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  • 101. Almér, S.
    et al.
    Mariéthoz, S.
    Morari, M.
    Jönsson, Ulf
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Application of model predictive control and analysis of switched systems to the direct voltage control of AC-DC converters2015In: 2009 European Control Conference, ECC 2009, 2015, p. 3593-3598Conference paper (Refereed)
    Abstract [en]

    Recent tools for control and analysis of hybrid systems are applied to an AC-DC converter. The topology poses particularly challenging problems since it is unusually complex and the circuit parameters are such that the dynamic coupling between the AC and DC sides cannot be ignored. The paper proposes a model predictive control scheme for direct voltage control which circumvents the bandwidth limitations associated with classical cascade control. The stability and harmonic properties of the resulting closed loop system are investigated using new tools for the analysis of switched systems.

  • 102. Sadeghian, A.
    et al.
    Lim, D.
    Karlsson, Johan Mikael
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Li, J.
    Automatic target recognition using discrimination based on optimal transport2015In: ICASSP, IEEE International Conference on Acoustics, Speech and Signal Processing - Proceedings, IEEE conference proceedings, 2015, p. 2604-2608Conference paper (Refereed)
    Abstract [en]

    The use of distances based on optimal transportation has recently shown promise for discrimination of power spectra. In particular, spectral estimation methods based on ℓ1 regularization as well as covariance based methods can be shown to be robust with respect to such distances. These transportation distances provide a geometric framework where geodesics corresponds to smooth transition of spectral mass, and have been useful for tracking. In this paper we investigate the use of these distances for automatic target recognition. We study the use of the Monge-Kantorovich distance compared to the standard ℓ2 distance for classifying civilian vehicles based on SAR images. We use a version of the Monge-Kantorovich distance that applies also for the case where the spectra may have different total mass, and we formulate the optimization problem as a minimum flow problem that can be computed using efficient algorithms.

  • 103. Wang, Lin
    et al.
    Wang, Xiaofan
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Connectivity maintenance and distributed tracking for double-integrator agents with bounded potential functions2015In: International Journal of Robust and Nonlinear Control, ISSN 1049-8923, E-ISSN 1099-1239, Vol. 25, no 4, p. 542-558Article in journal (Refereed)
    Abstract [en]

    In this paper, we develop a set of decentralized control laws with bounded potential functions. The basic control law is a combination of attractive, repulsive, and alignment forces, which can keep connectivity, avoid collision, synchronize all agents, and further track a constant moving leader. Furthermore, we investigate the distributed tracking problem with a varying-velocity leader, where the acceleration of the leader can not be measured. Two cases are considered; the acceleration of the leader is bounded, and the acceleration function satisfies Lipschitz condition. In the first case, the relative velocities of neighbors are integrated and transmitted as a new variable to account for the uncertain time-varying acceleration. In the second case, two distributed estimators are added for the leader's position and velocity. Finally, some simulations are presented to show the effectiveness of the proposed algorithms.

  • 104.
    Enqvist, Per
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Covariance interpolation and geometry of power spectral densities2015In: 2009 European Control Conference, ECC 2009, 2015, p. 4505-4510Conference paper (Refereed)
    Abstract [en]

    When methods of moments are used for identification of power spectral densities, a model is matched to estimated second order statistics such as, e.g., covariance estimates. There is an infinite family of power spectra consistent with such an estimate and in applications, such as identification, we want to single out the most representative spectrum. Here, we choose a prior spectral density to represent a priori information, and the spectrum closest to it in a given quasi-distance is determined. Depending on the selected quasi-distance, the geometry of the space of power spectral densities varies, and the structure of the minimizing spectral density changes with it. Recently, the Kullback-Leibler divergence, the Itakura-Saito divergence and Hellinger distances has been shown to determine power spectral densities of rational form and with tractable properties. Here, starting instead with the structure of the power spectral density, different (quasi-)distances and geometries for power spectral densities are derived.

  • 105. Wenjun, Song
    et al.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Xiaoming, Hu
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Yiguang, Hong
    Distributed high-gain attitude synchronization using rotation vectors2015In: Journal of Systems Science and Complexity, ISSN 1009-6124, E-ISSN 1559-7067, Vol. 28, no 2, p. 289-304Article in journal (Refereed)
    Abstract [en]

    This paper addresses attitude synchronization problems for systems of multiple rigid-body agents with directed interconnection topologies. Two scenarios which differ in available information are considered. In the first scenario the agents can obtain their rotations and angular velocities relative to an inertial reference frame and transmit these information to their neighbors, while in the second scenario the agents can only obtain their own angular velocities and measure the relative rotations and relative angular velocities of their neighbors. By using rotation vectors and the high gain control, the authors provide torque control laws asymptotically synchronizing the rotations of the system almost globally for the first scenario and with initial rotations of the agents contained in a convex ball of SO(3) for the second scenario. An illustrative example is provided to show the synchronization results for both scenarios.

  • 106.
    Almér, Stefan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Jönsson, Ulf T.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Dynamic phasor analysis of a class of PWM systems2015In: 2007 European Control Conference, ECC 2007, 2015, p. 1940-1947Conference paper (Refereed)
    Abstract [en]

    The paper makes use of the so-called dynamic phasor model for stability and performance analysis of a class of PWM systems. The dynamic phasor model allows for the state to be represented in the frequency domain where a harmonic Lyapunov function is defined. The analysis covers both periodically switched systems and non-periodic systems where the switching is controlled by feedback.

  • 107.
    Dong, Yi
    et al.
    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.
    Generalized formation control for unicycles2015In: Chinese Control Conference, CCC, IEEE Computer Society, 2015, Vol. 2015, p. 7535-7540, article id 7260834Conference paper (Refereed)
    Abstract [en]

    This paper studies formation control of a group of unicycles, and proposes a distributed control law, depending only on the relative position and bearing angle of one nearby vehicle, to realize the circular formation and render more achievable general formations by designing the parameters in the control law. For circular formation, not only we can strictly prove that only two sets of equilibria are asymptotically stable, but also by choosing appropriate control parameters, determine the distances of two vehicles and the radius of the circle that all the unicycles converge to.

  • 108.
    Svärd, Henrik
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Interior value extrapolation: a new method for stress evaluation during topology optimization2015In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 51, no 3, p. 613-629Article in journal (Refereed)
    Abstract [en]

    This article presents a new method for evaluating stresses in the jagged structures that arise when using a fixed finite element mesh to optimize the topology of a structure. The new method, Interior Value Extrapolation, IVE, exploits the fact that in the interior of the structure, the stresses calculated by the finite element method are more accurate than at the boundary. The jagged nature of the mesh makes stresses at the boundary oscillate. Therefore, stresses at the boundary are instead extrapolated from results in the interior, resulting in a more stable and accurate stress measure. A restriction method in the form of a non linear density filter is also proposed, tailored to be used in conjunction with the new stress evaluation method. The new method is evaluated for accuracy using example geometries, for which the stresses are known. It is shown that IVE improves the accuracy of the stress calculation. Optimization examples are thereafter solved with and without IVE, and the results are discussed. It is shown that the change in stress evaluation can in fact cause changes in the solution of a typical stress minimization problem.

  • 109.
    Karasalo, Maja
    et al.
    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.
    Martin, C. F.
    Localization and mapping using recursive smoothing splines2015In: 2007 European Control Conference, ECC 2007, 2015, p. 2166-2171Conference paper (Refereed)
    Abstract [en]

    In this paper a recursive smoothing spline approach is proposed for mapping and localization in continuous-featured environments such as terrains. Periodic smoothing splines are used to approximate the contour of encountered obstacles and for trajectory planning around them. The splines are generated through minimizing a cost function subject to constraints imposed by a linear control system. The filtering effect of the smoothing splines allows for usage of noisy sensor data. An important assumption is that several data sets for the same closed contour can be obtained. Thus a recursive approach is used to obtain a series of splines with improved accuracy.

  • 110.
    Jönsson, Ulf T.
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Kao, C. -Y
    Fujioka, H.
    Low dimensional stability criteria for large-scale interconnected systems2015In: 2007 European Control Conference, ECC 2007, 2015, p. 2741-2747Conference paper (Refereed)
    Abstract [en]

    A low complexity stability criterion for a class of large-scale systems is derived from a spectral characterization of the interconnection operator. The interconnection is modeled such that it is easy to account for delay and bandwidth constraints. The result is applied to a heterogeneous vehicle platoon and a class of circularly interconnected systems.

  • 111.
    Oddsdóttir, Hildur Æsa
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Macroscopic Modeling of Metabolic Reaction Networks and Dynamic Identification of Elementary Flux Modes by Column Generation2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this work an intersection between optimization methods and animal cell culture modeling is considered. We present optimization based methods for analyzing and building models of cell culture; models that could be used when designing the environment cells are cultivated in, i.e., medium. Since both the medium and cell line considered are complex, designing a good medium is not straightforward. Developing a model of cell metabolism is a step in facilitating medium design.

    In order to develop a model of the metabolism the methods presented in this work make use of an underlying metabolic reaction network and extracellular measurements. External substrates and products are connected via the relevant elementary flux modes (EFMs). Modeling from EFMs is generally limited to small networks, because the number of EFMs explodes when the underlying network size increases. The aim of this work is to enable modeling with more complex networks by presenting methods that dynamically identify a subset of the EFMs.

    In papers A and B we consider a model consisting of the EFMs along with the flux over each mode. In paper A we present how such a model can be decided by an optimization technique named column generation. In paper B the robustness of such a model with respect to measurement errors is considered. We show that a robust version of the underlying optimization problem in paper A can be formed and column generation applied to identify EFMs dynamically.

    In papers C and D a kinetic macroscopic model is considered. In paper C we show how a kinetic macroscopic model can be constructed from the EFMs. This macroscopic model is created by assuming that the flux along each EFM behaves according to Michaelis-Menten type kinetics. This modeling method has the ability to capture cell behavior in varied types of media, however the size of the underlying network is a limitation. In paper D this limitation is countered by developing an approximation algorithm, that can dynamically identify EFMs for a kinetic model.

  • 112. Fredriksson, Albin
    et al.
    Forsgren, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hardemark, Bjorn
    Maximizing the probability of satisfying the clinical goals in radiation therapy treatment planning under setup uncertainty2015In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 7, p. 3992-3999Article in journal (Refereed)
    Abstract [en]

    Purpose: This paper introduces a method that maximizes the probability of satisfying the clinical goals in intensity-modulated radiation therapy treatments subject to setup uncertainty. Methods: The authors perform robust optimization in which the clinical goals are constrained to be satisfied whenever the setup error falls within an uncertainty set. The shape of the uncertainty set is included as a variable in the optimization. The goal of the optimization is to modify the shape of the uncertainty set in order to maximize the probability that the setup error will fall within the modified set. Because the constraints enforce the clinical goals to be satisfied under all setup errors within the uncertainty set, this is equivalent to maximizing the probability of satisfying the clinical goals. This type of robust optimization is studied with respect to photon and proton therapy applied to a prostate case and compared to robust optimization using an a priori defined uncertainty set. Results: Slight reductions of the uncertainty sets resulted in plans that satisfied a larger number of clinical goals than optimization with respect to a priori defined uncertainty sets, both within the reduced uncertainty sets and within the a priori, nonreduced, uncertainty sets. For the prostate case, the plans taking reduced uncertainty sets into account satisfied 1.4 (photons) and 1.5 (protons) times as many clinical goals over the scenarios as the method taking a priori uncertainty sets into account. Conclusions: Reducing the uncertainty sets enabled the optimization to find better solutions with respect to the errors within the reduced as well as the nonreduced uncertainty sets and thereby achieve higher probability of satisfying the clinical goals. This shows that asking for a little less in the optimization sometimes leads to better overall plan quality.

  • 113. Wang, Jinhuan
    et al.
    Zhang, Lei
    Shi, Qiongyu
    Yang, Peng
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Modeling and simulating for congestion pedestrian evacuation with panic2015In: Physica A: Statistical Mechanics and its Applications, ISSN 0378-4371, E-ISSN 1873-2119, Vol. 428, p. 396-409Article in journal (Refereed)
    Abstract [en]

    A new multi-agent based congestion evacuation model incorporating panic behavior is proposed in this paper for simulating pedestrian evacuation in public places such as a stadium. Different from the existing results, pedestrians in this model are divided into four classes and each pedestrian's status can be either normal, being overtaken, or casualty. The direction of action for each individual is affected by competitive ability, distance to the exits as well as number and density of occupants within the view field of the agent. Our simulations exhibit that during the evacuation process: (1) The agents gather in front of the exits spontaneously and present arched shapes close to the exits. (2) Under the panic state the agents cohere closely and almost do not change the target exit. So other alternative exits are ignored. (3) For the case without obstacle, the casualties under panic increase greatly. But if there are obstacles (chairs), the congestion can be alleviated. Thus the casualties are reduced. (4) If certain exit is partly clogged, the evacuation becomes more efficient when adding a virtual leader. The overall simulation results show that the proposed model can reproduce the real evacuation process in a stadium quite well.

  • 114.
    Odland, Tove
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    On Methods for Solving Symmetric Systems of Linear Equations Arising in Optimization2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In this thesis we present research on mathematical properties of methods for solv- ing symmetric systems of linear equations that arise in various optimization problem formulations and in methods for solving such problems.

    In the first and third paper (Paper A and Paper C), we consider the connection be- tween the method of conjugate gradients and quasi-Newton methods on strictly convex quadratic optimization problems or equivalently on a symmetric system of linear equa- tions with a positive definite matrix. We state conditions on the quasi-Newton matrix and the update matrix such that the search directions generated by the corresponding quasi-Newton method and the method of conjugate gradients respectively are parallel.

    In paper A, we derive such conditions on the update matrix based on a sufficient condition to obtain mutually conjugate search directions. These conditions are shown to be equivalent to the one-parameter Broyden family. Further, we derive a one-to-one correspondence between the Broyden parameter and the scaling between the search directions from the method of conjugate gradients and a quasi-Newton method em- ploying some well-defined update scheme in the one-parameter Broyden family.

    In paper C, we give necessary and sufficient conditions on the quasi-Newton ma- trix and on the update matrix such that equivalence with the method of conjugate gra- dients hold for the corresponding quasi-Newton method. We show that the set of quasi- Newton schemes admitted by these necessary and sufficient conditions is strictly larger than the one-parameter Broyden family. In addition, we show that this set of quasi- Newton schemes includes an infinite number of symmetric rank-one update schemes.

    In the second paper (Paper B), we utilize an unnormalized Krylov subspace frame- work for solving symmetric systems of linear equations. These systems may be incom- patible and the matrix may be indefinite/singular. Such systems of symmetric linear equations arise in constrained optimization. In the case of an incompatible symmetric system of linear equations we give a certificate of incompatibility based on a projection on the null space of the symmetric matrix and characterize a minimum-residual solu- tion. Further we derive a minimum-residual method, give explicit recursions for the minimum-residual iterates and characterize a minimum-residual solution of minimum Euclidean norm.

  • 115.
    Forsgren, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Odland, Tove
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    On the connection between the conjugate gradient method and quasi-Newton methods on quadratic problems2015In: Computational optimization and applications, ISSN 0926-6003, E-ISSN 1573-2894, Vol. 60, no 2, p. 377-392Article in journal (Refereed)
    Abstract [en]

    It is well known that the conjugate gradient method and a quasi-Newton method, using any well-defined update matrix from the one-parameter Broyden family of up- dates, produce identical iterates on a quadratic problem with positive definite Hessian. This equivalence does not hold for any quasi-Newton method. We define precisely the conditions on the update matrix in the quasi-Newton method that give rise to this behavior. We show that the crucial facts are, that the range of each update matrix lies in the last two dimensions of the Krylov subspaces defined by the conjugate gradient method and that the quasi-Newton condition is satisfied. In the framework based on a sufficient condition to obtain mutually conjugate search directions, we show that the one-parameter Broyden family is complete.

    A one-to-one correspondence between the Broyden parameter and the non-zero scaling of the search direction obtained from the corresponding quasi-Newton method compared to the one obtained in the conjugate gradient method is derived. In addition, we show that the update matrices from the one-parameter Broyden family are almost always well-defined on a quadratic problem with positive definte Hessian. The only exception is when the symmetric rank-one update is used and the unit steplength is taken in the same iteration. In this case it is the Broyden parameter that becomes undefined.

  • 116. Deng, J.
    et al.
    Song, Wenjun
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Liu, Z.
    Baras, J. S.
    Pose synchronization of rigid body networks with switching topologies2015In: 2015 34th Chinese Control Conference (CCC), IEEE Computer Society, 2015, Vol. 2015, p. 7639-7644Conference paper (Refereed)
    Abstract [en]

    Coordination control of multiple rigid bodies attracts much attention of researchers due to its wide applications. This paper presents the pose synchronization problem of the moving rigid bodies whose dynamics is described by the group SE(3). The case of bidirectional neighbor graphs with switching interconnection topologies is considered. We design a distributed control law based on relative rotation matrices and relative positions between the neighboring rigid bodies, and show that the SE(3) reaches pose synchronization if and only if the neighbor graphs are infinitely jointly connected, which relaxes the theoretical results in the existing literature.

  • 117.
    Bokrantz, Rasmus
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. RaySearch Laboratories, Sweden.
    Miettinen, Kaisa
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. University of Jyväskylä, Finland.
    Projections onto the Pareto surface in multicriteria radiation therapy optimization2015In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 42, no 10, p. 5862-5870Article in journal (Refereed)
    Abstract [en]

    Purpose: To eliminate or reduce the error to Pareto optimality that arises in Pareto surface navigation when the Pareto surface is approximated by a small number of plans. Methods: The authors propose to project the navigated plan onto the Pareto surface as a postprocessing step to the navigation. The projection attempts to find a Pareto optimal plan that is at least as good as or better than the initial navigated plan with respect to all objective functions. An augmented form of projection is also suggested where dose-volume histogram constraints are used to prevent that the projection causes a violation of some clinical goal. The projections were evaluated with respect to planning for intensity modulated radiation therapy delivered by step-and-shoot and sliding window and spot-scanned intensity modulated proton therapy. Retrospective plans were generated for a prostate and a head and neck case. Results: The projections led to improved dose conformity and better sparing of organs at risk (OARs) for all three delivery techniques and both patient cases. The mean dose to OARs decreased by 3.1 Gy on average for the unconstrained form of the projection and by 2.0 Gy on average when dose-volume histogram constraints were used. No consistent improvements in target homogeneity were observed. Conclusions: There are situations when Pareto navigation leaves room for improvement in OAR sparing and dose conformity, for example, if the approximation of the Pareto surface is coarse or the problem formulation has too permissive constraints. A projection onto the Pareto surface can identify an inaccurate Pareto surface representation and, if necessary, improve the quality of the navigated plan.

  • 118.
    Gustavi, Tove
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Karasalo, Maja
    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.
    Martin, C.
    Recursive identification of a hybrid system2015In: 2009 European Control Conference, ECC 2009, 2015, p. 4907-4912Conference paper (Refereed)
    Abstract [en]

    In this paper we consider the problem of retrieving information from a set of noisy and distorted measurements. More precisely we consider a scenario where a set of trajectories x1(t),...,xn(t) are observed using a single measuring device so that the output y(ti) of the device at each sampling point is an observation of precisely one of x1(t),...,xn(t). The process can be modeled as a discrete switched system. We develop a recursive Classification, Correction and Identification Algorithm (CCIA), that recovers xj(ti) and the switching parameter δj(ti) together with the ratio zj of measurements from each state xj(ti).

  • 119.
    Markdahl, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Rigid-Body Attitude Control and Related Topics2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This dissertation explores aspects of control in rigid-body and robotic systems. The first and second paper analyze the attitude stabilization problem and its generalization to n-dimensional rigid bodies. The third and fourth paper are on cooperative control design for systems that evolve on the n-sphere and related topics such as rigid-body reduced attitude synchronization. The fifth, and final, paper proposes a hybrid systems approach to task-priority based control for mobile manipulation.

    he first and second paper concern the problem of attitude tracking by kinematic actuation for a class of almost globally asymptotically stabilizing feedback laws on SO(n). The closed-loop systems are solved exactly for the rotation matrices as functions of time. Exact solutions provide insight into both the transient and asymptotical behavior of a system. Applications of these results are found in model predictive control and in sampled systems. The second paper also solves the optimal control problem of geodesic reduced attitude stabilization subject to full attitude stabilization.

    The third and fourth paper concern three cooperative control problems on the n-sphere with applications to reduced attitude synchronization and formation control. The global behavior of a consensus protocol is studied both forwards and backwards in time. The forward time stability properties of all equilibria are characterized for a non-trivial class of graph topologies. The reverse time behavior in the case of cyclic graph topologies results in two types of formations depending on the parity of the number of agents. A third control protocol renders the centroid of agent states constant.

    The fifth, and final, paper proposes a hybrid control approach to task priority based planar mobile manipulation, i.e., control on the n-torus. The end-effector path following problem for a nonholonomic mobile manipulator is solved subject to constraints on the input norm, feasible joint configurations, and distance to singularities. The hybrid system is well-posed; there is no Zeno behavior or chattering. A continuous, time-independent feedback law is derived based on the hybrid control design.

  • 120.
    Yang, Yuecheng
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Dimarogonas, Dimos
    KTH, School of Electrical Engineering (EES), Automatic Control.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Shaping up crowd of agents through controlling their statistical moments2015In: 2015 European Control Conference, ECC 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, p. 1017-1022Conference paper (Refereed)
    Abstract [en]

    In a crowd model based on leader-follower interactions, where positions of the leaders are viewed as the control input, up-to-date solutions rely on knowledge of the agents' coordinates. In practice, it is more realistic to exploit knowledge of statistical properties of the group of agents, rather than their exact positions. In order to shape the crowd, we study thus the problem of controlling the moments instead, since it is well known that shape can be determined by moments. An optimal control for the moments tracking problem is obtained by solving a modified Hamilton-Jacobi-Bellman (HJB) equation, which only uses the moments and leaders' states as feedback. The optimal solution can be solved fast enough for on-line implementations.

  • 121.
    Möller, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Jönsson, Ulf T.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Stability of systems under interference feedback2015In: 2009 European Control Conference, ECC 2009, 2015, p. 525-530Conference paper (Refereed)
    Abstract [en]

    Distributed power control problems in wireless communications can be modeled as a highly nonlinear feedback system. The nonlinear coupling appear when a large number of mobile stations interact through interference and it is well known that this may lead to instabilities. In this paper a number of results on existence and uniqueness of solution and boundedness and convergence of the solution are derived for a higher order model class.

  • 122. Gattami, Ather
    et al.
    Ringh, Emil
    Ericsson Research, Sweden.
    Karlsson, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Time localization and capacity of faster-than-nyquist signaling2015In: 2015 IEEE Global Communications Conference, GLOBECOM 2015, Institute of Electrical and Electronics Engineers (IEEE), 2015, article id 7417358Conference paper (Refereed)
    Abstract [en]

    In this paper, we consider communication over the bandwidth limited analog white Gaussian noise channel using non-orthogonal pulses. In particular, we consider non-orthogonal transmission by signaling samples at a rate higher than the Nyquist rate. Using the faster-than- Nyquist (FTN) framework, Mazo showed that one may transmit symbols carried by sinc pulses at a higher rate than that dictated by Nyquist without loosing bit error rate. However, as we will show in this paper, such pulses are not necessarily well localized in time. In fact, assuming that signals in the FTN framework are well localized in time, one can construct a signaling scheme that violates the Shannon capacity bound. We also show directly that FTN signals are in general not well localized in time. We also consider FTN signaling in the case of pulses that are different from the sinc pulses. We show that one may use a precoding scheme of low complexity, in order to remove the intersymbol interference. This leads to the possibility of increasing the number of transmitted samples per time unit and compensate for spectral inefficiency due to signaling at the Nyquist rate of the non sinc pulses. We demonstrate the power of the precoding scheme by simulations.

  • 123.
    Svärd, Henrik
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Topology Optimization of Fatigue-Constrained Structures2015Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Fatigue, or failure of material due to repeated cyclic loading, is one of the most common causes of mechanical failures. The risk of fatigue in a load carrying component is often lowered by adding material, thereby reducing stresses. This increases the component weight, reducing the performance of the component and increasing its manufacturing cost. There is thus a need to design components to be as light as possible, while keeping the risk of fatigue at a low enough level, i.e. there is a need for optimization of the component subject to fatigue constraints

    This thesis deals with design against fatigue using topology optimization, which is a form of structural optimization where an optimal design is sought by using mathematical programming to decide which parts of a design domain should be filled with material, and which should not. 

    To predict fatigue, accurate representation of the geometry and accurate stress computation are of utmost importance. In this thesis, methods for imposing constraints such as minimum inner radii and minimum member sizes in the form of four new density filters are proposed. The filters are able to generate a very sharp representation of the structural boundary. A method for improving the accuracy of stress results at the structural boundary is also proposed, based on extrapolation of results from the interior of the structure. The method gives more accurate stresses, which affects the resulting structures when solving optimization problems. 

    A formulation for fatigue constraints in topology optimization is proposed, based on the weakest link integral. The formulation avoids the problem of choosing between accurate but costly local constraints, and efficient but approximate aggregated constraints, and gives a theoretical motivation for using expressions similar to the p-norm of stresses. 

    For verifying calculations of the fatigue probability of an optimized structure, critical plane criteria are commonly used. A new method for evaluating such criteria using optimization methods is proposed, and is proved to give results within a user given error tolerance. It is shown that compared to existing brute force methods, the proposed method evaluates significantly fewer planes in the search of the critical one.

     

  • 124. Aleman, Dionne M.
    et al.
    Wallgren, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Romeijn, H. Edwin
    Dempsey, James F.
    A fluence map optimization model for restoring traditional fractionation in IMRT treatment planning2014In: Optimization Letters, ISSN 1862-4472, E-ISSN 1862-4480, Vol. 8, no 4, p. 1453-1473Article in journal (Refereed)
    Abstract [en]

    One of the core problems in intensity modulated radiation therapy (IMRT) treatment planning is the fluence map optimization (FMO) problem, which determines a fluence map (or profile) for each beam used in the delivery of treatment. Radiation therapy is administered in multiple so-called daily fractions to allow for healthy tissue to recover from damage caused by the treatment. Before the advent of IMRT, the treatment was designed to ensure a constant dose to cells in the target (the areas in the patient where cancerous cells are present or suspected). In the presence of multiple targets with different prescribed doses, this design meant that treatment had to be delivered in a sequence of unequal fractions, one per prescription dose level. For example, in case of two targets treatment would consist of an initial plan aimed at treating both targets to a lower total dose, followed by a so-called boost plan aimed at delivering the additional dose at the target with higher prescribed dose. In contrast, IMRT treatment plans are often delivered with equal treatment plan for each fraction, which means that the dose per fraction cannot be the same for all targets. The important problem of restoring traditional fractionation to IMRT treatments has not yet received much attention in the literature. In this paper we propose a new optimization model that explicitly restores fractionation into the FMO problem, yielding an optimal set of fluence maps for each fraction. We illustrate the capabilities of our approach on clinical head-and-neck cancer cases.

  • 125.
    Markdahl, Johan
    et al.
    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.
    A globally stabilizing hybrid control algorithm for mobile manipulation subject to joint-space constraints2014In: Proceedings - IEEE International Conference on Robotics and Automation, IEEE conference proceedings, 2014, p. 5152-5157Conference paper (Refereed)
    Abstract [en]

    This paper proposes a hybrid control algorithm for mobile manipulation subject to joint-space constraints. More specifically, we consider the problem of making the end-effector of a planar manipulator attached to a nonholonomic mobile platform reach a set-point or equivalently follow a given path in workspace by means of kinematic actuation. A switched control strategy allows the robot to avoid singular joint configurations and execute a pseudo-inverse based feedback using bounded and continuous control signals. The switching scheme is also utilized to maintain feasible joint configurations. Numerical examples provide an intuitive understanding of the algorithm's workings.

  • 126. McGarry, Conor K.
    et al.
    Bokrantz, Rasmus
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. RaySearch Labs, Stockholm, Sweden.
    O'Sullivan, Joe M.
    Hounsell, Alan R.
    Advantages and limitations of navigation-based multicriteria optimization (MCO) for localized prostate cancer IMRT planning2014In: Medical Dosimetry, ISSN 0958-3947, E-ISSN 1873-4022, Vol. 39, no 3, p. 205-211Article in journal (Refereed)
    Abstract [en]

    Efficacy of inverse planning is becoming increasingly important for advanced radiotherapy techniques. This study's aims were to validate multicriteria optimization (MCO) in RayStation (v2.4, RaySearch Laboratories, Sweden) against standard intensity-modulated radiation therapy (IMRT) optimization in Oncentra (v4.1, Nucletron BV, the Netherlands) and characterize dose differences due to conversion of navigated MCO plans into deliverable multileaf collimator apertures. Step-and-shoot IMRT plans were created for 10 patients with localized prostate cancer using both standard optimization and MCO. Acceptable standard IMRT plans with minimal average rectal dose were chosen for comparison with deliverable MCO plans. The trade-off was, for the MCO plans, managed through a user interface that permits continuous navigation between fluence-based plans. Navigated MCO plans were made deliverable at incremental steps along a trajectory between maximal target homogeneity and maximal rectal sparing. Dosimetric differences between navigated and deliverable MCO plans were also quantified. MCO plans, chosen as acceptable under navigated and deliverable conditions resulted in similar rectal sparing compared with standard optimization (33.7 +/- 1.8 Gy vs 35.5 +/- 4.2 Gy, p = 0.117). The dose differences between navigated and deliverable MCO plans increased as higher priority was placed on rectal avoidance. If the best possible deliverable MCO was chosen, a significant reduction in rectal dose was observed in comparison with standard optimization (30.6 +/- 1.4 Gy vs 35.5 +/- 4.2 Gy, p = 0.047). Improvements were, however, to some extent, at the expense of less conformal dose distributions, which resulted in significantly higher doses to the bladder for 2 of the 3 tolerance levels. In conclusion, similar IMRT plans can be created for patients with prostate cancer using MCO compared with standard optimization. Limitations exist within MCO regarding conversion of navigated plans to deliverable apertures, particularly for plans that emphasize avoidance of critical structures. Minimizing these differences would result in better quality treatments for patients with prostate cancer who were treated with radiotherapy using MCO plans.

  • 127.
    Markdahl, Johan
    et al.
    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.
    Analytical solutions to a class of feedback systems on SO(n)2014In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2014, p. 445-450Conference paper (Refereed)
    Abstract [en]

    This paper provides analytical solutions to the closed loop system kinematics for a class of almost globally asymptotically stable feedback laws on SO(n). The resulting closed loop kinematics are solved for the respective matrices as functions of time, the initial conditions and the gain parameters of the control laws. The analytical solutions provide insight into the transient dynamics of the system and can be used to prove almost global attractiveness of the identity matrix. We consider an application of these results towards model predictive control where the transient phase of the system is utilized to attempt to complete a task of secondary importance by choosing the gain parameters as functions of time and the initial conditions.

  • 128.
    Thunberg, Johan
    et al.
    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.
    Attitude consensus using networks of uncalibrated cameras2014In: 2014 33rd Chinese Control Conference (CCC), IEEE Computer Society, 2014, p. 1444-1451Conference paper (Refereed)
    Abstract [en]

    This paper addresses the problem of consensus on SO(3) for networks of uncalibrated cameras. Under the assumption of a pinhole camera model, we prove convergence to the consensus manifold for two types of kinematic control laws, when only conjugate rotation matrices KRK-1 are available among the agents. In these conjugate rotations, the rotation matrices are distorted by the (unknown) intrinsic parameters of the cameras. For the conjugate rotations, we introduce distorted versions of well known local parameterizations of SO(3) and show consensus by using three types of control laws. The control laws are similar to the standard consensus protocol used for systems of agents with single integrator dynamics, where pairwise differences between the states of neighboring agents are used. By considering the restriction to the planar case (when all the rotations have the same rotational axes), we weaken the assumptions on the cameras in the system and consider networks where the camera matrices differ between agents.

  • 129. Hu, J.
    et al.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Zhang, Y.
    Bipartite consensus strategies for coopetition networks2014In: 2014 33rd Chinese Control Conference (CCC), IEEE Computer Society, 2014, p. 1505-1510Conference paper (Refereed)
    Abstract [en]

    Cooperation and competition are two typical interactional relationships in natural and engineering networked systems. Some interesting collective behaviors can emerge through local interactions within the networked systems. This paper focuses on the coexistence of competition and cooperation (i.e., coopetition) at the network level and, simultaneously, the collective dynamics on such coopetition networks. The coopetition network is represented by a directed signed graph. The interaction relationship between the agents on the coopetition network is described by a multi-agent system. We investigate two bipartite consensus strategies for the agents such that all the agents converge to a final state characterized by identical modulus but opposite sign. Under a weak connectivity assumption that the signed network has a spanning tree, some sufficient conditions are derived for bipartite consensus of multi-agent systems with the help of a structural balance theory. Finally, simulation results are provided to demonstrate the bipartite consensus formation.

  • 130. Zhong, Jianghua
    et al.
    Kleijn, W. Bastiaan
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Camera Control in Multi-Camera Systems for Video Quality Enhancement2014In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 14, no 9, p. 2955-2966Article in journal (Refereed)
    Abstract [en]

    Multiview video captured by multi-camera systems has been widely used in many applications such as surveillance, 3D television, and free viewpoint television. After capturing, the multi-camera system generally requires to compress a large amount of multiview data due to limitations on the transmission capacity. To improve video quality, previous work on video compression focused on algorithm refinements when a raw multiview video is compressed at a limited bit-rate, whereas existing work on instrumentation generally introduced higher resolution capture and additional hardware. In contrast, this paper uses camera control to enhance video quality. Individual cameras are controlled to adjust their pan angles and focal lengths to compensate for both object location difference and color inconsistency among camera views at the video acquisition stage. Such a compensation makes the camera images more similar and therefore improves the video quality when a raw multiview video is compressed at a limited bit-rate. The optimal pan angle and focal length are designed for each camera. An example application is to monitor a potted plant in real time for ornamental horticulture study. Experimental results validate the camera control method.

  • 131.
    Thunberg, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Consensus and Pursuit-Evasion in Nonlinear Multi-Agent Systems2014Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Within the field of multi-agent systems theory, we study the problems of consensus and pursuit-evasion. In our study of the consensus problem, we first provide some theoretical results and then consider the problem of consensus on SO(3) or attitude synchronization.

    In Chapter 2, for agents with states in R^m, we present two theorems along the lines of Lyapunov’s second method that, under different conditions, guarantee asymptotic state consensus in multi-agent systems where the interconnection topologies are switching. The first theorem is formulated by using the states of the agents in the multi-agent system, whereas the second theorem is formulated by using the pairwise states for pairs of agents in the multi-agent system.

    In Chapter 3, the problem of consensus on SO(3) for a multi-agent system with directed and switching interconnection topologies is addressed. We provide two different types of kinematic control laws for a broad class of local representations of SO(3). The first control law consists of a weighted sum of pairwise differences between positions of neighboring agents, expressed as coordinates in a local representation. The structure of the control law is well known in the consensus community for being used in systems of agents in the Euclidean space, and here we show that the same type of control law can be used in the context of consensus on SO(3). In a later part of this chapter, based on the kinematic control laws, we introduce torque control laws for a system of rigid bodies in space and show that the system reaches consensus when these control laws are used.

    Chapter 4 addresses the problem of consensus on SO(3) for networks of uncalibrated cameras. Under the assumption that each agent uses a camera in order to measure its rotation, we prove convergence to the consensus set for two types of kinematic control laws, where only conjugate rotation matrices are available for the agents. In these conjugate rotations, the rotation matrix can be seen as distorted by the (unknown) intrinsic parameters of the camera. For the conjugate rotations we introduce distorted versions of well known local parameterizations of SO(3) and show consensus by using control laws that are similar to the ones in Chapter 3, with the difference that the distorted local representations are used instead. In Chapter 5, we study the output consensus problem for homogeneous systems of agents with linear continuous time-invariant dynamics. We derive control laws that solve the problem, while minimizing a cost functional of the control signal. Instead of considering a fixed communication topology for the system, we derive the optimal control law without any restrictions on the topology. We show that for all linear output controllable homogeneous systems, the optimal control law uses only relative information but requires the connectivity graph to be complete and in general requires measurements of the state errors. We identify cases where the optimal control law is only based on output errors.

    In Chapter 6, we address the multi-pursuer version of the visibility pursuit-evasion problem in polygonal environments. By discretizing the problem and applying a Mixed Integer Linear Programming (MILP) framework, we are able to address problems requiring so called recontamination and also impose additional constraints, such as connectivity between the pursuers. The proposed MILP formulation is less conservative than solutions based on graph discretizations of the environment, but still somewhat more conservative than the original underlying problem. It is well known that MILPs, as well as multi-pursuer pursuit-evasion problems, are NP-hard. Therefore we apply an iterative Receding Horizon Control (RHC) scheme, where a number of smaller MILPs are solved over shorter planning horizons. The proposed approach is illustrated by a number of solved examples.

  • 132. Jinhuan, Wang
    et al.
    Zhixin, Liu
    Xiaoming, Hu
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Consensus control design for multi-agent systems using relative output feedback2014In: Journal of Systems Science and Complexity, ISSN 1009-6124, E-ISSN 1559-7067, Vol. 27, no 2, p. 237-251Article in journal (Refereed)
    Abstract [en]

    This paper studies the consensus problem of multi-agent systems in which all agents are modeled by a general linear system. The authors consider the case where only the relative output feedback between the neighboring agents can be measured. To solve the consensus problem, the authors first construct a static relative output feedback control under some mild constraints on the system model. Then the authors use an observer based approach to design a dynamic relative output feedback control. If the adjacent graph of the system is undirected and connected or directed with a spanning tree, with the proposed control laws, the consensus can be achieved. The authors note that with the observer based approach, some information exchange between the agents is needed unless the associated adjacent graph is completely connected.

  • 133. Jiangping, Hu
    et al.
    Xiaoming, Hu
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Tielong, Shen
    Cooperative shift estimation of target trajectory using clustered sensors2014In: Journal of Systems Science and Complexity, ISSN 1009-6124, E-ISSN 1559-7067, Vol. 27, no 3, p. 413-429Article in journal (Refereed)
    Abstract [en]

    In this paper, a mathematical model for target tracking using nonlinear scalar range sensors is formulated first. A time-shift sensor scheduling strategy is addressed on the basis of a k-barrier coverage protocol and all the sensors are divided into two classes of clusters, active cluster, and submissive cluster, for energy-saving. Then two types of time-shift nonlinear filters are proposed for both active and submissive clusters to estimate the trajectory of the moving target with disturbed dynamics. The stochastic stability of the two filters is analyzed. Finally, some numerical simulations are given to demonstrate the effectiveness of the new filters with a comparison of EKF.

  • 134.
    Thunberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Song, Wenjun
    Montijano, Eduardo
    Hong, Yiguang
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Distributed attitude synchronization control of multi-agent systems with switching topologies2014In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 50, no 3, p. 832-840Article in journal (Refereed)
    Abstract [en]

    This paper addresses the attitude synchronization problem in multi-agent systems with directed and switching interconnection topologies. Two cases for the synchronization problem are discussed under different assumptions about the measurable information. In the first case the agents can measure their rotations relative to a global reference coordinate frame, whilst in the second case they can only measure the relative rotations between each other. Two intuitive distributed control laws based on the axis-angle representations of the rotations are proposed for the two cases, respectively. The invariance of convex balls in SO(3) is guaranteed. Moreover, attitude synchronization is ensured under the well-known mild switching assumptions, the joint strong connection for the first case and joint quasi-strong connection for the second case. To show the effectiveness of the proposed control schemes, illustrative examples are provided.

  • 135.
    Thunberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Song, W.
    Hong, Y.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Distributed attitude synchronization using backstepping and sliding mode control2014In: Journal of Control Theory and Applications, ISSN 1672-6340, Vol. 12, no 1, p. 48-55Article in journal (Refereed)
    Abstract [en]

    We consider the problem of attitude synchronization for systems of rigid body agents with directed topologies. Two different scenarios for the rotation matrices of the agents are considered. In the first scenario, the rotations are contained in a convex subset of SO(3), which is a ball of radius less than π/2, whereas in the second scenario the agents are contained in a subset of SO(3), which is a ball of radius less than π. Using a control law based on backstepping and sliding mode control, we provide distributed, semi-global, torque control laws for the agents so that the rotations asymptotically synchronize. The control laws for the agents in the first scenario only depend on the relative rotations between neighboring agents, whereas the control laws in the second scenario depend on rotations defined in a global coordinate frame. Illustrative examples are provided where the synchronization is shown for both scenarios.

  • 136. Glentis, G. -O
    et al.
    Karlsson, Johan
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Jakobsson, A.
    Li, J.
    Efficient spectral analysis in the missing data case using sparse ML methods2014Conference paper (Refereed)
    Abstract [en]

    Given their wide applicability, several sparse high-resolution spectral estimation techniques and their implementation have been examined in the recent literature. In this work, we further the topic by examining a computationally efficient implementation of the recent SMLA algorithms in the missing data case. The work is an extension of our implementation for the uniformly sampled case, and offers a notable computational gain as compared to the alternative implementations in the missing data case.

  • 137.
    Karlsson, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Rowe, William
    Xu, Luzhou
    Glentis, George-Othon
    Li, Jian
    Fast Missing-Data IAA With Application to Notched Spectrum SAR2014In: IEEE Transactions on Aerospace and Electronic Systems, ISSN 0018-9251, E-ISSN 1557-9603, Vol. 50, no 2, p. 959-971Article in journal (Refereed)
    Abstract [en]

    Recently, the spectral estimation method known as the iterative adaptive approach (IAA) has been shown to provide higher resolution and lower sidelobes than comparable spectral estimation methods. The computational complexity is higher than methods such as the periodogram (matched filter method). Fast algorithms have been developed that considerably reduce the computational complexity of IAA by using Toeplitz and Vandermonde structures. For the missing-data case, several of these structures are lost, and existing fast algorithms are only efficient when the number of available samples is small. In this work, we consider the case in which the number of missing samples is small. This allows us to use low-rank completion to transform the problem to the structured problem. We compare the computational speed of the algorithm with the state of the art and demonstrate the utility in a frequency-notched synthetic aperture radar imaging problem.

  • 138. Yang, Xiaoxia
    et al.
    Dong, Hairong
    Wang, Qianling
    Chen, Yao
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Guided crowd dynamics via modified social force model2014In: Physica A: Statistical Mechanics and its Applications, ISSN 0378-4371, E-ISSN 1873-2119, Vol. 411, p. 63-73Article in journal (Refereed)
    Abstract [en]

    Pedestrian dynamics is of great theoretical significance for strategy design of emergency evacuation. Modification of pedestrian dynamics based on the social force model is presented to better reflect pedestrians' behavioral characteristics in emergency. Specifically, the modified model can be used for guided crowd dynamics in large-scale public places such as subway stations and stadiums. This guided crowd model is validated by explicitly comparing its density-speed and density-flow diagrams with fundamental diagrams. Some social phenomena such as gathering, balance and conflicts are clearly observed in simulation, which further illustrate the effectiveness of the proposed modeling method. Also, time delay for pedestrians with time-dependent desired velocities is observed and explained using the established model in this paper. Furthermore, this guided crowd model is applied to the simulation system of Beijing South Railway Station for predictive evacuation experiments.

  • 139. Wang, J.
    et al.
    Shi, Q.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Electrical Engineering (EES), Centres, ACCESS Linnaeus Centre.
    Yang, P.
    Modeling and simulating for congestion evacuation based on multi-agent approach2014In: 2014 33rd Chinese Control Conference (CCC), IEEE Computer Society, 2014, p. 6421-6426Conference paper (Refereed)
    Abstract [en]

    A new multi-agent based congestion evacuation model is proposed in this paper to simulate the pedestrian evacuation process in stadium with or without obstacles, in which each agent's state can be normal, being overtaked and dead. In this model, all the pedestrians are divided into four kinds and each kind has six different classes of competitive ability. The individual action direction is affected by their competitive ability, the distance to the exits and the occupant number and density within the view field of the agent. In the simulation, we present the congestion and death phenomenon as well as the effects of obstacles in evacuation process. We also add a virtual leader to regulate the evacuation effectively. Simulation results show that the proposed model can reproduce exactly the real evacuation process in stadium.

  • 140.
    Lindquist, Anders
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Picci, Giorgio
    Modeling of Periodic Time Series by Bilateral ARMA Representations2014In: INTERNATIONAL WORK-CONFERENCE ON TIME SERIES (ITISE 2014), 2014, p. 861-865Conference paper (Refereed)
    Abstract [en]

    In this extended abstract for an oral presentation we describe a moment-based approach to modeling of stationary, periodic time series from a finite sequence of covariance lags. We present a complete parameterization of a family of solutions and a convex optimization procedure to determine each solution, which is seen to be represented as a bilateral ARMA model.

  • 141. Hanebeck, U. D.
    et al.
    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.
    Moment-based dirac mixture approximation of circular densities2014In: IFAC Proceedings Volumes (IFAC-PapersOnline), 2014, p. 5040-5048Conference paper (Refereed)
    Abstract [en]

    Given a circular probability density function, called the true probability density function, the goal is to find a Dirac mixture approximation based on some circular moments of the true density. When keeping the locations of the Dirac points fixed, but almost arbitrarily located, we are applying recent results on the circulant rational covariance extension problem to the problem of calculating the weights. For the case of simultaneously calculating optimal locations, additional constraints have to be deduced from the given density. For that purpose, a distance measure for the deviation of the Dirac mixture approximation from the true density is derived, which then is minimized while considering the moment conditions as constraints. The method is based on progressive numerical minimization, converges quickly and gives well-distributed Dirac mixtures that fulfill the constraints, i.e., have the desired circular moments.

  • 142.
    Carlsson, Fredrik
    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.
    On column generation approaches for approximate solutions of quadratic programs in intensity-modulated radiation therapy2014In: Annals of Operations Research, ISSN 0254-5330, E-ISSN 1572-9338, Vol. 223, no 1, p. 471-481Article in journal (Refereed)
    Abstract [en]

    This paper deals with numerical behavior of a recently presented column generation approach for optimization of so called step-and-shoot radiotherapy treatment plans. The approach and variants of it have been reported to be efficient in practice, finding near-optimal solutions by generating only a low number of columns. The impact of different restrictions on the columns in a column generation method is studied, and numerical results are given for quadratic programs corresponding to three patient cases. In particular, it is noted that with a bound on the two-norm of the columns, the method is equivalent to the conjugate-gradient method. Further, the above-mentioned column generation approach for radiotherapy is obtained by employing a restriction based on the infinity-norm and non-negativity. The column generation method has weak convergence properties if restricted to generating feasible step-and-shoot plans, with a "tailing-off" effect for the objective values. However, the numerical results demonstrate that, like the conjugate-gradient method, a rapid decrease of the objective value is obtained in the first few iterations. For the three patient cases, the restriction on the columns to generate feasible step-and-shoot plans has small effect on the numerical efficiency.

  • 143.
    Oddsdóttir, Hildur Æsa
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Hagrot, Erika
    KTH, School of Biotechnology (BIO), Industrial Biotechnology.
    Chotteau, Véronique
    KTH, School of Biotechnology (BIO), Industrial Biotechnology.
    Forsgren, Anders
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    On dynamically generating relevant elementary flux modes in a metabolic network using optimization2014In: Journal of Mathematical Biology, ISSN 0303-6812, E-ISSN 1432-1416Article in journal (Refereed)
    Abstract [en]

    Elementary flux modes (EFMs) are pathways through a metabolic reaction network that connect external substrates to products. Using EFMs, a metabolic network can be transformed into its macroscopic counterpart, in which the internal metabolites have been eliminated and only external metabolites remain. In EFMs-based metabolic flux analysis (MFA) experimentally determined external fluxes are used to estimate the flux of each EFM. It is in general prohibitive to enumerate all EFMs for complex networks, since the number of EFMs increases rapidly with network complexity. In this work we present an optimization-based method that dynamically generates a subset of EFMs and solves the EFMs-based MFA problem simultaneously. The obtained subset contains EFMs that contribute to the optimal solution of the EFMs-based MFA problem. The usefulness of our method was examined in a case-study using data from a Chinese hamster ovary cell culture and two networks of varied complexity. It was demonstrated that the EFMs-based MFA problem could be solved at a low computational cost, even for the more complex network. Additionally, only a fraction of the total number of EFMs was needed to compute the optimal solution.

  • 144.
    Karlsson, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Ning, L.
    On robustness of ℓ1-regularization methods for spectral estimation2014In: Proceedings of the IEEE Conference on Decision and Control, IEEE conference proceedings, 2014, no February, p. 1767-1773Conference paper (Refereed)
    Abstract [en]

    The use of ℓ<inf>1</inf>-regularization in sparse estimation methods has received huge attention during the last decade, and applications in virtually all fields of applied mathematics have benefited greatly. This interest was sparked by the recovery results of Candès, Donoho, Tao, Tropp, et al. and has resulted in a framework for solving a set of combinatorial problems in polynomial time by using convex relaxation techniques. In this work we study the use of ℓ<inf>1</inf>-regularization methods for high-resolution spectral estimation. In this problem, the dictionary is typically coherent and existing theory for robust/exact recovery does not apply. In fact, the robustness cannot be guaranteed in the usual strong sense. Instead, we consider metrics inspired by the Monge-Kantorovich transportation problem and show that the magnitude can be robustly recovered if the original signal is sufficiently sparse and separated. We derive both worst case error bounds as well as error bounds based on assumptions on the noise distribution.

  • 145. Georgiou, T. T.
    et al.
    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.
    On time-reversibility of linear stochastic models2014In: IFAC Proceedings Volumes (IFAC-PapersOnline), IFAC Papers Online, 2014, p. 10403-10408Conference paper (Refereed)
    Abstract [en]

    Reversal of the time direction in stochastic systems driven by white noise has been of central importance throughout the development of stochastic realization theory, filtering and smoothing. Similarly, in connection with certain problems in the theory of moments, the duality induced by time reversal was the key to deriving a compact parametrization and an interpretation of solutions. By combining ideas from these two lines of development we present herein a general view and a simplified account of time-reversal in stochastic models.

  • 146.
    Yang, Yuecheng
    et al.
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Dimarogonas, Dimos V.
    KTH, School of Electrical Engineering (EES), Automatic Control. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Hu, Xiaoming
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory. KTH, School of Computer Science and Communication (CSC), Centres, Centre for Autonomous Systems, CAS.
    Opinion consensus of modified Hegselmann-Krause models2014In: Automatica, ISSN 0005-1098, E-ISSN 1873-2836, Vol. 50, no 2, p. 622-627Article in journal (Refereed)
    Abstract [en]

    We consider the opinion consensus problem using a multi-agent setting based on the Hegselmann-Krause (H-K) Model. Firstly, we give a sufficient condition on the initial opinion distribution so that the system will converge to only one cluster. Then, modified models are proposed to guarantee convergence for more general initial conditions. The overall connectivity is maintained with these models, while the loss of certain edges can occur. Furthermore, a smooth control protocol is provided to avoid the difficulties that may arise due to the discontinuous right-hand side in the H-K model.

  • 147.
    Osseiran, Afif
    et al.
    Ericsson Research, Sweden.
    Boccardi, F.
    Braun, V.
    Kusume, K.
    Marsch, P.
    Maternia, M.
    Queseth, O.
    Schellmann, M.
    Schotten, H.
    Taoka, H.
    Tullberg, H.
    Uusitalo, M. A.
    Timus, B.
    Fallgren, Mikael
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    Scenarios for 5G mobile and wireless communications: The vision of the METIS project2014In: IEEE Communications Magazine, ISSN 0163-6804, E-ISSN 1558-1896, Vol. 52, no 5, p. 26-35Article in journal (Refereed)
    Abstract [en]

    METIS is the EU flagship 5G project with the objective of laying the foundation for 5G systems and building consensus prior to standardization. The METIS overall approach toward 5G builds on the evolution of existing technologies complemented by new radio concepts that are designed to meet the new and challenging requirements of use cases today¿s radio access networks cannot support. The integration of these new radio concepts, such as massive MIMO, ultra dense networks, moving networks, and device-to-device, ultra reliable, and massive machine communications, will allow 5G to support the expected increase in mobile data volume while broadening the range of application domains that mobile communications can support beyond 2020. In this article, we describe the scenarios identified for the purpose of driving the 5G research direction. Furthermore, we give initial directions for the technology components (e.g., link level components, multinode/multiantenna, multi-RAT, and multi-layer networks and spectrum handling) that will allow the fulfillment of the requirements of the identified 5G scenarios.

  • 148.
    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.

  • 149.
    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.

  • 150. 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.

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