Byeongsik Ko | Inha - Academia.edu (original) (raw)

Papers by Byeongsik Ko

Research paper thumbnail of Finite Element Analysis of Carbon Diffusion at 880 ℃ in an Ultra-Low Carbon Steel for Solid Quenching

Korean Journal of Metals and Materials, 2013

Research paper thumbnail of Noise Source Localization by Applying MUSIC with Wavelet Transformation

In inverse acoustic problem with nearfield sources, it is important to separate multiple acoustic... more In inverse acoustic problem with nearfield sources, it is important to separate multiple acoustic sources and to measure the position of each target. This paper proposes a new algorithm by applying MUSIC(Multiple Signal Classification) to the outputs of discrete wavelet transformation with sub-band selection based on the entropy threshold, Some numerical experiments show that the proposed method can estimate the more precise positions than a conventional MUSIC algorithm under moderately correlated signal and relatively low signal-to-noise ratio case.

Research paper thumbnail of Analysis of Structure-borne Noise and Acoustic Sensitivity

한국자동차공학회 1997년 학술강연초록집, Sep 1, 1997

Research paper thumbnail of Replication of Automotive Vibration Target Signal Using Iterative Learning Control and Stewart Platform with Halbach Magnet Array

Transactions of The Korean Society for Noise and Vibration Engineering, 2013

This paper presents the replication of a desired vibration response by iterative learning control... more This paper presents the replication of a desired vibration response by iterative learning control (ILC) system for a vibration motion replication actuator. The vibration motion replication actuator has parameter uncertainties including system nonlinearity and joint nonlinearity. Vehicle manufacturers worldwide are increasingly relying on road simulation facilities that put simulated loads and stresses on vehicles and subassemblies in order to reduce development time. Road simulation algorithm is the key point of developing road simulation system. With the rapid progress of digital signal processing technology, more complex control algorithms including iterative learning control can be utilized. In this paper, ILC algorithm was utilized to produce simultaneously the six channels of desired responses using the Stewart platform composed of six linear electro-magnetic actuators with Halbach magnet array. The convergence rate and accuracy showed reasonable results to meet the requirement...

Research paper thumbnail of Holographic reconstruction of active sources in a three-dimensional enclosure

Journal of the Acoustical Society of America, 1998

This study proposes a method to identify active sources. The theory itself does not limit the app... more This study proposes a method to identify active sources. The theory itself does not limit the application of the proposed method, but the main aim has been focused on finding the location of active noise sources and their strengths in an enclosure. The acoustic holography method can be applied to identify active sources, primary sources if one prefers to call them. However, it will depict all sources including imaginary sources, which one may call secondary sources, which are due to reflections on the walls of enclosure. In this study, an attempt has been made to distinguish these two different types of sources. First, boundaries are modeled by the inhomogeneous Robin boundary condition. It makes it possible to represent the boundaries as the combination of general ideal sources and passive boundaries; the ideal sources imply the primary sources, while the passive boundaries control the secondary sources. Second, this general boundary condition is combined with the holography method...

Research paper thumbnail of Enhancing nearfield acoustic holography using wavelet transform

KSME International Journal

When there are low signal to noise relationships or low coherences between measured pressure and ... more When there are low signal to noise relationships or low coherences between measured pressure and a reference sensor, a pressure field measured and estimated by NAH (Nearfield Acoustic Holography) becomes noisy on the hologram and source planes. This paper proposes a method to obtain the high coherent de-noised pressure signals from low coherent noisy ones by combining a wavelet algorithm with NAH. The proposed method obtains the de-noised field from acoustic fields on a noise source plane reconstructed through backward propagation of NAH. Thus this method does not need high coherent pressure signals on the hologram surface while the conventional nearfield acoustic holography requires high-coherent signals. The proposed method was verified by numerical simulation using noisy signals, composed of original signals and imposed noises distributed on the hologram surface.

Research paper thumbnail of A study on iterative learning control for vibration of Stewart platform

International Journal of Control, Automation and Systems

This paper presents the replication of a desired vibration response by iterative learning control... more This paper presents the replication of a desired vibration response by iterative learning control system for a Stewart platform. The Stewart platform is a multi-input, multi-output system with parameter uncertainties including system nonlinearity and joint nonlinearity. Most vehicle manufacturers are relying on road test simulation facilities in order to reduce development time and to enhance product quality. Road simulation algorithm is essential for developing road test simulation system. With digital signal processing technology, more complex control algorithms including iterative learning control can be utilized. In this paper, a controller based on iterative learning control (ILC) algorithm was developed to produce the desired target response in case of a single actuator as the first experiment after programmed with C language. As a next experiment, the control algorithm was implemented in a road test simulation system using a Stewart platform. A real test was carried out to replicate total six channels of acceleration signals measured at top and left side points of audio player system installed to a car running on Belgian road. The convergence rate and test simulation accuracy higher than 90% showed that the algorithm was acceptable to replicate the target vibration response.

Research paper thumbnail of Design of a slim-type optical pick-up actuator using PMN-PT bimorphs

Smart Materials and Structures, Nov 12, 2006

In this paper, a new optical pickup actuator is proposed using PMN-PT (lead magnesium niobate-lea... more In this paper, a new optical pickup actuator is proposed using PMN-PT (lead magnesium niobate-lead titanate) bimorphs for slim and small form factor optical disk drives. We suggest a novel structure enabling both tracking and focusing motions by changing the moving directions of the two parallel bimorphs. A cymbal-type flextensional structure is used as a displacement amplifier in order to meet the stroke requirement for optical pickup actuators. We have performed the theoretical analyses for the bimorph actuator and displacement amplifier to predict the resultant force and displacement. The proposed actuator based on PMN-PT bimorphs and displacement amplifier has been manufactured, and the experimental results are compared to the analytical predictions. Experimental results agree well with the analytical predictions, showing that the cymbal structure amplifies the displacement twice and the focusing stroke is 52 μm at 10 V.

Research paper thumbnail of Reconstruction of surface velocity field using wavelet transformation and boundary-element method

Archive Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science 1989 1996, Feb 1, 2007

This paper shows the application of discrete wavelet transformation (DWT) to inverse acoustics fo... more This paper shows the application of discrete wavelet transformation (DWT) to inverse acoustics for reconstructing the surface velocity of a noise source. This approach uses the boundary-element analysis based on the measured sound pressure at a set of field points, the Helmholtz integral equations, and wavelet transformation to reconstruct the normal surface velocity field. The reconstructed velocity field can be diverged due to the small measurement errors in the case of nearfield acoustic holography using an inverse boundary-element method. In order to bypass the instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the order of an optimal truncation is difficult to determine. In this paper, a DWT is applied to reduce the computation time for inverse acoustics and to enhance the reconstructed surface velocity field. The computational speed-up is achieved, with solution time being reduced to 14.3 per cent.

Research paper thumbnail of Enhancing the reconstruction of acoustic source field using wavelet transformation

Journal of Mechanical Science and Technology - J MECH SCI TECHNOL, 2005

This paper shows the use of wavelet transformation combined with inverse acoustics to reconstruct... more This paper shows the use of wavelet transformation combined with inverse acoustics to reconstruct the surface velocity of a noise source. This approach uses the boundary element analysis based on the measured sound pressure at a set of field points, the Helmholtz integral equations and wavelet transformation for reconstructing the normal surface velocity field. The reconstructed field can be diverged due to the small measurement errors in the case of nearfield acoustic holography (NAH) using an inverse boundary element method. In order to avoid this instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the order of an optimal truncation is difficult to determine. In this paper, a wavelet transformation is applied to reduce the computation time for inverse acoustics...

Research paper thumbnail of A Method for Determining the Optimal Location of a Distributed Sensor/Actuator

Shock and Vibration, 1994

The optimal location problem of distributed sensor/actuator for observation and control of a flex... more The optimal location problem of distributed sensor/actuator for observation and control of a flexible structure is investigated. Using a property of controllability and observability grammian matrices, this approach employs a nonlinear optimization technique to determine the optimal placement of a distributed sensor/actuator. The effect of unimportant modes that do not strongly affect the structural behavior of a system is minimized and the effect of important modes is maximized. The final objective function is expressed as the combinational form of two different objective functions. This technique is applied to several types of beam support conditions and the corresponding optimal locations are determined.

Research paper thumbnail of Theoretical modeling, experiments and optimization of piezoelectric multimorph

Smart Materials & Structures, 2005

This paper deals with the static and dynamic electromechanical responses of piezoelectric layered... more This paper deals with the static and dynamic electromechanical responses of piezoelectric layered structures (multimorphs). Based on the Bernoulli-Euler plate model including the dynamics of piezoelectric, electrode and substrate layers, we obtain the natural frequencies, maximum displacement and resultant force of a symmetric cantilevered multimorph. The proposed theoretical model is verified by experiments using a 20-layered PZT (plumbum-zirconate-titanate) multimorph, and it is compared to the conventional bimorph model. Experimental results agree with the analytical predictions on the natural frequencies and vertical displacement. With the analytical solution for multimorph, we investigate the effects of the layer number and the layer thickness on natural frequency, maximum deflection and output force. It is found that there exists an optimum number of piezoelectric layers to maximize the transverse deflection. There also exists a specific value of the thickness ratio between piezoelectric and structure layers to maximize both the tip deflection and force.

Research paper thumbnail of Finite Element Analysis of Carbon Diffusion at 880 ℃ in an Ultra-Low Carbon Steel for Solid Quenching

Korean Journal of Metals and Materials, 2013

Research paper thumbnail of Noise Source Localization by Applying MUSIC with Wavelet Transformation

In inverse acoustic problem with nearfield sources, it is important to separate multiple acoustic... more In inverse acoustic problem with nearfield sources, it is important to separate multiple acoustic sources and to measure the position of each target. This paper proposes a new algorithm by applying MUSIC(Multiple Signal Classification) to the outputs of discrete wavelet transformation with sub-band selection based on the entropy threshold, Some numerical experiments show that the proposed method can estimate the more precise positions than a conventional MUSIC algorithm under moderately correlated signal and relatively low signal-to-noise ratio case.

Research paper thumbnail of Analysis of Structure-borne Noise and Acoustic Sensitivity

한국자동차공학회 1997년 학술강연초록집, Sep 1, 1997

Research paper thumbnail of Replication of Automotive Vibration Target Signal Using Iterative Learning Control and Stewart Platform with Halbach Magnet Array

Transactions of The Korean Society for Noise and Vibration Engineering, 2013

This paper presents the replication of a desired vibration response by iterative learning control... more This paper presents the replication of a desired vibration response by iterative learning control (ILC) system for a vibration motion replication actuator. The vibration motion replication actuator has parameter uncertainties including system nonlinearity and joint nonlinearity. Vehicle manufacturers worldwide are increasingly relying on road simulation facilities that put simulated loads and stresses on vehicles and subassemblies in order to reduce development time. Road simulation algorithm is the key point of developing road simulation system. With the rapid progress of digital signal processing technology, more complex control algorithms including iterative learning control can be utilized. In this paper, ILC algorithm was utilized to produce simultaneously the six channels of desired responses using the Stewart platform composed of six linear electro-magnetic actuators with Halbach magnet array. The convergence rate and accuracy showed reasonable results to meet the requirement...

Research paper thumbnail of Holographic reconstruction of active sources in a three-dimensional enclosure

Journal of the Acoustical Society of America, 1998

This study proposes a method to identify active sources. The theory itself does not limit the app... more This study proposes a method to identify active sources. The theory itself does not limit the application of the proposed method, but the main aim has been focused on finding the location of active noise sources and their strengths in an enclosure. The acoustic holography method can be applied to identify active sources, primary sources if one prefers to call them. However, it will depict all sources including imaginary sources, which one may call secondary sources, which are due to reflections on the walls of enclosure. In this study, an attempt has been made to distinguish these two different types of sources. First, boundaries are modeled by the inhomogeneous Robin boundary condition. It makes it possible to represent the boundaries as the combination of general ideal sources and passive boundaries; the ideal sources imply the primary sources, while the passive boundaries control the secondary sources. Second, this general boundary condition is combined with the holography method...

Research paper thumbnail of Enhancing nearfield acoustic holography using wavelet transform

KSME International Journal

When there are low signal to noise relationships or low coherences between measured pressure and ... more When there are low signal to noise relationships or low coherences between measured pressure and a reference sensor, a pressure field measured and estimated by NAH (Nearfield Acoustic Holography) becomes noisy on the hologram and source planes. This paper proposes a method to obtain the high coherent de-noised pressure signals from low coherent noisy ones by combining a wavelet algorithm with NAH. The proposed method obtains the de-noised field from acoustic fields on a noise source plane reconstructed through backward propagation of NAH. Thus this method does not need high coherent pressure signals on the hologram surface while the conventional nearfield acoustic holography requires high-coherent signals. The proposed method was verified by numerical simulation using noisy signals, composed of original signals and imposed noises distributed on the hologram surface.

Research paper thumbnail of A study on iterative learning control for vibration of Stewart platform

International Journal of Control, Automation and Systems

This paper presents the replication of a desired vibration response by iterative learning control... more This paper presents the replication of a desired vibration response by iterative learning control system for a Stewart platform. The Stewart platform is a multi-input, multi-output system with parameter uncertainties including system nonlinearity and joint nonlinearity. Most vehicle manufacturers are relying on road test simulation facilities in order to reduce development time and to enhance product quality. Road simulation algorithm is essential for developing road test simulation system. With digital signal processing technology, more complex control algorithms including iterative learning control can be utilized. In this paper, a controller based on iterative learning control (ILC) algorithm was developed to produce the desired target response in case of a single actuator as the first experiment after programmed with C language. As a next experiment, the control algorithm was implemented in a road test simulation system using a Stewart platform. A real test was carried out to replicate total six channels of acceleration signals measured at top and left side points of audio player system installed to a car running on Belgian road. The convergence rate and test simulation accuracy higher than 90% showed that the algorithm was acceptable to replicate the target vibration response.

Research paper thumbnail of Design of a slim-type optical pick-up actuator using PMN-PT bimorphs

Smart Materials and Structures, Nov 12, 2006

In this paper, a new optical pickup actuator is proposed using PMN-PT (lead magnesium niobate-lea... more In this paper, a new optical pickup actuator is proposed using PMN-PT (lead magnesium niobate-lead titanate) bimorphs for slim and small form factor optical disk drives. We suggest a novel structure enabling both tracking and focusing motions by changing the moving directions of the two parallel bimorphs. A cymbal-type flextensional structure is used as a displacement amplifier in order to meet the stroke requirement for optical pickup actuators. We have performed the theoretical analyses for the bimorph actuator and displacement amplifier to predict the resultant force and displacement. The proposed actuator based on PMN-PT bimorphs and displacement amplifier has been manufactured, and the experimental results are compared to the analytical predictions. Experimental results agree well with the analytical predictions, showing that the cymbal structure amplifies the displacement twice and the focusing stroke is 52 μm at 10 V.

Research paper thumbnail of Reconstruction of surface velocity field using wavelet transformation and boundary-element method

Archive Proceedings of the Institution of Mechanical Engineers Part C Journal of Mechanical Engineering Science 1989 1996, Feb 1, 2007

This paper shows the application of discrete wavelet transformation (DWT) to inverse acoustics fo... more This paper shows the application of discrete wavelet transformation (DWT) to inverse acoustics for reconstructing the surface velocity of a noise source. This approach uses the boundary-element analysis based on the measured sound pressure at a set of field points, the Helmholtz integral equations, and wavelet transformation to reconstruct the normal surface velocity field. The reconstructed velocity field can be diverged due to the small measurement errors in the case of nearfield acoustic holography using an inverse boundary-element method. In order to bypass the instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the order of an optimal truncation is difficult to determine. In this paper, a DWT is applied to reduce the computation time for inverse acoustics and to enhance the reconstructed surface velocity field. The computational speed-up is achieved, with solution time being reduced to 14.3 per cent.

Research paper thumbnail of Enhancing the reconstruction of acoustic source field using wavelet transformation

Journal of Mechanical Science and Technology - J MECH SCI TECHNOL, 2005

This paper shows the use of wavelet transformation combined with inverse acoustics to reconstruct... more This paper shows the use of wavelet transformation combined with inverse acoustics to reconstruct the surface velocity of a noise source. This approach uses the boundary element analysis based on the measured sound pressure at a set of field points, the Helmholtz integral equations and wavelet transformation for reconstructing the normal surface velocity field. The reconstructed field can be diverged due to the small measurement errors in the case of nearfield acoustic holography (NAH) using an inverse boundary element method. In order to avoid this instability in the inverse problem, the reconstruction process should include some form of regularization for enhancing the resolution of source images. The usual method of regularization has been the truncation of wave vectors associated with small singular values, although the order of an optimal truncation is difficult to determine. In this paper, a wavelet transformation is applied to reduce the computation time for inverse acoustics...

Research paper thumbnail of A Method for Determining the Optimal Location of a Distributed Sensor/Actuator

Shock and Vibration, 1994

The optimal location problem of distributed sensor/actuator for observation and control of a flex... more The optimal location problem of distributed sensor/actuator for observation and control of a flexible structure is investigated. Using a property of controllability and observability grammian matrices, this approach employs a nonlinear optimization technique to determine the optimal placement of a distributed sensor/actuator. The effect of unimportant modes that do not strongly affect the structural behavior of a system is minimized and the effect of important modes is maximized. The final objective function is expressed as the combinational form of two different objective functions. This technique is applied to several types of beam support conditions and the corresponding optimal locations are determined.

Research paper thumbnail of Theoretical modeling, experiments and optimization of piezoelectric multimorph

Smart Materials & Structures, 2005

This paper deals with the static and dynamic electromechanical responses of piezoelectric layered... more This paper deals with the static and dynamic electromechanical responses of piezoelectric layered structures (multimorphs). Based on the Bernoulli-Euler plate model including the dynamics of piezoelectric, electrode and substrate layers, we obtain the natural frequencies, maximum displacement and resultant force of a symmetric cantilevered multimorph. The proposed theoretical model is verified by experiments using a 20-layered PZT (plumbum-zirconate-titanate) multimorph, and it is compared to the conventional bimorph model. Experimental results agree with the analytical predictions on the natural frequencies and vertical displacement. With the analytical solution for multimorph, we investigate the effects of the layer number and the layer thickness on natural frequency, maximum deflection and output force. It is found that there exists an optimum number of piezoelectric layers to maximize the transverse deflection. There also exists a specific value of the thickness ratio between piezoelectric and structure layers to maximize both the tip deflection and force.