| 21. |
- Andersson, Christian, et al.
(författare)
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Derivative-free Parameter Optimization of Functional Mock-up Units
- 2012
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Ingår i: [Host publication title missing].
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Konferensbidrag (refereegranskat)abstract
- Representing a physical system with a mathematical model requires knowledge not only about the physical laws governing the dynamics but also about the parameter values of the system. The parameters can sometimes be measured or calculated, however some of them are often difficult or impossible to obtain in these ways. Finding accurate parameter values is crucial for the accuracy of the mathematical model. Estimating the parameters using optimization algorithms which attempt to minimize the error between the response from the mathematical model and the physical system is a common approach for improving the accuracy of the model. Optimization algorithms usually requires information about the derivatives which may not always be available or not be appropriate for estimation which forces the use of derivative-free optimization algorithms. In this paper, we present an implementation of derivative-free optimization algorithms for parameter estimation in the JModelica.org platform. The implementation allows the underlying dynamic system to be represented as a Functional Mock-up Unit (FMU), thus enables parameter estimation of models designed in modeling tools following the standardized interface, the Functional Mock-up Interface (FMI), such as Dymola.
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| 22. |
- Andersson, Christian, et al.
(författare)
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Efficient Predictor for Co-Simulation with Multistep Sub-System Solvers
- 2016
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- An industrial model of a dynamic system is usually not just a set of differential equations. External inputs acting on the system are common, such as an external force acting on a body or wind pressing on a car. Update of these inputs needs to be handled by the numerical solver in an efficient way.In dynamical simulation, multistep methods are commonly used. A multistep method uses the solution history in order to predict the future solution. When an input is changed, the history is no longer a good approximation for the future solution which may result in order reductions and simulation failure.In this paper, a modification of the predictor is presented. Modifying the predictor, instead of restarting the method, results in an increased performance of the method. The cost of the modification must be weighed with the cost of restarting the method. Experiments show that the benefit of modifying the predictor outweighs the cost of a restart.
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| 23. |
- Andersson, Christian, et al.
(författare)
-
Import and Export of Functional Mock-up Units in JModelica.org
- 2011
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Ingår i: [Host publication title missing]. - 9789173930963
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Konferensbidrag (refereegranskat)abstract
- Different simulation and modeling tools often use their own definition of how a model is represented and how model data is stored. Complications arise when trying to model parts in one tool and importing the resulting model in another tool or when trying to verify a result by using a different simulation tool. The Functional Mock-up Interface (FMI) is a standard to provide a unified model execution interface. In this paper we present an implementation of the FMI specification in the JModelica.org platform, where support for import and export of FMI compliant models has been added. The JModelica.org FMI import interface is written in Python and offers a complete mapping of the FMI C API. JModelica.org also offers a set of Pythonic convenience methods for interacting with the model in an object-oriented manner. In addition, a connection to the simulation environment Assimulo which is part of JModelica.org is offered to allow for simulation of models following the FMI specification using state of the art numerical integrators. Generation of FMI compliant models from JModelica.org will also be discussed.
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| 24. |
- Andersson, Christian, et al.
(författare)
-
PyFMI: A Python Package for Simulation of Coupled Dynamic Models with the Functional Mock-up Interface
- 2016
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- With the advent of the Functional Mock-up Interface (FMI) standard, exchanging dynamic models between modeling and simulation tools has been greatly simplified. At the core of it, FMI is a standardized and unified model execution interface for dynamic models. FMI has gained widespread adoption among users and numerous commercial and open source tools implement support for the standard. In this article, the Python package PyFMI is introduced. PyFMI supports loading and execution of models compliant with the FMI standard, called Functional Mock-up Units (FMUs). It includes a master algorithm for simulation of coupled FMUs together with connections to both Assimulo, for simulation of single FMUs, and to SciPy, for performing parameter estimation. Accessing models compliant with FMI in Python, which is an open and accessible scripting language, is intended to further spread the standard and also promote and facilitate future development of the standard. This is due to Python being a convenient language for experimentation and prototyping of numerical algorithms. PyFMI is also demonstrated on a number of problems that highlights its viability for solving industrial grade simulation problems with FMUs.
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| 25. |
- Andersson, Joel, et al.
(författare)
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CasADi -- A symbolic package for automatic differentiation and optimal control
- 2012
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Ingår i: Recent Advances in Algorithmic Differentiation. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 9783642300226 ; , s. 297-307
-
Konferensbidrag (refereegranskat)abstract
- We present CasADi, a free, open-source software tool for rapid, yet efficient solution of optimization problems in general and dynamic optimization problems in particular. To the developer of algorithms for numerical optimization and to the advanced user of such algorithms, it offers a level of abstraction which is notably lower, and hence more flexible, than that of algebraic modeling languages such as AMPL or GAMS, but higher than working with a conventional automatic differentiation (AD) tool. CasADi is best described as a minimalistic computer algebra system (CAS) implementing automatic differentiation in eight different flavors. Similar to algebraic modelling languages, it includes high-level interfaces to state-of-the-art numerical codes for nonlinear programming, quadratic programming and integration of differential-algebraic equations. CasADi is implemented in self-contained C++ code and contains full-featured front-ends to Python and Octave for rapid prototyping. In this paper, we show how CasADi can be used for optimal control using either a collocation approach or a shooting approach.
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| 26. |
- Andersson, Joel, et al.
(författare)
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Dynamic optimization with CasADi
- 2012
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Ingår i: IEEE 51st Annual Conference on Decision and Control (CDC), 2012. - 0743-1546. - 9781467320658 ; , s. 681-686
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Konferensbidrag (refereegranskat)abstract
- We demonstrate how CasADi, a recently devel- oped, free, open-source, general purpose software tool for nonlinear optimization, can be used for dynamic optimization in a flexible, interactive and numerically efficient way. CasADi is best described as a minimalistic computer al- gebra system (CAS) implementing automatic differentiation (AD) in eight different flavors. Similar to algebraic modeling languages like AMPL or GAMS, it includes high-level interfaces to state-of-the-art numerical codes for nonlinear program- ming, quadratic programming and integration of differential- algebraic equations. CasADi is implemented in self-contained C++ code and contains full-featured front-ends to Python and Octave for rapid prototyping. In this paper, we discuss CasADi from the perspective of the developer or advanced user of algorithms for dynamic optimization for the first time, leaving out details on the implementation of the tool. We demonstrate how the tool can be used to model highly complex dynamical systems directly or import existing models formulated in the algebraic modeling language AMPL or the physical modeling language Modelica. Given this symbolic representation of the process models, the resulting optimal control problem can be solved using a vari- ety of methods, including transcription methods (collocation), methods with embedded integrators (multiple shooting) as well as indirect methods.
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| 27. |
- Andersson, Joel, et al.
(författare)
-
Integration of CasADi and JModelica.org
- 2011
-
Konferensbidrag (refereegranskat)abstract
- This paper presents the integration of two open source softwares: CasADi, which is a framework for efficient evaluation of expressions and their derivatives, and the Modelica-based platform JModelica.org. The integration of the tools is based on an XML format for exchange of DAE models. The JModelica.org platform supports export of models in this XML format, wheras CasADi supports import of models expressed in this format. Furthermore, we have carried out comparisons with ACADO, which is a multiple shooting package for solving optimal control problems. CasADi, in turn, has been interfaced with ACADO Toolkit, enabling users to define optimal control problems using Modelica and Optimica specifications, and use solve using direct multiple shooting. In addition, a collocation algorithm targeted at solving large-scale DAE constrained dynamic optimization problems has been implemented. This implementation explores CasADi's Python and IPOPT interfaces, which offer a convenient, yet highly efficient environment for development of optimization algorithms. The algorithms are evaluated using industrially relevant benchmark problems.
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| 28. |
- Andersson, Niklas, et al.
(författare)
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Calibration of a polyethylene plant for grade change optimisations
- 2011
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Ingår i: 21ST European Symposium on Computer Aided Process Engineering. - 1570-7946. - 9780444538956 ; 29, s. 673-677
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Konferensbidrag (refereegranskat)abstract
- A polyethylene plant model coded in Modelica and based on a nonlinear MPC model currently used at Borealis AB is considered for calibration. A case study of model calibration at steady-state for four different operating points are analysed, both when looking at one operating point separately, but also to calibrate several simultaneously. Both model parameters and reactor inputs are calibrated for true plant measurement data. To solve the parameter estimation problem, the JModelica.org platform is used, offering tools to express and solve calibration problems. Calibration was obtained with narrow confidence intervals and shows a potential to improve the model accuracy by changing the parameter values. The results will be used for dynamic optimisations of grade changes.
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| 29. |
- Andersson, Niklas, et al.
(författare)
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Parameter Selection in the Parameter Estimation of Grade Transitions in a Polyethylene Plant
- 2015
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Ingår i: Studies in Engineering and Technology. - : Redfame Publishing. - 2330-2038 .- 2330-2046. ; 3:1, s. 1-18
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Tidskriftsartikel (refereegranskat)abstract
- A polyethylene plant at Borealis AB is modelled in the Modelica language and considered for parameter estimations at grade transitions. Parameters have been estimated for both the steady-state and the dynamic case using the JModelica.org platform, which offers tools for steady-state parameter estimation and supports simulation with parameter sensitivies. The model contains 31 candidate parameters, giving a huge amount of possible parameter combinations. The best parameter sets have been chosen using a parameter-selection algorithm that identified parameter sets with poor numerical properties. The parameter-selection algorithm reduces the number of parameter sets that is necessary to explore. The steady-state differs from the dynamic case with respect to parameter selection. Validations of the parameter estimations in the dynamic case show a significant reduction in an objective value used to evaluate the quality of the solution from that of the nominal reference, where the nominal parameter values are used.
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| 30. |
- Anderzén, Johan
(författare)
-
Differences in glycemic control in type 1 diabetes children and adolescents : in a national and international perspective and the effect on microvascular complications in young adults
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on glycemic control measured as HbA1c in type 1 diabetes (T1D) patients during childhood and especially during adolescence, both in a Swedish and an international context, and relates the glycemic control to the risk of complications in young adults. In studies I and II, the Swedish Pediatric Diabetes Quality Register (SWEDIABKIDS) and the Swedish National Diabetes Register (NDR) were used. More than 4000 young adults with T1D and data on HbA1c in NDR both in 2011 and 2012 as well as data on HbA1c in SWEDIABKIDS were used. The T1D patients with poor glycemic control during their teenage period had a risk for retinopathy several times higher than those with good glycemic control. The risk for micro- and macroalbuminuria was also higher in those with poor glycemic control and was most pronounced in the T1D patients with high HbA1c in both registers. Females had worse glycemic control than males during the teenage period and an increased risk of retinopathy as young adults. In studies III and IV, pediatric diabetes quality register data from, respectively, eight and seven Western high-income countries were collected in the year 2013. Data on about 60 000 T1D patients were analyzed according to mean HbA1c levels in the countries and related to actual age and T1D duration to determine if there were differences in glycemic control between the countries. There were large differences in mean HbA1c between the countries, both when related to age and T1D duration. Despite the differences in mean HbA1c, the increase in mean HbA1c with increasing age and T1D duration was very similar in all countries. The overall picture of these studies is that good glycemic control is very important to avoid complications of T1D as young adults, and it seems particularly important to maintain a good glycemic control during adolescence. Furthermore large differences in glycemic control in T1D patients in Western high-income countries were found. Despite the differences in glycemic control, the pattern of rising HbA1c with increasing age and duration of T1D was very similar in all countries. Females have worse glycemic control than males during their teenage period, both in Sweden and internationally, and they also have more retinopathy as young adults. This thesis shows that it is of the utmost importance to treat T1D patients intensively directly after diagnosis, to treat the young T1D patients intensely and to reduce the rise in HbA1c with increasing age and duration of T1D in order to avoid complications early in life. Diabetes quality registers give the opportunity to compare results and share experiences, both within and between countries, so treatment of T1D can be designed in the best possible way and thereby minimize T1D complications.
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