• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 2003.04a
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• pp.79-85
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• 2003

One of unique characteristics of guided waves is a dispersive behavior that guided wave velocity changes with an excitation frequency and mode. In practical applications of guided wave techniques, it is very important to identify propagating modes in a time-domain waveform for determination of defect location and size. Mode identification can be done by measurement of group velocity in a time-domain waveform. Thus, it is preferred to generate a single or less dispersive mode But in many cases, it is difficult to distinguish a mode clearly in a time-domain waveform because of superposition of multi modes and mode conversion phenomena. Time-frequency analysis is used as efficient methods to identify modes by presenting wave energy distribution in a time-frequency. In this study, experimental guided wave mode identification is carried out in a steel plate using time-frequency analysis methods such as wavelet transform. The results are compared with theoretically calculated group velocity dispersion curves. The results are in good agreement with analytical predictions and show the effectiveness of using the wavelet transform method to identify and measure the amplitudes of individual guided wave modes.

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.26-29
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• 2008

In the fields of electronics and displays where inkjet printing has demonstrated its capability to fabricate colorant subpixels of thin film transistor liquid crystal(TFT LCD) color filters and organic light emitting diode (OLED) displays, conducting tracks and TFTs, the production of satellite droplets is one of primary things to eliminate because they generally deteriorate the pattern quality. To understand the production mechanism of satellite droplets in this paper, driving waveforms such as monopolar and bipolar were employed and the influence of the pulse duration time were investigated in both experimental and numerical aspects.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.7
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• pp.127-133
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• 2001

The effects of triangular and rectangular discharge current waveforms on anode erosion in wire electric discharge machining is investigated based on heat transfer analysis of half-space subject to time and space-dependent heat flux. The thermal load on the anode is simulated by a hear flux that is calculated from time-dependent discharge channel radius and energy. Evolution of the melting front during discharge, the molten volume at the experimental result for qualitative verification of the analysis. It is demonstrated that the triangular current waveform is more efficient in eroding the anode than the rectangular one.

• International Journal of Precision Engineering and Manufacturing
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• v.7 no.3
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• pp.14-17
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• 2006

To diagnosis abnormal compressor conditions in an air-conditioner, the acoustic emission (AE) signal, which is derived from wear condition, compressed air, and assembly error, was analyzed experimentally. Burst and continuous type AE signals resulted from metal contact and compressed air, and the raw AE signal of compressors was acquired in the production line. After extracting samples using waveforms, the Early Life Test (ELT) was conducted and the waveform was classified as normal or abnormal. Efficient parameters in the waveform pattern were investigated in time and frequency domains and a diagnosis algorithm for air-conditioners using Neural Network estimation is suggested.

• Journal of the Korean Society of Safety
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• v.15 no.2
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• pp.8-12
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• 2000

The fatigue crack propagation tests were performed in triangular and holding-time stress waveforms at $650^{\circ}C$. The behavior of fatigue crack propagation was investigated according to waveform. The analysis of high temperature fatigue crack propagation by the stress intensity factor range ${\Delta}K$, elastic fracture mechanics parameter, was not available. The behaviors of high temperature fatigue crack propagation by the J-integral(${\Delta}J_f$, J' and ${\Delta}J_c$), elasto-plastic fracture mechanics parameter, were investigated in a number of stress waveforms. The fast-fast waveform exhibited cycle-dependent(fatigue type), the slow-fast and the hold time with 500sec waveforms appear to be time-dependent(creep type) and the fast-slow and the hold time with 5, 25sec waveforms exhibited conbined behavior of both types(fatigue-creep conbined type).

• Proceedings of the KIEE Conference
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• 1993.07b
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• pp.840-843
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• 1993

In this paper, an LPM controller using microprocessor was implemented and adopted Microstep method to control the LPW. In Microstep method, current waveform is sinusoidal not square. This method is characterized by less vibrating, less noisy, and more precise position control. Also, we simulate the static thrust characteristics for each waveform. The experiment was performed according to the mechanical vibration at the acceleration mode and slewing mode. The current tracks reference sinusoidal waveform well and stability was improved as we expected. Therefore, the possibility of the open loop position control was proposed.

• Structural Engineering and Mechanics
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• v.76 no.5
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• pp.591-598
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• 2020

Ultrasonic waves provide a non-destructive and sensitive way to monitor the concrete hydration. However, limited works are reported to monitor the evolution of the mechanical parameter at early ages. In this study, modified piezoelectric aggregates are embedded inside a concrete beam to excite and receive primary waves. A hydration index, namely, the variation of ultrasonic waveform (VUW) is developed to characterize the variation of the transmitted waves during the hydration process. The recorded hydration indices are compared with the compressive strength measured by destructive test at different ages. The results show that the VUW is closer to the compressive strength than the other two traditional hydration indices, ultrasonic velocity and wave packet energy. The proposed VUW provides a simple and accurate way to monitor the concrete hydration at early ages.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.11-11
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• 2009

GMA용접은 높은 생산성과 자동화 효율로 널리 사용되고 있다. 그러나 종래의 인버터 용접기는 스패터 발생이 높고 고속용접이 어려운 단점을 가지고 있다. 본 연구에서는 이러한 문제점을 해결하기 위해 와이어 송급 전후진을 가능하게 하는 장치를 개발하였고 개발된 장치를 H사의 펄스용접기에 장착하여 기계적 제어(와이어 전후진 고속 송급제어)를 통해 저스패터와 고속 용접이 가능한 기계적 전류파형을 개발하였다. 스패터는 단락과 아크발생시에 높은 전류로 인해 발생되기 때문에 기계적 제어를 통하여 Fig. 1에 보는 바와 같이 낮은 전류에서 단락과 아크발생이 일어나 스패터 발생이 줄어들게 되고 기계적 제어를 통해 단락과 아크발생 주기가 일정하게 되어 아크의 안정성 및 고속용접에 유리하게 된다. 개발된 기계적 파형을 이용하여 가장 스패터 발생이 많은 재료인 스틸과 $CO_2$를 보호가스로 하여 용접전류와 용접 속도를 변화시키며 실험한 결과 저스패터 및 1.5m/min이상의 용접 속도에서 안정된 비드를 얻을 수 있었다.

• Steel and Composite Structures
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• v.17 no.5
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• pp.561-572
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• 2014

In this paper, we present the scattering characteristics of infinite and finite array using method of moment (MoM) with Floquet harmonics and asymptotic waveform evaluation (AWE) technique. First, infinite cylindrical dipole array is analyzed using the MoM with entire domain basis function and cylindrical Floquet harmonics. To provide the validity of results, we fabricated the cylindrical dipole array and measured the transmission characteristics. The results show good agreements. Second, we analyzed the scattering characteristics of finite array. A large simulation time is needed to obtain the scattering characteristics of finite array over wide frequency range because Floquet harmonics can't be applied. So, we used the MoM with AWE technique using Taylor series and Pade approximation to overcome the shortcomings of conventional MoM. We calculated the radar cross section (RCS) as scattering characteristics using the proposed method in this paper and the conventional MoM for finite planar slot array, finite spherical slot array, and finite cylindrical dipole array, respectively. The compared results agree well and show that the proposed method in this paper is good for electromagnetic analysis of finite FSS.

• Proceedings of the KWS Conference
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• 2007.11a
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• pp.291-293
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• 2007