IX Draconis - a curious ER UMa-type dwarf nova

We report results of an extensive world-wide observing campaign devoted to a very active dwarf nova star - IX Draconis. We investigated photometric behaviour of the system to derive its basic outburst properties and understand peculiarities of IX Dra as well as other active cataclysmic variables, in particular dwarf novae of the ER Uma-type. In order to measure fundamental parameters of the system, we carried out analyses of the light curve, O-C diagram, and power spectra. During over two months of observations we detected two superoutbursts and several normal outbursts. The V magnitude of the star varied in the range 14.6 - 18.2 mag. Superoutbursts occur regularly with the supercycle length of 58.5+/-0.5 d. When analysing data over the past 20 years, we found that the supercycle length is increasing at a rate of P_dot = 1.8 * 10^{-3}. Normal outbursts appear to be irregular, with typical occurrence times in the range 3.1 - 4.1 d. We detected a double-peaked structure of superhumps during superoutburst, with the secondary maximum becoming dominant near the end of the superoutburst. The mean superhump period observed during superoutbursts equals 0.066982(36) d, which is constant over the last two decades of observations. Based on the power spectrum analysis, the evaluation of the orbital period was problematic. We found two possible values: the first one, 0.06641(3) d, which is in agreement with previous studies and our O-C analysis (0.06646(2) d), and the second one, 0.06482(3) d, which is less likely. The evolutionary status of the object depends dramatically on the choice between these two values. A spectroscopic determination of the orbital period is needed. We updated available information on ER UMa-type stars and present a new set of their basic statistics. Thereby, we provide evidence that this class of stars is not uniform.Comment: Accepted for publication in MNRAS; 15 pages, 15 figures, 6 tables; typo correcte

Systems Structure and Control

The title of the book System, Structure and Control encompasses broad field of theory and applications of many different control approaches applied on different classes of dynamic systems. Output and state feedback control include among others robust control, optimal control or intelligent control methods such as fuzzy or neural network approach, dynamic systems are e.g. linear or nonlinear with or without time delay, fixed or uncertain, onedimensional or multidimensional. The applications cover all branches of human activities including any kind of industry, economics, biology, social sciences etc

Design of wide-area damping control systems for power system low-frequency inter-area oscillations

The recently developed robust control theories and wide-area measurementtechnologies make the wide-area real-time feedback control potentially promising. Theobjective of this research is to develop a systematic procedure of designing a centralizeddamping control system for power grid inter-area oscillations by applying wide-areameasurement and robust control techniques while putting emphasis on several practicalconsiderations.The first consideration is the selection of stabilizing signals. Geometric measuresof controllability/observability are used to select the most effective stabilizing signals andcontrol sites. Line power flows and currents are found to be the most effective inputsignals. The second consideration is the effects of time-delay in the communication ofinput/output signals. Time-delays reduce the efficiency of the damping control system. Insome cases, large delays can destabilize the system. Time-delays should be modeled inthe controller design procedure so that the resulting controller can handle a range of timedelays.In this work, time-delays are modeled by Padé Approximations and the delayuncertainty is described by Linear Fractional Transformations (LFT). The thirdconsideration is the controller robustness. The synthesis of the controller is defined as aproblem of mixed H2/H∞ output-feedback control with regional pole placement and isresolved by the Linear Matrix Inequality (LMI) approach. The controller designed byrobust control techniques has satisfactory performance in a wide range of operatingpoints. The fourth consideration is the efficiency of the controller designed by lineartechniques in realistic nonlinear discrete environments. A tuning process and nonlinearsimulations are used to modify the controller parameters to ensure the performance androbustness of the controller designed with linear techniques. The last consideration is theselection of PMU data reporting rates. The performance of controllers designed in the sdomainis tested in digital environments and proper PMU data reporting rates are selectedwith consideration of the effects of time-delay.The design procedure of wide-area damping systems is illustrated by three studysystems. The first study system is a two-area four-machine system. The second one is theNew England 39-bus 10-machine system. The last one is a 29-generator 179-bus studysystem, which is a reduced order model of the Western Electricity Coordinating Council(WECC) system

Analysis and design of robust stabilizing modified repetitive control systems

In control system practice, high precision tracking or attenuation for periodic signals is an important issue. Repetitive control is known as an e.ective approach for such control problems. The internal model principle shows that the repetitive control system which contains a periodic generator in the closed-loop can achieve zero steady-state error for reference input or completely attenuate disturbance. Due to its simple structure and high control precision, repetitive control has been widely applied in many systems. To improve existing results on repetitive control theory, this thesis presents theoretical results in analysis and design repetitive control system. The main work and innovations are listed as follows: We propose a design method of robust stabilizing modi.ed repetitive controllers for multiple-input/multiple-output plants with uncertainties. The parameterization of all robust stabilizing modi.ed repetitive controllers for multiple-input/multiple-output plant with uncertainty is obtained by employing H∞ control theory based on the Riccati equation. The robust stabilizing controller contains free parameters that are designed to achieve desirable control characteristic. In addition, the bandwidth of low-pass .lter has been analyzed. In order to simplify the design process and avoid the wrong results obtained by graphical method, the robust stability conditions are converted to LMIs-constraint conditions by employing the delay-dependent bounded real lemma. When the free parameters of the parameterization of all robust stabiliz-ing controllers is adequately chosen, then the controller works as robust stabilizing modi.ed repetitive controller. For a time-varying periodic disturbances, we give an design method of an opti-mal robust stabilizing modi.ed repetitive controller for a strictly proper plant with time-varying uncertainties. A modi.ed repetitive controller with time-varying delay structure, inserted by a low-pass .lter and an adjustable parameter, is developed for this class of system. Two linear matrix inequalities LMIs-based robust stability con-ditions of the closed-loop system with time-varying state delay are derived for .xed parameters. One is a delay-dependent robust stability condition that is derived based on the free-weight matrix. The other robust stability condition is obtained based on the H∞ control problem by introducing a linear unitary operator. To obtain the desired controller, the design problems are converted to two LMI-constrained opti-mization problems by reformulating the LMIs given in the robust stability conditions. The validity of the proposed method is verified through a numerical example.学位記番号：工博甲46

Measurements, Models, Systems and Design

531 s.

Advances in Control of Power Electronic Converters

This book proposes a list of contributions in the field of control of power electronics converters for different topologies: DC-DC, DC-AC and AC-DC. It particularly focuses on the use of different advanced control techniques with the aim of improving the performances, flexibility and efficiency in the context of several operation conditions. Sliding mode control, fuzzy logic based control, dead time compensation and optimal linear control are among the techniques developed in the special issue. Simulation and experimental results are provided by the authors to validate the proposed control strategies

New Low Accretion-Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables

Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray faint, magnetic accretion binaries that accrete at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post common-envelope systems whose orbits have not yet decayed to the point of Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems exhibit spin/orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a ``magnetic siphon''. Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous Intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.Comment: 25 pages, 13 figures, Accepted to Ap