bluuebunny/crossref_metadata_2025_split huggingface dataset:
How platform leaders win
Authors: Hidding Gezinus J. , Williams Jeff , Sviokla John J. 
Published: February 2011

PurposeTo study successful strategies in platform industries, which are IT products that enable (a network of) users to communicate with each other, and that, consequently, exhibit network effects.Design/methodology/approachThe authors studied 15 platform industries, including their first‐mover, early entrants and current leaders. Also studied were historical documents, for example to find the time of market entry.FindingsUnlike traditional products (e.g. consumer products), platforms evolve over time by technically integrating separate platforms (“embrace and extend”). Two key patterns were found among platform leaders: follower advantage and staircase strategies. While follower advantage is also prevalent in traditional industries, staircase strategies are not. Complementary resources (i.e. resources the firm possesses outside of the product in question, for example R&D skills or customer relations) did not explain why the current leaders won.Research limitations/implicationsFirst‐mover advantage was largely illusionary and follower advantage prevalent. Some leaders of platform industries entered the industry early and some entered late, but it is not clear why a particular entry timing was successful in one platform industry but not in another. The authors cannot (yet) predict how a platform industry evolves, particularly in the presence of network effects.Practice implicationsTo become the leader in a platform industry, do not become the first mover, but enter some time afterwards, and manage the “embrace and extend” platform evolution using staircase strategies.Originality/ valueThis study analyzed a sample of 15 platform industries, i.e. more than a few case studies.
Testing the weak‐form efficiency in African stock markets
Authors: Ntim Collins G. , Opong Kwaku K. , Danbolt Jo , Senyo Dewotor Frank 
Published: February 2011

PurposeThe purpose of this paper is to investigate and compare the weak‐form efficiency of a set of 24 African continent‐wide stock price indices and those of eight individual African national stock price indices.Design/methodology/approachVariance‐ratio tests based on ranks and signs were used to examine the weak‐form efficiency of the 32 stock price indices investigated.FindingsOn average, it was found that irrespective of the test employed, the returns of all the 24 African continent‐wide stock price indices examined in the study are less non‐normally distributed compared to the eight individual national stock price indices examined. The authors also report evidence of the African continent‐wide stock price indices having significantly better weak‐form informational efficiency than their national counterparts.Practical implicationsThe policy implication of this evidence is that the African equity price discovery process can be significantly improved if African stock markets integrate their operations. Economically, this may contribute to improved liquidity and more efficient allocation of capital, which in turn can be expected to have a positive impact on economic growth.Originality/valueThe paper makes two major contributions to the extant literature. First, it offers for the first time a comparative analysis of the informational efficiencies of a sample of national stock price indices as against African continent‐wide stock price indices. Second, there is no prior evidence as to whether African stock markets can improve their informational efficiencies by integrating their operations. The paper fills this gap by demonstrating that the African equity price formation process can be improved if African stock markets integrate their operations.
Measuring volatility persistence and long memory in the presence of structural breaks
Authors: McMillan David , Thupayagale Pako 
Published: February 2011

PurposeThe purpose of this paper is to estimate volatility in African stock markets (ASMs), taking account of periodic level shifts in the mean level of volatility, where the regime shifts are determined endogenously.Design/methodology/approachVolatility estimates are incorporated into standard volatility models to assess the impact of structural breaks on volatility persistence, long memory and forecasting performance for ASMs.FindingsThe results presented here indeed suggest that persistence and long memory in volatility are overestimated when regime shifts are not accounted for. In particular, application of breakpoint tests and a moving average procedure suggest that unconditional volatility displays substantial time variation.Practical implicationsA modification of the standard generalised autoregressive conditional heteroscedasticity model to allow for time variation in the unconditional variance generates improved volatility forecasting performance for some African markets.Originality/valueThis paper describes one of the first studies to incorporate endogenously determined regime shifts into volatility estimates and assess the impact of structural breaks on volatility persistence, long memory and forecasting performance for ASMs.
The rare event risk in African emerging stock markets
Authors: Tolikas Konstantinos 
Published: February 2011

PurposeThe purpose of this paper is to investigate the asymptotic distribution of the extreme daily stock returns in African stock markets over the period 1996‐2007 and examine the implications for downside risk measurement.Design/methodology/approachExtreme value theory methods are used to model adequately the extreme minimum daily returns in a number of African emerging stock markets.FindingsThe empirical results indicate that the generalised logistic distribution best fitted the empirical data over the period of study.Practical implicationsUsing the generalised extreme value and normal distributions for risk assessment could lead to an underestimation of the likelihood of extreme share price declines which could potentially lead to inadequate protection against catastrophic losses.Originality/valueTo the best of the author's knowledge, this is the first study to examine the lower tail distribution of daily returns for African emerging stock markets.
Solution of Raman fiber amplifier equations using MATLAB BVP solvers
Authors: Serdar Gokhan Fikri , Yilmaz Gunes 
Published: February 2011

PurposeThe purpose of this paper is to demonstrate an effective and robust numerical solution for Raman fiber amplifier (RFA) equations which have no explicit solution. MATLAB BVP solvers are addressed for the solution.Design/methodology/approachThe continuation method proposed for the solution of RFA equations using MATLAB BVP solvers is explained. Scripts for improving the power values at the boundaries with continuation, extending fiber length with continuation and calculation of the analytical partial derivatives using the MATLAB Symbolic toolbox are introduced. Comparisons among the different MATLAB BVP solvers have been made. Using the continuation method, signal evolutions for different kinds of RFA amplifier configurations are plotted.FindingsThe paper finds that MATLAB BVP solver with the continuation method can be used in the design of various kinds of RFAs for high powers/long gain fiber spans.Research limitations/implicationsThe paper will assist the fiber optic research community who suffer from two or more point boundary‐value problems. Moreover, the stiffness of the signal evolution which is faced with high pump powers and/or long fiber lengths can be solved with continuation. This superiority of the solver can be used to overcome any stiff changes of the signals for future studies.Practical implicationsThe increased research interests and practical demands for RFAs have been calling for reasonable and efficient means for the performance evaluation of RFAs before the real amplifiers are fabricated. The solution method presented in this paper will be an efficient means for the solution of this issue.Originality/valueMATLAB BVP solvers have been proven to be effective for the numerical solution of RFAs with multiple pumps and signal waves. Using the continuation method, in a distributed RFA with ten pump sources, 2,400 mW total input pump power is achieved. The improvement of the total power is about 1.4 times compared with those of the previously reported methods. Using the MATLAB BVP solvers, total power/fiber span can be improved further using the continuation process with the cost of computational time. This is a notable and promising improvement from a RFA designer's point of view.
A DWT‐based approach for detection of interturn faults in power transformers
Authors: Behjat Vahid , Vahedi Abolfazl 
Published: February 2011

PurposeInterturn winding faults, one of the most important causes of power transformers failures, cannot be detected by existing detection methods until they develop into high‐level faults with more severe damage to the transformer. The purpose of this paper is to describe development of a new discrete wavelet transform (DWT) based approach for detection of winding interturn faults.Design/methodology/approachThe following approach was accomplished for development of the proposed fault detection method in this study. The DWT was first applied to decompose the terminal current signals of a transformer, which in turn were obtained from simulations using a finite elements method model of the transformer, into a series of wavelet components. Based on the characteristic features associated with interturn faults extracted from the decomposed waveforms of the terminal currents, a detection scheme was developed. An experimental setup was used to validate the proposed detection method.FindingsThe results of this study demonstrate the efficacy of DWT applied on terminal currents of the transformer to identify interturn faults on the windings well before such faults lead to a catastrophic failure. It is believed that, based on the present findings, there definitely exists scope for improving interturn fault diagnosis with wavelet transform.Research limitations/implicationsPerforming more detailed studies to find all relevant characteristics of the wavelet transform in this application, identifying the location of the faulted turns along winding, applying the method for indicating early stages of turn insulation deterioration and evaluating other type of wavelets for this application would be some future directions of this research.Practical implicationsWith the proposed method, it is becoming possible to detect early signs of the fault occurrence, so that the necessary corrective actions can be taken to prevent long‐lasting outages and reduce down times of the faulty power transformer. The method will be particularly useful as a complement for the classical protection devices of the power transformers.Originality/valueSome recent studies have been carried out regarding the application of DWT for discrimination between an internal fault and other disturbances such as magnetizing inrush and external faults. This paper extends those studies for the detection of interturn faults using more quantitative and qualitative characteristics features.
A novel PSO‐based technique for optimal design of protective current transformers
Authors: Rashtchi Vahid , Bagheri Amir , Shabani Arash , Fazli Saeed 
Published: February 2011

PurposeThe purpose of this paper is to present a new approach for the optimal design of protective current transformers (CTs). The proposed technique is applied to the design of typical CTs and results are compared with traditional CT design method and genetic algorithm optimization algorithm.Design/methodology/approachThe aim of CT design is to make current measurements more accurately, particularly for the high fault currents which arise in short circuits, and more efficient CT in terms of both size and cost. The present work formulates these objectives as an optimization problem to be solved by particle swarm optimization.FindingsSimulation results demonstrate the effectiveness of this technique in optimizing the CT design parameters. The designed CTs have smaller measurement errors compared to standard values and respond well to high fault currents. Manufacturing costs have also been reduced.Originality/valueIn addition to improved efficiency, the benefits of this method are its treatment of CT design in terms of an equivalent circuit and design parameters. The proposed algorithm also extends the linear operation area of the CT and guarantees its good response to high fault currents that may occur in power systems.
Flexible approximation schemes with numerical and semi‐analytical bases
Authors: Dai Jianhua , Pinheiro Helder , Webb Jonathan P. , Tsukerman Igor 
Published: February 2011

PurposeThe purpose of this paper is to extend the generalized finite‐difference calculus of flexible local approximation methods (FLAME) to problems where local analytical solutions are unavailable.Design/methodology/approachFLAME uses accurate local approximations of the solution to generate difference schemes with small consistency errors. When local analytical approximations are too complicated, semi‐analytical or numerical ones can be used instead. In the paper, this strategy is applied to electrostatic multi‐particle simulations and to electromagnetic wave propagation and scattering. The FLAME basis is constructed by solving small local finite‐element problems or, alternatively, by a local multipole‐multicenter expansion. As yet another alternative, adaptive FLAME is applied to problems of wave propagation in electromagnetic (photonic) crystals.FindingsNumerical examples demonstrate the high rate of convergence of new five‐ and nine‐point schemes in 2D and seven‐ and 19‐point schemes in 3D. The accuracy of FLAME is much higher than that of the standard FD scheme. This paves the way for solving problems with a large number of particles on relatively coarse grids. FLAME with numerical bases has particular advantages for the multi‐particle model of a random or quasi‐random medium.Research limitations/implicationsIrregular stencils produced by local refinement may adversely affect the accuracy. This drawback could be rectified by least squares FLAME, where the number of stencil nodes can be much greater than the number of basis functions, making the method more robust and less sensitive to the irregularities of the stencils.Originality/valuePrevious applications of FLAME were limited to purely analytical basis functions. The present paper shows that numerical bases can be successfully used in FLAME when analytical ones are not available.
An iterative method for optimal resolution‐constrained polar quantizer design
Authors: Petković Marko D. , Perić Zoran H. , Jovanović Aleksandra Ž. 
Published: February 2011

PurposeThe purpose of this paper is to address the problem of polar quantization optimization. Particularly, the aim is to find the method for the optimal resolution‐constrained polar quantizer design.Design/methodology/approachThe new iterative algorithm for determination of the optimal decision and representation magnitude levels and algorithm for optimization of number of phase cells within each magnitude level, is proposed.FindingsAt high rates, the new optimal polar quantizer outperforms the optimal polar compander for 0.2 dB, while the more significant gain should be expected at lower rates. In this paper, in order to enable practical implementation of quantizer model, algorithm which transforms real values for the optimal numbers of phase cells within magnitude levels into integer ones is also proposed. Moreover, the approximate closed form of signal‐to‐quantization ratio is derived.Practical implicationsSince circularly symmetric sources and complex presentation of signals arise in numerous applications, it can be concluded that the usage area of the suggested proposal is very wide (audio coding, image coding, spectral phase coding, synthetic aperture radars systems, coding of the discrete Fourier transform).Originality/valueIt should be emphasized that in contrast to earlier work, where models have been designed under high‐rate assumption, the obtained nonuniform unrestricted polar quantizer is optimal for all rates.
A novel reactive navigation strategy for mobile robots based on chaotic exploration and TTM self‐construction
Authors: Yu Xiaolei , Zhao Zhimin 
Published: February 2011

PurposeThe purpose of this paper is to present a novel method for integrating of chaotic exploration and thinning‐based topological mapping to deal with the “traverse targets and return” problem applied for robot navigation in unknown environments. This new strategy can guarantee the robot stronger ability of exploring unknown environments, as well as recording and selecting optimal trajectory to return.Design/methodology/approachThe chaotic dynamic evolution of controlled multi‐scroll system is linked to the multi‐sensory perception and reactive behaviors of a mobile robot. The thinning‐based topological map (TTM), as the contextual layer of the cognitive system, is adopted to achieve the environmental recording in the process of robot exploration and navigation. Once the robot arrives at the terminal target via avoiding all the obstacles, the TTM has been built in real time. Based on the records in the topological map, a short and smooth point‐to‐point path is generated to achieve the exit from target and to move back to the starting point.FindingsThe simulation results confirmed that the proposed solution is suitable to resolve the robot's tasks of obstacle avoidance, target retrieving, and return, also has better performance than traditional strategies.Originality/valueThe presented novel method focuses integration of chaotic exploration and TTM self‐construction. The chaotic perception and control technique permits the robot to explore most of the environmental information within the smallest explored area. The introduced topological map, generated by applying a thinning algorithm, guarantees a short and smooth returning trajectory for the robot.
Pointwise completeness and pointwise degeneracy of 2D standard and positive Fornasini‐Marchesini models
Authors: Kaczorek Tadeusz 
Published: February 2011

PurposeThe purpose of this paper is to formulate and solve the pointwise completeness and pointwise degeneracy of standard and positive 2D Fornasini‐Marchesini and 2D Roesser models.Design/methodology/approachA procedure is proposed for investigation of the pointwise completeness and pointwise degeneracy of 2D standard and positive Fornasini‐Marchesini and Roesser models.FindingsNecessary and sufficient conditions for pointwise completeness and pointwise degeneracy of standard and positive Fornasini‐Marchesini and Roesser models are established.Originality/valueThe paper proposes a method for investigation of the pointwise completeness and pointwise degeneracy of the standard and positive Fornasini‐Marchesini and Roesser models.
Determining degrees of freedom for eccentricity fault in SRM based on nonlinear static torque function
Authors: Torkaman Hossein , Afjei Ebrahim 
Published: February 2011

PurposeThe purpose of this paper is to determine the degrees of freedom (DOF) for eccentricity fault in switched reluctance motor (SRM) based on nonlinear static torque function to minimize torque ripple for maximum performance in motoring operation.Design/methodology/approachAn algorithm based on nonlinear torque function versus rotor position and percent of rotor eccentricity for the SRM operation is introduced. This algorithm enables accurate determination of different modes of motor operations namely, healthy and faulty conditions. In this approach, SRM is first analyzed by a 3D finite element method for estimation of nonlinear torque function and then the function is approximated by least square, cubic spline and piecewise cubic Hermitian methods. The minimization is performed by random search method and genetic algorithm.FindingsA new procedure for computing the DOF of eccentricity in SRM based on nonlinear torque function is proposed and analyzed. It computes the legal intervals for the radial airgap length as well as the regions of the motor operation under different conditions. The functionability and the feasibility of this algorithm is illustrated by employing it on a three‐phase 6/4 SRM.Practical implicationsThe proposed method and its results can improve the motor control while the implementation is simple and practical. The proposed method can be used for other motors as well.Originality/valueThe boundary of motor operation under fault must be calculated to attain smooth control on motor to achieve high performance expected from the machine. To the best knowledge of the authors, this is the first time such a study has been conducted on SRM.
Speed sensorless induction motor drive with motor choke and predictive control
Authors: Guzinski Jaroslaw , Abu‐Rub Haitham 
Published: February 2011

PurposeThe purpose of this paper is to present a complete solution for speed sensorless AC drive with voltage source inverter, induction machine, and motor choke. Major problems with adjustable speed drives are underlined and the use of motor choke is justified. An AC drive with motor choke can work only if specific modifications in the control algorithms are done.Design/methodology/approachThe goal of the paper is to present new nonlinear vector control method for induction motor drive. In the control system, the presence of motor choke is taken into account. The choke changes the structure of the predictive controller and state observer. The new concept of integrating the predictive controller with electromagnetic forces observer is presented. The paper presents theoretical description of the system as well the simulation and experimental verification.FindingsThe paper shows that the suggested decoupled AC drive control system is operating better than a system without decoupling. The system with motor choke requires modifications in the current controller and observer system. With omitting the motor choke a speed sensorless drive cannot work properly.Practical implicationsThe solution is oriented for industrial applications because in numerous industrial dives the motor choke is utilized. However, with motor choke many sophisticated control algorithms cannot work properly. The concept presented in the paper solves such practical problems.Originality/valueThe paper presents a completely new decoupled field‐oriented control system with load angle controller, predictive current controller and state observer for AC drive with motor choke.
Modeling and optimal control of variable reluctance stepper motor
Authors: Stepien Slawomir , Bernat Jakub 
Published: February 2011

PurposeThe purpose of this paper is to present a method of modeling the variable reluctance stepper motor using the time‐stepping finite element technique. The proposed model is used to obtain the optimal control law for the input circuit solving the linear‐quadratic problem.Design/methodology/approachA strongly coupled field‐circuit model of the stepper motor is presented. Also, the method of the optimal control that minimizes the power loss in the motor windings is proposed.FindingsThe proposed optimal control method can be applied to the electrical machines connected to the electronic converters. Calculated control signals may be used to obtain the optimal waveforms of the input voltages at each phase of the analyzed machine.Originality/valueThe paper examines the application of the presented control method to minimize the power loss in the stator windings of the four‐phased variable reluctance stepper motor.