| 51. |
- Bottegal, Giulio, et al.
(författare)
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Outlier robust kernel-based system identification using l1-Laplace techniques
- 2015
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Ingår i: 2015 54th IEEE Conference on Decision and Control. - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 2109-2114
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Konferensbidrag (refereegranskat)abstract
- Regularized kernel-based methods for system identification have gained popularity in recent years. However, current formulations are not robust with respect to outliers. In this paper, we study possible solutions to robustify kernel-based methods that rely on modeling noise using the Laplacian probability density function (pdf). The contribution of this paper is two-fold. First, we introduce a new outlier robust kernel-based system identification method. It exploits the representation of Laplacian pdfs as scale mixture of Gaussians. The hyperparameters characterizing the problem are chosen using a new maximum a posteriori estimator whose solution is computed using a novel iterative scheme based on the expectation-maximization method. The second contribution of the paper is the review of two other robust kernel-based methods. The three methods are compared by means of numerical experiments, which show that all of them give substantial performance improvements compared to standard kernel-based methods for linear system identification.
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| 52. |
- Bottegal, Giulio, et al.
(författare)
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Outlier robust system identification : A Bayesian kernel-based approach
- 2014
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Ingår i: IFAC Proceedings Volumes (IFAC-PapersOnline). - : IFAC Papers Online. - 9783902823625 ; , s. 1073-1078
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Konferensbidrag (refereegranskat)abstract
- In this paper, we propose an outlier-robust regularized kernel-based method for linear system identification. The unknown impulse response is modeled as a zero-mean Gaussian process whose covariance (kernel) is given by the recently proposed stable spline kernel, which encodes information on regularity and exponential stability. To build robustness to outliers, we model the measurement noise as realizations of independent Laplacian random variables. The identification problem is cast in a Bayesian framework, and solved by a new Markov Chain Monte Carlo (MCMC) scheme. In particular, exploiting the representation of the Laplacian random variables as scale mixtures of Gaussians, we design a Gibbs sampler which quickly converges to the target distribution. Numerical simulations show a substantial improvement in the accuracy of the estimates over state-of-the-art kernel-based methods.
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| 53. |
- Briat, Corentin, et al.
(författare)
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An axiomatic fluid-flow model for congestion control analysis
- 2011
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Ingår i: 2011 50th IEEE Conference on Decision and Control andEuropean Control Conference (CDC-ECC). ; , s. 3122-3129
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Konferensbidrag (refereegranskat)abstract
- An axiomatic model for congestion control isderived. The proposed four axioms serve as a basis for theconstruction of models for the network elements. It is shownthat, under some assumptions, some models of the literature canbe recovered. A single-buffer/multiple-users topology is finallyderived and studied for illustration.
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| 54. |
- Briat, Corentin, et al.
(författare)
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Nonlinear state-dependent delay modeling and stability analysis of internet congestion control
- 2010
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Ingår i: 49TH IEEE CONFERENCE ON DECISION AND CONTROL (CDC). - 9781424477463 ; , s. 1484-1491
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Konferensbidrag (refereegranskat)abstract
- It is shown that the queuing delay involved in the congestion control algorithm is state-dependent and does not depend on the current time. Then, using an accurate formulation for buffers, networks with arbitrary topologies can be built. At equilibrium, our model reduces to the widely used setup by Paganini et al. Using this model, the delay-derivative is analyzed and it is proved that the delay time-derivative does not exceed one for the considered topologies. It is then shown that the considered congestion control algorithm globally stabilizes a delay-free single buffer network. Finally, using a specific linearization result for systems with state-dependent delays from Cooke and Huang, we show the local stability of the single bottleneck network.
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| 55. |
- Briat, C., et al.
(författare)
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The Conservation of Information, Towards an Axiomatized Modular Modeling Approach to Congestion Control
- 2015
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Ingår i: IEEE/ACM Transactions on Networking. - 1063-6692 .- 1558-2566. ; 23:3, s. 851-865
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Tidskriftsartikel (refereegranskat)abstract
- We derive a modular fluid-flow network congestion control model based on a law of fundamental nature in networks: the conservation of information. Network elements such as queues, users, and transmission channels and network performance indicators like sending/acknowledgment rates and delays are mathematically modeled by applying this law locally. Our contributions are twofold. First, we introduce a modular metamodel that is sufficiently generic to represent any network topology. The proposed model is composed of building blocks that implement mechanisms ignored by the existing ones, which can be recovered from exact reduction or approximation of this new model. Second, we provide a novel classification of previously proposed models in the literature and show that they are often not capable of capturing the transient behavior of the network precisely. Numerical results obtained from packet-level simulations demonstrate the accuracy of the proposed model.
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| 56. |
- Burström, Lage, et al.
(författare)
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Daily text messages used as a method for assessing low back pain among workers
- 2016
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Ingår i: Journal of Clinical Epidemiology. - : Elsevier BV. - 0895-4356 .- 1878-5921. ; :70, s. 45-51
-
Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: To evaluate a method for collecting data concerning low back pain (LBP) using daily text messages and to characterize the reported LBP in terms of intensity, variability, and episodes.STUDY DESIGN AND SETTING: We conducted a cohort study of LBP among workers used by a mining company. The participants were asked to answer the question "How much pain have you had in your lower back in the last 24 hours on a scale from 0 to 10, where 0 = no pain and 10 = the worst pain imaginable" once a day for 5 weeks, with this process being repeated 6 months later.RESULTS: A total of 121 workers participated in the first period of data collection, and 108 participated in the second period. The daily response rate was 93% for both periods, and cluster analysis was shown to be a feasible statistical method for clustering LBP into subgroups of low, medium, and high pain. The daily text messages method also worked well for assessing the episodic nature of LBP.CONCLUSION: We have demonstrated a method for repeatedly measuring of LBP using daily text messages. The data permitted clustering into subgroups and could be used to define episodes of LBP.
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| 57. |
- Chotteau, Veronique, 1963-, et al.
(författare)
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Tuning of dissolved oxygen and pH PID control parameters in large scale bioreactor by lag control
- 2008
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Ingår i: Proceedings of the Cell Culture Engineering XI Conference.
-
Konferensbidrag (refereegranskat)abstract
- Achieving satisfying DO and pH controllers are often challenges for pilot and large scale mammalian cultivation. Unsatisfactory DO or pH controls can imply fatal effects for the culture. Large scale bioreactors have long response times due to long mixing times compared to small scale systems where control tuning of DO and pH is not so challenging. A method was developed to tune the DO controller PID parameters of a 50 L bioreactor (wv) controlled by a continuous oxygen flow of microbubbles. DO control by continuous flow of pure oxygen microbubbles can oscillate quite widely showing instable behaviour. The method, called lag control here, was based on a lead lag control design by Bode analysis where the prediction part, i.e. ‘lead’ part was omitted. A comparison of this method with a pole placement approach showed the advantage of the lag control. It was decided to omit the derivate part which could lead to instability caused by the long delay observed between the applied oxygen flow and the response detected by the DO probe. Applying the lag control method resulted in a highly satisfactory DO control. In this system, the oxygen microbubbles were almost completely consumed before reaching the liquid surface as demonstrated by the absence of foam. So the oxygen flow used to maintain the DO gave an excellent indication of the cellular oxygen consumption. The control system was robust against all the perturbations of this system, i.e. cell growth, cell bleed, addition of air-saturated fresh medium, DO set point change and a second gas sparger used to strip out the carbon dioxide. The method was first tested with the sulphite oxidation method simulating the oxygen consumption with copper as catalyst to establish the PID parameters. Then the selected parameters were successfully used during cell cultivation. Following this, an adaptation of the method was done in order to avoid the sulphite oxidation method, which leaves copper traces in the bioreactor. This was successfully used in a 400 L bioreactor (wv) for the DO controller by continuous oxygen flow of microbbubles. The lag controller method was finally modified to tune the pH controller of the same 400 L bioreactor with control upward by alkali addition or downwards by pulsed carbon dioxide addition.
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| 58. |
- Chotteau, Véronique, et al.
(författare)
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Tuning of dissolved oxygen and pH PID control parameters in large scale bioreactor by lag control
- 2012
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Ingår i: Proceedings of the 21st Annual Meeting of the European Society for Animal Cell Technology (ESACT). - Dordrecht : Springer Netherlands. - 9789400708839 ; , s. 327-330
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Konferensbidrag (refereegranskat)abstract
- A method has been developed to tune the DO and pH controller PID parameters for pilot / large scale mammalian cultivation. Our approach is to identify a model of the variable to be controlled (e.g. DO, pH) and to design several possible PID controllers based on this model. The controllers were first tested in computer simulations, followed by wet simulation and finally the best controller was tested on the real process. The approach is developed for the tuning of the DO controller of a 50 L bioreactor using microbubble continuous oxygen flow. The method, called lag control here, is based on a lead lag control design using Bode analysis where the prediction part is omitted. Experiments show that the approach results in a highly satisfactory DO control. The oxygen microbubbles were almost completely consumed before reaching the liquid surface so the oxygen flow used to maintain the DO gave an excellent indication of the cellular oxygen consumption. The control system was robust against all the perturbations, i.e. cell growth, cell bleed, addition of air-saturated fresh medium, DO set point change and a second gas sparger used to strip out the carbon dioxide. This approach was also successfully used for the tuning of a 400 L bioreactor DO controller and pH controller.
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| 59. |
- Colin, Kevin, et al.
(författare)
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Gaussian process modeling of macroscopic kinetics : a better-tailored kernel for Monod-type kinetics
- 2022
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Ingår i: 10th Vienna International Conference on Mathematical Modelling MATHMOD 2022 Vienna Austria, 27–29 July 2022. - : Elsevier BV. ; , s. 397-402
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Konferensbidrag (refereegranskat)abstract
- In bioprocesses, it is important to model the kinetics of the macroscopic rates of reactions since these are required to catch the dynamical aspects of a process. In [Wang et al. 2020], a modeling method involving Gaussian processes has been developed, using a kernel especially designed for the modeling of Monod-type kinetics (activation, inhibition, double component, neutral effect). However, as will be illustrated in this paper, when the number of training data is limited or the metabolite concentration data do not have large variations (which is generally the case for real-life data), this kernel can yield inaccurate models for the kinetics. In this paper, we develop a new kernel better tailored for the modeling of Monod-type kinetics and we show that it has good modeling performances in the case of a limited number of data. The idea is to use the particular structure of Monod-type functions in the design of the kernel, i.e., we incorporate prior knowledge in the modeling.
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| 60. |
- Colin, Kevin, et al.
(författare)
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Optimal exploration strategies for finite horizon regret minimization in some adaptive control problems
- 2023
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Ingår i: 22nd IFAC World Congress Yokohama, Japan, July 9-14, 2023. - : Elsevier BV. ; , s. 2564-2569
-
Konferensbidrag (refereegranskat)abstract
- In this work, we consider the problem of regret minimization in adaptive minimum variance and linear quadratic control problems. Regret minimization has been extensively studied in the literature for both types of adaptive control problems. Most of these works give results of the optimal rate of the regret in the asymptotic regime. In the minimum variance case, the optimal asymptotic rate for the regret is log(T) which can be reached without any additional external excitation. On the contrary, for most adaptive linear quadratic problems, it is necessary to add an external excitation in order to get the optimal asymptotic rate of √T. In this paper, we will actually show from a theoretical study, as well as, in simulations that when the control horizon is pre-specified a lower regret can be obtained with either no external excitation or a new exploration type termed immediate.
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