Vol. 12, No. 1, February 2015


Miloš Bjelić, Miodrag Stanojević
Comparison of LMS Adaptive Beamforming Techniques in Microphone Arrays
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DOI: 10.2298/SJEE1501001B
Abstract: This paper discusses principles of microphone array beamforming, specifically the use of LMS algorithm with training sequence. The problem of wideband nature of acoustical signals and its impact on the techniques of beamforming are discussed. Detailed explanation of classic narrowband and wideband LMS beamformers is presented, as well as the modification of narrowband algorithm with pre-steering. Experimental testing and comparison of algorithm performances was conducted and measurement results are presented. The used microphone array is part of Brüel & Kjær acoustical camera, and is comprised of 18 omnidirectional non-uniformly spaced microphones.
Keywords: Microphone array, Adaptive beamforming, LMS, Pre-steering.

Dejan Reljić, Josif Tomić, Željko Kanović
Application of the Goertzel’s Algorithm in the Airgap Mixed Eccentricity Fault Detection
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DOI: 10.2298/SJEE1501017R
Abstract: In this paper, a suitable method for the on-line detection of the airgap mixed eccentricity fault in a three-phase cage induction motor has been proposed. The method is based on a Motor Current Signature Analysis (MCSA) approach, a technique that is often used for an induction motor condition monitoring and fault diagnosis. It is based on the spectral analysis of the stator line current signal and the frequency identification of specific components, which are created as a result of motor faults. The most commonly used method for the current signal spectral analysis is based on the Fast Fourier transform (FFT). However, due to the complexity and memory demands, the FFT algorithm is not always suitable for real-time systems. Instead of the whole spectrum analysis, this paper suggests only the spectral analysis on the expected airgap fault frequencies employing the Goertzel’s algorithm to predict the magnitude of these frequency components. The method is simple and can be implemented in real-time airgap mixed eccentricity monitoring systems without much computational effort. A low-cost data acquisition system, supported by the LabView software, has been used for the hardware and software implementation of the proposed method. The method has been validated by the laboratory experiments on both the line-connected and the inverter-fed three-phase fourpole cage induction motor operated at the rated frequency and under constant load at a few different values. In addition, the results of the proposed method have been verified through the motor’s vibration signal analysis.
Keywords: Induction motor, Airgap mixed eccentricity, MCSA, Goertzel’s algorithm.

Nenad Petrović, Velibor Pjevalica, Vladimir Vujičić
The Theorem about the Transformer Excitation Current Waveform Mapping into the Dynamic Hysteresis Loop Branch for the Sinusoidal Magnetic Flux Case
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DOI: 10.2298/SJEE1501033P
Abstract: This paper analyses aspects of the approximation theory application on the certain subsets of the measured samples of the transformer excitation current and the sinusoidal magnetic flux. The presented analysis is performed for single-phase transformer case, Epstein frame case and toroidal core case. In the paper the theorem of direct mapping the transformer excitation current in the stationary regime is proposed. The excitation current is mapped to the dynamic hysteresis loop branch (in further text DHLB) by an appropriate cosine transformation. This theorem provides the necessary and satisfactory conditions for above described mapping. The theorem highlights that the transformer excitation current under the sinusoidal magnetic flux has qualitatively equivalent information about magnetic core properties as the DHLB. Furthermore, the theorem establishes direct relationship between the number of the transformer excitation current harmonics and their coefficients with the degree of the DHLB interpolation polynomial and its coefficients. The DHLB interpolation polynomial is calculated over the measured subsets of samples representing Chebyshev nodes of the first and the second kind. These nonequidistant Chebyshev nodes provides uniform convergence of the interpolation polynomial to the experimentally obtained DHLB with an excellent approximation accuracy and are applicable on the approximation of the static hysteresis loops and the DC magnetization curves as well.
Keywords: Excitation Transformer Current, Dynamic Hysteresis Loop, Sinusoidal Magnetic Flux, Approximation, Chebyshev nodes.

Kosta Jovanović, Jovana Vranić, Nadica Miljković
Hill’s and Huxley’s Muscle Models – Tools for Simulations in Biomechanics
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DOI: 10.2298/SJEE1501053J
Abstract: Numerous mathematical models of human skeletal muscles have been developed. However, none of them is adopted as a general one and each of them is suggested for some specific purpose. This topic is essential in humanoid robotics, since we firstly need to understand how human moves and acts in order to exploit human movement patterns in robotics and design human like actuators. Simulations in biomechanics are intensively used in research of locomotion, safe human-robot interaction, development of novel robotic actuators, biologically inspired control algorithms, etc. This paper presents two widely adopted muscle models (Hill’s and Huxley’s model), elaborates their features and demonstrates trade-off between their accuracy and efficiency of computer simulations. The simulation setup contains mathematical representation of passive muscle structures as well as mathematical model of an elastic tendon as a series elastic actuation element. Advanced robot control techniques point out energy consumption as one of the key issues. Therefore, energy store and release mechanism in elastic elements in both tendon and muscle, based on the simulation models, are considered.
Keywords: Biomechanics, Musculoskeletal Modeling, Computer Simulation, Energy Distribution.

Miloš Marjanović, Danijel Danković, Aneta Prijić, Zoran Prijić, Nebojša Janković, Vojkan Davidović
Modeling and PSPICE Simulation of NBTI Effects in VDMOS Transistors
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DOI: 10.2298/SJEE1501069M
Abstract: In this paper the results of modeling and simulation of NBTI effects in p-channel power VDMOS transistor have been presented. Based on the experimental results, the threshold voltage shifts and changes of transconductance during the NBT stress have been modeled and implemented in the PSPICE model of the IRF9520 transistor. By predefining the threshold voltage value before the NBT stress, and by assigning the stress time, transfer characteristics of the transistor are simulated. These characteristics are within (1.33÷11.25)% limits in respect to the measured ones, which represents a good agreement.
Keywords: NBTI, Modeling, PSPICE simulation, VDMOS transistor.

Petar B. Petrović, Nikola Lukić, Ivan Danilov
Compliant Behaviour of Redundant Robot Arm - Experiments with Null-Space
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DOI: 10.2298/SJEE1501081P
Abstract: This paper presents theoretical and experimental aspects of Jacobian nullspace use in kinematically redundant robots for achieving kinetostatically consistent control of their compliant behavior. When the stiffness of the robot endpoint is dominantly influenced by the compliance of the robot joints, generalized stiffness matrix can be mapped into joint space using appropriate congruent transformation. Actuation stiffness matrix achieved by this transformation is generally nondiagonal. Off-diagonal elements of the actuation matrix can be generated by redundant actuation only (polyarticular actuators), but such kind of actuation is very difficult to realize practically in technical systems. The approach of solving this problem which is proposed in this paper is based on the use of kinematic redundancy and nullspace of the Jacobian matrix. Evaluation of the developed analytical model was done numerically by a minimal redundant robot with one redundant d.o.f. and experimentally by a 7 d.o.f. Yaskawa SIA 10F robot arm.
Keywords: Kinematic redundancy, Compliance control, Null space stiffness.

Nikola Bošković, Branka Jokanović, Aleksandar Nešić
Frequency Scanning Antenna Arrays with Pentagonal Dipoles of Different Impedances
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DOI: 10.2298/SJEE1501099B
Abstract: In this work we present the benefits of using pentagonal dipoles as radiating elements instead of classical printed dipoles in the design of frequency scanning antenna arrays. We investigate how impedance of pentagonal dipoles, which can be changed in a wide range, influences the overall characteristics of the uniform antenna array. Some very important antenna characteristics such as side lobe level, gain and scanning angle are compared for three different antenna arrays consisting of identical pentagonal dipoles with impedances of 500Ω, 1000Ω and 1500Ω.
Keywords: Antenna array, Frequency scanning, Pentagonal dipoles, Phase shifter, Split-ring resonator.

Nebojša Perić
Fuzzy Logic and Fuzzy Set Theory Based Edge Detection Algorithm
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DOI: 10.2298/SJEE1501109P
Abstract: In this paper we will show a way how to detect edges in digital images. Edge detection is a fundamental part of many algorithms, both in image processing and in video processing. Therefore it is important that the algorithm is efficient and, if possible, fast to carry out. The fuzzy set theory based approach on edge detection is good for use when we need to make some kind of image segmentation, or when there is a need for edge classification (primary, secondary, ...). One example that motivated us is region labeling; this is a process by which the digital image is divided in units and each unit is given a unique label (sky, house, grass, …, etc.). To accomplish that, we need to have an intelligent system that will precisely determine the edges of the region. In this paper we will describe tools from image processing and fuzzy logic that we use for edge detection as well as the proposed algorithm.
Keywords: Fuzzy, Logic, Edge, Detection, Image, Processing.

Željko Jovanović
Data Stream Management System for Moving Sensor Object Data
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DOI: 10.2298/SJEE1501117J
Abstract: Sensor and communication development has led to the development of new types of applications. Classic database data storage becomes inadequate when data streams arrive from multiple sensors. Then, data querying and result presentation are not efficient. The desired results are obtained with a delay, and the database is filled with a large amount of unnecessary data. To adequately support the above applications, Data Stream Management System (DSMS) applications are needed. DSMSs provide real-time data stream processing. In this paper, a client-server system is presented with DSMS realized on the Java WebDSMS application server side. WebDSMS functionalities are tested with simulated data and in real-life usage.
Keywords: CEP, DBMS, DSMS, Data Streams, Simulation.

©2015 Serbian Journal of Electrical Engineering