• Smart Structures and Systems
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• v.9 no.2
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• pp.189-206
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• 2012

This paper focuses on the actuation system combined with a piezoelectric transducer and an electric circuit, which leads to a new insight; the electric actuation system is equivalent to mechanical variable-stiffness actuation systems. By controlling the switch in the circuit, the electric status of the piezoelectric transducer is changed, and consequently a variable-stiffness mechanism is achieved on the electric actuator. This proposed actuator features a shift in the equilibrium point of force, while conventional electrically-induced variable-stiffness actuators feature the variation of the stiffness value. We intensively focus on the equilibrium shift in the actuation system, which has been neglected. The stiffness of the variable-stiffness actuator is periodically modulated by controlling the switch, to suppress the vibration of the system in an open-loop way. It is proved that this electric actuator is equivalent to its mechanical counterpart, and that the electrical version has some practical advantages over the mechanical one. Furthermore, another kind of electrically-induced variable-stiffness actuator, using an energy-recycling mechanism is also discussed from the viewpoint of open-loop vibration control. Extensive numerical simulations provide comprehensive assessment on both electrically-induced variable-stiffness actuators employed for open-loop vibration control.

• Earthquakes and Structures
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• v.1 no.1
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• pp.43-67
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• 2010

A gradient-based evolutionary optimization methodology is presented for finding the optimal design of both the added dampers and their supporting members to minimize an objective function of a linear multi-storey structure subjected to the critical ground acceleration. The objective function is taken as the sum of the stochastic interstorey drifts. A frequency-dependent viscoelastic damper and the supporting member are treated as a vibration control device. Due to the added stiffness by the supplemental viscoelastic damper, the variable critical excitation needs to be updated simultaneously within the evolutionary phase of the optimal damper placement. Two different models of the entire damper unit are investigated. The first model is a detailed model referred to as "the 3N model" where the relative displacement in each component (i.e., the spring and the dashpot) of the damper unit is defined. The second model is a simpler model referred to as "the N model" where the entire damper unit is converted into an equivalent frequency-dependent Kelvin-Voigt model. Numerical analyses for 3 and 10-storey building models are conducted to investigate the characters of the optimal design using these models and to examine the validity of the proposed technique.

• Structural Engineering and Mechanics
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• v.59 no.6
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• pp.1037-1054
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• 2016

Structural parameter evaluation and external force estimation are two important parts of structural health monitoring. But the structural parameter identification with limited input information is still a challenging problem. A new simultaneous identification method in time domain is proposed in this study to identify the structural parameters and evaluate the external force. Each sampling point in the time history of external force is taken as the unknowns in force evaluation. To reduce the number of unknowns for force evaluation the time domain measurements are divided into several windows. In each time window the structural excitation is decomposed by orthogonal polynomials. The time-variant excitation can be represented approximately by the linear combination of these orthogonal bases. Structural parameters and the coefficients of decomposition are added to the state variable to be identified. The extended Kalman filter (EKF) is augmented and selected as the mathematical tool for the implementation of state variable evaluation. The proposed method is validated numerically with simulation studies of a time-invariant linear structure, a hysteretic nonlinear structure and a time-variant linear shear frame, respectively. Results from the simulation studies indicate that the proposed method is capable of identifying the dynamic load and structural parameters fairly accurately. This method could also identify the time-variant and nonlinear structural parameter even with contaminated incomplete measurement.

• Journal of the Institute of Electronics Engineers of Korea SP
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• v.38 no.5
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• pp.576-582
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• 2001

The Multi-Band Excitation(MBE) speech coder uses a different approach for the representation of the excitation signal. It replaces the frame-based single voiced/unvoiced classification of a classical speech coder with a set of such decision over harmonic intervals in the frequency domain. This enables each speech segment to be a mixture of voiced and unvoiced, and improves the synthetic speech quality by reducing decision errors that might occur on the frame-based single voiced and unvoiced decision process when input speech is degraded with noise. The IMBE-LP, improved version of MBE with linear prediction, represents the spectral information of MBE model with linear prediction coefficients to obtain low bit rate of 2.4 kbps. In this Paper, we proposed a variable rate IMBE-LP vocoder that has lower bit rate than IMBE-LP without degrading the synthetic speech quality. To determine the LP order, it uses the spectral band information of the MBE model that has something to do with he input speech's characteristics. Experimental results are riven with our findings and discussions.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.7C
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• pp.962-970
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• 2004

This paper presents an efficient implementation of the 2.4 kbps EHSX(Enhanced Harmonic Stochastic Excitation) speech coder on a TMS320C6701$^{TM}$ floating-point digital signal processor. The EHSX speech codec is based on a harmonic and CELP(Code Excited Linear Prediction) modeling of the excitation signal respectively according to the frame characteristic such as a voiced speech and an unvoiced speech. In this paper, we represent the optimization methods to reduce the complexity for real-time implementation. The complexity in the filtering of a CELP algorithm that is the main part for the EHSX algorithm complexity can be reduced by converting program using floating-point variable to program using fixed-point variable. We also present the efficient optimization methods including the code allocation considering a DSP architecture and the low complexity algorithm of harmonic/pitch search in encoder part. Finally, we obtained the subjective quality of MOS 3.28 from speech quality test using the PESQ(perceptual evaluation of speech quality), ITU-T Recommendation P.862 and could get a goal of realtime operation of the EHSX codec.c.

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.4
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• pp.348-352
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• 2016

This paper addresses the efficient improvement of the Switched Reluctance Motor(SRM) by the proper voltage excitation. In the case of loads with large operational motor-speed differences such as washing machine, an SRM system driven by a constant DC-link voltage is not useful for improving the efficiency. To reduce the effect of the excess DC-link voltage, AC-DC control converter that uses a silicon controlled rectifier instead of diode rectifier is employed in the SRM driver system. AC-DC control converter supplies a proper link voltage for low-speed operation. The experimental results demonstrated that the efficiency of the system was improved at low speeds.

• Structural Engineering and Mechanics
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• v.66 no.2
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• pp.161-172
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• 2018

The passive control of structures using a pendulum tuned mass damper has been extensively studied in the technical literature. As the frequency of the pendulum depends only on its length and the acceleration of gravity, to tune the frequency of the pendulum with that of the structure, the pendulum length is the only design variable. However, in many cases, the required length and the space necessary for its installation are not compatible with the design. In these cases, one can replace the classical pendulum by a virtual pendulum which consists of a mass moving over a curved surface, allowing thus for a greater flexibility in the absorber design, since the length of the pendulum becomes irrelevant and the shape of the curved surface can be optimized. A mathematical model for a building with a pendular tuned mass damper and a detailed parametric analysis is conducted to study the influence of this device on the nonlinear oscillations and stability of the main system under harmonic and seismic base excitation. In addition to the circular profiles, different curved surfaces with softening and hardening characteristics are analyzed. Also, the influence of impact on energy dissipation is considered. A detailed parametric analysis is presented showing that the proposed damper can not only reduce sharply the displacements, and consequently the internal forces in the main structure, but also the accelerations, increasing user comfort. A review of the relevant aspects is also presented.

• Structural Monitoring and Maintenance
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• v.9 no.2
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• pp.107-127
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• 2022

Structural health monitoring (SHM) has been related to damage identification with either operational loads or other environmental loading playing a significant complimentary role in terms of structural safety. In this study, a non-parametric method of time frequency analysis on the measurement is used to address the time-frequency representation for modal parameter estimation and system damage identification of structure. The method employs the wavelet decomposition of dynamic data by using the modified complex Morlet wavelet with variable central frequency (MCMW+VCF). Through detail discussion on the selection of model parameter in wavelet analysis, the method is applied to study the dynamic response of both steel structure and reinforced concrete frame under white noise excitation as well as earthquake excitation from shaking table test. Application of the method to building earthquake response measurement is also examined. It is shown that by using the spectrogram generated from MCMW+VCF method, with suitable selected model parameter, one can clearly identify the time-varying modal frequency of the reinforced concrete structure under earthquake excitation. Discussions on the advantages and disadvantages of the method through field experiments are also presented.

• The Journal of the Acoustical Society of Korea
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• v.38 no.3
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• pp.340-343
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• 2019

In this paper, a problem of mechanical resonance between traction motor's rigid body mode and traction motor's excitation force is introduced, and a bogie design variable affecting the control of resonance response is reviewed numerically. To solve the resonance problem in rotating machinery with variable rotational speeds, resonance frequency should be out of rotational machine's operation range or dynamic stiffness of structures should be increased for resonance response enough to be low. In general, operation range of a traction motor is from 0 r/min to 4800 r/min. It is not possible that all bogie modes are more than 80 Hz. Therefore, it is very important to find design factor affecting resonance response of traction motor's rigid body modes. It is found that key design variable is the gab between transom pipes from finite element analysis. The larger gab is, the higher resonance response when resonance between traction motor's excitation force and traction motor's rigid body mode is happened.

• The Transactions of the Korean Institute of Electrical Engineers B
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• v.54 no.12
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• pp.609-615
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• 2005

A 30kW electrical power conversion system is developed for a variable speed wind turbine. In the wind energy conversion system(WECS) a synchronous generator with field current excitation converts the mechanical energy into electrical energy. As the voltage and the frequency of the generator output vary according to the wind speed, a 6-bridge diode rectifier and a PWM boost chopper is utilized as an ac-dc converter maintaining the constant dc-link voltage with only single switch control. An input current control algorithm for maximum power generation during the variable speed operation is proposed without any usage of speed sensor. Grid connection type PWM inverter converts dc input power to ac output currents into the grid. The active power to the grid is controlled by q-axis current and the reactive power is controlled by d-axis current with appropriate decoupling. The phase angle of utility voltage is detected using software PLL(Phased Locked Loop) in d-q synchronous reference frame. Experimental results from the test of 30kW prototype wind turbine system show that the generator power can be controlled effectively during the variable speed operation without any speed sensor.