• 한국소음진동공학회논문집
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• 제20권7호
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• pp.677-684
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• 2010

Backlashes of gears provide gears for good lubrication and for removal of the interference between teeth by the wear and manufacturing errors. The backlash is the strong nonlinear factor to gears. This study deals with nonlinear modeling of helical gears with backlash. Excitation of helical gears comes from torque variation, the tooth surface error, and the periodical change of mesh stiffness. To study the effect of torque fluctuation, equation of motion for the single degree of freedom torsional model of helical gears with the periodical change of mesh stiffness and the backlash was derived. The Newmark beta method and the Newton-Raphson method were used to obtain the nonlinear behaviors of mesh forces of helical gears. All excitation frequencies initially caused the tooth separation and single-sided impacts of the gear pair and eventually led to the normal tooth contact. However, some special excitation frequencies caused the single-sided impacts in the entire time as well as the initial time. Damping increase reduced the duration of single-sided impacts, and the backlash increase caused those in the entire time domain.

• Journal of Power Electronics
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• 제17권6호
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• pp.1683-1693
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• 2017

This paper analyses the harmonic pollution to power grids caused by thyristor-controlled devices. It also formulates a mathematic derivation for the voltage spikes in thyristor-controlled branches to explain the harmonic and EMI derived from the reverse-recovery characteristics of the thyristor. With an equivalent nonlinear time-varying voltage source, a detailed simulation model is established, and the periodic dynamic switching characteristic of the thyristor can be explicitly implied. The simulation results are consistent with the probed results from on-site measurements. An improved trigger system with gate-shorted circuit structure is proposed to reduce the voltage spikes that cause EMI. The experimental results indicate that a prototype with the improved trigger system can effectively suppress the voltage spikes.

• 한국철도학회:학술대회논문집
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• 한국철도학회 1999년도 추계학술대회 논문집
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• pp.318-325
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• 1999

Modern AC electric car has PWM(Pulse Width Modulation)-controlled converters, which give rise to higher harmonics. The current harmonics injected from AC electric car is propagated through power feeding circuit, As the feeding circuit is a distributed constant circuit composed of RLC, the capacitance of the feeding circuit and the inductance on the side of power system cause a parallel resonance and a magnification of current harmonics at a specific frequency. The magnified current harmonics usually brings about various problems. That is, the current harmonics makes interference in the adjacent lines of communications and the railway signalling system. Furthermore, in case it flows on the side of power system, not only overheating and vibration at the power capacitors but also wrong operation at the protective devices can occur. Therefore, the exact assessment of the harmonic current flow must be undertaken at design and planning stage for the electric traction systems. From these point of view, this study presents an approach to model and to analyse traction power feeding system focused on the amplification of harmonic current The proposed algorithm is applied to a standard AT(Auto-transformer)-fed test system in which electric car with PWM-controlled converters is running.

• Nuclear Engineering and Technology
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• 제49권5호
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• pp.1006-1009
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• 2017

We discuss the problems of scattering in this framework, and show that the applied method is very useful in the investigation of the effect of the resonance in the observed scattering cross sections. In this study, not only the scattering cross sections but also the decomposition of the scattering cross sections was computed for the ${\alpha}-{\alpha}$ system. To obtain the decomposition of scattering cross sections into resonance and residual continuum terms, the complex scaled orthogonality condition model and the extended completeness relation are used. Applying the present method to the ${\alpha}-{\alpha}$ and ${\alpha}-n$ systems, we obtained good reproduction of the observed phase shifts and cross sections. The decomposition into resonance and continuum terms makes clear that resonance contributions are dominant but continuum terms and their interference are not negligible. To understand the behavior of observed phase shifts and the shape of the cross sections, both resonance and continuum terms are calculated.

• Smart Structures and Systems
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• 제23권6호
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• pp.683-693
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• 2019

In August 2012, during the passage of the typhoon Haikui (1211), large amplitude vibrations were observed on long hangers of the Xihoumen suspension Bridge, which destroyed a few viscoelastic dampers originally installed to connect a pair of hanger ropes transversely. The purpose of this study is to identify the cause of vibration and to develop countermeasures against vibration. Field measurements have been conducted in order to correlate the wind and vibration characteristics of hangers. Furthermore, a replica aeroelastic model of prototype hangers consisting of four parallel ropes was used to study the aeroelastic behavior of hanger ropes and to examine the effect of the rigid spacers on vibration mitigation. It is shown that the downstream hanger rope experiences the most violent elliptical vibration for certain wind direction, and the vibration is mainly attributed to wake interference of parallel hanger ropes. Based on wind tunnel tests and field validation, it is confirmed that four rigid spacers placed vertically at equal intervals are sufficient to suppress the wake-induced vibrations. Since the deployment of spacers on hangers, server hanger vibrations and clash of hanger ropes are never observed.

• Journal of Information Processing Systems
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• 제15권3호
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• pp.670-681
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• 2019

Orthogonal frequency division multiplexing (OFDM) is a system which is used to encode data using multiple carriers instead of the traditional single carrier system. This method improves the spectral efficiency (optimum use of bandwidth). It also lessens the effect of fading and intersymbol interference (ISI). In 1995, digital audio broadcast (DAB) adopted OFDM as the first standard using OFDM. Later in 1997, it was adopted for digital video broadcast (DVB). Currently, it has been adopted for WiMAX and LTE standards. In this project, a Verilog design is employed to implement an OFDM transmitter (DAC block) and receiver (FFT and ADC block). Generally, OFDM uses FFT and IFFT for modulation and demodulation. In this paper, 16-point FFT decimation-in-frequency (DIF) with the radix-2 algorithm and direct summation method have been analyzed. ADC and DAC in OFDM are used for conversion of the signal from analog to digital or vice-versa has also been analyzed. All the designs are simulated using Verilog on ModelSim simulator. The result generated from the FFT block after Verilog simulation has also been verified with MATLAB.

• Advances in aircraft and spacecraft science
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• 제6권1호
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• pp.31-49
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• 2019

Flutter is a dangerous phenomenon encountered in flexible structures subjected to aerodynamic forces. This includes aircraft, helicopter blades, engine rotors, buildings and bridges. Flutter occurs as a result of interactions between aerodynamic, stiffness and inertia forces on a structure. The conventional method for designing a rotor blade to be free from flutter instability throughout the helicopter's flight regime is to design the blade so that the aerodynamic center (AC), elastic axis (EA) and center of gravity (CG) are coincident and located at the quarter-chord. While this assures freedom from flutter, it adds constraints on rotor blade design which are not usually followed in fixed wing design. Periodic Structures have been in the focus of research for their useful characteristics and ability to attenuate vibration in frequency bands called "stop-bands". A periodic structure consists of cells which differ in material or geometry. As vibration waves travel along the structure and face the cell boundaries, some waves pass and some are reflected back, which may cause destructive interference with the succeeding waves. In this work, we analyze the flutter characteristics of a helicopter blades with a periodic change in their sandwich material using a finite element structural model. Results shows great improvements in the flutter forward speed of the rotating blade obtained by using periodic design and increasing the number of periodic cells.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제12권12호
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• pp.5898-5916
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• 2018

In this paper, the downlink performance of multi-cell distributed antenna systems (DAS) with a single user in each cell is investigated. Assuming the channel state information is available at the transmitter, four transmission modes are formulated as combinations of remote antenna units (RAUs) selection and cooperative transmission, namely, non-cooperative transmission without RAU selection (NCT), cooperative transmission without RAU selection (CT), non-cooperative transmission with RAU selection (NCT_RAUS), and cooperative transmission with RAU selection (CT_RAUS). By using probability theory, the cumulative distribution function (CDF) of a user's signal to interference plus noise ratio (SINR) and the system ergodic capacity under the above four modes are determined, and their closed-form expressions are obtained. Furthermore, the system energy efficiency (EE) is studied by introducing a realistic power consumption model of DAS. An expression for determining EE is formulated, and the closed-form tradeoff relationship between spectral efficiency (SE) and EE is derived as well. Simulation results demonstrate their consistency with the theoretical analysis and reveal the factors constraining system EE, which provide a scientific basis for future design and optimization of DAS.

• Journal of Power Electronics
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• 제19권6호
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• pp.1527-1535
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• 2019

Because of the shortcomings of the PID controllers and traditional drive systems of permanent magnet synchronous motors (PMSMs), a PMSM passivity-based control (PBC) drive system based on a quasi-Z source matrix converter (QZMC) is proposed in this paper. The traditional matrix converter is a buck converter with a maximum voltage transmission ratio of only 0.866, which limits the performance of the driven motor. Therefore, in this paper a quasi-Z source circuit is added to the input side of the two-stage matrix converter (TSMC) and its working principle has also been verified. In addition, the controller of the speed loop and current loop in the conventional vector control of a PMSM is a PID controller. The PID controller has the problem since its parameters are difficult to adjust and its anti-interference capability is limited. As a result, a port controlled dissipative Hamiltonian model (PCHD) of a PMSM is established. Thereafter a passivity-based controller based on the interconnection and damping assignment (IDA) of a QZMC-PMSM is designed, and the stability of the equilibrium point is theoretically verified. Simulation and experimental results show that the designed PBC control system of a PMSM based on a QZMC can make the PMSM run stably at the rated speed. In addition, the system has strong robustness, as well as good dynamic and static performances.

• ETRI Journal
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• 제42권6호
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• pp.976-986
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• 2020

Internet of Things (IoT) is considered the future network to support wireless communications. To realize an IoT network, sufficient spectrum should be allocated for the rapidly increasing IoT devices. Through cognitive radio, unlicensed IoT devices exploit cooperative spectrum sensing (CSS) to opportunistically access a licensed spectrum without causing harmful interference to licensed primary users (PUs), thereby effectively improving the spectrum utilization. However, an open access cognitive IoT allows abnormal IoT devices to undermine the CSS process. Herein, we first establish a hard-combining attack model according to the malicious behavior of falsifying sensing data. Subsequently, we propose a weighted sequential hypothesis test (WSHT) to increase the PU detection accuracy and decrease the sampling number, which comprises the data transmission status-trust evaluation mechanism, sensing data availability, and sequential hypothesis test. Finally, simulation results show that when various attacks are encountered, the requirements of the WSHT are less than those of the conventional WSHT for a better detection performance.