| 1. |
- Nilsson, Lena, 1950-
(författare)
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Studies on skin blood flow and evaporative water loss : a bioengineering approach
- 1986
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Skin blood flow (SBF), temperature, evaporative water loss (EWL) and skin conductance (SC) measured during stimulus-response experiments reveal valuable information regarding the skin's regulatory capacity. SBF and EWL play powerful roles in keeping the human body within an adequate temperature range. EWL and SC reflect the thermal and mental state of the subject. SBF, EWL and SC were measured using noninvasive laser Doppler flowmetry, vapour pressure gradient estimation and a DC-voltage method, respectively.Short acoustic stimuli evoked responses in both EWL and SC at the palm; their response amplitudes were strongly correlated (r = 0.87). Both latency and time to the peak of EWL responses were delayed by 1.1 sec and 2.3 sec, respectively, in comparison to SC responses.EWL measurements at the forearm revealed rhythmical variations in the recorded signal. The period was approximately 0.74 sec at rest. At a higher evaporation rate, induced by exercise, a longer period (0.84 sec) was detected. The period was independent of the sex of the subject and ambient temperature, and remained unchanged for repeated measurements on the same subject.In the skin temperature range 12 to 38°C, the relationship between SBF and local temperature, at the thigh, palm and fingertip, was best fitted to exponential equations. For a narrower temperature interval (23 to 36°C), palmar SBF and temperature followed linear equations within subjects. However, a large variability in regression coefficients was revealed between subjects. When SBF values were normalized to room climate level, they correlated strongly to skin temperature values (r=0.88).Radiative cooling (17°C) of the palm from room climate conditions doubled local heat loss and left SBF virtually unchanged. When cold convective air currents (19°C, 0.5 or 1.0 m/s) were applied, SBF decreased to 60 and 53% while heat loss dropped to 68 and 70%. Rewarming after local cooling revealed discrepancies'in the restoration of both SBF and skin temperature between the measurement sites.
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| 2. |
- Salerud, Göran, 1954-
(författare)
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Laser doppler tissue flowmetry : fiberoptic methods in microvascular research
- 1986
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Laser Doppler flowmetry (LDF) constitutes a method for measuring the flux of blood cells in the microcirculatory bed. Photons are scattered in the moving blood cells, where they undergo a frequency shift according to the Doppler principle. Light is brought to the tissue under study by one optical fiber and transmitted back to photodetectors by two separate fibers. LDF is a particulary useful method to study blood flow patterns and vascular regulatory mechanisms, since it is non-invasive and assesses blood flow in a very small volume of tissue (1 mm3).Blood flow recordings from skin areas, such as forearm and forehead, have revealed large spatial and temporal variations in microvascular blood flow. If the flowmeter probe is moved as little as 2.5 mm, the spatial heterogeneity of the vasculature result in a significant (p<0.001) difference in flowmeter output signal.Spontaneous rhythmical blood flow patterns (vasomotion) with different amplitudes, appeared in all of the 8 subjects studied. Some subjects had a continuous vasomotion pattern, while others showed only "bursts" of the pattern. Recorded blood flow in two juxtaposed skin sites sometimes demonstrated simultaneous variations in both sites, while in other instances the rhythmical flow patterns were out of phase or of different frequencies. To overcome the problem with a large spatial variation in skin blood flow in relation to the geometrical dimension of the probe, a multifiber probe was developed. It was designed to integrate the blood flow over an area enlarged approximately seven times compared to the standard probe. Measurements with this probe reduced the spatial differences as theoretically expected. No averaging effect was found, however, on the temporal variations.In skin, the capillary bed is located superficially, while in other tissues, such as the intestine, the inner wall (mucosa) is the most perfused. In an experiment on cat small intestine, the blood flow was measured both from the mucosal and serosal side. The results showed that it was possible to record the total blood flow of the intestinal wall, irrespective of whether the probe was placed on the mucosal or serosal side of the bowel wall.For tissues like muscle, liver and brain it may be of interest to assess the deep tissue perfusion. The LDF standard probes are, however, too large and blunt to be inserted into the tissue, without disturbing the flow. Therefore a single fiber LDF was developed, with oneoptical diber (Ø =0.5 mm) guiding the light to and from the tissue under study. In a flow model resembling tissue perfusion, the usefulness of the single fiber LDF based on the differential technique was evaluated. When a mathematical model was used to evaluate the single versus the differential channel operation, the differential technique was found most powerful if the laser broadband noise has a substantial rms-value or if many coherence areas are detected. The dynamic responses of the single fiber LDF were studied in a pig experimental model under different physiological conditions. The results agreed well with known reference blood flow patterns
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| 3. |
- Stål, Olle, 1952-
(författare)
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Static and flow cytometry for tumor DNA analysis
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The analysis of cellular DNA content in human tumors has shown to be of prognostic importance. The techniques used involve measurements in slide preparations by absorption cytophotometry and static cytofluorometry as well as the analysis of cell suspensions by flow cytometry. The aim of this work has been to improve these techniques by the development of computer aided systems in order to facilitate the use in a larger scale for clinical purposes. Another aim has been to study the prognostic significance of DNA analysis in breast cancer.Software for determination of DNA content and nuclear area by scanning absorption cytophotometry was developed. The system, HISTOSCAN, is insensitive to light scattering and may therefore be used in tissue sections. Thus, measurements in morphologically well-defined areas may be performed, but the method is less suitable for extensive use due to the slow procedure of the mechanical scanning.More rapid analyses are achieved by the system developed for static cytofluorometry in cytocentrifuged specimens. Cells to be measured are not positioned in the ordinary way, but are just passed through the excitation light beam as the specimen is visually scanned. DNA content and nuclear size are estimated simultaneously from the fluorescence recorded. For estimation of proliferative activity a sufficient number of cells may be analysed within a reasonable time.Similar results were obtained by static cytofluorometry and flow cytometry in a series of primary breast cancers. A close correlation was found for DNA index and, if 200 cells or more were measured, the same was true for S-phase fraction.Tumors from 472 women with primary breast cancer were analysed by flow cytometry. DNA ploidy showed significant association with disease recurrence and mortality but did not show a prognostic value in addition to that of traditional factors. The prognostic significance of S-phase fraction was independent of nodal status, tumor size and estrogen receptor content concerning early relapse and mortality. The survival of 116 women with recurrent breast cancer was correlated with both DNA ploidy and S-phase fraction in a multivariate analysis including nodal status, tumor size, ER content and site of recurrence.
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| 4. |
- Åhlfeldt, Hans, 1955-
(författare)
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Computer-based modeling and simulation in the analysis of cardiac arrhythmias and cardiac pacing
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most systems for ECG analysis deal with QRS classification and arrhythmia detection but do not address the problem of finding the underlying mechanism responsible for the manifest arrhythmia. Although it is possible to distinguish among different arrhythmogenic mechanisms on the cellular level, it is far more difficult in the clinic based on ECG recordings. A computer model which is well suited for rhythm studies has been developed. The heart is modeled as a network of finite elements in which the impulse propagation is described mathematically, as well as several arrhythmogenic mechanisms. These include modulated parasystole, macro and micro reentry and different kinds of block. Since modulated parasystole provides a unified explanation of a variety of different arrhythmias, a stepwise procedure is presented by which this mechanism can be detected. Modulated parasystole can be described mathematically with a phase response curve, and the model has proven valuable for deduction of biand triphasic phase response curves from clinical cases with frequent ventricular premature complexes.Cardiac pacing is a very important therapy for rhythm disorders and since modern pacemakers interact with the heart in a complex way, the problem of cardiac pacing and pacemaker follow-up has also been studied. An ambulatory recording system has been developed, including a pacemaker spike detector and a computer program which can detect possible events of pacemaker malfunction. The computer model has been used to analyse the interaction between the heart and different types of pacemakers. The model has also been integrated with a hypertext system, allowing the system to be used for computer-aided education of cardiac pacing and cardiac arrhythmias.
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| 5. |
- Bigün, Josef, 1961-
(författare)
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Local symmetry features in image processing
- 1988
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The extraction of features is necessary for all aspects of image processing and analysis such as classification, segmentation, enhancement and coding. In the course of developing models to describe images, a need arises for description of more complex structures than lines. This need does not reject the importance of line structures but indicates the need to complement and utilize them in a more systematic way.In this thesis, some new methods for extraction of local symmetry features as well as experimental results and applications are presented. The local images are expanded in terms of orthogonal functions with iso-value curves being harmonic functions. Circular, linear, hyperbolic and parabolic structures are studied in particular and some two-step algorithms involving only convolutions are given for detection purposes. Confidence measures with a reliability verified by both theoretical and experimental studies, are proposed. The method is extended to symmetric patterns fulfilling certain general conditions. It is shown that in the general case the resulting algorithms are implementable through the same computing schemes used for detection of linear structures except for a use of different filters.Multidimensional linear symmetry is studied and an application problem in 3-D or in particular, optical flow, and the solution proposed by this general framework is presented. The solution results in a closed form algorithm consisting of two steps, in which spatio-temporal gradient and Gaussian filtering are performed. The result consists of an optical flow estimate minimizing the linear symmetry criterion and a confidence measure based on the minimum error. The frequency band sensitivity of the obtained results is found to be possible to control. Experimental results are presented.
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| 6. |
- Driankov, Dimiter
(författare)
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Towards a many-valued logic of quantified belief
- 1988
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We consider a logic which "truth-values" are represented as quantified belief/disbelief pairs, thus integrating reports on how strongly the truth of a proposition is believed, and how strongly it is disbelieved. In this context a major motive for the logic proposed is, that it should not lead (as in classical logic) to irrelevant conclusions when contradictory beliefs are encountered. The logical machinery is built around the notion of the so-called logical klattice: a particular partial order on belief/disbelief pairs and fuzzy set-theoretic operators representing meet and join. A set of principles (semantically valid and complete) to be used in making inferences is proposed, and it is shown that they are a many-valued variant of the tautological entailments of relevance logic.To treat non truth-functional aspects of knowledge we introduce also the notion of the information lattice together with particular meet and join operators. These are used to provide answers to three fundamental questions: how to represent knowledge about belief/disbelief in the constituents of a formula when supplied with belief/disbelief about the formula as a whole; how to determine the amount of belief/disbelief to be assigned to formulas in an epistemic state (or a state of knowledge), that is, a collection of partial interpretations, and finally, how to change the present belief/disbelief in the truth of formulas, when provided with an input bringing in new belief/disbelief in the truth of these formulas. The answer to all these questions is given by defining a formula as a mapping from one epistemic state to a new state. Such a mapping is constructed as the minimum mutilation of the given epistemic state which makes a formula to be believed true (or false) in the new one. The entailment between formulas is also given the meaning of an input and its properties are studied.We study also if- then inference rules that are not pure tautological entailments, but rather express the causal relationship between the beliefs held with respect to the truth and falsity of the antecedent and the conclusion. Detachment operators are proposed to be used in cases when: (i) it is firmly believed that belief/disbelief in the validity of the conclusion follows from belief and/or disbelief in the validity of the antecedent, and (ii) it is believed, but only to a degree, that belief/disbelief in the validity of the conclusion follows from belief/disbelief in the validity of the antecedent. It is shown that the following four modes of inference are legitimated within the setting of these rules: modus ponens, modus tollens, denial, and confirmation.We consider also inference rules augmented with the so-called exeption condition: if IA/ then /BI unless IC/. The if- then part of the rule expresses the major relationship between A and B, i.e., it is believed (up to a degree) that belief and/or disbelief in Bfollows from belief and/or disbelief in A. Then the unless part acts as a switch that transforms the belief/disbelief pair of B from one expressing belief in its validity to oneindicating disbelief in the validity of B, whenever there is a meaningful enough belief in the exception condition C.We also give a meaning to the inference rules proposed as mappings from epistemic states to epistemic states, thus using them as a tool for changing already existing beliefs as well as for deriving new ones.
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| 7. |
- Gunnarsson, Svante
(författare)
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Frequency Domain Aspects of Modeling and Control in Adaptive Systems
- 1988
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis various aspects of modeling and control in adaptive systems are presented from a frequency domain viewpoint.The thesis consists of three parts, where the first part contains a general introduction and background information concerning the problems that will be treated. In the second part some recursive identification algorithms are studied with respect to their ability to track time-varying systems and their disturbance sensitivity. Simple and illustrative frequency domain expressions that describe these properties are derived using asymptotic methods. The algorithms that are treated are the constant gain gradient (LMS) algorithm, the recursive least squares algorithm with constant forgetting factor and the Kalman filter respectively. The behavior of these methods when applied to FIR and ARX systems are studied. In the third part of the thesis adaptive control based on low order models is studied. The adaptive control algorithm that is investigated is the recursive least squares algorithm combined with pole placement regulator design. Starting from frequency domain expressions, that describe how a low order model obtained by system identification approximates a higher order system, the consequences for adaptive control are investigated. It is shown how a proper use of filtering in the identification part of the adaptive regulator can improve the robustness properties of the adaptive regulator with respect to unmodelled dynamics.
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| 8. |
- Isaksson, Alf
(författare)
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On System Identification in one and two Dimensions with Signal Processing Applications
- 1988
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of four parts, with system identification as the common theme. The first part studies the asymptotic properties of two-dimensional identification methods. In the second part an approach to identification of time varying systems is presented. Part three applies system identification to the problem of transmission line protection. Finally part four deals with input estimation in speech coding.Part I is devoted to system identification in two dimensions. First we study the asymptotic properties of the estimates as the number of data tends to infinity. The main objective is to investigate what happens if the model order also tends to infinity. The focus is on frequency expressions of the extimation variance. The analysis covers both the least squares method for causal models, and the maximum likelihood method for noncausal models.In Part II we study one approach to identification of time varying sytems. The parameter variations are modelled as process noise in a state space model, and identified using adaptive Kalman filtering. A method for adaptive Kalman filtering is derived and analysed. The simulations indicate that this new approach is superior to previous methods based on adjusting the forgetting factor. The improvement is however gained at the price of a significant increase in computational complexity.Part III describes the use of recursive identification in protective relaying. The Fourier coefficients of voltage and current are estimated using recursive least squares identification. The estimates are then used to detect short circuits. The method is evaluated using data generated by the standard program EMTP.In Part IV a method for inverse glottal filtering is presented. The basis of the method is to use a parameterized model of the input signal, i.e. the glottal pulses. The algorithm simultaneously estimates the parameters of the input signal and the parameters of the system transfer function, the vocal tract model. The presentation is restricted to transfer functions of all-pole type.
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| 9. |
- Jönsson, Ulf
(författare)
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Immobilization and Interaction of Biomolecules at the Solid-Liquid Interface : Applications to Solid-Phase Analytical Techniques
- 1986
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The development of solid-phase-analytical techniques based on surface-concentration-measuring devices requires a stable and reproducible immobilization of biomolecules on well-characterized solid surfaces. This study reports on the immobilization of Immunoglobulin G, Protein A and Fibronectin on chemically modified silica surfaces. Reactive groups were introduced to the silica surfaces by chemical-vapour deposition of silane. The silanized surfaces were characterized by ellipsometry, contact angle measurements and scanning electron microscopy, which revealed smooth, stable and reproducible silane films of monolayer character. Immobilization by adsorption as well as covalent immobilization by the use of thiol-disulphide exchange reactions, thereby controlling the maximum number of covalent bonds to the surface, were studied. The immobilization was quantitated in a study applying both ellipsometry and radiotracer techniques. The amounts of immobilized proteins obtained from the radiolabeling experiments agreed well with those calculated from ellipsometric data. The results outline the possibilities of obtaining a controlled covalent binding of biomolecules to solid surfaces with an optimal stability and biological activity of the immobilized molecules.
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| 10. |
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| 11. |
- Nordström, Kjell
(författare)
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Uncertainty, Robustness and Sensitivity Reduction in the Design of Single Input Control Systems
- 1987
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Feedback control of systems modelled as single input, time invariant, linear and continuous time systems is the subject of this thesis. A growing interest in the design of feedback control systems that can cope with model uncertainties has been the motivation for the study. Some commonly used design approches for various kinds of systems are investigated with respect to robustness properties.The Smith controller, often propsed to controlling open loop stable plants with an input delay, state feedback control based on linear quadratic optimization techniques and output feedback control using a Kalman filter are considered.Also some new design strategies are given. A graphical design philosphy suitable for open loop stable plants with no modelled input delay, feedback stabilization of systems with a known number of unstable poles and modification of the Kalman filter based controller are proposed. The objective in common for these control strategies is to obtain control systems with good robustness properties.
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| 12. |
- Philipson, Lennart, 1952-
(författare)
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The electromyographic signal used for control of upper extremity prostheses and for quantification of motor blockade during epidural anaesthesia
- 1987
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A contribution of biomedical research engineering has been the development of detection and analysis methods for the electromyographic (EMG) signal and its application to health care problems. The present thesis describes the use of EMG signals for two such applications: Aid in the control of multifunctional upper limb prostheses and for quantification of motor blockade during epidural anaesthesia. A myoelectric control system operated by synergies from the EMG signals generated by two muscles is presented in papers I, II, and III. Results from paper I suggest that microprocessor based control systems for upper extremity prostheses could emulate all previous controllers based on discrete components. The study presented in paper II includes a graphic presentation of the EMG signal to this computer based control system and a visualization of the control algorithm and prosthetic movement. A series of single subject case studies of amputees showed successful operation of the prosthesis using the described system. Paper III presents a method for the quantification of success rate and response time for multiple-state control systems when applied to a group of non-disabled volunteers. Results showed that an increase in complexity of the control system was correlated to a decrease in performance. A prosthesis with four myoelectrically controlled joint movements was shown to be operable with a mean control accuracy of 53 percent subsequent to introductory training. In paper IV a presentation is made of the relationships between the isometric force exerted by the biceps brachii and six different quantities of the EMG simultaneously detected with bipolar surface and concentric needle electrodes. Results implied that amplitude sensitive parameters may be the best descriptors of isometric force from 0 to 100 percent of maximal voluntary contraction. In addition it was shown that bipolar surface electrodes may be superior to concentric needle electrodes for detection of EMG as a substitute measure of force. Paper V describes the use of EMG signals as a method for quantifying motor blockade during epidural anaesthesia. Surface EMG was detected from muscles with innervations from spinal levels T7, T9, T11 and L2-L4. Quantification of the recorded EMG during onset, duration and regression of anaesthesia appeared to enable quantitative mapping of both the temporal and spatial motor blockade behavior of the drug used in epidural anaesthesia.It is concluded that the applications of detected and quantified EMG signals as presented in this thesis may lead to improvements both in the areas of prosthesis operation for amputees and for detection of motor blockade during epidural anaesthesia.
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| 13. |
- Reinfrank, Michael, 1958-
(författare)
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Fundamentals and logical foundations of truth maintenance
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Despite their importance in AI problem solving, nonmonotonic truth maintenance systems (TMSs) still lack sufficiently well-understood logical foundations. In this thesis, I present a rigorous logical theory of TMSs. I pursue a two-step, bottom-up approach. First, I specify a direct, but implementation-independent, theory of truth maintenance. This theory, then, is used to• draw a connection between TMSs and Autoepistemic Logic, thus closing a gap between theory and implementation in Nonmonotonic Reasoning,• provide a correctness proof for an encoding of nonmonotonic justifications in an essentially monotonic assumption-based TMS,• design a uniform framework for truth maintenance and nonmonotonic inference based on the concept of justification-schemata,• discuss a model theory of TMSs in terms of stable, maximally preferred model sets.At the time of writing, no comprehensive introductory readings on truth maintenance are available. Therefore, the present thesis begins with a set of lecture notes which provide the necessary background information for the subsequent formal treatment of foundational issues.
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| 14. |
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| 15. |
- Teriö, Heikki, 1953-
(författare)
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Urodynamic modelling and measurement techniques for assessment of urethral function during micturition
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- According to Griffiths' theory of flow through elastic tubes the urinary flow is controlled by an elastic constriction. The mechanical properties of this flow-controlling zone are described by the intrinsic urethral pressure as a function of cross-sectional area given by the pressure/area relation p(A)=pmo + KnAn, where pmo is the minimal urethral opening pressure, A the cross-sectional area and Kn and n are parameters describing the distensibility of this zone. Values of the three parameters were estimated from pressure/flow data measured during micturition. The detrusor pressure was measured with suprapubic catheters connected to external transducers and urinary flow was recorded with a balance-type flowmeter with a rotating disc. The detrusor pressure as a function of the volume flow was described by pdet(Q)= pmo + LmQm, where Q is the volume flow and Lm and m parameters, and this function was fitted to recorded data. Using this model it is possible to describe urethral flow properties in a standardized way and to identify different biomechanical changes that may cause obstruction.21 randomly selected elderly men without voiding problems were examined urodynamically and the measured pressure/flow data was analysed according to the described model. 19 of these men had pressure/area relations with a low slope indicating high distensibility of the flow-controlling zone.The method has also been used to investigate 28 men with benign prostatic hypertrophy, 23 of whom were also studied postoperatively. Preoperatively the minimal opening pressure was substantially elevated compared to normal, parameter Lm was moderately increased and m usually had a low value. The flow-controlling zone therefore had a somewhat reduced distensibility. Postoperatively, the minimal opening pressure was much reduced and the slope of the pressure/flow relation also was lower. The pressure/area relations had lower slopes and the flow-controlling zone could be distended to larger maximum cross-sectional areas. However, in many cases the curve shape suggested that the distension might have been restricted by fibrosis.A new, non-invasive method based on detection of pressure variations from turbulent urethral flow was described. The method was studied in a urethral flow model for different degrees of obstruction and volume flows. The power spectrum for the detected signals showed increasing intensity and increased frequency content for increasing volume flow and degree of obstruction. The mean power frequency, which can be seen as a limit between the low and the high frequency parts of the spectrum, was used to characterize the spectrum. It was found to be related to the degree of obstruction and the volume flow.
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| 16. |
- Viberg, Mats
(författare)
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Subspace Fitting Concepts in Sensor Array Processing
- 1989
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The area of sensor array processing has recently received considerable attention in the litterature. Much interest stems from the potential of applying advanced concepts from signal processing to a variety of real-world problems. In these applications, spatially distributed sensors are used to detect and locate multible emitters and/or to seperate the individual encountered in communication and radar applications, when the signal transmission is subject to undesired interference. Acoustic devices (hydrophones) are used in, e.g., underwater and seismic applications.A vast number of methods have been proposed for estimating unknown parameters of the received wavefronts. In Part II of this thesis, an attempt is made to examine the relations among many of these techniques, as well as the accuracy of the resulting estimates. Several estimation methods are posed as solutions to different versions of a basic subspace fitting problem. The asymptotic (for large data records) performance of the multidimensional subspace fitting methods is investigated in Parts II, IV, and V. A new estimator, termed the Weighted Subspace Fitting (WSF) method is introduced and analysed. The WSF method is shown to be a unification of the subspace fitting techniques and the stochastic Maximum Likelihood method, valid for large amounts of data. The analysis is carried out for the case of Gaussian noise and Gaussian/nonGaussian signal waveforms.In Part III, a numerical procedure for calculating the WSF estimates is proposed. A detection scheme based on the WSF method is also suggested, and shown to yield a consistent estimate of the number of coherent/non-coherent wavefronts.Part VI deals with the problem of separating a desired signal from unwanted disturbances. The desired signal is assumed to be absent in certain time (or frequency) intervals, and the proposed method requires no calibration of the array and involves no numerical search.
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| 17. |
- Wahlberg, Bo
(författare)
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On the Identification and Approximation of Linear Systems
- 1987
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis consists of four parts. In the first one, the connections between system identification and model reduction are discussed. The second part deals with the problem of estimating ARMA models for narrow band processes. In the third part we study how to build models of continuous time dynamical models form discrete time measurements, and in the fourth part we show how to affect the bias distribution in transfer function estimation.No mathematical models are perfect descriptions of physical systems. Recently there has been a growing interest in how approximate models will affect the results, in e.g. control design. System identification deals with the problem of building mathematical models of dynamical systems based on observed data from the system. The theme of this thesis is to study how the fact that the physical system cannot be exactly represented within the chosen model set will influence the identified model.In parts I and II the application of model reduction in system identification is investigated. It is showed how the fact that the high order model is obtained from an identification experiment will affect the choice of model reduction procedure. A by-product will be an identification algorithm based on an high order ARX estimate and model reduction.In part III the problem of building models of continuous time linear dynamical systems based on discrete time observations of the system is considered. By studying continuous time prediction error methods it is shown how the choice of model structure and sampling interval will affect the resulting estimate in case of fast sampling.In part IV it is shown how the use of prefilters, noise model, sampling interval and prediction horizon will affect the distribution of bias transfer function estimation. An important aspect is that the true sytem is not assumed to be exactly represented within the chosen model set. It is shown how the distribution of bias in the frequency domain is governed by a weighting function that emphasizes different frequency bands.
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