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  • 151.
    Ringh, Axel
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Lindquist, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. Shanghai Jiao Tong University, China .
    Spectral estimation of periodic and skew periodic random signals and approximation of spectral densities2014In: Proceedings of the 33rd Chinese Control Conference, CCC 2014, 2014, p. 5322-5327Conference paper (Refereed)
    Abstract [en]

    This paper discusses extensions of the theory of rational covariance extension to periodic and skew-periodic processes. It is also shown how these methods can be used to construct fast algorithms for approximate spectral estimation of (non-periodic) processes.

  • 152.
    Miettinen, Kaisa
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Survey of methods to visualize alternatives in multiple criteria decision making problems2014In: Or-Spektrum, ISSN 0171-6468, E-ISSN 1436-6304, Vol. 36, no 1, p. 3-37Article in journal (Refereed)
    Abstract [en]

    When solving decision problems where multiple conflicting criteria are to be considered simultaneously, decision makers must compare several different alternatives and select the most preferred one. The task of comparing multidimensional vectors is very demanding for the decision maker without any support. Different graphical visualization tools can be used to support and help the decision maker in understanding similarities and differences between the alternatives and graphical illustration is a very important part of decision support systems that are used in solving multiple criteria decision making problems. The visualization task is by no means trivial because, on the one hand, the graphics must be easy to comprehend and not too much information should be lost but, on the other hand, no extra unintentional information should be included. In this paper, we survey and analyze different ways of visualizing a small set of discrete alternatives graphically in the context of multiple criteria decision making. Some of the ways discussed are widely used and some others deserve to be brought into a wider awareness. This survey provides a starting point for all those who deal with multiple criteria decision making problems and need information of what kind of visualization techniques could be put to use in order to support the decision maker better.

  • 153. Liu, Z.
    et al.
    Wang, J.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Synchronization of unicycle robots with proximity communication networks2014In: IFAC Proceedings Volumes (IFAC-PapersOnline), IFAC Papers Online, 2014, p. 9197-9202Conference paper (Refereed)
    Abstract [en]

    In this paper synchronization of both the orientation and velocity for a group of unicycle robots is studied. It is assumed that a robot can only detect and obtain information from those robots that lie in the proximity (within certain distance). A minimum dwell time is imposed on the updating of neighborhood relation in the controller in order to avoid introducing chattering in the closed-loop system that may be caused by abrupt changes of the relation, which as a consequence implies that the neighbor information will be updated only at discrete time instants in the control. In the paper a distributed feedback control law is designed for each agent, and a sufficient condition for uniformly and independently distributed initial states is provided for reaching the synchronization, which depends on the neighborhood radius, the maximum initial translational velocity and the dwell time.

  • 154. Wei, A.
    et al.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Wang, Y.
    Tracking control of leader-follower multi-agent systems subject to actuator saturation2014In: IEEE/CAA Journal of Automatica Sinica, ISSN 2329-9266, Vol. 1, no 1, p. 84-91Article in journal (Refereed)
    Abstract [en]

    This paper addresses the tracking problem of leader-follower multi-agent systems subject to actuator saturation. A leader node or command generator is considered, which generates the desired tracking trajectory. To track such a leader, a new family of scheduled low-and-high-gain feedback controllers is designed for each follower, provided that the linear dynamic mode is asymptotically null controllable with bounded controls, and such control laws rely on the asymptotic property of a class of parametric algebraic Ricatti equations. We show that if the associated undirected graph of the system is connected, with the proposed control law, all the followers can track the leader eventually. A simulation example is finally given to illustrate the performance of the proposed control scheme.

  • 155. Zhong, J.
    et al.
    Kleijn, W. B.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Video quality improvement for multi-camera systems using camera control2014In: Proceedings of the 33rd Chinese Control Conference, CCC 2014, 2014, p. 1924-1931Conference paper (Refereed)
    Abstract [en]

    Multiview video captured by a multi-camera system has been widely used in many applications such as environmental surveillance, industrial inspection and 3D television. A multiview video contains a vast amount of data required to be transmitted and/or stored, and therefore compression is essential. However, distortion or artifacts are usually witnessed in the reconstructed multiview video after a raw multiview video is compressed at a limited bit-rate. Previous results on video compression mainly focus on the refinements of compression algorithms to improve the quality of reconstructed multiview videos. This paper uses a camera control for video capture to improve the quality of reconstructed multiview videos at a limited bit-rate. Individual cameras are controlled to adjust their pan angles and focal lengths at the video acquisition stage to compensate for the difference in feature-point locations. The optimal pan angle and focal length are designed for each camera. Experimental results validate that the proposed method capturing with camera control outperforms the conventional method capturing without camera control and the heuristic method of 'keeping the object of interest in the image center'.

  • 156.
    Bokrantz, Rasmus
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Forsgren, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    An algorithm for approximating convex Pareto surfaces based on dual techniques2013In: INFORMS journal on computing, ISSN 1091-9856, E-ISSN 1526-5528, Vol. 25, no 2, p. 377-393Article in journal (Refereed)
    Abstract [en]

    We consider the problem of approximating Pareto surfaces of convex multicriteria optimization problems by a discrete set of points and their convex combinations. Finding the scalarization parameters that optimally limit the approximation error when generating a single Pareto optimal solution is a nonconvex optimization problem. This problem can be solved by enumerative techniques but at a cost that increases exponentially with the number of objectives. We present an algorithm for solving the Pareto surface approximation problem that is practical with 10 or less conflicting objectives, motivated by an application to radiation therapy optimization. Our enumerative scheme is, in a sense, dual to a family of previous algorithms. The proposed technique retains the quality of the best previous algorithm in this class while solving fewer subproblems. A further improvement is provided by a procedure for discarding subproblems based on reusing information from previous solves. The combined effect of the enhancements is empirically demonstrated to reduce the computational expense of solving the Pareto surface approximation problem by orders of magnitude. For problems where the objectives have positive curvature, an improved bound on the approximation error is demonstrated using transformations of the initial objectives with strictly increasing and concave functions.

  • 157.
    Markdahl, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.).
    Hoppe, Jens
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Mathematics (Div.).
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Analytical solutions to feedback systems on the special orthogonal group SO(n)2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 5246-5251Conference paper (Refereed)
    Abstract [en]

    This paper provides analytical solutions to the closed-loop kinematics of two almost globally exponentially stabilizing attitude control laws on the special orthogonal group SO(n). By studying the general case we give a uniform treatment to the cases of SO(2) and SO(3), which are the most interesting dimensions for application purposes. Working directly with rotation matrices in the case of SO(3) allows us to avoid certain complications which may arise when using local and global many-to-one parameterizations. The analytical solutions provide insight into the transient behaviour of the system and are of theoretical value since they can be used to prove almost global attractiveness of the identity matrix. The practical usefulness of analytical solutions in problems of continuous time actuation subject to piece-wise unavailable or discrete time sensing are illustrated by numerical examples.

  • 158.
    Avventi, Enrico
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Lindquist, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Wahlberg, Bo
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    ARMA Identification of Graphical Models2013In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 5, p. 1167-1178Article in journal (Refereed)
    Abstract [en]

    Consider a Gaussian stationary stochastic vector process with the property that designated pairs of components are conditionally independent given the rest of the components. Such processes can be represented on a graph where the components are nodes and the lack of a connecting link between two nodes signifies conditional independence. This leads to a sparsity pattern in the inverse of the matrix-valued spectral density. Such graphical models find applications in speech, bioinformatics, image processing, econometrics and many other fields, where the problem to fit an autoregressive (AR) model to such a process has been considered. In this paper we take this problem one step further, namely to fit an autoregressive moving-average (ARMA) model to the same data. We develop a theoretical framework and an optimization procedure which also spreads further light on previous approaches and results. This procedure is then applied to the identification problem of estimating the ARMA parameters as well as the topology of the graph from statistical data.

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  • 159. Wang, Lin
    et al.
    Wang, Xiaofan
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Connectivity Preserving Flocking without Velocity Measurement2013In: Asian journal of control, ISSN 1561-8625, E-ISSN 1561-8625, Vol. 15, no 2, p. 521-532Article in journal (Refereed)
    Abstract [en]

    In this paper, we address the design of a decentralized controller for connectivity-preserving flocking, where each agent only can access to the position information of the agents within its sensing zone. An output vector, based on the position information alone, is constructed to replace the role of velocity, and some bounded attractive and repulsive forces are integrated together to design the controller. We prove that the controller not only synchronizes all agents in a stable formation, but also enables collision avoidance and connectivity preserving all of the time, when the initial condition meets certain requirements. Moreover, a leader-follower method is used to guide the group to a desired direction, where the followers can sense the leader only if the distance between them is less than the communication radius.

  • 160. Wei, Airong
    et al.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Wang, Yuzhen
    Consensus of Linear Multi-Agent Systems Subject to Actuator Saturation2013In: International Journal of Control, Automation and Systems, ISSN 1598-6446, E-ISSN 2005-4092, Vol. 11, no 4, p. 649-656Article in journal (Refereed)
    Abstract [en]

    This paper is aimed at studying the consensus of linear multi-agent systems subject to actuator saturation. In order to solve the consensus problem, a new family of scheduled low-and-high-gain decentralized control laws are designed, provided that the dynamics of each agent is asymptotically null controllable with bounded controls, and such control laws rely on the asymptotic property of a class of parametric algebraic Ricatti equations. It is shown that the consensus of the systems with connected and fixed topology can be achieved semi-globally asymptotically via the local error low-and-high-gain feedback. An illustrative example with simulations shows that our method as well as control protocols is effective for the consensus of the linear multi-agent systems subject to actuator saturation.

  • 161.
    Bokrantz, Rasmus
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Fredriksson, Albin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Controlling robustness and conservativeness in multicriteria intensity-modulated proton therapy optimization under uncertainty2013Report (Other academic)
  • 162.
    Fredriksson, Albin
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Bokrantz, Rasmus
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Deliverable navigation for multicriteria IMRT treatment planning by combining shared and individual apertures2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 21, p. 7683-7697Article in journal (Refereed)
    Abstract [en]

    We consider the problem of deliverable Pareto surface navigation for step-and-shoot intensity-modulated radiation therapy. This problem amounts to calculation of a collection of treatment plans with the property that convex combinations of plans are directly deliverable. Previous methods for deliverable navigation impose restrictions on the number of apertures of the individual plans, or require that all treatment plans have identical apertures. We introduce simultaneous direct step-and-shoot optimization of multiple plans subject to constraints that some of the apertures must be identical across all plans. This method generalizes previous methods for deliverable navigation to allow for treatment plans with some apertures from a collective pool and some apertures that are individual. The method can also be used as a post-processing step to previous methods for deliverable navigation in order to improve upon their plans. By applying the method to subsets of plans in the collection representing the Pareto set, we show how it can enable convergence toward the unrestricted (non-navigable) Pareto set where all apertures are individual.

  • 163.
    Fredriksson, Albin
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Bokrantz, Rasmus
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Deliverable navigation for multicriteria intensity-modulated radiation therapy planning by combining shared and individual apertures2013Report (Other academic)
  • 164.
    Svanberg, Krister
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Svärd, Henrik
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Density filters for topology optimization based on the Pythagorean means2013In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 48, no 5, p. 859-875Article in journal (Refereed)
    Abstract [en]

    In topology optimization, restriction methods are needed to prohibit mesh dependent solutions and enforce length scale on the optimized structure. This paper presents new restriction methods in the form of density filters. The proposed filters are based on the geometric and harmonic means, respectively, and possess properties that could be of interest in topology optimization, for example the possibility to obtain solutions which are almost completely black and white. The article presents the new filters in detail, and several numerical test examples are used to investigate the properties of the new filters compared to filters existing in the literature. The results show that the new filters in several cases provide solutions with competitive objective function values using few iterations, but also, and perhaps more importantly, in many cases, different filters make the optimization converge to different solutions with close to equal value. A variety of filters to choose from will hence provide the user with several suggested optimized structures, and the new filters proposed in this work may certainly provide interesting alternatives.

  • 165.
    Bokrantz, Rasmus
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Distributed approximation of Pareto surfaces in multicriteria radiation therapy treatment planning2013In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 58, no 11, p. 3501-3516Article in journal (Refereed)
    Abstract [en]

    We consider multicriteria radiation therapy treatment planning by navigationover the Pareto surface, implemented by interpolation between discretetreatment plans. Current state of the art for calculation of a discreterepresentation of the Pareto surface is to sandwich this set between inner andouter approximations that are updated one point at a time. In this paper, wegeneralize this sequential method to an algorithm that permits parallelization.The principle of the generalization is to apply the sequential method to anapproximation of an inexpensive model of the Pareto surface. The informationgathered from the model is sub-sequently used for the calculation of pointsfrom the exact Pareto surface, which are processed in parallel. The model isconstructed according to the current inner and outer approximations, and givena shape that is difficult to approximate, in order to avoid that parts of the Paretosurface are incorrectly disregarded. Approximations of comparable quality tothose generated by the sequential method are demonstrated when the degree ofparallelization is up to twice the number of dimensions of the objective space.For practical applications, the number of dimensions is typically at least five,so that a speed-up of one order of magnitude is obtained.

  • 166. Song, W.
    et al.
    Hong, Y.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Distributed relative attitude formation control of multiple rigid-body agents with switching topologies2013In: Chinese Control Conf., CCC, 2013, p. 7125-7130Conference paper (Refereed)
    Abstract [en]

    This paper formulates and investigates the relative attitude formation problem of a group of rigid-body agents with directed and switching sensing topologies. Each agent can only measure the attitudes of its neighbors from the body-fixed frame of the agent. Based on the axis-angle representation of the error attitude, we present a distributed protocol and then show how to achieve the desired relative attitude formation, provided that the inter-agent topology graph is jointly connected and each relative attitude between agents initially is contained within a geodesic ball of radius less than 2p/3 centered at its desired relative attitude. A numerical example is provided to show the effectiveness of the proposed angular velocity control scheme.

  • 167. Wei, A.
    et al.
    Wang, Y.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Disturbance tolerance and H∞ Control of Port-Controlled Hamiltonian systems in the presence of actuator saturation2013In: Chinese Control Conf., CCC, 2013, p. 828-833Conference paper (Refereed)
    Abstract [en]

    This paper investigates the disturbance tolerance and H∞ control of multi-input Port-Controlled Hamiltonian (PCH) systems in the presence of actuator saturation which may be not open-loop stable. A simple condition is derived under which trajectories starting from the origin will remain inside an ellipsoid. The disturbance tolerance ability of the closed-loop system under a given feedback control law is measured by the size of this ellipsoid. Based on the above mentioned condition, the problem of disturbance tolerance can be expressed in the form of the linear matrix inequalities (LMIs) optimization problem with constraints. In addition, an H∞ control approach is presented to attenuate the disturbances, and disturbance rejection ability in terms of L2 gain is also determined by the solution of an LMI optimization problem. Study of an illustrative example with simulations shows the effectiveness of the methods proposed.

  • 168.
    Wei, Airong
    et al.
    Shandong Univ, Sch Control Sci & Engn, Jinan 250061, Peoples R China..
    Wang, Yuzhen
    Shandong Univ, Sch Control Sci & Engn, Jinan 250061, Peoples R China..
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. Royal Inst Technol, Optimizat & Syst Theory, S-10044 Stockholm, Sweden..
    Disturbance Tolerance and H-infinity Control of Port-Controlled Hamiltonian Systems in the Presence of Actuator Saturation2013In: 2013 32ND CHINESE CONTROL CONFERENCE (CCC), IEEE , 2013, p. 828-833Conference paper (Refereed)
    Abstract [en]

    This paper investigates the disturbance tolerance and H-infinity control of multi-input Port-Controlled Hamiltonian (PCH) systems in the presence of actuator saturation which may be not open-loop stable. A simple condition is derived under which trajectories starting from the origin will remain inside an ellipsoid. The disturbance tolerance ability of the closed-loop system under a given feedback control law is measured by the size of this ellipsoid. Based on the above mentioned condition, the problem of disturbance tolerance can be expressed in the form of the linear matrix inequalities (LMIs) optimization problem with constraints. In addition, an H-infinity control approach is presented to attenuate the disturbances, and disturbance rejection ability in terms of L-2 gain is also determined by the solution of an LMI optimization problem. Study of an illustrative example with simulations shows the effectiveness of the methods proposed.

  • 169. Montijano, Eduardo
    et al.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Saguees, Carlos
    Epipolar Visual Servoing for Multirobot Distributed Consensus2013In: IEEE Transactions on robotics, ISSN 1552-3098, E-ISSN 1941-0468, Vol. 29, no 5, p. 1212-1225Article in journal (Refereed)
    Abstract [en]

    In this paper, we give a distributed solution to the problem of making a team of nonholonomic robots reach consensus about their orientations using monocular cameras. We consider a scheme where the motions of the robots are decided using nearest neighbor rules. Each robot is equipped with a camera and can only exchange visual information with a subset of the other robots. The main contribution of this paper is a new controller that uses the epipoles that are computed from the images provided by neighboring robots, eventually reaching consensus in their orientations without the necessity of directly observing each other. In addition, the controller only requires a partial knowledge of the calibration of the cameras in order to achieve the desired configuration. We also demonstrate that the controller is robust to changes in the topology of the network and we use this robustness to propose strategies to reduce the computational load of the robots. Finally, we test our controller in simulations using a virtual environment and with real robots moving in indoor and outdoor scenarios.

  • 170. Hu, J. -P
    et al.
    Liu, Z. -X
    Wang, J. -H
    Wang, L.
    Hu, Xiao-Ming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Estimation, intervention and interaction of multi-agent systems2013In: Zidonghua Xuebao/Acta Automatica Sinica, ISSN 0254-4156, Vol. 39, no 11, p. 1796-1804Article in journal (Refereed)
    Abstract [en]

    In this paper we provide a brief survey on recent research on multi-agent systems. We focus on results in three areas of the research, namely, estimation and filtering, intervention by external means, and interactive control.

  • 171. Zhong, J.
    et al.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Kleijn, W. B.
    Gaze and zoom controls for visual tracking2013In: Chinese Control Conf., CCC, 2013, p. 811-816Conference paper (Refereed)
    Abstract [en]

    Visual tracking is widely used in many applications such as security surveillance, robot navigation and industrial inspection. In this paper gaze and zoom controls for visual tracking with a monocular camera are considered. The gaze control keeps the camera pointing to the centroid of a moving object of interest to keep the object centered in each image. The zoom control adjusts the camera focal length to preserve the object at a desired constant image size. Both gaze control and zoom control are explicitly designed with simple formulas. Simulation results validate the controls.

  • 172.
    Möller, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Centres, Center for Industrial and Applied Mathematics, CIAM.
    Jönsson, Ulf T.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Input-Output Analysis of Power Control in Wireless Networks2013In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 4, p. 834-846Article in journal (Refereed)
    Abstract [en]

    In a wireless communication network, different users share a common resource. An objective of radio resource management is to assign the resources in an effective way among the users. Power control is an important component in this setting that has been extensively studied over the last two decades. In many real networks, there are inherent time delays due to filtering of signals and control signalling. Time delays can affect stability and convergence properties of the power control algorithms. We therefore consider power control laws of higher order to include models with delays and delay compensation. The main contribution of this paper is to exploit more structure of the interference feedback to prove less conservative conditions for system stability. Our primary tool to reduce conservativeness is to use scalings in an input-output framework for stability analysis.

  • 173.
    Nilsson, Petter
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Boscain, U.
    Sigalotti, M.
    Newling, J.
    Invariant sets of defocused switched systems2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 5987-5992Conference paper (Refereed)
    Abstract [en]

    We consider affine switched systems as perturbations of linear ones, the equilibria playing the role of perturbation parameters. We study the stability properties of an affine switched system under arbitrary switching, assuming that the corresponding linear system is uniformly exponentially stable. It turns out that the affine system admits a minimal invariant set , whose properties we investigate. In the two dimensional bi-switched case when both subsystems have nonreal eigenvalues we are able to characterize completely and to prove that all trajectories of the system converge to . We also explore the behavior of minimal-time trajectories in by constructing optimal syntheses.

  • 174.
    Olsson, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Georgiev, P.
    Pardalos, P. M.
    Kernel Principal Component Analysis: Applications, implementation and comparison2013In: Models, Algorithms, and Technologies for Network Analysis: Proceedings of the Second International Conference on Network Analysis, Springer, 2013, p. 127-147Conference paper (Refereed)
    Abstract [en]

    Kernel Principal Component Analysis (KPCA) is a dimension reduction method that is closely related to Principal Component Analysis (PCA). This report gives an overview of kernel PCA and presents an implementation of the method in MATLAB. The implemented method is tested in a transductive setting on two data bases. Two methods for labeling data points are considered, the nearest neighbor method and kernel regression, together with some possible improvements of the methods.

  • 175.
    Fredriksson, Albin
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Forsgren, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hårdemark, Björn
    Maximizing the probability of satisfying the planning criteria in radiation therapy under setup uncertainty2013Manuscript (preprint) (Other academic)
    Abstract [en]

    We consider intensity-modulated photon and proton therapy in the presence of setup uncertainty. The uncertainty is accounted for by worst case optimization, in which the planning criteria are constrained to be satisfied under all setup errors from a specified set. To handle that the set may contain errors under which the planning criteria cannot be satisfied, a method is introduced that includes the magnitudes of the setup errors within the set as variables in the optimization, which is aimed at making these magnitudes as large as possible (within specified bounds) while satisfying all planning criteria under the errors. This is equivalent to maximizing the probability of satisfying the planning criteria.

    The method is studied with respect to photon and proton therapy applied to a prostate case and a lung case, and compared to worst case optimization with respect to an a priori determined set of errors. For both modalities, slight reductions of the magnitudes of the considered setup errors resulted in plans that satisfied a substantially larger number of planning criteria under the retracted errors, and also a larger number of criteria under the a priori errors: for the prostate case, the plans accounting for retracted errors satisfied 1.5 (photons) and 1.2 (protons) times as many planning criteria as the method accounting for a priori errors, and for the lung case, the numbers were 2.7 (photons) and 1.6 (protons).

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    scenariooptimization
  • 176.
    Bokrantz, Rasmus
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Multicriteria optimization for managing tradeoffs in radiation therapy treatment planning2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Treatment planning for radiation therapy inherently involves tradeoffs, such as between tumor control and normal tissue sparing, between time-efficiency and dose quality, and between nominal plan quality and robustness. The purpose of this thesis is to develop methods that can facilitate decision making related to such tradeoffs. The main focus of the thesis is on multicriteria optimization methods where a representative set of treatment plans are first calculated and the most appropriate plan contained in this representation then selected by the treatment planner through continuous interpolation between the precalculated alternatives. These alternatives constitute a subset of the set of Pareto optimal plans, meaning plans such that no criterion can be improved without a sacrifice in another.

    Approximation of Pareto optimal sets is first studied with respect to fluence map optimization for intensity-modulated radiation therapy. The approximation error of a discrete representation is minimized by calculation of points one at the time at the location where the distance between an inner and outer approximation of the Pareto set currently attains its maximum. A technique for calculating this distance that is orders of magnitude more efficient than the best previous method is presented. A generalization to distributed computational environments is also proposed.

    Approximation of Pareto optimal sets is also considered with respect to direct machine parameter optimization. Optimization of this form is used to calculate representations where any interpolated treatment plan is directly deliverable. The fact that finite representations of Pareto optimal sets have approximation errors with respect to Pareto optimality is addressed by a technique that removes these errors by a projection onto the exact Pareto set. Projections are also studied subject to constraints that prevent the dose-volume histogram from deteriorating.

    Multicriteria optimization is extended to treatment planning for volumetric-modulated arc therapy and intensity-modulated proton therapy. Proton therapy plans that are robust against geometric errors are calculated by optimization of the worst case outcome. The theory for multicriteria optimization is extended to accommodate this formulation. Worst case optimization is shown to be preferable to a previous more conservative method that also protects against uncertainties which cannot be realized in practice.

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  • 177.
    Lindquist, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Engineering Sciences (SCI), Centres, Center for Industrial and Applied Mathematics, CIAM. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. Shanghai Jiao Tong University, Shanghai, China .
    Masiero, C.
    Picci, G.
    On the multivariate circulant rational covariance extension problem2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 7155-7161Conference paper (Refereed)
    Abstract [en]

    Partial stochastic realization of periodic processes from finite covariance data leads to the circulant rational covariance extension problem and bilateral ARMA models. In this paper we present a convex optimization-based theory for this problem that extends and modifies previous results by Carli, Ferrante, Pavon and Picci on the AR solution, which have been successfully applied to image processing of textures. We expect that our present results will provide an enhancement of these procedures.

  • 178.
    Yang, Yuecheng
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Dimarogonas, Dimos V.
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Optimal leader-follower control for crowd evacuation2013In: 2013 IEEE 52nd Annual Conference on Decision and Control (CDC), IEEE conference proceedings, 2013, p. 2769-2774Conference paper (Refereed)
    Abstract [en]

    Crowd evacuation has become a primary safety issue in many public places in a metropolitan area. Experts from different fields have worked on modeling and designing evacuating policies by using different tools and methods. In this paper, an optimal control approach is used to derive guiding strategies for the rescue agents under different circumstances. Various optimal control problems are formulated to handle different assumptions of the scenario. Both the analytic solutions and the numerical simulation confirm the efficiency of this approach, which in turn can be potentially used as decisionmaking support in practical applications.

  • 179.
    Fredriksson, Albin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Robust optimization of radiation therapy accounting for geometric uncertainty2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Geometric errors may compromise the quality of radiation therapy treatments. Optimization methods that account for errors can reduce their effects.

    The first paper of this thesis introduces minimax optimization to account for systematic range and setup errors in intensity-modulated proton therapy. The minimax method optimizes the worst case outcome of the errors within a given set. It is applied to three patient cases and shown to yield improved target coverage robustness and healthy structure sparing compared to conventional methods using margins, uniform beam doses, and density override. Information about the uncertainties enables the optimization to counterbalance the effects of errors.

    In the second paper, random setup errors of uncertain distribution---in addition to the systematic range and setup errors---are considered in a framework that enables scaling between expected value and minimax optimization. Experiments on a phantom show that the best and mean case tradeoffs between target coverage and critical structure sparing are similar between the methods of the framework, but that the worst case tradeoff improves with conservativeness.

    Minimax optimization only considers the worst case errors. When the planning criteria cannot be fulfilled for all errors, this may have an adverse effect on the plan quality. The third paper introduces a method for such cases that modifies the set of considered errors to maximize the probability of satisfying the planning criteria. For two cases treated with intensity-modulated photon and proton therapy, the method increased the number of satisfied criteria substantially. Grasping for a little less sometimes yields better plans.

    In the fourth paper, the theory for multicriteria optimization is extended to incorporate minimax optimization. Minimax optimization is shown to better exploit spatial information than objective-wise worst case optimization, which has previously been used for robust multicriteria optimization.

    The fifth and sixth papers introduce methods for improving treatment plans: one for deliverable Pareto surface navigation, which improves upon the Pareto set representations of previous methods; and one that minimizes healthy structure doses while constraining the doses of all structures not to deteriorate compared to a reference plan, thereby improving upon plans that have been reached with too weak planning goals.

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  • 180. Cantoni, Michael
    et al.
    Jönsson, Ulf T.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Khong, Sei Zhen
    Robust Stability Analysis For Feedback Interconnections Of Time-Varying Linear Systems2013In: SIAM Journal of Control and Optimization, ISSN 0363-0129, E-ISSN 1095-7138, Vol. 51, no 1, p. 353-379Article in journal (Refereed)
    Abstract [en]

    Feedback interconnections of causal linear systems are studied in a continuous-time setting. The developments include a linear time-varying (LTV) generalization of Vinnicombe's nu-gap metric and an integral-quadratic-constraint-based robust L-2-stability theorem for uncertain feedback interconnections of potentially open-loop unstable systems. These main results are established in terms of Toeplitz-Wiener-Hopf and Hankel operators, and the Fredholm index, for a class of causal linear systems with the following attributes: (i) a system graph (i.e., subspace of L-2 input-output pairs) admits normalized strong right (i.e., image) and left (i.e., kernel) representations, and (ii) the corresponding Hankel operators are compact. These properties are first verified for stabilizable and detectable LTV state-space models to initially motivate the abstract formulation, and subsequently verified for frequency-domain multiplication by constantly proper Callier-Desoer transfer functions in analysis that confirms consistency of the developments with the time-invariant theory. To conclude, the aforementioned robust stability theorem is applied in an illustrative example concerning the feedback interconnection of distributed-parameter systems over a network with time-varying gains.

  • 181. Rowe, W.
    et al.
    Karlsson, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Xu, L.
    Glentis, G. -O
    Li, J.
    SAR imaging in the presence of spectrum notches via fast missing data IAA2013In: Automatic Target Recognition XXIII, SPIE - International Society for Optical Engineering, 2013, p. UNSP 87440X-Conference paper (Refereed)
    Abstract [en]

    A synthetic aperture radar system operating in congested frequency bands suffers from radio frequency inter- ference (RFI) from narrowband sources. When RFI interference is suppressed by frequency notching, gaps are introduced into the fast time phase history. This results in a missing data spectral estimation problem, where the missing data increases sidelobe energy and degrades image quality. The adaptive spectral estimation method Iterative Adaptive Approach (IAA) has been shown to provide higher resolution and lower sidelobes than comparable methods, but at the cost of higher computationally complexity. Current fast IAA algorithms reduce the computational complexity using Toeplitz/Vandermonde structures, but are not applicable for missing data cases because these structures are lost. When the number of missing data samples is small, which often is the case in SAR with RFI, we use a low rank completion to restore the Toeplitz/Vandermonde structures. We show that the computational complexity of the proposed algorithm is considerably lower than the state-of-the-art and demonstrate the utility on a simulated frequency notched SAR imaging problem.

  • 182.
    Lindquist, Anders G.
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Engineering Sciences (SCI), Centres, Center for Industrial and Applied Mathematics, CIAM. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Picci, Giorgio
    The Circulant Rational Covariance Extension Problem: The Complete Solution2013In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 11, p. 2848-2861Article in journal (Refereed)
    Abstract [en]

    The rational covariance extension problem to determine a rational spectral density given a finite number of covariance lags can be seen as a matrix completion problem to construct an infinite-dimensional positive-definite Toeplitz matrix the northwest corner of which is given. The circulant rational covariance extension problem considered in this paper is a modification of this problem to partial stochastic realization of periodic stationary process, which are better represented on the discrete unit circle Z(2N) rather than on the discrete real line Z. The corresponding matrix completion problem then amounts to completing a finite-dimensional Toeplitz matrix that is circulant. Another important motivation for this problem is that it provides a natural approximation, involving only computations based on the fast Fourier transform, for the ordinary rational covariance extension problem, potentially leading to an efficient numerical procedure for the latter. The circulant rational covariance extension problem is an inverse problem with infinitely many solutions in general, each corresponding to a bilateral ARMA representation of the underlying periodic process. In this paper, we present a complete smooth parameterization of all solutions and convex optimization procedures for determining them. A procedure to determine which solution that best matches additional data in the form of logarithmic moments is also presented.

  • 183.
    Georgiou, Tryphon
    et al.
    University of Minnesota, United States.
    Lindquist, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Engineering Sciences (SCI), Centres, Center for Industrial and Applied Mathematics, CIAM. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    The separation principle in stochastic control, redux2013In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 58, no 10, p. 2481-2494Article in journal (Refereed)
    Abstract [en]

    Over the last 50 years, a steady stream of accounts have been written on the separation principle of stochastic control. Even in the context of the linear-quadratic regulator in continuous time with Gaussian white noise, subtle difficulties arise, unexpected by many, that are often overlooked. In this paper we propose a new framework for establishing the separation principle. This approach takes the viewpoint that stochastic systems are well-defined maps between sample paths rather than stochastic processes per se and allows us to extend the separation principle to systems driven by martingales with possible jumps. While the approach is more in line with "real-life" engineering thinking where signals travel around the feedback loop, it is unconventional from a probabilistic point of view in that control laws for which the feedback equations are satisfied almost surely, and not deterministically for every sample path, are excluded.

  • 184. Björnsson, H.
    et al.
    Magnusson, Sverker
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Arason, P.
    Petersen, G. N.
    Velocities in the plume of the 2010 Eyjafjallajökull eruption2013In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 118, no 20, p. 11698-11711Article in journal (Refereed)
    Abstract [en]

    The eruption of the Icelandic volcano Eyjafjallajökull in the spring of 2010 lasted for 39 days with an explosive phase (14-18 April), an effusive phase (18 April to 4 May) and a phase with renewed explosive activity (5-17 May). Images every 5 s from a camera mounted 34 km from the volcano are available for most of the eruption. Applying the maximum cross-correlation method (MCC) on these images, the velocity structure of the eruption cloud has been mapped in detail for four time intervals covering the three phases of the eruption. The results show that on average there are updrafts in one part of the cloud and lateral motion or downdrafts in another. Even within the updraft part, there are alternating motions of strong updrafts, weak updrafts, and downward motion. These results show a highly variable plume driven by intermittent explosions. The results are discussed in the context of integral plume models and in terms of elementary parcel theory. Key Points Velocities in a volcanic cloud based on analysis of image data from the eruption Velocities in the eruption cloud are inhomogeneous and updrafts intermittent Intermittent updrafts are important for the dynamics and the lofting of ash.

  • 185.
    Fredriksson, Albin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    A characterization of robust radiation therapy treatment planning methods-from expected value to worst case optimization2012In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 39, no 8, p. 5169-5181Article in journal (Refereed)
    Abstract [en]

    Purpose: To characterize a class of optimization formulations used to handle systematic and random errors in radiation therapy, and to study the differences between the methods within this class. Methods: The class of robust methods that can be formulated as minimax stochastic programs is studied. This class generalizes many previously used methods, ranging between optimization of the expected and the worst case objective value. The robust methods are used to plan intensity-modulated proton therapy (IMPT) treatments for a case subject to systematic setup and range errors, random setup errors with and without uncertain probability distribution, and combinations thereof. As reference, plans resulting from a conventional method that uses a margin to account for errors are shown. Results: For all types of errors, target coverage robustness increased with the conservativeness of the method. For systematic errors, best case organ at risk (OAR) doses increased and worst case doses decreased with the conservativeness. Accounting for random errors of fixed probability distribution resulted in heterogeneous dose. The heterogeneities were reduced when uncertainty in the probability distribution was accounted for. Doing so, the OAR doses decreased with the conservativeness. All robust methods studied resulted in more robust target coverage and lower OAR doses than the conventional method. Conclusions: Accounting for uncertainties is essential to ensure plan quality in complex radiation therapy such as IMPT. The utilization of more information than conventional in the optimization can lead to robust target coverage and low OAR doses. Increased target coverage robustness can be achieved by more conservative methods.

  • 186. Wang, L.
    et al.
    Markdahl, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Kragic, Danica
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    A high level decentralized tracking algorithm for three manipulators subject to motion constraints2012In: Intelligent Control and Automation (WCICA), 2012 10th World Congress on, IEEE , 2012, p. 1920-1924Conference paper (Refereed)
    Abstract [en]

    This paper considers a tracking problem for three manipulators grasping a rigid object. The control objective is to coordinate the movements of the manipulators using local information in order to align the object attitude with a desired rest attitude and the object position with a time parameterized reference trajectory. The object rigidity is modelled as a constraint on the motion of the end-effectors saying that the distance between any pair of end-effectors must be constant in time. The control law consists of a rotational part and a translational part. The translational part also incorporates a linear observer of the reference trajectory. We prove stability and illustrate the system dynamics by simulation.

  • 187.
    Miettinen, Kaisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Nikulin, Yury
    University of Turku, Finland.
    Mäkelä, Marko M
    Department of Mathematics, University of Jyväskylä.
    A new achievement scalarizing function based on parameterization in multiobjective optimization2012In: OR Spectrum, ISSN 0171-6468, Vol. 34, no 1, p. 69-87Article in journal (Refereed)
    Abstract [en]

    This paper addresses a general multiobjective optimization problem. One of the most widely used methods of dealing with multiple conflicting objectives consists of constructing and optimizing a so-called achievement scalarizing function (ASF) which has an ability to produce any Pareto optimal or weakly/properly Pareto optimal solution. The ASF minimizes the distance from the reference point to the feasible region, if the reference point is unattainable, or maximizes the distance otherwise. The distance is defined by means of some specific kind of a metric introduced in the objective space. The reference point is usually specified by a decision maker and contains her/his aspirations about desirable objective values. The classical approach to constructing an ASF is based on using the Chebyshev metric L 8. Another possibility is to use an additive ASF based on a modified linear metric L 1. In this paper, we propose a parameterized version of an ASF. We introduce an integer parameter in order to control the degree of metric flexibility varying from L 1 to L 8. We prove that the parameterized ASF supports all the Pareto optimal solutions. Moreover, we specify conditions under which the Pareto optimality of each solution is guaranteed. An illustrative example for the case of three objectives and comparative analysis of parameterized ASFs with different values of the parameter are given. We show that the parameterized ASF provides the decision maker with flexible and advanced tools to detect Pareto optimal points, especially those whose detection with other ASFs is not straightforward since it may require changing essentially the reference point or weighting coefficients as well as some other extra computational efforts.

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  • 188.
    Wang, Yuquan
    et al.
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    A transformation of the Position Based Visual Servoing Problem into a convex optimization problem2012In: 2012 IEEE 51st Annual Conference on Decision and Control (CDC), IEEE , 2012, p. 5673-5678Conference paper (Refereed)
    Abstract [en]

    Here we address the problem of moving a camera from an initial pose to a final pose. The trajectory between the two poses is subject to constraints on the camera motion and the visibility, where we have bounds on the allowed velocities and accelerations of the camera and require that a set of point features are visible for the camera. We assume that the pose is possible to retrieve from the observations of the point features, i.e., we have a Position Based Visual Servoing Problem with constraints. We introduce a two step method that transforms the problem into a convex optimization problem with linear constraints. In the first step the rotational motion is restricted to be of a certain type. This restriction allows us to retrieve an explicit solution of the rotational motion that is optimal in terms of minimizing geodesic distance. Furthermore, this restriction guarantees that the rotational motion satisfies the constraints. Using the explicit solution, we can formulate a convex optimization problem for the translational motion, where we include constraints on workspace and visibility.

  • 189.
    Miettinen, Kaisa
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    A Two-Slope Achievement Scalarizing Function for Interactive Multiobjective Optimization2012In: Computers & Operations Research, ISSN 0305-0548, E-ISSN 1873-765X, Vol. 39, no 7, p. 1673-1681Article in journal (Refereed)
    Abstract [en]

    The use of achievement (scalarizing) functions in interactive multiobjective optimization methods is very popular, as indicated by the large number of algorithmic and applied scientific papers that use this approach. Key parameters in this approach are the reference point, which expresses desirable objective function values for the decision maker, and weights. The role of the weights can range from purely normalizing to fully preferential parameters that indicate the relative importance given by the decision maker to the achievement of each reference value. Technically, the influence of the weights in the solution generated by the achievement scalarizing function is different, depending on whether the reference point is achievable or not. Besides, from a psychological point of view, decision makers also react in a different way, depending on the achievability of the reference point. For this reason, in this work, we introduce the formulation of a new achievement scalarizing function with two different weight vectors, one for achievable reference points, and the other one for unachievable reference points. The new achievement scalarizing function is designed so that an appropriate weight vector is used in each case, without having to carry out any a priori achievability test. It allows us to reflect the decision maker’s preferences in a better way as a part of an interactive solution method, and this can cause a quicker convergence of the method. The computational efficiency of this new formulation is shown in several test examples using different reference points.

  • 190. Wei, A.
    et al.
    Wang, Y.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Adaptive robust parallel simultaneous stabilization of two uncertain port-controlled Hamiltonian systems subject to input saturation2012In: Chinese Control Conference, CCC, IEEE , 2012, p. 727-732Conference paper (Refereed)
    Abstract [en]

    This paper deals with the problem of adaptive robust parallel simultaneous stabilization(ARPSS) of two uncertain nonlinear port-controlled Hamiltonian systems subject to input saturation, and proposes a number of results on the control design. Firstly, an adaptive H∞ control design approach is presented by using both the dissipative Hamiltonian structural and saturated actuator properties for the case that there are both parametric uncertainties and external disturbances in the systems. Secondly, the results obtained for Hamiltonian systems are applied to ARPSS problem for two uncertain nonlinear affine systems subject to input saturation, and several interesting results are obtained. Finally, study of an example with simulations shows that the adaptive controller proposed in this paper is effective.

  • 191. Wei, Airong
    et al.
    Wang, Yuzhen
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Adaptive simultaneous stabilization of two Port-Controlled Hamiltonian systems subject to actuator saturation2012In: Intelligent Control and Automation (WCICA), 2012 10th World Congress on, IEEE , 2012, p. 1767-1772Conference paper (Refereed)
    Abstract [en]

    This paper investigates the adaptive parallel simultaneous stabilization (APSS) of two multi-input nonlinear Port-Controlled Hamiltonian (PCH) systems subject to actuator saturation, and proposes a number of results on the design of the APSS controllers. Using both the dissipative Hamiltonian structural and saturated actuator properties, the two systems are combined to generate an augmented PCH system subject to actuator saturation, with which some results on the control designs are then obtained. Study of an illustrative example with simulations shows that the APSS controller obtained in this paper is effective.

  • 192.
    Fallgren, Mikael
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    An Optimization Approach to Joint Cell, Channel and Power Allocation in Multicell Relay Networks2012In: IEEE Transactions on Wireless Communications, ISSN 1536-1276, E-ISSN 1558-2248, Vol. 11, no 8, p. 2868-2875Article in journal (Refereed)
    Abstract [en]

    In this paper, we consider joint resource allocation of a multicell OFDMA-based networks, with fixed two-hop decode-and-forward relay stations. A joint cell, channel and power allocation problem is formulated as an overall optimization problem, where the objective is to maximize the minimum user throughput. Based on previous complexity results for the setting without relays, the overall optimization problem is shown not to be approximable, unless P is equal to NP. We propose a method for solving this challenging problem. First, a feasible cell allocation is obtained, either via greedy allocation or an exhaustive search. Thereafter, the channel and power allocations are alternately updated, either using a heuristic or optimization-based approach while holding the other two allocations fixed. These alternating channel and power allocations are repeated until no further improvement is obtained. The impact of relay stations is investigated by considering a model with relays as well as one without relays. The simulations show that substantial performance improvement can be made by introducing relays. They also indicate, somewhat surprisingly, that the heuristic channel allocation leads to better overall solutions than the corresponding optimization approach in the relay setting. The optimization-based power allocation significantly outperforms the heuristic power approach with as well as without relay stations.

  • 193.
    Fredriksson, Albin
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Automated improvement of radiation therapy treatment plans by optimization under reference dose constraints2012In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 57, no 23, p. 7799-7811Article in journal (Refereed)
    Abstract [en]

    A method is presented that automatically improves upon previous treatment plans by optimization under reference dose constraints. In such an optimization, a previous plan is taken as reference and a new optimization is performed toward some goal, such as minimization of the doses to healthy structures under the constraint that no structure can become worse off than in the reference plan. Two types of constraints that enforce this are discussed: either each voxel or each dose-volume histogram of the improved plan must be at least as good as in the reference plan. These constraints ensure that the quality of the dose distribution cannot deteriorate, something that constraints on conventional physical penalty functions do not. To avoid discontinuous gradients, which may restrain gradient-based optimization algorithms, the positive part operators that constitute the optimization functions are regularized. The method was applied to a previously optimized plan for a C-shaped phantom and the effects of the choice of regularization parameter were studied. The method resulted in reduced integral dose and reduced doses to the organ at risk while maintaining target homogeneity. It could be used to improve upon treatment plans directly or as a means of quality control of plans.

  • 194. Laukkanen, Timo
    et al.
    Tveit, Tor-Martin
    Ojalehto, Vesa
    Miettinen, Kaisa
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Fogelholm, Carl-Johan
    Bilevel heat exchanger network synthesis with an interactive multi-objective optimization method2012In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 48, p. 301-316Article in journal (Refereed)
    Abstract [en]

    Heat exchanger network synthesis (HENS) has been an active research area for more than 40 years because well-designed heat exchanger networks enable heat recovery in process industries in an energy-and cost-efficient manner. Due to ever increasing global competition and need to decrease the harmful effects done on the environment, there still is a continuous need to improve the heat exchanger networks and their synthesizing methods. In this work we present a HENS method that combines an interactive multi-objective optimization method with a simultaneous bilevel HENS method, where the bilevel part of the method is based on grouping of process streams and building aggregate streams from the grouped streams. This is done in order to solve medium-sized industrial HENS problems efficiently with good final solutions. The combined method provides an opportunity to solve HENS problems efficiently also regarding computing effort and at the same time optimizing all the objectives of HENS simultaneously and in a genuine multi-objective manner without using weighting factors. This enables the designer or decision maker to be in charge of the design procedure and guide the search into areas that the decision maker is most interested in. Two examples are solved with the proposed method. The purpose of the first example is to help in illustrating the steps in the overall method. The second example is obtained from the literature.

  • 195.
    Miettinen, Kaisa
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Ruuska, Sauli
    University of Jyväskylä.
    Wiecek, Margaret M
    Clemson University.
    Connections Between Single-Level and Bilevel Multiobjective Optimization2012In: Journal of Optimization Theory and Applications, ISSN 0022-3239, E-ISSN 1573-2878, Vol. 153, no 1, p. 60-74Article in journal (Refereed)
    Abstract [en]

    The relationship between bilevel optimization and multiobjective optimization has been studied by several authors and there have been repeated attempts to establish a link between the two. We unify the results from the literature and generalize them for bilevel multiobjective optimization. We formulate sufficient conditions for an arbitrary binary relation to guarantee equality between the efficient set produced by the relation and the set of optimal solutions to a bilevel problem. In addition, we present specially structured bilevel multiobjective optimization problems motivated by real-life applications and an accompanying binary relation permitting their reduction to single-level multiobjective optimization problems.

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  • 196.
    Jönsson, Ulf T.
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Kao, C. -Y
    Consensus of heterogeneous LTI agents2012In: IEEE Transactions on Automatic Control, ISSN 0018-9286, E-ISSN 1558-2523, Vol. 57, no 8, p. 2133-2139Article in journal (Refereed)
    Abstract [en]

    A robust stability criterion is derived for a class of higher order consensus algorithms. In distinction to the standard consensus problem where the dynamics of all individual agents are the same and of low dimension, the criterion for reaching consensus now depends on the full spectrum of the communication graph and not only the largest nonzero eigenvalue. The new consensus criterion is posed as a Nyquist-like criterion that provides a clear separation between the individual dynamics and the spectrum of the graph. Our results are illustrated on a formation control problem for a target encircling task.

  • 197.
    Markdahl, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Karayiannidis, Yiannis
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Cooperative object path following control by means of mobile manipulators: A switched systems approach2012In: IFAC Proceedings Volumes (IFAC-PapersOnline): Robot Control, Vol 10, Part 1, IFAC Papers Online, 2012, p. 773-778Conference paper (Refereed)
    Abstract [en]

    This paper proposes a switched control algorithm for distributed cooperative manipulation of rigid bodies in a planar setting. More specifically, we consider the problem of making a grasped object follow a desired position and orientation path. The system contains N robots, possibly of heterogeneous designs, each of which consists of a manipulator arm and a nonholonomic mobile platform. Control is based on local information, is carried out on a kinematic level and partly utilizes inverse kinematics. We use Lyapunov-like arguments to prove that the algorithm is almost globally stable and also show that its parameters can be chosen so that any input saturation level is met. The time between certain switches is proved to be bounded below, and the system is shown to be free of any Zeno behavior.

  • 198.
    Thunberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Song, W.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Distributed attitude synchronization control of multi-agent systems with directed topologies2012In: Intelligent Control and Automation (WCICA), 2012 10th World Congress on, IEEE , 2012, p. 958-963Conference paper (Refereed)
    Abstract [en]

    In the present paper we consider the problem of attitude synchronization for a system of rigid body agents. We provide distributed kinematic control laws for two different synchronization problems. In the two problems the objective is the same, i.e., to synchronize the orientations of the agents, but what is assumed to be measurable by the agents differs. In problem 1 the agents measure their own orientations in a global reference frame, and obtain the orientations of their neighbors by means of communication. In problem 2 the agents only measure the relative orientations to their neighbors. By using the axis-angle representation of the orientation, we show that simple linear control laws solve both synchronization problems. Moreover we show that our proposed control laws work for directed and connected topologies on almost all SO(3) for problem 1 and on convex balls in SO(3) for problem 2.

  • 199. Song, W.
    et al.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hong, Y.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Distributed attitude synchronization control of multi-agent systems with time-varying topologies2012In: Intelligent Control and Automation (WCICA), 2012 10th World Congress on, IEEE , 2012, p. 946-951Conference paper (Refereed)
    Abstract [en]

    This paper addresses the attitude synchronization problem of multiple rigid body agents in SO(3) with directed and switching interconnection topologies. Using the axis-angle representation of the orientation, a distributed controller based on differences between the orientations of agents in a global frame is proposed. In the case of the balanced interconnection graph, the attitude synchronization is achieved with the wellknown mild assumption (that is, uniform joint strong connection). To demonstrate the effectiveness of the proposed control scheme, an illustrative example is provided.

  • 200.
    Markdahl, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Karayiannidis, Yiannis
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Kragic, Danica
    KTH, School of Computer Science and Communication (CSC), Computer Vision and Active Perception, CVAP.
    Distributed Cooperative Object Attitude Manipulation2012In: 2012 IEEE International Conference on Robotics and Automation (ICRA), IEEE Computer Society, 2012, p. 2960-2965Conference paper (Refereed)
    Abstract [en]

    This paper proposes a local information based control law in order to solve the planar manipulation problem of rotating a grasped rigid object to a desired orientation using multiple mobile manipulators. We adopt a multi-agent systems theory approach and assume that: (i) the manipulators (agents) are capable of sensing the relative position to their neighbors at discrete time instances, (ii) neighboring agents may exchange information at discrete time instances, and (iii) the communication topology is connected. Control of the manipulators is carried out at a kinematic level in continuous time and utilizes inverse kinematics. The mobile platforms are assigned trajectory tracking tasks that adjust the positions of the manipulator bases in order to avoid singular arm configurations. Our main result concerns the stability of the proposed control law.

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