http://www.ifac-papersonline.net/ en Copyright 08:00 PM Wednesday 22, 2012 IFAC-PapersOnline http://www.ifacpapersonline.com 08:00 PM Wednesday 22, 2012 08:00 PM Wednesday 22, 2012 ET webmaster@ifac-papersonline.net webmaster@ifac-papersonline.net http://www.ifac-papersonline.net/Detailed/44701.html 04:00 PM Wednesday 31, 1969 This paper formulates the problems of stability testing and stabilization of an infinite platoon of vehicles within the 2D polynomial framework, that is, the dynamics of the problem is described using a fraction of two bivariate polynomials. Recent computational tools for testing positivity of polynomial matrices are suggested for stability testing and stabilization. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44667.html 04:00 PM Wednesday 31, 1969 System identification for networked control is considered. Due to the random time-delays in the network, it can be difficult to work with a discrete-time model and a continuous-time model is therefore chosen here. A covariance function based method is proposed for estimating the parameters, which relies on the second order statistical properties of the output signal and that the input signal samples can be taken from a discrete-time white noise sequence. Numerical studies indicate that the method is robust to additive measurement noise and packet drop-outs. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44655.html 04:00 PM Wednesday 31, 1969 To reduce their energy costs, many buildings are equipped with multiple heating sources with different dynamics, gains and energy prices, an example is the use of a hot water based central heating and local electric convectors as a complementary heating source. The control problem objective is to minimize the heating energy bills while maintaining a certain indoor thermal comfort. In order to attain this goal, a model predictive approach was used to optimally control the available heating systems regarding their energy prices and the heat demand. This paper presents a distributed model predictive control (dMPC) algorithm based a modified version of Benders' decomposition for temperature regulation in buildings in order to consider the thermal coupling between the adjacent zones. Simulation studies are presented in order to illustrate the effectiveness of the proposed control strategy. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44669.html 04:00 PM Wednesday 31, 1969 The problem of maximizing a utility function while limiting the outage probability below an appropriate threshold is investigated. A coded-division multi access wireless network under mixed Nakagami-lognormal fading is considered. Solving such a utility maximization problem is difficult because the problem is non-convex and non-geometric with mixed integer and real decision variables and no explicit functions of the constraints are available. In this paper, two methods to the solution of the utility maximization problem are proposed. By the first method, a simple explicit outage approximation is used and the constraint that rates are integers is relaxed yielding a standard convex programming optimization that can be solved quickly but at the price of a reduced accuracy. The second method uses a more accurate outage approximation, which allows one solving the utility maximization problem by the Lagrange duality for non-convex problems and contraction mapping theory. Numerical results show that the first method performs well for average values of the outage requirements, whereas the second one is always more accurate, but is also more computationally expensive. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44709.html 04:00 PM Wednesday 31, 1969 In this paper, we propose a new approach to the problem of community detection in graphs. It is based on a model of opinion dynamics with decaying confidence. This model is a multi-agent system where each agent receives the opinions of its neighbors and then updates its opinion by taking a weighted average of its own opinion and those of its neighbors that are within some confidence range. The confidence ranges are getting smaller at each time step: an agent gives repetitively confidence only to the neighbors that approach sufficiently fast its own opinion. Under that constraint, global consensus may not be achieved and the agents may only reach local agreement organizing themselves in communities. A characterization of these communities is given in terms of eigenvalues of normalized Laplacian matrices of graphs. This shows that our model of opinion dynamics with decaying confidence provides an appealing approach to community detection in graphs. Experimental results show that our approach is also effective. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44627.html 04:00 PM Wednesday 31, 1969 In this paper we present a stochastic model predictive control (SMPC) approach for networked control systems (NCSs) that are subject to time-varying sampling intervals and time-varying transmission delays. These network-induced uncertain parameters are assumed to be described by random processes, having a bounded support and an arbitrary continuous probability density function. Assuming that the controlled plant can be modeled as a linear system, we present a SMPC formulation based on scenario enumeration and quadratic programming that optimizes a stochastic performance index and provides closed-loop stability in the mean-square sense. Simulation results are shown to demonstrate the performance of the proposed approach. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44659.html 04:00 PM Wednesday 31, 1969 We study the synchronization of a set of SISO subsystems interconnected via a time-invariant Laplacian matrix. By synchronization we mean that the outputs of all subsystems must be asymptotically equal to each other and behave in the same manner. We assume that the subsystems can be represented as the sum of a common nonzero transfer function plus a perturbation. A Nyquist-type criterion is established which ensures synchronization provided that the convex hull of the frequency responses of the subsystems does not intersect a certain region defined by the spectrum of the interconnection matrix. The result is applied to a variety of examples of different nature for which synchronization takes place. A counter example for which complete synchronization is impossible is also presented. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44677.html 04:00 PM Wednesday 31, 1969 As a consequence of the new requirements for low energy consumption in buildings, intelligent embedded control systems will be widely deployed for domestic and office aerologic applications. Our work is concerned with the real-time tuning and application protocol synthesis for a wireless controller PAN (Personal Area Network) based on the IEEE 802.15.4 MAC (Medium Access Control) layer. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44733.html 04:00 PM Wednesday 31, 1969 In this paper we study the consensus problem previously introduced in (Nourian, et. al.(2009)) by the use of Stochastic Mean Field (MF) (Nash Certainty Equivalence (NCE)) Control framework. We explicitly compute the unique solution of the MF (NCE) equation system corresponding to the large population dynamic game model. We also show that the set of the MF (NCE) control laws possesses an epsilon_N-Nash equilibrium property where epsilon_N goes to zero as the population size, N, goes to infinity. These control strategies drive each agent towards track the overall population's initial state distribution in which is reached asymptotically, thus achieving mean consensus. In the MF (NCE) set-up each agent has a priori information on the initial state distribution of the overall population; relaxing this a priori information gives rise to the localized feedback MF (NCE) control laws where each agent observes a time-varying random subset of the overall population. These analyses begin to bridge between (i) the stochastic MF (NCE) control methodology with a priori information and only individual state feedback, and (ii) standard consensus algorithms which involve real time observations of other agent states. Finally, we present the centralized optimal control model of the problem and compare it with the (decentralized) MF (NCE) dynamic game model. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44735.html 04:00 PM Wednesday 31, 1969 We consider the problem of designing distributed output-feedback controllers that achieve H2 and H_infty performance objectives for a group of plants dynamically interconnected via an arbitrary directed communication network. For a particular class of discrete-time linear time-invariant interconnected systems that are characterized by a structural property of their state-space matrices, we design stabilizing distributed controllers which can use the available network along with the plants of the interconnected system. This is achieved by means of parametrization for the output-feedback linear controllers that linearizes the closed-loop H2 and H_infty norm conditions and provides equivalent linear matrix inequalities (LMIs). Using these LMIs, we formulate the minimization of H2 and H_infty norms as semi-definite programs (SDPs) that can be efficiently solved using well-established and standard techniques and tools. The solutions of these SDPs allow us to synthesize the corresponding controllers that are realizable over the given network. Even though we provide only sufficiency conditions for the design of stabilizing distributed controllers, simulations show that the synthesized controllers we obtain provide good performance in spite of being suboptimal compared to the centralized controller. In essence, we gain the advantage of designing realizable distributed controllers at the expense of some performance degradation compared to the centralized solutions. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44645.html 04:00 PM Wednesday 31, 1969 This paper presents a tracking algorithm for PTZ (pan-tilt-zoom) cameras. The tracked objects are Mini 1:43 scale RC cars that have been described by a unicycle model. The algorithm is based on the combination of EKF (Extended Kalman Filter) and PF (Particle Filters). A scanning procedure is used to explore the environment. Once targets are detected, EKF is used to predict their future position. PTZ are then moved in order to guarantee a certain probability of targets detection at the next time instant. If a target is lost, particle filters is exploited. If target is found again EKF is restored. If this does not happen in a predefined number of steps, the scanning procedure restarts. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44649.html 04:00 PM Wednesday 31, 1969 In this paper alternative frequency-domain criteria are provided for the stability of discrete-time networked control systems with time-varying delays. These criteria are in various situations less conservative than the existing frequency-domain conditions as is demonstrated by means of an example. In addition, new stability conditions are presented that allow for multiple sensor-to-controller and controller-to-actuator channels exhibiting different delay characteristics. The stability conditions are formulated in terms of the H_infinity norm and the structured singular value. As a result, the obtained results can be used directly for controller synthesis via standard robust control techniques. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44687.html 04:00 PM Wednesday 31, 1969 This paper is concerned with discrete-time distributed average-consensus with limited communication data-rate and switching communication topologies. We design a distributed encoding-decoding scheme based on difference quantization with dynamic scaling and a control protocol based on a symmetric compensation method. We develop an adaptive scheme to select the numbers of quantization levels. The number of quantization levels of each quantizer is tuned on-line according to whether the associated communication channel is active or not at the last step. We prove that if the network is jointly connected, then under the protocol designed, average-consensus can be asymptotically achieved without steady-state error, and the convergence rate is quantified. Especially, if the duration of link failures of all communication channels is bounded, then the control gain and the scaling function can be selected properly such that 5-level quantizers suffice for asymptotic average-consensus with an exponential convergencerate. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44695.html 04:00 PM Wednesday 31, 1969 In this paper, we analyze distributed average consensus algorithms, both deterministic and gossip based, with respect to a new metric related to the energy cost of communication among agents. We first introduce a new notion of communication complexity as a metric to assess the energy efficiency properties of consensus algorithms. We provide explicit formulas to compute the communication complexity of deterministic algorithms and gossip based algorithms depending on different stopping criteria. We also show that the gossip based algorithms have less communication complexity than the deterministic counterparts under mild conditions, usually satisfied by a large number of networks. We also show that the gossip algorithm with minimum communication complexity can be effectively computed as the solution of a convex optimization problem. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44685.html 04:00 PM Wednesday 31, 1969 This paper deals with the double integrator consensus problem. The objective is the design of a new consensus algorithm for continuous-time multi-agent systems. The dynamic of agents is assumed to be of double integrator type. The proposed algorithm considers that there are no sensors to measure the velocity of the agents. Thus the classical double integrator consensus algorithm leads to an oscillatory behavior if the communication graph is undirected and to instability if the graph is directed. The novel algorithm proposes to sampled, in an appropriate manner, part of the multi-agent systems state such that the algorithm converges. An expression of the consensus equilibrium is provided. Some examples are provided to show the efficiency of the new algorithm. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44703.html 04:00 PM Wednesday 31, 1969 In this paper we study formation control of a network of communicating mobile agents with double integrator dynamics. All agents run a consensus algorithm, several leader agents are further subject to an external input. We put this problem in a control theoretical framework and show that the controllability of the entire system via external control input depends only on the controllability of the follower system through the leaders. We then show necessary and sufficient conditions for the follower system to be controllable and relate the obtained conditions to the existing results on topological properties of the follower communication graph. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44715.html 04:00 PM Wednesday 31, 1969 We address the design of optimally decentralized controllers for linear discrete-time systems. Decentralized controllers are designed that use local measurements and a minimal number of additional measurement links between the subsystems and the controllers. The structure of the decentralized controller, i.e. the additional links between the subsystems and the controllers, is not specified in advance but included into the controller design. We consider state feedback for higher order subsystems and define a pattern matrix to deal with the block structure of the controller. We formulate this problem as a maximization of the degree of decentralization, subject to a given error performance in terms of the Hinf-norm between the centralized and the decentralized closed loop. For the resulting non-convex optimization problem, numerically tractable convex relaxations are provided and an example shows the effectiveness of this approach. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44671.html 04:00 PM Wednesday 31, 1969 This paper considers formation shape control of a team of four point agents, for the most part in the plane. Control laws based on specified interagent distances are used. For a complete graph, specification of all interagent distances determines the formation shape uniquely. Krick, Broucke and Francis showed that for a now almost standard control law, there may exist equilibrium formation shapes with incorrect interagent distances. This paper studies such equilibria, shows that in some cases they are necessarily unstable. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44711.html 04:00 PM Wednesday 31, 1969 The most effective solution for energy saving in low-rate wireless sensor networks is maintaining each node in a doze state as long as possible. In order to guarantee network connectivity, the intervals at which the network sensors are turned on and off have to be coordinated. Therefore, synchronization is a very critical function for sensor networks. In this paper, we analyze the Zigbee MAC performance in sensor networks deployed in multi-hop topologies. For this networks, critical inefficiencies can arise due to transmissions performed by hidden nodes. We evaluate the impact of different synchronization schemes on the network performance, both in terms of network capacity and in terms of energy consumption. We also show how the synchronization function can be opportunistically configured in order to emulate a token-like access mechanism, able to significantly improve the network performance. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44637.html 04:00 PM Wednesday 31, 1969 In this paper the sampled-data stabilization of linear time-invariant systems with feedback delay is considered. We assume that the delay is time-varying and that its value is approximatively known. We investigate how to use the available information about the evolution of delays for adapting the control law. Numerical methods for the design of a delay-dependent controller are presented. This procedure may be used for providing a control for some cases in which the stabilization cannot be ensured using a controller with a fixed structure. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44693.html 04:00 PM Wednesday 31, 1969 In this paper a novel consensus based distributed recursive algorithm is proposed for real time change detection using sensor networks. Convergence of the algorithm to the optimal centralized solution defined by a weighted sum of the results of local signal processing is proved in the cases of constant and time varying forgetting factors of the underlying recursions, assuming correlated data and different local values of the parameter changes. Using the same methodology, a consensus based recursive generalized likelihood ratio change detection scheme is also proposed. Simulation results illustrate characteristic properties of the algorithms. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44723.html 04:00 PM Wednesday 31, 1969 This paper deals with a distributed coordination problem including collision avoidance. The problem is solved by using a Command Governor strategy based on mixed integer optimization. First, we present an algorithm to find an appropriate command in the centralized case, then a distributed sequential procedure is described. Simulations are reported for comparisons. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44713.html 04:00 PM Wednesday 31, 1969 This work considers the problem of obtaining optimal estimates via distributed computation in a large scale system. The electric power system, the transportation system, and generally any computer or network system, are examples of large scale systems: a decentralized estimation of signals based on observations acquired by spatially distributed sensors is the basis for a wide range of important applications. In this work, we focus on the problem of reconstructing the initial state of a linear network in the presence of process and measurement noise. We consider a local model information setup, in which the entire dynamical and measurement model is nowhere available and cannot be reconstructed for the computation. Our estimation procedure relies upon a novel technique to solve a consistent system of linear equations, for which we prove correctness and convergence. In the second part of the paper we consider the problem of detecting anomalies in a large scale network driven by noise. Despite the theoretical advances in this field of research, the currently available procedures to enforce security in large scale systems are computationally inefficient and numerically unreliable. Using our optimal estimation scheme, we describe a distributed procedure with performance guarantees that only requires local knowledge of the system model. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44731.html 04:00 PM Wednesday 31, 1969 In this paper, we consider the data detection problem involved in distributed estimation over unknown non-orthogonal fading channel. In general, when studying the distributed estimation problem, the impairments induced by communication channels are restricted to additive noise, quantization or packet loss. In addition, communication protocols are often of TDMA or FDMA type. Herein, by modulating the local data with doubly spread waveforms, we show that although each node receives a mixture of data transmitted by its neighbors, these data exhibit a trilinear structure, which can be used for separating the neighbors contributions. We state identifiability conditions and study the embedding of the data detection steps in a distributed estimation problem. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif http://www.ifac-papersonline.net/Detailed/44631.html 04:00 PM Wednesday 31, 1969 We consider the optimal network frequency control in electrical power systems. The overall power system consists of several subsystems distributed over a network. We implement a method of synthesizing a distributed dynamic output feedback controller (Langbort et al., 2004a) for such a system consisting of several subsystems distributed over an arbitrary graph. This H-infinity-based distributed approach is then compared with the classical AGC controller, the centralized H-infinity solution and different MPC approaches. http://www.ifac-papersonline.net/static/luna/images/ifac/icon-download.gif