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1.	Åström, Karl Johan, et al. (författare) Theory and Applications of Self Tuning Regulators 1977 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 13:5, s. 457-476 Tidskriftsartikel (refereegranskat)abstract This paper reviews work on self-tuning regulators. The regulator algorithms, their theory and industrial applications are reviewed. The paper is expository—the major ideas are covered but detailed analysis is given elsewhere.
2.	Bauer, Dietmar (författare) Order Estimation for Subspace Methods 2001 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 37:10, s. 1561-1573 Tidskriftsartikel (refereegranskat)abstract Three different order estimation criteria in the context of subspace algorithms are introduced and sufficient conditions for strong consistency are derived. A simulation study points to open questions.
3.	Bauer, Dietmar, et al. (författare) Some Facts about the Choice of the Weighting Matrices in Larimore Type of Subspace Algorithms 2002 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 38:5, s. 763-773 Tidskriftsartikel (refereegranskat)abstract In this paper the effect of some weighting matrices on the asymptotic variance of the estimates of linear discrete time state space systems estimated using subspace methods is investigated. The analysis deals with systems with white or without observed inputs and refers to the Larimore type of subspace procedures. The main result expresses the asymptotic variance of the system matrix estimates in canonical form as a function of some of the user choices, clarifying the question on how to choose them optimally. It is shown, that the CCA weighting scheme leads to optimal accuracy. The expressions for the asymptotic variance can be implemented more efficiently as compared to the ones previously published.
4.	Forssell, Urban, et al. (författare) Some Results on Optimal Experiment Design 2000 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 36:5, s. 749-756 Tidskriftsartikel (refereegranskat)abstract The problem of designing the identification experiments to make them maximally informative with respect to the intended model use is studied. The focus is on how to identify models that are good for control, so called `Identification for Control'. A main result is that we derive explicit expressions for the optimal controller and the optimal reference signal spectrum to use in the identification experiment for the case that only the misfit in the dynamics model is penalized and when a linear combination of the input and output variances is constrained.
5.	Frisk, Erik, et al. (författare) A minimal polynomial basis solution to residual generation for fault diagnosis in linear systems 2001 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 37:9, s. 1417-1424 Tidskriftsartikel (refereegranskat)abstract A fundamental part of a fault diagnosis system is the residual generator. Here a new method, the minimal polynomial basis approach, for design of residual generators for linear systems, is presented. The residual generation problem is transformed into a problem of finding polynomial bases for null-spaces of polynomial matrices. This is a standard problem in established linear systems theory, which means that numerically efficient computational tools are generally available. It is shown that the minimal polynomial basis approach can find all possible residual generators and explicitly those of minimal order. © 2001 Elsevier Science Ltd. All rights reserved.
6.	Gevers, Michel, et al. (författare) Asymptotic Variance Expressions for Closed-Loop Identification 2001 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 37:5, s. 781-786 Tidskriftsartikel (refereegranskat)abstract Asymptotic variance expressions are analyzed for models that are identified on the basis of closed-loop data. The considered methods comprise the classical `direct' method, as well as the more recently developed indirect methods, employing coprime factorized models, dual Youla/Kucera parametrizations and the two-stage approach. The variance expressions are compared with the open-loop situation, and evaluated in terms of their relevance for subsequent model-based control design.
7.	Gevers, Michel, et al. (författare) Optimal Experimental Designs with Respect to the Intended Model Application 1986 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 22:5, s. 543-554 Tidskriftsartikel (refereegranskat)abstract The purpose of the design of identification experiments is to make the collected data maximally informative with respect to the intended use of the model, subject to constraints that might be at hand. When the true system is replaced by an estimated model, there results a performance degradation that is due to the error in the transfer function estimates. Using some recent asymptotic expressions for the bias and the variance of the estimated transfer function, it is shown how this performance degradation can be minimized by a proper experiment design. Several applications, where it is beneficial to let the experiment be carried out in closed loop, are highlighted.
8.	Ghandhari, Mehrdad, et al. (författare) A control strategy for controllable series capacitor in electric power systems 2001 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 37:10, s. 1575-1583 Tidskriftsartikel (refereegranskat)abstract It has been verified that a controllable series capacitor with a suitable control scheme can improve transient stability and help to damp electromechanical oscillations. A question of great importance is the selection of the input signals and a control strategy for this device in order to damp power oscillations in an effective and robust manner. Based on Lyapunov theory a control strategy for damping of electromechanical power oscillations in a multi-machine power system is derived. Lyapunov theory deals with dynamical systems without inputs. For this reason, it has traditionally been applied only to closed-loop control systems, that is, systems for which the input has been eliminated through the substitution of a predetermined feedback control. However, in this paper, we use Lyapunov function candidates in feedback design itself by making the Lyapunov derivative negative when choosing the control. This control strategy is called control Lyapunov function for systems with control inputs. Also, two input signals for this control strategy are used. The first one is based on local information and the second one on remote information derived by the single machine equivalent method.
9.	Ghulchak, Andrey, et al. (författare) Computer control systems. Analysis and design with process-oriented nodels : E. Rosenwasser and B. Lampe; Springer, London, 2000, ISBN 1-85233-307-3 2002 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 38, s. 2031-2035 Recension (övrigt vetenskapligt/konstnärligt)
10.	Glad, Torkel, 1947-, et al. (författare) Bounds on the Response Time under Control Constraints 2001 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 37:12, s. 2073-2076 Tidskriftsartikel (refereegranskat)abstract We consider the problem of transferring the output of a linear system from one equilibrium value to another under control amplitude constraint. It is possible to give a simple lower bound on the time required, expressed in the longest time constant, the gain of the system, the change in y and the control bound. This theoretical bound appears to be useful as a practical lower bound of the rise time.
11.	Glad, Torkel, 1947- (författare) Robustness of Nonlinear State Feedback : A Survey 1987 Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 23:4, s. 425-435 Tidskriftsartikel (refereegranskat)abstract A survey of robustness of nonlinear state feedback is given. For series perturbations there are fairly complete results, showing that under mild restrictions an optimal controller can tolerate an infinite increase in gain. For gain reductions there are some results for systems linear in the control. In particular there is a 50% reduction tolerance if the control penalty is quadratic. Usually the optimal controller cannot be computed exactly. There are some results showing the effects of truncation on the robustness. Essentially robustness is maintained but in a reduced (computable) part of the state space.
12.	Gunnarsson, Svante, et al. (författare) On the Design of ILC Algorithms using Optimization 2001 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 37:12, s. 2011-2016 Tidskriftsartikel (refereegranskat)abstract Iterative learning control (ILC) based on minimization of a quadratic criterion in the control error and the input signal is considered. The focus is on the frequency domain properties of the algorithm, and how it is able to handle non-minimum phase systems. Experiments carried out on a commercial industrial robot are also presented.
13.	Gustafsson, Fredrik, et al. (författare) Closed Loop Performance Monitoring in the Presence of System Changes and Disturbances 1998 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 34:11, s. 1311-1326 Tidskriftsartikel (refereegranskat)abstract This paper addresses issues in closed-loop performance monitoring. Particular attention is paid to detecting whether an observed deviation from nominal performance is due to a disturbance or due to a control relevant system change. This is achieved by introducing a novel performance measure that allows feasible application of a standard CUSUM change detector. The paper includes explicit results on the risk of mistaking a disturbance for a system change. The algorithm has been implemented in real time on a DSP and evaluated on a DC motor.
14.	Gustafsson, Fredrik (författare) Slip-Based Estimation of Tire-Road Friction 1997 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 33:6, s. 1087-1099 Tidskriftsartikel (refereegranskat)abstract An approach to estimate the tire-road friction during normal drive using only the wheel slip, that is, the relative difference in wheel velocities, is presented. The driver can be informed about the maximum friction force and be alarmed for sudden changes. Friction-related parameters are estimated using only signals from standard sensors in a modern car. An adaptive estimator is presented for a model linear in parameters, which is designed to work for periods of poor excitation, errors in variables, simultaneous slow and fast parameter drifts and abrupt changes. The physical relation between these parameters and the maximal friction force is determined from extensive field trials using a Volvo 850 GLT as a test car.
15.	Gustafsson, Fredrik, et al. (författare) Twenty-one ML estimators for model selection 1995 Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 31:10, s. 1377-1392 Tidskriftsartikel (refereegranskat)abstract Classical approaches to determine a suitable model structure from observed input-output data are based on hypothesis tests and information-based criteria. Recently, the model structure has been considered as a stochastic variable, and standard estimation techniques have been proposed. The resulting estimators are closely related to the aforementioned methods. However, it turns out that there are a number of prior choices in the problem formulation, which are crucial for the estimators' behavior. The contribution of this paper is to clarify the role of the prior choices, to examine a number of possibilities and to show which estimators are consistent. This is done in a linear regression framework. For autoregressive models, we also investigate a novel prior assumption on stability, and give the estimator for the model order and the parameters themselves. Copyright Â© 1995 Elsevier Science Ltd All rights reserved.
16.	Gustafsson, T.K., et al. (författare) Modelling of uncertain systems via linear programming 1996 Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 32:3, s. 319-335 Tidskriftsartikel (refereegranskat)abstract Real plants are, in general, time-varying and uncertain. Yet most industrial control loops are designed using linear time-invariant (LTI) plant models. The present work formalizes the problem of best approximate LTI modelling of BIBO-stable linear time-varying (LTV) systems in a way that is compatible with the notion of induced l∞ norm used in robust l1 control. This setup is closely associated with certain identification methods and controlmotivated model validation/invalidation procedures that can be efficiently implemented with special-purpose linear programming (LP) techniques. Results are given for approximate modelling of BIBO-stable LTV and LTI systems using LTI models, especially BIBO-stable fixed-pole state-space model parametrizations. It is shown that such parametrizations are satisfactory from an approximate modelling point of view, and can be used in output-error-type LP identification techniques and in LP model validation procedures. Various useful constraints on model parameters etc. can be included, as long as they are linear in the unknown parameters, and both insensitive (statistically robust) and sensitive criteria can be used in identification and model validation. Simulation examples are included to illustrate that such techniques indeed give good results and can be used to solve problems of realistic size.
17.	Gustavsson, Ivar, et al. (författare) Identification of Processes in Closed Loop-Identifiability and Accuracy Aspects 1977 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 13, s. 59-75 Tidskriftsartikel (refereegranskat)abstract It is often necessary in practice to perform identification experiments on systems operating in closed loop. There has been some confusion about the possibilities of successful identification in such cases, evidently due to the fact that certain common methods then fail. A rapidly increasing literature on the problem is briefly surveyed in this paper, and an overview of a particular approach is given. It is shown that prediction error identification methods, applied in a direct fashion will given correct estimates in a number of feedback cases. Furthermore, the accuracy is not necessarily worse in the presence of feedback; in fact optimal inputs may very well require feedback terms. Some practical applications are also described.
18.	Ho, W. K., et al. (författare) Relay auto-tuning of PID controllers using iterative feedback tuning 2003 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 39:1, s. 149-157 Tidskriftsartikel (refereegranskat)abstract In this paper, ideas from iterative feedback tuning (IFT) are incorporated into relay auto-tuning of the proportional-plus-integal-plus-derivative (PID) controller. The PID controller is auto-tuned to give specified phase margin and bandwidth. Good tuning performance according to the specified bandwidth and phase margin can be obtained and the limitation of the standard relay auto-tuning technique using a version of Ziegler-Nichols formula can be eliminated. Furthermore, by using common modelling assumptions for the relay system, some of the required derivatives in the IFT algorithm can be derived analytically. The algorithm was tested in the laboratory on a coupled tank and good tuning result was demonstrated.
19.	Hu, Xiaoming (författare) Hybrid control approach to action coordination for mobile robots 2002 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 38:1, s. 125-130 Tidskriftsartikel (refereegranskat)abstract In this paper, the problem concerning how to coordinate the contributions from concurrent controllers, when controlling mobile robots, is investigated. It is shown how a behavior based control system for autonomous robots can be modeled as a hybrid automaton, where each node corresponds to a distinct robot behavior, This type of construction gives rise to chattering executions, but it is shown how regularized automata can be used to solve this problem. As an illustration. the obstacle-negotiation problem is solved by using a combination of a robust path-following behavior and a reactive obstacle-avoidance behavior that move the robot around a given obstacle at a predefined safety distance.
20.	Johansson, Andreas, et al. (författare) An observer for systems with nonlinear output map 2003 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 39:5, s. 909-918 Tidskriftsartikel (refereegranskat)abstract A nonlinear observer, with the feedback gain weighted by the sensitivity of the output with respect to the state, is developed for systems with nonlinear output map. The observer can be obtained from the extended Kalman filter by a special choice of time-varying weighting matrices. It is shown that the estimation error dynamics are asymptotically stable and a region of attraction is derived. The observer is applied to the top blown converter process for estimating the content of impurities in the liquid metal. Using plant data from the converter at SSAB Oxelösund AB, the observer is shown to provide accurate estimates of the carbon content.
21.	Johansson, Karl Henrik, et al. (författare) Fast switches in relay feedback systems 1999 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 35:4, s. 539-552 Tidskriftsartikel (refereegranskat)abstract Relays are common in automatic control systems. Even linear systems with relay feedback are, however, far from fully understood. New results are given about the behavior of these systems via a state-space approach. It is proved that there exist multiple fast switches if and only if the sign of the first nonvanishing Markov parameter of the linear system is positive. Fast switches are shown to occur as part of stable limit cycles. An analysis is developed for these limit cycles that illustrates how they can be predicted.
22.	Johansson, Karl H. (författare) Interaction bounds in multivariable control systems 2002 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 38:6, s. 1045-1051 Tidskriftsartikel (refereegranskat)
23.	Juditsky, A., et al. (författare) Nonlinear black-box models in system identification: Mathematical foundations 1995 Ingår i: Automatica. - Linköping : Elsevier BV. - 0005-1098 .- 1873-2836. ; 31:12, s. 1725-1750 Tidskriftsartikel (refereegranskat)abstract We discuss several aspects of the mathematical foundations of the nonlinear black-box identification problem. We shall see that the quality of the identification procedure is always a result of a certain trade-off between the expressive power of the model we try to identify (the larger the number of parameters used to describe the model, the more flexible is the approximation), and the stochastic error (which is proportional to the number of parameters). A consequence of this trade-off is the simple fact that a good approximation technique can be the basis of a good identification algorithm. From this point of view, we consider different approximation methods, and pay special attention to spatially adaptive approximants. We introduce wavelet and 'neuron' approximations, and show that they are spatially adaptive. Then we apply the acquired approximation experience to estimation problems. Finally, we consider some implications of these theoretical developments for the practically implemented versions of the 'spatially adaptive' algorithms. Copyright Â© 1995 Elsevier Science Ltd All rights reserved.
24.	Jönsson, Ulf T. (författare) New results for analysis of systems with repeated nonlinearities 2001 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 37:5, s. 739-747 Tidskriftsartikel (refereegranskat)abstract This work presents new results for analysis of systems with repeated monotonic or slope restricted nonlinearities. We first give new integral quadratic constraints (IQCs) that are satisfied by the inputs and outputs of diagonal operators with equal, monotonic or slope restricted, diagonal entries. Then, we present new analysis results For systems with repeated nonlinearities, obtained using the new IQCs. For the type of nonlinearities considered in this work, the new results are less conservative than previously known results. The computation of the new conditions is discussed. The results in this paper apply also to problems with nonrepeated nonlinearities whenever these are representable as the interconnection of multiple repeated nonlinearities and a constant matrix.
25.	Laila, Dina Shona, et al. (författare) Changing supply rates for input-output to state stable discrete-time nonlinear systems with applications 2003 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 39:5, s. 821-835 Tidskriftsartikel (refereegranskat)
26.	Lindquist, Anders (författare) A covariance extension approach to identification of time series 2000 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 36:3, s. 379-398 Tidskriftsartikel (refereegranskat)abstract In this paper we consider a three-step procedure far identification of time series, based on covariance extension and model reduction, and we present a complete analysis of its statistical convergence properties. A partial covariance sequence is estimated from statistical data. Then a high-order maximum-entropy model is determined, which is finally approximated by a lower-order model by stochastically balanced model reduction. Such procedures have been studied before, in various combinations, but an overall convergence analysis comprising all three steps has been lacking. Supposing the data is generated from a true finite-dimensional system which is minimum phase, it is shown that the transfer function of the estimated system tends in H-infinity to the true transfer function as the data length tends to infinity, if the covariance extension and the model reduction is done properly. The proposed identification procedure, and some variations of it, are evaluated by simulations.
27.	Lindskog, Peter, et al. (författare) Ensuring Monotonic Gain Characteristics in Estimated Models by Fuzzy Model Structures 2000 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 36:2, s. 311-317 Tidskriftsartikel (refereegranskat)abstract We consider the situation where a non-linear physical system is identified from input-output data. In case no specific physical structural knowledge about the system is available, parameterized grey-box models cannot be used. Identification in black-box type of model structures is then the only alternative, and general approaches like neural nets, neuro-fuzzy models, etc., have to be applied. However, certain non-structural knowledge about the system is sometimes available. It could be known, e.g., that the step response is monotonic, or that the steady-state gain curve is monotonic. The main question is then how to utilize and maintain such information in an otherwise black-box framework. In this paper we show how this can be done, by applying a specific fuzzy model structure, with strict parametric constraints. The usefulness of the approach is illustrated by experiments on real-world data.
28.	Ljung, Lennart, 1946-, et al. (författare) Adaptation and Tracking in System Identification : A Survey 1990 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 26:1, s. 7-21 Tidskriftsartikel (refereegranskat)abstract To track the time-varying dynamics of a system or the time-varying properties of a signal is a fundamental problem in control and signal processing. Many approaches to derive such adaptation algorithms and to analyse their behaviour have been taken. This article gives a survey of basic techniques to derive and analyse algorithms for tracking time-varying systems. Special attention is paid to the study of how different assumptions about the true system's variations affect the algorithm. Several explicit and semi-explicit expressions for the mean square error are derived, which clearly demonstrate the character of the trade-off between tracking ability and noise rejection.
29.	Ljung, Lennart, 1946- (författare) Analysis of a general Recursive Prediction Error Identification Algorithm 1981 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 17:1, s. 89-99 Tidskriftsartikel (refereegranskat)abstract A general class of algorithms for recursive identification of (stochastic) dynamical systems is studied. In this class, the discrepancy between the measured output and the output, predicted from previous data according to a candidate model (‘the prediction error’) is minimized over the model set using a stochastic approximation approach. It is proved that this class of methods has the same convergence properties as its off-line counterparts under mild and general assumptions. These assumptions do not, for example, include stationary conditions or conditions that the true system can be exactly represented within the model set.The considered class of methods contains as special cases several well-known algorithms. Other common recursive identification methods, like the extended least squares method and the extended Kalman filter can be interpreted as approximate prediction error methods with simplified gradient calculations. Therefore, the approach taken here, may also serve as a basis for unified description of many recursive identification methods.
30.	Ljung, Lennart, 1946-, et al. (författare) Closed-Loop Identification Revisited 1999 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 35:7, s. 1215-1241 Tidskriftsartikel (refereegranskat)abstract Identification of systems operating in closed loop has long been of prime interest in industrial applications. The problem offers many possibilities, and also some fallacies, and a wide variety of approaches have been suggested, many quite recently. The purpose of the current contribution is to place most of these approaches in a coherent framework, thereby showing their connections and display similarities and differences in the asymptotic properties of the resulting estimates. The common framework is created by the basic prediction error method, and it is shown that most of the common methods correspond to different parameterizations of the dynamics and noise models. The so-called indirect methods, e.g., are indeed “direct” methods employing noise models that contain the regulator. The asymptotic properties of the estimates then follow from the general theory and take different forms as they are translated to the particular parameterizations. We also study a new projection approach to closed-loop identification with the advantage of allowing approximation of the open-loop dynamics in a given, and user-chosen frequency domain norm, even in the case of an unknown, nonlinear regulator.
31.	Ljung, Lennart, 1946- (författare) Control Theory 1984-1986 : A Progress Report from IFACS's Technical Commitee on Theory 1988 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 24:4, s. 573-583 Tidskriftsartikel (refereegranskat)abstract Members of IFAC's Technical Committee on Theory prepared this review of papers published during 1984–1986 for the participants of the 10th IFAC World Congress in Munich. The review is limited to the scope of the Theory Committee. The idea is to give a status report on Control Theory and to give some guidance into the extensive literature on this topic.
32.	Ljung, Lennart, 1946-, et al. (författare) Frequency Domain versus Time Domain Methods in System Identification 1981 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 17:1, s. 71-86 Tidskriftsartikel (refereegranskat)abstract The basic techniques of time domain and frequency domain identification, including the maximum entropy methods, are outlined. Then connections and distinctions between the methods are explored. This includes the derivation of some analytic relationships together with a discussion of the restrictions inherent in choosing certain methods, and their ease of use in different experimental conditions. It is concluded that these are complementary rather than competing techniques.
33.	Ljung, Lennart, 1946-, et al. (författare) On Global Identifiability for Arbitrary Model Parametrizations 1994 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 30:2, s. 265-276 Tidskriftsartikel (refereegranskat)abstract It is a fundamental problem of identification to be able—even before the data have been analyzed—to decide if all the free parameters of a model structure can be uniquely recovered from data. This is the issue of global identifiability. In this contribution we show how global identifiability for an arbitrary model structure (basically with analytic non-linearities) can be analyzed using concepts and algorithms from differential algebra. It is shown how the question of global structural identifiability is reduced to the question of whether the given model structure can be rearranged as a linear regression. An explicit algorithm to test this is also given. Furthermore, the question of ‘persistent excitation’ for the input can also be tested explicitly is a similar fashion. The algorithms involved are very well suited for implementation in computer algebra. One such implementation is also described.
34.	Ljung, Lennart, 1946- (författare) On the Estimation of Transfer Functions 1985 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 21:6, s. 677-696 Tidskriftsartikel (refereegranskat)abstract This paper treats the close conceptual relationships between basic approaches to the estimation of transfer functions of linear systems. The classical methods of frequency and spectral analysis are shown to be related to the well-known time domain methods of prediction error type via a common “empirical transfer function estimate”. Asymptotic properties of the estimates obtained by the respective methods are also described and discussed. An important feature that is displayed by this treatment is a frequency domain weighting function that determines the distribution of bias in case the true system cannot be exactly described within the chosen model set. The choice of this weighting function is made in terms of noise models for time-domain methods. The noise model thus has a dual character from the system approximation point of view.
35.	Ljung, Lennart, 1946-, et al. (författare) Subspace-based Identification of Infinite-dimensional Multivariable Systems from Frequency-response Data 1996 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 32:6, s. 885-902 Tidskriftsartikel (refereegranskat)abstract A new identification algorithm which identifies low complexity models of infinite-dimensional systems from equidistant frequency-response data is presented. The new algorithm is a combination of the Fourier transform technique with the recent subspace techniques. Given noisefree data, finite-dimensional systems are exactly retrieved by the algorithm. When noise is present, it is shown that identified models strongly converge to the balanced truncation of the identified system if the measurement errors are covariance bounded. Several conditions are derived on consistency, illustrating the trade-offs in the selection of certain parameters of the algorithm. Two examples are presented which clearly illustrate the good performance of the algorithm.
36.	Ljung, Lennart, 1946-, et al. (författare) Vibration Data Analysis for a Commercial Aircraft 1996 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 32:12, s. 1689-1700 Tidskriftsartikel (refereegranskat)abstract Using data from extensive vibrational tests of the new Saab 2000 aircraft, a combined method for vibration analysis is studied. The method is based on a realization algorithm followed by standard prediction error methods (PEM). We find that the realization algorithm gives good initial model parameter estimates that can be further improved by the use of PEM. We use the method to get insights into the vibrational eigenmodes.
37.	Ljung, Stefan, et al. (författare) Error Propagation Properties of Recursive Least Squares Adaptation Algorithms 1985 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 21:2, s. 157-167 Tidskriftsartikel (refereegranskat)abstract The numerical properties of implementations of the recursive least-squares identification algorithm are of great importance for their continuous use in various adaptive schemes. Here we investigate how an error that is introduced at an arbitrary point in the algorithm propagates. It is shown that conventional LS algorithms, including Bierman's UD-factorization algorithm are exponentially stable with respect to such errors, i.e. the effect of the error decays exponentially. The base of the decay is equal to the forgetting factor. The same is true for fast lattice algorithms. The fast least-squares algorithm, sometimes known as the ‘fast Kalman algorithm’ is however shown to be unstable with respect to such errors.
38.	Medvedev, Alexander (författare) Disturbance attenuation in finite spectrum assignment controllers 1997 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 33:6, s. 1163-1168 Tidskriftsartikel (refereegranskat)abstract Two finite-spectrum-assignment controllers for continuous linear dynamic systems with multiple time delays in input signal are introduced. The controllers are shown to comprise least-squares state estimators. Disturbance attenuation properties stipulated by the choice of shift operator in the observers are studied.
39.	Medvedev, A (författare) Fault detection and isolation by a continuous parity space method 1995 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 31:7, s. 1039-1044 Tidskriftsartikel (refereegranskat)abstract The well-known parity space method for fault detection and isolation is generalized to a continuous-time case. As in the discrete-time case, it is shown that the parity space equations can be implemented using a number of time delays, and are equivalent to a continuous deadbeat observer. Algorithms for isolation of sensor and actuator faults are derived and illustrated by a numerical example.
40.	Mossberg, Magnus, et al. (författare) Non-parametric identification of viscoelastic materials from wave propagation experiments 2001 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 37:4, s. 511-521 Tidskriftsartikel (refereegranskat)abstract We investigate how the frequency-dependent wave propagation coefficient and complex modulus for a linearly viscoelastic material can be estimated from wave propagation experiments. The strains at different sections of a bar specimen are measured as functions of time. The time series are transformed into the frequency domain, where a non-parametric identification is made. A thorough analysis of the quality of the non-parametric estimate is made, in which approximate expressions for the covariance matrices of the wave propagation coefficient and the complex modulus are derived. The validity of these expressions are confirmed by numerical studies and real experiments.
41.	Mäkilä, Pertti, et al. (författare) Worst-case control-relevant identification 1995 Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 31:12, s. 1799-1819 Tidskriftsartikel (refereegranskat)abstract This paper introduces the reader to several recent developments in worst-case identification motivated by various issues of modelling of systems from data for the purpose of robust control design. Many aspects of identification in H∞ and ℓ1 are covered including algorithms, convergence and divergence results, worst-case estimation of uncertainty models, model validation and control relevancy issues.
42.	Nazin, Alexander V., et al. (författare) Asymptotically Optimal Smoothing of Averaged LMS Estimates for Regression Parameter Tracking 2002 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 38:8, s. 1287-1293 Tidskriftsartikel (refereegranskat)abstract The sequence of estimates formed by the LMS algorithm for a standard linear regression estimation problem is considered. It is known since earlier that smoothing these estimates by simple averaging will lead to, asymptotically, the recursive least-squares algorithm. In this paper, it is first shown that smoothing the LMS estimates using a matrix updating will lead to smoothed estimates with optimal tracking properties, also in case the true parameters are slowly changing as a random walk. The choice of smoothing matrix should be tailored to the properties of the random walk. Second, it is shown that the same accuracy can be obtained also for a modified algorithm, SLAMS, which is based on averages and requires much less computations.
43.	Ottersten, Björn, et al. (författare) Statistical signal processing and control 1994 Ingår i: Automatica. - : Elsevier BV. - 0005-1098 .- 1873-2836. ; 30:1, s. 9- Tidskriftsartikel (refereegranskat)
44.	Partington, J.R., et al. (författare) Analysis of linear methods for robust identification in l1 1995 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 31:5, s. 755-758 Tidskriftsartikel (refereegranskat)abstract We consider worst-case analysis of system identification by means of the linear algorithms such as least-squares. We provide estimates for worst-case and average errors, showing that worst-case robust convergence cannot occur in the l1 identification problem. The case of periodic inputs is also analysed. Finally a pseudorandomness assumption is introduced that allows more powerful convergence results in a deterministic framework.
45.	Poznyak, Alex S., et al. (författare) On-Line Identification and Adaptive Trajectory Tracking for Nonlinear Stochastic Continuous Time Systems using Differential Neural Networks 2001 Ingår i: Automatica. - Linköping : Elsevier. - 0005-1098 .- 1873-2836. ; 37:8, s. 1257-1268 Tidskriftsartikel (refereegranskat)abstract Identification of nonlinear stochastic processes via differential neural networks is discussed. A new "dead-zone" type learning law for the weight dynamics is suggested. By a stochastic Lyapunov-like analysis the stability conditions for the identification error as well as for the neural network weights are established. The adaptive trajectory tracking using the obtained neural network model is realized for the subclass of stochastic completely controllable processes linearly dependent on control. The upper bounds for the identification and adaptive tracking errors are established.
46.	Reinelt, Wolfgang, et al. (författare) Comparing Different Approaches to Model Error Modeling in Robust Identification 2002 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 38:5, s. 787-803 Tidskriftsartikel (refereegranskat)abstract Identification for robust control must deliver not only a nominal model, but also a reliable estimate of the uncertainty associated with the model. This paper addresses recent approaches to robust identification, that aim at dealing with contributions from the two main uncertainty sources: unmodeled dynamics and noise affecting the data. In particular, non-stationary Stochastic Embedding, Model Error Modeling based on prediction error methods and Set Membership Identification are considered. Moreover, we show how Set Membership Identification can be embedded into a Model Error Modeling framework. Model validation issues are easily addressed in the proposed framework. A discussion of asymptotic properties of all methods is presented. For all three methods, uncertainty is evaluated in terms of the frequency response, so that it can be handled by H8 control techniques. An example, where a nontrivial undermodeling is ensured by the presence of a nonlinearity in the system generating the data, is presented to compare these methods.
47.	Sjöberg, Jonas, et al. (författare) Nonlinear black-box modeling in system identification: A unified overview 1995 Ingår i: Automatica. - Linköping : Elsevier BV. - 0005-1098 .- 1873-2836. ; 31:12, s. 1691-1724 Tidskriftsartikel (refereegranskat)abstract A nonlinear black-box structure for a dynamical system is a model structure that is prepared to describe virtually any nonlinear dynamics. There has been considerable recent interest in this area, with structures based on neural networks, radial basis networks, wavelet networks and hinging hyperplanes, as well as wavelet-transform-based methods and models based on fuzzy sets and fuzzy rules. This paper describes all these approaches in a common framework, from a user's perspective. It focuses on what are the common features in the different approaches, the choices that have to be made and what considerations are relevant for a successful system-identification application of these techniques. It is pointed out that the nonlinear structures can be seen as a concatenation of a mapping form observed data to a regression vector and a nonlinear mapping from the regressor space to the output space. These mappings are discussed separately. The latter mapping is usually formed as a basis function expansion. The basis functions are typically formed from one simple scalar function, which is modified in terms of scale and location. The expansion from the scalar argument to the regressor space is achieved by a radial- or a ridge-type approach. Basic techniques for estimating the parameters in the structures are criterion minimization, as well as two-step procedures, where first the relevant basis functions are determined, using data, and then a linear least-squares step to determine the coordinates of the function approximation. A particular problem is to deal with the large number of potentially necessary parameters. This is handled by making the number of 'used' parameters considerably less than the number of 'offered' parameters, by regularization, shrinking, pruning or regressor selection. Copyright Â© 1995 Elsevier Science Ltd All rights reserved.
48.	Stenman, Anders, et al. (författare) Adaptive Smoothing Methods for Frequency-Function Estimation 2001 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 37:5, s. 675-685 Tidskriftsartikel (refereegranskat)abstract The determination of resolution parameter when estimating frequency functions of linear systems is a trade-off between bias and variance. Traditional approaches, like `window-closing' employ a global resolution parameter - the window width - that is tuned by ad hoc methods, usually visual inspection of the results. Here we explore more sophisticated estimation methods, based on local polynomial modeling, that tune such parameters by automatic procedures. A further benefit is that the tuning can be performed locally, i.e., that different resolutions can be used in different frequency bands. The approach is thus a conceptually simple and useful alternative to more established multi-resolution techniques like wavelets. The advantages of the method are illustrated in a numerical simulation.
49.	Stoica, P., et al. (författare) Approximate maximum-likelihood frequency estimation 1994 Ingår i: Automatica. - 0005-1098 .- 1873-2836. ; 30:1, s. 131-145 Tidskriftsartikel (refereegranskat)abstract The high-resolution frequency estimators most commonly used, such as MUSIC, ESPRIT and Yule-Walker, determine estimates of the sinusoidal frequencies from the sample covariances of noise-corrupted data. In this paper, a frequency estimation method termed Approximate Maximum Likelihood (AML) is derived from the approximate likelihood function of sample covariances. The statistical performance of AML is studied, both analytically and numerically, and compared with the Cramer-Rao bound as well as the statistical performance corresponding to the aforementioned methods of frequency estimation. AML is shown to provide the minimum asymptotic error variance in the class of all estimators based on a given set of covariances. The implementation of the AML frequency estimator is discussed in detail. The paper also introduces an AML-based procedure for estimating the number of sinusoidal signals in the measured data, which is shown to possess high detection performance.
50.	Söderström, Torsten, et al. (författare) A Theoretical Analysis of Recursive Identification 1978 Ingår i: Automatica. - : Elsevier. - 0005-1098 .- 1873-2836. ; 14:231-244 Tidskriftsartikel (refereegranskat)abstract In this paper five different recursive identification methods will be analyzed and compared, namely recursive versions of the least squares method, the instrumental variable method, the generalized least squares method, the extended least squares method and the maximum likelihood method. They are shown to be similar in structure and need of computer storage and time. Making use of recently developed theory for asymptotic analysis of recursive stochastic algorithms, these methods are examined from a theoretical viewpoint. Possible convergence points and their global and local convergence properties are studied. The theoretical analysis is illustrated and supplemented by simulations.

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