| 1. |
- González, R. A., et al.
(författare)
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Optimal enforcement of causality in non-parametric transfer function estimation
- 2017
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Ingår i: IEEE Control Systems Letters. - : Institute of Electrical and Electronics Engineers Inc.. - 2475-1456. ; 1:2, s. 268-273
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Tidskriftsartikel (refereegranskat)abstract
- Traditionally, non-parametric impulse and frequency response functions are estimated by taking the ratio of power spectral density estimates. However, this approach may often lead to non-causal estimates. In this letter, we derive a closed form expression for the impulse response estimator by smoothed empirical transfer function estimate, which allows optimal enforcement of causality on non-parametric estimators based on spectral analysis. The new method is shown to be asymptotically unbiased and of minimum covariance in a positive semidefinite sense among a broad class of linear estimators. Numerical simulations illustrate the performance of the new estimator.
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| 2. |
- Ramírez, Jaime A., et al.
(författare)
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Aportes a la Teoría y la Implementación del Método LSCR
- 2010
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Ingår i: RIAI - Revista Iberoamericana de Automatica e Informatica Industrial. - 1697-7912. ; 7:3, s. 83-94
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Tidskriftsartikel (refereegranskat)abstract
- The LSCR method (Leave-out-Sign-dominant-Correlation-Regions) provides confidence regions for the parameters of a system by evaluating a set of correlation functions calculated for the available data. To do the approximation for the whole region, the procedure must be repeated for each value of the parameter vector. The main attributes of LSCR are its validity for a finite set of data and the scarce assumptions on the noise. However, the procedure needs much computational effort, which limits its application to very simple cases. In this work some theoretical aspects of the LSCR method are improved and some implementation alternatives are suggested. It is also compared, in terms of computational effort, with Bootstrap, another way to obtain confidence regions.
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| 3. |
- Rojas, Cristian R., et al.
(författare)
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A Critical View on Benchmarks based on Randomly Generated Systems
- 2015
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Ingår i: IFAC-PapersOnLine. - : Elsevier. - 2405-8963. ; 48:28, s. 1471-1476
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Tidskriftsartikel (refereegranskat)abstract
- In data-based modelling communities, such as system identification, machine learning, signal processing and statistics, benchmarks are essential for testing and comparing old and new techniques for the estimation of models. During the last years, it has become customary in system identification to rely on data sets built from randomly generated systems. In this article we discuss the implications of this practice, in particular when using data sets generated with the MATLABr command drss, and advocate the cautious use of comparisons based on these benchmarks.
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| 4. |
- Rojas, Cristian R., 1980-, et al.
(författare)
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Equivalence between Transfer-Matrix and Observed-State Feedback Control
- 2006
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Ingår i: IEE Proceedings - Control Theory and Applications. - : Institution of Engineering and Technology (IET). - 1350-2379 .- 1359-7035. ; 153:2, s. 147-155
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Tidskriftsartikel (refereegranskat)abstract
- An observed-state feedback is built for a given multiple input-multiple Output (MIMO) control loop, where the controller is specified in transfer-matrix form. This contribution solves for the first time, for MIMO systems, the classical problem of finding a feedback gain and an observer gain such that the observed-state feedback control loop has the same sensitivity as that provided by a one-degree-of-freedom classical control loop.
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| 5. |
- Rojas, Ricardo A., et al.
(författare)
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The inverse of sampling revisited
- 2001
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Ingår i: Proceedings of the IASTED Conference on Intelligent Systems and Control (ISC-2001).
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Konferensbidrag (refereegranskat)
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| 6. |
- Abdalmoaty, Mohamed R., 1986-, et al.
(författare)
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Identification of a Class of Nonlinear Dynamical Networks⁎
- 2018
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Ingår i: IFAC-PapersOnLine. - : Elsevier B.V.. - 2405-8963. ; 51:15, s. 868-873
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Tidskriftsartikel (refereegranskat)abstract
- Identification of dynamic networks has attracted considerable interest recently. So far the main focus has been on linear time-invariant networks. Meanwhile, most real-life systems exhibit nonlinear behaviors; consider, for example, two stochastic linear time-invariant systems connected in series, each of which has a nonlinearity at its output. The estimation problem in this case is recognized to be challenging, due to the analytical intractability of both the likelihood function and the optimal one-step ahead predictors of the measured nodes. In this contribution, we introduce a relatively simple prediction error method that may be used for the estimation of nonlinear dynamical networks. The estimator is defined using a deterministic predictor that is nonlinear in the known signals. The estimation problem can be defined using closed-form analytical expressions in several non-trivial cases, and Monte Carlo approximations are not necessarily required. We show, that this is the case for some block-oriented networks with no feedback loops and where all the nonlinear modules are polynomials. Consequently, the proposed method can be applied in situations considered challenging by current approaches. The performance of the estimation method is illustrated on a numerical simulation example.
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| 7. |
- Agüero, Juan C., et al.
(författare)
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Accuracy of linear multiple-input multiple-output (MIMO) models obtained by maximum likelihood estimation
- 2012
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Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 48:4, s. 632-637
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we study the accuracy of linear multiple-input multiple-output (MIMO) models obtained by maximum likelihood estimation. We present a frequency-domain representation for the information matrix for general linear MIMO models. We show that the variance of estimated parametric models for linear MIMO systems satisfies a fundamental integral trade-off. This trade-off is expressed as a multivariable 'water-bed' effect. An extension to spectral estimation is also discussed.
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| 8. |
- Agüero, Juan C., et al.
(författare)
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Fundamental Limitations on the Accuracy of MIMO Linear Models Obtained by PEM for Systems Operating in Open Loop
- 2009
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Ingår i: Proceedings of the Joint 48th IEEE Conference on Decision and Control (CDC’09) and 28th Chinese Control Conference (CCC’09). - 9781424438716 ; , s. 482-487
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Konferensbidrag (refereegranskat)abstract
- In this paper we show that the variance of estimated parametric models for open loopMultiple-Input Multiple-Output (MIMO) systems obtained by the prediction error method (PEM) satisfies a fundamental integral limitation. The fundamental limitation gives rise to a multivariable 'water-bed' effect.
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| 9. |
- Ansell, Brendan R. E., et al.
(författare)
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Time-Dependent Transcriptional Changes in Axenic Giardia duodenalis Trophozoites
- 2015
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Ingår i: PLoS Neglected Tropical Diseases. - : Public Library of Science (PLoS). - 1935-2727 .- 1935-2735. ; 9:12
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Tidskriftsartikel (refereegranskat)abstract
- Giardia duodenalis is the most common gastrointestinal protozoan parasite of humans and a significant contributor to the global burden of both diarrheal disease and post-infectious chronic disorders. Although G. duodenalis can be cultured axenically, significant gaps exist in our understanding of the molecular biology and metabolism of this pathogen. The present study employed RNA sequencing to characterize the mRNA transcriptome of G. duodenalis trophozoites in axenic culture, at log (48 h of growth), stationary (60 h), and declining (96 h) growth phases. Using similar to 400-times coverage of the transcriptome, we identified 754 differentially transcribed genes (DTGs), mainly representing two large DTG groups: 438 that were down-regulated in the declining phase relative to log and stationary phases, and 281 that were up-regulated. Differential transcription of prominent antioxidant and glycolytic enzymes implicated oxygen tension as a key factor influencing the transcriptional program of axenic trophozoites. Systematic bioinformatic characterization of numerous DTGs encoding hypothetical proteins of unknown function was achieved using structural homology searching. This powerful approach greatly informed the differential transcription analysis and revealed putative novel antioxidant-coding genes, and the presence of a nearcomplete two-component-like signaling system that may link cytosolic redox or metabolite sensing to the observed transcriptional changes. Motif searching applied to promoter regions of the two large DTG groups identified different putative transcription factor-binding motifs that may underpin global transcriptional regulation. This study provides new insights into the drivers and potential mediators of transcriptional variation in axenic G. duodenalis and provides context for static transcriptional studies.
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| 10. |
- Bjurgert, Johan, et al.
(författare)
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On Adaptive Boosting for System Identification
- 2018
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Ingår i: IEEE Transactions on Neural Networks and Learning Systems. - : Institute of Electrical and Electronics Engineers (IEEE). - 2162-237X .- 2162-2388. ; 29:9, s. 4510-4514
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Tidskriftsartikel (refereegranskat)abstract
- In the field of machine learning, the algorithm Adaptive Boosting has been successfully applied to a wide range of regression and classification problems. However, to the best of the authors' knowledge, the use of this algorithm to estimate dynamical systems has not been exploited. In this brief, we explore the connection between Adaptive Boosting and system identification, and give examples of an identification method that makes use of this connection. We prove that the resulting estimate converges to the true underlying system for an output-error model structure under reasonable assumptions in the large sample limit and derive a bound of the model mismatch for the noise-free case.
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