| 1. |
- Auvert, Marine, et al.
(författare)
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On Router Control for Congestion Avoidance
- 2002
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Konferensbidrag (refereegranskat)abstract
- This short paper deals with active queue management for computer networks. The goal is to develop control mechanisms for routers in heterogeneous networks that reduce traffic fluctuations. The proposed control strategy operates with local information (such as estimated arrival rates) and actively use the buffers to smooth traffic, and thus it avoids the buildup and propagation of traffic bursts.
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| 2. |
- Gerencser, Laszlo, et al.
(författare)
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Randomization methods in optimization and adaptive control - Dedicated to Tyrone Duncan on occasion of his 60th birthday
- 2002
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Ingår i: STOCHASTIC THEORY AND CONTROL. - : Springer-Verlag New York. - 3540437770 ; , s. 137-153
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Bokkapitel (refereegranskat)abstract
- We consider simultaneous perturbation stochastic approximation (SPSA) methods applied to noise-free problems in optimization and adaptive control. More generally, we consider discrete-time fixed gain stochastic approximation processes that are defined in terms of a random field that is identically zero at some point theta*. The boundedness of the estimator process is enforced by a resetting mechanism. Under appropriate technical conditions the estimator sequence converges to theta* with geometric rate almost surely. This result is in striking contrast to classical stochastic approximation theory where the typical convergence rate is n(-1/2). For the proof a discrete-time version of the ODE-method is used and the techniques of [10] are extended. A simple variant of noise free-SPSA is applied to extend a direct controller tuning method named Iterative Feedback Timing (IFT), see [16]. Using randomization, the number of experiments required to obtain an unbiased estimate of the gradient of the cost function can be reduced significantly for multi-input multi-output systems.
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| 3. |
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| 4. |
- Hansson, Anders, et al.
(författare)
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On Methods for Gradient Estimation in IFT for MIMO Systems
- 2002
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Ingår i: Proceedings of 15th IFAC World Congress. - 9783902661746 ; , s. 1047-1047
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Konferensbidrag (refereegranskat)abstract
- Iterative feedback tuning (IFT) is a model free control tuning method using closed loop experiments. For single-input single-output (SISO) systems only 2 or 3, depending on the controller structure, closed loop experiments are required. However for multivariable systems the number of experiments increase to a maximum of 1 + m times p, where m times p is the dimension of the controller. In this contribution several methods are proposed to reduce the experimental time by approximating the gradient of the cost function. The local convergence for a method which uses the same technique as in IFT for SISO systems is analyzed. It is shown that even if there are commutation errors due to the approximation method, the numerical optimization may still converge to the true optimum.
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| 5. |
- Henriksson, Björn, et al.
(författare)
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Control relevant identification of nonlinear systems using linear models
- 2001
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Ingår i: PROCEEDINGS OF THE 2001 AMERICAN CONTROL CONFERENCE. - 0780364953 ; , s. 1178-1183
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Konferensbidrag (refereegranskat)abstract
- In this contribution we consider control of nonlinear systems by means of linear time-invariant model based control design. It is shown that a desired closed loop response of the nonlinear system for a given reference signal can be achieved using a controller based on a linear model. This is possible if the linear model used in the control design is able to capture the input/output behaviour of the nonlinear system for the desired response. An identification method, designed for this purpose, is presented and illustrated on an numerical example.
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| 6. |
- Hjalmarsson, Håkan, 1962-, et al.
(författare)
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An exact finite sample variance expression for a class of frequency function estimates
- 2004
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Ingår i: 43RD IEEE CONFERENCE ON DECISION AND CONTROL. - 0780386825 ; , s. 370-375
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Konferensbidrag (refereegranskat)abstract
- An explicit expression for the variance of finite sample frequency function estimates is presented. The expression is applicable to models estimated using the least-squares method under the following conditions: 1) The system is in the model set; 2) The model structure is linear in the parameters (c.f. FIR, Laguerre and Kautz models); 3) The measurement noise is white; 4) The number of estimated parameters coincides with the number of non-zero spectral lines of the input. The new variance expression gives insight into how to choose the input spectrum in order to obtain reliable frequency function estimates. In the paper this variance expression is also used to compare FIR modeling with Empirical Transfer Function Estimation.
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| 7. |
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| 8. |
- Hjalmarsson, Håkan, 1962-, et al.
(författare)
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Identification for control: L-2 and L-infinity methods
- 2001
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Ingår i: PROCEEDINGS OF THE 40TH IEEE CONFERENCE ON DECISION AND CONTROL. - 0780370619 ; , s. 2701-2706
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Konferensbidrag (refereegranskat)abstract
- Identification of restricted complexity models for linear time-invariant systems is considered. A method, with ties to model reduction and the extended instrumental variable method, which uses a cost function based on cross-correlations between the prediction error and some auxiliary variable such as the input is presented. In open loop and under linear feedback, this method is able to asymptotically approximate the underlying system in L-2 or L-infinity norm without an explicit noise model under noisy conditions. Arbitrary frequency weighting, also depending on the true frequency function, can be used. The method is applied to identification of models suited for control design. It is shown that some model free methods can be fit into this framework. It is also pointed out that closed loop stability is not taken into account in the bias tuning due to the fact that L-norms are used.
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| 9. |
- Hjalmarsson, Håkan, et al.
(författare)
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Identification of performance limitations in control
- 2001
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Ingår i: 2001 European Control Conference, ECC 2001. - : IEEE conference proceedings. - 9783952417362 ; , s. 1446-1451
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Konferensbidrag (refereegranskat)abstract
- Non-minimum phase zeros and poles of a process put upper and lower constraints on the bandwidth of a closed loop system. It is thus of great interest to be able to identify these quantities. In this contribution it is shown that non-minimum phase zeros and unstable poles can be identified using high order models without the standard o(n) (n is the model order) variance penalty for over modeling. An asymptotic, in the model order and the number of data, expression for the variance of non-minimum phase zeros is derived. This result shows that the problem of determining the performance limits of a system from experimental data is considerably easier than identifying the complete system. As a direct application of this an optimal experiment design problem for identification of performance limits is posed and solved.
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| 10. |
- Hjalmarsson, Håkan, 1962-
(författare)
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Iterative feedback tuning - an overview
- 2002
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Ingår i: International journal of adaptive control and signal processing (Print). - : Wiley. - 0890-6327 .- 1099-1115. ; 16:5, s. 373-395
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Tidskriftsartikel (refereegranskat)abstract
- Adaptive and iterative control algorithms based on explicit criterion minimization are briefly reviewed and an overview of one such algorithm, iterative feedback tuning (IFT), is presented. The basic IFT algorithm is reviewed for both single-input/single-output and multi-input/multi-output systems. Subsequently the application to non-linear systems is discussed. Stability and robustness aspects are covered. A survey of existing extensions, applications and related methods is also provided.
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