input andoutput. These problems are often further classified as inverseproblems of the first and second kind, respectively. Aspects ofthese two neighbor problems, namely controllability considerationsin the case of response reconstruction and error metricconsiderations in the case of model validation, are treated.
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