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- Andersson, Torbjörn
(författare)
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Concepts and Algorithms for Non-Linear System Identifiability
- 1994
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recently, commutative algebra and differential algebra have come to use as mathematical tools for solving problems in automatic control. We will use these tools to answer questions regarding identifiability for models given as a set of differential polynomials. A constructive algorithm, Ritt's algorithm, has been modified for this specific situation. Furthermore, comparisons between Ritt's algorithm and Buchberger's algorithm, to answer the identifiability question when the model structures are given in state space form, are performed. The basic problem is that the computational complexity rapidly increases with the problem size. We examine various ways to simplify the computations in this respect, but it must also be stressed that the complexity increase is inherent in the problem.In identification from a deterministic point of view an algorithm is said to be robustly convergent if the true system is regained when the noise level tends to zero. In this thesis we introduce a concept close to this performance measure; robust global identifiability. A model structure, i.e., a smoothly parameterized set of models, is said to be robustly globally identifiable if there exist an identification algorithm such that the true parameters are regained when the noise level tends to zero. In this thesis we show that global identifiability implies robust global identifiability when the considered model structure is a characteristic set of differential polynomials. This means that any model structure with parameters, that can be uniquely estimated from data has this robustness property.Finally, a method for estirnation of residence time in continuous flow systems with varying dynamics is discussed. By resampling, i.e., choosing time instants different from the given sampling instants, and interpolation between measured data points, we obtain a continuous fl.ow system with constant residence time expressed in a new resampled time vector. We assume that the fiow patterns in the systems are invariant. The new data set is then used for identification of parameters in a chosen model structure. From the identified model, the residence time is readily calculated and a procedure for that is briefly described. The presented method is readily extended to enable use in recursive identification. In that case the modified recursive identification method is an improvement of the tracking ability compared to an ordinary recursive routine.
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- Andersson, Torbjörn, et al.
(författare)
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Effects of Interferon on Tumor Tissue Content in Liver Metastases of Human Carcinoid Tumors
- 1990
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Ingår i: Cancer Research. - 0008-5472 .- 1538-7445. ; :50, s. 3413-3415
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Tidskriftsartikel (refereegranskat)abstract
- In 21 patients ultrasound-guided cutting biopsies, from carcinoid metastases of the liver, were taken before and after therapy with α-interferon. Each biopsy was examined under light microscopy and the amount of tumor tissue and connective tissue was quantified and then correlated to objective response to interferon therapy. A significant reduction of the amount of tumor tissue, in spite of unaltered metastatic size and a corresponding increase in connective tissue, was seen after interferon therapy. A more pronounced reduction of tumor tissue occurred after long-term interferon therapy. A positive correlation between objective therapy response and tumor tissue reduction was also present. Patients responding poorly, or not at all, to therapy did not show any significant decrease in tumor tissue. Since treatment with immune response modifiers is expected to increase in the near future, it is important to choose the right investigations for therapy monitoring, and since all patients in this investigation had unchanged tumor size on repeated radiological examinations, it is obvious that microscopic examination of core biopsies is a better method for evaluating effects of long-term therapy than tumor size measurement with radiological techniques. Further, the results may indicate that interferon exerts a cytotoxic effect on carcinoid tumor cells in vivo.
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- Andersson, Torbjörn, et al.
(författare)
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Estimation of Residence Time in Continuous Flow Systems with Dynamics
- 1994
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Ingår i: Proceedings of the 10th IFAC Symposium on System Identification. - Linköping : Linköping University. ; , s. 401-406
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Konferensbidrag (refereegranskat)abstract
- A method for estimation of residence time in continuous flow systems with varying dynamics is presented. By resampling, i.e., choosing time instants different from the given sampling instants, and interpolation between measured data points, we obtain a continuous flow system with constant residence time expressed in the new resampled time vector. We assume the flow patterns in the systems are invariant. The new data set is then used for identification of parameters in a chosen model structure. From the identified model, the residence time is readily calculated and a procedure for that is briefly described. The presented method is readily extended to enable use in recursive identification. In that case, however, as an improvement of tracking ability of an ordinary recursive routine.
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- Andersson, Torbjörn, et al.
(författare)
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Estimation of Residence Time in Continuous Flow Systems with Varying Flow and Volume
- 1993
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A method for estimation of residence time in continuous flow vessels with variable flow and volume is presented. By resampling, i.e., choosing time instants different from the given sampling instants, and interpolation of measured data points, we obtain a continuous flow system with constant residence time expressed in the new resampled time vector, assuming the flow patterns in the vessels and tanks are invariant. The new data set is then used for identification of parameters in a chosen model structure. From the identified model the residence time is easily calculated and a procedure for that is briefly described. The presented method is easily extended to enable use in recursive identification but then as an improvement of tracking ability of an ordinary recursive routine.
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- Andersson, Torbjörn, et al.
(författare)
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Identification Aspects of Inter-Sample Input Behavior
- 1994
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Ingår i: Proceedings of the 10th IFAC Symposium on System Identification. - Linköping : Linköping University. - 9780080422251 ; , s. 137-142
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Konferensbidrag (refereegranskat)abstract
- In this contribution aspects of inter-sample input signal behavior are examined. The starting point is that parametric identification always is performed on basis of discrete-time data. This is valid for identification of discrete-time models as well as continuous-time models. The usual assumptions on the input signal are; i) it is band-limited, ii) it is piecewise constant or iii) it is piecewise linear. One point made in this paper is that if a discrete-time model is used, the best possible (in the model structure) adjustment to data is made. This is independent of the assumption on the input signal. However, a transformation of the obtained discrete model to a continuous one is not possible without additional assumptions on the input signal. The other point made is that the frequency functions of the discrete models very well coincides with the frequency functions of the discretized continuous time models and the continuous time transfer function fitted in the frequency domain.
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- Andersson, Torbjörn, et al.
(författare)
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Identifying Models using Piecewise Linear Approximation of Input Signals
- 1992
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Very often in system identification problems it is assumed that the input signal is piecewise constant but in many practical cases this is not the fact. In such cases when the input signal is continuous it shows that a piecewise linear approximation of the input signal leads to a better model. In this report it is shown how to handle system identification problems using state space descriptions and the assumption of piecewise linear input signals with MathWork's system identification software.
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- Ljung, Lennart, 1946-, et al.
(författare)
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Identifiability Implies Robust Identifiability
- 1993
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Ingår i: Proceedings of the 32nd IEEE Conference on Decision and Control. - Linköping : Linköping University. - 0780312988 ; , s. 567-569 vol.1
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Konferensbidrag (refereegranskat)abstract
- In identification from a deterministic point of view an algorithm is said to be robustly convergent if the true system is regained when the noise level tends to zero. In this paper we introduce a concept close to this performance measure: robust global identifiability. A model structure, i.e. a smoothly parametrized set of models, is said to be robustly globally identifiable if there exist an identification algorithm such that the true parameters are regained when the noise level tends to zero. We show that global identifiability implies robust global identifiability when the model structure in consideration is a characteristic set of differential polynomials.
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- von Schantz, Torbjörn, et al.
(författare)
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Genetics of long-spurred pheasants - Reply
- 1990
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 343:6259, s. 600-600
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Tidskriftsartikel (refereegranskat)abstract
- Abstract is not available
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